HEATING APPARATUS

Abstract: The heating apparatus comprises a plurality of elongate linear heating elements(16,17,18) which are substantially parallel to and equally spaced from the surface of a thermoplastic sheet supported across a gap between a pair of sheet locating members. Each of the heating elements if mounted between longitudinally separated rotatable rollers (5) having means for adjustment of the separation between the elements as a result of the rotation of the rollers. This means for adjustment is preferably a series a part helical grooves around the outer periphery of the relevant roller, each groove being arranged to receive one of the heating elements.

Description: The present invention is concerned with heating apparatus and, in particular, such apparatus incorporating elongate linear radiant heating elements. It is often desired to locally heat thermoplastic sheet in a straight line, in order to produce a line of thermally softened material which can be used as a bend line. One way of achieving such a linear heating effect is by the use of bare wire radiant elements (typically of electrically energised nickel-chromium resistance wire) disposed in closed proximity to the thermoplastic sheet, which is typically supported above the heating element(s). There is a wide range of thermoplastic sheet materials which can be locally heated and subsequently bent using such heating elements; included in such materials are cast acrylics, extruded acrylics, polystyrene, PVC, expanded PVC, ABS, polypropylene and polycarbonates. Sheets of these thermoplastic materials are generally in the thickness range 1 to 20mm, but as the thickness increases, a wider band of material must be heated to ensure a satisfactory bend line. As a general rule, a heated band of width about 1.5 times the thickness of the sheet is required in order to ensure a neat 90°C bend without blemishes, whereas for a 180° bend, the heated band should preferably have a width of about three times the thickness of the sheet. In conventional equipment for heating thermoplastic sheets by means of non-contacting elongate radiant heating elements, the width of the heated band is controlled either by controlling the electrical power supply, or by controlling the width of a slot in a heat-resisting material in which the heating element is located. Both of these methods involve considerable wastage of electrical energy when a wide band is heated. Accordingly, when it is desired to heat a wide band of thermoplastic sheet material, heating tapes or other contact heaters are often used. Unfortunately such contact heaters are unsuitable for some plastics (such as polystyrene) because they tend to adhere to, and make marks at, the line of contact. Other plastics, such as polycarbonates, require very uniform heating , within close tolerances, which is difficult to achieve with such contact heaters. We have now devised improved apparatus for non-contact radiant heating of thermoplastic sheet material, which enables some of the disadvantages of prior art apparatus to be alleviated. The apparatus according to the invention comprises a plurality of elongate linear electrical heating elements arranged substantially parallel to, and equally spaced from, the surface of a thermoplastic sheet supported across a gap between a pair of sheet locating members, each of said elements being mounted between longitudinally separated rotatable rollers having means for adjustment of the separation between said elements as a result of rotation of said rollers. The sheet locating members are typically a pair of supports for the sheet, with a longitudinally extending gap there between in which the heating elements are disposed. The means for adjustment of the separation between the heating elements are typically part-helical grooves around the outer periphery of the respective roller, each such groove being arranged to receive one such heating element. The electrical supply to the heating elements is preferably via the rollers; that is, at least the outer surface of each roller is electrically conductive and arranged to be connected to an electrical power source. Typically, the rollers are of brass or similar conductive metal. The electrical heating elements are preferably bare wire conductors of, for example nickel-chrome wire, each mounted in tension to span two of the above-mentioned rollers. At least two such elements are arranged to face at least one surface the sheet to be heated; typically three such elements are employed to face both surfaces of the thermoplastic sheet (that is, a total of six heating elements). Preferred embodiments of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows schematically, the general construction of an exemplary arrangement of apparatus according to the invention; Figure 2 shows in more detail an end assembly for use in apparatus such as that shown in Figure 1; Figure 3 shows in more details the roller employed in the end assembly of Figure 1; and Figures 4a to 4d show various heating patterns achievable using apparatus according to the invention. Referring to Figure 1, there are shown two arrays 1, 1′ of elongate bare wire heating elements extending between opposed ends 2,3 of a worktable 4. The ends of each heating element pass over respective rollers 5,6 and 5′,6′, which both support and position the elements below respective pairs of sheet supports 7,8 and 7′,8′ supported on a worktable. Referring to Figure 2, the roller 5 is adjustable for height using a screw adjuster 9 acting on an upstanding post 10, at the top end of which is a bracket 11 receiving the roller 5. The latter roller is rotatable, for example, by means of a hexagon key to be placed in socket 12. Referring to Figure 3, the roller 5 has a series of part-helical grooves 13,14,15 each capable of receiving a respective heating element 16,17,18 (Figure 2). As will be seen, rotation of the roller 5 in one sense will cause a heating element present in either of the outer grooves 13,15 to diverge from a heating element present in the central groove 14; rotation in the opposite sense causes the outer elements to move closer to the central element. Reverting to Figure 2, heating elements 16, 17,18 are connected to tensioning roller 19 which is coaxial with a tensioning pulley 20 connected via tension wire 21 to tension spring 22 which is substantially within shroud 23. The tension wire is wound around the pulley by an amount necessary to provide the required tension. An indicating marker (not shown) may be provided on the tension spring 22 to provide an indication of the required cold tension (that is, before energising the electrical connection). The spring 22 is arranged to compensate for expansion of the heating element as the latter reaches its operating temperature. the electrical power supply is preferably no more than 50V for safety reasons. Referring now to Figures 4a to 4d, there are shown various heating patterns achievable using apparatus according to the invention with heating elements located above and below the sheet 23 being heated. Specifically, Figure 4a shows the use of a pair of heating elements 17,17′ above and below the sheet 23 (such elements being as used in the central groove 14 as described above, with the outer elements 16, 18 removed). By this means a single bend line 24 can be achieved. Figure 4b is similar and shows the use of two pairs of heating elements 16, 17, 16′, 17′ whereby a wider heating band 25 can be achieved. Figure 4c is again similar and shows the use of three pairs of heating elements 16,17,18,16′,17′,18′ whereby an even wider heating band 26 can be achieved. Figure 4d shows the use of the two outer pairs of heating elements 16,18,16′,18′, only so as to achieve two separate heated lines 27,28.

