Extrusion surge suppressor and method

An apparatus and method for extruding an elastomeric polymer which is subject to periodic pressure surges. The apparatus includes a barrel having an upstream portion and a downstream portion, a hopper positioned for delivering polymer to the upstream portion of the barrel, a shaft rotationally mounted within the barrel and a drive means for rotating the shaft. The shaft has a conveying screw flight for conveying polymer downstream from the hopper through the barrel. The barrel has a discharge port located downstream of the hopper. A surge suppressor is provided on the shaft for reducing the pressure and flow rate surges in the polymer. The surge suppressor includes a screw flight downstream of the discharge port for urging the polymer upstream toward the discharge while permitting a portion of the polymer to flow downstream into the surge suppressor. A polymer seal is provided downstream of the surge suppressor.

BACKGROUND OF THE INVENTION 
This invention provides an apparatus for reducing or eliminating pressure 
and flow rate surges in polymer extrusion machines. It also relates to a 
method of extruding with a substantially constant polymer pressure. In 
particular, this invention provides an extrusion apparatus and method 
utilizing a surge suppressor incorporated into the extruder screw. 
FIELD OF THE INVENTION 
Surges within polymer extruders are recognized as a major problem faced by 
the extrusion industry. Surges are output variations from an extruder 
screw corresponding to variations in polymer pressure and changes in 
polymer flow rate. Accordingly, surges are nearly synonymous in the 
extrusion industry with pressure and flow variations. Put simply, surges 
are like waves wherein maximum output and pressure occur at the top of the 
wave and minimum output and pressure occur at the bottom of the wave. When 
a wave-like surge arrives at the discharge end of the extrusion screw, 
there will be a corresponding surge in discharge pressure and flow rate. 
Accordingly, an instantaneous pressure or flow rate surge will produce an 
instantaneous surge at the extrusion die. 
Pressure and flow variations at the extrusion die are know to result in 
dimensional variations in the extruded product. Such dimensional 
variations create severe problems, especially when it is desired or 
necessary to extrude tube, rod or other shapes having tight tolerances. 
Dimensional variations may result in the extrusion of large quantities of 
expensive materials into useless products. Moreover, pressure and flow 
changes at the extrusion die cause dimensional variations along the length 
of an extrusion. Inspection of one portion of the extruded product may 
result in different results from other portions, reducing predictability. 
Dimensional and other variations resulting from polymer surging ultimately 
results in material waste, product rejection, and other inefficiencies. 
Various attempts have been made to efficiently and effectively control 
variations in extruder output. For example, valve control of extruder 
output was considered in Patterson et al, The Dynamic Behaviour of 
Extruders, SPE ANTEC, pp. 483-487 (1978). Also, control of melt 
temperature and pressure by continuously varying screw speed coupled with 
infrequent variations in die resistance was considered in Parnaby et al, 
Development of Computer Control Strategies for Plastic Extruders, Polym. 
Eng. Sci., Volume 15, No. 8, pp. 594-605 (1975). 
Lee, in U.S. Pat. No. 4,118,163, also recognized difficulties in 
controlling the uniformity of pumping zone pressure and attempted to 
minimize pressure and flow surges at the discharge end of an extruder. Lee 
provided a complicated screw extruder apparatus having two separate 
pumping zones, a first zone for feeding plastic from a hopper and a second 
zone for pumping plastic back toward the first zone and out a lateral exit 
orifice. The first and second zones were connected by a central bore 
formed in the second zone of the screw which communicated with the first 
zone through radial passageways. The Lee extruder was an expensive device 
requiring a highly specialized extruder screw. 
These attempts failed to provide a practical and effective apparatus or 
method for reducing polymer surging. Accordingly, there is a great and 
thus far unsatisfied demand for a practical apparatus and method for 
reducing or eliminating surging of polymer in extrusion processes. 
OBJECTS OF THE INVENTION 
It is an object of this invention to provide an apparatus which overcomes 
the problems associated with conventional extruders. 
