THERMALLY INSULATED HEATING DEVICE

Abstract: An electrical heating device for use, for exemple, on a plastics injection moulding machine or a plastics extrusion machine including dies, comprises a heating element (10), a flexible heat barrier (12), a sheet or strap of flexible material (14), an inner covering (16) of flexible, heat-resistant material, a layer (18) of thermally insulating material, and an outer covering (20) of flexible material. The heating device may be in wrap-around form and have a clamping arrangement (22) which bridges the longitudinal split in the device and enables the device to be securely clamped around a feed barrel or pipe, for example. The heating element (10) is applied directly to the object to be heated, thus saving energy, and the outer covering (20) remains cool enough for a thermosensitive strap (not shown) to be sewn thereto.

Description: THERMALLY INSULATED HEATING DEVICE This invention relates to a thermally insulated heating device, and more particularly but not solely to a thermally insulated heating device in wrap-around form for application around a feed barrel of a plastics injection moulding machine or plastics extrusion machinery including dies, for example- Plastics injection moulding machines typically include a feed barrel housing a rotating auger, for feeding pellets of plastics material into the machine: a plurality of electrical heater devices are wrapped around the barrel, and apply heat to melt the plastics pellets as they are fed towards the machine. Typically metal heaters of this type have a metal outer casing and no thermal insulation: accordingly a substantial amount of heat escapes radially outwards and is wasted; furthermore, the outer surface of the wrap-around heater becomes very hot (sometimes up to 250°C) and therefore represents a danger to operating personnel. We have now devised an arrangement which overcomes the problems outlined above. In accordance with the present invention there is provided an electrical heating device comprising an electrical heating element having a first surface for application directly to an object to be heated, and a thermally insulating layer which is enclosed within a cover which includes a sheet of heat-resistant material disposed over a second opposite surface of said electrical heating element. Preferably the heating device is of wrap-around form exhibiting a degree of flexibility so that it can be opened sufficiently to slip over, for example, the feed barrel of a plastics injection moulding machine, then closed up to embrace the feed barrel. The heating device may similarly be used on a plastics extrusion machine or on pipes. Preferably, a sheet or one or more straps of flexible material (preferably metal) is provided between the heating element and the thermally insulating layer, this sheet being provided with clamping means which bridge the gap between the two ends of the wrap-around heating device, such that when the heating device is fitted over the object to be heated, it may be tightened and secured in position by activating the clamping means. Preferably a heat barrier, for example a sheet of heat insulating paper or ceramics, is disposed between the heating element and the clamping sheet or strap. Preferably the cover of the heating device includes a sheet of plastics-impregnated fabric disposed over the layer of thermally insulating material. This forms an exterior surface of the heating device, in use thereof: the surface is therefore impervious to moisture and resistant to the adherence of molten plastics. Preferably this sheet comprises a woven fibreglass fabric having its outer surface impregnated with PTFE. Preferably the heat-resistant sheet material is capable of withstanding a continuous temperature of 250°C or more. Preferably the heat-resistant sheet material comprises a fabric woven from mineral or ceramic fibres. Preferably the thermally insulating material is arranged such that the exterior surface of the heating device does not exceed a temperature of 75°C whilst the opposite surface of the device runs at 300°C. Preferably the thermally insulating material comprises compressed mineral, ceramic or glass fibres. Preferably the thermally insulating layer has a thickness of between 20 and 30mm, most preferably 25mm. Preferably the heating element comprises a planar array of electrically insulating bricks through which electrical resistance heating wires run. Preferably these bricks interfit with each other in a manner enabling relative flexing, so that the heating element exhibits a degree of flexibility. The heating device of this invention provides substantial environmental benefits, including substantial energy saving, particularly since the heating element is applied directly to the object to be heated. Furthermore, the processing temperature of the apparatus, to which the heating device is applied, is relatively immune to changes in ambient temperature. Another advantage of the heating device of the present invention is that the terminal box, to which the electrical resistance wires of the heating element are brought and via which the heating element is powered, may be made of plastics since the outer surface of the heating device remains cool enough such that such a terminal box would not melt. Thus the device is inherently safer than the prior art devices mentioned above since it is not necessary to insulate the electrical components within the box from the box itself and, since plastics is not as dangerous to the touch as metal at equivalent temperature, the terminal box can be worked upon by personnel while the machine to which the heating device is fitted is still running. Moreover, preferably the heating device of this invention includes a thermosensitive element which is fitted to the outer surface of the device and indicates when the latter surface rises above and/or drops below a predetermined threshold temperature. Thus operating personnel can easily see whether the heating device is working correctly or not. The thermosensitive element preferably comprises a thermosensitive strap which is sewn onto the outer surface of the heating device. It has not been possible to incorporate such an element