WO2009000032A1

WO2009000032A1 - Pressure thermoforming process

Info

Publication number: WO2009000032A1
Application number: PCT/AU2008/000921
Authority: WO
Inventors: Nicholas Barson Mebberson
Original assignee: Nicholas Barson Mebberson
Priority date: 2007-06-27
Filing date: 2008-06-26
Publication date: 2008-12-31

Abstract

The present invention relates to a pressure thermoforming process and a pressure thermoforming apparatus for thermoforming plastic film into thin-walled products. The pressure thermoforming process includes the steps of transporting a plastic film between a mould body and a platten, the mould body having one or more mould cavities and at least one cutter, and the platten being arranged in a facing relationship with the mould cavities; moving the mould body and platten together to clamp the plastic film between at least one cutter and the platten at a clamping force; heating the plastic film to a softening temperature; and pressure forming the plastic film into the shape of the mould cavities. The mould body and platten in the process can be relatively moved to provide at least two different clamping forces on the plastic film. Each of the clamping forces are selected to be greater than the minimum force necessary to clamp the plastic film between at least one cutter and the platten and less than the minimum force which allows the cutter to substantially cut through the plastic film.

Description

PRESSURE THERMOFORMING PROCESS

Field of the Invention

The present invention generally relates to a pressure thermoforming process. The invention is applicable for use in pressure thermoforming machinery for thermoforming plastic film into thin-walled products. It will therefore be convenient to hereinafter disclose the invention in relation to that exemplary application, although it is to be appreciated that the invention is not limited thereto.

Background of the Invention

The following discussion of the background to the invention is intended to facilitate an understanding of the invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of the application.

Pressure thermoforming is a moulding process in which a pressurised fluid, typically compressed air, is used to deform a heated flat thermoplastic sheet into a more complex three-dimensional shape. The process equipment typically includes a die body having one or more mould cavities, trimming knives which extend about the mould cavities and a heated cutting platten arranged in a facing relationship with openings of the mould cavities in the die body. The die body and cutting platten are relatively movable toward and away from one another.

In a contact heat pressure thermoforming process, the plastic film is transported in successive steps along a path between the die body and cutting platten during operating cycles of the process. In each operating cycle, the die body and cutting platten are relatively moved toward one another until a section of film is clamped between the cutting platten and trimming knives so as to create a seal therebetween. Compressed air ("heating air") is injected into the mould cavities at a pressure of about 1 bar forcing the film into direct contact with the cutting platten. The heating air is then vented. The film is either preheated or heated through the cutting platten so as to heat soften the film section. Pressurised "forming air" is then introduced through small delivery ports in the cutting platten at a pressure of about 4 bar to rapidly blow the heat film into the contours of the mould cavities to adopt the shape thereof.

The now formed portions of the film are cooled through contact with the mould cavities and the forming air vented from the mould cavities. The die body and cutting platten are then again relatively moved toward one another to allow the trimming knives to cut entirely through the film section to separate the formed products from the film.

The die body and cutting platten are then moved apart, and compressed air is forced between the formed products and the mould cavities to eject the products therefrom. As the products clear the cavities, the film is transported forward along the path carrying the formed and trimmed products clear of the die body and cutting platten, and a sequential section of film is introduced between the die body and the cutting platten to commence a new cycle.

Traditionally, the cutting platten is raised and lowered using a mechanical toggle arrangement driven by a servomotor. The toggle arrangement is mechanically configurable to provide a force setting through a positional setting driven by a servomotor during the operating cycle. A first positional setting provides a clamping force to clamp the film between the platten and knives. A second positional setting provides a cutting force to cut through the film. As can be appreciated, the use of a single clamping force is not optimal, because the selected clamping force must accommodate several process factors over a number of operational steps of the pressure thermoforming process. At minimum, the clamping force must be greater than the force produced by the heating air and the force produced by the forming air in order to maintain a seal between the cutting platten and trimming knives, and also be less than the minimum force required to cut through the film. The selection of such a force within such rigid limitations allows for little flexibility in accommodating changes in process steps or operational variations.

In addition, the clamping force in a traditional mechanical toggle system is determined by mechanical positioning of the toggle mechanism using a servomotor. Accordingly, obtaining a precise force setting is difficult because by its nature the toggle force setting is non-linear having a range where small position changes in the high mechanical advantage region result in large force changes. Force settings in mechanical toggle systems are also susceptible to changes in temperature. Changes in the ambient temperature during the day can vary the dimensions of the toggle arrangement and therefore change the force setting.

It would therefore be desirable to provide a pressure thermoforming process which provides greater operational flexibility during the processing steps, in particular during heating air and forming air operation steps, of a pressure thermoforming machine.

