Note: Descriptions are shown in the official language in which they were submitted.
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Title of the Invention
UNIPOLARITY POWDER COATING SYSTEMS INCLUDING
IMPROVED TRIBOCRARGING AND CORONA GUNS
RELATED APPLICATION
This application is a continuation-in-part of pending United States patent
application serial no.
09/724,363 filed on November 2~, 2000 for UNIPOLARITY POWDER COATING SYSTEMS
INCLUDING M'ROVED TRIBOCHARGING AND CORONA GUNS, the entire disclosure of
which is
fully incorporated herein by reference. This application also claims the
benefit of United States
Provisional patent application serial no. 60/217,216 filed on July 11, 2000
for A UN1POLARITY
POWDER COATING SYSTEM INCLUDING AN IMPROVED TRIBOCHARGING GUN,
UNIfOLARITY GUN AND METHOD FOR MAKING SAME, the entire disclosure of which is
fully
incorporated herein by reference.
Field of the Invention
This invention relates to powder coating systems which use corona and
tribocharging powder
spray guns to apply an electrostatic charge to powder for deposition on a
substrate.
Background of the Invention
There are two basic types of powder spray guns which are commonly used in the
electrostatic
2.0 powder spray coating of articles. The most common type of spray gun is the
corona type, which has a
high voltage charging electrode which produces a corona to charge the powder.
Typically, corona guns
are designed to charge the powder negatively. One major disadvantage of corona
guns is that they do not
coat the interior corners of parts well due to the strong electrostatic field
or Faraday caging effect
produced by the corona electrode. A second disadvantage to corona guns is that
back ionization may
occur due to the formation of free ions which results in pinholing or an
orange peel surface of the part to
be coated. Another disadvantage to these type of guns is that the system
components such as the nozzle,
and diffuser as well as the powder deliver system components such as the pump,
hopper and other parts in
contact with the powder delivery system are typically made of materials such
as polyethylene or
polytetrafluoroethylene (PTFE). While these materials have the advantage of
low impact fusion, they
have the disadvantage of positively charging the powder, which can impair the
negative corona charging
process because the final or maximum charge on the powder is diminished.
Further, more voltage is often
required in order to counteract the positive polarity charging of the system.
In addition, this positive
polarity tribocharging may cause breakdown of the powder conveying components
such as the hose,
which connects the pump to the spray gun.
A second type of gun which is also commonly used is a tribocharging gun in
which the powder is
charged by frictional contact with the interior surfaces of the gun. One
advantage to triboelectric guns is
that the powder can easily penetrate corners of parts to be coated because the
gun does not produce a
strong electric field like a corona gun does.
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Summary of the Present Invention
The invention provides novel electrostatic powder coating guns and system
components in which
powder is pre-charged to the same polarity as a charge applied by the powder
spray gun in order to
increase and enhance the applied charge and the transfer efficiency. Also
novel powder coating methods
are described.
In accordance with one aspect of the invention, an apparatus for spraying
powder coating material
is described. The apparatus has a powder flow path, wherein the powder flow
path has a charging surface
for triboelectrically charging powder coating material which comes in contact
with the charging surface,
and the charging surface comprises a negative tribocharging material selected
from polyamide resin
blends, fiber reinforced polyamides, aminoplastic resins and acetal polymers.
In accordance with another aspect of the invention, an apparatus for spraying
powder coating
material has a powder flow path, wherein the powder flow path has a charging
surface for triboelectrically
charging powder coating material which comes in contact with the charging
surface, and wherein one or
more air passages are formed through the charging surface, the air passages
being in a fluid
communication with a source of compressed air.
In accordance with another aspect of the invention, an apparatus for spraying
powder coating
material is described. The apparatus has a powder flow path through which the
powder coating material
flows, wherein the powder flow path has a first charging surface for
triboelectrically charging powder
coating material which comes in contact with the first charging surface, the
first charging surface
comprising a tribocharging material having a first charging polarity, the
apparatus fizrther comprising a
component through which powder coating material also flows, the component
having a second charging
surface which also comprises a tribocharging material having the first
charging polarity.
In accordance with another aspect of the invention, a system for applying
powder coating
materials to articles is described. The system includes a powder feed
apparatus for supplying powder
coating material and an apparatus for spraying powder coating material
received from the feed apparatus.
The spraying apparatus has an electrode for charging the powder coating
material a first charging polarity.
The feed apparatus includes a component having a charging surface for
triboelectrically charging powder
coating material which comes in contact with the charging surface, the
charging surface comprising a
tribocharging material having the first charging polarity.
In accordance with another aspect of the invention, a system for applying
powder coating
materials to articles is described. The system includes at least one corona
charging spraying apparatus
and at least one tribocharging spraying apparatus. The corona charging
spraying apparatus has an
electrode for charging the powder coating material a first charging polarity.
The tribocharging spraying
apparatus has a powder flow path, wherein the powder flow path has a charging
surface for
triboelectrically charging powder coating material which comes in contact with
the charging surface, the
powder coating material being charged to the first polarity by the charging
surface of the tribocharging
spraying apparatus.
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In accordance with another aspect of the invention, a tribocharging powder
spraying apparatus is
described. The apparatus includes a body having an internal bore, a wear tube
located within the internal
bore, and an open passageway provided between the internal bore and the wear
tube, with at least one air
jet passageway being provided through the wear tube. The air jet passageway
provides fluid
communication between the open passageway and the interior of the wear tube.
The wear tube has a
charging surface for triboelectrically charging powder coating material which
comes in contact with the
charging surface. The open passageway is in fluid communication with a source
of compressed air,
whereby compressed air flows from the open passageway through the air jet
passageway into the interior
of the wear tube to affect the flow of powder coating material through the
wear tube.
In accordance with another aspect of the invention, a system for applying
powder coating
materials to articles is described. The system includes a powder feed
apparatus for supplying powder
coating material and an apparatus for spraying powder coating material
received from the feed apparatus.
The feed apparatus includes a component having a charging surface for
triboelectrically charging powder
coating material that comes in contact with the charging surface. The
component charging surface is
comprised of a negative tribocharging material selected from polyamide resin
blends, fiber reinforced
polyamides, aminoplastic resins and acetal polymers.
In accordance with another aspect of the invention, a triboelectric powder
coating gun has a
component which includes a triboelectric charging surface, wherein the
component is capable of assembly
into the gun in at least two different positional orientations. Still a
further aspect of the invention provides
a triboelectric powder coating gun having a triboelectric charging surface and
an air jet which impinges on
the charging surface, further including a ground element which is positioned
upstream of the charging
surface.
These and other aspects of the invention are herein described in detail with
reference to the
accompanying Figures.
Description of the Figures
Figure 1 is a cross-sectional view of a tribocharging gun which incorporates
the novel
unconventional materials of the invention;
Figure 2 is a cross-sectional view of a novel short barrel tribocharging gun
of the present
invention;
Figures 3A through 3D illustrate a portion of the insert of the gun of Figure
2 in which the airjets
are arranged in various opposed configurations;
Figure 4A illustrates a cross-sectional view of the insert of the short barrel
tribocharging gun of
Figure 2, aft looking forward, in which the airjets are not vertically offset
from each other;
Figures 4B through 4E illustrate cross-sectional views of the insert of the
short barrel
tribocharging gun of Figure 2, aft looking forward, in which the airjets are
vertically offset from each
other a perpendicular distance H;
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Figures SA and SB each illustrate a cross-sectional view of the insert of the
short barrel
tribocharging gun of Figure 2, aft looking forward, in which a first set of
airjets as shown in Figure SA are
not rotationally offset from a second set of downstream airjets as shown in
Figure SB;
Figures 5E through SF each illustrate a cross-sectional view of the insert of
the short barrel
tribocharging gun of Figure 2, aft looking forward, in which a first set of
airjets as shown in Figures SC
and SE are rotationally offset from a second set of downstream airjets as
shown in Figures SD, and SF,
respectively;
Figures SG and SH each illustrate a cross-sectional view of the insert of the
short barrel
tribocharging gun of Figure 2, aft looking forward, in which a first set of
airjets as shown in Figures SG
are not rotationally offset from a single downstream airjet as shown in Figure
SH;
Figure 6 illustrates a cross-sectional view of a corona gun which incorporates
the novel
unconventional materials of the invention;
Figure 7 illustrates a cross-sectional view of a flat spray nozzle which
incorporates the novel
unconventional materials and one or more airjets of the invention;
Figure 8 is a cross-sectional view of a powder pump of a powder coating system
which
incorporates the novel unconventional materials of the invention;
Figure 9 illustrates a perspective schematic view of powder coating system
which includes a
corona and tribocharging gun which charge the powder to the same polarity;
Figure 10 is a cross-sectional view of an alternate embodiment of a
tribocharging gun of the
present invention which incorporates airjets;
Figure 10A is a cutaway view of the gun shown in Figure 10 in the direction
l0A-10A;
Figure 11 is a cross-sectional view of yet another alternate embodiment of a
tribocharging gun of
the present invention which incorporates airjets arranged in a helical
pattern;
Figure 11A is a cutaway view of the gun shown in Figure 11 in the direction
11A-1 1A;
Figure 12 is a cross-sectional illustration of another embodiment of a
tribocharging gun using air
j ets;
Figure 13 is a cross-sectional illustration of a modified version of the gun
in Figure 12 having a
portion with air jets and a tribocharging post-charge portion;
Figure 14 is another cross-sectional illustration of a modified version of the
gun in Figure 12 in
which there is a pre-charge portion with air jets followed by a tribocharging
portion;
Figures 15 and 16 are cross-sectional views of two embodiments of an inside-
out gun in
accordance with the invention;
Figure 17 illustrates an embodiment of an air jet induced charging gun in a
conventional manual
spray gun configuration;
Figures 18A-D illustrate additional embodiments of the gun style of Fig. 17
using different
extension lengths;
Figure 19 illustrates an inside-out gun in a manual gun configuration;
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Figure 20 illustrates a spray gun that incorporates an inside-out
configuration with an outside-in
configuration; and
Figures 21-24 illustrate another embodiment of the invention.