It is another object of the invention to provide an apparatus for reducing 
or eliminating surges of polymer during extrusion processes. 
It is a further object of the invention to provide an extruder having an 
integral surge suppressor. 
It is another object of the invention to provide a surge suppressing 
extruder having an inexpensive and reliable means for reducing or 
eliminating surges before pressurized polymer reaches the extrusion die. 
It is still another object of the invention to provide a method for 
reducing or eliminating surges known to occur in conventional extrusion 
processes. 
It is a further object of the invention to provide a method for reducing or 
eliminating polymer surging by providing a modified extrusion screw within 
an otherwise standard extruder. 
Other important objects of the invention will become apparent to one of 
skill in this art in view of the following descriptions, the appended 
figures and the claims. 
SUMMARY OF THE INVENTION 
This invention provides an extruder having a barrel and a shaft mounted for 
rotation within the barrel. The shaft has a conveying screw flight for 
conveying and melting polymer pellets introduced into the barrel through a 
hopper. A metering screw flight meters the melted polymer and delivers the 
polymer to an extrusion die. A surge suppressing screw flight on the shaft 
downstream of the die urges polymer toward the die while permitting a 
portion of the polymer to flow into or past the surge suppressing screw 
flight. A seal is optionally provided near the surge suppressing screw 
flight to prevent further polymer flow. 
The surge suppressor absorbs instantaneous pressure and flow increases. The 
surge suppressor also compensates for instantaneous pressure and flow 
drops. Accordingly, the surge suppressor dampens pressure and flow surges 
to maintain a substantially uniform pressure at the extrusion die. 
This invention also provides a method for reducing or eliminating polymer 
pressure and flow surges during extrusion processes. A shaft of an 
extruder is provided with a surge suppressing screw flight. The surge 
suppressing screw flight generates a pressure less than or equal to the 
extruder metering screw and a portion of the pressurized polymer flows 
into the surge suppressing screw flight. A seal is optionally provided 
downstream of the surge suppressing screw flight to prevent further 
polymer flowing.

DETAILED DESCRIPTION OF THE INVENTION 
The following description is intended to refer to the specific embodiments 
of the invention illustrated in the drawings. This description is not 
intended to define or limit the scope of the invention, which is defined 
separately in the claims that follow. Also, the drawings referred to 
throughout the following description are not to scale and are not intended 
to reflect actual dimensions or proportions. 
FIGS. 1a and 1b are provided to illustrate features found in conventional 
extrusion screws utilized in conventional extruders. The extrusion screws 
shown in FIGS. 1a and 1b both have an upstream portion to the right and a 
downstream portion to the left. 
Referring to FIG. 1a, an extrusion screw S is driven from a drive end DR 
located at the upstream end of extrusion screw S. Just downstream of drive 
end DR, hopper pellets HP are introduced into the extruder barrel (not 
shown) within which extrusion screw S is rotationally mounted. Hopper 
pellets HP are conveyed downstream and melted into molten polymer in 
conveying and melting zone CM. Melted polymer is then metered in metering 
zone M, sometimes referred to as a pumping zone, located downstream of 
conveying and melting zone CM. Melted polymer is then discharged to an 
extrusion die through an axial discharge D. 
Surges in pressurized melted polymer occur within metering zone M in the 
form of pressure surges and flow surges. Such surges are caused, as 
described above, by extrusion screw rotation speed variations, variations 
in polymer temperature, variations in polymer supply, and other commonly 
encountered parameter changes. Such surges commonly result in pressure and 
flow rate surges at the extrusion die. 
Referring to FIG. 1b, extrusion screw S is driven from a drive end DR at 
the downstream end of extrusion screw S. Hopper pellets HP are introduced 
into the barrel (not shown) and are conveyed and melted in conveying and 
melting zone CM at the upstream portion of extrusion screw S. Melted 
polymer is metered in metering zone M just upstream of a radial extrusion 
die discharge D. 