Summary of the Invention

According to the present invention, there is provided a pressure thermoforming process including the steps of: transporting a plastic film between a mould body and a platten, the mould body having one or more mould cavities and at least one cutter, and the platten being arranged in a facing relationship with the mould cavities; moving the mould body and platten together to clamp the plastic film between at least one cutter and the platten at a clamping force; heating the plastic film to a softening temperature; and pressure forming the plastic film into the shape of the mould cavities, wherein the mould body and platten can be relatively moved to provide at least two different clamping forces on the plastic film, each of the clamping forces being selected to be greater than the minimum force necessary to clamp the plastic film between at least one cutter and the platten and less than the minimum force which allows the cutter to substantially cut through the plastic film.

The pressure thermoforming process according to the present invention therefore includes the application of at least two different clamping forces between the cutter(s) and the platten. Each clamping force is preferably selected to optimise clamping and sealing properties required at the particular stage in the process and substantially prevent the at least one cutter cutting the plastic film. In this respect, the first clamping force can be optimised for the step of feeding fluid into the mould cavities in order to force the plastic film into direct contact with the platten, while substantially preventing the at least one cutter cutting the plastic film. Similarly, the second clamping force can be optimised for the step of feeding fluid through the platten to mould the plastic film into the shape of the mould cavities while substantially preventing the at least one cutter cutting the plastic film. Accordingly, the use of at least these two different clamping forces provides more flexibility in the selection of a clamping force for steps during the operating cycle of the pressure thermoforming process, and substantially prevents damage to the plastic film until the at least one cutter is preferably used in a subsequent cutting step to cut the plastic film.

In one preferred embodiment, the pressure thermoforming process according to the present invention utilises fluid, preferably compressed air, to pressure form the product. In this embodiment, the process can include the further steps of: moving the mould body and platten together to clamp the plastic film between at least one cutter and the platten at a first clamping force; feeding fluid into the mould cavities in order to force the plastic film into direct contact with the platten; moving the mould body and platten together to clamp the plastic film between at least one cutter and the platten at a second clamping force; and feeding fluid through the platten to mould the plastic film into the shape of the mould cavities. Accordingly, the use of at least two different clamping forces can allow a clamping force to be selected to accommodate the specific pressure and force demands of each of these individual steps (heating air and forming air) while substantially preventing the at least one cutter cutting the plastic film.

In those embodiments of the process in which fluid is used, the clamping force is selected to be high enough to counteract the force ("pressure force") within the mould cavities provided by the application of feeding fluid (typically compressed air) into the mould cavities, but not great enough to cut through the plastic film and thereby wreck the seal. In this respect, the two clamping forces can be selected such that the first clamping force is greater than the pressure force created by feeding fluid into the mould cavities in order to force the plastic film into direct contact with the platten and less than the minimum force which allows the cutter to cut through the plastic film; and the second clamping force is greater than the pressure force created by feeding fluid through the platten to mould the plastic film into the shape of the mould cavities and less than the minimum force which allows the cutter to cut through the plastic film.

During processing, it is preferable for the second clamping force to be greater than the first clamping force. Of course, this is typically a result of process conditions where the application of heating air injected into the mould cavities to force the plastic film into direct contact with the cutting platten is typically at a lower pressure than the forming air. In this respect, heating air is usually injected at a lower pressure than the forming air to substantially prevent the heated plastic film from obtaining an imprint of the platten on the side abutting the platten. As should be appreciated, such an imprint could produce undesirable imperfections on the inner surface of the pressure thermoformed product. In a preferred embodiment, heating air is fed into the mould cavity at a pressure of 1 to 2 bar and the forming air is fed into the mould cavity at a pressure of 3 to 4 bar. Accordingly, the first clamping force must be greater than the pressure force exerted by a fluid pressure of between 0.5 bar and 4 bar within the mould cavities and the second clamping force must be greater than the pressure force exerted by a fluid pressure of greater than 4 bar within the mould cavities. In this regard, the actual magnitude of the clamping force producing the reaction force must be sufficient to oppose the force created by the pressure on the face area of the tool/mould. As should be appreciated, the clamping force should be at least equal to the force created by the pressure within the tool (heat/form) multiplied by the face area of the tool. Of course, the clamping force can be altered as required to suit various process requirements in the pressure thermoforming process as it can be appreciated that the pressures used may change over time (ramp up or down). In practice, these variations would preferably not be step changes.

Further clamping forces, in addition to the first and second clamping force, can be applied between the at least one cutters and platten. In some cases, a third, fourth, fifth or more clamping force can be used in order to accommodate different operations during the pressure thermoforming process. Preferably, the clamping forces also include at least a third force which allows the cutter to cut through the plastic film.