Detailed Description of Preferred and Alternate Embodiments
The following Detailed Description of Preferred and Alternate Embodiments is
divided into the
following sections. Section I provides a detailed description of a novel
tribocharging gun which charges a
powder to a negative polarity by frictional contact with novel use of
unconventional materials as
described in more detail below. Section II provides a detailed description of
a novel short barrel
tribocharging gun which can charge powder to a positive or negative polarity
depending upon the
materials selected for frictional contact with the tribocharging surfaces of
the gun. Sections III and IV
concern a corona gun and powder supply system, respectively, with the corona
gun and system including
components which charge the powder to the same polarity as the corona gun by
fractionally contacting the
powder with tribocharging surfaces comprised of the desired positive or
negative tribocharging material.
Section V provides a detailed description of a powder coating system which
includes corona and
tribocharging guns which charge the powder to the same polarity so that the
tribocharging gun can be
used in conjunction with the corona gun to coat the same workpiece. Finally,
Section VI provides a
detailed description of an alternate tribocharging gun embodiment which
utilizes air jets.
I. NEGATIVE TRIBOCHARGING GUN CONSTRUCTED FROM
UNCONVENTIONAL MATERIALS.
A. UNCONVENTIONAL NEGATIVE CHARGING
TRIBOMATERTALS
A part of this invention is the discovery of what will be referred to herein
as "unconventional
negative charging tribomaterials". These materials are useful as powder
contact surfaces for negatively
charging powder coating material by frictional contact with the powder contact
surfaces of a powder spray
gun. The term "negative charging tribomaterials" means materials which impart
a negative charge to
powders, such as powdered paints, upon frictional impact with the surface of
the negative charging
tribomaterials.
As described in more detail in this application, the unconventional negative
charging
tribomaterials could be used as the interior surfaces of tribocharging or
corona powder spray guns, as well
as spray gun components and powder delivery system components such as the
diffuser, powder tube, feed
hopper, and pump as described in more detail in Section IV. Although the
unconventional negative
charging tribomaterials are known generally, they have not been previously
known to be useful in spray
guns in order to tribocharge powder coating materials.
The non-conventional negative charging tribomaterials are selected from
polyamide blends, fiber
reinforced polyamide resins, the aminoplastic resins, acetal polymers or
mixture thereof, and are described
in more detail, below. These materials not only charge well negatively but
they also do not experience
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impact fusion problems as significant as negative tribo charging materials
which have been used in the
past such as nylon.
1. The Polyamide Blend
The polyamide blend comprises a blend of a polyamide polymer and a second
polymer selected
from the group consisting of: polyethylene, polypropylene, halogenated
hydrocarbon resin, and mixtures
thereof. The polyamide polymer is preferably present in the polyamide blend
from 50% to 96%, more
preferably from 70% to 90%, by weight. The second polymer is preferably
present in the polyamide
blend from about 4% to about 50%, more preferably from about 10% to about 30%,
most preferably from
about 15% to about 25% by weight.
The halogenated hydrocarbon resin is preferably a fluorinated hydrocarbon
resin, such as for
example, polytetrafluoroethylene, (also known as PTFE); a copolymer of
tetrafluoroethylene and
hexafluoropropylene (also known as FEP); and a copolymer of
tetrafluoroethylene and perfluorinated
vinyl ether (also known as PFA). Suitable fluorinated resins are commercially
available under the
tradename TEFLON~ from DuPont.
The polyamide polymer in the polyamide blend is preferably a nylon. Preferred
grades of nylon
are nylon 6/6, nylon 6/12, nylon 416 and nylon 11. A suitable polyamide blend
is a 20%
polytetrafluoethylene and 80% nylon 6/6 commercially available under the trade
name Lubricon RL 4040
from LNP Engineering Plastics, Division of ICI Advanced Materials, Exton,
Pennsylvania. A suitable
blend is about a 5% polytetrafluoethylene and about a 95% nylon 6/6
commercially available under the
trade name Lubricon RL 4010 from LNP Engineering Plastics, Division of ICI
Advanced Materials,
Exton, Pennsylvania.
Example 1
Individual discs of a 20% polytetrafluoethylene and 80% nylon 6/6,
polyamide/halogenated
hydrocarbon resin blend were prepared. For comparison, coupons of conventional
material, that is, nylon
and Teflon were also prepared.
The relative transfer efficiency was determined by spraying powder paint from
a flat spray nozzle
with a 0.450 inch by 0.065 inch slot at an air flow rate of 4 cubic feet per
minute onto a disc at a 45°
angle. The powder impacted the surface of the disc of the tribocharging
material and was deflected from
the disc onto a grounded metal target. The powder exiting the nozzle had a
measured initial charge of
zero. Thus, all of the powder charging was due to impacting the tribomaterial.
The amount of powder
adhered to the target as compared to the total powder sprayed is defined as
the relative transfer efficiency.
Typically, 50 grams of polyester epoxy powder from Ferro Corporation was the
powder used for the tests.
Since this relative transfer efficiency test is done by a single impact from a
coupon, the values tend to be
lower than for numerous contacts using a tribocharging gun.
The powder used in the evaluation was a polyester epoxy powder, designated
153W-121, from
Ferro Corporation. The results are shown below in Table I.
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example 2
Individual discs of a 5% PTFE and 95% nylon 616, polyamide blend were prepared
and the
transfer efficiency was evaluated as in Example 1. The results are shown below
in Table I.
The advantage of using the polyamide blends in powder spray guns is that they
increase the
powder charging due to increased discharging of the tribocharged gun surfaces.
The increased surface
discharging is due to the incompatible polymers which provide for a leakage
path that is not present in the
homogeneous polymer. Another advantage of using these polyamide blends is that
reduced moisture
absorption of nylons occur when they are filled with PTFE or polyethylene.
2. The Fiber Reinforced Polyamide Resin
The fiber reinforced polyamide resin comprise a polyamide polymer filled with
polyaramide
fibers. Preferably there is from about 50% to about 99%, more preferably from
about 85% to about 95%
of the polyamide polymer. Preferably there is from about 1% to about 50%, and
more preferably from
about 5% to about 15% of the polyaramide fiber in the polyamide polymer.
The polyamide polymer in the fiber reinforced polyamide resin is preferably
commercially
available polyamide polymers. Suitable polyamides are for example, nylons.
The polyaramide fibers are long chain synthetic aromatic polyamides in which
at least 85% of the
amide linkages are attached directly to two aromatic rings. A suitable
polyaramide fiber is a poly(p-
phenylene terephthalamide) commercially available under the trade name
KEVLAR~, from DuPont. The
polyaramide fiber, poly(m-phenylene terephthalamide), commercially available
under the trade name
Nomex, from DuPont, is less preferred. Examples of other polyaramide fibers
are the polymer comprising
polymerized units of p-aminobenzhydrazide and terephthaloyl chloride; a
suitable such polymer is
commercially available under the trade name PABH-T X-500 from Monsanto.
A suitable fiber reinforced polyamide resin is 10% KEVLAR~ in 90% nylon 6,6
available under
the trade name Lubricon RA from LNP Engineering Plastics, Division of ICI
Advanced Materials, Exton,
Pennsylvania.
Example 3
Individual discs of the fiber reinforced polyamide resin were prepared. For
comparison, coupons
of conventional, non fiber containing nylon and Teflon were also prepared. The
relative transfer
efficiency was determined as in Example 1. The results are shown below in
Table I.
TABLE I
EXAMPLE MATERIAL DISK POLARITY RELATIVE
THICKNESS TRANSFER EFFICIENCY
IN
Com arativeN lon 6,6 0.155 - 16.5
1 5% PTFE in N lon 0.250 - 21.3
6,6
2 20% PTFE in N lon 0.250 - 24.7
6,6
3 10% KEVLAR~ in N 0.123 - 39.2
lon 6,6
Comparative100% KEVLAR~ tow --- + 54.3
fibers
4 N lon R MoS2 filled0.118 - 22.4
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Surprisingly, despite the fact that the KEVLAR~ tow fiber charges powder
positively in the
comparative example, the addition of such fiber to the nylon which charges
negatively, increased the
relative transfer efficiency.
3. The Aminonlastic Resins
The aminoplastic resins are comprised of polymerized units of an amine monomer
and an
aldehyde monomer. Preferred aminio plastic resins are aniline formaldehyde
resins, urea formaldehyde
resins and melamine formaldehyde resins. Optionally, the aminoplastic resins
fiufiher comprise cellulose
such as alpha-cellulose and pigments.
Suitable molding grade melamine formaldehyde resins filled with alpha
cellulose, are
commercially available under the trade name Perstorp 752026 white melamine or
Perstorp 775270 red
melamine available from Perstorp Compounds, Inc. in Florence, Massachusetts.
Another suitable
melamine resin is a melamine phenol-formaldehyde copolymer, commercially
available under the trade
name Plenco 00732, from Plenco Plastics Engineering Company in Sheboygan,
Wisconsin.
Another suitable melamine resin is a melamine formaldehyde polymer, Perstop
752-046,
available from Perstorp Compounds, Inc. in Florence, Massachusetts.
Example 4
Individual discs of the white melamine formaldehyde resin, Perstorp 752026,
filled with alpha
cellulose were obtained. For comparison, discs of conventional nylon 6/6 were
also prepared. Relative
transfer efficiency was determined as in Example 1. The results are shown
below in Table II.
Example 5
Individual discs of the red peppercorn melamine formaldehyde resin, Perstorp
775270, filled with
alpha cellulose were obtained. For comparison, discs of conventional nylon
were also prepared. The
relative transfer efficiency was determined as in Example 1. The results are
shown below in Table II.
Example 6
Individual discs of the melamine phenol-formaldehyde resin, Plenco 00732 were
obtained. For
comparison, discs of conventional nylon were also prepared. The relative
transfer efficiency was
determined as in Example 1. The results are shown below in Table II.
Examule 7
Individual discs of the white melamine formaldehyde resin Perstorp 752-046,
were obtained. For
comparison, discs of conventional nylon were also prepared. The relative
transfer efficiency was
determined as in Example 1. The results are shown below in Table II.