In order to prevent pressurized polymer from flowing downstream of 
extrusion die discharge D and into a transmission mechanism (not shown) at 
drive end DR, a dynamic seal DS is provided downstream of extrusion die 
discharge D. Seals similar to dynamic seal DS, also known in the industry 
as seal screws or viscous seals, are commonly used on gear pumps and on 
some drum extruders. Dynamic seals are also used in internal mixers and 
vertical single screw extruders to keep polymer melt away from critical 
parts of process machinery. 
Most extruders drive the extrusion screw from the end opposite the 
extrusion die. In other words, the extrusion screw begins at the 
transmission at an upstream portion and terminates in a point at the 
opposite, downstream end (see FIG. 1a). An example of such an extrusion 
screw was illustrated by Adderley, Jr., in U.S. Pat. No. 4,465,451. 
Other extruders drive the extrusion screw from its downstream portion and 
have an extrusion die discharge between the upstream and downstream ends 
of the screw. This type of extrusion screw S is shown in FIG. 1b. An 
example of such an extruder was also illustrated by Li et al., in U.S. 
Pat. No. 4,695,240. Dynamic seals have been used in such extruders to 
prevent melted polymer from entering and fouling transmission mechanisms 
attached to drive the screw. 
Referring to FIG. 1b, dynamic seal DS prevents polymer flow past extrusion 
die discharge D to drive end DR. Accordingly, dynamic seal DS is designed 
to maximize pressure so as to generate pressure greater than that 
developed in metering zone M. Because the pitch of the screw in dynamic 
seal DS is opposite that of metering zone M and the screw is designed to 
generate maximum pressure, dynamic seal DS pumps melted polymer back 
upstream to the extrusion die and seals against downstream polymer flow. 
A variety of extrusion screws, many of which included dynamic or viscous 
seals, were disclosed in the following patents: Geier et al., U.S. Pat. 
No. 3,023,455; Kasting et al., U.S. Pat. No. 3,632,256; Latinen, U.S. Pat. 
No. 3,797,550; Okada et al., U.S. Pat. No. 3,802,670; Shinmoto, U.S. Pat. 
No. 3,924,841; Markel et al., U.S. Pat. No. 4,689,187; Kolossow, U.S. Pat. 
No. 4,730,935; Shogenji et al., U.S. Pat. No. 4,766,676; Pena, U.S. Pat. 
No. 4,966,539; and Klein, U.S. Pat. No. 5,106,286. The extrusion screw 
shown in U.S. Pat. No. 3,924,841, incorporated herein by reference, has a 
reverse thread portion which serves to force back the molten resin toward 
the mixing zone to prevent polymer leakage past the extruder screw shank. 
FIG. 2 illustrates structural elements of conventional extrusion screws. 
Extrusion screw S has a flight F helically arranged at a helix angle H. 
Flight F is also known as a thread or screw. Flight F has a flight width 
FW. The space between adjacent flights defines a channel C between 
extrusion screw S and an extruder barrel B. Channel C has a channel depth 
CD (sometimes known as thread depth) and a channel width CW. Extrusion 
screw S has a shaft diameter D.sub.s and extrusion screw S is sized to fit 
within barrel B having a barrel diameter D.sub.b. 
Rotation of extrusion screw S shown in FIG. 2 conveys polymer (not shown) 
in channel C in a downstream direction. For example, rotating extrusion 
screw S in a counter-clockwise direction from the right-hand side of FIG. 
2 conveys polymer toward the left-hand side of FIG. 2. 
Extrusion screws can of course be provided with a wide variety of 
dimensions, configurations and shapes. Meyer, in U.S. Pat. No. 5,215,374, 
illustrated a variety of extrusion screw shapes. 
FIGS. 3a, 3b, 4a, 4b, 5 and 6 illustrate several embodiments of the 
extrusion surge suppressor according to this invention. This invention is 
not, however, limited to the embodiments illustrated in the figures, but 
instead is defined separately in the appended claims. 