Various clamping forces can be used during the various process steps in the pressure thermoforming process. During the process steps, it is preferable for the clamping force on the plastic film between the mould body and platten to be altered in response to changes in the pressure thermoforming process. In this way, the process of the present invention allows an optimal clamping force to be selected for each step in the operating cycle of the process.

In other embodiments, the clamping force includes a plurality of settings which are used to accommodate and compensates for change in pressure between the cutter and the platten. In this sense, the process can be configured with the clamping forces being reactionary to changes in the forces between the cutters and platten. Of course in most processes according to the present invention, the greatest force changes occur during the steps of feeding fluid into the mould cavity. Consequently, it is preferable for the clamping force between the cutter and the platten to be moved from the first clamping force to the second clamping force as soon as fluid is fed through the platten to mould the plastic film into the shape of the mould cavities. Preferably, this change in clamping force occurs simultaneously with (and is reactionary to) the pressure changes between the mould body and the platten.

A number of different actuation devices can be used to actuate relative movement (together and apart) of the mould body and cutting platten. In some embodiments, the actuation device includes a servomotor and a mechanical toggle arrangement is used. A mechanical toggle can be used as it can be extended and locked under little load by a small actuation force and can then resist the high clamp and cut forces with little or no force required by the toggle actuation mechanism to resist it. In other embodiments, the actuation device includes at least one of a screw arrangement, mechanical wedge or a hydraulic means such as for example a hydraulic cylinder. Preferably, the steps of moving the mould body and cutting platten together are accomplished using an actuation device which includes a hydraulic means. In this regard, the actuation device could comprise only hydraulic means, such as for example one or more hydraulic cylinders. In other embodiments, the actuation device can comprise a combination of devices, one or more of which could include hydraulic means. For example, in one particular embodiment the actuation device includes a mechanical lifting means to actuate rapid, broad relative movement between the platten and mould body and a hydraulic means to actuate smaller, specific relative movement between the platten and mould body. Preferably, the mechanical lifting means includes a motor and mechanical toggle arrangement. Preferably, the hydraulic means includes at least one hydraulic cylinder.

The process of the present invention is intended to be suitable to the various types of pressure thermoforming processes presently used in industry. In this respect, the method according to the present invention is applicable to pressure thermoforming processes wherein the platten is heated prior to feeding fluid into the mould cavities in order to force the plastic film into direct contact with the platten. The method according to the present invention is also intended to be applicable to pressure thermoforming processes where the plastic film is heated prior to being transported between the mould body and the platten. It is preferable for the process to include at least one step where the moulded section of the plastic film is substantially separated from the unmoulded sections of the plastic film. In this respect, it is preferable after moulding the plastic film into the shape of the mould cavities that the process further includes the steps of: moving the mould body and platten together to clamp the plastic film between at least one cutter and the platten at a cutting force which allows the cutter to cut through the plastic film; and moving the mould body and platten apart to release the plastic film from between the at least one cutter and the platten.

These steps separate one or more pressure thermoformed products from the plastic film and allow the product to be extracted for further processing or packaging. In some embodiments, the cutters do not provide a complete through-cut through the plastic film, but rather leave small connection or bridging sections between the plastic film and the pressure thermoformed product. These bridging sections allow the pressure thermoformed product to be transported with the plastic film in successive steps out from between the die body and cutting platten to a further processing section during the operating cycles of the process.

According to another aspect of the present invention there is provided a pressure thermoforming apparatus including: a mould body having one or more mould cavities and at least one cutter; a platten arranged in a facing relationship with the mould cavities; and an actuation device for actuating relative movement between the mould body and cutting platten, wherein the actuation device is configured to move the mould body and platten together to clamp a plastic film between at least one cutter and the platten at at least two different clamping forces, each of the clamping forces being selected to be greater than the minimum force necessary to clamp the plastic film between at least one cutter and the platten and less than the minimum force which allows the cutter to substantially cut through the plastic film. Again, it is preferable for the cutters to be able to cut through the plastic film when the plastic film is located between the mould body and the platten. In this respect, the actuation device can also be configured to move the mould body and platten together to clamp the plastic film between at least one cutter and the platten at a cutting force which allows the cutter to cut through the plastic film.

It can be advantageous in some embodiments for the actuation device to be operated to provide a clamping force that accommodates and compensates for changes in pressure between the cutter and the platten. Any plastic film located between the cutter and the platten can therefore be clamped therein by a substantially constant clamping force. In one embodiment, the actuation device includes a control device operatively associated with a force sensing device, such as a strain gauge or load cell, measuring the clamping force, the control device operating the actuation device in reaction to changes in the clamping forced sensed by the force sensing device.

