Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
12~30284
The present invention relates to a paint spraying shroud
and to paint spraying apparatus employing the shroud.
Known paint spraying apparatus, used for the continuous
painting of elongated articles such as extrusions, shelving,
racking components, building panels and the like, comprises a
spinner, driven at a high speed of rotation, to which the paint
is fed. The spinner is reciprocable along a vertical axis. An
overhead conveyor extends toward and away from the spinner and
in a partial loop around the axis of the spinner. The articles
to be painted are suspended from the conveyor and passed in
succession therealong. Usually, an electrical potential
difference is maintained between the spinner and the articles
so that the paint, which is spun off from the spinner in the
form of a fine mist ofpain-tdroplets, is attracted electro-
statically to the articles. Normally, in order to collectpaint mist which is not deposited on the articles, i.e. the
so-called "overspray", a paint mist collecting shroud is
disposed adjacent the paint spinner. The conventional form of
shroud has side walls which extend in a partial arc around the
axis of the paint spinner leaving a gap at the front through
which the conveyor and the articles suspended therefrom enter
and leave the shroud. An air plenum extends around the base
of the side walls on their outer side and this communicates
with the interior of the shroud through openings formed
through the lower ends of the side walls.
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A fan at the rear of the plenum exhausts air from the
plenum and passes it to a stack connected to the plenum
adjacent the rear of the shroud. The overspray paint mist
tends to gravitate downwards, and is drawn downwards by the
current of air drawn inwardly through the openings in the
plenum. Usually, the openings are fitted with filters which
trap the paint droplets to reduce build-up of paint on the
inside of the plenum, or on the fan blades, and to reduce
emissions of paint mist up the stack.
The applicants have observed that the known apparatus is
inefficient, as the downward current of air within the shroud
tends to distribute the paint mist non-uniformly on the
articles to be painted, and tends to result in the lower
portions of the articles receiving a larger proportion of the paint,
while the upper portions tend to receive a smaller proportion,
so that there is a difference in the paint coating thickness
from top to bottom. This, therefore, increases the consumption
of paint required to achieve a desired coating thickness.
Further, with the conventional form of shroud, the cross-
sectional area of the plenum is too small to permit uniformsuction on all of the filters, and the air flow through the
filters ad~acent the front of the shroud tends to be less than
that through the filters adjacent the rear. This results in
considerable quantities of the paint mist tending to escape
through -the front opening of the shroud and to be deposited on
adjacent floor or other surfaces, necessitating frequent
clean-ups, which are inconvenient, time-consuming and
expensive. The loss of paint mist through the front opening
also renders the application of the paint less efficient, as it
reduces the quantity of paint available for coating on the
articles.
The present invention relates to a shroud whereby the
above-noted disadvantages can be avoided, and provides a paint
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spraying shroud comprising vertically extending side walls
defining an enclosure having a vertically extending relatively
narrow front opening, a cover over the top of the enclosure,
means defining an air exhaust opening on a rear side of the
enclosure opposite the front opening, the air exhaust opening
extending parallel to and to approximately the same vertical
extent as the front opening, and the enclosure narrowing in
its width from a relatively wide region intermediate the front
and the air exhaust openings toward said relatively narrow
front opening.
The means defining an air exhaust opening on the rear
side of the shroud may comprise a series of closely vertically
adjacent exhaust openings. With the present shroud,
substantially uniform suction can be applied to each of the
exhaust openings, or over substantially the entire area of
other means defining the air exhaust opening at the rear side
of the shroud, using, for example, a plenum at the rear of
the shroud co-extensive with and in communication with the air
exhaust opening, and with air being exhausted from the upper
end of the plenum. With this arrangement, substantially the
entire air flow through the shroud takes place in a direction
transverse to the vertical line of reciprocation of the
paint-dispensing spinner, and the paint droplets entrained by
the air flow tend to be distributed substantially uniformly
over the entire length of the articles to be painted.
Further, the air entering the filters is, with the present
arrangement, incident on the front face of the filter at an
angle of substantially 90 with respect to the plane of the
front face. Applicants have found that this can greatly
increase the filtration efficiency, resulting in considerably
less penetration of paint droplets through the filter and
into the air exhaust plenumt as compared with the conventional
arrangement, where the air containing the overspray is
incident at an oblique angle on most of the filters.
