IES950717A2 - Vacuum forming - Google Patents

Abstract

A vacuum forming system comprises a chamber (1) having an aperture (2) which is sealingly occluded against the passage of air during a vacuum forming operation by a plastics sheet to be vacuum formed, heaters (31,32) for heating a plastics sheet occluding the aperture (2), a mechanism (21) for sealingly positioning a mould (11) located within the chamber (1) in proximity to a heated plastics sheet occluding the aperture (2) against the passage of air between a frame (12) enclosing the mould and remainder of the interior of the chamber (1), and means for exhausting air from the region within the frame (12) surrounding and underlying mould (11), so that a heated plastics sheet occluding the aperture (2) is applied to the mould (11) by atmostpheric pressure to take up a required shape. The chamber (1) is provided with a valve (41) for relieving excess air pressure within the chamber (1) during the heating of a plastics sheet to minimise any departure by the softened heated plastics sheet from a substantially planar configuration. The valve (41) is suitably a weighted flap valve, while the mechanism (21) is suitably a double toggle mechanism (23,24) actuated by a central expanding cylinder (22) for displacement between a mould-lowered disposition and an upper mould-raised disposition, which is also an over-centre disposition of the toggle mechanism. <Fig.1>

Description

This invention relates to vacuum forming. The invention is particularly directed to the vacuum formation of moulded plastics panels for assembly into door or like units.

Vacuum forming of plastics panels for use in forming doors or like structural elements is known as such. In the vacuum forming process, a sheet of plastics is clamped around its periphery and heated until it reaches a desired level of formability. It is then pulled down by vacuum action onto a mould, so as to assume as closely as possible the shape defined by the mould, which typically includes detail features to define elements equating to the sub-panels of doors manufactured from timber by traditional methods. It is important for successful production of a quality product that peripheral sealing of the sheet is complete, so that a uniform vacuum effect is created underneath the softened plastics panel or sheet, thus allowing it to be deformed consistently across its surface area to assume the required profile. It is further of importance in the initial heating of the plastics sheet that the sheet be maintained as far as possible in a level disposition, so that the heating effect is also uniform and that OPEN TO PUBLK 5NSPEC Ϊ ION jND- R SECTION 28 AND RULE 23 JNL No 1.7- U OF 2-OFF έ MtTCI- η;»» 3lzo si ho 950717s - 2 excessive stretching and consequential local reduction of thickness in the sheet does not take place.

The present invention is especially directed to specific improvements in the areas of sheet clamping and uniform heating.

According to the invention, there is provided a vacuum forming system comprising (a) a chamber having an aperture which is sealingly occluded against the passage of air during a vacuum forming operation by a plastics sheet to be vacuum formed, (b) means for heating a plastics sheet occluding said aperture, (c) means for sealingly positioning a mould located within the chamber in proximity to a heated plastics sheet occluding said aperture against the passage of air between a region enclosing the mould and the remainder of the interior of the chamber, and (d) means for exhausting air from a region surrounding and underlying said mould, so that a heated plastics sheet occluding said aperture is applied to said mould by atmospheric pressure, wherein the chamber is provided with valve means for release of air from within said chamber during the heating of a said plastics sheet developed by said heating to minimise any departure by the softened heated plastics sheet from a substantially planar configuration.

The aperture in the chamber is typically a horizontal opening in the top of the chamber. The plastics sheet is held horizontally in this opening during the vacuum-forming process. Application of heat results in the plastics sheet becoming softened. The heating step also leads to the air within the chamber being heated up. If the chamber is closed against the exterior, the air will expand and there is thus an upward force applied to the plastics sheet in the upper opening of the chamber, so that the sheet will tend to bubble up. As previously indicated, it is important that deformation of the sheet at this stage be avoided and that it be maintained as far as possible in a - 3 horizontal flat or planar disposition. To this end, the chamber is therefore according to the invention provided with the above noted valve means which releases air from within the chamber when the air expands during the heating step. Effectively, there is an imperceptible increase in air pressure, which is relieved by the valve means, such as by outward opening of the preferred flap valve embodiment subsequently described.