A number of different actuation devices can be used to actuate relative movement (together and apart) of the mould body and cutting platten. In some embodiments, the actuation device includes a mechanical wedge. In other embodiments, the actuation device includes a screw arrangement. In one preferred embodiment, the actuation device includes a hydraulic means. As should be appreciated, the actuation device could include only hydraulic means such as for example a series of hydraulic cylinders or be a combination of different actuators. In one preferred embodiment, the actuation device includes a mechanical lifting means for rapid, broad relative movement between the platten and mould body and a hydraulic means for smaller, specific relative movement between the platten and mould body. Preferably, the hydraulic means includes a hydraulic cylinder.

In some embodiments, the clamping force is set by an actuation means that does not involve setting the force by mechanical positioning. In this respect, it is preferable for the actuation means to include a mechanical wedge, screw arrangement or air/hydraulic cylinder which is used to set the clamping force of the pressure thermoforming apparatus.

Brief Description of the Drawings

The present invention will now be described with reference to the figures of the accompanying drawings, which illustrate particular preferred embodiments of the present invention, wherein:

Figure 1 illustrates a pressure thermoforming device which can be operated in accordance to one preferred embodiment of the method according to the present invention.

Figure 2 provides a series of schematic diagrams illustrating a pressure thermoforming process in accordance to one preferred embodiment of the present invention.

Figure 3 shows an eccentric crank and hydraulic cylinder combination which can be used in a pressure thermoforming device shown in Figure 1.

Figure 4 shows an eccentric crank, toggle and hydraulic cylinder combination which can be used in a pressure thermoforming device shown in Figure 1.

Figure 5 shows three different arrangements (5A, 5B and 5C) of the actuator which can be used in a pressure thermoforming device shown in Figure 1.

Detailed Description

Illustrated in Figure 1 is one embodiment of a pressure thermoforming apparatus 10 which can be operated in accordance with the pressure thermoforming process according to the present invention. The illustrated apparatus 10 is a contact heat thermoforming machine which houses a number of integrated and operatively associated processing stations. The illustrated apparatus 10 includes an external plastic film roll lifter and feeder station 12, and an enclosed machinery housing 13 containing a film preheating station 14, pressure thermoforming station 15, product separation station 16 and scrap feed winder 18. An externally mounted operator control panel/pendant 20 is provided for controlling each station and also to monitor the operating parameters from each station. Compressed air is fed to the pressure thermoforming station 15 from pressure vessel 22 located above this station 15 and movement of the components in the pressure thermoforming station 15 are actuated by an actuation device 22 housed underneath the pressure thermoforming station 15.

In operation, a roll of plastic film 26 is mounted on a spindle of the feeder station 12 and lifted using hydraulic cylinders 30 to a preferred operation height H. Plastic film from the roll 26 is then fed through forward guide rollers 34 and into the machinery housing 13 through a front feed aperture 36. The plastic film passes under the film preheating station 14 where radiant heaters (not shown) heat the film to a particular temperature that softens the plastic film 50. It should be appreciated that this step is optional, and that the plastic film can be heated within the pressure thermoforming station 15. The plastic film is then feed through the pressure thermoforming station 15 where the plastic film is pressure thermoformed into a desired moulded shape (as will be described in more detail later in the specification), and the plastic film around each moulded product (not illustrated) is substantially cut to assist in releasing the formed product from the film. It should be understood that the cut around the moulded product is not a complete through-cut, but rather is only substantially complete leaving several uncut or bridging portions between the moulded product and film so as to allow the moulded product to be transported through and out of the pressure thermoforming station 15 with the sequential movement of the plastic film through the various remaining stations. The plastic film and moulded product then passes into the product separation station 16 where the moulded product is separated from the unmoulded section of the plastic film. This is typically achieved by pulling the unmoulded section of the plastic film at a sharp angle (typically 90 degrees downwardly or more) away from the moulded product so as to break the bridging portions along the cut between the moulded product and the film. The unmoulded section of the plastic film is then wound around a further scrap winding spindle 38 in the scrap feed winder 18 section. As can be appreciated, the sequential movement of the plastic film though the various stations between the feed spindle 28 and scrap winding spindle 38 moves the plastic film and moulded product through the machine 10.

Referring now to Figure 2, there is shown a series of figures (a) to (e) illustrating the various stages involved in a pressure thermoforming process according to one particular embodiment of the present invention. The illustrated figures provide a schematic view of the major components of a pressure thermoforming station 15 similar to that used in the apparatus shown in Figure 1.

Referring firstly to Figure 2(a) it can be seen that the heat thermoforming station 15 includes a die body 40 and a cutting platten 41.

