MOULDING OF THERMOPLASTIC SHEET MATERIALS

Abstract: A method of moulding thermoplastic sheet material (17) includes: floating the sheet material (17) on a liquid (12), the density of the liquid (12) being greater than that of the sheet material (17); maintaining the liquid (12) at a temperature at which the sheet material (17) is plastic, and bringing a mould (14) progressively into engagement with the sheet material (17), so that the sheet material (17) is deformed to the shape of the mould (14).

Description: MOULDING OF THERMOPLASTIC SHEET MATERIALS The present invention relates to moulding of thermoplastic sheet materials, that is materials which soften and become mouldable upon application of heat. The invention is particularly applicable to glass but may be used with thermoplastic plastics materials. Laminate sheet materials are often required to be moulded to shape such that the material is deformed to a three dimensional topography whilst maintaining an approximate uniformity in thickness. This is particularly important when the product • is required to be transparent and produce little optical distortion. Conventional methods of producing such forms include vacuum moulding, sag bending and press bending. These processes require a mould and care must be taken to avoid abrading the surface of the material. During the formation of the three dimensional product from a flat sheet local stretching and compaction will occur in the material producing a tendency to tear or buckle. This limits the complexity and accuracy of the forming process. The conventional methods of forming to a mould do not provide a simple method of controlling such local movement. The present invention provides a means of moulding thermoplastic sheet materials whilst maintaining control on the local deformation. According to one aspect of the present invention a method of moulding a thermoplastic sheet material is characterised by; floating the sheet material on a liquid, the density of the liquid being greater than that of the sheet material; maintaining the liquid at a temperature at which the sheet material is plastic; and bringing a mould progressively into engagement with the sheet material so that the sheet material is deformed to the shape of the mould. According to a preferred embodiment of the invention a mould is filled with liquid, the density of the liquid being greater than that of the sheet material and the temperature being maintained at a temperature at which the sheet material is plastic; the sheet material is floated on the liquid so that the periphery of the sheet material sealingly engages the periphery of the mould and the liquid is permitted to drain from the mould in controlled manner, so that the thermoplastic sheet is drawn down into engagement with the surface of the mould. Preferably the mould is submerged in a bath of the liquid maintained at an appropriate temperature whilst the sheet material floats on the surface. The moulding process is accomplished by raising the mould so that the lip engages the floating sheet and is sealed with respect thereto, the mould then being raised in a controlled manner. A drain in the base. of the mould allows liquid to escape as the sheet material deforms to the shape of the mould above the level of the liquid in the mould whilst still being supported by the liquid above the submerged portions of the mould. The interface between the sheet material and the mould surface will in general be lubricated by the liquid which will be retained in the gap by capillary action provided the surfaces are wetted by the liquid. This action should prevent abrasion of the surface. The "working" liquid must be more dense than the laminate sheet so that the sheet will float. As the liquid is removed during the moulding process air pressure acting on the sheet material will deform the material to the mould. The largest pressure differential that acts on the sheet material depends on the vapour pressure and temperature of the liquid and a suitable liquid should be chosen accordingly. Furthermore the maximum height of liquid that can be supported by atmospheric pressure depends on the density; thus with mercury this height is 76 cm at 1 atmosphere whilst with water this height is approximately 10 m. Thus, if the liquid is drained by gravity then the density is very important in terms of the lift of the mould. Generally the liquid should be as dense as possible, be inert and have a high boiling point. Liquid metals such as mercury and tin and some alloys such as Woods metal could be suitable as might some heavy oils and solvents such as carbon tetrabromide. According to an alternative embodiment of the present invention the sheet material is floated on the liquid, the liquid having a density greater than that of the sheet material and being maintained at a termperature at which the sheet material is plastic and a mould is lowered onto the sheet material, so that the buoyancy of the sheet material will deform the sheet material so that it conforms to the surface of the mould. Preferably the sheet material, which might be precut to a plan shape, is floated on a bath of liquid, maintained at the correct temperature for moulding, and so heating the sheet material by contact with the liquid. The material is registered relative to the mould and the mould submerged