GB2187132A

GB2187132A - Method and apparatus for moulding plastic sheet

Info

Publication number: GB2187132A
Application number: GB08704178A
Authority: GB
Inventors: Christopher Anthony Jonas
Original assignee: CARAVELL MANUFACTURING Ltd
Current assignee: CARAVELL MANUFACTURING Ltd
Priority date: 1986-02-27
Filing date: 1987-02-23
Publication date: 1987-09-03
Also published as: GB8604857D0; GB2187132B; GB8704178D0

Abstract

A method of moulding a plastic sheet comprises shaping the sheet over a tool and then forming edge shapes of the sheet by vacuum moulding. In a particular embodiment a plastics sheet is held between outer (12) and inner (16) parts of a tenter frame (10), end sections (12a, b and 16a, b) of which are pivotable relative to central section (12c, 16c). As a tool (32) with undercuts edges (34) is raised the end sections (12a, b and 16a, b) pivot to bring the plastics sheet against the sides of the tool. A vacuum is then applied to the tool undercuts to shape the edges of the plastics sheet therearound. <IMAGE>

Description

SPECIFICATION Method and apparatus for moulding plastic sheet This invention concerns a method and apparatus for moulding plastic sheet, particularly for the formation of boxes and cabinets that may be suitable for freezer and refrigerator bodies as well as other "white goods".

Freezers and refrigerators can be made from plastics material usually by forming inner and outer skins with a foamed plastics insulating layer therebetween. The inner and outer skins may be formed in one of two ways.

The first way is conventional vacuum forming but this has several drawbacks. A 1'taperis needed to enablethe tool and moulding to be parted. Thus, the open box produced would need capping mouldings top and bottom to nullify the tapers. These mouldingsduplicatethewalls in certain areas and so increase weight and costs. Secondly, the plastics sheet being moulded has typically to be about 4.5mm thick so that the moulding is sufficiently thick on deep stretch areas, such as corners and runners so as to have the desired strength. Thirdly, withdrawal of, the tool from the moulding means that the edges of the moulding cannot be turned inwards to give the desired neat finish on the vertical doorshutfaces i.e. the moulded sheet would unwrap around anytool under it.

The second way is conventional injection moulding. However, the vacuum moulding disadvantages regarding draw taper and undercuts also apply to injection moulding. Admittedlywall thickness can be reduced butthetooling cost is very high compared with vacuum moulding. Sometimes an injection mould can be forty times the cost ofan equivalent vacuum mould.

The object of the invention isto provide a method and apparatusforforming thin plastics sheet into boxorcabinetmouldingsparticularlyofthetypethat comprises a back, sides and turned-in front edges.

According to the invention a plastics sheet is firstly shaped over a tool and then edge shapes formed by vacuum moulding.

Preferably the plastics sheet is held in a tenter frame, that has pivotable end sections and a tool for shaping the sheet causes the end sections to pivot to draw the ends of the sheet towards the sides ofthe tool. Further means, such as pneumatic pistons may be provided to force the end sections oftheframe against the tool whereupon the edges of the sheet can be vacuum formed as desired. Means for heating the plastics sheet so as to make it pliant may also be provided.

The principle but not exclusive use envisaged for the invention is in the formation of box or cabinet body mouldings, such as for freezers, refrigerators and otherwhite body goods, and in particular in the formation of outer body mouldings provided a back, sides and turned-in front edges, the various sides to be at 900 to each other. To convert such mouldings into an open ended box or cabinet, top and bottom mouldings, an inner wall moulding and an insulating filling, such as of polyurethanefoam,would be added.

The tool to produce such a moulding may be a generally rectangular blockwith an under cutto which a vacuum can be applied to form the turned-in edges. After moulding, the tenterframe may be opened slightly to allowthetool and the moulding to separate, usually by movement away of the tool.

The preferred tenterframe has inner and outer parts that are separable for insertion of a plastics sheet and removal of a moulding, clamps, preferably pneumatically operated, being used to hold the two parts together to retain the plastics sheet during moulding.

This invention will now be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a plan view of a plastics sheet clamping frame; Figure 2 is a side view of the frame of Figure 1; and Figures 3, 4and show in sequencethe steps of moulding a plastics sheet in the frame if Figures 1 and 2 to form part of a box or cabinet.

Referring to Figures 1 and 2, an open rectangular tenterframe 10that has a lower outer L-section frame 12 supported on fourwheels 14 one at each corner and an innerflatframe 16that sits in the outer frame 12. The outer and inner frames 12 and 16 respectively are separable so that a plastics sheet can be placed therebetween. Adjustable pneumatic clamps 20 connect the two frames 12 and 16 whereby the plastics sheet can be clamped securely in place. The clamps 20 comprise a cylinder50 pivotably attached to the side of frame 12 and a piston 52 movable therein pneumatically which is pivotably attached to two arms 53 and 54, the one 53 pivotablyattachedtotheinnerframe 16 and the other 54 pivotably attached to the outer frame 12.In its retracted state, the piston 52 holds the innerframe totheouterframeinordertoclampasheetof plastics material between the inner and outer frames and in its extended state the inner and outer frames are held apartfor a sheet of plastics material to be loaded or a finished moulding removed.

