GB2166999A - Apparatus for moulding and conveying a pre-form - Google Patents

Abstract

A pre-form 9 of plastic materials for use in the compression moulding of video discs is formed by extruding, from an extruder outlet 3, a shot of molten plastics material 7 vertically upwards against a single horizontally disposed concave mould member 1. The mould member rises as the plastics material is extruded until, at a pre-set height, the extruder is switched off and the shot of plastics material flows to form a globule whose lower surface is naturally defined by the flow of the plastics material. The globule solidifies to form a substantially blemish-free pre-form which is severed by a reciprocating knife 10. The knife is double-edged and fixed beneath a lever arm 24 which alternately conveys severed pre-forms towards one or other of a pair of record presses 11,12. <IMAGE>

Description

SPECIFICATION Apparatus for moulding and conveying a pre-form The present invention relates to apparatus for moulding a pre-form of plastics material and for conveying the moulded pre-form to a position between the heated mould blocks of a compression moulding press. The invention is particularly suitable for use in the manufacture of high information density disc records such as video discs in which the recorded information is typically represented by closely packed shallow pits of approximately one micrometre diameter. When manufacturing such a disc, it is highly desirable that the pre-form should be free of defects such as voids or cracks, since such defects can give rise to blemished regions in the information region of a disc record made from the pre-form.

Pre-forms used in the compression moulding of video discs have hitherto been formed by injecting molten plastics material from an extruder into a substantially closed cavity formed by mating recesses in a pair of separable mould halves. The plastics material may be injected horizontally through an opening at the side of the cavity or, preferably, injected vertically upwards through an opening at the bottom of the cavity, as is disclosed, for example, in UK Patent Application No.20481 58A. The extruder generally comprises an upwardly directed motor driven screw conveyor which is fed from a hopper with granules of a vinyl based plastics material.A pre-form moulding station has hitherto been provided with each record press, a horizontal transfer sled being provided to convey a newly moulded pre-form to a position centrally between the heated mould blocks of the press. An individual extruder may be provided for each record press, in which case the preform mould may be conveniently mounted directly above the extruder and filled from the bottom. It is more economical, however, for a single extruder to feed a pair of record presses operating in anti-phase, in which case a vertical extruder may be placed between the two presses and the molten plastics material conveyed horizontally from the extruder outlet to side inlets at each of the pre-form moulds, as is disclosed, for example, in British Patent Specification No. 1498801.While such an arrangement is satisfactory for the production of conventional gramophone records, it has been found thatthe conveying of molten plastics material through a right angle bend above the extruder can cause small sharp edged cracks to appear in a pre-form which may cause defects in a pressed video disc. Furthermore, it has been found that, when injecting molten plastics material into a substantially closed cavity, voids may occur in the pre-form due to entrapment of air or gases within the cavity.

It is an object of the present invention to provide an improved apparatusforthe moulding of preforms which are suitable for use in the manufacture of high information density disc records.

It is a further object of the invention to provide a single pre-form moulding station with means for conveying newly moulded pre-forms to a pair of record presses.

According to one aspect of the invention there is provided apparatus for moulding a pre-form of plastics material for use with a compression moulding press, said apparatus comprising a substantially horizontal mould memberwhose lower surface has a concave recess, an outlet for molten plastics material mounted beneath said mould member, means for upwardly injecting a controlled shot of molten plastics material from said outlet to impinge on said lower surface such that, in use, said shot of molten plastics material forms a globule having an upper convex surface conforming to said concave recess and a lower convex surface which is naturally defined by the flow of molten plastics material within the globule under the influence of gravity, said globule, when solidified constituting said preform.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a side sectional view of a preform moulding apparatus in a first position, Figure 2 is a side sectional view of the apparatus of Figure 1 in a second position, Figure 3 is a side view of a pre-form moulding apparatus with transfer means for conveying preforms to a pair of record presses, and Figure 4 is a plan view of the apparatus of Figure 3 viewed along the line XX.

