GB1595965A

GB1595965A - Multiple screw extruders

Info

Publication number: GB1595965A
Application number: GB18518/77A
Authority: GB
Original assignee: Matthews & Co Worcester Ltd K
Current assignee: Matthews & Co Worcester Ltd K
Priority date: 1978-05-03
Filing date: 1978-05-03
Publication date: 1981-08-19

Description

(54) MULTIPLE SCREW EXTRUDERS (71) We, K. R. MATTHEWS & COMPANY (WORCESTER) LIMITED, a British Company, of Moseley Road, Hallow, near Worcester, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to the general field of screw-type extrusion machines, herein termed "screw extruders", as used in the plastics and rubber industry.

Such screw extruders can be classified as being either "single screw extruders" or "multiple screw extruders".

Single screw extruders are characterised by the provision of a single feed screw, having at least one generally helical feed channel. which is rotatably driven to operate within a barrel terminating in an extrusion die head.

Since they are capable of handling practically all extrudable materials covering a wide range of physical properties, they are extensively used in the plastics and rubber industries but their efficiency does vary for different materials and can be relatively low for certain "difficult" materials, especially certain forms of rigid P.V.C. for example.

With all types of screw extruders, in order to achieve a high efficiency in operation it is necessary not only that the machine should be capable of providing a high "throughput" measured in terms of the mass of material which can be passed through in unit time, but also it is necessary that there should be a high usable output, that is, that the quality of the material discharged should be satisfactory, under the high "thoughput" conditions.

Frequently, especially with "difficult" materials for which critical processing conditions are required for satisfactory extrusion, it is this need for a high usable output that is a principal limiting factor in curtailing the operating efficiency of screw extruders, and single screw extruders in particular are sometimes inherently incapable of yielding the desired results notwithstanding various special designs of feed screw which can be provided for endeavouring to improve efficiency.

Satisfactory quality, at least for plastics materials, depends not only on achieving complete melting of the raw material within the extruder but also on the avoidance of overheating, especially localised overheating, which will cause degradation of the material and, in some cases, on achieving a thorough or adequate "mixing" or "working" of the material which involves the input of work modifying the internal energy and physical structure of the material to produce the desired physical characteristics of the output product.

For example, for producing extruded high tensile rigid P.V.C., such as is required for high pressure P.V.C. pipes or tubing, the material must be adequately worked during processing in an extruder otherwise the product will not possess the correct physical qualities. But this is a "difficult" material to work because, on the one hand, it must be melted within a very narrow temperature range-if the temperature is too low the material will not be sufficiently fluid and if the temperature is too high the material oxidises and is degraded-and on the other hand, it has a high shear strength so that a substantial amount of heat tends to be generated internally during working which tends to lead to serious localised overheating. The difficulty of satisfactory and adequate working of this material to achieve a high usable output seems to be especially severe with all types of single screw extruders, and this has led to the alternative use of "multiple screw extruders" often being deemed preferable as they are usually inherently capable of giving a higher usable output.

Multiple screw extruders are generally characterised by the provision of two rotatably driven feed screws disposed in side-byside relationship within a common bore portion of the extruder barrel so as to mutually co-operate throughout a working region so as in use to compress or squeeze and crush between them the material being fed which is also heated to promote melting.

Known types of such multiple screw extruders fall into two main categories.

In one category, the feed screws have a parallel-sided cylindrical exterior profile and are arranged in closely adjacent side-by-side parallel relationship throughout their length.

In the second category, the feed screws have a conically tapering exterior profile, tapering in the downstream direction, and are arranged with their longitudinal axes mutually convergent in the direction of the extruder outlet such that the two screws co-operate throughout their length along a common tangential median plane.

In both categories, means are provided for introducing raw material and delivering it to the two feed screws at a place where they co-operate and both feed screws are fitted within a common specially profiled bore portion of the extruder barrel.