at a predetermined rate, below the surface of the liquid. The sheet material floating on the liquid, is moulded to the form of the mould, by the buoyant forces acting on the material. Additional moulding force may be achieved by providing rapid downward impetus to the mould once submerged to ensure good contact of the material with the mould surface. Such force will depend on the area of the mould, the restriction on motion of the liquid and the speed of motion. Sound waves may also be introduced into the liquid to provide an impressed additional oscillating force to the buoyant force acting on the sheet material. In order to facilitate removal of the moulded sheet material from the bath, the sheet material may be maintained in contact with the mould after forming by suction between the mould and the material. This suction being achieved by ensuring atmospheric pressure cannot be exerted at the mould/material interface. The release of the material from the mould will be achieved by allowing the pressure at the mould/material interface to rise and so reduce the suction. According to a further aspect of the present invention a method of moulding a thermoplastic sheet material is characterised by supporting the sheet material, directing a jet of fluid onto the sheet material, the fluid being maintained at a temperature at which the sheet material is plastic, and controlling the jet so that the sheet material is deformed to the required shape. The position and direction of the nozzle are preferably automatically controlled to follow a prescribed pattern. The pattern being determined to give an acceptable moulding process and to minimise stretching and buckling. The automatic control being accomplished by means of a computer or microprocessor. Preferably the support for the sheet material is in the form of a mould underlaying the sheet and registered to it so that the stream of fluid leaving the nozzle progressively forms the sheet against the mould in accordance with the spatial pattern followed by the nozzle. Additionally the pressure of the pumped fluid might be altered to vary the force exerted by the fluid on the sheet material. Alternatively the sheet material might be supported by a ring frame and the fluid stream is then used to form the material to shape in the areas where it is not underlayed. Reproduction is achieved by having a repeatable process and constant material properties. The fluid material may be gas or liquid. The force of the fluid acting on the sheet material will depend on the density, flow rate and velocity. It is also intended that the fluid will partially or wholly heat the sheet material to facilitate deforming to shape. The specific heat of the fluid is therefore important. Variable parameters which might be used to facilitate the forming process are flow rate, temperature, velocity, nozzle shape. According to a further aspect of the present invention an apparatus for moulding thermoplastic sheet material is characterised by a container for liquid, means for maintaining the liquid at a temperature at which the sheet material is plastic, a mould and means for moving the mould into engagement with the sheet material when floated on the surface of the liquid. Various embodiments of the invention are now described, by way of example only, with reference to the accompanying drawings, in which:- Figure 1 illustrates a method of forming glass sheet in accordance with the present invention; Figures 2A, 2B and 2C illustrate stages of the forming process illustrated in Figure 1; Figure 3 illustrated a view similar to Figure 2B, but with different mould configuration; Figure 4 illustrates a modification to the apparatus used in SUBSTITUTE SHEET the method illustrated in Figure 1; Figure 5 illustrates an alternative method of forming glass sheet, in accordance with the present invention; Figure 6A and 6B illustrate stages of the method illustrated in Figure 5; Figure 7 illustrates an alternative method of forming glass sheet, in accordance, with the present invention; and Figure 8 illustrates a modification to the method illustrated in Figure 7. As illustrated in Figure 1, the apparatus 10 comprises a bath 11 filled with molten tin 12 which is maintained at an appropriate temperature by a heater 13. A mould 14 is suspended belov; the surface of the molten tin 12 with a drain 15 let into sump 16. A sheet of glass 17 is floated on the surface of the molten tin 12 and is maintained in a plastic condition by heat transfer therefrom. Lifting frame 18 attached to the mould 14 is connected to a hoist 19 supported on a gantry 20. In operation, the mould 14 is lifted by means of hoist 19 until the lip 21 of the mould 14 engages the glass sheet 17 SUBSTITUTE SHEET and forms an airtight seal therewith. Additional clamping means (not shown) may be provided to improve the seal, if required. Continued lifting of the mould 14 permits the molten tin 12 to drain from the mould 14 back into the bath 11 and draw the glass sheet 17 back into the mould 14, until it is formed to the surface of the mould 14. The rate at which the mould 14 is raised out of the bath 11 is varied depending on the amount of deformation required for the glass sheet 17 to settle onto the surface of the mould 14. The drain orifice 15 remains below the surface of the molten tin 12 in the bath 11, throughout