Claims: 1. Heating apparatus comprising a plurality of elongate linear electrical heating elements arranged to be substantially parallel to, and equally spaced from, the surface of a thermoplastic sheet supported across a gap between a pair of sheet locating members, each of said elements being mounted between longitudinally separated rotatable rollers having means for adjustment of the separation between said elements as a result of rotation of said rollers. 2. Heating apparatus according to claim 1, in which said sheet locating members comprise a pair of supports for said thermoplastic sheet, with a longitudinally extending gap between said supports in which the heating elements are disposed. 3. Heating apparatus according to claim 1 or 2, in which said means for adjustment of the separation between the heating elements comprise part-helical grooves around the outer periphery of the respective roller, each groove being arranged to receive one of said heating elements. 4. Heating apparatus according to any of claims 1 to 3, in which at least the outer surface of each roller is electrically conductive and arranged to be connected to an electrical power supply. 5. Heating apparatus according to any of claims 1 to 4, in which said electrical heating elements are bare wire conductors, mounted in tension. 6. Heating apparatus according to any of claims 1 to 5 , in which at least six of said elements are employed so arranged that at least three face each surface of the thermoplastic sheet. 7. heating apparatus for line heating of thermoplastic sheet material, substantially as described herein with reference to Figures 1 to 3 of the accompanying drawings.

Inventors: CLARKE, CHRISTOPHER R.ÂFFORESTFACH FARM, Clarke, Christopher R.§Fforestfach Farm
Applicants: C.R. CLARKE & COMPANY (UK) LIMITED
Application year: 1987
Main classification: B29C53
IPC codes: B29C53, B29C35, H05B3