The embodiments of the invention to be described areforthe moulding and conveyance of pre-forms for high information density disc records such as video discs. The pre-forms are substantially disc shaped with convex major surfaces, and are of substantially oval cross-section.

Referring to Figures 1 and 2, a single horizontal disc shaped mould member 1 is supported above the horizontal upper surface of an extruder 2. The extruder is motor driven and may be switched on or off in a controllable manner. The mould member is supported by means of a vertical piston rod 5 whose upper region extends into a pneumatic cylinder 6 (shown only in Figure 3) which is controllable such that the mould member may be raised or lowered or held stationary at a pre-determined height relative to the extruder. The lower surface of the mould block has a concave recess 4 of substantially part-spherical form, the centre of the recess lying directly above the outlet 3 of the extruder. The recess 4 may alternatively be non-spherical and of substantially circular periphery.

In a pre-form moulding cycle, the mould member 1 is first lowered to a position on, or in close proximity to, the horizontal upper surface of the extruder, as shown in Figure 1, with the pneumatic cylinder exerting a downward pressure. The extruder is switched on and molten plastics material 7 is extruded towards the mould with an upward pressure which exceeds the downward pressure of the pneumatic cylinder, causing the mould to rise and the molten plastics material to spread over the concave mould surface 4 and into the cavity under the mould member.The upward pressure of the molten plastics material acting against the concave mould surface 4 is sufficient to substantially remove any entrapped air or gases from the extruded plastics material, and, once the mould member periphery has left the extruder surface, any such air or gases may readily escape to atmosphere at the peripheral gap between the mould and the extruder surface. A proximity switch mounted to the air cylinder senses when the mould member has reached a pre-determined height above the upper surface of the extruder and switches off the extruder.

The mould member is held stationary at this predetermined height for a period of approximately 20 seconds, in the position shown in Figure 2.

The flow characteristics and volume of the shot of plastics material are such that a substantially disc shaped globule of substantially oval section is formed whose convex upper surface conforms to the concave surface 4 of the mould member and whose lower convex surface 8 is naturally defined by the flow of the molten plastics material within the globule under the influence of gravity. The globule cools and partially solidifies during the 20 second period to form a substantially blemish-free pre-form 9 attached to the extruder by a sprue at the extruder outlet 3. The pneumatic cylinder then operates to raise the mould member, parting the mould member from the pre-form and a horizontally reciprocating knife 10 severs the sprue to free the pre-form for transfer to a position between the horizontal mould blocks of a record press.

Referring to Figures 3 and 4, an embodiment of the invention will now be described in which a single pre-form moulding station as described above is mounted above an extruder 2 activated between two record presses 11, 112 and means are provided for automatically conveying moulded pre-forms from the moulding station to both presses. The two presses operate in anti-phase, and the pre-forms are delivered alternately to one press or the other. The apparatus includes a pair of horizontal disc shaped tables, 13, 14, each of which is mounted between the extruder and an associated press. The tables may be controllably raised or lowered or held stationary by means of pneumatic cylinders 15, 16.In a fully lowered position, shown at 13 and 14, a pre-form may rest on one of the tables in a position which is at the same level as the interface between the mould blocks of the associated press. A horizontal transfer sled is provided between each table and its associated press to convey moulded pre-forms from the table to a central position between the opened mould blocks. Each transfer sled includes a pair of openable jaws, 17, 18,through which a table 13,14 may freely pass and which will support a pre-form when closed around it. The detailed construction of the record presses and of their associated transfer sleds is well known as, for example, described in British Patent Specification No. 11184111, and will not be described further.The upper surface of the extruder 2 lies above the level of the transfer sleds, and two chutes 19, 20 are provided for conveying pre-forms from the extruder top towards the tables 13, 14. The chutes each have a lining 21 of a low friction material such as PTFE or nylon.