The parallel cylindrical screw type multiple screw extruders are, however, subject to a serious disadvantage in that their mechanical design is restricted by an inherent inability to incorporate large diameter thrust bearings because of the fixed spacing of the longitudinal rotational axes of the feed screws.

Since the back pressures developed in an axial direction can be very high during operation under high input driving power, this restriction severely limits the practical power rating for such machines.

This disadvantage can be overcome with the tapering screw type multiple screw extruders because, since the longitudinal rotational axes of the feed screws diverge in the rearwards upstream direction, the necessary thrust bearings can be placed at the rear at a position where there is a relatively wide separation between these axes whereby these bearings can have a relatively large diameter giving a large bearing area.

Nevertheless, with the multiple screw extruders in both categories, a disadvantage can still arise from the fact that the raw material, which is normally in a solid state, is introduced and delivered to the feed screws at a position where they are in co-operative relationship with one another. This results in the raw material being compacted and crushed between the two screws before it has begun to become heated and soften or melt so that, especially when the raw material is initially in the form of pellets or finely powdered or granular solid matter, high lateral thrust pressures can be generated which can, inter alia, cause a high rate of wear of the bearings or of the interior surface of the bore of the extruder barrel. In consequence, with both types of multiple screw extruder it is usually necessary to control very carefully the amount and rate at which the raw material is introduced, and usually it is necessary to provide special additional "dosing devices" for this purpose, An object of the present invention is to provide multiple screw extruders in which this disadvantage can be reduced or overcome.

According to the present invention, in a multiple screw extruder having a barrel containing a pair of rotary feed screws mounted and arranged with their longitudinal axes mutually converging in the direction of the extruder outlet, at least one of the feed screws has a profile which tapers conically towards the extruder outlet in a working region throughout which the feed screws mutally cooperate in a common bore portion so as in use to compress and squeeze or work with a milling action between them the material being fed, and upstream of said working region, in an adjacent feed and melting region, the feed screws diverge away from one another and are housed in separate spaced-apart divergent bore portions of the barrel, said feed and melting region extending in the upstream direction to an input region where inlet means are provided for introducing raw material and delivering it to said feed screws within the barrel, whereby in use raw material introduced is conveyed separately by the two feed screws through the feed and melting region, in which at least partial melting can take place, before reaching the working region.

This arrangement, with the angular relationship between the axes of the two feed screws, retains the advantage of the tapering screw type extruders in permitting relatively large diameter thrust bearings to be provided to the rear of the input means at a position where there is a relatively wide separation between the axes of the two screws, but at the same time the raw material introduced can be heated and at least partially melted or softened, in passing along each screw through the feed and melting region, before it is subjected to the compression or milling action of both screws where they co-operate in the working region whereby the production of high lateral thrust pressures can be avoided even with high input feed rates.

In practical embodiments of the invention, in the rearwards direction upstream of the working region, throughout the feed and melting region, both feed screws preferably have a substantially parallel-sided cylindrical exterior profile, and throughout the working region, both feed screws have a conically tapering exterior profile. Advantageously, both feed screws may be of identical profile with screw threads of the same or opposite hand, and may be driven in the same or in opposite directions.

Also, the input means in some embodiments preferably eomprises a single input aperture or port in the extruder barrel which com municates with both the separate bore portions enclosing the two feed screws so that the raw material is delivered to each simultaneously, and there may be a central partitioning guide ridge to deflect the material into the respective bore portions.

By way of example, one form of multiple screw extruder in accordance with the invention is illustrated, somewhat diagrammatically in the accompanying drawing, in which: Figure 1 shows the general arrangement of the two feed screws in the extruder of this embodiment; Figure la is a fragmentary longitudinal sectional view, on a larger scale, through a section of the barrel portion of the extruder of Figure 1; and Figures 2, 3, and 4 are diagrammatic cross-sectional views, in which some dimensions are rather exaggerated, on lines Il-Il, III--III, and IV-IV respectively of Figure 1.