The vacuum forming system according to the invention preferably comprises means for maintaining said valve means in a closed condition at least during a period following heating of the plastics sheet and preparatory to the air exhausting phase of a vacuum forming operation. Said valve means suitably comprises a flap valve hinged in an upper edge region for pivoting opening movement for outward of flow of air from the chamber and closing movement under the action of gravity, and in an especially preferred arrangement, said flap valve has an arm extending from outwardly from said flap for mounting at least one weight at a selected spacing from the axis of pivoting of the flap for enhancing the gravity-actuated valve-closing action. Said means for maintaining said valve means in a closed condition at least during a period following heating of the plastics sheet and preparatory to the air exhausting phase of a vacuum forming operation is then suitably defined by an actuator for engaging said arm of the flap valve during said period to prevent opening movement of the flap.

Said means for sealingly positioning a mould located within the chamber in proximity to a heated plastics sheet occluding said aperture against the passage of air between a region enclosing the mould and the remainder of the interior of the chamber preferably comprises a mechanism for displacing the mould between a first lowered disposition within the chamber and a second raised sealing disposition, said mechanism being lockable in the raised sealing disposition to withstand any downward forces resulting from the sealing occlusion of said aperture by the plastics sheet. 950717 - 4 In the preferred arrangement previously described in which the aperture is defined in the roof of the chamber, the plastics sheet is sealingly clamped over this aperture for the vacuum forming process.

A substantial downward force is thereby applied. By arranging for the mould displacing mechanism to be lockable in its raised disposition, a firm base is thereby provided to resist the downward sealing forces exerted against the sheet as positioned at the aperture.

In a further aspect, the invention also extends to a vacuum forming system comprising (a) a chamber having an aperture which is sealingly occluded against the passage of air during a vacuum forming operation by a plastics sheet to be vacuum formed, (b) means for heating a plastics sheet occluding said aperture, (c) means for sealingly positioning a mould located within the chamber in proximity to a heated plastics sheet occluding said aperture against the passage of air between a region enclosing the mould and the remainder of the interior of the chamber, and (d) means for exhausting air from a region surrounding and underlying said mould, so that a heated plastics sheet occluding said aperture is applied to said mould by atmospheric pressure, where i n said means for sealingly positioning a mould located within the chamber in proximity to a heated plastics sheet occluding said aperture against the passage of air between a region enclosing the mould and the remainder of the interior of the chamber comprises a mechanism for displacing the mould between a first lowered disposition within the chamber and a second raised sealing disposition, said mechanism being lockable in the raised sealing disposition to withstand any downward forces resulting from the sealing occlusion of said aperture by the plastics sheet.

In this second aspect of the invention, the chamber may again be provided with valve means for release of air from within said chamber during the heating of a said plastics sheet to minimise any 950717« - 5 departure by the soft plastics sheet from a substantially planar configuration, and optionally also means for maintaining said valve means in a closed condition at least during a period following heating of the plastics sheet and preparatory to the air exhausting phase of a vacuum forming operation. Said valve means preferably comprises a flap valve hinged in an upper edge region for pivoting opening movement for outward flow of air from the chamber and closing movement under the action of gravity, and said flap valve may have an arm extending from outwardly from said flap for mounting at least one weight at a selected spacing from the axis of pivoting of the flap for enhancing the gravity-actuated valve-closing action. Said means for maintaining said valve means in a closed condition at least during a period following heating of the plastics sheet and preparatory to the air exhausting phase of a vacuum forming operation may again be defined by an actuator for engaging said arm of the flap valve during said period to prevent opening movement of the flap.

In either aspect of the invention, said mechanism may be a double toggle mechanism comprising two pairs of links pivoted to one another and located between the mould and the floor of the chamber, the upper link of each pair being pivotably associated with the mould and the lower link of each pair being pivotably associated with the floor, and the mutual pivot points of the two links being interconnected by an actuator, so that the mould is moved from the lowered disposition to the raised disposition by expansion of the actuator. In a favoured arrangement, the expanded condition of the actuator corresponds to over-centre positions of the pairs of pivoted links, so that the mould is locked in the raised sealing disposition while the actuator remains in the expanded condition. In a practical construction, the mould is mounted on a mould frame, and the association of the linkages with the mould is effected by the links being pivotally connected to the mould frame, which carries the mould for the upward and downward movement.

The invention also extends to a vacuum forming system ®5θ7ί 7 - 6 substantially as described herein with reference to and as shown in the accompanying drawings.

