PLASTIC STRETCHING METHOD AND APPARATUS FOR USE IN VACUUM FORMING TECHNIQUES

Abstract: Machine and method for stretching and then vacuum forming acrylic sheets. Machine has clamping bars (1-4), each of which has a plurality of secondary clamps (5) which move along the clamping bars proportionately to the stretching of the plastic sheet as it stretches but act to prevent bowing of the plastic sheet. Primary clamps (13, 14) hold the edges of the sheet in place after stretching during the vacuum forming process.

Description: PLASTIC STRETCHING METHOD AND APPARATUS FOR USE IN VACUUM FORMING TECHNIQUES The present invention relates in general to technology for stretching plastic sheets. In particular, the invention relates to the stretching of plastic sheets as a preparation step for use with vacuum forming techniques . Vacuum forming techniques are used in a variety of industries; for example, bathroom furnishings such as baths can be vacuum formed from a single sheet of thermoplastic material. The thermoplastic material most commonly used for vacuum forming bathroom furnishings at the present time is acrylic. In this manufacturing process, the cost of the thermoplastic sheet represents approximately 70% of the manufacturer's final costs. It would therefore be beneficial to find a method of using less thermoplastic in the manufacturing process. However, if one simply used a thinner sheet, one would expect to produce a final product with reduced mechanical strength. It is therefore an aim of the present invention to provide a method and apparatus for producing vacuum formed goods that uses less thermoplastic than conventional techniques whilst retaining, or even improving, mechanical strength. We have found that a thermoplastic sheet, such as an acrylic sheet, which has been stretched before vacuum forming provides a final product which has greater mechanical strength than an unstretched sheet which has the same thickness at the start of the vacuum forming step. However, when one considers how to implement this stretching step industrially, a technical difficulty becomes apparent . When a plastic sheet is stretched it will scallop, that is to say that it will narrow in the middle with the sides bowing inwards. This means that it will not have a constant width and thickness throughout and cannot be cut efficiently into smaller square or rectangular portions. For example, US Patent No. 5,271,352 to Critical Sciences (Australia) Limited discloses a plastic stretching device for use in vacuum forming boat hulls. This device pulls plastic apart without any means for preventing scalloping. It is therefore impractical to use for any significant amount of stretching. PCT/US92/07408 describes a plastic stretching device which has movable clamping means holding all sides of a plastic sheet. However, the edges of the plastic sheet would be unable to expand with the material as a whole, distorting its shape. JP60049921 to Toyota Jidosha KK goes part of the way to solving this problem by having a plurality of clamps along at least one side of a plastic sheet. However, the movement of each clamps needs individually controlled, leading to a complex and expensive machine. In JP63197626 to Shinto Kogyo KK, a plurality of clamps along opposite sides of a sheet of plastic are controlled automatically by their being mounted on endless roller chains which function to move the sheet of plastic into and out of a heating means as well as holding the plastic sheet. However, when material is stretched by this machine moving the two roller chains apart, the material edges held by the chains are unable to contract proportionately, leading to distortion of the edges and scalloping of the sheet as a whole. The aim of this invention is therefore to provide a method of stretching plastic sheets which can be used to expand the total surface area of the sheet without distorting the sheet, wasting material or having a non- uniform thickness. In particular, the invention aims to provide a simple controllable mechanism. Furthermore, this invention aims to stretch the plastic sheet in a manner adapted for immediate use in vacuum forming to gain the strength enhancing benefits of the stretching step. In this application, the term "plastic" refers to any thermoplastic material. According to a first aspect of the present invention there is provided a machine for stretching a rectangular sheet of plastic material, the machine having a first and second orthogonal pairs of opposed elongate clamping means which engagably cooperate with the sides of said sheet and thereby hold said sheet, the machine including a means to controllably urge apart a first pair of opposed elongate clamping means and thereby stretch a sheet held therebetween characterized m that the second pair of opposed elongate clamping means is adapted to allow said sheet to stretch uniformly m a direction parallel to the lengths of the second pair of elongate clamping means whilst resisting movement of the clamped sides of the sheet in a direction orthogonal to the lengths of the second pair of elongate clamping means. Preferably, the machine includes means to controllably urge apart both pairs of opposed clamping means and thereby stretch a sheet held therein in two dimensions, each elongate clamping means being adapted to allow said sheet to stretch uniformly m a direction parallel to the length of said individual elongate clamping means whilst resisting movement of the side of the rectangular sheet clamped by said individual elongate clamping means m a direction orthogonal to the length of said individual elongate clamping means. The machine may be adapted to stretch