~802~
In the preferred form, the shroud has a front opening of
width which is less than about half the width of the
enclosure at its intermediate region. As a result, the
velocity of the air entering through the constricted front
opening is relatively high, thus reducing or eliminating
escape of overspray paint mist through the front opening. In
the interior of the shroud, the air velocity drops as the flow
passes through the relatively wide region toward the rear
exhaust opening. The relatively low air velocity in the
intermediate region is conducive to uniform distribution of
the paint spray over the articles to be painted.
In the accompanying drawings, which show, by way of
example only, one form of paint spraying shroud in accordance
with the invention:
Figure 1 shows a partially fragmentary perspective view
of a shroud equipped with a conveyor, reciprocating paint
spinner, an exhaust plenum and an air exhaust system;
Figure 2 shows a horizontal section along the line 2-2 of
Figure 1 through the shroud and plenum; and
Figure 3 shows a partially fragmentary perspective view
of a preferred form of air filter used in the shroud of
Figure 1.
Referring to the drawings, the shroud 10, as best seen
in plan in Figure 2, comprises an enclosure 11 which is
symmetrical about a longitudinal median. The enclosure has
two intermediate planar side panels 12 and 13. Forward
planar side panels 14 and 16 incline inwardly from a front
edge of the panels 12 and 13. Each of the panels 14 and 16
continues forwardly in a flange 18, the flanges 13 being
parallel to one another, and terminates in a lip, 19 or 20,
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directed outwardly, and connected at its end to a vertically
extending box-section front support 22. The 'lanyes 18 define
between them a front opening which is desirably less than
about half the width of the intermediate region of the
5 enclosure 11 defined between the panels 12 and 13. In the
example illustrated, the width between flanges 18 is about
0.49 the width between the panels 12 and 13.
A rear side panel 23 or 24 inclines inwardly from the
rear edge of each panel 12 and 13, and the panels 23 and 24
are each connected at their rear edges to a rear panel 26 and
27, respectively. As will be seen from Figure 2, -the general
planes of the front faces of the panels 26 and 27 incline at
a relatively shallow angle with respect to the transverse
direction, as compared with the panels 14 and 16 or 23 and 24,
which incline at a relatively sharp angle.
The adjacent edges of the panels 12, 13, 14, 16, 23, 24,
26 and 27 are each formed with a short outwardly directed edge
flange 28, and these edge flanges are connected by mechanical
fasteners e.g. nuts and bolts 29.
Each rear panel 26 and 27 is formed with a series of
closely vertically adjacent rectangular openings 31. A paint
aroplet-entrapping air filter 32, shown in more detail in
: Figure 3, is connected on the rear face of each panel 26 and
27, in register with each opening 31. As shown, each filter
32 may have a front edge flange 33 which may be removeably
connected, by a releasable fastening, not shown, in air-tight
engagement to the rear face of the panel 26 or 27, permitting
the filters to be replaced when necessary.
Adjacent the rear of the shroud, there are a pair of
vertically extending box section rear supports 34. A
connector plate 36 is connected to the inner side of each
support 34 and is connected to the enclosure 11 through the
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bolts used to bolt together the edge flanges at the connection
between the adjacen-t rear side panel 23 or 24 and the rear
panel 26 or 27.
An air exhaust plenum chamber 36 is provided on the rear
side of the shroud, in communication with the openings 31. The
chamber 36 comprises side walls 37, connected at their front
edges to the supports 34, a rear wall 38, and a roof 39. One
side wall 37 is provided with a door 41 through which access
may be had to the rear of the filters 32, for the purposes of
inspecting and replacing the fil~ers when necessary. The roof
of the chamber has a circular opening 42 to which is connected
in air-tight fashion, a duct section 43 housing a fan 44 which
can be actuated to exhaust air from the chamber 36 and pass it
to a stack 46 connected to the section 43, which vents to the
atmosphere. The lower edges of the chamber 36 and of the
enclosure 11 are in substantially air--tight contact with the
floor or other surface on which the shroud is supported.
The upper end of the enclosure 11 is substantially
entirely closed by a cover in the form of a rectangular plate
47, which has openings through which the supports 22 and 34
pass, and which rests in air-tight fashion on the upper edges
of the forward side, rear side, and rear panels 14, 16, 23, 24,
26 and 27. The side edges of the plate 47 are each spaced
inwardly a small distance from the adjacent intermediate side
panel 12 or 13, thus defining a narrow elongate opening 48 at
each side of the upper end of the intermediate region of the
enclosure 11.
