Filter screen pack for extrusion die

The filter intercepts dirt emanating from a molten-polymer extruder. The filter is clamped between perforated support plates, and a rotor sweeps the holes in the upstream plate. The rotor defines a cavity which communicates with a small sector of the holes at any one time. The cavity is openable to drain, via a valve. The melt is under high pressure, to which the cavity is exposed. The rotor is rotated by a drive-shaft, and the drain is located co-axially with the drive-shaft.

BACKGROUND TO THE INVENTION 
Screen Changers are known, in which a screen is placed in the conduit 
conveying the flow of melt to the die. In a conventional screen changer, 
two separate screens are provided, together with a changeover system, 
which may be manually or otherwise operated. 
When the extruder is shut down temporarily, for example when cleaning the 
die nozzle or to change the colour of the material or to a different melt 
material, the opportunity can be taken to snap the screens over, whereby 
the clean screen is interposed in the conduit, in place of the dirty 
screen. Now, the dirty screen can be taken out, cleaned, and replaced, 
ready for re-use. 
In some cases, it is desired to change the screen at a time when the 
extruder is not otherwise shut down. Designs have been made in which it is 
possible to change the screens while melt continues to flow. One major 
problem with change the screen while the melt flow continues is that the 
change affects the flow. Even if the change is done very quickly, the 
pulse caused by the change can affect the resulting extruded product. 
Besides, the momentary disruption is not the only problem. Even if the 
short pulse disruption can be minimized, the pressure/flow characteristics 
of the new screen are never quite the same as those of the old (dirty) 
screen. As a result, the final product emerging from the die nozzle might 
not, after the change-over, have the same film thickness, through-flow 
rate etc. 
Also, on the practical side, the type of screen system that permits 
change-over of the screens has been notoriously difficult to seal, and the 
problem of leakage of melt from the screen changer housing can be 
tiresome. 
It is recognized that when the die is producing short batch runs, the 
screen can be changed between batches, and the screen can remain clean for 
the length of the batch. But for long or continuous production, the need 
to change the screen during operation arises; the quick change screen 
system is not very satisfactory, but is in widespread use for long runs 
because of the lack of an alternative. 
U.S. Pat. No. 4,332,541 (Anders, 1982) shows an example of a prior art 
screen pack, in which a reverse-pressure back-flushing phase of operation 
is automatically incorporated into the extrusion process, and this is used 
to clean out debris that may have accumulated in the screen. The idea is 
that, as a result of the periodic back-flushing, the screen can be kept 
cleaner, longer.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
This invention can be described in detail via explanation of apparatuses 
and exemplary embodiments as shown in accompanying drawings. It should be 
noted that the scope of the invention is defined by the accompanying 
claims, and not necessarily by specific features of exemplary embodiments. 
FIG. 1 shows a self-cleaning screen unit 20, which is suitable for 
installation between the extruder screw and the extrusion die. 
The unit 20 includes a static housing 23, which has a port 25 for receiving 
the hot melt from the screw. The filtered and screened melt, having passed 
through the unit, leaves via an exit port 27, whence it enters the 
extrusion die. 
A screen or filter pack 29 comprises several layers of filter screen mesh. 
The pack 29 lies sandwiched between the upper 30 and lower 32 screen 
supports. In line with the terminology used with conventional screen 
changers, the upper screen support 30 may be termed the breaker plate, and 
the lower screen support 32 may be termed the facing plate. 
A rotor 34 includes a shaft 36, which is mounted and guided for rotation 
within the housing component 23a. At the bottom end of the shaft is a 
spider 38, having three radial arms, one 40 of which is shown in FIG. 1. 
When the shaft 36 rotates, the spider 38 sweeps the chamber 43 created 
between the lower screen support 32 and the base plate 23b of the housing 
23. 
In use of the unit 20, hot liquid melt from the screw enters, under high 
pressure, via port 25. The melt enters and fills the chamber 43. From the 
chamber 43, the melt passes through the filter mesh pack 29, and passes 
into discharge chamber 47. From there, the melt exits to the die via port 
27. 
The filters in the filter pack 29 gradually become clogged with debris that 
is present in the melt emanating from the screw. The filters are cleaned 
by a pressure-reverse, back-flush procedure, as will now be described. 
The radial arm 40 actually comprises two arms 40a, side by side, with a 
cavity 49 between. FIG. 3 shows a comparable shape of arm 85, in another 
self-cleaning screen unit. As the spider 38 rotates, some of the holes 52 
in the lower screen support 32 lie open to the cavity 49 between the arms 
40a, the rest of the holes 52, i.e. the holes in the screen support that 
do not lie over the cavity 49 being open, of course, to the chamber 43. 
Melt therefore enters and fills the. cavity 49, and this charge of melt is 
carried around with the radial arm 40 as the spider rotates. 
The cavity 49 is normally closed, but the cavity can be opened to exhaust 
by opening the exhaust valve 54. 
To open the valve 54, valve stem 56 is activated upwards. The activation is 
done by means of a pneumatic ram, or other suitable mechanism 58. The stem 
56 is normally (i.e. when not activated upwards), urged downwards by means 
of a spring within the mechanism 58, whereby the valve head 60 is urged 
normally into sealing contact with the valve seat. 