the sheet first m one direction and then subsequently m a second direction orthogonal to the first direction. Preferably, the elongate clamping means having a plurality of secondary clamps distributed along their length, the secondary clamps engagably clamping a portion of the edge of the rectangular sheet of plastic and being adapted to move along the length of the elongate clamping means proportionately to the stretching of the sheet parallel to the length of the clamping means. More preferably, the secondary clamps are moved along the length of the elongate clamping means by the motion of the sheet . Preferably, the clamping means have a primary clamping means which engageably and fixedly clamps a side of the sheet of the plastic material. Preferably also, the machine has a heating means for evenly heating a plastic sheet held therein. According to a second aspect of the present invention there is provided a machine for stretching and vacuum forming a rectangular sheet of plastic comprising a machine for stretching a rectangular sheet of plastic as claimed in any preceding Claim and an engagable vacuum forming means, wherein the engagable vacuum forming means is adapted to vacuum form a rectangular sheet of plastic stretched by the machine for stretching a rectangular sheet of plastic whilst said rectangular sheet of plastic is still held within the elongate clamping means of the machine for stretching a rectangular sheet of plastic. Preferably, the machine has a means for selectively cooling zones of the plastic sheet held therein. According to a third aspect of the present invention, there is provided a method of stretching a rectangular sheet of plastic material, the method comprising the steps of: (a) engagably holding the sides of said sheet; and (b) stretching said sheet along a first axis of said sheet whilst holding the sides of said sheet parallel to the first axis so as to allow the sheet to stretch proportionately along the first axis but so as to resist deformation of the sides of said sheet orthogonal to the first axis. Preferably, the method further has the step of: (c) stretching said sheet along a second axis of said sheet whilst holding the sides of said sheet parallel to the second axis so as to allow the sheet to stretch proportionately along the second axis but so as to resist deformation of the sides of said sheet orthogonal to the second axis, the second axis being orthogonal to the first axis. Preferably, the method involves, simultaneous to stretching said sheet parallel to a first axis, also stretching said sheet along a second axis of said sheet whilst holding the sides of said sheet parallel to the second axis so as to allow the sheet to stretch proportionately along the second axis but so as to resist deformation of the sides of said sheet orthogonal to the second axis, the second axis being orthogonal to the first axis. Preferably, the sides of said sheet are engageably held by a first and second orthogonal pairs of elongate clamping means which each engagably cooperate with a side of said sheet through a plurality of secondary clamps. Preferably also, the secondary clamps are adapted to move along the length of the clamping means proportionately to the stretching of the sheet along an axis parallel to the clamping means . Most preferably, the secondary clamps wherein the secondary clamps are moved along the length of the elongate clamping means by the motion of the sheet. Typically, the method will have the step of heating the sheet of plastic material. According to a fourth aspect of the present invention, there is provided a method of stretching and vacuum forming a rectangular sheet of plastic material comprising the steps of stretching a rectangular sheet of plastic according to a method as claimed in any of Claims 10 to 16 and then vacuum forming said stretched sheet of plastic. Preferably, the edges of said sheet are engagably held by a plurality of primary clamping means before vacuum forming. Zones of the plastic material may be selectively cooled. An example embodiment of the present invention will now be illustrated with reference to the following figures in which: Figure 1 is a plan view of a plastic sheet stretching machine; Figure 2 is a cross-section through a plastic stretching machine along line AA; Figure 3 is a perspective view of a plastic stretching machine according to the present invention; Figure 4 is a table of results from a Notched Charpy Impact Strength test performed to BS EN ISO 179/1eA- 1997; and Figure 5 shows a plan for cutting up sheets of material in a conventional fashion and in a fashion for use with the present invention. A plastic stretching machine shown in plan view in Figure 1 is intended for use with a rectangular sheet of plastic material or any other vacuum formable material. Elongate clamping means are provided in the form of four clamping bars (1 to 4), one to clamp each edge of the rectangular plastic sheet. Clamping bars 1 and 3 oppose each other, as do clamping bars 2 and 4. Clamping bars 1 and 2 are fixed in place, whereas clamping bars 3 and 4 may be moved, as controlled by a programmable logic controller. Each clamping bar 1, 2, 3 and 4 comprises a plurality of secondary clamping means 5 (only some of which are labelled in Figure 1 ) . These clamping means are mounted upon each clamp bar and may move along its length with a low co-efficient of friction. A rectangular hot thermoplastic sheet is placed within the device and is clamped around its four edges by the secondary clamps 5. This apparatus is shown in cross- section along line AA in Figure 2. The machine may also have means to itself heat the