GB2173441A

GB2173441A - Mixing and extrusion machinery

Info

Publication number: GB2173441A
Application number: GB08603663A
Authority: GB
Inventors: Geoffrey Francis Holder
Original assignee: BICC PLC
Current assignee: Balfour Beatty PLC
Priority date: 1985-02-19
Filing date: 1986-02-14
Publication date: 1986-10-15
Also published as: GB8603663D0; AU5226786A; GB8504158D0

Abstract

An extruder (2) incorporated in its feed a mixing device (4) that does not of itself drive the material forward and a screw conveyor (5) which operates in a hopper (6) to drive the material forward through the mixing device. The screw (5) is moveable along its own axis against a force e.g. a pneumatic motor (7), biassing it downwardly. Associated with the axial displacement of the screw (5) are detecting means, e.g. simple limit switches (12, 13, 14, 15), which reduce the speed of the screw and/or the rate of feed of material from a means (11) to the hopper, or stop either or both completely, when the screw conveyor (5) rises to an upper limit and increase or restart when it reaches a lower limit. Preferably separate relatively displaced limit switches are provided so that, when the conveyor rises, the feed of material is stopped before the rotation of the screw, and when it falls rotation is restarted before feed of material is. <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 elastomerbase materials, that cannot be adequately mixed solely by shearing in a screw extruder.

Conventionally such materials have been mixed off-line, usually 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 prior to 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 our prior British Patent Application publication No. 2116865A (our prior application) we have described and claimed precision mixing and extrusion machinery comprising an extruder incorporating in its feed a mixing device that does not of itself drive the material downward and a feed device to push material through the extruder distinguished by the fact that the feed device is a continuously-acting screw conveyor.

The present invention is a modification or improvement of that invention which provides still further uniformity of feed rate.

The modified machinery in accordance with the present invention is distinguished by the fact that the feed device is a screw conveyor operable to urge material in a downward direction though a hopper and itself moveable along its own axis against a force biassing it downwardly and is associated with detecting means operable to reduce the rate of feed of material to the hopper and/or the rate of rotation of the screw conveyor when the screw conveyor rises against the biassing force and to increase the rate of feed and/or the rate of rotation of the screw conveyor when it falls under the biassing force.

The verb 'reduce' is used herein in a broad sense so as to include reduction to zero (stopping completely), and similarly 'increase' includes re-starting. In fact the preferred form of the invention is one in which the detector means are simple limit switches arranged to stop the feed of material and/or rotation of the screw on reaching a predetermined upper limit and re-starting on reaching a predetermined lower limit, so that the conveyor acts alternately as a ram (descending) and as a screw (rising).

Preferably both the feed of material and the rotation of the screw are stopped and restarted (or decreased and increased) on reaching the appropriate limit. Preferably separate limits are set in such positions that when the screw conveyor rises the feed of material is stopped before the rotation of the conveyor and when it descends rotation is re-started before feed of material. This makes it easier to avoid accumulation of material in the hopper. A similar result can be obtained by using one limit switch controlling the material feed immediately and the conveyor drive after a preset time period at the upper limit, and vice versa at the lower limit.

If preferred however, the position of the screw conveyor could be determined by any suitable analogue or digital detector (electronic, electrical, mechanical, pneumatic etc.) either over its whole range of movement or over selected parts thereof and pre-programmed feed rates and/or rotation rates applied accordingly by standard control techniques.

If, when the screw conveyor rises, its rotation is stopped or slowed down but the feed of material to the hopper is unchanged, separate provision may need to be provided to prevent or deal with overflow of material if the hopper becomes completely filled.

In the practice of the invention, at least for some positions of the screw conveyor, there will be a risk of material lodging in parts of the hopper that are not swept by the flights of the screw conveyor, and to eliminate or reduce this risk we prefer to provide projecting rods or other stirring/dislodging devices at an appropriate height on the screw conveyor.

Except as expressly taught herein, the teaching of our prior application is preferably followed.

The accompanying drawing is a diagrammatic representation, by way of example, of a Farrel-Bridge MVX machine modified in accordance with the present invention.

The machine comprises a screw 1, barrel 2, cross-head 3 and twin-rotor mixer 4, all of unmodified conventional form.

In accordance with the invention, the material is fed to the mixer by a screw conveyor 5 operating in a hopper 6 and biassed downwards by a pneumatic motor 7 acting through thrust bearings 8. The hopper may be fitted with a vibrator if desired. The drive 9 for rotating the screw is arranged to acommodate the required up-and-down movement of the screw shaft 10. A conventional feed mechanism 11 supplies material to the hopper at an adjustable (preset) rate, when energised.

The motor 7 comprises a non-magnetic cylinder 17 co-operating with a magnetic piston 18 and fitted with four magnetic reed switches 12, 13, 14, 15 which respond to the proximity of the piston. These reed switches (or other limit switches could be used) are connected to conventional electronic switchgear so that: reed switch 12, when activated on rising of the shaft 10, switches off the feed device 11; reed switch 13, when activated on rising of the shaft 10 switches off the screw drive 9; reed switch 14, when activated on falling of the shaft 10 switches on the screw drive 9; and reed switch 15, when activated on falling of the shaft 10 switches on the feed device 11.

A series of stirring rods 16 eliminates any substantial risk of material bridging across the tapered base of the hopper.

On start-up, the biassing force from the pneumatic motor 7 holds the conveyor screw in its lowest position and the screw drive 9 and material feed device 11 are energised.

The feed rate is set such that a slow build up material occurs in the hopper, and the backpressure built up as the machine comes into operation is such that the screw rises against the biassing force. In due course reed switch 12 is activated and the feed device 11 stopped. If the screw continues to rise, in a short time reed switch 13 is activated and the screw conveyor drive 9 also stopped. Pneumatic motor 7 ensures that a substantially constant thrust nevertheless continues to be applied to the material under the screw and as material is consumed by the mixer/extruder the screw descends, normally acting as a feed ram without rotating. In due course, as the screw approaches the lower limit of its travel, reed switch 14 is activated and the screw drive 9 re-started. On some cycles, the screw will immediately begin to rise; otherwise reed switch 15 is activated shortly thereafter to restart the feed device 11, whereupon (in either case) the cycle repeats.

In this way, a substantially uniform feed pressure is achieved without the need for precise matching of the output of the feed device 11 to the demand of the extruder.

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 screw conveyor operable to urge material in a downward direction through a hopper and itself moveable along its own axis against a force biassing it downwardly and is associated with detecting means operable to reduce the rate of feed of the material to the hopper and/or the rate of rotation of the screw conveyor when the screw conveyor rises against the biassing force and to increase the rate of feed and/or the rate of rotation of the screw conveyor when it falls under the biassing force.

2. Machinery as claimed in Claim 1 in which, when the screw conveyor rises, the rate of feed of the material and/or the rate of rotation of the screw is reduced to zero.

3. Machinery as claimed in Claim 2 in which separate limits are set in such positions that when the screw conveyor rises the feed of material is stopped before the rotation of the conveyor and when it descends rotation is re-started before the feed of material.

4. Machinery as claimed in any one of the preceding Claims in which the screw conveyor is provided with stirring/dislodging devices to inhibit lodging of material in parts of the hopper that are not swept by the flights of the screw conveyor.

5. Precision mixing and extrusion machinery substantially as described with reference to and as shown in the accompanying diagrammatic drawing.