When the valve 54 is opened, the cavity 49 is connected to drain 65, and 
the pressurized melt in the cavity spurts out of the drain 65. The sudden 
drop in pressure within the cavity causes a surge of melt from the 
discharge chamber 47 to pass through whichever of the wholes 52 happen to 
coincide with the cavity at the time when the valve is opened. It is this 
surge of reverse pressure that back-washes the dirt out of the filter 
pack; that is to say, the surge back-washes the dirt out of that portion 
of the filter pack that coincides with, the cavity. 
The valve 54 should not be kept open for long time, because when the valve 
is open the melt has an easy escape path to the drain, with the result 
that the melt flow to the die might be attenuated, or perhaps might cease 
altogether. Therefore, the valve should only be opened for short pulse 
time, e.g. for a few milliseconds. The shorter the pulse, the less likely 
it is that there will be any perceptible change in the flow of melt 
through the nozzle of the extrusion die. 
It is important that the flow through the die nozzle not be affected, 
because a sudden drop in melt pressure at the die nozzle will inevitably 
have an effect on the quality of the extruded product. 
It is recognized that the pulse should be of short enough duration that no 
significant pressure or flow effect is felt at the die nozzle, and yet the 
pulse should be long enough duration to create enough of a back-flush 
effect to make sure the dirt trapped in the filter pack is vigorously 
dislodged and cleared. 
It is recognized that in the apparatus as described an optimized pulse 
length can be achieved that is neither too long nor too short from these 
standpoints. 
One of the reasons the pulse can be so effective to clear the dirt, and yet 
not affect the flow through the die nozzle, is that the volume of the 
cavity 49 is much smaller than the volume of the bulk of the pressurized 
melt. The volume of the cavity 49 is a few cos, whereas the total volume 
of the pressurized melt, in the unit 20 and in the die, is of the order of 
a liter or more. 
Because the valve 54 is so close to the cavity 49, and the volume of the 
cavity is so small, the instant the valve 54 opens, the pressure in the 
cavity 49 drops very rapidly. However, there is a much larger volume of 
melt at the high pressure i.e. the melt emerging from the screw, the melt 
in the chambers 43, 47, the melt in the conduits leading to the die, and 
melt in the die itself etc. This large quantity of high pressure melt 
serves as a reservoir, to dampen out pressure pulses or surges. The 
reservoir effect is not large, of course, but it is recognized that, in 
the unit, the reservoir of high pressure melt in the system is so much 
greater than the volume of the cavity 49 that the pulse can be short 
enough to cause very little, or no, pressure drop in the melt in the die, 
and yet that same pulse is long enough to allow the pressure in the cavity 
to drop to zero. 
The important effect of the pulse is to create a vigorous back-flow through 
some of the holes in the screen supports. The back-flow only lasts for a 
moment, but it is recognized that is sufficient to dislodge the dirt from 
the filter pack. 
The valve 54 should be large. If the valve seat were small (for example, 
less than about 6 mm diameter), flow through the valve might be 
over-restricted, and prevent the desired rapid drop in pressure in the 
cavity 49 when the valve is opened. On the other hand, if the seat were 
too large (e.g. more than about 12 mm diameter), the pressure might drop 
so rapidly in the cavity that the pressure in the bulk of the melt would 
be affected. Such an effect could be alleviated by making the pulse 
duration even shorter (i.e. by moving the valve head very quickly), but 
then mechanical problems might arise. 
The pulse length may be set such that the quantity of melt discharged 
through the valve during one pulse is less than the volume of the cavity 
49. In that case, if the cavity were, say, 6 cc in volume, and the volume 
discharged per pulse were, say, 2 ccs, not all the melt in the cavity 
would be discharged each pulse, whereby the melt in the cavity would work 
its way progressively to the drain. 
Although the pulses should be of short duration, the pulses may, on the 
other hand, be frequent. If the melt is especially dirty, the frequency 
may be stepped up. Generally, the designer would arrange that the control 
of the pulse-duration, pulse-frequency, speed of rotation of the spider, 
etc., would be under the automatic control of pressure-differential 
sensors which detect when the filters are becoming clogged. 
The rotor 34 is caused to rotate (slowly) by means of a suitable rotary 
drive, which engages the gear wheel 67 attached to the shaft 36. 
As the rotor 34 rotates, the cavity 49 comes under the different ones of 
the holes 52, and the designer should arrange the pattern of holes in 
relation to the profiles of the cavity, such that all the holes 52 are 
exposed from time to time. This can be done by random, or programmed, 
operation, as required. 
An option that may be preferred, when they are, say, three radial arms to 
the spider, is to assign the respective arms each to sweep only a 
particular annulus. Thus, the cavity in the first arm only covers the 
radially-outermost holes in the screen support, the cavity in the second 
arm covers only the holes that lie within the middle diameters of the 
screen support, and the cavity in the third arm covers the holes in the 
innermost diameters. 
