GB2269149A

GB2269149A - Screw feed mechanism.

Info

Publication number: GB2269149A
Application number: GB9215482A
Authority: GB
Inventors: Michael David Prosser
Original assignee: Courtaulds Packaging Ltd
Current assignee: Albea UK Ltd
Priority date: 1992-07-21
Filing date: 1992-07-21
Publication date: 1994-02-02
Anticipated expiration: 2012-07-21
Also published as: GB2269149B; GB9215482D0

Abstract

A screw feed apparatus for conveying flowable materials in an extrusion or moulding machine comprises a rotatable helical feed screw (16) located in a cylindrical barrel (15) which has an inlet opening (28) and an outlet (31) spaced along the axis of the barrel from the inlet opening. The screw (16) is displaceable bodily, axially along the barrel as a direct consequence of the pressure exerted on the screw by the material being conveyed. Speed control for the screw, is based on a monitoring of the axial position of the screw (16) in its barrel (15). <IMAGE>

Description

Screw Feed Mechanism This invention relates to apparatus for feeding material in an extruder or in an injection moulding machine and to a method of operating such apparatus.

In the plastics industry, plastics components are usually either extruded using an extruder or are injection moulded using an injection moulding machine. Both extruders and injection moulding machines usually employ a feed screw which receives feedstock material supplied to a hopper at one end and conveys that feedstock material through a heating and compaction zone to the other end and then either into a sealed mould or through an extrusion die.

It is commonplace to use virgin feedstock material, that is to say, unused feedstock material in the form of granules or small particles. The virgin material is usually of uniform shape and size granules and is fairly freeflowing.

In an attempt to economise on production costs, waste material from the moulding or extrusion process, is ground or chopped into fine particles to form revert material which can be mixed with the virgin material and fed to the hopper of the feed screw of the extruder or injection moulding machine.

It is extremely difficult to grind or cut waste plastics moulded materials into exactly the same uniform shape and size particles as one obtains with virgin material. Consequently the density of revert material can be considerably less than that of the virgin material. In some cases the loose virgin material can have a density 70% greater than the revert granular material. Furthermore, the density of the revert material in any given batch can vary over a wide range of densities.

The flow characteristics of the revert material are considerably different from those of the virgin material.

Therefore, in the past it has required skilled operators to blend various amounts of revert material with the virgin material to feed into the feed hopper of the extruder or injection moulding machine. Even so, without careful monitoring of the pressures developed in the injection mould or in the compaction zone of an extruder it is difficult to ensure uniform density of product.

Many attempts have been made in the past to control rotational speed of an extruder in an attempt to obtain a uniform extruded product.

An object of the present invention is to provide apparatus for conveying flowable materials which have varying flow characteristics in an extruder or in an injection moulding machine.

A further object of the present invention is to provide a method of controlling the feed of flowable materials from an inlet to an outlet.

According to one aspect of the present invention there is provided a screw feed apparatus suitable for conveying flowable materials in an extrusion or moulding machine, the mechanism comprising a rotatable helical feed screw located in a cylindrical barrel which has an inlet opening and an outlet spaced along the axis of the barrel from the inlet opening, said screw being displaceable bodily, axially along the barrel as a direct consequence of the pressure exerted on the screw by the material being conveyed, speed control means for varying the speed of rotation of the screw, and control means for monitoring the axial position of the screw in the barrel and for varying the rotational speed of the screw in dependence on said axial position of the screw.

A method of controlling the feed of flowable material from an inlet to an outlet represents a further aspect of this invention.

The invention will now be described, by way of example only, with reference to the accompanying drawing which shows a screw feed apparatus constructed in accordance with the present invention arranged as an add-on unit to a conventional continuous extruder.

Referring to the drawing there is shown, schematically, an existing extruder consisting of an extrusion barrel 10 having an outlet die 11 and a helical extruder screw 12 driven by an electric motor (not shown) via a belt 13 and a gearbox 13a. The extrusion barrel 10 is fairly conventional except that the throat casting, which would normally be provided with a material feed hopper, is replaced by a casting 14 which is essentially a T-shaped coupling which connects the barrel 10 to a feed barrel 15 of an apparatus according to the present invention. The barrel 10 is provided with heaters in conventional manner but these are not shown in the drawing.

