GB2101518A - Vacuum forming machine - Google Patents

Abstract

A vacuum forming machine comprises an array of infra-red heaters 10 for heating a sheet 34 of plastics material to be formed. A forming table 16 has a planar surface supporting a baseplate 18 of a mould or pattern block 19 just clear of itself with a peripheral gasket 17 air-tightly sealing the small space between the forming table and the baseplate. A pump 32 is directly connected to an orifice 22 in the forming table 16 by which air can be evacuated from the said small space, and a clamping frame 33 for the sheet of plastics material 34 is interposed between the heaters 10 and the forming table 16. Either the frame 33 or the forming table 16 is movable so that the frame 33 is clear of the baseplate 18 during softening of the plastics sheet 34 but thereafter is in contact with the baseplate 18 so that air can be evacuated from between the plastics sheet 34 and the mould or pattern block 19. <IMAGE>

Description

SPECIFICATION Compact vacuum forming machine The present invention relates to a vacuum forming machine.

Vacuum forming is a process where a sheet of plastic is softened by infra-red radiation. A vacuum pump then exhausts the air between the material and a pattern block. The plastic is forced to take up the shape of the pattern block by the resulting air pressure. Once the plastic has cooled it will retain its new shape.

Previously described vacuum forming machines evacuate air from additional vessels, e.g. vacuum storage receivers and a box containing a moving forming table, as well as the air between the plastic and the pattern block. These machines require substantial vacuum pumps and thus the machines are physically large.

The present invention is based on the realisation that baseplate for the pattern block can simply be supported on a forming table of sheet metal having an orifice through which air can be withdrawn and a peripheral gasket that makes an air-tight seal, and that the orifice can be connected directly to the pump. By this means only a relatively small space between the baseplate and the forming table need be evaucated in addition to the space between the pattern block and the plastics sheet being formed.

Broadly stated the invention provides a vacuum forming machine comprising an array of infra-red heaters for heating a sheet of plastics material to be formed, a forming table having a planar surface for supporting a baseplate of a mould or pattern block just clear of itself with a peripheral gasket air-tightly sealing the small space between the forming table and the baseplate, a pump directly connected to an orifice in the forming table by which air can be evacuated from the said small space, and a clamping frame for the sheet of plastics material interposed between the heaters and the forming table, said frame or forming table being relatively movable so that the frame is clear of the baseplate during softening of the plastics sheet but is in contact with the baseplate so that air can be evacuated from between the plastics sheet and the mould or pattern block.

Preferably both the inlet and exhaust sides of the pump are connected to ports in a two position valve another port of which is connected to the orifice in the forming table so that in one position of the valve air is evacuated from the space between the baseplate and forming table and discharged through a free port in the valve and in the other position air enters through a free port in the valve and is pumped into the space between the baseplate and the forming table. By this means the action of the pump can be reversed when it is desired to bring about release of a finished moulding from the forming block.

The array of infra-red heaters may be pivotally supported on one end of a pair of parallel levers whose other end is fixed to frame members of the machine so that the heater array may be moved from a working position above the forming table to a stand-by position clear of the forming table. A heat shield may be pivoted to the parallel levers for movement between a generally vertical attitude spanning the gap between the heaters and the baseplate when the heater array is in the working position to a generally horizontal attitude underlying the heater array when in the stand-by position. In this arrangement which is primarily intended for use as a bench top machine the forming table will normally be stationary and the clamping frame movable.

Alternatively the array of heaters may be supported on rollers that run on horizontal guide rails for movement from a working position above the forming table to a stand-by position clear of the forming table. The clamping frame will normally be stationary and the forming table moved up and down relative to the clamping frame. Since there is no vacuum box for the pattern block whose dimensions limit the vertical travel thereof it is possible to produce deep drawn mouldings without significantly adding to the complexity and expense of the machine.

Various embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 shows schematically the major components of a vacuum forming machine according to the invention prior to forming; Figure 2 is similar to Figure 1 but shows the major components after forming has been carried out; Figure 3 is a side view of a first practical embodiment of a forming machine according to the invention with the heater in its working position (solid lines) and stand-by position (chain dotted lines); Figure 4 is a front view of the vacuum forming machine of Figure 3; Figure 5 is a front view of part of a second practical embodiment of a vacuum forming machine with the cover removed; and Figure 6 is a side view of the machine of Figure 5.

In Figures 1 and 2 a heater assembly includes a central area of electrical infra-red heaters 10 and a peripheral zone of heaters 13 controlled by respective thermostatic power supplies 1 1, 14 whose temperature levels can be set by control knobs 12, 15. If desired there can be three concentric zones of heaters. In the embodiment illustrated with the knob 12 for the central heaters set at about half and the outer zone 13 switched on the whole of the array of heaters 10, 13 will be radiating an even heat. The temperature of the central heaters 10 can be raised or lowered by turning the control knob 12. This will affect the wall thickness of the resulting moulding which is a useful facility where the tool has a deep draw requiring more or less plastics material in one area.

The heater unit cooperates with a forming table 1 6 of thin sheet metal having a peripheral gasket 1 7 that makes an air-tight seal with the lower face of a baseboard 18 that carries a moulding tool or pattern block 19. Within the gasket 17 may be positioned a louvred air distribution grille 20. Vent holes 21 in baseboard 18 and in tool 19 are formed to permit air to be removed from beneath the moulding and to prevent air from being trapped in corners and recesses of the tool.

Provided that these vent holes are kept small they will not leave any marks on the mouldings. The central region of forming table 1 6 is formed with a vent hole 17 connected via union 22 and air pipe 23 with the centre of a change-over pneumatic valve having a centre port 26 and vent ports 26, 26a. Ports 27, 28 on valve 24 are connected to electrically driven pump 29 via lines 30 and 31.