The illustrated die body 40 comprises a metallic body having an upper plate 44 from which extends two mould cavities 42 and a number of trimming knives 46. The illustrated mould cavities 42 have a frustoconical shape used to form frustoconical shaped tray containers 54. It should however be appreciated any suitable mould shape could be used. The upper plate 44 of the die body 40 includes a number of smaller feed air apertures (not illustrated) through which compressed air can be fed. The trimming knives 46 extend perpendicularly from the upper plate 44 about and around the mould cavities 42. Each trimming knife 46 has a sharpened distal end 48 which is used to clamp a plastic film 50 between the cutting platten 41 and knife 46 and also cut a moulded product 54 from the plastic film 50.

The cutting platten 41 comprises a generally flat plate including a heating arrangement (not illustrated) and a plurality of air feed apertures (not illustrated) through which compressed air can be fed. The cutting platten 41 is arranged in a facing relationship with the openings of the mould cavities 42 in the die body 40. The die body 40 and cutting platten 41 are relatively movable toward and away from one another through the operation of one or more actuation devices (not shown in Figures 1 or 2). Examples of suitable actuation devices are shown in Figures 3, 4 and 5 and will be described in more detail later in the specification. In the illustrated apparatus 10 (Figure 1 ), the die body 40 is fixed in position, and the cutting platten 41 is raised towards and lowered away from the die body by the actuation device.

During operation of the pressure thermoforming apparatus 10 (shown in Figure 1 ), the plastic film 50 is transported in successive steps along a path between the die body 40 and cutting platten 41 during operating cycles of the process. These steps will now be described with reference to Figures 2(a) to

2(e):

As shown in Figure 2(a), at the start of each operating cycle a flat sheet of plastic film 50 is transported between the die body 40 and cutting platten 41. The actuation device is then operated to raise the cutting platten 41 toward the die body 40 until a section of plastic film 50 is clamped between the cutting platten 41 and trimming knives 46 at a first clamping force (F_Ciam_Pi) so as to create a seal therebetween. As shown by arrows H, compressed air ("heating air") is injected into the mould cavities through the apertures in the top plate 44 at a pressure of around 1 bar to force/clamp the plastic film 50 into direct contact with the cutting platten 41. As should be appreciated, feeding compressed air into the mould cavities 42 exerts a pressure force on the cutting platten 41 which must be resisted by the first clamping force (F_Ciam_Pi) in order to maintain the seal between the cutting platten 41 , plastic film 50 and trimming knives 46. Accordingly, the first clamping force (F_Ciam_Pi) is selected to be greater than the pressure force created by the heating air, in this case around 1 bar, but less than the minimum force needed to cut through the plastic film 50. It should be appreciated that the minimum force is dependent on the sharpness of the trimming knives 46 and the material property of the plastic film 50. In the illustrated embodiment, the first clamping force (F_C|_amp1) is used to provide a pressure equivalent to around 2 bar. The plastic film 50 can be either preheated prior to entering between the cutting platten 41 and die body 40 (i.e. a non-contact process), or as occurs in the illustrated embodiment, is heated through contacting the cutting platten 41 which is heated to a temperature which soften the plastic film 50 (i.e. a contact process).

As shown by the arrows F in Figure 2(b), pressurised "forming air" is then introduced through a plurality of apertures in the cutting platten 41 at a pressure of around 4 bar to rapidly blow the heat plastic film 50 into the mould cavities to adopt the shape thereof. Simultaneous with the application of the forming air, the actuation device is operated to raise the cutting platten 41 further toward the die body 40 to clamp the plastic film 50 between the cutting platten 41 and trimming knives 46 at a second clamping force (F_Ciam_P2) so as to maintain a seal therebetween. The second clamping force (F_Ciam_P2) is selected to be greater than the pressure force created by the forming air, in this case around 4 bar, but be less than the minimum force needed to cut through the plastic film 50. Again, it should be appreciated that the minimum force is dependent on the sharpness of the trimming knives 46 and the material property of the plastic film 50. In the illustrated embodiment, the first clamping force (F_Ciam_P2) is used to provide a pressure equivalent to around 5 bar.

Subsequently, the moulded portions 54 of the plastic film 50 are cooled and the forming air vented from the mould cavities 42 through the plurality of apertures in the cutting platten 41.

As the flow of forming air is vented, the actuation device can in some embodiments be operated to move the cutting platten 41 away from the die body 40 to reduce the clamping force between the plastic film 50, cutting platten 41 and trimming knives 46. If this occurs, a third clamping force (F_Ciam_P3) can be selected which maintains a seal therebetween and is less than the minimum force needed to cut through the plastic film 50.