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TABLE H. RELATIVE TRANSFER EFFICIENCY OF FERRO 153W-121
ON CONTACT WITH AMINO RESIN COUPONS
EXAMPLE MATERIAL POLARITY RELATIVE
TE
Com arative Nylon 6/6 Negative 16.5
4 Perstorp 752026 white Negative 37.7
Melamine
Perstorp 775270 red Negative 37.0
Pe ercorn melamine
6 Plenco 00732 melamine/ Negative 28.7
henol formaldehyde
7 Perstorp 752-046 Negative 44.9
Melamine-formaldeh de
Powder flow rate = 1.5 g/s
5 Examples 8-10
A short barrel tribo gun as described herein in Section II and shown in Figure
2, was fabricated, in
which the interior surfaces of the gun, specifically the interior surface of
the powder conduit insert and flat
spray nozzle, were made of red peppercorn, melamine formaldehyde, designated
Perstorp 775270 from
Perstorp Compounds Inc., Florence, Massachusetts. The gun used in the test had
two pairs of air jets and
two electrodes. The air jets were offset from the centerline which is
perpendicular to the longitudinal axis
by one jet diameter and the second set of air jets was rotated about the
longitudinal axis by 5 degrees
relative from the first set of air jets. The angle of the air jets was 90
degrees.
The relative transfer efficiency was determined by spraying a set amount of
powder at a target,
moving perpendicular to the spray gun at the rate of 10 feet per minute. The
powder in the spray gun was
an epoxy polyester powder, designated 153W-121 from Ferro Corporation. The
results are presented
below.
TABLE HI.
EXAMPLE NO. MELAMINE FORMALD. POLARITY RELATIVE TRANSFER
GRADE EFFICIENCY
Com arative N lon 6/6 Ne ative 79.3
Ex. 8 Melamine G-9 from AtlasNegative 80.6
Fibre
Co. of Skokie, Illinois
Ex. 9 Red peppercorn melamineNegative 74.3
Persto 775270
Ex. 10 White melamine 700 Negative 74.7
Series
Molding Compound from
Persto
4. Acetal Resins
The acetal resin is a polyoxymethylene engineering thermoplastic polymer. The
acetal resin is a
homopolymer or a copolymer. The acetal resin is optionally combined with
polytetrafluorethylene,
polytetrafluoroethylene fibers, and polyethylene, or other polymers or
additives. Suitable acetal
homopolymers are commercially available under the trademark Delrin~ from E.I.
DuPont de Nemours &
Co., in Wilmington, Delaware. A suitable example is an acetal homopolymer
resin comprising 20%
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'~'eflon PTFE fibers, and is commercially available under the trade name
Delrin AF. One advantage of
this material is that electrical shocks from stored capacitance to operators
handling this gun are less with
this material than other materials tested.
A suitable modified copolymer resin is an acetal copolymer modified with an
ultra high molecular
weight polyethylene (UIIMWPE) which is commercially available under the trade
name Ultraform~
N2380X available from BASF Corp., Parsippany, New Jersey. Another suitable
acetal copolymer is
commercially available under the trade name Celcon~ from the Hoechst Celanese
Corp. in Chatam, New
Jersey.
Example 11
A short barrel tribocharging gun as described below in Section II and shown in
Figure 2, was
fabricated, in which the interior surfaces of the gun, specifically the
interior surface of the insert were
made from the acetal polymer Delrin 150 from DuPont.
The powder in the spray gun was an epoxy polyester powder, designated 153W-121
from Ferro
Corporation or a polyester/urethane powder, designated 153W-281 from Ferro
Corporation. The transfer
efficiency was determined as in the Examples 8-10. The results are presented
below.
Transfer efficiency results are about 62% for both powders as shown in Table
IV. below at a flow
rate of 2.5 g/s.
TABLE IV.
AVERAGE TRANSFER EFFICIENCY
OF DELRIN SHORT TRIBO
GUN
SAMPLE AVERAGE TE
153W-121 61.9
155W-281 62.3
One advantage to these acetal resins is that they are capable of being
injection molded, thus
making it possible to fabricate a low cost powder spray gun. The Delrin acetal
resin relative transfer
efficiency results were surprising and unexpected because the Delrin resin
does not contain nitrogen
atoms, which are typically found in negatively charging materials such as
nylon and melamines. It was
also discovered that the presence of PTFE fibers in the Delrin acetal resin,
such as with the Delrin AF
acetal resin, resulted in an increase in transfer efficiency over the Delrin
acetal resin.
B. NEGATIVE TRIBOCHARGING GUN WITH UNCONVENTIONAL
MATERIALS
Referring now to Figure 1, there is shown a tribocharging powder spray gun 10
for use with the
method and apparatus of the present inventions. The gun 10 includes a gun body
12 having a central
opening extending therethrough. The gun 10 may be supported by a suitable gun
mount assembly which
is known by those skilled' in the art. The gun 10 comprises a powder feed
portion 20, a tribocharging
portion 30 and a sprayhead portion 40 at the outlet end of the gun.
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The tribocharging portion 30 of the gun comprises an inner core 34 positioned
within an outer
cylinder 32 in which the surfaces 34a, 32a cooperate to provide an annular
charging path for the powder
flowing through the charging path of the gun. As shown in Figure 1, the
surfaces 34a, 32a may optionally
comprise a wavy or undulating surface so that the annular gap provides a
tortuous path for the powder,
thereby enhancing powder contact with the surfaces 34a,32a so that charge is
imparted to the powder.
In the preferred embodiment of the invention, some or all of the powder
contact surfaces of the
gun are comprised of a material selected from the group consisting of a
polyamide blend, a fiber
reinforced polyamide resin, an acetal polymer, an acetal polymer homopolymer,
a copolymer, preferably
filled with PTFE fibers (hereinafter collectively referred to as acetyl
polymer), an aminoplastic resin or
mixtures thereof. These are the unconventional negative charging tribo
materials of this invention which
have been found to charge well. Thus the powder contact surface may be coated
with the above
mentioned material or the respective component having the powder contact
surface may be constructed in
whole or in part from the above mentioned materials. Thus as shown in Figure
1, the powder contact
surfaces of the outer cylinder 32, the inner core 34 and the nozzle 40 may be
comprised of a material
selected from the group consisting of a polyamide blend, fiber reinforced
polyamide resin, acetal polymer,
aminoplastic resin or mixtures thereof. Additionally, the powder contact
surfaces of the inner wear sleeve
38, the outer wear sleeve 40, the inlet wear sleeve 41, the inlet distributor
36, the outlet distributor 37, and
the outlet wear sleeve 42 may be coated with or made entirely of a material
selected from the group
consisting of a polyamide blend, fiber reinforced polyamide resin, acetal
polymer, aminoplastic resin or
mixtures thereof. Other powder contact surfaces not specifically referenced
herein may also comprise the
above referenced materials.
A grounded electrode 43, discharge ring or other means know to those skilled
in the art (not
shown) may be utilized to discharge the powder contact surfaces of the inner
core and outer cylinder from
the build up of charge. The grounded electrode or discharge ring may be placed
in any position known to
those skilled in the art.
As shown in Figure 1, powder and the conveying air is fed to the powder feed
portion 20. Powder
enters the charging portion of the gun from the feed portion 20 and is
channeled into the annular charging
path located between the inner core 34 and the outer cylinder 32. As the air
entrained powder repeatedly
contacts the powder contact surfaces 32a, 34a of the outer cylinder 32 and
inner core 34, the powder is
tribocharged to a negative polarity. Finally, the tribocharged powder is
discharged into the sprayhead
portion 40 of the gun. In that unconventional negative charging tribo
materials are used, the powder will
be negatively charged, but the gun will not experience unacceptable impact
fusion of the powder on the
charging surface.
II. SHORT BARREL TRIBOCHARGING POWDER SPRAY GUN CONSTRUCTED FROM
EITHER POSITIVE OR NOVEL NEGATIVE TRIBOCHARGING MATERIALS.
As shown in Figure 2, a first embodiment of the short barrel tribocharging gun
200 of this
invention provides a novel powder spray gun of relatively simple construction
and small size which
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charges powder by the tribocharging process. The invention has the advantage
of a removable insert 220
which can be easily changed for fast color change of the powder. One important
advantage to the short
barrel tribogun is that it does not have the disadvantages of strong electric
fields or back ionization issues
which are present with corona guns. The gun as described in more detail below
can positively or
negatively charge a powder. The triboelectric powder charging gun, indicated
generally at 200, has an
overall length in a range of approximately one to ten inches from the powder
inlet to the nozzle tip, and
more preferably in the range of one to six inches, which is substantially less
than the overall length of
tribocharging guns of the prior art, which typically run from 14-36 inches in
length.
The main components of the gun are a body 210, a powder conduit insert 220
which fits within
the body 210, and a nozzle 230 which also fits within or is otherwise attached
to the body 210. The insert
220 and nozzle 230 together form the barrel of the gun. The body 210 can be
fabricated out of any
structurally suitable material. The body 210 has an intake end 212 having an
opening adapted to receive
an insert 220, and an output end 214 adapted to receive or connect to the
nozzle 230. For manual use, a
handle or pistol grip (not shown) may be attached to or formed as an integral
part of the body 210.
The powder conduit insert 220 is preferably a cylindrical tube having an
interior powder
passageway 222. The inner diameter of the powder passageway 222 may preferably
be in the range of
about 0.25 inches to about 1.5 inches, and most preferably is 0.5".
It is preferred that the insert 220 be removably or releasably connected to
the body by
conventional methods. For a negative polarity gun, it is preferred that the
insert 220 be entirely made of,
or have an interior surface 222 coated with, the materials selected from the
polyamides, preferably nylon
6/6, a polyamide blend, fiber reinforced polyamide resin, acetal polymer,
aminoplastic resin or mixtures
thereof. For a positive charging gun, the insert 220 may be entirely made of,
or have an interior surface
222 coated with a tribo-charging material such as, but not limited to,
fluoropolymers particularly
polytetrafluoroethylene, or mixtures thereof. Thus depending upon the type of
tribocharging material
selected, a negative or positive charge is imparted to the powder particles
upon contact with the interior
powder contact surfaces of the insert 220.
The spray gun 200 may further comprise one or more air jets 240 which are
provided within the
interior passageway 222, 234 of the gun. The air jets 240 may be located
within the insert 220 or the
nozzle 230, and function to create turbulence resulting in the increase of
frictional contact of the powder
with the walls 222 of the insert 220 or the nozzle 230. Air or other fluid
(hereinafter air) is supplied to the
air jets 240 via air passage 250 formed in the body 210, which leads to a
chamber 252 about the insert
220 or nozzle (not shown). One or more air jets 240 lead from chamber 252 to
the powder passageway
222, 234 in insert 220 or nozzle 230 (not shown).