Thus during moulding the pump sucks air via lines 23, 30 and ports 25, 28 and vents air via line 31 and ports 27. 26. To inject air beneath a finished moulding to bring about release thereof from the mould the valve 24 is changed over and air is pumped in via lines 30, 31, 23 to beneath the mould. Power supply to pump 29 is via pump switch 32.

A two part hinged clamping frame 33 for a sheet 34 of plastics material to be moulded is interposed between the mould and the working position of the heater assembly.

In use a sheet of plastics material to be moulded is interposed between the upper and lower halves of frame 33 and the heaters 10, 13 positioned as in Figure 1 to soften the plastics sheet. When the plastics sheet has lost its rigidity the frame 33 is lowered onto the baseplate 18 so that its periphery makes a seal therewith, after which the pump 29 is switched on, if necessary with the heaters still in place. Air is withdrawn through lines 23 and 30 and exhausted through line 31 from the small space between the lowerface of baseboard 18 and forming table 16 and the sheet is sucked onto the baseboard and the forming tool. The heaters 10, 13 are withdrawn at the appropriate time (Figure 2) after which the position of valve 24 is changed over to force air between the moulded plastics sheet and the mould tool and thereby to release the moulded sheet.

It is a particular advantage of the invention that there are no additional vessels such as vacuum storage receivers or a box containing a moving forming table and all that the pump 29 has to do is to evacuate the air between the softened sheet 34 and the forming tool 18, 19. The pump inlet is connected directly through tubes 23, 30 to the orifice 22 in the flat forming table 16. In consequence only a very small vacuum pump is required and the apparatus can be made more compact than existing commercially available forming machines.

Figures 3 and 4 show a compact bench top vacuum forming machine that will produce inexpensive high quality plastics mouldings up to 100 mm in height from a variety of materials up to 300 mm square and which can operate from a 240 volts supply. The pump 29, switch 32, changeover valve 24 and forming table 16 are housed in a base 37 that has a lever 38 linked to the clamp frame 33 so that the clamp frame may be raised or lowered relative to the forming table.

The heater assembly 39 is pivotally mounted on frame member 41 by means of pivoted parallel links 42, 43 which also carry a heat shield 44 that in a stand-by position of the heater assembly underlies the heaters to shield the bench top from direct heat and in the working position folds behind the machine as shown.

Figures 5 and 6 show a free standing machine of somewhat greater size. Forming table 16 having union 22 for connection to pipe 23 (omitted from this view) is supported at either side on guide rods 50, 51 and is urged by springs 52 upwardly towards clamping frame 33. The forming table has pivoted thereto a crossbar 53 connected to a crossbar 54 pivoted to the frame of the machine by a pair of levers 55, 56 that can be rotated to move the forming table from its raised (solid lines) to its lowered (chain dotted lines) position, the links 55, 56 passing to an overcentre position when the forming table is fully raised. The heater assembly includes an element tray 60 in which the infra-red heating elements are located and a so-called chimney 61 above the heater box in which air can circulate. It is supported on guide rails 62 by means of rollers 63 so that it can slide from a working position overlying forming tray 16 to a stand-by position in which it no longer overlaps the forming tray. The rails 62 are formed in two sections pivoted together so that the projecting portion 62a can be folded upwardly for storage. A heat shield 63 is associated with rails 62.

Claims (9)

1. A vacuum forming machine comprising an array of infra-red heaters for heating a sheet of plastics material to be formed, a forming table having a planar surface for supporting a baseplate of a mould or pattern block just clear of itself with a peripheral gasket air-tightly sealing the small space between the forming table and the baseplate, a pump directly connected to an orifice in the forming table by which air can be evacuated from the said small space, and a clamping frame for the sheet of plastics material interposed between the heaters and the forming table, said frame or forming table being relatively movable so that the frame is clear of the baseplate during softening of the plastics sheet but is in contact with the baseplate so that air can be evacuated from between the plastics sheet and the mould or pattern block.

2. A vacuum forming machine according to Claim 1 in which both the inlet and exhaust sides of the pump are connected to ports in a two position valve another port of which is connected to the orifice in the forming table so that in one position of the valve air is evacuated from the space between the baseplate and forming table and discharged through a free port in the valve and in the other position air enters through a free port in the valve and is pumped into the space between the baseplate and the forming table.

3. A vacuum forming machine according to Claim 1 or 2, wherein the heater array comprises a central zone and at least one zone of heaters surrounding the central zone, the temperature of each said zone of heaters being independently controlled.

4. A vacuum forming machine according to any preceding claim, wherein the array of infra-red heaters may be pivotally supported on one end of a pair of parallel levers whose other end is fixed to frame members of the machine so that the heater array may be moved from a working position above the forming table to a stand-by position clear of the forming table.

5. A vacuum forming machine according to Claim 4 wherein a heat shield is pivoted to the parallel levers for movement between a generally vertical attitude spanning the gap between the heaters and the baseplate when the heater array is in the working position to a generally horizontal attitude underlying the heater array when in the stand-by position.

6. A vacuum forming machine according to any preceding claim wherein the forming table is stationary and the clamping frame is vertically movable.

7. A vacuum forming machine according to any of Claims 1 to 3 wherein the array of heaters is supported on rollers that run on horizontal guide rails for movement from a working position above the forming table to a stand-by position clear of the forming table.

8. A vacuum forming machine according to Claim 7, wherein the clamping frame is stationary and the forming table is mounted for up and down motion relative to the clamping frame.

9. A vacuum forming machine substantially as hereinbefore described with reference to and as illustrated in Figures 1 and 2 and either Figures 3, 3a and 4 of Figures 5 and 6 of the accompanying drawings.