sheet and/or keep the sheet hot . Actuator 6 (one shown in detail as an example) activates the clamping mechanism. This causes the clamping head to be rotated around joint 7 on support assembly 8 and the plastic sheet is held in the secondary clamps 5 between plates 9 and 10. Alternate pairs of clamp bars are then pulled apart. For example, whilst clamp bar 1 is held m place, clamp bar 3 may be moved away from it, stretching the plastic sheet m the longitudinal direction. Clamp bars 2 and 4 do not move. However, the secondary clamps 5 are free to move along the clamp bars. They are pulled along by the material sheet and therefore move proportionately to the movement of clamp bar 3. They act to resist scalloping of the plastic sheet. Similarly, clamp bar 4 can be moved away from clamp bar 2, whilst clamp bar 1 and clamp bar 3 remain m place. Again, the clamping means move proportionately to the stretching of the sheet cause by movement m clamping bar 4 and prevent scalloping. In order for this to work there has to be very low friction preventing the movement of the secondary clamping means. Once the stretching phase has been completed, an actuator 11 acting through a thrust pm 12 engages a primary clamping means which is provided on each clamped set and comprises a top plate 13 and bottom plate 14 within which the plastic sheet s securely held. All four sides are held at once. A mould is then driven up through the clamped sheet forming a seal. The mould is then vacuumed by conventional means to form the resulting mould. At the end of this complete product cycle the secondary clamp carriages 5 are returned to their initial positions by the action of the primary clamp bars. A series of linkages prevent the secondary clamps 5 from getting too close to each other. Figure 4 shows a perspective view of the combined stretching and vacuum forming machine. In this Figure, conventional vacuum forming apparatus 15 is provided underneath the stretching apparatus. The machine also provides zoned cooling for control of thickness distribution in the final product. This is a technique used in the prior art and cooling apparatus 16 is mounted above the stretching apparatus. An electronic controller 17 is provided to allow automation of the procedure. The entire process cycle involving both the timings and movements is controlled by a computer or controlled programme logic controller. As a result of this machine it becomes possible to make considerable savings in raw material costs; for example, experiments have indicated that savings of 30% - 50% can be readily achieved. This could not be achieved simply by vacuum forming a thinner sheet of thermoplastic as only the strength benefits provided by the stretching step allow this lower volume of plastic to be used. Figure 4 shows a table of results from a Notched Charpy Impact Strength (N.C.I.S.) test performed to BS EN ISO 179/1eA - 1997 on samples of 5mm Acrylic Cappers ABS taken through the above process. In this table, Flexural Modulus tests were performed to BS EN ISO 178 - 1997 at a test speed of 2mm/min. Sample 10 test bars were nominally 4mm thick x 10mm wide. Sample 12 test bars nominally 2.3mm thick x 15mm wide. Fallen Ball. All Notched Charpy samples that gave a valid result exhibited a partial break type. Figure 5 (a) shows a conventional method for cutting up a sheet of plastic that is 3100mm x 1800mm to form four rectangular pieces 1750mm x 750mm for use m making a product sized 1700mm x 700mm. Note that offcuts are left around the edge. Figure 5 (b) shows how this same sheet can be cut into six rectangular pieces 1550mm x 600mm without offcuts, that may be stretched then used to give a product sized 1700mm x 700mm as before. This reduces the time required and the space requirements for storing raw material. Additionally, fewer different sizes of sheet will have to be stocked by a manufacturer. These benefits represent a substantial cut m the cost of manufacturing the product, therefore providing an important commercial advantage. It will be clear to one skilled in the art that this technique can be used with acrylic plastic or any other vacuum formable material. Furthermore, this plastic stretching technique will find applications m areas other than just vacuum forming. Further modifications and improvements may be incorporated without departing from the scope of the invention herein described. Throughout this application, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising" will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Claims: CLAIMS 1. A machine for stretching a rectangular sheet of plastic material, the machine having a first and second orthogonal pairs of opposed elongate clamping means which engagably cooperate with the sides of said sheet and thereby hold said sheet, the machine including a means to controllably urge apart a first pair of opposed elongate clamping means and thereby stretch a sheet held therebetween characterized m that the second pair of opposed elongate clamping means is adapted to allow said sheet to stretch uniformly m a direction parallel to the lengths of the second pair of elongate clamping means whilst resisting movement of the clamped sides of the sheet m a direction orthogonal to the lengths of the second pair of elongate clamping means. 2. A machine for stretching a rectangular sheet of plastic material as Claimed m Claim 1 , the machine including means to controllably urge apart both pairs of opposed clamping means and thereby stretch a sheet held therein m two dimensions, each elongate clamping means being adapted to allow said sheet to stretch uniformly m a direction parallel to the length of said individual elongate clamping means whilst resisting movement of the side of the rectangular sheet clamped by said individual elongate clamping means m a direction orthogonal to the length of said individual elongate clamping means . 