The air jets 240 may comprise any orifice shape such as round, rectangular,
square or oval. Each
air jet cross-sectional area may range from about 0.001 to about 0.03 square
inches (which corresponds to
a round hole size of about 0.03 to about .2 inches in diameter). More
preferably, each air jet cross-
sectional area may be in the range of about .003 to about .005 square inches
(which corresponds to a
round hole size diameter of about 0.06 to about 0.08 inches). Most preferably,
the air jet cross-sectional
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area may be about 0.0038 square inches, which corresponds to a round hole size
diameter of about 0.07
inches.
As shown in Figure 2, the air jets 240 define an angle O with respect to the
longitudinal axis or
insert or nozzle side wall of the internal passageway 222 in the range of
about 0 to about 90 degrees, and
more preferably in the range of about 45 to about 90 degrees, and most
preferably about 60 degrees.
The air jets may be arranged in one or more groups of air jets with the same
or differing
diameters. A group may be two or more air jets which may be arranged in either
an opposed or
unopposed configuration. Figures 3A-3D illustrate alternate configurations of
the arrangements of upper
and lower air jets 240 of the insert 220. Figure 3A illustrates an upper and
lower air jet 240 in which the
air flow from the jets intersect on the longitudinal axis (or centerline CL).
Both the upper and lower air
jets form an angle of 45 degrees with the insert sidewall 222. Figure 3B is
almost the same configuration
as Figure 3A except that the center of the upper air jet is longitudinally
offset from center of the lower air
jet, resulting in the air flow from the air jets intersecting at a point
offset from the longitudinal axis.
Figure 3C illustrates that the air jets may have different air jet angles
which results in the flow of the air
jets intersecting at a point offset from the longitudinal axis. Figure 3D
illustrates that the upper and lower
air jets may be longitudinally offset and have different angles yet result in
the flow of the jets intersecting
at the longitudinal axis.
If two or more air jets are utilized, one air jet may be offset relative to
another air jet a distance H
perpendicular to the longitudinal axis as shown in Figures 4B-4E. Thus, in
Figures 4B-4E the air jets are
vertically offset from one another by varying the perpendicular (or vertical)
distances H relative to the
longitudinal axis. The distance H may vary from 0 (no offset) as shown in
Figure 4A, to one diameter of
the insert as shown in Figure 4E.
As shown in Figures SA through SH, if two or more groups of air jets are
utilized, one group of
air jets may be angularly rotated about the longitudinal axis relative to the
first group of air jets in the
clockwise or counterclockwise direction. It is preferred that the downstream
group of air jets be angularly
rotated in the range of about 0 to about 90 degrees relative to the first
group in either the clockwise or
counterclockwise direction. Figures SA, SC, SE and SG each illustrate a first
or upstream group of air jets
located within the insert 220 of Figure 2. Figures SB, SD, SF and SH,
represent a second or downstream
group of air jets which are rotated 0, 45, 90 and 0 degrees in the counter-
clockwise direction with respect
to the corresponding first set of air jets of Figures SA, SC, SE and SG,
respectively. Figure SH also
illustrates that the second group of air jets need only comprise one air jet.
The total air flow to the four air jet orifices 240 in Figure 2 may range from
about 0.3 cubic feet
per minute (CFM) to about 6.5 cubic feet/minute. If two pairs of air jets are
utilized, the total air flow rate
to the air jets is preferably 4.2 CFM. The air jet orifices 240 typically have
an air velocity in the range of
about 100 to about 1,000 feet/second, and more preferably in the range of
about 400 to about 800
feet/second, and most preferably about 655 feet/second. These variables can be
scaled appropriately for
different diameter tubes.
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The internal charging gun 200 is further provided with one or more electrodes
260 or other means
known to those skilled in the art which function to discharge the
tribocharging surfaces 222, 234 due to
the build up of charge as a result of frictional contact with the powder. For
example, the electrode may be
a conductive pin, a pressed solid metal ring, an air washed porous ring, or a
metal strip located along the
longitudinal axis inside the charging tube. The one or more electrodes are
preferably electrically
grounded. However, the electrode 260 may also be charged to either a positive
or negative electrical
potential as shown in Fig. 2, preferably in the range of about 0 to about 10
kilovolts (kv). The electrode
260 may be positioned within the interior of the insert 220 or the nozzle 230,
however it is preferred that
the electrode be positioned upstream from the air jets. The one or more
electrodes 260 may be airwashed,
i.e., an air flow is provided from chamber 250 through passages 262 and 264 to
blow powder off of the
electrode 260.
A flat spray nozzle 230 is shown in Figure 2 in conjunction with the
invention, although other
prior art nozzles would also work for the invention. The nozzle 230 has a slot
232 which creates a
generally flat spray pattern, and an interior passageway 234 which is in fluid
communication with the
interior passageway 222 of the insert 220. It is preferred that the nozzle 230
be removably or releasably
connected to the gun body 210 by any conventional methods. Because the nozzle
is a high powder
contact area, for a negative tribo charging gun, it is also preferred that the
nozzle 230 be entirely made of,
or have an interior surface 234 coated with a tribo-charging material such as
a polyamide, particularly
nylon 6/6, a polyamide blend, fiber reinforced polyamide resin, acetal
polymer, aminoplastic resin or
mixtures thereof. For a positive tribo charging gun, it is also preferred that
the nozzle 230 be entirely
made of, or have an interior surface 234 coated with a tribo-charging material
such as fluoropolymers
particularly PTFE. Thus depending upon the type of tribocharging material
selected, a negative or
positive charge is transferred to the powder particles upon contact vv~ith the
interior surface 234 of the
nozzle 230. Thus the nozzle 230 works in conjunction with the insert 220 to
tribocharge the powder
particles to the desired polarity as they contact the inner surface of the gun
200.
Although not shown, the insert 220 and nozzle 230 may be formed as an integral
one piece unit
which is releasably connected to the body 210 (not shown). Alternatively, the
insert 220 and nozzle 230
may be releasably connected together and then releasably connected to the
body. Thus, a particular
advantage of the short internal charging gun 200 of the invention is the
simple configuration of the insert
220 and nozzle 230, which allows these components to be fabricated out of, or
coated with any of the
described tribocharging materials and easily interchanged with the gun body
210. An array of inserts 220
and nozzles 230, made of or coated with different tribocharging materials, can
be provided for use with a
single gun body. An appropriate insert and nozzle can then be selected
according to the type of powder to
be sprayed, and according to the type of polarity to be applied to the powder.
Since powders charge
differently from one another depending on their chemistry, a material-specific
insert can be used for a
particular powder chemistry. For example, epoxies tend to charge positively,
so a PTFE insert would be
ideal for this powder. Polyesters, on the other hand, tend to charge
negatively, and would therefore be
charged better using a nylon insert.
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The following examples illustrate several gun configurations having varying
placement of air jets,
type and position of electrodes and use of tribocharging materials. However,
the invention is not limited
to these examples, as many other combinations and configurations are possible.
Example 12
In one example of the invention, a tribocharging gun 200 having an insert 220
was fabricated out
of nylon 6/6 material. The insert had two pairs of aligned, opposed air jets,
with each air jet angled in the
insert sidewall at an angle O of 60 degrees, and having a velocity of about
655 feet/second and a total air
flow rate of 4.2 cubic foot/minute. The centerline of the first pair of air
jets is longitudinally spaced
0.625" apart from the centerline of the second pair of air jets. A grounded
electrode was mounted flush
with the internal surface of the powderflow passageway and was angularly
offset from the air jets by 60
degrees. The gun was 5.75 inches long as measured from the powder inlet to the
tip of a flat spray nozzle.
The powder flow rate was 20 lbs/hr using Ferro 153W-108 polyester urethane
powder. The transfer
efficiency for this configuration was 78.0%.
Examule 13
In another example of the invention using the same gun configuration as
described in Example 12,
the electrode was charged to -8 ITV. The transfer efficiency was measured at
84%.
Examule 14
In another example of the invention, a short barrel tribocharging gun was
fabricated out of Delrin
100 AF material. The total combined length of the insert and nozzle was 3.375
inches. A 4 mm Delrin
100AF flat spray nozzle was used. As shown in Figure 2, the insert inlet
diameter was 0.375 inches for a
length of 1.25 inches, and was followed by a 45 degree step opening the insert
diameter to .5 inches for
the remainder of the tube length of 2.125 inches. Two pairs of opposing air
jets were used, with each air
jet having a diameter of 0.07 inches, and having an angle O of 60 degrees. The
downstream set of air jets
was rotated about the longitudinal axis by 5 degrees relative to the first
pair of air jets. All of the air jets
were offset a perpendicular distance from the longitudinal axis by .035
inches. Each air jet had an airflow
rate of about 1 standard cubic feet per minute and a velocity of 655 ft/sec. A
single grounded sharp tipped
electrode was located upstream from the air jets as shown in Figure 2. The
electrode was angularly
rotated about the longitudinal axis by 60 degrees relative to the first set of
air jets. The transfer efficiency
for this configuration was 70% using Ferro 153W-121 at 20 lbs/hour.
In summary, the above described short barrel tribocharging gun provides a
novel lightweight
spray gun which is easily maneuverable into tight spaces due to the guns
shorter length and smaller
diameter. Conventional tribcharging guns are typically 14-36 inches in length,
while the short
tribocharging gun provides a gun of about 6 inches long. The gun lends itself
as a manual gun or use as a
low cost automatic gun. ~ The straight flow powder path allows for easy
cleaning, as well as a removable
insert which can be easily replaced by an inexpensive insert for quick color
changes. The novel materials
CA 02413267 2002-12-19
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which are used to make the gun are injection moldable, thus reducing the
machining costs significantly.
Thus the invention provides a short barrel tribocharging gun which can
accommodate a powder flow rate
of up to about 30 lbs/hour and a reasonable transfer efficiency.
The invention further provides a short barrel negative tribocharging gun which
can be used alone
or in conjunction with a negative corona gun as described in more detail
below. While providing all of
the above described advantages, the short barrel negative tribocharging gun
further provides the advantage
of excellently applying and charging polyester powders such as TGIC
polyesters, epoxy/polyester hybrid
powders, and polyester urethanes, as well as thermoplastic powders such as PVC
and PTFE powders.
HI. UNIPOLARITY CORONA GUN WITH TRIBO-CHARGING COMPONENTS.