the moulding process. The process continues until the molten tin 12 is drained from the mould and the glass sheet 17 is fully moulded. The moulded glass sheet 17 may then be permitted to cool until it is set and removed from the mould 14, so that the process can then be repeated. Figures 2Λ, 2B and 2C show the glass sheet 17 at various stages through the forming process. Figure 2A shows the inital stage in which the lip 21 of the mould 14 is in contact with the glass sheet 17 before deformation has occurred. Figure 2B shows the process at an intermediate stage while Figure 2C shows the glass sheet 17 fully conforming with the mould 14. As illustrated in Figure 3, the glass sheet 17 is in a SUBSTITUTE SHEET position where the surface of the mould 14 is steep. The glass sheet 17 at position A is fully supported by the mould 14 whilst at position B it is supported by the molten tin 12. The amount of deformation and stretching of the glass sheet 17 in the locality of position C will depend upon the time available for the deformation to occur and how much of the surface force is transmitted through the glass sheet 17. The time available for deformation and stretching may be controlled by controlling the rate at v/hich molten tin 12 drains from the mould 14, the rate being slower for steeper surfaces of the mould 14. The surface force is primarily related to the viscosity and hence to shear rate of the glass sheet 17. In the modification illustrated in Figure 4, a cooling channel 26 is provided in the lip 25 of mould 14. This cooling channel 26 is adapted to be connected to a cooling water supply. A heating strip 28 is also provided in lip 25 of mould 14 adjacent the surface D which is engaged by the sheet material during moulding. With this modification, when the surface D of lip 25 of the mould 14 has been brought into engagement with the glass sheet 17, cooling water is passed through the channel 26 to freeze the small layer of molten tin 12 between the surface D and glass sheet 17, to freeze and bond the glass sheet 17 to the surface D, thereby ensuring an airtight seal therebetween. When the SUBSTITUTE SHEET oulding process is completed, the heating strip 28 may be used to melt the frozen metal and permit removal of the moulded glass sheet 17 from the mould 14. As illustrated in Figure 5, a sheet of glass 30 is floated upon a bath 35 of molten tin 31. The molten tin 31 is heated to the required forming temperature which in turn heats the glass sheet 30 by contact. A mould 32 is mounted above the glass sheet 30 and is attached to a hoist 33 enabling the mould 32 to be lowered onto the glass sheet 30. The glass sheet 30 is registered against lugs 34 attached to the mould 32. The buoyant force acting on the glass sheet 30 forces the glass sheet 30 to conform v/ith the mould 32. The trapped gas and/or molten tin 31 in volume 39 is expelled through hole 38. The process is controlled by the rate of introducing the mould 32 into contact with the glass sheet 30 and the temperature of operation. Figure 6A illustrates the mould 32 partly engaged v/ith the glass sheet 30 floating on molten tin 31. The buoyancy force so exerted causes the glass sheet 30 to deform towards the mould. Air and molten tin 31 caught in the volume above the glass sheet v/ill be expelled and drained by breather hole 38. Figure 6B illustrates the glass sheet material 30 fully SUBSTITUTE SHEET formed to the mould 32, at this point the brather hole 38 is closed and the mould 32 is lifted out of the bath of molten tin 31. Air might be introduced underneath the mould 32 or the mould 32 might be tipped in order to ease the removal of the mould 32 from the molten tin 31. In order to remove the sheet glass 30 from the mould 32 after the glass 30 has been cooled sufficiently, the breather hole 38 is opened v/hich reduces the suction and the moulded glass form is removed. In the method illustrated in Figure 7, a robotic arm 51 holds an adjustable nozzle 52 above glass sheet material 53 supported on mould 55. Mould 55 is inclined so that liquid will drain from the surface to sump 56. Molten tin 57 initially in sump 56 is pumped via heater 58 to nozzle 52. The nozzle 52 is moved spatially and directionally by robotic arm 51 in a pattern v/hich forces the glass sheet 53 to conform to mould 55. The temperature and velocity of the stream of molten tin 57 leaving the nozzle 52 and controlled to facilitate the forming process. This apparatus may be used in the following manner:- a. A gentle diffuse spray of hot molten tin 57 is first sprayed onto the surface of the glass sheet 53 to allow the sheet to reach its moulding temperature; SUBSTITUTE SHEET b. The pressure of the jet is then increased while still maintaining a wide angle to deform the glass sheet 53 and shape it approximately to the mould; c. The jet is finally narrowed and a high pressure jet directed by the robot arm 51 to areas requiring further forming to the mould. The temperature of the molten tin 57 may also be increased at this stage. Figure 8 illustrates an alternative means of supporting the sheet material 53. Glass sheet 53 is held in ring frame 62 at an angle so that liquid will drain from the surface to sump 56, nozzle 52 supported by robotic arm 51 directs molten tin 57 at a temperature and velocity that might be varied to suit the process. The