As shown in the drawing, the extruder of this embodiment has two feed screws l0a. lOb, rotatably housed within the interior of the extruder barrel 12 which is made in three sections 12a, 12b and 12c mounted within a cavity 13 of the extruder. The screws 1 ova, lOb, extend rearwards beyond the relatively short drive rearmost barrel section 12c and are connected to shafts 14a, 14b respectively, in alignment therewith.

The screws lOa, lOb, are arranged with their longitudinal rotational axes in a common plane mutually converging in the direction towards the extruder outlet 15 at point A, and the driving shafts 14a, 14b, are each rotatably mounted by spaced apart bearings 16 and 18. These driving shafts are also fitted with axial thrust bearings 20 and with meshing bevel pinions 22 for transmission of rotary drive from the shaft 14a to the shaft 14b. The driving shaft 14a is itself connected, at its rearmost end to power drive input means diagrammatically represented at 24, and it will be clear that with this arrangement the two feed screws 10a and lOb are driven simultaneously in opposite directions. In an alternative arrangement the power drive input means may be mounted on the rear section of the feed screw lOa as indicated in broken lines at 24.

Both feed screws l0a and lOb have conventional helical screw threads 26a, 26b, of opposite hand which define helical feed channels 27a, 27b, and over the forwards section immediately upstream of the extruder outlet 15, between the points A and B, the feed screws both have an exterior profile which tapes conically towards the extruder outlet and they intermesh along a common tangential median plane. This section A-B represents the working region in which the two screws lOa, lOb fit within a common bore portion 28 of substantially figure-of-eight cross-sectional configuration (see Figure 2) in the barrel section 12a and mutually cooperate so as in use to compress and squeeze or work with a milling action between them the material passing along the feed channels 27a, 27b.

Rearwards of point B in the upstream direction, however, the feed screws l0a and lOb each have a cylindrical exterior profile so that, with the angle between their longitudinal axes, the two screws diverge away from one another and are housed in separate spaced-apart divergent bore portions 30a 30b, in sections 12b and 12c of the barrel 12.

The helical screw threads 26a, 26b, termi-. nate at substantially the point D to the rear of which the feed screws each have cylindrical shank portion 32a or 32b ending in the coupling connection to the respective driving shaft, and an input region is defined by the section C-D, within barrel section 12c, which is in registry with an inlet aperture 36, conveniently of rectangular cross-section, in the top of barrel section 12c. As shown in Figure 4, this inlet aperture 36 communicates with both the bore portions 30a and 30b, and serves for the input and delivery of raw material, from a hopper or like feed device 38, to each of the two feed screws. A ridge partition 39 between the bore portions 30a and 30b in this region assists in guiding and deflecting the raw material into these bore portions.

The section B-C intermediate the working region A-B and the input region C-D, where each feed screw has a cylindrical profile and fits within its individual bore portion, 30a or 30b, then defines a feed and melting region.

Thus, the barrel 12 will be provided with heating means (not shown) in a conventional manner, and in operation solid raw material delivered to the feed screws through the input aperture 36 will be fed through this region B C as in a single screw extruder and will be heated sufficiently to at least begin to melt or soften by the time it reaches and enters the working region A-B.

Thereby, as previously indicated, the production of high lateral pressures can be avoided in the region A-B, even with finely powdered raw material, without having to closely control the rate of input by special dosing devices; in fact, the input feed of the raw material may need no greater degree of control than it does with a conventional single screw extruder so that the disadvantages in this respect shown by conventional parallel or tapered screw type multiple screw extruders are avoided. But at the same time, it is seen that the thrust bearings 20, in their rearward location where there is a relatively large spacing between the axes of the feed screws l0a and lOb, can have relatively large diameters providing an extensive thrust bearing area so that the extruder can be designed for safe operation with a high power drive.