Referring now to Figure 6, a unipolarity corona spray gun 300 is provided for
spraying fluidized
powder that has been charged to either a positive or negative polarity. The
term "unipolarity" refers to a
powder spray gun or powder supply system wherein the components are selected
to charge the powder
coating material to a single polarity. An example would be a corona gun with a
negative polarity power
supply which includes tribocharging components such as the spray nozzle which
also charges the powder
negatively. The gun 300 comprises a rearward barrel 328 which may be secured
to a mounting block.
The rearward barrel 328 has an internal bore 332 and an angled bore 333 for
connection to a powder
supply tube 334. The powder supply tube 334 functions to introduce fluidized
powder through the angled
bore 333 into the throughbore 332 of the rearward barrel member 328. The
forward end of the rearward
barrel member 328 is connected to a forward barrel member 338, which further
comprises a throughbore
346 which is axially aligned with bore 332 to form a powder flow passageway
350 for transfernng
powder from the powder supply tube 334 towards the forward end of the gun 300.
A flat spray nozzle
394 is located on the forward end of the forward barrel member 380.
A barrel liner 352 extends axially within the powder passageway 350 which is
mounted within the
end of the rearward barrel member 328. The barrel liner 352 receives and
supports a high voltage
electrostatic cable assembly 358. An electrode 362 is mounted at the forward
end of the cable assembly
352 and extends through a bore 396 of the of the nozzle tip 390 and extends
forward of the spray nozzle
394 between the rectangular slot 398. The electrode 362 extending forward of
the spray nozzle 380,
produces a strong electrostatic field between it and the object to be coated.
The electrode may be charged
positively or negatively depending upon the desired gun polarity. It is
preferred that the electrode be
charged to the desired polarity in the range of about 60 to about 100 kv.
The powder contact surfaces of the corona gun 300 are the barrel liner 352,
the powder
passageway 350, the powder supply tube 334, and the passageway 372 through
nozzle 380. For a positive
polarity corona gun which charges the powder to a positive polarity, one or
more powder contact surfaces
334, 350, 352, or 372, for example, are comprised of materials which
tribocharge the powder positively.
These materials are selected from the group consisting of: polyethylene, a
fluoropolymer or mixtures
thereof. It is preferred that the fluoropolymer comprise
polytetrafluoroethylene. For a negative polarity
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corona gun which charges the powder to a negative polarity, one or more of the
powder contact surfaces
334, 350, 352, or 372, for example, of the corona gun 300 are selected to be
of a material which
tribocharges the powder negatively. These surfaces are comprised of a material
selected form the group
consisting o~ a polyamide, a polyamide blend, a fiber reinforced polyamide
resin, an acetal polymer, an
aminoplastic resin or mixtures thereof, as described in detail in Section I.
Thus the unipolarity corona gun of the present invention utilizes
tribocharging to charge the
powder as well as the corona charging. The tribocharging which occurs is of
the same polarity as and
therefore increases the charge on the powder which results from the corona
charging electrode. Because
the powder contact surfaces add to the charge on the powder produced by the
corona electrode, less
electrode voltage is needed to produce the same amount of charge as in prior
art guns. Thus for a negative
polarity gun, reduced back ionization occurs because the voltage is lower.
This results in an improved
surface finish. This reduction in electrode voltage also reduces the Faraday
Cage effect. In addition, a
smaller power supply can be used to produce the same voltage.
In an alternate embodiment of the invention, the corona gun 300 may
additionally include an
enhanced tribocharging nozzle 400 as shown in Figure 7. Tribocharging nozzle
400 may be used with
other prior art corona or tribocharging guns and is not limited to the corona
gun 300 as described above.
Tribocharging nozzle 400 provides a large interior surface area which may be
utilized in order to
tribocharge the powder. The powder may be charged positively or negatively as
desired depending upon
the triboelectric material selected, as described in more detail, below.
The nozzle shown generally at 400 has a powder inlet end 410 and an interior
flow passageway
412 which is in fluid communication with the interior passageway of a prior
art corona gun or triboelectric
gun (not shown). The inlet end 410 may be threaded or otherwise configured to
be releasably connected
to the body of a prior art spray gun. The interior passageway 412 is
preferably cylindrically shaped with a
transition surface 414 leading to the nozzle slot 420. The nozzle 400 has a
slot 420 shaped to create a
generally flat spray pattern. The depth and width of the nozzle slot 420 may
be sized as needed for the
particular application.
Because the nozzle surfaces 412, 414 are in contact with the powder, it is
preferred that the
nozzle 400 be entirely made of, or have an interior surface coated with a
tribo-charging material. For a
positive polarity corona gun, it is preferred that the nozzle be made or have
interior powder contact
surfaces coated with a material selected from the group consisting of:
fluoropolymers particularly PTFE.
For use with a negative polarity gun, it is more preferable that the nozzle
400 be entirely made of, or have
interior surfaces 412, 414 coated with the materials selected from the group
consisting of: a polyamide,
particularly nylon 6/6, a polyamide blend, a fiber reinforced polyamide resin,
an acetal polymer, an
aminoplastic resin, or mixtures thereof. Thus depending upon the type of
tribocharging material selected,
a negative or positive charge is transferred to the powder particles upon
contact with the interior surfaces
412, 414 of the nozzle 400. Thus the nozzle 400 can work in conjunction with
the corona charging
electrode of the prior art spray guns in order to charge the powder with the
same polarity as the corona
electrode.
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The nozzle 400 may preferably include one or more air jet orifices 430 which
are positioned for
fluid communication with the internal passageway 412 of the nozzle. Air or
other fluid is provided to the
air jet orifices 430 for example by chamber 440 which is connected to an
external fluid source (not
shown) via port 450. It is preferred that the air jet orifices 430 be sized
and configured to provide an air
velocity in the range of about 100 to about 1,000 feet/second, and more
preferably in the range of about
400 to about 800 feet/second. It is additionally preferred that the air jet
orifices) 430 comprise an angle a
with respect to the longitudinal axis of the insert internal passageway in the
range of about 0 to about 90
degrees, and more preferably in the range of about 45 to about 90 degrees. It
is preferred that the angle of
the air jet orifices 430 be such that the air jets intersect to provide
turbulence resulting in increased
frictional contact with the charging surface. It is preferred that the impact
angle [3 of the air jets upon the
transition surface 414 should be in the range of about 45 to about 90 degrees,
and more preferably about
60 degrees.
The nozzle 400 may additionally comprise one or more electrodes 460 or other
means known to
those skilled in the art to discharge the interior surface 412 from charge
build-up. The one or more
electrodes is preferably grounded. Alternatively, the one or more electrodes
may have a positive or
negative charge in the range of about 0 to about 100 KV, and more preferably
in the range of about 0 to
about 10 kv. The high voltage electrodes) is charged positively if an
electronegative charging material is
utilized, and the electrodes are charged negatively if an electropositive
charging material is utilized on the
interior surface of the nozzle. As shown in Figure 7, the electrode may be
positioned within an electrode
holder 490. The electrode holder 490 has an outer surface 492 made of the
materials described for the
internal passageway 412 of the nozzle described above. However, it is
important to note that other
electrode configurations are possible such as for example, a ground ring, or a
blunt or sharp tipped
conductive pin. If a conductive pin is used, it may be positioned at a right
angle to the fluid passageway
anywhere in the nozzle 400. The electrodes are positioned upstream within
about 2 inches of the air jet
impingement on the wall.
In a preferred embodiment of the nozzle, the electrode is grounded and
positioned upstream of 2
pairs of aligned, opposed air jets which are laterally spaced one diameter
apart. The air jets are angled at
60 degrees with respect to the longitudinal axis.
IV. TRIBO-CHARGING COMPONENTS OF POWDER DELIVERY SYSTEMS
The invention further provides tribocharging powder contact surfaces in
various components
throughout a powder delivery system which can be used to tribocharge the
powder to the same polarity as
the corona powder supply. Tribocharging at several areas along the delivery
system incrementally
increases the charge on the powder as it passes through each tribocharging
area. This benefits corona gun
systems with increased transfer efficiency. This idea can also be used with
tribocharging gun systems.
The tribocharging areas of the powder supply system tribocharge the powder to
the same polarity as is
used in the triboguns of the system.
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As shown in FIG. 9, a typical powder spray system 500 includes a spray gun 510
connected by a
powder supply hose 540 to a hopper 520, through a powder pump 530 mounted on
top of the hopper. The
spray gun 510 is, for example a negative charging corona type powder spray
gun, but may alternatively be
a positive charging corona gun, or a negative or positive tribo-charging
powder spray gun.
An electrical line 544 is connected to the gun 510 from control system 550
which regulates air
pressure to pump 530 and the voltage of the corona electrode in gun 510.
Within the powder hopper 520,
a diffuser plate 521 is configured to extend over a cross-sectional area
within the hopper, and is formed of
a porous material through which air passes to fluidize the powder. Because the
hopper sidewalk 522 and
the diffuser plate 521 are high contact areas of the powder, the invention
includes constructing the plate
521 and sidewalls 522 out of the negative tribo pre-charging materials
selected from the group consisting
of polyamides, particularly nylon 6/6, a polyamide blend, fiber reinforced
polyamide resin, acetal
polymer, aminoplastic resin or mixtures thereof. Thus contact of the powder
with the diffuser plate 521
and sidewalls within the hopper 520 pre-charges the powder negatively before
it is transported to negative
corona gun 510.
The pump 530, shown in cross-section in Figure 8, includes a body 531 with a
powder inlet tube
532 leading to a cavity 533 which is intersected by an ejector or venturi
nozzle 534 and a venturi throat
535. The venturi throat 535 is held in the pump body 531 by a throat holder
536 which extends out of the
pump body to provide an attachment fitting 537 for a hose. Within the
attachment fitting 537 is a wear
sleeve 538, also referred to as a wear tube, downstream of the pump throat.
The wear sleeve prevents
impact fusion on the inside wall of the throat holder. An atomizing air inlet
539 intersects with the throat
holder 536 to provide air flow which joins the powder air mixture from the
venturi throat.