As shown in Figure 2(c), the actuation device is operated to raise the cutting platten 41 further toward the die body 40 to exert a further clamping force (Fcut) to cut the plastic film 50 between the cutting platten 41 and trimming knives 46. In this step, the trimming knives 46 cut through the plastic film 50 to separate the moulded products 54 from the plastic film 50. As mentioned previously, the trimming knives 46 are configured to leave bridging portions between the plastic film 50 and moulded sections 54. This can be achieved by the trimming knives 46 having one or more notches along the length of the knife 46. The third clamping force (F_cut) is selected to be greater than the minimum force needed to cut through the plastic film 50.

Referring now to Figure 2(d), it can be seen that the die body 40 and cutting platten 41 are then moved apart, and if necessary, as shown by arrows R, compressed air is forced between the moulded sections 54 and the mould cavities 42 to eject the moulded sections 54 therefrom. As the moulded sections 54 clear the cavities, the plastic film 50 is transported forward along the path carrying the moulded sections 54 clear of the die body 40 and cutting platten 41 , and as shown in Figure 2(e) a new sequential section of plastic film 50 is introduced between the cutting platten 41 and die body 40 to commence a new cycle of the pressure thermoforming process.

It should be appreciated that the magnitude of the forming pressure that can be used determines the speed of the forming step. In this regard, a 6 bar forming pressure provides a better and faster forming process than a 4 bar forming pressure. For some processes, it can take 1 second @ 4 bar to pressure form a product as compared to 0.7 seconds at 6 bar. It is therefore desirable to use as high as possible forming air pressure to optimise operating cycle times. Accordingly, an apparatus in which the clamping force can be maximized allows for a higher forming air pressure to be used, for example forming air pressures of 6, 7, 8 or more bar. Such pressures and associated clamping forces can be problematic for traditional mechanical toggle arrangements. However, as will now be described, actuation devices including a hydraulic component can be advantageously used to provide these types of clamping forces. Referring now to Figures 3 and 4, there is shown two different actuation devices 60 and 70 which can be used to actuate movement of the cutting platten 41 relative to the die body 40.

The actuation device 60 shown in Figure 3 includes two operatively cooperative actuation means 62 and 64. The first actuation means is a servomotor (not illustrated) driven eccentric crank 62 which operates about an eccentric pivot point 65A to provide fast movements to raise and lower the platten 41 towards and away from the die body 40. The second actuation means is a hydraulic cylinder 64 which is used to for more precise relative movements required when varying the clamping force between the trimming knives 46 and the cutting platten 41. The piston rod 65 of the hydraulic cylinder 64 is connected to the cutting platten 41 through a pivot connection 66.

The actuation device 70 shown in Figure 4 similarly includes two operatively co-operative actuation means 72 and 64. The first actuation means is a servomotor (not illustrated) driven crank 72 which operates about an pivot point 75 to actuate a mechanical toggle 77 which is linked to the cutting platten 41. Operation of the crank 72 raises or lowers the cutting platten 41 through the mechanical toggle 77. Again, the crank 72 provides fast movements to raise and lower the platten 41 towards and away from the die body 40. The second actuation means is a hydraulic cylinder 74 which is used to for more precise relative movements required when varying the clamping force between the trimming knives 46 and the cutting platten 41. The piston rod 75 of the hydraulic cylinder 74 is connected also linked to the mechanical toggle 77 in order to move the cutting platten 41.

In each case, the clamping force provided by the hydraulic component 64, 74 of the actuation device 60, 70 can be operated to be reactionary to changes in pressure between the trimming knives 46 and the platten 41. In this respect, a hydraulic control device can be included in the hydraulic circuit of the respective hydraulic cylinder 64, 74 to maintain a constant force setting between the trimming knives 46 and the platten 41. Any operational variations would be compensated. In addition, automatic control of the actuation device 60, 70 and in particular the hydraulic component 64, 74 thereof can allow for more accurate and easy force setting adjustments than is currently possible in the traditional mechanical toggle arrangements used to actuate movement between the platten 41 and die body 40.

Referring now to Figure 5, there is shown three further embodiments of the actuation device 8OA, 8OB and 8OC which can be used to which can be used to actuate movement of the cutting platten 41 relative to the die body 40.

Like components in Figure 5 have been given the same reference numerals as used in relation to the same components shown in Figures 1 to 4.

Referring firstly to Figure 5A and 5B, it can be observed that each device 8OA and 8OB includes two operatively co-operative actuation means 74 and 77. The first actuation means is a servomotor 72 driven mechanical toggle arrangement 77 which provides fast movements to raise and lower the platten 41 towards and away from the die body 40 as shown by arrow J. The second actuation means is a hydraulic cylinder 74 which is used for more precise relative movements required when varying the clamping force between the trimming knives 46 and the cutting platten 41 as shown by arrow I. In the embodiment shown in Figure 5A, the hydraulic cylinder 74 directly moves the die body 40. In the embodiment shown in Figure 5B, the hydraulic cylinder 74 moves the platform 81 on which the mechanical toggle 77 is positioned, thereby indirectly moving the position of the cutting platten 41. The gross mechanical position of the die body 40 can be adjusted through positional adjustment of the supporting platform 82 as shown by arrow H. It should be appreciated that the servomotor 72 driven mechanical toggle arrangement 77 in each of these figures could be equally be replaced using a hydraulic cylinder without any alteration to the function of the embodiment.