The barrel 15 extends at right angles to the barrel 10 and has within it a feedscrew 16. The barrel 15 has a flange 17 located at the end of the barrel 15 remote from the barrel 10. The flange 17 is provided with two fixed rods 18 which extend in an axial direction relative to the barrel 15. Located at the far end of each rod 18 is a second flange 19.

The feedscrew 16 is rotatable in the barrel 15 and is also movable axially along the length of the barrel. The feedscrew 16 comprises a deep helical vane 20 which extends over the operating length of the feedscrew 16 and a spindle 21 which is mounted in a plain bearing 22 carried by the flange 17. The bearing 22 supports the feedscrew 16 via its spindle 21 and the feedscrew 16 is stabilised centrally in the barrel 15 because of a small clearance provided between the periphery of the vane 20 and the inner bore of the barrel 15.

Mounted on the rods 18 is a screw carriage 23. The carriage 23 is movable axially along the length of the rods 18 and is moved along the rods by means of a pneumatic or hydraulic piston-in-cylinder assembly 24 mounted on the flange 19. The carriage 23 carries an electric motor 25 which drives, via a chain or belt drive 26, a pinion gear which rotates the feedscrew. A linear transducer 27, which monitors the position of the screw carriage 23 and hence the axial position of the feedscrew 16 in the barrel 15, is mounted in the vicinity of the carriage 23.

A feed hopper 28 is provided at the upstream end of the feed barrel 15 remote from the barrel 10. Safety cut-out switches 29 are provided on the flange 19 and operate to stop the electric motor 25 in the event that the carriage 23 reaches the end of its travel towards the flange 19.

In use, the extruder is operated by driving the extruder screw 12 at constant speed and the heaters are brought up to temperature. The feedscrew 16 is rotated at a predetermined speed with the screw carriage 23 set towards the flange 17 in a first predetermined position. 100% virgin feedstock material is first supplied to the hopper 28, and is taken by the feedscrew 16 to a chamber 31 formed at the outlet end of the feedscrew 16, where it is fed into the supply end of the extruder screw 12. The resistance to flow of the feedstock material acts on the feedscrew 16 and tends to displace the feedscrew 16 in a direction away from the barrel 10. This tendency is resisted by the pressure exerted by the piston in the assembly 24, which biases the feedscrew 16 in a direction towards the barrel 10. Assuming that the virgin feedstock material, fed in at hopper 28, is of a uniform granule shape and size, then the feedscrew 16 will stabilise at a predetermined axial position maintaining the feedstock material at a predetermined pressure in the chamber 31.

If revert material is now added to the virgin material and supplied to the hopper 28, then the flow characteristics of the feedstock material alter. With increased amounts of revert material in the hopper 28, the force exerted by the feedstock material on the feedscrew 16 will decrease and the feedscrew 16 will be displaced axially in a direction towards the barrel 10. If nothing were done, the pressure of the feedstock material in chamber 31 would drop and a non-uniform product would be extruded at the die 11.

However, the linear transducer 27, which monitors the axial position of the screw carriage 23 detects the change in the axial position of the feedscrew 16 in the barrel 15 caused by the noted pressure drop and operates to increase the speed of rotation of the feedscrew 16, in order to stabilise the pressure of the feedstock material in chamber 31.

The apparatus described therefore self compensates automatically and allows one to vary the blends of feedstock material from 100% virgin to 100% revert if required and regardless of the virgin granule or revert granule shape or size, the linear transducer 27 will control the speed of rotation of the feedscrew 16 and the axial position of the feedscrew 16 for varying amounts between these extremes.

In the event of larger than normal granule sizes of revert material being fed into the hopper 28, then the apparatus may be set so that axial displacement of the feedscrew 16 to a predetermined second position may be sensed and used to operate the piston-in-cylinder assembly 24 so as to give a short, sharp nudge to the feedscrew 16 axially in a direction towards the barrel 10, in order to dislodge the oversized granule. However, in the event that a foreign object such as a spanner or other object falls into the hopper 28, then the screw 16 would automatically advance in a direction away from the barrel 10 causing the carriage 23 to contact the safety cut out switch 29 and stop the motor 25 which drives the feedscrew 16 and the motor which drives the extruder screw 12.The limit position where the switch 29 is actuated can thus be considered as a predetermined third position.

