Improvements in cold feed rubber extruders.

This invention relates to cold feed rubber extruders of the kind comprising a barrel in which a screw is rotatably mounted and driven to coact with the barrel, and having an inlet for the material to be extruded, a compression section, a plasticizing section and an exit section leading to an extrusion head.

Such extruders of the kind set forth have been replacing warm feed rubber extruders throughout the Industry, though for certain compounds and applications the latter still persist. With advances in design, mainly in the plasticizing section, the range has been extended to include more rubber compounds which had been found difficult to extrude on account of high viscosity (Mooney values), nerve (natural rubber compounds), hardness, high filler content or special sensitivity to heat build-up.

An improved plasticizing section is known as the Transfermix, and is shown in US-A- 2 744 287, (Reissue No. 26 147), GB-A-842 692, US-A-4 136 969 and US-A-4 184 772. It has the generic feature that in a transfer zone formed over a common length of screw and barrel, in the screw a helical groove varies in cross-section from full area to zero area and in the barrel the cross-section of an opposite handed helical groove varies from zero area to full area, whereby in the transfer zone the compound is transferred from the screw into the barrel, while being mixed and plasticized. The plasticizing section may comprise only one such transfer zone or more, where a second one is conversely adapted by way of changes in cross-sections of helices to effect a return-transfer from barrel to screw.

For a given cross-sectional area between the bottom of the screw helix and that of the barrel helix, such a Transfermix geometry provides a uniform action of flow division and rearrangement, treating on one transfer each subdivision just once, and none repeatedly, unlike other plasticizing systems, which operate non-uniformly to different degrees and are therefore less effective.

Mathematics can be developed to show that for the earlier (1956) generation of Transfermix having invariant numbers of helical grooves in each transfer zone, this number is limited by the condition of maintaining continuity of transport by shear stress transfer in the rubber. Expressed simply, this means avoiding grooves which get too deep and narrow so that there can be no more forward transport in the bottoms of the grooves due to the relative rotation of the screw and barrel and the grooves are no longer self-cleaning.

This limitation is, to a large measure, overcome in the second (1979) generation of Transfermix, where the number of helical grooves varies inversely as between screw and barrel within the same transfer zone, whereby the width/depth ratio of individual helical channels can be kept such that transport flow - and thereby the important practical feature of self-cleaning - is maintained while effecting a much more intense flow-division and rearrangement, which in equal intensity can be realised for any diameter of Transfermix. Hence the designation "Multi Cut Transfermix" and the considerable widening of its plasticizing action into the range of difficult-to-extrude compounds, as well as its solution of the scale-up problem, enabling an equal plasticizing action to be designed into any diameter of a range of extruders similar in outline dimensions, such as L/D ratio.

Taking as an example a 90 mm Transfermix; in the first generation the maximum number of helical grooves in both screw and barrel for an output typical for this size would be 4-6 in the screw and 4-6 in the barrel, making 16 to 36 subdivisions, whereas the Multi Cut Transfermix would bring about 40 to 80 subdivisions.

This concentrated action, compared to other types of plasticizing section, has for both generations led to transfer zones of a length of 1 to 5 screw diameters, which has allowed "easy to extrude" compounds to be plasticized homogeneously at relatively low temperatures with throughput restricted by the die-resistance only, while for more "difficult" compounds various degrees of throttling in addition to the die-resistance bring uniformity into the plasticization, thus greatly extending the range of compounds.

As a Transfermix plasticizing section, on account of having opposite handed helices in screw and barrel, exerts a powerful pumping effect in both screw and barrel especially on partially plasticized and hence tough media, giving rise to a multiple of the normal throughput, a high degree of throttling has been found necessary, up to restricting 95% of the flow-area.

A useful device for doing this has been a set of radially arranged conically shaped pins, set in a cut in the screw following a Transfermix section, and which when moved radially inwards to almost touch the bottom of the cut-out in the screw, reduce the projected area of flow down to even less than 5% and when moved radially outwards, leave this projected area fully open.