glass sheet material 53 is progressively deformed to desired shape by the action of the stream of molten tin 57 impinging upon the surface. Repeatability is ensured by having accurate repeatable action of the nozzle 52, pump 59 and heater 58, and by uniformity of material properites of the glass sheet 53. Various improvements may be made without departing from the invention. For example,. while in the embodiment illustrated in Figure 1 the mould is lifted from the bath, the level of SUBSTITUTE SHEET liquid in the bath 11 may alternatively be reduced or the bath 11 itself lowered. In the embodiment described with reference to Figure 1, the sheet material 17 is over-sized and the periphery which is engaged by the lip 21 of the mould 14 will be trimmed after moulding. Alternatively, the sheet material 17 may be cut to size prior to moulding and suitable registering formations may be provided on the lip 21 of the mould 14 to accurately align the cut sheet 17 v/ith the mould 14. For example, a recess may be provided around the inner periphery of the lip 21 of the 'mould 14 in which the sheet material 17 will be located v/hen the mould 14 is raised to engage the sheet 17. Vϊith the embodiments illustrated in Figures 7 and 8, instead of the nozzle 52 moving, it may remain stationary while the sheet material is moved. With these methods after the moulding process is completed the nozzle may be used to spray a coolant fluid in a diffused pattern to temper the sheet material. While in the above embodiments, glass sheet is moulded using molten tin, other molten metals, for example lead, or other substances with suitable melting points, for example fused salts, may be used. The method is also applicable to other SUBSTITUTE SHEET thermoplastic materials, for example plastics materials where mercury or lov/ melting point alloys may be used as the liquid media. A flux may be provided as an interlayer between the sheet material and molten metal, in order to avoid oxidation. SUBSTITUTE SHEET

Claims: CLAI S 1. A method of moulding a thermoplastic sheet material characterised by floating the sheet material (17; 30) on a liquid (12; 31), the density of the liquid (12; 31) being greater than that of the sheet material (17; 30); maintaining the liquid at a temperature at which the sheet material (17; 30) is plastic; and bringing a mould (14; 32) progressively into engagement with the sheet material (17; 30) so that the sheet material (17; 30) is deformed to the shape of the mould (14; 32). 2. A method according to Claim 1 characterised in that a mould (14) is filled with a liquid (12), the density of the liquid (12) being greater than that of the sheet material (17), maintaining the liquid (12) at a temperature at which the sheet material (17) is plastic, floating the sheet material (17) on the liquid (12) so that the periphery of the sheet material sealingly engages the periphery (21) of the mould (14) and permitting the liquid (12) to drain from the mould (14) in controlled manner, so that the thermoplastic sheet (17) is drawn down into engagement with the surface of the mould (14). 3. A method according to Claim 1 characterised in that the mould (14) is submerged in a bath (11) of liquid SUBSTITUTE SHEET (12), the sheet material (17) is floated on the surface of the liquid (12) in bath (11) and the mould (14) is raised so that the periphery (21) of the mould (14) is first brought into contact with the sheet material (17) and produces a seal with respect thereto, liquid (12) draining from the mould (14) upon continued upward movement of the mould (14). 4. A method according to claim 3 characterised in that the mould (14) has a drain (15) which extends to an outlet which is below the lowest point of the surface of the mould (14), the outlet from the drain (15) being maintained below the level of liquid (12) in the bath (11) throughout the moulding process. 5. A method according to any one of the preceding claims characterised in that the rate at which liquid (12) is permitted to drain from the mould (14) is varied. 6. A method according to Claim 5 characterised in that the rate at which liquid (12) drains from the mould (14) is slower in the portions of the mould (14) which define a surface of steeper inclination. 7. A method according to any one of the preceding claims characterised in that the lip (25) of the mould (14) is cooled when it engages the sheet material (17) in order to freeze the liquid (12) and produce a seal between the lip (25) of the mould (14) and sheet material (17). 8. A method according to Claim 1 characterised in that the sheet (30) is floated on a liquid (31), the liquid (31) having a density greater than that of the sheet material (30) and being maintained at a temperature at which the sheet material (30) is plastic and a mould (32) is lowered onto the sheet material (30), so that the buoyancy of the sheet material (30) will deform the sheet material (30) so that it conforms to the surface of the mould (32). 9. A method according to Claim 8 characterised in that the rate at which the mould (32) is lowered is controlled in relation to the amount of deformation required. 10. A method according to Claim 9 characterised in that lowering of the mould (32) is accelerated to increase the moulding force. 11. A method according to any one of Claims 8 to 10 characterised in that sound waves are introduced into the liquid (31). 