Note: Descriptions are shown in the official language in which they were submitted.
" 1327687
`~FIELD OF THE INVENTION
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~ The invention relates to the processing of
'~ particulate plastics in extrusion or injection-molding
machines and in particular to the elimination of
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volatiles contained in such plastics. `
~;` BACKGROUND OF THE INVEN~ION
A major problem in the injection molding of
plastic relates to the rémoval of volatiles (primarily
moisture) commonly contained in particulate feed
materials. Failure to adequately remove moisture
tends to produce bubbles in the resultant product
which seriously degrade the quality of the product.
In the prior art, it has been common to
pre-dry particulate feed material in a reservoir
through which heated air flows are directed. Several
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hours of drying are commonly re~uired with a considerable ~;
expenditure of energy. Warm damp air that has been
exhausted from the reservoir is sometimes passed
through a dessicant filter, re-heated and re-circulated
through the reservoir to provide a very dry air flow.
The dessicant filters must periodically be purged of -~
water, and elaborate systems had been proposed for
moving such filters between an operative position and
purging apparatus without significantly interrupting
the drying process.
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-` A method has been proposed to eliminate pre-
drying. In U.S. patent No. 3,826,477 which issued on
2 July 30, 1974 to Xunogi et al there is described a
method for venting the barrel of a m~lding machine to
remove volatiles prior to injection. The method
`i~ involves forming the barrel in sections of stepped
internal diameter and adapting the associated screw-
y drive to reduce the accumulation of plasticized
material in a section of increased diameter. This
section is subject to reduced internal pressure and
a vent connected to a vacuum pump removes volatiles
from this section without significant expulsion
of plasticized material. Such a technique has apparently
not been entirely successful, and in some instances a
measure of pre-drying has been felt necessary to obtain
a satisfactory molded product.
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Accordingly, it is an object of the present
invention to provide a method that permits damp
particulate plastic to be processed without pre-drying
in an extruder or injection molder and to provide a
device embodying the method which can eliminate the
need for pre-drying without requiring modification
of an existing extruder or injection molder or
cons'ruction of a plastic processing machine with a -
! 25 stepped barrel and complex screw-drive.
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`- 1 BRIEF SUM~RY OF THE `INVENTION
In a first aspect, the invention provides a
process which permits damp particulate plastic to be
~`~ processed without pre-drying in an extruding or injection-
molding machine. The process involves heating the
damp particulate plastic externally of the barrel to a
pre-selected temperature which can be empirically
determined in a manner described more ~ully below without
knowleage of the actual value. In typical applications,
the temperature may be in the order ot 150-200F. The
heated damp particulate material is delivered downwardly
through generally vertically extending guide means to
the material inlet of the machine wherein contact with
the heated barrel and the screw-drive releases volatiles
from the damp particulate plastic; and applying controlled
suction to the guide means whereby heated air is drawn
inwardly through the particulate plastic flowing downward
t~rough the guide means, and substantially àll of the
released volatiles are removed from the material inlet
upwardly through the guide means. -~
In a second aspect the invention provides a
device adapted to permit dàmp particulate plastic to ;~
be processed`without pre-drying in an extruding or
¦ injection-molding m~chine. Guide means are provided
1 25 for directing movem~nt of the particulate plas~ic to
the material inlet of the machine, including an inlet
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1 for receiving the particulate plastic and an outlet
adapted to be placed in communication with the machine's
material inlet. Means are provided for raising the
temperature of the particulate plastic during movement
between the inlet and outlet of the guide means, including
temperature control means which permit selection of the
temperature to which the particulate plastic is heated.
Means are provided ~or applying controlled suction to
the guide means whereby air is drawn into the guide
means through the particulate plastic flowing downward
~ through the guide means, and substantially all of the
`~ volatiles released from the damp particulate plastic
in the barrel of the machine are removed upwardly through
the material inlet and the guide means, and heating
means for raising the temperature of the air being
drawn into the guide means~
The term "heating" as used in this disclosure
and the appended claims should be distinguished
~rom ~pre-drying~. The latte~ term is intended to
designàte substantial removal of volatiles (primarily
water~ from d ~ particulatc plastlc prior to
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introduction of the materials into an extruding or
injection-molding machine, a prior art practice. The
. principle of operation embodied in devices of the
-~ present invention is to heat damp particulate plastic
externally of the barrel to a temperature (typically
between about 150-200F) sufficient to ensure that .
contact with the heated barrel and screw-drive of .
the plastic processina machine "adjacent" the material
inlet of the n;achine, that is, prior to an extensive
plasticization and transfer by the screw-drive, causes
release of substantially all volatiles contained in .
the damp particulate plastic. In such circumstances,
suction applied at the material inlet where feed material ;;
j is introduced into the interior of the barrel (which
¦ 15 suction obviously cannot be so great as to interfere
with delivery and processing of the feed material)
which obviously cannot be at a level which can ::`
suffice to remove substantially all the released
volatiles.