This area in the powder delivery system is thus a suitable site for use of one
of the described pre-
charging materials. Thus it is desired that the venturi throat 535, wear
sleeve 538, pump suction tube 532,
and powder hose (not shown) be coated with or fabricated from the materials
selected from the group
consisting of a polyamide, polyamide blend, fiber reinforced polyamide resin,
acetal polymer,
aminoplastic resin or mixtures thereof, as described in more detail above, to
precharge the powder
triboelectrically with a negative polarity. It is additionally preferred that
the length of the venturi throat
535 and the throat holder 536 be extended by, for example, from one to flue
inches beyond the edge of the
pump body. Optimally, this extended length provides for substantial additional
negative tribocharging of
powder at this region of the powder delivery system.
Powder pre-charged in the powder delivery system in the hopper and/or pump as
described in this
section flows through the hose to arrive at the gun with a pre-established
negative charge. This pre-
charging augments the additional negative charge applied at the gun by the
corona electrode.
V. UNIPOLARITY POWDER COATING SYSTEM INCLUDING CORONA AND
TRIBOCHARGING GUNS
As shown in Figure 9, a corona gun 510 is shown together in use with a tribo-
charging powder
spray gun 10 of the invention, which has been described in detail, above. The
corona gun 510 and the
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tribocharging gun 10 have the same polarity. This unique combination allows
for the tribocharging gun
to be used as a touch up gun, for example, to penetrate the corners or hard to
reach parts that the corona
gun 510 has not effectively coated. This exemplary combination of a negative
corona gun 510 and a
negative tribo-charging gun 10 is preferably connected to a common powder
delivery system 520, which
5 pre-charges the powder negatively as described above. Alternatively, the
tribocharging gun may
comprise the short barrel gun 200 (not shown) which is described in more
detail, above. This novel
combination of one or more negative corona guns with one or more negative
tribo guns, optimally with a
negative pre-charging powder delivery system, used to coat different parts of
the same workpiece is one
important embodiment of this invention.
VI. TRIBOCHARGING GUN WITH AIR JETS
As shown in Figure 10, a novel tribocharging gun 600 is provided which
comprises a powder feed
section 610, a powder charging section 620, and a spray nozzle 630 located at
the outlet of the gun. The
powder charging section 620 of the tribocharging gun 600 further comprises a
cylindrically shaped body
622 having an internal bore 623 for housing the internal components of the
gun. Housed within the bore
623 of the body 622 is a powder tube connector 612 having an internal bore
626a. A first end 616 of the
connector 612 is connected to a powder supply tube (not shown) for supplying
fluidized powder to the
powder flow passageway 626a,b,c of the gun 600. The second end 618 of the
powder tube connector 612
is connected to an inlet air entry 640. The inlet air entry 640 has an
internal passageway 626b and one or
more angled holes or air jets 642 which are connected to an air manifold 628
located in the body 622 for
supplying pressurized air to the air jets 642 in order to increase the
velocity and induce turbulence of the
fluidized powder entering the gun. Connected to the inlet air entry 640 is an
outer wear tube 650 which
has an internal passageway which is part of the powder flow passageway 626 of
the gun. The outer wear
tube 650 further comprises one or more air jets 652. Pressurized air is
provided to the air jets 652 via
passageway 654 which is in fluid communication with air manifold 628. The gun
600 may further be
provided with an optional inner wear surface 660 which forms an annular powder
flow path. As shown in
a cross sectional view in Figure 10A, a plurality of air jets 652 are arranged
in an opposed configuration at
one or more longitudinal stations. Preferably the air jets 652 comprise an
angle y ( as measured
counterclockwise from the longitudinal axis) preferably in the range of about
90 to about 135 degrees.
The air jet velocity is preferably high enough to induce turbulence and cause
the powder flowing through
passageway to contact the wall opposite the air j et, in order to increase the
tribocharging of the powder. It
is preferred that the air jet velocity be in the range of about 100 to about
1,000 feet/second and more
preferably in the range of about 400 to about 800 feet/second.
In order to provide tribocharging of the powder, the powder contact surfaces
of the gun such as
the internal surface of the powder flow passageway 626a-c, the nozzle 630 and
the outer surface of the
inner charge tube 660 are constructed from or coated with a tribocharging
material. For a positive polarity
tribocharging gun the powder contact surfaces are preferably selected from the
group consisting of:
fluoropolymers particularly PTFE. For a negative polarity tribocharging gun
the powder contact surfaces
CA 02413267 2002-12-19
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ire preferably selected from the group consisting of: nylon, particularly
nylon 616, a polyamide blend, a
fiber reinforced polyamide resin, an acetal polymer, an aminoplastic resin or
mixtures thereof.
In yet another embodiment of the invention as shown in Figure 11, the
tribocharging gun is the
same as described above, except for the following differences. First, no inner
charge tube 660 is utilized.
Second, the air jets 652 of the tribocharging gun 600 located within the outer
wear tube 650 are arranged
in a helical pattern about the longitudinal axis as shown in Figures 11 and
11A. Optionally, the air jets
652a located on the upper portion of the tube 650 can have a different angular
orientation than the air jets
652b located on the lower portion of the tube 650 (not shown). The air jets
652a, 652b when configured
in this manner, are designed to impact the fluidized powder against the
opposite wall in a staggered or
wave fashion in order to increase the tribocharging of the powder. It is
preferred that there be 3-4 sets of
holes arranged in the configuration, with each set comprising 2 or more holes.
This helical configuration
functions to induce turbulence and swirl the fluidized powder in a helical
fashion so that the relatively
heavier powder is spun or induced to impact the wall via centrifugal forces
into contact with the
passageway wall.
An advantage of this embodiment is that to cause each powder particle to
impact the charging
surface numerous times and thereby increase the charge on the powder, instead
of forming mechanical
waves on the charging surface such as shown in the Figure 1 gun, the charging
surface is a straight
cylinder which is easy to manufacture, while the air jets 652 cause the powder
particles to take a turbulent
route through the flow passage 626a,b,c, impacting the surface many times to
increase the triboelectrically
induced charge on the powders.
With reference to Fig..,12, another embodiment of the short barrel
tribocharging gun 200 of Fig. 2
is illustrated. In the embodiment of Fig. 12, the modified gun 200' includes a
gun body 210' that retains a
powder conduit insert 800 that is somewhat different from the insert 220 in
Fig. 2. The insert 800
includes a powder feed inlet 802 and an optional diffuser air inlet 804.
Diffuser air may be used as
required to increase the velocity of the powder through the gun 200'. This
increased velocity increases
the tribocharge charging effect on the powder, and also helps diffuse the
powder, and also may be used to
affect the spray pattern. Diffuser air however is not required in all
situations, and depends on several
factors among which are notably the velocity and pressure of the powder
entering the gun 200' from the
powder supply hose 540 and related powder supply components (see Fig. 9 and
the discussion herein
related thereto) as well as how much additional diffusion of the powder is
required, if any, through the
gun. In many cases where the air jets are incorporated into a tribocharging
type gun, the pressure drop
created by the air flow through the air jets may be sufficient to obviate the
use of diffuser air. This is
particularly the case when the air jets are forwardly angled to direct a
significant air flow in the axially
forward direction through the gun, thereby inducing a suction effect at the
powder inlet end of the gun.
Reducing overall air use in a spray gun is usually beneficial as it reduces
operating costs associated with
shop air, impact fusion and wear. Reducing impact fusion helps speed up color
change and cleaning
operations.
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The inner end 800a of the powder conduit insert 800 slideably receives a first
end of a charging
tube 806. The charging tube 806 is preferably made of any one of the various
materials described herein
to apply either a positive or negative charge to the powder as desired for a
particular application. The
charging tube inlet 806a may include an optional internal diametric reduction
or neck down 808 which
serves to increase powder velocity (without needing to increase diffuser air
volume or pressure) and also
to re-center the powder in the central volume of the charging tube 806 before
the powder enters the main
portion of the charging tube.
A solid or hollow shaft 810 is longitudinally and preferably coaxially
positioned within the
charging tube 806. This shaft 810 is preferably but not necessarily
cylindrical in shape, and includes an
optional taper to a conical end 810a to facilitate discharge of the shaft 810.
The charging tube 806
includes a metallic discharge or grounding ring 812 that is connected to a
grounded discharge pin 814.
The pin 814 permits the charging tube 806 and the shaft 810 to self discharge
during a spraying operation
as charge builds up on the tribocharging surfaces. A bore 816 is provided to
receive a grounded pin or
wire (not shown) that contacts the grounding ring 812.
The body 210' includes an air inlet port 250' much in the same manner as the
port 250 in the
embodiment of Fig. 2 herein. This port 250' opens into an annulus 817. The
annulus 817 is in fluid
communication with and surrounds another annulus 818 that is generally defined
by the space between the
outer circumference of the shaft 810 and the inner surface of the charging
tube 806. The annulus 818
preferably forms a rather narrow gap between the charging tube 806 and the
shaft 8I0. A series of air jets
240' are provided through the wall of the charging tube 806, in a manner
similar to the embodiment of
Fig. 2 herein, and pressurized air flows from the outer annulus 817 to the
inner annulus 818 therethrough.
The exact location, number, angle and orientation of the jets 240' may be
determined based on various
factors as previously described herein. In accordance with one aspect of the
invention, the smaller
annulus 818, as compared, for example to the diameter of the tubular insert
220 in Fig. 2, significantly
reduces the travel distance for powder particles that are forced by air from
the jets 240' toward the shaft
810. Thus, less air is required to cause the powder to impact the
tribocharging surface of the shaft 810 at
a comparable velocity to the embodiment of Fig. 2. This not only reduces the
air requirements, but also
reduces impact fusion effects. Additionally, use of the shaft 810
substantially increases the total surface
area of tribocharging material to which the powder particles are exposed,
because the powder will impact
both the surface area of the shaft 810 as well as the inner surface area of
the charging tube 806. The air
jets 240' may be angled forwardly and radially as in Fig. 12 (relative to the
longitudinal axis of the gun
200') or may also be offset to create a spinning air movement around the shaft
810, as previously
described herein. The narrower annulus 818 also permits conventional
tribocharging effects on the
powder as it passes through the gun 200', much in an analogous manner that a
prior art tribocharging gun
uses a tortuous or wavy path for the powder to pass through. By way of
example, the annulus 818 may
22
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nary from about 0.02 inches to about 0.5 inches, although the exact dimensions
selected will depend.on
the overall performance characteristics and requirements of each gun design.