Referring to FIG. 3a, an extrusion screw is provided with a metering zone M 
downstream from a conveying and melting zone (not shown) into which hopper 
pellets are introduced. Downstream of metering zone M is a radially 
extending extrusion die discharge D. Farther downstream from extrusion die 
discharge D is a surge suppressor SS for suppressing melted polymer 
pressure and flow rate surges. Surge suppressor SS in this embodiment 
generates a polymer pressure less than that generated in metering zone M, 
thereby allowing some melted polymer to flow downstream through surge 
suppressor SS. Surge suppressor SS is in the form of a screw flight having 
a direction opposite that in metering zone M. Accordingly, surge 
suppressor SS pumps a substantial portion of melted polymer back toward 
metering zone M and extrusion die discharge D. Details of a preferred 
surge suppressor SS are provided below. 
A dynamic seal DS is provided on extrusion screw S downstream from surge 
suppressor SS. Dynamic seal DS seals against downstream flow of the melted 
polymer that passes through surge suppressor SS. Dynamic seal DS is formed 
from a helical groove cut into extrusion screw S in a direction opposite 
to the screw flights in metering zone M. Dynamic seal DS generates a high 
polymer pressure greater than that generated in metering zone M. 
To generate high polymer pressure, dynamic seal DS is provided with a small 
helix angle H (FIG. 2), a shallow channel depth CD and/or a narrow channel 
width CW. Dynamic seal DS is preferably formed with a small axial length 
to permit a shorter extrusion screw S. Dynamic seal DS is provided with a 
helix angle H not exceeding about half that of metering zone M. Channel 
depth CD in dynamic seal DS does not exceed about half that of metering 
zone M. Also, the axial length of dynamic seal DS is less than or equal to 
about 25% that of metering zone M. Finally, channel width CW in dynamic 
seal DS does not exceed about 10% of the screw diameter. A dynamic seal DS 
so designed generates a pressure much greater than metering zone M and 
prevents flow of melted polymer to the drive end DR of extrusion screw S 
and into the screw drive mechanism (not shown) attached to drive end DR. 
Referring to FIG. 3b, an extrusion screw S is similar to that shown in FIG. 
3a except that extrusion screw S is driven from a drive end DR at the 
upstream end of extrusion screw S. Drive end DR is provided upstream from 
a conveying and melting zone CM into which hopper pellets (not shown) are 
introduced. Downstream from conveying and melting zone CM is a metering 
zone M for metering melted polymer and delivering the polymer to a 
radially extending extrusion die discharge D. Downstream from extrusion 
die discharge D is a surge suppressor SS similar to that described with 
reference to FIG. 3a. As in FIG. 3a, the surge suppressing screw 
embodiment shown in FIG. 3b has a dynamic seal DS located downstream from 
surge suppressor SS. Dynamic seal DS has a structure similar to that 
described with reference to FIG. 3a. Dynamic seal DS prevents downstream 
flow of melted polymer that flows through surge suppressor SS. 
Accordingly, dynamic seal DS prevents flow of pressurized melted polymer 
downstream into downstream portions of barrel B. 
FIGS. 3a and 3b both illustrate embodiments having a surge suppressor 
permitting downstream flow of some polymer with a dynamic seal which seals 
against farther downstream flow. FIG. 3a shows such an embodiment driven 
from a downstream end of the extrusion screw. FIG. 3b shows an embodiment 
wherein the extrusion screw is driven from its upstream end. 
Referring to FIG. 4a, another extrusion screw embodiment is provided with a 
metering zone M downstream from a conveying and melting zone (not shown) 
into which hopper pellets are introduced. Metering zone M meters melted 
polymer and delivers it to a radially extending extrusion die discharge D. 
Downstream from extrusion die discharge D is a surge suppressor SS similar 
to those shown in FIGS. 3a and 3b. Surge suppressor SS pumps a portion of 
the melted polymer back upstream toward metering zone M and extrusion die 
discharge D. Surge suppressor SS also permits downstream flow of a portion 
of melted polymer. 