An embodiment of the invention and a method by which the invention may be carried out will now be described having regard to the accompanying drawings, in which Figure 1 is an exploded partially diagrammatic pictorial view of a vacuum moulding machine according to the invention, Figure 2 is a front view of the machine of Figure 1 showing the mould in a lowered disposition, Figure 3 is a view similar to that of Figure 2, with the mould in a raised or active disposition, Figure 4 is a diagrammatic front view of the system of the invention during the plastics sheet heating phase, Figure 5 is a detail view of an air valve according to the invention for regulating sheet position during the heating stage, and Figure 6 shows the system of the invention in front diagrammatic view during the pressurisation or blowing stage preparatory to the final vacuum forming step.

As shown in Figure 1, and further in Figures 2 and 3, a vacuum forming system according to the invention has a substantially enclosed heating chamber 1, with a substantially rectangular top opening 2 where a plastics sheet 3 (Figures 2 and 3) is placed in position for the vacuum forming step. The effective working periphery of the top opening 3 is defined by a so-called window frame 4, the dimensions of the opening of which are slightly less than those of the plastics sheet. Frame 4 is also of rectangular configuration. The frame 4 is held in position on the top wall 5 of the chamber 1 by clamps 6, which 9s07lf - 7 - ' are mounted on the top wall 5 by means of fasteners 7, for example screws or bolts. A displaceable upper sheet clamping member 8, again of generally rectangular configuration and formed from a series of rectangular cross-section bar portions, arranged with their narrow edges downwards, serves to hold the sheet 3 in position with its periphery engaged against the top or outer surface of the window frame 4, for the heating and moulding steps. The periphery of the window frame 4 is bounded throughout its generally rectangular shape by a resilient sealing strip 9, for example formed of rubber or plastics, against which the peripheral edge of the sheet 3 is urged and held by the clamping member 8 during the forming process.

Underneath the window frame 4, the mould 11 against which the plastics sheet 3, when softened, is shaped, is mounted within the machine on a mould frame 12. The mould frame 12 is defined by a rectangular box, sealed along its upper peripheral edge 13 in the operating disposition against the underside of the periphery of the window frame 4, again with interposition of a resilient rubber or plastics sealing strip 14. Air is exhausted from within the mould frame 12 and from the space 15 above the mould 11 and underlying the plastics sheet 3 in use of the machine in part through apertures provided in the mould 11 itself. These apertures (not shown) are of relatively small dimensions and extend along the various raised or moulding features 16 on the mould 11 surface.

It is important for producing a good finished product in the vacuum moulding process that the periphery of the plastics sheet be effectively sealed against the passage of air throughout its extent.

The interposition of the upper sealing strip 9 underneath the periphery of the sheet 3, between the edge region of the sheet 3 and the upper surface of the so-called window frame 4, achieves this required sealing. Effective sealing is also required between the upper periphery 13 of the mould frame 12 and the underside of the window frame 4, and this is again achieved by the interposition of the resilient sealing strip 14 which underlies the periphery of the window 07 1 Τ’ - 8 frame 4. It is further of importance that there be minimal distortion or dimensional imprecision in the mould frame 12 and the clamping member 8, although the presence of the resilient strips 9, 14 ensures that this criterion can be achieved within appropriate tolerances. It is further of importance that a suitable level of sealing force is applied between the upper clamping member 8, which is urged downwardly against the window frame 4, and the mould frame 12, which is urged upwardly against the window frame. It is particularly important that a uniform and effective upward force is exerted on the mould frame 12, since the effect of gravity will tend to break the seal between the mould frame 12 and the window frame 4, unless a positive sealing force is exerted, preferably in a locked or latched manner. The invention provides an arrangement for achieving such a sealing force and for locking the mould frame 12 into a raised and sealing disposition, as will now be described.

The mould 11 carried on the mould frame 12 is raised and lowered for engagement of the seal 14 against the underlying peripheral edge of the window frame 4 by means of a double toggle clamp or scissors type mechanism 21, the toggle features of which are pushed outwards in opposition to one another by expansion of a centrally mounted hydraulic cylinder 22, to terminate in a slightly over-centre position as shown in Figure 3, this being the raised or clamping disposition of the mould frame 12, in which the mould frame 12 remains effectively locked or latched until such time as the mechanism 21 is released by contraction of cylinder 22.