The shroud 10 is used in association with an endless
conveyor track which passes through a loading sta-tion, where
articles to be painted are suspended from the track, through
one or more paint-spraying shrouds of the type shown in the
drawings, through a drying oven where the paint coating or
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coatings are dried, and to an unloading station where the
painted and dried items are unloaded, and then returns to the
loading station. A portion of the track 49 passing through
the paint-spraying shroud 10 is shown in Figure 1. In the
example shown, the track 49 is of inverted T-section. The
vertical limb of the track may be suspended from the underside
of the plate 47 by connectors (not shown). The portion of
the track shown comprises an arcuate and part semi-circular
loop 51 passing around the centre of the intermediate region
of the enclosure 11 and merging with straight sections 52
passing outwardly and parallel to one another through the
entrance opening defined between the flanges 17 and 18.
Carriers 53 are disposed in rolling engagement on the track
49. Each carrier has a pair of upper arms, each supporting a
roller. The rollers engage on the horizontal limbs of the
track 49 on opposite sides of the vertical limb, respectively.
For the sake of clarity of illustration, only a few carriers
53 are shown in Figure 1. ~s will be appreciated, in practice,
a carrier 53 is supported at each few inches on the track 49
forming an endless line of carriers extending around the
endless track 49. Each carrier 53 is connected to a flexible
endless chain 54, and has a downwardly depending hook 56
connected to it. Items to bepainted may be suspended from the
hook 56. For example, elongate panels 57 to be painted may be
provided with a hole through them adjacent one end, the hook
56 being passed through the hole to connect the panel 57 to the
mobile carrier 53. In use, the chain 54, together with the
carriers 53 attached to it, is normally driven continuously
longitudinally relative to the track 49, so that items to be
painted and suspended from the carriers 53 are passed
continuously through the shroud 10. Periodically the drive
to the chain 54 will be interrupted to permit painted and dried
items to be removed from the carriers 53 at the unloading
station and to permit fresh items to be painted to be attached
to the carriers at the loading station.
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In use, paint is applied to the panels 57, or other items
to be painted, from a paint dispensing head 58 which is recip-
rocable along a vertical axis aligned with the centre of the
intermediate region of the enclosure 11 and coinciding with the
centre of the semi-circle which the track 49 describes around
the rear portion of the enclosure. Thus, as seen in Figure 2,
the items 57 pass in a semi-circular arc around the rear of
the enclosure 11, and receive a paint coating at least
predominantly on the side facing toward the paint dispenser 58.
Usually, the paint spraying line includes two shrouds each
similar to the shroud 10 illustrated, but each disposed
oppositely to the other, and with one being disposed with its
paint dispenser 58 on the inner side of the endless loop formed
by the conveyor and the other with its dispenser 58 on the
outer side thereof, so that items receive a coating of paint
predominantly on one side in one shroud and pre~ominantly on the
opposite side in the other.
In the example illustrated, the paint dispensing structure
comprises a frame connected to the upper ends of front and rear
supports 22 and 34, the frame comprising a pair of longitudinally
extending members 59, each connected at each end to a support
22 or 34, and a pair of spaced transversely extending members 61
connected to the members 59. On the upper side of the members
61 is connected, e.g. by bolts 62, a plate 63, to which is
connected e.g. by bolts 64, a pressure cylinder 66. The axis of
the cylinder is aligned with the centre of the intermediate
region of the enclosure 11. A piston disposed in the cylinder
is connected to a vertically-extending rod 67 which can be
extended downwardly relative to the cylinder 66 or retracted
upwardly relative thereto by controlling a flow of operating
fluid, e.g. hydraulic fluid or compressed air, to the cylinder
66. The rod 67 passes with some clearance through a hole 68
in the plate 47 and is connected at its lower end to the paint
dispensing head 58. This comprises a circular plate 69, or
spinner, connected to a spindle which is connected to a motor
71, whereby the spinner can be rotated at high speed about the
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axis of the rod 67. As shown, the motor 71 is connected to
flexible lines 72, which enter the enclosure 11 between the
sections 52 of the track 49 at the entrance to the shroud, and
which supply power to the motor, e.g. compressed air to a
turbine constituting the motor 71, paint to outlet or outlets
therefor adjacent the upper side of the spinner 69, and an
electrical conductor which applies an electrostatic potential
to the head 58, so that there is a potential difference between
the head 58 and the items 57 to be painted.