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i BRIEF DE~CRIPTION OF THE DRAWINGS
: 20 The invention will be better understood
with refer~nce to drawings which illustrate a preferred
embodiment of a de~ice constructed according to
the invention, in which:
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~ Fig. 1 is a fragmented, elevational view
-~ illustrating the device mounted on an injection molding
machine;
Fig. 2 is a cross-sectional view of the
device in a plane parallel to the plane of Fig. 1 and
substantially through the centre of the device; and,
~ig. 3 is a fragmented, elevational view
illustrating the device adapted for self-feeding from
a reservoir of particulate plastic material.
DESCRIPTION OF THE PRE~ERRED EMBODIMENT
Reference is made to Fig. 1 which illustrates
a device embodying the invention and generally indicated
by the reference numeral 10. The device 10 is ceated
about the material inlet 12 of an injection malding
machine 14 having a heated barrel 16 and a screw-drive `
18 that serve to plastici~e particulate plastic.
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The device 10 includes means adapted to
guide part~culate plastic under the influence of
gravity to the material inlet 12 of the machine 14.
The guide means include a tubular guide member 20
constructed in four steel rings 22, 24, 26, 28 about
4 inches in diameter and 1/4 inch in thickness. The
S rings are spaced vertically to provide apertures 30 "~
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~` which are generally annular except for weld points
which are not illustrated in the drawings. The
apertures 30 are angled upwardly at about 45 to the
~- interior surface of the tubular guide member 20 to
permit the passage of air but preclude the escape
of particulate materials (because of the natural angle
of repose of the materials). An annular heating
band 32 is secured with a ~trap connector to the outer
surface of each of the rings, and the heating bands
32 are coupled to a temperature controller 34 which
~ can be used to vary the power supplied to the heatinq
¦ bands 32. Together, the bands 32 have a maximum
heat generating capacity of about 400 watts and serve
to heat air flows drawn into the guide member through
the apertures 30 as described more fully below.
The guide means also include a bell-shaped
housing 36 which is snuggly fit about and bolted to
the upper ring 22. An opening 38, circumscribed
by an abuttment flange 40, serves as an inlet for
the guide~means. A hopper 42 ~fragmented) is bolted
to the abuttment flange 40 and serves as a reservoir
for materials to be delivered by the device 10. A
second bell-shaped housing 44 is fitted in a similar
manner about the lower ring 28, and has an opening
46 which serves as an outlet for the guide means. The
opening 46 is circumscribed by an abuttment flange 48
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that is bolted to the machine 14 to place the opening
46 in communication with the material inlet 12.
~' Means are provided for generating a flow of
-~ heated air through materials guided in the tubular
guide member 20. These means include the heating
bands 32 and temperature controller 34 described above.
A cylindrical sheet metal housing 50 defines a
t~substantially closed compartment about the tubular `-
guide member 20 and heating bands 32: leakage of -~
air from the housing is not particularly critical as
the general object in providing the housing 50 is -
to contain heat radiated by the bands 32. The housing
50 is constructed in two housing portions 52, 54 which
can be located about the guide member 20 during
assembly. The housing portions 52, 54 are formed
with longitudinal flanges 56 lonly one pair illustrated)
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which can be secured to one another by means of bolts
58 (only one specifically indicated~. The housing 50
is dimensioned to fit snuggly between annular surfaces
60 of the~bell-shaped housings 36, 40 but, as mentioned
above, strict sealing engagement is not necessary.
- Air can be drawn into the housing 50 through an inlet
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pipe 62 $ormed with the housing portion 52 for heating ;~
by the bands 32. A damper 64 pivotally mounted within `-
the inlet pipe 62 and rotatable by means of a control `
arm 66 external to the inlet pipe 62 permits regulation
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of air intake. Air is drawn from the interior of
the housing 50 into the guide member 20 by application
of suction to a conduit 68 which is centrally located
in the guide member 20. The conduit 68 includes
a meshed body portion 70 with apertures that permit
passage of air but preclude the entry of particulate
plastic typically used in injection molding or extrusion.
