Plastics extrusion apparatus

In plastics extrusion apparatus including a rotating screw in a cylinder forming a channel receiving solid material and eventually expressing it through a die, the channel comprising a feed zone, a melting and compression zone and a melt pumping zone, the improvement wherein a solids decompression zone is between the feed and the melting and compression zones which has increased unit volume relative to that of the feed zone, with the channel of the melting and compression zone communicating with the feed inlet through the decompression and feed zones such that solids are initially metered producing a starved condition so as to melt at substantially atmospheric pressure without filling the channel of the melting and compression zone until close to its end.

BACKGROUND OF THE INVENTION 
This invention relates to extrusion apparatus for melting solid plastic and 
generating hydrostatic pressure to force the melt through a die, and more 
particularly to an improved screw for use in such apparatus. 
Continuously operating screw extruders for processing synthetic resinous 
plastic material are conventionally gravity fed from a source of stock in 
solid or particulate form to a screw portion beneath the feed inlet having 
the same or greater conveying capacity as that beyond the inlet. Systems 
under such conditions are said to operate in a flood feed mode, and 
traditionally exhibit a certain level of mass flow surging in the outlet 
die particularly at high throughput rates which results in corresponding 
undesirable fluctuations in extrudate weight. Such surging as manifested 
by fluctuating melt pressure in the delivery tube to the die is believed 
caused by a fluctuation in the axial position of the solid-melt interface 
along the screw. More specifically, when such interface moves rearwardly 
toward the feed end of the system, pressure and therefore mass flow 
through the die is increased whereas forward movement decreases pressure 
and mass flow. This fluctuation of the interface is a function of a number 
of variables such as, for example, extruder barrel temperature profile, 
head of material in the hopper feeding the extruder, temperature and bulk 
density of the feed stock, level of buildup of volatile products within 
the extruder and the like. Likewise characteristic of systems operating 
under flood feed conditions at high throughput rates are reduced power 
economy (mass extrustion rate divided by mechanical power) and increased 
occlusion of air which, if present, results in bubbles in the extrudate. 
SUMMARY OF THE INVENTION 
Now, however, improvements have been developed in plastics extrusion 
apparatus which reduce or substantially overcome the aforementioned prior 
art shortcomings. 
Accordingly, it is a principal object of this invention to provide an 
improved screw member of novel construction for use in extrusion apparatus 
which is capable of self-regulating a feed of particulate material from 
the extruder inlet to the downstream zone of the extruder. 
Another object is to provide such a screw member which permits operation of 
the extruder hopper in the flood feed mode while achieving the 
metered-starved state with respect to the screw without the use of 
apparatus external to the extruder. 
Another object is to provide a self-starving screw member for use in a 
plastics extruder which substantially reduces torque and mechanical power 
requirements, improves volatiles venting and provides a high degree of 
uniformity of mass flow and pressure at the die. 
Other objects of this invention will in part be obvious and will in part 
appear from the following description and claims. 
These and other objects are accomplished in plastics extrusion apparatus 
which includes a hollow cylinder having an inlet for containing a bed of 
solid plastic, a rotatable screw within the cylinder defining with the 
cylinder a helical channel for the plastic comprising in series, a feed 
zone beneath the inlet and a melting zone, by providing the improvement 
which comprises a solids decompression zone between the feed zone and the 
melting zone having increased free volume relative to the feed zone 
whereby before significant melting occurs the bed of solid plastic is 
decompressed to the extent that the channel in the solids decompression 
zone is partially filled.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS 
Referring now to the drawing, a plastic extrusion apparatus in FIG. 1 is 
collectively identified as 10 and includes horizontally elongated, hollow 
cylinder 12 formed with feed inlet 14 preferably adjacent its rearward end 
for receiving plastic material 16 from a reservoir in the form of feed 
hopper 18. Except for inlet 14, cylinder 12 is preferably otherwise 
without vent ports along its length. Die 20 at the opposite, forward end 
of cylinder 12 is formed with through-opening 22 which may be of any cross 
sectional configuration, such as cylindrical, tubular, substantially 
rectangular or the like through which the plastics material in melt form 
exits the system. Melt issuing from opening 22 may be either in finished 
form or a semi-finished product where further shaping is contemplated 
before it assumes its finished condition. Though not shown, an annular 
chamber surrounding cylinder 12 may be conventionally formed by providing 
a jacket around cylinder 12 for circulation of a suitable heat transfer 
medium such as steam or water to either add to or remove heat from the 
plastics material being processed. Electric heaters in heat transfer 
contact with the outer surface of cylinder 12 are also suitable for this 
purpose. 