Each end of the mould frame 12 is supported on a pair of links 23, 24, of substantially equal length, which are interposed between the underside of the mould frame 12 and the floor or base 25 of the chamber 1. The links 23 and 24 are in each case of substantially equal length and are pivoted to one another at a central pivot point 26 midway between the underside of the mould frame and the floor 25. The hydraulic cylinder 22 is mounted between the central pivots 26 of the two pairs of links 23, 24, so that the cylinder portion of the unit 22 0 7 1 7 Μ - 9 is associated with the righthand central pivot 26 in the views of Figures 2 and 3 and the piston rod 27 of the unit 22 is pivotally associated with the lefthand central pivot 26. At the upper end of each upper link 23, the link 23 is pivotally connected to the underside of the mould frame 12 at a mounting point 28. A similar pivotal connection 29 of the lower link 24 at the floor or base 25 of the chamber 1 is also provided.

Figure 2 shows the lowered condition of the mould frame in which the hydraulic device 22 is contracted so that the central pivots 26 are brought towards one another and the mould frame 12 is brought downwards, by virtue of the inwardly pivoting movement of the links 23 and 24. As shown in Figure 3, the hydraulic device 22 has been expanded to its maximum extent, so that the pivots 26 are moved apart from one another and the linkage mechanisms 23, 24 thus result in the mould frame 12 being displaced upwards from the position of Figure 2. The central pivots 26 are moved apart from one another into a final slightly over-center disposition, shown somewhat exaggerated in Figure 3, so that the mechanism is substantially locked in the raised condition on maximum extension of cylinder 22 against any downward load on the mould frame 12. Release of the mechanism 21 from this locked over-center condition can only be achieved by contraction of the cylinder 22 to bring the central pivots 26 towards one another past their dead centres, at which time, downward movement of the mould frame 12 may progress. This locking arrangement thus ensures that when the mould frame 12 is in the upper position, it cannot be displaced downwardly when the clamping force is applied by the upper clamping member 8 above the window frame 4. Effectively therefore the raised mould frame 12 provides a substantially rigid base against which the downward clamping force can be exerted by member 8 to achieve the required fully effective seal at all points on the peripheries of the sheet 3, window frame 4 and mould frame upper edge region 13.

The system also includes electrical heating elements 31, 32 to be disposed respectively above (31) and below (32) the window frame 4 950717, - 10 or sheet locating area in operation of the system, to heat up the plastics sheet 3 and soften it preparatory to the vacuum forming operation. These heaters 32, 32 are displaceable forwardly and rearwardly, between an inoperative disposition towards the rearward region of the chamber 1, as shown for heater 31 in Figure 1, and a forward or active disposition in which they carry out their heating function, shown for heater 32 in Figure 1. In the actual installation, the upper and lower heaters 31, 32 are moved forward and back in unison, and the relative dispositions of these items shown in Figure 1 is not in fact normally feasible. Thus the positions shown in Figure 1 are indicated only for the purposes of explanation. Other structural and service features of the system not shown in the drawings provide for exhausting of the air from the vacuum chamber 1 and mould frame 12 and for supplying hydraulic pressure to the actuator 22 for the toggle clamp 21 for the mould frame 12 and for displacing the heaters 21, 22 forwardly and rearwardly. Electrical supplies to these suitably ceramic electric radiant heaters 31, 32 are also present. Finally, the chamber 1 is also provided with an air exhaust valve feature 41, further described in connection with a subsequent Figure.

In carrying out a vacuum forming operation according to the invention, a sheet of plastics 3 of appropriate thickness and quality is clamped in position on the window frame 4 by means of the overlying clamping mechanism 8, the operating mechanism for which is not shown. The ceramic electric radiant heaters 31, 32 are then advanced above and below the sheet of plastics 3, which is heated up. As shown in Figure 4, reference 3a, there is a tendency for the softened sheet of plastics 3 to sag in the profile shown. It is an important feature of all vacuum forming systems to minimise this sag 3a and to keep the sheet 3 as level as possible during the initial heating step, as shown in dotted outline in Figure 4. In one known arrangement, the chamber 1 exhausts freely to the ambient pressure externally of the system, so that the sheet 3 will sag 3a according as it is heated up. A photoelectric detector senses sag of the sheet and then boosts the air pressure within the chamber 1 above ambient to a sufficient extent as 950717: - 11 to restore the sheet to a substantially horizontal condition 3. This system presents difficulties in that injection of cooler ambient air into the heated environment of the chamber 1 is not desirable during the heating process, and there is also the potential problem of ensuring control stability, so that oscillatory displacement of the sheet between a sag disposition 3a and an upwardly extending bubble-shaped disposition may be avoided or prevented.