The shaft 810 is positioned and held in the charging tube 806 by any
convenient mechanism, such
as for example centering pins (not shown). Furthermore, in the embodiment of
Fig. 12, the insert 800, the
charging tube 806 and the nozzle 820 form the gun barrel and may all be made
of the various materials
described herein to produce positive or negative charging of the powder
particles as desired, as will the
shaft 810 be made of such tribocharging materials. The embodiment of Fig. 12
uses a conventional flat
spray nozzle 820 having a slot 821 but any suitable nozzle design may be used.
With reference to Fig. 13, an alternative embodiment of the Fig. 12 version is
illustrated. Like
parts are given like reference numerals and the description thereof is not
repeated. Tn the embodiment of
Fig. 13, the charging tube 822 and the shaft 824 have been modified at their
forward ends to cooperate
with a corresponding configuration of a nozzle body 826 to define a
tribocharging parallel wave path 828
that is downstream of the annulus 818. The wave path 828 is realized in the
form of an hourglass type
reduced diameter in the nozzle body cavity 820. , The shaft 824 is formed with
a corresponding geometry,
and the charging tube 822 forward end simply abuts the backward end of the
nozzle body 826 to form a
smooth continuous contour. A spider 830 is centered and supported in the
nozzle body 826 cavity by a
plurality of radial legs 832. The spider 830 may be joined or assembled with
the shaft 824 if so required,
by a pin insert 834, and at its forward end the spider 830 may be used to
support a conventional conical
nozzle 836. The spider 830 preferably is made of a suitable tribocharging
material such as those
described herein. In this embodiment then, the gun 200" operates with both the
air jets 240', the charging
tube 822 and the shaft 824 initially charging the powder, as well as a
tribocharging post-charge function
produced by the parallel wave path 828. Although in the embodiment of Fig. 13
the tribocharging section
828 is illustrated as a parallel wave pattern, such illustration is intended
to be exemplary in nature and
should not be construed in a limiting sense. Those skilled in the art will
readily appreciate that the
tribocharging section may be realized utilizing any number of known
tribocharging arrangements.
Fig. 14 illustrates another modification of the gun 200' in Fig. 12. In this
version, the shaft 810 is
installed in a slightly axially forward position as compared to the shaft 810
in Fig. 12. This has the effect
of positioning the conical rearward tip 810a of the shaft 810 nearer the
grounding pin 814. This
significantly increases the ease with which the shaft 810 may discharge during
a spraying operation.
Fig. 14 further includes the concept of incorporating both an initial air jet
assisted or induced
tribocharging function and an additional tribocharging function into the gun
200'. Note in Fig. 14, as
compared for example to Fig. 13, that the air jets 240' are positioned aft of
the shaft 810. This places the
air jet induced tribocharging function first, followed by a subsequent
tribocharging function in the annulus
818. The air jets apply sufficient energy to the powder particles to cause
impact against the charging tube
and shaft surfaces to charge the powder. The air flow produced by the air jets
is sufficient to allow a
23
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tribocharging effect downstream via the annulus 818 without needing a
tortuous, wavy or other
conventional tribocharging path, although such tribocharging techniques and
configurations may be used
if so required.
With reference next to Fig. 15, another gun embodiment is illustrated. The
basic concept
illustrated in this drawing is referred to herein as an "inside-out" gun
because, as compared to the
embodiments previously described herein, the flow direction of the air jets is
reversed. Thus the prior
embodiments herein can for convenience be referred to as "outside-in" gun
configurations. In the
embodiment of Fig. 15 then, the gun 840 includes a gun body 842 that has a
rearward end 842a and a
forward end 842b. The rearward end 842a includes a counterbore that slideably
receives and retains a
powder conduit insert 844. The powder insert 844 supports a powder tube
connection nipple 846 and an
air inlet connector 848. The insert 844 receives and supports a first end of a
charging tube 850 that is
made of a suitable tribocharging material as previously described herein. The
charging tube 850 extends
through the gun body 842 to a nozzle assembly 852. The particular design of
the nozzle assembly 852
may be selected as required for a specific spray pattern. In the example of
Fig. 15, the nozzle assembly
852 includes a nozzle body 852a that retains a spider 852b which at one end
supports a conventional
conical nozzle 852c. The spider 852b may include radial legs 852d or other
suitable elements to such as
pins to support the spider 852b within the nozzle body 852a.
The insert 844 receives and supports a first or inlet end of an air tube 854
which in this example is
realized in the form of a hollow shaft. The air tube 854 includes one or more
air jets 856 that are formed
at appropriate angles and orientations as described herein before with respect
to the other embodiments
herein. In the example of Fig. 15, the air jets 856 produce a forward air flow
towards the front of the gun
840, but are radially angled to direct powder against the inner surface 858 of
the charging tube 850. The
inlet end 854a of the air tube 854 is in fluid communication with the air
inlet coupling 848. Therefore,
pressurized air fed into the air inlet 848 via an air hose (not shown) enters
the air tube 854 and exits
through the various air jets 856. The air tube 854 generally coextends with
the charging tube 850 and has
a forward end 854b of the air tube 854 is closed and supported by the spider
852a.
As compared to the embodiments, for example, of Figs. 2, 7, 3A-3D, 4A-4H, and
1 l, the concept
of the inside-out gun is that the powder particles have a substantially
shorter travel distance under the
influence of the pressurized air from the air jets 856 before the particles
impact the tribocharging surface
of the charging tube 850. This reduces the amount of air to achieve adequate
impact velocity to effect
adequate charging of the powder and also reduces the amount of lost energy
from the particles traveling
down the gun. The air tube 854 may be also made of tribocharging material to
further increase the
tribocharging effect of the design. Another advantage of the inside-out design
is that the gun is simpler to
manufacture as it uses fewer parts.
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WO 02/04127 PCT/USO1/21572
Fig. 16 shows a variation of the inside-out gun of Fig. 15. In Fig. 16, the
gun 840' has a central
gun body 860 that also functions as the charging tube. The powder insert 844'
is attached at an inlet end
of the body and a nozzle assembly 852' is attached at an opposite end of the
gun body 860. The nozzle
assembly 852' may be similar to that shown in Fig. 15 or may be of some other
suitable design.
In both Figs. 15 and 16, a grounding pin 862 extends through the gun body
842/860 to discharge
the tribocharging surfaces and components inside the guns. The pin 862 is
illustrated in Fig. 16 with the
pin omitted in Fig. 15 to illustrate the pin bore 862a.
Fig. 17 illustrates an embodiment of the invention in a hand operated gun
configuration. Previous
embodiments herein are illustrated as automatic gun conftgurations such as are
mounted on gun supports
and gun movers, although the main elements of those embodiments may be
incorporated into a manual
gun handle, as exemplified in Figs. 17 and 18.
In Fig. 17 then, the gun 870 includes a handle portion 872 having a trigger
874 or other control
device for controlling the flow of powder through the gun 870. A gun body 876
supports a powder feed
hose connector 878 to which a powder feed hose (not shown) may be connected.
Powder flows down a
powder extension tube 880 which may be made of tribocharging material. The
extension tube 880 is
supported within a gun body extension 882 that at an opposite end supports a
nozzle assembly 883. The
extension tube 880 is generally concentrically mounted within the gun body 876
and extension 882 to
provide an annulus 884. This annulus 884 receives pressurized air through an
air fttting 886 that is
connected to an air line 886a extending up through the handle 872. A diffuser
air passageway 888 is
formed through the wall of the powder extension tube 880. The passageway 888
is sized so as to effect a
desired balance between diffuser air entering the powder extension tube 880
and air that will travel down
the annulus 884 to the charging portion 890 of the gun 870.
The charging portion 890 in this example is in the form of an outside-in gun,
and includes a
charging tube 892 that is inserted at one end into the forward end of the
powder extension tube 880. The
forward end of the charging tube 892 is assembled to the nozzle assembly 883.
The charging tube 892 is
supported by ribs or legs 894 that include or permit the air from the annulus
884 to pass through a series
of air jets 896. The air entering the charging tube 892 directs the powder
particles to impact the
tribocharging surface 892a of the charging tube 892 as in the earlier
described embodiments. It is
contemplated that the extension tube 880 and the nozzle assembly 882 may also
be made of suitable
tribocharging materials to enhance the charging effect of the gun 870. The use
of the internal diffuser air
passageway 888 requires only a single air supply to the gun 870 for both
diffuser air and air for the jets
896, thus eliminating any need for a second air port into the side of the gun
at the portion 890. Although
not shown in Fig. 17, a shaft similar in concept to the shaft 810 in Fig. 15
may be used in the gun
configuration of Fig. 17.
CA 02413267 2002-12-19
WO 02/04127 PCT/USO1/21572
The embodiment of Figure 17 has a ground pin 893 which is connected to the
extension 882
which is electrically conductive. The extension 882 is in turn connected to a
grounding screw 885 which
is electrically grounded by a ground wire 887. Placing the ground pin 893 at a
location just behind, or
upstream, of the location where tribocharging air assist jets 896 first impact
the charging surface is
preferred in that in this location the surface charge which builds up on the
tribocharging surface due to the
tribocharging of the powder can be readily discharged by ground pin 893 to
promote tribocharging of the
powder. If the ground pin is placed too far upstream from the point of air jet
impingement, the surface
charge which builds up on the surface will not be discharged by the ground
pin. If the ground pin is
placed in front of, or downstream of, the place where the tribocharging air
jets impinge on the charging
surface, the powder charged by impinging that surface will be discharged by
the ground pin as the powder
flows downstream over the ground pin.
In a typical tribocharging gun, extending the length of the gun barrel
downstream of the
tribocharging portion tends to cause a loss of charge before the powder is
ejected through the nozzle. In
Figs. 18A-D we illustrate an alternative arrangement wherein for different gun
lengths, the air jet induced
tribocharging portion 890 is kept positioned closer to the nozzle, therefore
the charge loss is minimized.
In all of these embodiments, it is preferred that the ground pin or other
ground element (not shown) be
placed at a location just behind the place where tribocharging air assist jets
first impact the charging
surface as is done in the Figure 17 embodiment.