Screw 24 within cylinder 12 is fixed against axial movement along cylinder 
12 and mounted for rotation about its longitudinal axis 28 via suitable 
motor means, bearings and similar rotary motion transmitting components, 
not shown, conventionally associated with rearward end 26. Screw 24 has a 
longitudinal shaft of varying diameters with helical thread 30 formed 
thereon which on rotation of the shaft moves in close proximity to and 
with minimal clearance from the inner surface of the bore of cylinder 12. 
The helical thread 30 of screw 24 in combination with the smooth inner wall 
delimiting the bore of cylinder 12 defines an unobstructed, forwardly 
directed helical channel between inlet 14 and die 22 through which 
material 16 passes in being converted to melt form. Such helical channel 
comprises, in series, feed zone 32 beneath and receiving solid particulate 
material 16 through feed inlet 14, melting or melting and compression zone 
34 progressively melting material 16 received from zone 32, and melt 
pumping zone 36 pumping melted material 38 under pressure through die 
opening 22. In the representative embodiment of FIG. 1 the diameter of the 
shaft portion of screw 24 in zone 34 progressively increases to a maximum 
at the inlet end of zone 36 along which such shaft diameter is constant. 
The configuration of screw 24 in zones 34 and 36 may, however, vary from 
that shown and the apparatus may include more than one of each of such 
zones to accomplish melting and pumping. 
In accordance with the invention, a special decompression section or zone 
31 (FIG. 1) is incorporated into the screw to decompress the solid bed of 
plastic 16 contained within inlet 14 to the extent that the channel in 
such decompression zone is only partially filled in the region forward of 
feed hopper 18 and feed zone 32 and before melting has made little if any 
significant progress. Such decompression is accomplished by increasing the 
free or open volume of the helical channel in such decompression zone, for 
example, by varying the geometric parameters defining the shape of screw 
24 according to any of a number of ways to be now described. 
In the embodiment of FIG. 1 decompression is accomplished by decreasing the 
root or shaft diameter of screw 24 in zone 31 relative to that in feed 
zone 32 while holding all other parameters constant. More specifically, 
feed zone 32 includes annular channel section 40 disposed across feed 
inlet 14 having reduced unit volume relative to that of solids 
decompression zone 31 immediately downstream of such channel section 40. 
Such reduced unit volume of channel 40 is calculable as the product of 
pitch length 42 in zone 32 minus the width of the screw flight or thread 
therein times the annular cross-sectional area 44 of such channel 40. 
Diameter 48 of screw shaft 26 of reduced unit volume section 40 is greater 
than that in zone 31 which in FIG. 1 extends forwardly to where 
compression commences via an increase at 50 in the diameter of shaft 26. 
Pitch 42 in FIG. 1 of thread 30 is substantially constant throughout feed 
zone 32 and preferably is constant along the full length of screw 24, as 
shown. 
In the embodiment of FIG. 2, decompression is accomplished by increasing 
the helix angle of the thread of the screw in the solids decompression 
zone relative to that in the feed zone, holding all other parameters 
constant. More specifically, the feed metering process to be further 
explained hereafter will be the same as achieved with that of FIG. 1 in 
that though shaft diameter 51 is less than at 48 in FIG. 1, helix angle 
.theta..sub.1 in feed zone 52 beneath feed inlet 53 is different from and 
less than .theta..sub.2 in solids decompression zone 55. Stated reversely 
.theta..sub.2 in zone 55 is greater than .theta..sub.1 in zone 52. Channel 
section 56 disposed across feed inlet 53 thus has reduced unit volume 
defined as the product of the annular cross-sectional area of channel 56 
times pitch length 58 minus the flight thickness, such channel of reduced 
volume being substantially constant in feed zone 52. 
With respect to the embodiment of FIG. 4, the reduced unit volume of the 
channel in feed zone 92, or conversely the increased unit volume in solids 
decompression zone 94 relative to feed zone 92, is achieved by increasing 
the lateral thickness of screw flight 88 in zone 92 relative to the 
reduced thickness shown at 90 in zone 94. 
In operation, extrudable, solid, resinous thermoplastic material, 
preferably in particulate form, is continuously flood fed by gravity 
through inlet 14 to feed zone 32 of apparatus 10 and advanced via helical 
thread 30 (FIG. 1) along solids decompression zone 31, melting and 
compression zone 34 and melt pumping zone 36 until eventually forced in 
melt form through die opening 22. As shown in FIG. 1, the solid plastic 
bed present in feed zone 32 is decompressed or expanded in zone 31 in that 
zone 31 has a larger unit volume than that in the immediately preceding 
zone 32 with the result that the channel in zone 31 is only partially 
filled with plastic. In zone 34, the solids impinge on the inside surface 
of cylinder 12 forming a melt film 61 which collects in a melt pool at the 
rearward end of the channel between adjacent flight sections so that there 
exists simultaneously in each such channel section a pool of melted 
plastic, a solids portion and an open channel which extends throughout 
substantially the entire melting and compression zone 34. 