The portion of melted polymer that flows downstream past surge suppressor 
SS exits barrel B through a radially extending polymer discharge port 10. 
Radial discharge port 10 may also take the form of a bleed hole for the 
escape of small amounts of melted polymer. Radial polymer discharge port 
10 optionally leads to a restriction valve (not shown) or any other known 
means of restricting melted polymer flow. Discharge of the portion of 
polymer that passes downstream through surge suppressor SS and out port 10 
prevents fouling of transmission mechanisms at a drive end DR at the 
downstream end of extrusion screw S. 
Referring to FIG. 4b, an extrusion screw S similar to that shown in FIG. 4a 
is shown, differing mainly in that extrusion screw S in FIG. 4b is driven 
from a drive end DR at the upstream end of extrusion screw S. Downstream 
from drive end DR is a conveying and melting zone CM into which hopper 
pellets are introduced. Melted polymer is metered in a metering zone M 
downstream from conveying and melting zone CM for delivery to a radially 
extending extrusion die discharge D. Downstream from extrusion die 
discharge D is a surge suppressor SS similar to that shown in FIG. 4a. 
The portion of melted polymer that flows downstream through and past surge 
suppressor SS exits barrel B through an axial polymer discharge 20. Axial 
polymer discharge port 20 optionally terminates at a restriction valve or 
other known restriction device. Port 20 may also be referred to as a bleed 
hole. Accordingly, the small portion of melted polymer that flows 
downstream from surge suppressor SS exits the extruder through port 20 
while the majority of melted polymer exits extrusion die discharge D 
upstream from surge suppressor SS. 
FIGS. 4a and 4b both show extrusion screw embodiments wherein a surge 
suppressor which permits downstream passage of melted polymer is provided 
in conjunction with a bleed hole and optional restriction device. FIG. 4a 
shows an embodiment having a surge suppressor combined with a radially 
extending bleed hole. FIG. 4b shows an embodiment having a surge 
suppressor combined with an axially extending bleed hole. 
Referring to FIG. 5, yet another embodiment of an extrusion surge 
suppressor according to this invention is illustrated. This embodiment 
provides an extrusion screw S having a metering zone M downstream from a 
conveying and melting zone (not shown) which supplies melted polymer. 
Metering zone M meters and delivers pressurized polymer to a radially 
extending extrusion die discharge D. Downstream from extrusion die 
discharge D is a surge suppressor SS. Surge suppressor SS pumps some 
melted polymer back upstream toward metering zone M and extrusion die 
discharge D while permitting a portion of melted polymer to flow farther 
downstream through surge suppressor SS. 
An O-ring 30 is captured within an O-ring groove 40 formed in barrel B. 
O-ring 30 prevents downstream flow of the melted polymer that passes 
through surge suppressor SS. O-ring 30 is preferably formed from any known 
elastomeric material. Whatever material is selected, however, O-ring 30 
should be capable of withstanding the elevated temperatures maintained 
during extrusion processes. 
O-ring 30 provides a circumferential seal against an outermost surface of 
O-ring groove 40 and a circumferential seal against the surface of 
extrusion screw S. These seals provided by O-ring 30 prevent passage of 
melted polymer to the drive end DR of extrusion screw S, thereby 
preventing fouling of any transmission mechanism attached to drive end DR. 
The extrusion surge suppressor embodiment shown in FIG. 5 has a surge 
suppressor in combination with an O-ring seal which seals-off the polymer 
that flows through the surge suppressor. It is of course contemplated 
(although not shown) that the screw in FIG. 5 could also be driven from a 
drive end located at the upstream end of the screw. It is also 
contemplated that any other known mechanical seal device can be 
substituted for O-ring 30 and O-ring groove 40. 