A cycle of operation will now be described in which a first pre-form is moulded and covneyed to the left hand press 12 and a second pre-form is moulded and conveyed to the right hand press 11. At the commencement of the first half-cycle, the left hand table 14 is fully lowered and the mould member 1 is fully raised with the extruder switched off. A reciprocating knife 10 is in a stationary position to the right of the extruder outlet 3. The left hand transfer sled, with jaws 18 opened, moves to the right and remains stationary when the jaws 18 are positioned immediately above the table 14. The table 14 then rises through the jaws 18 and through a circular opening at the bottom of the chute 20 to an upper position substantially level with the bottom of the chute, shown at 14A in Figure 3.The mould member 1 is then lowered to meet the upper surface of the extruder 2 at its periphery, shown at 1 A in Figure 3. The extruder is then switched on and a shot of molten plastics material is injected into the cavity beneath the mould member as described earlier. A proximity switch 22 mounted to the air cylinder 6 switches off the extruder when the mould cup reaches its pre-set height for globule formation, and the mould cup is held stationary at this position for approximately 20 seconds while a pre-form is formed. The mould member then rises to its fully raised position and the reciprocating knife 10, shown in Figure 4 is actuated. The knife blade has two cutting edges 22, 23 and is fixed to the underside of a lever arm 24 which is joined to a pneumatically controlled reciprocating actuator 25 at a pivot point 26.The knife blade moves to the left along the arc AB and the left hand cutting edge 22 severs the sprue joining the pre-form to the extruder. When the severing is completed, the lever arm 24 strikes the pre-form, conveying the pre-form to position B. At this position, the pre-form impinges a curved portion 27 of the side wall of chute 20 and is deflected down the chute on to the table 14A. The knife blade remains in position B and the extruder top surface is now clearforthe moulding of the next pre-form to commence. The table 14 is lowered, carrying the pre-form to a central position between the opened jaws 18. The jaws 18 close to support the pre-form and the transfer sled moves to the left to convey the pre-form to a central position between the opened mould blocks of the left hand press. The press 12 operates to form a disc record in coventional manner. Moulding a pre-form for conveyance to the right hand press in the second half-cycle may commence immediately the knife has reached position B and the first pre-form has been discharged down the chute 20. It will be apparent that the first pre-form has been discharged very rapidly from the moulding station, and this rapid discharge enables two presses to be fed in anti-phase from a single moulding station with a short cycle time. The second half-cycle is operated in identical manner to the first half-cycle, with right and left handed components interchanged and the reciprocating knife 10 operating from left to right to sever the spool with cutting edge 23. The cycle may be repeated continuously, and it has been found in practice that, using this invention, a pair of video disc presses may be reliably operated with a cycle time of less than one minute and in a manner which overcomes disadvantages which have been encountered with prior art apparatus.

Other embodiments of the invention will be apparent to those skilled in the art. For example, the mould member may alternatively be held stationary at the pre-set height shown in Figure 2 during the whole of the time of plastics extrusion, with the extruder controlled such that each shot of molten plastics material injected into the cavity is of a pre-set volume.

Claims (25)

1. Apparatus for moulding a pre-form of plastics material for use with a compression moulding press, said apparatus comprising a substantially horizontal mould member whose lower surface has a concave recess, an outlet for molten plastics material mounted beneath said mould member, means for upwardly injecting a controlled shot of molten plastics material from said outlet to impinge on said lower surface such that, in use, said shot of molten plastics material forms a globule having an upper convex surface conforming to said concave recess and a lower convex surface which is naturally defined by the flow of molten plastics material within the globule underthe influence of gravity, said globule, when solidified constituting said preform.

2. Apparatus according to Claim 1 wherein said pre-form is substantially disc shaped and is of substantially oval cross-section.

3. Apparatus according to Claim 2 wherein the periphery of said recess is substantially cicrcular and the centre of said recess lies directly above said outlet.

4. Apparatus according to Claim 3 wherein said recess is substantially part-spherical.

5. Apparatus according to any preceding claim wherein said means for upwardly injecting a shot of plastics material comprises a substantially vertical extruder, said extruder being motor driven such that it may be switched on or off in a controllable manner.

6. Apparatus according to any preceding claim wherein means are provided for controllably moving said mould member in an upward or downward direction relative to said outlet and for holding said mould member in a stationary position at one or more pre-determined heights.