It should be also appreciated that in other embodiments, one or more hydraulic cylinders (not illustrated) could be operatively connected to the cutting platten 41 in order to actuate movement of the cutting platten 41. The one or more hydraulic cylinders could be operated to provide both the fast movements and for more precise relative movements when relatively moving the die body 40 and the cutting platten 41. For example Figure 5C shows one embodiment where one hydraulic cylinder 84 is used to provide fast movements to raise and lower the platten 41 towards and away from the die body 40 as shown by arrow J and also for more precise relative movements required when varying the clamping force between the trimming knives 46 and the cutting platten 41 as shown by arrow I.

The process of the present invention therefore allows two or more clamping forces to be used to clamp the plastic film 50 between the trimming knives 46 and the cutting platten 41. The use of at least two different clamping forces (F_C|_ampi and F_C|_amp2) allows a user to select the most appropriate clamping force for each operational step. In comparison, traditional process arrangements only allowed for a single clamping force to be used which therefore had to be selected to accommodate all the various force demands during every operational step of the process. A compromise would therefore have to be made in order to suit all the operational parameters of the different process steps. The process of the present invention therefore allows for greater flexibility and optimisation of the operational steps of the pressure thermoforming process. In this respect, it should be appreciated that the apparatus can include a control system that can calculate and predict and then accurately control (for example using proportional regulators) the optimal clamping forces.

Advantageously, when a hydraulic actuator is included in the actuation arrangement for actuating movement of the cutting platten 41 relative to the die body 40 the described pressure thermoforming apparatus is: • Easy and fast to setup as clamp forces can be set via a Control System (e.g. Regulators) rather than mechanical positioning as is presently used in mechanical toggle type arrangements; • When using an automated control system, the apparatus can predict and limit/control clamping/cutting forces so as not to overload the cutting knives damaging them and reducing operational life. In this regard, it should be appreciated that different knife linear lengths can cope with different cut forces. For example, two metres of knife can accommodate twice the total cut force (with the same force per unit length) as one metre of knife. The maximum allowable force before knife damage is related to the linear length of the knife;

• Provides a consistent clamp force setting which is not significantly effected by mechanical changes to the machine, or external temperature; and

• Provides the ability to use a plurality of clamp forces which means that apparatus can clamp / hold / use successfully (relatively) higher forming pressures and therefore provide faster operation cycle times.

It should be understood that the above described process can be used to pressure thermoform a variety of thermoplastic films including but not limited to Acrylonitrile Butadiene Styrene (ABS), Polyethylene Terephthalate Glycol (PETG), Styrene, Polycarbonate, Polypropylene, Kydex®, Poly Vinyl Chloride (PVC), Polystyrene, APET, thermoformable biopolymers such as Plantic™ and Natureworks (a polylactic acid)or the like.

It should also be understood that while not explicitly described in the foregoing description, the present invention is also applicable to non-contact heat thermoforming machines and also plug assisted thermoforming processes.

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is understood that the invention includes all such variations and modifications which fall within the spirit and scope of the present invention.

Throughout the description and claims of the specification the word "comprise" and variations of the word, such as "comprising" and "comprises", is not intended to exclude other additives, components, integers or steps.

Claims

CLAIMS:

1. A pressure thermoforming process including the steps of: transporting a plastic film between a mould body and a platten, the mould body having one or more mould cavities and at least one cutter, and the platten being arranged in a facing relationship with the mould cavities; moving the mould body and platten together to clamp the plastic film between at least one cutter and the platten at a clamping force; heating the plastic film to a softening temperature; and pressure forming the plastic film into the shape of the mould cavities, wherein the mould body and platten can be relatively moved to provide at least two different clamping forces on the plastic film, each of the clamping forces being selected to be greater than the minimum force necessary to clamp the plastic film between at least one cutter and the platten and less than the minimum force which allows the cutter to substantially cut through the plastic film.

2. A pressure thermoforming process according to claim 1 , further including the steps of: moving the mould body and platten together to clamp the plastic film between at least one cutter and the platten at a first clamping force; feeding fluid into the mould cavities in order to force the plastic film into direct contact with the platten; moving the mould body and platten together to clamp the plastic film between at least one cutter and the platten at a second clamping force; and feeding fluid through the platten to mould the plastic film into the shape of the mould cavities.