Referring to FIG. 6, an extrusion screw S is again provided with a metering 
zone M which receives melted polymer from a conveying and melting zone 
(not shown). Metering zone M meters and delivers pressurized and melted 
polymer to radially extending extrusion die discharge D. Downstream from 
extrusion die discharge D is provided a surge suppressor SS similar to 
that shown in FIG. 5. Surge suppressor SS pumps a portion of melted 
polymer back towards metering zone M and out extrusion die discharge D. 
Another portion of the melted polymer flows downstream through at least a 
portion of surge suppressor SS. 
A coolant reservoir 50 is provided within barrel B at a position which 
preferably overlaps with surge suppressor SS on extrusion screw S. Coolant 
is circulated through coolant reservoir 50 to cool the melted polymer in a 
portion of surge suppressor SS. As the melted polymer is cooled, it tends 
to prevent farther downstream flow. Accordingly, the portion of melted 
polymer which flows downstream through a portion of surge suppressor SS is 
sealed against flowing farther downstream, thereby preventing fouling of 
screw transmission mechanisms attached at drive end DR. 
The embodiment shown in FIG. 6 illustrates the combination of a surge 
suppressor with polymer cooling to reduce or eliminate polymer surging 
while preventing polymer leakage. It is of course contemplated that drive 
end DR could also be located at the upstream end of extrusion screw S. It 
is also contemplated that coolant reservoir can be substituted for any 
known cooling means, including but not limited to a coiled coolant flow 
passage or even convection cooling induced by air flow around or through 
the extruder barrel. Also, coolant reservoir 50 or any other known cooling 
means can be positioned to coincide with surge suppressor SS, can overlap 
with surge suppressor SS or can be positioned downstream of surge 
suppressor SS. 
Operation of the extrusion surge suppressor according to this invention 
will now be described with reference to FIGS. 2 and 3a. In essence, the 
surge suppressor portion of the extrusion screw provides a uniform output 
pressure at the extrusion die by absorbing surges in polymer pressure and 
flow rate. In conventional extruders, surges are known to occur when the 
extruder speed is increased or when other extrusions parameters such as 
temperature are varied. Such changes result in fluctuations in output. 
Accordingly, in conventional extruders, any change in extruder output is 
transmitted directly to the extruder die, thereby causing the severe 
disadvantages described above. 
The extrusion surge suppressor of this invention eliminates the peaks and 
valleys of wave-like surges to provide an output that is uniform. More 
specifically, the surge suppressor acts to store surging polymer 
associated with pressure or flow rate increases so that excess polymer 
does not travel directly from the metering zone to the extrusion die. 
Accordingly, the surge suppressor absorbs the surge while preventing 
transmission of the surge directly to the extrusion die. When, on the 
other hand, the surge represents a pressure drop or flow reduction, the 
surge suppressor gives up some of its stored polymer to the extrusion die 
discharge to even-out the die output. 
The surge suppressor is preferably formed with a length sufficient to allow 
molten polymer pressure generation approaching that of the metering zone. 
The surge suppressor's ability to generate pressure increases with length. 
As polymer enters the surge suppressor (pushed into the surge suppressor 
by pressure generated in the metering zone), the pressure in the surge 
suppressor approaches the pressure in the metering zone. Accordingly, a 
surge of molten polymer flows into the surge suppressor before it reaches 
the discharge die. 
It is believed that the function of the surge suppressor and method 
according to this invention is founded upon fundamentals of polymer flow. 
In a steady state the drag flow of polymer in the surge suppressor relates 
to the pressure flow of the polymer according to the following equations: 
##EQU1## 
wherein quantity (1) is polymer drag flow in the surge suppressor and 
quantity (2) is polymer pressure flow. CW and CD are defined in FIG. 2. P 
is the pressure developed in the screw metering zone, z is the helical 
length over which pressure P is developed in the surge suppressor, and 
v.sub.sb is the relative velocity between the extruder screw and extruder 
barrel. In steady state operation of the surge suppressor, drag flow and 
pressure flow are approximately equal: 
##EQU2## 
According to relationship (3), there is an increase in flow into the surge 
suppressor whenever an instantaneous pressure increase occurs at the surge 
suppressor entrance. This flow increase causes an increase in filled 
length z. If the pressure increase is maintained for sufficient time, a 
new equilibrium will be reached with the new filled length z until drag 
flow is again proportional to pressure flow. 