~he conduit 68 include~ an elbow 72 which extends
through a sidewall of the bell-shaped housing 36 and
which is sealed to the housing 36 by welding.
Application of suction to the interior of the conduit
- 68 causes air heated by the bands 32 to be drawn
through the apertures 30 formed in the guide member
20.
j 15 The conduit 68 also serves to apply suction
at the material inlet 12 of the injection molding
machine 14. Thus, the conduit 68 is formed at a
lower end with an extension conduit 74 that extends
into the material inlet 12 to direct suction for removal
2~ of volatiles~ The extension conduit 74 is a telescoping
member with an outer tube 76 slidably mounted over
an inner tube 78. A simple lock assembly is provided
i to prevent telescoping of the outer tube 76 into the
screw-drive 18. The lock assembly consists of a rod
80 hingedly connected to the outer tube 76 and extending
through an aperture provided in the bell-shaped housing
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` 44. A knob 81 is secured to an end of the rod 80 to
prevent telescoping of the tube 75 beyond a pre-
determined position relative to the remainder of the
-~ conduit 68. The length of the conduit 74 is selected
to locate the point of application of suction just above
the screw drive 18 as illustrated in fig. 1.
A heat gathering housing 82 (extensively
fragmented in the view of fig. 1) is mounted about
the barrel 16 of the machine 14 to gather heat radiated
by the barrel 16 during operation. The housing 82 is
~t coupled by a flexible conduit 84 to the inlet pipe 62
of the housing 50. This arrangement permits pre-
heating of air which is further heated by the bands 32. :.
~ In typical applications, the air may be pre-heated
¦ 15 to a temperature of about 120-130F thereby significantly
reducing the power requirements imposed on the heater
bands 32.
An exhaust pipe 86 of about 2 inch diameter
is strapped to the elbow ?2 and carries a substantially
radia~ly symmetric metal nozzle 88 to which pressurized
air can be applied to produce suction in the
3 interior of the conduit 68. The nozzle 88 has a first :
annular portion 90 which contacts the interior :`
surface of the exhaust pipe 86 in substantially
conforming engagement, a second annular portion 92
which is spaced from the interior surface of the -
i exhaust pipe 86 to define an annular space 94, and ~
a third annular portion 96 with an annular outer
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surface 98 marginally spaced from the interior surface
of the exhaust pipe 86 to define an annular nozzle
:~ outlet. An air hose 80 secured to the exhaust pipe
86 places a regulator 102 in pressure communication
with the annular space 94. The regulator 102 can be
connected to any convenient source of compressed air
(not illustrated) to apply air under pressure to the
annular space 94 of the nozzle 88 to cause a stream
of air to flow about the surface 98 into the exhaust
pipe 86. In typical applications 3-5 pounds of
pressure are applied to the hose 100 by the regulator
102 and generates sufficient suction in the interior
of the conduit 68 both to draw heated air flows and
to exhaust volatiles at the material inlet 12 of the
machine 14, as well as permitting a material delivery
function described more fully below~ A nozzle of
simpler construction ~for example a right-angled tube)
can be used, but the nozzle 88 is relatively simple
t~o install and cannot be rotated by vibration or the
. 20 like into a position in which air flows would be
misdirectèd.
Operation of the device 10 in combination
with the machine 14 is essentially as follows.
