GB2116865A

GB2116865A - Mixing and extrusion machinery

Info

Publication number: GB2116865A
Application number: GB08235126A
Authority: GB
Inventors: Barry George Howell; Stuart Malcolm Castle
Original assignee: BICC PLC
Current assignee: Balfour Beatty PLC
Priority date: 1981-12-10
Filing date: 1982-12-09
Publication date: 1983-10-05

Abstract

Apparatus comprises a mixing device 6 attached to an extruder 1, 2, and a feeding device 11 for feeding material to the mixer in a controlled continuous manner. The feeding device is in the form of a screw conveying means 13 and ensures a continuous conform supply of material to the mixer and hence to the extruder. <IMAGE>

Description

SPECIFICATION Mixing and extrusion machinery This invention relates to machinery for the mixing and extrusion of polymer-base material, more especially (though not exclusively) materials of high viscosity, such as elastomer-base materials, that cannot be adequately mixed solely by shearing in a screw extruder.

Conventionally such materials have been mixed off-line, usally by means of a Banbury mixer, which requires much auxiliary equipment and is energetically inefficient partly because at least some of the material will cool after mixing and priorto extrusion and will need to be re-heated to establish suitable extrusion conditions, and which may lead to major operating problems if the mixture is liable to premature vulcanisation, ageing or other changes while awaiting extrusion.

In one known type of machinery (the Farrel-Bridge MVX machine), this problem of intermediate storage is avoided by incorporating in the feed to the extruder a mixing device that does not of itself drive the material forwards, but this requires the provision of a feed device to push material through the mixing device, for which purpose a reciprocating ram has always been used. This arrangement mixes effectively and is versatile because the extruder, mixing device and ram can be controlled independently, but the reciprocations of the feed ram are inevitably reflected in variations in operating conditions that make it very difficult to extrude a product of high uniformity. The art has accepted this as an inherent limitation of the system, and it has been used extensively in mixer/granulators and for the lower grades of direct extrusion, but not for extrusion of the highest precision.

In accordance with the present invention, precision mixing and extrusion machinery comprising an extruder incorporating in its feed a mixing device that does not of itself drive the material dorward and a feed device to push material through the extruder is distinguished by the fact that the feed device is a continuously-acting screw conveyor.

A suitable feed device is already commercially available from an extrusion machine manufacturer (Mapre) who has for many years recommended it for feeding twin-screw extruders.

As with the known machinery described above, the extruder, mixing device and feed device can be adjusted independently to optimise the mixing and extrusion of a wide variety of materials, but the feed rate can be as uniform as in conventional extruders and a uniform, high-precision product can be obtained. This is especially important in the cable industry, where close dimensional tolerances may be essential to subsequent operations, but allows significant material savings where dimensions are not of themselves critical.

Materials are preferably supplied to the feed device at a uniform rate using one or more than one gravimetric orvolumetric dispenser (one is sufficient if all the ingredients are tumble-mixed), but in some cases a simple hopper feed might be adequate. The feed device may be controlled by conventional means to obtain a constant pressure at its exit to the masticating device, a constant throughput rate, a constant level at its inlet, or other desired parameter.

The preferred type of mixing device is that incor porated in the Farrel-Bridge MVX machine which comprises parallel shafts of triangular cross-section, driven in opposite angular directions and having an inlet near one end of the shafts and an outlet near the other; but other types can be used, at least for some materials.

Since the extruder is supplied with ready-mixed, hot and fully plastified material and only has to meter it (and de-gassify if required), it may be short and simple, e.g. a simple screw with a length:diameter ratio of about 6 is suitable for most materials.

Amount the materials for which the machinery of the invention is particularly recommended may be mentioned (1 ) natural or synthetic rubber compounds, fed as powders or in a finely-granulated state (2) thermoplastic rubbers (3) blends of polymers with disparate physical properties, such as mixtures of ethylene-propylene rubber with either polyethylene or polypropylene, and (4) silane-grafting compounds for condensation cross-linking after hydrolysis, for example those described in Dow Corning UK Patent No. 1286460.

In the accompanying drawings Figure 1 is a diagrammatic cross-section of the Farrel-Bridge MVX machine, included for comparison, and Figure 2 is a corresponding view of the same machine modified to form a machine in accordance with the invention.

Referring first to Figure 1, the machine comprises a screw 1 operating in a barrel 2 to advance material to and through an extrusion head 3. Material to be extruded is fed from a conveyor 4 to a hopper 5, and between the hopper and the barrel it is passed through and processed by a mxing device 6. This comprises two parallel shafts 7 and 8, each of broadly triangular cross-section, rotating in opposite directions as indicated in a closely fitting chamber.

This type of mixing device requires positive feeding of material into its input side, and in the known machine this function is performed by a ram 9 reciprocated by a pneumatic cylinder 10 so that material is steadily pushed into the mixer except for short periods during the return stroke of the ram.

Inevitably, the interruption for this return stroke is reflected in the throughput of the machine.

In accordance with the present invention (Figure 2) a screw conveyor arrangement 11 replaces the hopper 5, ram 9 and its operating mechanism 10.

This screw conveyor 11 comprises an appropriately shaped hopper 12 within which a conveyor screw comprising two sections 13 and 14 in series operates. The screw is steadily rotated by a conventional drive mechanism 15.

By modifying the machine in this way constant operating conditions and a precisely uniform output can be obtained.

Claims

1. Precision mixing and extrusion machinery comprising an extruder incorporating in its feed a mixing device that does not of itself drive the material forward and a feed device to push material through the extruder distinguished by the fact that the feed device is a continuously acting screw conveyor.

2. Precision mixing and extrusion machinery comprising an extruder incorporating in its feed a mixing device comprising parallel shafts of triangular cross-section driven in opposite angular directions and having an inlet near one end of the shafts and an outlet near the other and a feed device to push material through the extruder distinguished by the fact that the feed device is a continuously acting screw conveyor.

3. Precision mixing and extrusion machinery substantially as described with reference to and as shown in Figure 2 of the drawings.