12. A method according to any one of Claims 8 to 11 characterised in that suction is applied between the mould (32) and sheet material (30) after the sheet material (30) has been deformed to the shape of the mould (32), in order to permit removal of the moulded sheet material (30) from the liquid (31) . 13. A method according to any one of Claims 8 to 12 characterised in that the moulded sheet material (30) is permitted to cool and set in the mould (32), air being permitted to penetrate between the sheet material (30) and mould (32) to release the sheet (30) from the mould (32). 14. A method according to any one of Claims 8 to 13 characterised in that the sheet material (17; 30) is first cut to shape, the mould (14; 32) being brought into engagement with the floating sheet (17; 30) so that locating formations (34) engage the periphery of the sheet (17; 30) to accurately locate the sheet (17; 30) relative to the mould (14; 32). 15. A method according to any one of the preceding claims characterised in that the theremoplastic sheet material (17; 30) is glass and the liquid (12; 31) is molten metal. 16. A method according to Claim 15 characterised in that the molten metal is tin or lead. 17. A method according to any one of Claims 1 to 16 characterised in that the theremoplastic sheet material (17; 30) is a plastics material and the liquid (12; 31) is mercury or a molten low melting point alloy. 18. An apparatus for moulding a thermoplastic sheet material (17; 30) characte ised by a container (11; 35) for liquid (12; 31), means (13) for maintaining the liquid (12; 31) at a temperature at which the sheet material (17; 30) is plastic, a mould (14; 32) and means (19; 33) for moving the mould (14; 32) into engagement with the sheet material (17; 30) when floated on the surface of the liquid (12; 31). 19. An apparatus according to Claim 18 characterised by a mould (14) v/hich may be filled with a liquid (12) of density greater than the density of the sheet material (17), means (13) for maintaining the temperature of the liquid (12) at a temperature at which the sheet material (17) is plastic, and means (15) for permitting liquid to drain from the mould (14) . 20. An apparatus according to Claim 19 characterised in that the mould (14) is εubmergible in a bath (11) of the liquid (12), means (19) being provided to raise the mould (14) out of the bath (11) and permit liquid (12) to drain from the mould (14) into the bath (11). 21. An apparatus according to Claim 20 characterised in that the mould (14) has a downwardly extending drain (15) which may be maintained below the level of liquid (12) in the bath (11) throughout the moulding process. 22. An apparatus according to any one of Clair.s 19 to 21 characterised in that the mould (14) has a cooling channel (26) around its lip (25) through which a cooling medium may be passed to freeze the sheet material (17) to the lip (25) of the mould (14). 23. An apparatus according to Claim 22 characterised in that a heating element (28) is provided around the lip (25) of the mould (14) so that frozen liquid (12) between the sheet material (17) and lip (25) may be melted to permit removal of the moulded sheet material (17) from the mould (14). 24. A method of moulding a thermoplastic sheet material (53) characterised by supporting the sheet material (53), directing a jet of fluid (57) onto the sheet material (53), the fluid (57) being maintained at a temperature at which the sheet material (53) is plastic and controlling the jet so that the sheet material (53) is deformed to the required shape. 25. A method according to Claim 24 characterised in that the sheet material (53) is supported on a mould (55), the jet deforming the sheet material (53) onto the surface of the mould (55 ) . 2G. A method according to Claim 24 characterised in that the sheet material (53) is supported around the edge, the middle portions of the sheet material (53) being unsupported and formable to the desired shape. 27. A method according to any one of Claims 24 to 26 characterised in that a nozzle (52) is controlled automatically to follow a predetermined pattern, thereby directing the jet of fluid (57) over different areas of the sheet material (53). 28. A method according to any one of Claims 24 to 27 characterised in that the rate of flow of fluid (57) is varied to control deformation of the sheet material (53). 29. A method according to any one of Claims 24 to 28 characterised in that the spread of the jet of fluid (57) is varied to control deformation of the sheet material (53). 30. A method according to any one of Claims 24 to 29 characterised in that the temperature of the fluid (57) is varied to control deformation of the sheet material (53). 31. A method according to any one of Claims 24 to 30 characterised in that the sheet material (53) is glass and the fluid (57) is molten tin or lead. 32. A method according to any one of Claims 24 to 30 characterised in that the sheet material (53) is a plastics material and the fluid (57) is mercury or a molten low melting point alloy.

Inventors: HELFET EDSEL KEITH, PARSONS BRYAN NIGEL VICTOR, HELFET, EDSEL, KEITH, PARSONS, BRYAN, NIGEL, VICTOR
Applicants: JAGUAR CARS, JAGUAR CARS LIMITED
Application year: 1990
Main classification: B29C51
IPC codes: B29C51, C03B18, C03B23