If molten polymer is presumed to be incompressible, an instantaneous 
pressure surge will be accompanied by an instantaneous flow rate increase. 
Such an instantaneous flow rate surge is absorbed by the surge suppressor 
of this invention. The initial flow surge into the surge suppressor is 
large and then gradually tapers. Accordingly, initial flow increase into 
the surge suppressor immediately reduces flow into the die, thereby 
reducing output variations at the extrusion die. In other words, a step 
change in pressure or flow rate in the metering zone will not produce a 
step change in the discharge pressure or flow rate at the extrusion die 
when a surge suppressor according to this invention is used. 
If the duration of the surge is very short (less than five seconds, for 
example), flow into the surge suppressor and pressure in the extrusion die 
will still be building before the surge ends. Accordingly, pressure and 
flow will start to reduce even before a new steady state is reached and 
the amplitude of the pressure and flow surge is dramatically reduced or 
eliminated. 
It has been discovered that it is easiest to suppress surges when the time 
required for flow into the surge suppressor to reach equilibrium 
(.DELTA.t.sub.eq) is longer than the time duration of the surge 
(.DELTA.t.sub.surge). For example, surges having a duration as long as an 
hour can be limited by pressure feedback control. Accordingly, it is most 
preferable to design the surge suppressor according to this invention for 
reduction of short-term surges lasting only a few seconds. 
Preferred Embodiment 
Surge suppressors according to this invention are preferably designed to 
maximize polymer volumetric capacity so as to absorb larger pressure and 
volumetric surges. This is preferably accomplished by adjusting the 
flights in the surge suppressor by optimizing channel width CW, channel 
depth CD, and helix angle H (FIG. 2). 
Surge suppressors according to this invention generate less pressure than 
dynamic seals, such as the one described above with reference to FIG. 3a, 
which are intended to seal against polymer flow. As compared to dynamic 
seals, the surge suppressor of this invention will have a greater helix 
angle H (FIG. 2), a greater channel depth CD and/or a wider channel width 
CW. 
Wider channels provide increased polymer storage capacity during a surge. 
Increased polymer storage capacity also allows the surge suppressor to 
pump back into the extrusion die a greater volume of polymer during a 
pressure or flow rate drop. Another advantage of wider surge suppressor 
channels is that the overall axial length of the surge suppressor can be 
made smaller (because fewer channels are required) and allows the 
manufacture of a small extruder. Accordingly, the channel width of the 
preferred surge suppressor embodiment is greater than about 10% of the 
screw diameter. Referring to FIG. 2, channel width CW in the surge 
suppressor is preferably greater than about 10% of barrel diameter 
D.sub.b. 
The most preferable surge suppressor according to his invention generates 
lower pressures than that generated in the metering zone. Lower pressure 
generation maintains continuous flow of molten polymer downstream through 
the surge suppressor. Such continuous flow replenishes molten polymer in 
the surge suppressor, thereby preventing polymer degradation and burning. 
As described above, the flow of molten polymer downstream from the surge 
suppressor can be stopped with any sealing method or simply allowed to 
flow from the extruder barrel. To achieve these benefits, the preferred 
surge suppressor embodiment generates a pressure less than the metering 
zone and, most preferably, will only be capable of generating a pressure 
up to approximately 95% of the pressure generated in the metering zone. 