3. A pressure thermoforming process according to claim 2, wherein the second clamping force is greater than the first clamping force.

4. A pressure thermoforming process according to claim 2 or 3, wherein: the first clamping force is greater than the pressure force created by feeding fluid into the mould cavities in order to force the plastic film into direct contact with the platten and less than the minimum force which allows the cutter to cut through the plastic film; and the second clamping force is greater than the pressure force created by feeding fluid through the platten to mould the plastic film into the shape of the mould cavities and less than the minimum force which allows the cutter to cut through the plastic film.

5. A pressure thermoforming process according to claim 4, wherein the first clamping force is greater than the pressure force exerted by a fluid pressure of between 0.5 bar and 4 bar in the mould cavities and the second clamping force is greater than the pressure force exerted by a fluid pressure of greater than 4 bar in the mould cavities.

6. A pressure thermoforming process according to any one of claims 2 to 5, wherein the clamping force between the cutter and the platten is moved from the first clamping force to the second clamping force as soon as fluid is fed through the platten to mould the plastic film into the shape of the mould cavities.

7. A pressure thermoforming process according to any one of the preceding claims, wherein the clamping force on the plastic film between the mould body and platten is altered in response to changes in the pressure thermoforming process.

8. A pressure thermoforming process according to claim 7, wherein the clamping force between the cutter and the platten is reactionary to changes in pressure between the cutter and the platten.

9. A pressure thermoforming process according to any one of the preceding claims, wherein the steps of moving the mould body and cutting platten together are accomplished using an actuation device including at least one of a screw arrangement, mechanical wedge or a hydraulic means.

10. A pressure thermoforming process according to claim 9, wherein the actuation device includes a mechanical lifting means to actuate rapid, broad relative movement between the platten and mould body and a hydraulic means to actuate smaller, specific relative movement between the platten and mould body.

11. A pressure thermoforming process according to claim 10, wherein the mechanical lifting means includes a motor and mechanical toggle arrangement.

12. A pressure thermoforming process according to claim 9, 10 or 11 , wherein the hydraulic means includes at least one hydraulic cylinder.

13. A pressure thermoforming process according to any one of the preceding claims, wherein after moulding the plastic film into the shape of the mould cavities further including the steps of: moving the mould body and platten together to clamp the plastic film between at least one cutter and the platten at a cutting force which allows the cutter to cut through the plastic film; and moving the mould body and platten apart to release the plastic film between the at least one cutter and the platten.

14. A moulded product produced from a pressure thermoforming process according to any one of the preceding claims.

15. A pressure thermoforming apparatus including: a mould body having one or more mould cavities and at least one cutter; a platten arranged in a facing relationship with the mould cavities; and an actuation device for actuating relative movement between the mould body and cutting platten, wherein the actuation device is configured to move the mould body and platten together to clamp the plastic film between at least one cutter and the platten at at least two different clamping forces, each of the clamping forces being selected to be greater than the minimum force necessary to clamp the plastic film between at least one cutter and the platten and less than the minimum force which allows the cutter to substantially cut through the plastic film.

16. A pressure thermoforming apparatus according to claim 15, wherein the actuation device is also configured to move the mould body and platten together to clamp the plastic film between at least one cutter and the platten at a cutting force which allows the cutter to cut through the plastic film.

17. A pressure thermoforming apparatus according to claim 15 or 16, wherein the actuation device can be operated to provide a clamping force which accommodates and compensates for changes in pressure between the cutter and the platten.

18. A pressure thermoforming apparatus according to claim 17, wherein the actuation device includes a control device operatively associated with a force sensing device measuring the clamping force, the control device operating the actuation device in reaction to changes in the clamping forced sensed by the force sensing device.

19. A pressure thermoforming apparatus according to any one of claims 15 to 18, wherein the actuation device includes a hydraulic means.

20. A pressure thermoforming apparatus according to any one of claims 15 to 19, wherein the actuation device includes a mechanical lifting means for rapid, broad relative movement between the platten and mould body and a hydraulic means for smaller, specific relative movement between the platten and mould body.

21. A pressure thermoforming apparatus according to claims 19 or 20, wherein the hydraulic means includes at least one hydraulic cylinder.

22. A pressure thermoforming apparatus according to any one of claims 15 to 21 , wherein actuation device includes a mechanical wedge.

23. A pressure thermoforming apparatus according to any one of claims 15 to 22, wherein actuation device includes a screw arrangement.

24. A pressure thermoforming apparatus according to any one of claims 15 to 20, wherein the clamping force is set using an actuation device selected from a screw arrangement, mechanical wedge or a hydraulic means.

25. A pressure thermoforming apparatus according to any one of claims 15 to 24 operated using a method according to any one of claims 1 to 14.