With reference next to Fig. 19, a spray gun is illustrated that incorporates
the concept of an inside-
out gun in a hand held manual spray gun configuration. The gun 900 includes a
gun body 902 that has a
handle 904. The handle 904 may include conventional trigger mechanisms 906 for
controlling the flow of
powder into the gun 900. The body 902 supports a charging tube 908 within a
body extension 910. The
charging tube 908 is made of a suitable tribocharging material as set forth
hereinabove. At a rearward end
of the gun body 902 is attached a powder inlet cap assembly 912, that in a
manner similar to the
embodiments of Figs. 15 and 16, includes a powder hose connector 914 and an
air fitting 916 (the air and
powder supply lines being omitted from Fig. 19 for clarity). The air inlet 916
is in fluid communication
with an air tube 918 that extends longitudinally through the gun 900 from the
inlet head 912 to a nozzle
assembly 920. In this embodiment, the nozzle assembly includes a flat spray
nozzle 922 within which is
installed a spider 924 that may be similar in design to the spider 852b of
Fig. 15 herein. The spider 924
supports the forward end'of the air tube 918. The air tube extends generally
concentrically through the
gun 900, thus providing an annulus 926 between the outer surface of the air
tube 918 and the inner surface
908a of the charging tube 908. In a portion 928 of the gun 900 a number of air
jets 930 are provided
through the wall of the air tube 918 which are directed towards the forward
end of the gun near the nozzle.
The number, location, orientation and angles of the various air jets 930 may
be selected for a particular
gun design as explained hereinabove. The air jets 930 also need not be all at
the forward end of the gun
900 but may also be located more towards the gun handle.
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Powder enters the gun 900 through the coupling 914 and passes down the annulus
926.
Appropriate sizing of the annulus 926 may be used to provide a tribocharging
precharge to the powder
before it reaches the portion 928 of the gun 900. Pressurized air flow from
inside the air tube 918 out to
the annulus 926, causing powder particles to impact the tribocharging surface
of the charging tube 908.
The air tube 918 may also be constructed of tribocharging material to increase
the charging effect on the
powder. Although the gun 900 is illustrated as having a charging tube 918
disposed within a gun
extension 910, these two elements may if required be a single tube, as in the
embodiment of Fig. 16
herein.
As in the previous embodiments, a ground pin 931 is placed at a location just
behind the place
where tribocharging air assist jets 930 first impact the charging surface. The
grounding pin 931 is
connected to the extension 910 which is electrically conductive. The extension
910 is grounded through a
ground screw 933 to a ground wire 935.
Another advantage of the inside-out gun configurations illustrated herein is
that if impact fusion
should occur along portions of the charging tube surface, it is a
straightforward operation to simply rotate
the air tube 918 through an angle sufficient to reorient the air jets 930
towards "clean" tribocharging
surface areas where there is no impact fusion. This exposes clean charging
surface to the impacting
powder particles and will improve the charging efficiency as the gun is used.
Alternatively, the relative
axial position between the air jets 930 and the tribocharging surfaces could
be adjusted to expose clean
charging surface to the powder, or both the relative axial and rotational
positions could be changed.
Fig. 20 illustrates another embodiment of the invention that combines the
inside-out configuration
with an outside-in configuration in a single gun. In this embodiment, the gun
940 includes a gun body
942 that supports at one end a powder inlet cap assembly 944 and at an
opposite end a nozzle assembly
946. The nozzle assembly 946 is illustrated to be a conical nozzle type with a
nozzle 948 supported by a
spider 950 in a manner similar to other embodiments described herein.
The inlet assembly 944 includes a powder hose fitting 952 and an air fitting
954. The air fitting
954 is in fluid communication with an air tube 956 that extends through the
gun to the nozzle assembly
946 and is supported at the forward end by the spider 950. A charging tube 958
is also supported inside
the gun body 942 and concentrically surrounds the air tube 956 to form a
second or outer annulus 960
therebetween. The air tube 956 includes a plurality of inside-out air jets 957
that allow air to pass from
inside the air tube into the annulus 960. The charging tube 958 is sized with
a diameter that is Iess than
the diameter of the gun body 942, thereby providing an air passageway or
second outer annulus 962. The
charging tube 958 is also provided with a number of air jets 964 such that the
charging tube 958 also
functions as an outside-in air tube. Pressurized air flows from the second or
outer annulus 962 through
the charging tube air jets 964 into the first or inner annulus 960. Powder
from the inlet 952 flows into the
inner annulus 960 and is then entrained in the air flow produced by the air
jets 957 and 964. The two sets
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WO 02/04127 PCT/USO1/21572
of air jets, one outside-in and the other inside-out significantly increases
the turbulence of the powder and
causes impact with both the charging tube surface 958a and the air tube outer
surface 956a. A grounding
pin 966 is provided as previously described hereinabove.
Pressurized air enters the gun through the air fitting 954 and flows through
the air tube 956. In
addition, an air passageway 968 is provided that directs part of the air into
the outer annulus 962. In this
manner only a single air input is needed to the gun. If required, a portion or
the air may also be directed
into the inner annulus 960 to function as diffuser air, however this is
unlikely to be needed as the volume
of moving air from all the air jets will in most cases adequately diffuse the
powder. The gun 940 may also
include additional powder flow lengths prior to the charging operation to
incorporate a tribocharge pre-
charge or post-charge effect.
Figures 21-24 show another embodiment of the invention. In this embodiment, an
electronically
conductive extension 972 supports a nozzle 974 having a slot 976. A charge
sleeve 978 is installed
between the nozzle 974 and a charge sleeve holder 980. The powder feed tube
982 is inserted into the
charge sleeve holder 980 and is connected to a powder feed hose 984. A ground
pin 986 is connected to
the extension 972. The extension 972 is connected through a ground screw 988
to a ground wire 990.
The charge sleeve holder 980 includes air jets 981 which enhance the
tribocharging ability of the gun.
The jets 981 impinge upon the inside surface 979 of the charge sleeve 978
which is constructed from a
tribocharging material such as those described above. The ground pin 986 is
positioned just behind the
place where tribocharging air assist jets 981 impact the charging surface 979.
Figures 22 and 23 show the charge sleeve holder 980 in more detail. As shown
in Figure 23, the
air jets 981 are disposed at 90 degree intervals around the circumference of
the charge sleeve holder 980.
The passage 992 for the ground pin 986 is shown in Figure 23 as disposed
between two of the air jets 981.
Figure 24 shows a view of the charge sleeve 978 assembled to the charge sleeve
holder 980. A
locating pin 996 is frictionally received within the holder 980. When the
charge sleeve 978 is assembled
to the holder 980, the locating pin 996 is received within a slot 994 formed
within the exterior surface of
the sleeve 978. This permits the sleeve 978 to assume a particular positional
orientation in the holder 980
(hereinafter referred to as a first orientation). In this first orientation, a
certain portion of the interior
surface 979 of the sleeve 978 is impacted by the air jets 981 and worn away by
the frictional charging of
the powder. In order to be able to expose different parts of the interior
surface 979 to the air jets 981 a
number of such slots are formed on the exterior of sleeve 978. To reorient the
sleeve in holder 980 in a
different positional orientation, the sleeve 978 would be pulled out of the
holder 980 and rotated to align a
different slot formed in the exterior of sleeve 978 with the pin 996 and the
sleeve 978 would then be
pushed back into holder 980. In this way a new portion of the charging surface
979 would be impacted by
air jets 981 to be used for frictional, or triboelectric, charging of the
powder without the need for replacing
the charge sleeve 978. In addition, the sleeve 978 is symmetrical so that its
orientation within the holder
90 can be reversed with the opposite and of sleeve 978 being inserted into
holder 980. This doubles the
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number of different orientations the sleeve can assume within holder 980 to
permit an even greater portion
of the surface to be used for triboelectric charging before the sleeve 978
must be replaced.
Consequently, among the advantages of this embodiment is the employment of a
novel concept in
triboelectric gun of designing one or more components of the gun, which are
used as a triboelectric
charging surface, to be assembled into the gun in more than one orientation so
that more of the surface can
be used for tribocharging the powder before the component is replaced with a
new component. This saves
the customer money by enabling the customer to more fully utilize the
component before replacing it.
A further cost savings is provided to the customer by forming the
triboelectric charging assembly
in two pieces as a charge sleeve and a charge sleeve holder. By constructing
this component as a two
piece assembly, only the charge sleeve holder, which includes the air jets and
is more complicated to
manufacture, does not have to be replaced. Thus the charge sleeve 978 is a
much simpler part to
manufacture and replace than a charge sleeve such as the one shown in Figure
17 which includes the air
jets as well as the charging surface.
Note also that in the Figure 21-24 embodiment all of the air jets 981 are in a
single vertical plane.
This produces a number of advantages. The charge sleeve can be shorter than
charge sleeves with sets of
air jets provided along the length of the charge sleeve. Also, any air
introduced from the back of the gun
will feed all the air jets uniformly, which produces more even charging of the
powder. Further, all
powder impact areas within the sleeve are close to the ground pin. In
addition, a lower pressure can be
used for air jets in a~ single plane, which reduces energy requirements, since
there is no pressure drop
between the first set of air jets and the second set of air jets.
In accordance with another aspect of the invention then, various combinations
of air jet assisted
tribocharging and tribocharging techniques can be implemented in a spray gun.
These include but are not
necessarily limited to: air jet assisted tribocharging followed by
tribocharging; tribocharging followed by
air jet assisted tribocharging; an inside-out air jet assisted tribocharging
followed by tribocharging;
tribocharging followed by an inside-out air jet assisted tribocharging; inside-
out air jet assisted
tribocharging followed by an outside-in air jet assisted tribocharging; and
inside-out air jet assisted
tribocharging combined with outside-in air jet assisted tribocharging. Various
tribocharging material
combinations may also be used in a gun, including positive and negative
charging materials as required.
A significant advantage of the air jet assisted tribocharging guns is that
their short length design makes
them suitable for coating the insides of pipes and other enclosed surfaces.
The short gun length allows the
gun to travel through a pipe that even has bends of various angles, which is
difficult for prior art spray
guns of significant length.
While the invention has been described with reference to a preferred
embodiment, it should be
understood by those skilled in the art that various changes may be made and
equivalents may be
substituted for elements thereof without departing from the scope of the
invention. In addition, many
modifications may be made to adapt a particular situation or material to the
teachings of the invention
without departing from the essential scope thereof.
29
CA 02413267 2002-12-19
WO 02/04127 PCT/USO1/21572
Therefore, it is intended that invention not be limited to the particular
embodiment disclosed as
the best mode contemplated for carrying out this invention, but that the
invention will include all
embodiments falling within the scope of the appended claims.