A surge suppressor according to this invention is preferably formed with a 
helix angle H (FIG. 2) larger than that of the dynamic seal in FIG. 3a, or 
greater than about half that of the metering zone screw flight. Helix 
angle H of the surge suppressor is most preferably slightly larger than 
the helix angle H in the metering zone. Where the other surge suppressor 
dimensions coincide with those in the metering zone, the preferred surge 
suppressor embodiment has a helix angle H about 10% larger than the 
metering zone. Such a helix angle has been discovered to generate a 
maximum pressure of about 90% the pressure generated in the metering 
section if the other dimensions are the same. 
A surge suppressor according to this invention preferably has a channel 
depth CD larger than that of the dynamic seal in FIG. 3a, or greater than 
about half that of the metering zone. If helix angle H were the same as in 
metering zone M, the most preferred surge suppressor would have a channel 
depth CD about twice that in the metering zone. Such a channel depth CD 
has been discovered to generate about 25% the pressure generated in the 
metering zone. 
The axial length of the surge suppressor according to this invention is 
preferably greater than that of the dynamic seal of FIG. 3a, or greater 
than about 25% that of the metering zone. However, axial length of the 
surge suppressor is also preferably less than that of the metering zone. 
If channel depth CD and helix angle H were both the same in the surge 
suppressor as in the metering zone, reducing the surge suppressor length 
to about 95% of the metering zone length would generate about 95% of the 
metering zone pressure. 
Of course, any combination of channel depth CD, channel width CW, surge 
suppressor length and helix angle H can be used so long as the surge 
suppressor retains its surge suppressing function. However, it is most 
preferable that the surge suppressor is not designed to generate more than 
about 95% of the metering zone pressure. 
In any embodiment, the surge suppressor according to this invention 
provides significant benefits. The surge suppressor dramatically reduces 
or eliminates pressure and volumetric surges commonly known to occur in 
conventional extruders. The surge suppressor prevents these surges from 
transferring directly to the extrusion die, thereby reducing or 
eliminating variations in extruder product dimensions and quality. Also, 
the surge suppressor according to this invention is practical and 
inexpensive, merely requiring modification of the extrusion screw without 
complicated and expensive control systems such as those considered in the 
past. 
These surprising and significant benefits are conferred without 
disadvantage. The continuous flow of molten polymer through the surge 
suppressor prevents degradation or burning of the polymer. Also, the surge 
suppressor can be provided without requiring a significant increase in 
extruder length. Accordingly, the surge suppressor and method according to 
this invention provides a simple, effective and practical solution to the 
longstanding problem of surging in extruders. 
Many modifications to the surge suppressor embodiments described herein can 
be made without departing from the spirit and scope of this invention. For 
example, instead of forming helical grooves in the extrusion screw to 
produce the surge suppressor, a surge suppressing result can be produced 
by creating helical grooves in the cylindrical housing or barrel. Also, 
the surge suppressor is optionally a separately driven component. It is 
the relative motion between the shaft and the barrel that creates the 
important pumping effect of the surge suppressor. 
Also, more than one bleed hole can be provided for the embodiments shown in 
FIGS. 4a and 4b and any known conventional seal or restriction device can 
be used in conjunction with the surge suppressor in any embodiment. Of 
course, multiple seals can be used in combination if desirable or 
necessary. For example, a dynamic seal can be used in conjunction with 
polymer cooling. 
It is also contemplated that bleed holes 10 and 20 (FIGS. 4a and 4b, 
respectively) may also communicate with a flexible membrane covering the 
hole or a piston inserted into the hole as opposed to other forms of 
restriction device. Such a flexible membrane or piston could provide some 
additional surge reduction and/or sealing capability. 
The screw dimensions and configuration can be varied in any way in any 
combination in the surge suppressor so long as the surge suppressing 
function is maintained. It is also contemplated that the channel width CW, 
channel depth CD and helix angle H may vary over the surge suppressor's 
length. Although the shaft diameter Ds and is preferably constant 
throughout the surge suppressor, shaft diameter Ds (FIG. 2) may optionally 
be tapered. 
Many other modifications will be apparent to those of skill in the 
extrusion art. Such modifications are within the scope of this invention, 
which is defined in the following claims.