In operation, the spinner 69 may be rotated, for example
at about 20,000 r.p.m., while paint is fed along one of the
lines 72 to the upper surface of the spinner 69 and is flung
off therefrom as a fine mist of paint droplets. These tend to
be attracted toward the items 57 to be painted by reason of
electrostatic charge which the droplets assume when they leave
the electrostatically charged head 58, and the electrostatic
field created by the potential difference between the head 58
and the items 57. The piston rod 67 together with the head 58
are reciprocated up and down with controlled stroke and with
controlled frequency of reciprocation. The stroke is such that
at the extended or lower position, as shown in broken lines in
Figure 1, the head 58 is at approximately the level of the
lower edge of the items to be painted, and in its retracted or
upper position, at approximately the level of the upper edge of
the items. The frequency of reciprocation is matched to the
speed at which the conveyor chain 54 draws the items along the
conveyor rail 49 through the enclosure, so that, during the
passage of each item through the enclosure, the head 58 is
reciprocated upwardly and downwardly a sufficient number of
times to provide the items with an adequate paint coating.
Attached to the underside of the plate 47 there may be a
shield in the form of a vertically-extending plate 73 which is
spaced inwardly from and is curved -to conform to the shape of
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the track 49. The profile of this plate 73 is shown in dashed
and dotted lines in Figure 2. The plate 73 shields the track
49 from the head 58 when the latter is in an upper, retracted
position, to avoid accumulations of paint on the track 49
which would interfere with free rolling of the carriers 53
along the track.
As will be seen from Figure 2, where various arrows are
drawn to illustrate flows of air through the shroud, the
draft of the fan 44 results in air being drawn in through the
entrance of the enclosure 11. Some air will be drawn into the
enclosure through the elongate openings 48 at each side of the
cover plate 47 and through the circular opening 68 through
which the piston rod 67 passes. The openings 48 need only be
sufficient in area to provide a definite down draft adjacent
the upper portion of the sides of the enclosure 11, since it
is found that this prevents the tendency for a mist of very
fine paint droplets to rise toward the upper end of the
enclosure and to become deposited there or to escape through
any fine openings between the plate 47 and the enclosure 11.
Typically, however, the combined area of the openings 48 and
68 is less than about 10% of the entrance opening defined
between the flanges 18 and the plate 47, and thus substantially
all the demand of the fan 44 is met by air drawn in through the
front opening of the enclosure 11. Since the front opening is
relatively constricted as compared with the intermediate region
of the enclosure defined between the side panels 12 and 13, the
velocity of the air passing through the front opening is
relatively high as compared with the air velocity in the
intermediate region. Merely by way of example, it may be
mentioned that in one embodiment in which the draft of the fan
44 is 7600 cu. ft./min., and the cross-sectional area of the
front opening is about 66.5 sq. ft., and that of the inter-
mediate region, between the panels 12 and 13, about 140 sq. ft.,
the air velocity through the front opening will be about 114
ft./min., and that through the intermediate region about 54
2~3~)213~
ft./min. The relatively high velocity inflow of air through
the entrance opening eliminates or at least considerably reduces
any tendency for the overspray paint mist to escape from the
front opening of the enclosure. Loss of paint mist through the
front opening is further reduced with the preferred form of
enclosure 11, wherein the enclosure 11 extends, in plan,
through a relatively large arc about the centre of the
intermediate region i.e. the axis of the rod 67 and head 58,
and the front opening subtends a correspondingly small arc at
said centre. Preferably the enclosure 11 subtends an angle of
at least about 310, more preferably at least about 320 about
said centre. Within the enclosure, particularly in the relatively
wide intermediate region, the velocity of the air flow drops
considerably and this is conducive to flow of the paint mist,
dispensed from the head 58, in a direction generally transverse
of the air flowJ so that there is increased opportunity-for the
electrostatically-charged paint mist to become deposited on the
items to be painted. Because of the relatively low air
velocities within the enclosure 11, the so-called "wrap" or
extent of deposition of the paint on the transverse edges and on
the rear side of the items is improved, and there is better
penetration of the paint into recesses of the items to be
painted. In the preferred form, the front portion of the
enclosure, defined between the forward panels 14 and 16, narrows
continuously to the front opening so that the sides of the
enclosure are somewhat streamlined and there is little or no
turbulence in theair flow between the front opening and the
intermediate region.