However, it was found that when such a throttle was partially closed to different degrees, it would produce different actions. At first it would increase the forward transport by stopping the rotation of an outer ring of the material which on being gripped by the rotating screw would set up a local pressure- increase. Only when inserted more deeply would it start to throttle, but accompanied by a kneading action introducing non-uniformities over the depth of the screw.

It is an object of this invention to provide a throttling means for insertion into an exit section of a screw which exerts a flow-interrupting and kneading effect over the whole depth of the helical flow-channel.

According to the present invention, in an extruder of the kind set forth, the plasticizing section embodies a Transfermix geometry in which a helical groove in the screw varies in cross-section from full area to zero area and in a substantially corresponding length of the barrel the cross-section of an opposite-handed helical groove varies from zero area to full area, whereby in operation the material is transferred from the screw to the barrel while being mixed and plasticized, and in the exit section the screw has a circumferential cut whose radial depth is substantially the same as the radial depth of the helical screw groove, and the barrel at the position of the cut is provided with a set of flow interruptors, spaced around its circumference, and extending radially and substantially to the bottom of the cut, the flow interruptors being fixed in the barrel and fitting loosely into the cut in the screw, and throttling members are provided for co-operation with the flow interruptors the throttling members being mounted for radial movement by positioning means, for minimum throttling when positioned radially out and maximum throttling when positioned radially in.

The throttling members may be provided around, and closely fit, the flow interruptors so as to exert a cleaning action on the flow interruptors when being moved inwards, especially as part of a cleaning cycle at the end of operations. Furthermore, the flow interruptors may be round pins and the throttling members may be round pins of larger diameter with conical radially innermost ends so that when moved radially in these radial ends substantially close the flow-cross-section of the screw. Alternatively, the throttling members may be pins of rectangular cross-section, slanting inwards at their radially innermost ends. The flow interruptors may be vanes for redirecting the flow of material.

The invention will now be described in some detail with reference to the accompanying drawings in which: <ul id="ul0001" list-style="none"><li><u>Figure 1</u> is a sectional elevation of an extruder embodying a throttling means following a Transfermix plasticizing section;</li><li><u>Figure 2</u> is an enlarged view of the throttling means of Figure 1;</li><li><u>Figure 3</u> is a part-sectional side-elevation of the throttling means shown in Figure 2; and</li><li><u>Figure 4</u> is a section of a developed cross-sectional view of a throttle showing an alternative sectional shape of a throttling member and an alternative shape of a flow interruptor.</li></ul>

The extruder shown in Figure 1 comprises an extruder screw 1 (shown unsectioned) rotatably mounted in a sectioned housing barrel 2, and has a feed inlet 3 for material to be extruded, a compression section 4, a plasticizing section 5 and an exit section 6. The housing has passages 7 and 8 respectively for a heating/cooling fluid. The barrel components adjacent to the screw 1 are formed by sleeves. At the exit section 6 there is a plain cylindrical sleeve 9, while there is a Transfermix sleeve 10 in the plasticizing/mixing section 5 for co-operating with a corresponding Transfermix section 11 of the screw, as shown in US-A-4 136 969 and/or US-A- 4 184 772. In the compression section 4 there is a sleeve 12 and under the inlet opening 3 there is a sleeve 13. A feed-pocket 14 in the sleeve 13 extends under the inlet opening and runs into a spiral undercut 15 in sleeve 12 where the screw enters the barrel and carries on with its continuously reducing width for 360° round the screw, to end just before the end of the compression section, - as shown in US-A-4 462 692.

At least over the length of the inlet opening 3, but as shown, also extending over a part of the length of the compression section 4, the extruder screw flights are provided with notches 16 at an angle considerably greater than the lead-angle of the screw, - preferably at a lead-angle of 45°, as against a normal flight lead-angle between 20° and 25°. These notches may start as depths of about half the depth of the flight, and then, especially in the compression section, reduce gradually in depth to finally run out, which makes a reduction in volume along the length of the screw parallel to that provided by the spiral undercut in the compression section. In practice for example a length of notch of 20-30 mm measuring along the length of the screw-flight and spaced at the same distance, has been found effective. The main function of the notches is to provide edges to grip the fresh feed so that this is pulled into the screw strictly in ratio to the speed of rotation.