Both oftheframes 12 and 16 have end sections 1 2a, 1 2b and 1 6a, 1 6b respectively that are hingedly movable relative to central sections 12c and 1 6c respectively. The hinges 22 on frame 12 and 24own frame 16 are aligned so thatthe two frames 12 and 16 fold together.

Turning to Figures3,4and 5, thetenter frame 10 has its wheels 14 in tracks 30 fixed in a lower clamp frame of a vacuum forming machine so that once a plastics sheet has been clamped in place the center frame 10 can be moved into the vacuum moulding area, where the plastics sheet is heated.

At this stage the tenterframe 10 is positioned over a former 32 which in this instance is to form the back and sides with turned in edges of a box or cabinet for a freezer or refrigerator. The turned-in edges are to provide the upright faces of the freezer or refrigerator exposed on opening its door. The width of the turned-in edges is, therefore, approximately the intended thickness of the box or cabinet. The former 32 is, therefore, a generally rectangular block but undercut at 34 by the intended width of the turned-in edges.

Theformer32 is on a carriage 36 that is movable verticallyand the former 32 is connected to a source of vacuum via pipe 38. Pneumatic pistons 40 are situated on the carriage 36 eitherside oftheformer 32 and there are actuated by trigger switches 42 also on the carriage 36.

Asheet of plastics material, typically acrylonitrile butadiene styrene (ABS), will be moulded in the above described apparatus bythefollowing procedure. Firstly, the sheet of plastics material is clamped between the inner and outerframes 12 and 16 ofthe tenterframe and the tenterframe moved to the moulding area along the tracks 30. Here the plastics sheet is heated so as to be pliant, and, when sufficiently so, the former 32 is raised to contractthe underside of the plastic sheet. The plastics material is lifted by the former 32 and as it does so the end section of the tenter frame pivot inwards (Figure4) towards the former 32. Eventually the end sections of the frame 10 pass overthe respective trigger switches 42 which actuate the pneumatic pistons 40.

The pistons 40 force their respective end sections of the tenterframe hard againstthe former or tool 32.

The pivot points ofthetenterframe are actually slightly inboard of the vertical sides ofthe tool, so that the plastics material is stretched in forming corners. The action of the tool forcing through the normal place of the sheet on the corners obviates webbing.

When the end section of the frame are clamped againstthe tool by the pneumatic pistons 40 (Figure 5),vacuum is applied to drawthe ends of the plastics sheet into the undercut of the tool to form the turned-in front vertical walls of the box or cabinet.

The frame 10 is then opened enough to spring the moulding pastthe undercut ofthe tool as it returns to its start position. The outer and innerframes 12 and 16 are then separated for the moulding to be removed and excess material trimmed off.Thetenter frame is then returned to its flat state for loading of the next sheet of ABS.

Claims

1. Amethodofmouldinga plastics sheet comprising shaping said sheet over a tool and then forming edge shapes of said sheet by vacuum moulding.

2. A method as claimed in claim 1, wherein the plastics sheet is held in a tenterframe that has pivotable end sections and the tool causes the end-sections to pivot to draw the ends of the sheet towards the sides of the tool.

3. A method as claimed in claim 2, wherein said end sections are forced against the sides of the tool.

4. A method as claimed in claim 3, wherein said forcing is pneumatic.

5. A method as claimed in any one of claims 1 to 4 including the step of heating the plastics sheet so as to make it pliant before shaping.

6. Apparatusformouldinga plasticssheet comprising means for shaping said sheet over a tool and meansforvacuumformingedgeshapesofsaid sheet.

7. Apparatus as claimed in claim 6 comprising a tenterframe in which the plastics sheet is held, the frame having pivotable end sections and the tool being arranged to cause the end sections to pivot to draw ends of the sheet towards the tool sides.

8. Apparatus as claimed in claim 7 including pneumatic pistonsforforcing said end sections against the tool sides.

9. Apparatus as claimed in claim 7 or8, wherein the tenterframe has inner and outer parts that are separable for insertion of a plastics sheet and removal of a moulding and clamps holding thetwo parts together to retain the plastics sheet during moulding.

10. Apparatus as claimed in claim 9, wherein the clamps are pneumatically operated.

11. Apparatus as claimed in anyone of claims 6 to 10, wherein the tool is a generally rectangular block with an under cut to which a vacuum can be applied to form edge shape.

12. Apparatus as claimed in any one of claims 7 to 11, wherein the ends of the tenterframe are slidablyretained in guides.

13. Apparatus as claimed in any one of claims 6 to 12 including means for heating the plastics sheet.

14. A method of moulding a plastics sheet substantially as herein before described with reference to the accompanying drawings.

15. Apparatus for moulding a plastics sheet substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.