It will of course be understood that many variations and modifications can be made, within the scope of the invention as defined in the appended claims, in the particular design hereinabove specifically described, as for example in the precise profiling and angle of convergence of the feed screws, the form of the helical screw threads; and form and configuration of the raw material input means. If desired, the raw material could be fed and delivered to the two screws through separate independent inlet apertures or passages, and this can be useful for example for enabling the material to be compounded, within the extruder, of different components such as a pigment and a base material fed separately one component to one screw and the other component to the other screw. Or, in some cases it may be sufficient to deliver the raw material to one only of the two feed screws.

The embodiment described, however, should at least be sufficient to illustrate the basic features of a preferred embodiment of the invention.

WHAT WE CLAIM IS: 1. A multiple screw extruder having a barrel containing a pair of rotary feed screws mounted and arranged with their longitudinal axes mutually converging in the direction of the extruder outlet, wherein at least one of the feed screws has a profile which tapers conically towards the extruder outlet in a working region throughout which the feed screws mutually co-operate in a common bore portion so as in use to compress and squeeze or work with a milling action between them the material being fed, and upstream of said working region, in an adjacent feed and melting region, the feed screws diverge away from one another and are housed in separate spaced-apart divergent bore portions of the barrel, said feed and melting region extending in the upstream direction to an input region where inlet means are provided for introducing raw material and delivering it to said feed screws within the barrel, whereby in use raw material introduced is conveyed separately by the two feed screws through the feed and melting region, in which at least partial melting can take place, before reaching the working region.

2. A multiple screw extruder as claimed in claim 1, wherein throughout the feed and melting region, both feed screws have a substantially parallel-sided cylindrical exterior profile, and throughout the working region both feed screwshave a conically tapering exterior profile.

3. A multiple screw extruder as claimed in claim 1 or 2, wherein the input means comprises an input aperture or port in the extruder barrel which communicates with both the separate bore portions housing the two feed screws so that the raw material introduced is delivered to each screw simultaneously.

4. A multiple screw extruder as claimed in claim 3, wherein the input means is associated with a partitioning guide member adapted to deflect the raw material introduced and to guide it into the respective separate bore portions.

5. A multiple screw extruder as claimed in any of the preceding claims wherein the barrel is constructed from a plurality of separate barrel sections, namely, a first section having a common bore portion adapted to house the sections of the two feed screws which lie in said working region, a second section having separate spaced-apart divergent bore portions adapted to house the setions of the two feed screws which lie in said feed and melting region, and a third section which includes said inlet means.

6. A multiple screw extruder constructed and arranged substantially as herein described and illustrated with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. designed for safe operation with a high power drive. It will of course be understood that many variations and modifications can be made, within the scope of the invention as defined in the appended claims, in the particular design hereinabove specifically described, as for example in the precise profiling and angle of convergence of the feed screws, the form of the helical screw threads; and form and configuration of the raw material input means. If desired, the raw material could be fed and delivered to the two screws through separate independent inlet apertures or passages, and this can be useful for example for enabling the material to be compounded, within the extruder, of different components such as a pigment and a base material fed separately one component to one screw and the other component to the other screw. Or, in some cases it may be sufficient to deliver the raw material to one only of the two feed screws. The embodiment described, however, should at least be sufficient to illustrate the basic features of a preferred embodiment of the invention. WHAT WE CLAIM IS:

1. A multiple screw extruder having a barrel containing a pair of rotary feed screws mounted and arranged with their longitudinal axes mutually converging in the direction of the extruder outlet, wherein at least one of the feed screws has a profile which tapers conically towards the extruder outlet in a working region throughout which the feed screws mutually co-operate in a common bore portion so as in use to compress and squeeze or work with a milling action between them the material being fed, and upstream of said working region, in an adjacent feed and melting region, the feed screws diverge away from one another and are housed in separate spaced-apart divergent bore portions of the barrel, said feed and melting region extending in the upstream direction to an input region where inlet means are provided for introducing raw material and delivering it to said feed screws within the barrel, whereby in use raw material introduced is conveyed separately by the two feed screws through the feed and melting region, in which at least partial melting can take place, before reaching the working region.