With respect to melt pumping zone 36, the pressure generated therein is 
dependent on the speed of rotation of screw 24, the viscosity of the 
melted plastic and the annular cross section and length of such zone. 
In order to achieve the greatest reduction in pressure and mass flow 
variations at the die, it is preferred that the screw be designed for a 
given set of extruder operating conditions so that the depth of the 
channel at the point where the channel first becomes completely filled 
with melt is somewhat greater, for example twice as deep, as in the melt 
pumping section of the screw. 
Channel section 40 in FIG. 1 and 56 in FIG. 2 in feed zones 32 and 52 
respectively, and the similarly limited section in feed zone 92 of FIG. 4, 
all have reduced channel volume relative to that in the immediately 
adjacent solids decompression zones, and thus meter the particulate 
material into the melting and compression zones 34, 57 and 94 at a rate 
less than that at which it is capable of being conveyed away so as to 
produce a starved condition during compression and progressive melting in 
zones 34, 57, and the corresponding one in FIG. 4, not shown. As depicted 
in FIG. 1, channel 60 in melting and compression zone 34 is not completely 
filled until, as shown at 68, a point substantially at the end of zone 34. 
As mentioned, the channel in zone 34 rearwardly, of region 68 toward inlet 
14 between adjacent thread sections comprises an open portion for venting, 
in addition to a portion containing solid material and a portion 
containing molten polymer. Under such conditions, the helical channel up 
to point 68 where it is substantially completely filled with melted 
plastic and through which the material has just progressed forwardly, will 
be vented back through feed zone 32, 52 (FIG. 2) and 92 (FIG. 4) to the 
feed inlet. Stated differently, the unfilled portion of channel 60 in zone 
34 substantially spirally intercommunicates rearwardly back through the 
solids decompression and feed zones with the inlet. The vent path for air 
occluded in the particulate material or any gases emitted from the 
material working their way around back through the unfilled portion of the 
helical channel maintains the system up through substantially the end of 
the melting and compression zone at substantially atmospheric pressure, 
such path being in the opposite direction to the forwardly advancing 
material and constituting a portion of the channel also occupied by solid 
and melted material. The material being gradually converted to molten form 
is depicted by the increasing depth of melt pool at 63, 64, 66 and 68 in 
FIG. 1. Assuming sufficient gases being backwardly vented and percolating 
up through the solid bed in feed hopper 18, a nominal pressure slightly 
above atmospheric pressure can exist in zone 34. Such melting at 
essentially atmospheric pressure in a partially filled channel translates 
into reduced torque requirements to turn screw 24 which in turn can reduce 
motor and gear box sizes by from about 30 to 50% vis-a-vis conventional 
flood feed systems where melting occurs in a filled channel and high 
hydrostatic pressures are generated concurrently with the melting of the 
polymer. Such reductions in systems according to the invention in turn 
permit realization of significantly lower extrudate temperatures. 
In the embodiment of FIG. 3, baffle 72 is disposed in inlet 76 to confine 
material 78 being fed to the system through inlet 76 to the upstream side 
of such baffle 72. This creates a slot 80 in the solids decompression zone 
allowing gases 82 venting from cylinder 84 to escape to atmosphere and 
thus maintain atmospheric pressure during compression along channel 
portion 86. Such a feature avoids any venting gases having to percolate up 
through the feed hopper, otherwise the system is identical to and operates 
the same as those of FIGS. 1, 2 and 4. 
Though the ratios of unit volume of the channel in the section of the feed 
zone disposed across the feed inlet to that of the unit volume of the 
channel in the solids decompression zone immediately beyond such inlet may 
vary depending upon the extent of metered-starved feed mode desired to be 
accomplished with the self-starving screw configurations of the invention, 
it is preferred that such ratio be maintained between about 0.95 to about 
0.5. 
The present invention may be applied to any type of extruder screw and is 
not limited to the single flight, single stage metering screw shown in the 
drawing. Also, as opposed to relying on only one of the approaches 
described herein to achieve decompression, various combinations of the 
techniques disclosed in FIGS. 1, 2 and 4 for specifying the shape of the 
screw to achieve the metered-starved state can be used. For example, screw 
shaft diameter, flight angle and flight width may each be cooperatively 
specified to provide the particular degree of solids decompression desired 
for the system being designed. 
The above description and particularly the drawing is set forth for 
purposes of illustration only and is not to be taken in a limited sense. 
Various modifications and alterations will be readily suggested to persons 
skilled in the art. It is intended, therefore, that the foregoing be 
considered as exemplary only and that the scope of the invention be 
ascertained from the following claims.