Particulate plastic 104 is gravity fed from the hopper
42 into the tubular guide member 20 and ultimately
into the material inlet 12 of the machine 14. The :
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delivery rate is determined entirely by the take-up
rate of the machine 14. During start-up, materials
loaded into the tubular guide member 20 should be heate
for about 5-10 minutes with heated air flows prior to
starting operation of the machine 14. The machine 14
should then be purged with several shots. Resulting
-~i sample products should be inspected visually for
defects such as lines of bubbles characteristic of
unvented volatiles. If a new plastic is being introduced
~; 10 into the machine 14, the heat delivered by the bands
32 should be increased until a satisfactory product is
obtained substantially free of any deleterious
~ualities. In practice, the temperature to which
particulate plastic materials are to be heated is not
unduly critical: heating to temperatures in the - -
range of 150-200F is satisfactory for most plastics,
and exceeding some predetermined minimum temperature
at which a satisfactory product can be obtained has
not in testing adversely affected the resultant
product. The setting of the pressure in the air hose - ;
100 is also not particularly critical as pressure `~`
must simply exceed some empirically determined minimum ;~;
value at which volatiles are sufficiently removed ;`
from the material ir~let 12 that a satisfactory product `
is obtained. The general object is to heat particulate
plastic to a temperature sufficient to cause substantialy `
all volatiles to be liberated in the heated barrel 16
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~ prior to extensive plasticization and transfer along
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the screw-drive 18. In such circumstances, substantially
all volatiles can be removed through the material
~ inlet 14 by application of a properly selected amount
-~ 5 of suction, a level which of course does not disrupt
the processing or delivery of the particulate plastic.
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In practice, a pressure setting satisfactory
for one type of material will generally be satisfactory
~` for another; however, a case may arise where both
temperature and pressure must be adjusted to obtain a
satisfactory product when a new material is being
plasticized. Typical feed rates which can be achieved
using the device 10 are as follows: typical nylon
materials at 100 pounds per hour, ABS products at
120 pounds per hour, and polycarbonates at 70 pounds
per hour. Heat settings for the bands 32 may have
to be varied if plastic materials are stored or
plasticized in a particularly humid environment. Increased
feed rates can be obtained by constructing the guide
member 20 with additional aligned rings, or by an
overall increase in the dimensions of the device 10
(the housing 50 being about 10 inches in diameter, and
the distance between the inlet and outlet of the device
10 being about 28 inches).
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.~ Fig. 3 shows the device 10 adapted for self-
regulating uptake of feed material. The bell-
shaped housing 36 effectively defines in use a
. substantially sealed compartment about the inlet
. 5 opening 38 and about an apertured portion 106 of
the conduit 68. Thus, when the level of particulate
. plastic drops below the apertured conduit portion 106,
the suction applied to the interior of the conduit 68
produces greater negative pressure at the inlet
opening 38. This suction is applied to a supply
conduit 108 which has an upper end portion 110 in : :.
substantially sealed pressure communication with the
inlet opening 38 ~the hopper 42 having been removed)
and a lower end portion 112 located in a mass of particu- :late plastic (pelletized) 114 contained in a reservoir
116 (shown in cross-section). A tube 118 connected :to a source of compressed air directs a flow of air
upwardly in the material supply conduit 108, thereby
producing a negative pressure in the conduit 108 that
tends to draw the material 114 into the conduit 108.
The pressure applied to the tube 118 is regulated ! ~ `
so that material 114 is not drawn up the conduit 108 -~
until additional negative pressure is created within
the bell-shaped hou;sing 36 by suction applied from :the conduit 68. Comse~uently, material 114 is dxawn
into the device 10 only until the apertures of the
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conduit portion 106 are covered with the material 114.
. The rate of take-up of material 114 through the conduit
108 is thus constrained to correspond to the rate
; at which material is consumed by the machine 14. A
vibrator 119 in pressure communication with the tube
. 118 and operated by pressure in the tube 118 vibrates
the cond~uit 108 thereby agitating the material 114 in ~i
the reservoir 116 to prevent bare spots from forming
I about the lower end portion 112 of the conduit 108.
¦ 10 A second reservoir 1~0 which is a cyclone
type separator communicates with the exhaust pipe 86
via an inlet pipe 122 mounted about the exhaust pipe
86. An open upper end 124 of the reservoir 120
permits air exhausted from the tubular guide member
20 to escape. Particulate plastic materials entrained
by the air f lows deposit at the bottom of the
rleservoir 120, and are returned to the inlet opening
3B through a tube 126 which receives any suction applied
! in the interior of the bell-shaped housing 36.
! 20 It will be appreciated that a particular .
device has been described to exemplify the principles -.
of operation of the invention and its associated method,
and that many modifi.cations may be made in the device
described without departing from the spirit of the
j 25 invention and the scope of the appended claims. It should
~ also be appreciated that the term "damp particulate
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plastic" as used in this specification refers to
hygroscopic plastics which have not been subject to th
. "pre-drying" normally required for processing in a
.- conventional extruder or injection-molder, which has not
been fitted with venting apparatus for removal
of moistur~ and other volatiles present in the plastic.
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