In order to overcome this problem, the enclosed air space or chamber 1 within which the vacuum mould 11 is accommodated is, according to the invention, provided with an exit air valve 41 of flap type. The chamber 1 is otherwise closed during the heating step against ambient air, by virtue of the window frame 4 periphery being sealed by the top clamp 8 and the interposed sealing strip 9. The flap valve 41 functions similarly to a non-return valve at the exit from a river into the sea or a lake. When the external pressure is higher than the pressure inside the chamber 1, the flap valve 41 remains closed. When an overpressure condition develops within the chamber 1, the flap valve 41 opens and relieves the air pressure within the chamber 1. A suitable valve is shown in Figure 5. This is a simple swinging pivoted flap 42, provided with a lever arm 43 on which a weight 44 can be placed at a suitable spacing from the pivot axis 45, to exert an appropriate closing force on the flap valve 41. The flap 42 is suitably mounted on a spigot-type exhaust passage 46 from the chamber 1. The pivot axis 45 may be defined by a pin passing through a lug portion 47 extending from the flap 42, the pin being also retained in one or more lug portions 48 extending from the side wall 49 of the chamber 1. The weight 44 is suitably provided with a hook 51 for suspending it at any required axial location along the length of the lever arm 43. The weight 44 may be of fixed or variable weight, with the hook structure 51 being suitable for detachable association with one or more additional weight members 52, so that the closing force exerted on the flap 42 may be varied not only by moving the weight 44 to and fro along the lever arm 43, but also by varying the value of the weight 44 itself. By adjustment of the closing force on 950717 - 12 the valve, the internal air pressure within the chamber 1 at which the valve flap 42 will pivot towards a valve opening condition may be likewise varied and regulated or controlled.

A further feature of the valve described in more detail in respect of its action in connection with the subsequent Figure 6 is an actuator mechanism 61, suitably a cylinder or solenoid having a piston rod or other operating member 62 with a head portion 63 urgable against the lever arm 43 to hold the flap 42 of the valve 41 closed as require during the operating process.

The system functions as follows. As heating of the sheet takes place, the air within the closed chamber 1 is also heated up and expands. This expansion of the air and the slightly increased pressure within the chamber 1 will lead to the flap valve 41 pivoting open when the overpressure is sufficient. The exact degree of overpressure at which the flap valve 41 will open is determined by placement of the weight 44 on the pivot arm 43, and/or the value of weight 44. Thus as pressure increases within the chamber 1, this pressure is sufficient to overcome the sag force under gravity of the softened sheet of plastics 3 and to maintain the sheet substantially level. However, if the chamber 1 was fully sealed without any facility for egress of air, when heated, the overpressure would become excessive, so that the sheet 3 would bubble up above the level disposition at an undesired stage of the process. By selecting an appropriate opening force for the valve 41, the exact level of overpressure at which the overpressure within the chamber 1 is relieved may be determined and this may be selected with sufficient precision for the softened sheet 3 to remain substantially level throughout the heating process. This simple and easily adjustable valve arrangement 41 has been found to be extremely effective in practice and to provide better results than more complex control systems.

At the termination of the heating phase, the heaters 31, 32 are withdrawn from their positions above and below the sheet 3 and the 950717^ - 13 mould 11 is brought up by action of the toggle or scissors lift device 21. The periphery of the mould frame 12 is clamped against the underside of the window frame 4 by displacement of the toggle feature 21 into the over-center disposition, Figure 3. In this disposition, mechanism 21 is effectively locked and a high degree of sealing is achieved against the underside of the window frame 4. There is then injection of air to bubble up the sheet 3 to a predetermined extent, as shown in Figure 6, this stretching the sheet 3 to a slight extent before the vacuum stage, so that the sheet 3 will take up in substantially perfect manner, the required complex profile of the mould 11. During this stage, the air valve 41 is held closed by extension of the actuator shaft or rod 62, which may be for example the piston rod of a hydraulic device or an extension of the moving portion of a solenoid, the actuating end 63 of this shaft or rod 62 pressing down on the lever arm 43 of the flap valve 41, to lock the valve flap 42 shut. Under these conditions, the chamber is effectively sealed, so that the necessary brief boost in the internal air pressure can readily be achieved to bring about the required upward push on the sheet 3 and adequate stretching of it for the vacuum forming step. Vacuum is then applied to pull the sheet 3 firmly down and stretch it into all the recesses and profile features 16 of the mould 12. A pre-stressed sheet is better able to follow the mould profile in a uniform manner without undue local variations in thickness.