However, the notches, in co-operation with the undercut, also provide a pre-plasticizing action for tough or hard compounds, thereby providing a contiguous mass to form the squash-back to fill the spaces between pieces of unplasticized feed.

Immediately following the Transfermix plasticizing section 5, in the exit section 6 of the extruder, the screw 1 has a circumferential cut 17 extending substantially to the bottom of the groove, and a set of flow interruptors in the form of eight pins 18 also located in the cut 17. The pins 18 are fixed relative to the barrel, and extend radially and substantially to the bottom of the screw groove, in which they are a loose fit. Each pin 18 is located in a bore in a throttling member comprising a throttle pin 19 mounted for radial movement by positioning means. Each throttle pin 19 has a conical radially innermost end, shaped to substantially close the annular space of cut 17 when these throttle pins are moved radially inwards. This is shown above the horizontal centre-line; below the centre-line the throttle pin is shown moved out, as may be more clearly seen in Figure 2.

Figure 3 is an outside view of the end-flange of the exit section 6, partly sectioned to show the throttle pins 19 in outside view above the centre-line, and partly sectioned also through one of the throttle-pins below the centre-line where this is withdrawn.

The positioning means for moving the throttle-pins in and out comprises two annular plates 20 rotatably mounted on the barrel, with curved slots 21 engaged by round knobs 22 on each of the throttle pins. The annular plates 20 are held together by an outer ring 23 which, like each of the plates, is made in two halves so that the whole device can be assembled and bolted together from the outside. Means for rotating the annular plates 20 through one eighth of a circle for moving the throttle pins in and out and for arresting these at any position in between are indicated diagrammatically at 24.

In operation, the extruder is run with the throttle open which covers a range of normally extrudable compounds. For more difficult compound, insufficiently worked extrudate with cooler lumps will be transported at a higher output. Closing the throttle will make the extrudate smooth and uniform to the required degree. For tough, nervy and highly-loaded compounds which cannot be plasticized on other extruders, this may have to shut off up to 95% of the flow-area while still providing good throughputs for the size of extruder.

The advantage provided by the flow interruptor pins 17 over a throttle with radially movable pins only is that of uniform action over the depth of the screw-flight whether the throttle pins 19 are in or out, as well as an additional pressure-generation with positive effect on the output.

In a recent test on a 95 mm Transfermix this has amounted to over 10% on a nervy compound requiring the throttle pins to be about 85% in, at the highest screw-speed previously possible, with a reduced extrusion temperature. Running at a higher speed to come up to the permitted extrusion temperature provided another 12% of output, making an improvement of 20% at if anything better quality extrudate due to this invention.

While the Transfermix plasticizing section is self-cleaning for any but smeary compounds which would not clean on any single-screw extruder, this throttle with flow interruptor pins according to this invention can also be made self-cleaning at the end of a run. After the throttle pins 19 have been opened to allow the Transfermix to self-clean, they can then be shut gradually to expel compound caught between the throttle pins and to scrape compound off the interruptor pins. As is known from pin-barrel practice, already plasticized compound tends to stick on the trailing side of the pins especially in pin-planes near the screw-end. On the throttle being reopened gradually, the compound thus released gets caught by the screw and moved out in small rolled lumps.

Figure 4 shows an alternative throttle pin of rectangular shape 25, with the flow interruptor being shaped as a curved vane 26, adapted to deflect the flow of material so as to maximize the increase of pressure on the compound being gripped again by the rotating screw - analogous to a set of stator-vanes in an axial flow pump. The rectangular cross-section of the throttle pins provides a more intense throttling action by providing flow-channels of a length almost equal to width of the cut 17 in the screw.