3. A machine for stretching a rectangular sheet of plastic material as Claimed m Claim 1 or Claim 2, the machine being adapted to stretch the sheet first in one direction and then subsequently m a second direction orthogonal to the first direction. 4. A machine for stretching a rectangular sheet of plastic as claimed in any preceding Claim, the elongate clamping means having a plurality of secondary clamps distributed along their length, the secondary clamps engagably clamping a portion of the edge of the rectangular sheet of plastic and being adapted to move along the length of the elongate clamping means proportionately to the stretching of the sheet parallel to the length of the clamping means . 5. A machine for stretching a rectangular sheet of plastic as claimed m Claim 4, wherein the secondary clamps are moved along the length of the elongate clamping means by the motion of the sheet. 6. A machine for stretching a rectangular sheet of plastic as claimed m any preceding Claim, wherein the clamping means have a primary clamping means which engageably and fixedly clamps a side of the sheet of the plastic material. 7. A machine for stretching a rectangular sheet of plastic as claimed m any preceding Claim, the machine having a heating means for evenly heating a plastic sheet held therein. 8. A machine for stretching and vacuum forming a rectangular sheet of plastic comprising a machine for stretching a rectangular sheet of plastic as claimed m any preceding Claim and an engagable vacuum forming means, wherein the engagable vacuum forming means is adapted to vacuum form a rectangular sheet of plastic stretched by the machine for stretching a rectangular sheet of plastic whilst said rectangular sheet of plastic is still held within the elongate clamping means of the machine for stretching a rectangular sheet of plastic. 9. A machine for stretching and vacuum forming a rectangular sheet of plastic as claimed m Claim 8, the machine having a means for selectively cooling zones of the plastic sheet held therein. 10. A method of stretching a rectangular sheet of plastic material, the method comprising the steps of: (a) engagably holding the sides of said sheet; and (b) stretching said sheet along a first axis of said sheet whilst holding the sides of said sheet parallel to the first axis so as to allow the sheet to stretch proportionately along the first axis but so as to resist deformation of the sides of said sheet orthogonal to the first axis. 11. A method of stretching a rectangular sheet of plastic material as claimed in Claim 10, the method further having the step of: (c) stretching said sheet along a second axis of said sheet whilst holding the sides of said sheet parallel to the second axis so as to allow the sheet to stretch proportionately along the second axis but so as to resist deformation of the sides of said sheet orthogonal to the second axis, the second axis being orthogonal to the first axis. 12. A method of stretching a rectangular sheet of plastic material as claimed in Claim 10, the method involving, simultaneous to stretching said sheet parallel to a first axis, also stretching said sheet along a second axis of said sheet whilst holding the sides of said sheet parallel to the second axis so as to allow the sheet to stretch proportionately along the second axis but so as to resist deformation of the sides of said sheet orthogonal to the second axis, the second axis being orthogonal to the first axis. 13. A method of stretching a rectangular sheet of plastic material as claimed in any of claims 10 to 12, wherein the sides of said sheet are engageably held by a first and second orthogonal pairs of elongate clamping means which each engagably cooperate with a side of said sheet through a plurality of secondary clamps. 14. A method of stretching a rectangular sheet of plastic material as claimed in Claim 13 wherein the secondary clamps are adapted to move along the length of the clamping means proportionately to the stretching of the sheet along an axis parallel to the clamping means . 15. A method of stretching a rectangular sheet of plastic material as claimed in Claim 14 wherein the secondary clamps wherein the secondary clamps are moved along the length of the elongate clamping means by the motion of the sheet. 16. A method of stretching a rectangular sheet of plastic material as claimed in any of Claims 10 to 15, the method having the step of heating the sheet of plastic material. 17. A method of stretching and vacuum forming a rectangular sheet of plastic material comprising the steps of stretching a rectangular sheet of plastic according to a method as claimed in any of Claims 10 to 16 and then vacuum forming said stretched sheet of plastic. 18. A method of stretching and vacuum forming a rectangular sheet of plastic material as claimed in Claim 17 wherein the edges of said sheet are engagably held by a plurality of primary clamping means before vacuum forming. 19. A method of stretching and vacuum forming a rectangular sheet of plastic material as claimed in Claim 17 or Claim 18 wherein zones of the plastic material are selectively cooled.

Inventors: HALL GORDON, PRESTON JOHN MEEK, HALL, GORDON, PRESTON, JOHN, MEEK
Applicants: HALL GORDON, PRESTON ASSOC SCOTLAND LTD, PRESTON JOHN MEEK, HALL, GORDON, PRESTON ASSOCIATES (SCOTLAND) LIMITED, PRESTON, JOHN, MEEK
Application year: 1999
Main classification: B29C55
IPC codes: B29C51, B29C55