The system of the invention overcomes therefore in an effective and efficient manner deficiencies in the prior art, in that it enables uniform and non-sag heating of the sheet during the initial phase of the vacuum forming process, and, by virtue of the mechanically elegant mould displacement arrangement, it also ensures high quality sealing against ingress of air during the vacuum forming step proper. These results and advantages of the invention are achieved by the technical features described herein.

Claims (5)

1. A vacuum forming system comprising (a) a chamber having an aperture which is sealingly occluded against the passage of air during a vacuum forming operation by a plastics sheet to be vacuum formed, (b) means for heating a plastics sheet occluding said aperture, (c) means for sealingly positioning a mould located within the chamber in proximity to a heated plastics sheet occluding said aperture against the passage of air between a region enclosing the mould and the remainder of the interior of the chamber, and (d) means for exhausting air from a region surrounding and underlying said mould, so that a heated plastics sheet occluding said aperture is applied to said mould by atmospheric pressure, wherein the chamber is provided with valve means for release of air from within said chamber during the heating of a said plastics sheet developed by said heating to minimise any departure by the softened heated plastics sheet from a substantially planar configuration.

2. A vacuum forming system according to Claim 1, wherein said valve means comprises a flap valve hinged in an upper edge region for pivoting opening movement for outward flow of air from the chamber and closing movement under the action of gravity, said flap valve having an arm extending from outwardly from said flap for mounting at least one weight at a selected spacing from the axis of pivoting of the flap for enhancing the gravity-actuated valve-closing action, means are provided for maintaining said valve means in a closed condition at least during a period following heating of the plastics sheet and preparatory to the air exhausting phase of a vacuum forming operation, said means being defined by an actuator for engaging said arm of the flap valve during said period to prevent opening movement of the flap, and said means for sealingly positioning a mould located within the chamber in proximity to a heated plastics sheet occluding said aperture against the passage of air between a region enclosing the mould and the remainder of the 8507 1 7 - 15 interior of the chamber comprises a mechanism for displacing the mould between a first lowered disposition within the chamber and a second raised sealing disposition, said mechanism being lockable in the raised sealing disposition to withstand any downward forces resulting from the sealing occlusion of said aperture by the plastics sheet.

3. A vacuum forming system comprising (a) a chamber having an aperture which is sealingly occluded against the passage of air during a vacuum forming operation by a plastics sheet to be vacuum formed, (b) means for heating a plastics sheet occluding said aperture, (c) means for sealingly positioning a mould located within the chamber in proximity to a heated plastics sheet occluding said aperture against the passage of air between a region enclosing the mould and the remainder of the interior of the chamber, and (d) means for exhausting air from a region surrounding and underlying said mould, so that a heated plastics sheet occluding said aperture is applied to said mould by atmospheric pressure, wherein said means for sealingly positioning a mould located within the chamber in proximity to a heated plastics sheet occluding said aperture against the passage of air between a region enclosing the mould and the remainder of the interior of the chamber comprises a mechanism for displacing the mould between a first lowered disposition within the chamber and a second raised sealing disposition, said mechanism being lockable in the raised sealing disposition to withstand any downward forces resulting from the sealing occlusion of said aperture by the plastics sheet.

4. A vacuum forming system according to Claim 2 or Claim 3, wherein said mechanism is a double toggle mechanism comprising two pairs of links pivoted to one another and located between the mould and the floor of the chamber, the upper link of each pair being pivotably associated with the mould and the lower link of each pair being pivotably associated with the floor, and the mutual pivot points of the 950717 - 16 two links being interconnected by an actuator, so that the mould is moved from the lowered disposition to the raised disposition by expansion of the actuator, and the expanded condition of the actuator corresponds to over-centre positions of the pairs of pivoted links, so that the mould is locked in the raised sealing disposition while the actuator remains in the expanded condition.

5. A vacuum forming system substantially as described herein with reference to and as shown in the accompanying drawings.