A control system 33 monitors the current supply to the motor driving the extruder screw 12 and may receive other signals such as, for example, temperatures and pressures and can be used to control the speed of the feedscrew 16 in response to demand signals from the main extruder, for example, if a high load is being applied to the extruder screw 12 above a set parameter then the feedscrew 16 may be slowed down to idle until the load has dropped. Similarly, if insufficient material is supplied from the hopper 28, this may be detected to give a warning or to control the speed of rotation of the extruder screw 12.

An advantage of the present invention is that one may load varying amounts of virgin and revert materials into the hopper 28 (varying from 100% virgin to 100% revert) and the feedscrew mechanism automatically ensures that a uniform flow rate of feedstock material is supplied to the extruder barrel 10 without the usual delays by more conventional feed back control systems.

In the above-described embodiment of the invention, axial displacement of the feedscrew 16 is used to control the speed of rotation of the feedscrew 16 which supplies material to the extruder screw 12 and this works extremely well, and has enabled one to achieve a relative low cost modification to existing equipment without the need to rebuild the main extruder screw mechanism 12. It may be possible to utilise the present invention directly on the screw 12 instead of on the feedscrew 16 or as wG as-on the feedscrew 16. In this case the screw 12 would need to be mounted for axial displacement in the barrel 10 and a carriage, axial position sensors and the like similar to those described above for the feedscrew 16 would be required to be mounted adjacent to the barrel 10.

Furthermore, the present invention could also be used to control the feedscrew of an injection moulding machine during that part of the injection moulding cycle when the feedscrew is rotated to draw feedstock material into the compaction chamber of the moulding machine.

Claims

CT.AT}

1. A screw feed apparatus suitable for conveying flowable materials in an extrusion or moulding machine, the mechanism comprising a rotatable helical feed screw located in a cylindrical barrel which has an inlet opening and an outlet spaced along the axis of the barrel from the inlet opening, said screw being displaceable bodily, axially along the barrel as a direct consequence of the pressure exerted on the screw by the material being conveyed, speed control means for varying the speed of rotation of the screw, and control means for monitoring the axial position of the screw in the barrel and for varying the rotational speed of the screw in dependence on the axial position of the screw.

2. A screw feed apparatus according to claim 1 wherein the screw is biased in a direction towards the outlet of the barrel by a biasing means.

3. A screw feed apparatus according to claim 2 wherein the biasing means is a fluid-operated piston-in-cylinder assembly and the piston acts on the screw to urge the latter towards the outlet of the barrel.

4. A screw feed apparatus according to claim 2 or claim 3 wherein the biasing means is operable to produce a predetermined axial movement of the screw in the event that the screw reaches a predetermined first position along the barrel.

5. A screw feed apparatus according to claim 2, claim 3 or claim 4 wherein the biasing means is operable to nudge the feedscrew axially of the barrel in the event that the screw reaches a second predetermined position along the barrel.

6. A screw feed apparatus according to any preceding claim, in which means is provided to stop rotation of the screw in the event that the screw reaches a third predetermined position along the barrel.

7. A screw feed apparatus according to any preceding claim wherein the screw is mounted on an axially displaceable carriage, and the control means includes a transducer that monitors axial movement of the carriage.

8. A screw feed apparatus according to claim 7 when dependent on claim 2 wherein the biasing means comprises a fluid operated piston-in-cylinder assembly which biases the carriage, and hence the screw, in a direction towards the outlet of the barrel.

9. A method of controlling the feed of flowable material from an inlet to an outlet using apparatus constructed in accordance with any one of the preceding claims comprising the steps of supplying flowable material to the inlet, positioning the screw at a predetermined axial position in the barrel, biasing the screw towards a predetermined first axial position in the barrel, rotating the screw at a predetermined speed, monitoring the axial displacement of the screw in the barrel due to the pressure exerted by the flowable material on the screw, and adjusting the speed of rotation in dependence upon the axial position of the screw in the barrel.

10. A method according to claim 9 wherein the speed of rotation of the screw is adjusted so as to allow the screw to be moved to the first predetermined axial position.