With the shroud shown in the drawings, the exhaust
openings 31, provided with the filter elements 32, are disposed
directly opposite and in a closely-packed array substantially
co-extensive with the front opening of the enclosure 11. Thus,
except in the region immediately adjacent the openings 48 and
68 and above the level of the upper edge of the uppermost of
the openings 31, the air flow through the enclosure 11 tends to
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~28~Z8~L
be substantially uniformly parallel to a horizontal plane,
transverse to the axis of reciprocation of the paint dispensing
head 58. Further, the arrangement shown is readily adapted to
apply substantially equal suction to each of the openings 31
and hence to each of the filter elements 32. In the arrangement
shown, where suction is applied by the fan 44 to one end, in the
example shown the upper end, of the plenum chamber 36, the
transverse cross-section of the chamber can be made relatively
large in relation to its length, so that the pressure gradient
existing along the length of the chamber 36, if any, is
negligible, so that substantially equal suction can be applied
at each filter 32. The uniforrnly transverse air flQw t~us
created tends to distribute the paint mist uniformly over the
articles to be painted, thus achieving coatings with desired
uniformity of coating thickness, with substantially reduced
consumption of paint.
A preferred form of paint mist entrapping air filter is
shown in Figure 3. The filter 32 has a rectangular housing 74,
a forward edge of which provides the flange 33. Within the
housing is mounted a three-layer paint arresting unit comprising
three spaced sheets 76, 77 and 78 of stiff paper, card or the
like. The front sheet 76 has rearwardly extending edges 79 which
provide a box-like enclosure within which the other two sheets 77
and 78 are secured. Each sheet is formed with an array of
uniform and uniformly spaced rectangular holes 81. The holes in
the middle layer 77 are disposed out of register with those in
the first layer 76, preferably such that opposite each hole 81
in the first layer 76, there extends an area of the unperforated
paper or card material of the middle layer 77, and, similarly,
each hole 81 in the rear layer 78 is out of register with the
holes 81 in the middle layer. A three layer paint arresting
box-like structure of this kind is in itself known and is
available, for example, under the trade mark CORR AIR, from
Corr-Craft, Inc., Sanborn, New York. It is highly effective
in trapping fine paint mists on its surfaces. Rearwardly of the
paint arresting unit, and spaced therefrom by a rectangular
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spacer 82 extending around the inner side of housing 74 are
disposed two filter elements 83, each having a rectangular frame
84 received snugly within and secured within the inner side of
the housing 74 and each comprising a rectangular porous filter
mat 86, which may be formed of coarse fibrous material e.g.
shredded paper, mat formed into a coherent mat e.g. with
adhesives. While other forms of filter may be employed as the
fillers 32, those shown are highly preferred for their
excellent efficiency in trapping paint mist droplets, small
pressure drop and high capacity for receiving paint droplets
before requiring replacement as a result of impaired collection
efficiency or pressure drop characteristics.
One advantage of the present arrangement is that, as will
be appreciated from consideration of Figure 2 and the air flow
arrows shown therein, a substantial component of the air flow
incident on the front faces of the filter elments 32 is
incident at an angle perpendicular to the said front faces.
The applicants have found that this markedly increases the
paint droplet collection efficiency of the three layer paint
arresting unit comprising the spaced perforated sheets 76, 77
and 78, as compared with arrangements in which the paint
droplet-laden air flow is incident on the front faces at an
oblique angle.
Over a prolonged period of use, paint droplets become
deposited on the inner surfaces of the panels forming the
enclosure. In order to collect and to prevent unwanted
spreading of the paint thus deposited, which tends to run down
the panels, a trough 87 may be provided around the inner side
of the enclosure adjacent its lower end. As shown, the trough
may comprise a lip extending inwardly and upwardly from each
panel 12, 13, 14, 16, 23, 24, 26 and 27 and being secured e.g.
by welding to the inner side of the panel, the ends of the
adjacent lips co-operating to form a continuous trough. The
ends adjacent the flanges 18 at the front opening of the
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enclosure may be closed by end pieces 88. The trough can also
serve to collecting washings in the event that solvents,
thinners or the like are applied, e.g. through the dispensing head
58 in order to clean the inner surfaces of the shroud. I.iquid
accumulating in the trough 87 may be drawn off periodically or
continuously through a pipe placed in the trough or through an
opening formed through one of the panels in the region of the
trough.