7. Apparatus according to Claim 6 wherein said means comprise a pneumatic cylinder.

8. Apparatus according to either of Claims 6 or 7 including a horizontally reciprocating knife blade which is capable of severing a plastics sprue joining said pre-form to said outlet.

9. Apparatus according to Claim 8 wherein said knife blade is fixed to a reciprocating member, said reciprocating member being positioned such that, in use, said mould member may be raised and separated from a newly moulded pre-form, and said knife and said reciprocating member may be operated such that the sprue is severed and the separated pre-form is impacted by said reciprocating member to be deflected towards said press.

10. Apparatus according to Claim 9 wherein said reciprocating member comprises a lever arm which is pivotally mounted to an actuator, and wherein said knife blade is fixed to the underside of said lever arm.

11. Apparatus according to either of Claims 9 or 10 wherein said compression moulding press in cludes a pair of openable horizontal mould blocks and wherein said outlet lies above the interface between said mould blocks, said apparatus including a chute positioned between said outlet and said press such that, in use, said reciprocating member deflects a severed pre-form into said chute to slide towards said press.

12. Apparatus according to Claim 11 wherein said chute is lined with PTFE or nylon.

13. Apparatus according to either of Claims 11 or 12 wherein said chute is provided with an opening at its bottom such that a pre-form may pass through said opening to transfer means, said transfer means being capable of conveying said pre-form to a position between the opened mould blocks of said press.

14. Apparatus according to Claim 13 wherein said transfer means includes a horizontally reciprocating transfer sled mounted at substantially the level of said interface between said mould blocks.

15. Apparatus according to Claim 14 wherein the level of said transfer sled lies below the level of said opening and wherein a vertically reciprocating table is provided beneath said opening such that said table may convey a pre-form from said opening to said transfer sled.

16. Apparatus according to Claim 15 wherein said transfer sled is equipped with openable jaws which, when closed, may support a pre-form.

17. Apparatus according to Claim 9 wherein said comparison moulding press is a first press and a second compression moulding press is provided, said outlet and said mould member lying between said first and second presses such that said mould member may alternatively provide pre-forms for each of said presses, said knife blade being double edged such that one edge faces in the direction of said first press and the other edge faces in the direction of said second press, said reciprocating member and said knife blade being operable such that pre-forms may be alternately directed towards said first and second presses.

18. Apparatus according to Claim 17 wherein said reciprocating member comprises a lever arm and wherein said knife blade is fixed to the underside of said lever arm at a position substantially midway between said first and second edges.

19. Apparatus according to either of Claims 17 or 18 wherein each press includes a pair of horizontal openable mould blocks, said outlet lying above the level of the interface between either pair of mould blocks, said apparatus including a first chute directed towards said first press and a second chute directed towrds said second press, said chutes being positioned such that, in use, said reciprocating member may alternately deflect newly severed pre-forms into one or other of said chutes.

20. Apparatus according to Claim 19 wherein each chute is provided with an opening at its bottom through which a pre-form may pass, transfer means being provided between each opening and its associated press such that pre-forms may be alternately conveyed to a position between the opened mould blocks of one or other of said presses.

21. Apparatus according to Claim 20 wherein said transfer means include a pair of horizontally reciprocating transfer sleds.

22. Apparatus according to Claim 21 wherein each of said transfer sleds lies at a level below the level of the bottom of its associated chute and wherein a pair of vertically reciprocating tables are provided and positioned such that one of said tables may convey a pre-form from the opening in said first chute to its associated transfer sled and the other of said tables may convey a pre-form from the opening in said second chute to its associated transfer sled.

23. Apparatus according to either of Claims 21 or 22 wherein each of said transfer sleds is equipped with openable jaws which are capable of supporting a pre-form.

24. Apparatus according to any of the Claims 17 to 22 wherein said apparatus is capable of operating continuously with said first and second presses operating in anti-phase.

25. Apparatus substantially as described herein with reference to the accompaning drawings.