FIGS. 2 and 3 show another self-cleaning screen unit. Melt from the 
extruder screw enters via port 25, into the inlet chamber 69. Some of the 
melt then flows upwards, through the upper screen, into the upper 
discharge chamber 70, and the rest downwards, through the lower screen, 
into the lower discharge chamber 72. 
The melt passes through holes 52 in the respective inner and outer screen 
support plates 74, 76, and through the respective mesh screen filter packs 
78 sandwiched between the pairs of support plates. 
The melt flowing through the two chambers 70, 72 then comes together in 
conduit 79, and from there passes to the extrusion die. An operable 
selection valve 80 enables the flow from one or other of the chambers 70, 
72 to be cut off from entering the conduit 79. 
The spider 83 includes a radial arm 85, which is hollow inside, thereby 
defining a cavity 87. The radial arm is arranged to be a tight sliding fit 
between the inner screen support plates 74, whereby the cavity is sealed 
between the plates. The cavity is open to the holes 52 in the plates (FIG. 
3), under which the cavity lies, but the cavity is closed to the incoming 
melt present in the inlet chamber, and is closed to all the other holes 52 
in the plates. 
The passage 89 communicates the cavity 87 with a valve chamber 90. A valve 
92 is pressed against a seat, and closes the cavity 87 from the drain 94. 
The valve 92 is operated by means of a plunger 96, to which is coupled a 
suitable pneumatic ram or other actuator. 
The spider 83 is mounted on a shaft 98 for rotation, in the manner as 
described with reference to FIG. 1. The shaft is driven into rotation by 
means of a motor and gearbox, which may be mounted on top of the 
apparatus. 
The spider of the rotor is very tightly confined between the screen 
supports, in order to minimize leakage. The shaft of the rotor also is 
very tightly confined in the housing. 
In order to prevent binding, therefore, preferably the designer should see 
to it that the shaft is coupled to the spider by means of a rotary 
motion-transmitting connection with the spider, and not by being coupled 
rigidly to the spider. 
In the various designs, the rotor may be set to rotate continuously, or may 
be set to rotate only when the filters need cleaning. Continuous motion of 
the rotor is preferable, to make sure pockets of stagnant melt do not 
develop. 
In the alternative apparatus shown in FIG. 4, separate exhaust valves 100, 
one for each radial arm 102 are provided. In this case, the exhaust valves 
may be timed differently. The exhaust valve 100 corresponds to the arm 102 
whose cavity 104 sweeps the holes in the middle diameters of the screen 
support. Similarly, another of the exhaust valves corresponds to the arm 
whose cavity sweeps the holes in the innermost diameters of the screen 
support. Another exhaust valve handles the cavity that sweeps the 
outermost holes. The "middle" exhaust valve, and even more so the "outer" 
exhaust valve, may be programmed to open more frequently than the "inner" 
exhaust valve, to accommodate the greater number of holes at the larger 
diameters. 
In the embodiments as shown, one design aim is that any leakage that might 
occur past the screen-to-support interface goes into the melt-flow, not to 
the drain, and certainly not to the outside of the housing in which the 
screen is contained. Leakage past the filters is not important, since any 
gaps through which the melt may leak are themselves small enough to act as 
filters, the important aspect is that there are no gaps through which melt 
could leak to the outside, 
In the embodiments as shown, the spider is confined between the support 
plates. The spider should be made a shade thinner than the distance 
between the support plates, in order that the spider can rotate without 
binding. Accurate, precision machining of the components is therefore 
required; however, as shown, the components of the apparatus which have to 
be precisely sized, accurately flat, and parallel-faced, are all shaped so 
as to be readily manufacturable in that mode. 
In a case where it is desired not to rely on accurate-to-size machining, it 
may be preferred to spring-load the support plate or plates onto the 
spider. In FIG. 1, for example, it will be apparent that springs could 
readily be inserted between the upper plate of the housing and the screen 
support plate, so as to compress the spider between the screen support 
plate and the base plate. 
The torque needed to turn the spider depends on the friction between the 
spider and the facing plates. The more tightly the spider is squeezed 
between the facing plates, the more torque is required. In fact, to 
prevent the melt from by-passing the filters, the spider must be confined 
quite tightly between the facing plates. As the spider rubs heavily 
against the facing plates, some wear can take place, the debris from which 
passes into the melt stream. Therefore, the designer should arrange that 
the rubbed surfaces are upstream of the filters, as shown in the 
embodiments. 
The double-pack arrangement as depicted in FIGS. 2, 3 is more efficient in 
this sense than the single-pack arrangement of FIG. 1. Given that there 
are two rubbing surfaces per spider (i.e. top surface and bottom surface), 
the apparatus can be made more efficient, in terms of size, and in terms 
of torque required to turn the spider, for a given through-flow, if filter 
packs are placed both sides of the spider. 
The apparatuses as described are good for long runs, where colour changes 
of the extruded plastic material are not frequent. Although the units are 
not suited for frequent changes of extruded material, in fact colour 
changes etc., can be accommodated to some extent, in that, for 
change-over, the drain can be kept open a little longer than usual, to 
back-flush out the debris, and to flush out the last remnants of the 
previous colour. Besides, by the use of the invention, the filter may be 
so clean that no extra flushing is required.