in the prior art heating devices mentioned above, because those heating devices have lacked any thermal insulation and the thermosensitive element would therefore have melted. Embodiments of the present invention will now be described by way of examples only and with reference to the accompanying drawings, in which: FIGURE 1 is a schematic elevation, partly in section, showing the feed barrel of a plastics injection moulding machine fitted with a number of wrap-around heating devices in accordance with a first embodiment of the present invention; FIGURE 2 is and end view, partly in section, of a wrap¬ around heating device in accordance with the first embodiment of the present invention; FIGURE 3 is a perspective view of the wrap-around heating device of Figure 2 ; FIGURE 4 is a view of a heating element of the heating device of Figures 2 and 3 ; and FIGURE 5 is a perspective view, partly in section, of a heating device in accordance with a second embodiment of the present invention. Referring to Figure 1 of the drawings, there is shown, in diagrammatic form, a feed barrel 1 of a plastics injection moulding machine 2. The feed barrel 1 has a hopper 3 at its outer end, into which plastics pellets are introduced. A rotating auger 4 is housed within the feed barrel 1, to feed the plastics pellets along the barrel 1 and into the machine 2. A plurality of heating devices 5 are wrapped around the feed barrel 1, to apply heat and melt the plastics pellets aε they are fed along the barrel. Figures 2 and 3 show the construction of the heating devices 5. Thus each heating device 5 comprises a number of successive layers of different materials, together formed into a generally tubular shape, but longitudinally split and sufficiently flexible that it can be opened up, slipped around the feed barrel 1 and then closed up again. More particularly, the heating device comprises a heating element 10, a flexible heat barrier 12, a sheet or strap of flexible material 14, an inner covering 16 of flexible, heat resistant material, a layer 18 of thermally insulating material, and an outer covering 20 of flexible material. The inner and outer coverings 16, 20 are stitched together along the longitudinal free edges of the heating device: the device further comprises two end coverings 21 (one shown) of the same material as the inner covering 16 and stitched to the outer covering 20. The sheet 14 (or strap) of flexible material, which is formed, for example, of aluminium or stainless steel, or a zinc-plated metal, is provided with a clamping arrangement 22 which bridges the longitudinal split in the heating element. The flexible heat barrier 12 may be formed for example of paper, which is both electrically insulative and heat resistant. The clamping arrangement comprises a bolt 22a which passes through a pin 22b and is threaded into a pin 22c: the latter pin 22c is of brass or bronze and so resists the bolt 22a seizing in its threaded bore. The device further comprises a flap 11 formed of the same material as, and stitched to, the outer covering 20. The flap 11 is positioned so that it projects at 13 beyond one end of the device and also projects at 15 beyond one of the longitudinal edges of the device. In use, when the device is wrapped around, for example, the feed barrel 1 of the plastics injection moulding machine, and the clamping arrangement 22 is tightened, the projecting portion at 15 of the flap 11 overlies the other longitudinal edge of the device and a velcro fastener 17, 19 (preferably of stainless steel) is engaged to fasten the device in position: furthermore, the projecting portion at 13 of the flap 11 overlies the adjacent heating device 5 on the feed barrel, as shown in Figure 1. As shown in Figure 4, the heating element 10 comprises a plurality of electrically insulating ceramic bricks 30 fitted together, and having electrical resistance heating wires, e.g. 40 running through them. Each ceramic brick 30 is of elongate shape, having opposite edges 31, 32 which are respectively convex and concave curved, and holes 33 for the wires 40 to extend through the brick between its curved surfaces. The ceramic bricks are arranged in successive rows, the bricks of one row having their convex-curved edges engaged with the concave-curved edges of the bricks of the adjacent row: accordingly, the adjacent rows of ceramic bricks are able to flex to a limited degree relative to each other, imparting a degree of flexibility to the heating element to enable the heating device to be opened up, slipped over the feed barrel 1 and then closed up again, as described previously. It will also be noted that the ceramic bricks of each row are staggered relative to the bricks of the adjacent rows, lengthwise of those rows. At a central portion of the heating element 10, the heating wires 40 extend outwardly to a terminal box 50 (Figure 2) mounted to the outer surface of the heating device 5. It will be appreciated that the heating element 10 is prevented from being separated from the other layers forming the heating device of the present invention because the heating wires 40 extend through each layer to the terminal box 50. In use, the heating device 5 is slipped over the feed barrel 1 and the clamping member 22 is used to tighten the device around the barrel in order to maximise the contact between the heating element 10 and the barrel 1. When electrical current is passed through the electrical resistance heating wires 40 via terminals 51 of the terminal box 50, the heating element 10 generates substantial heat, typically around 250°C, which passes directly to the feed barrel 1, around which the heating device 5 is wrapped. The heat barrier 12 ensures that a large proportion of the heat generated by the heating element 10 is applied to the barrel 1 and substantially reduces the amount of energy permitted to escape. A further proportion of any heat which is permitted to escape through the heat barrier 12 is prevented from escaping to the outer surface of the heating device by the layer 18 of thermally insulating material. The thermally- insulating layer 18 prevents substantial amounts of heat passing to the outer surface of the device 5, such that the latter surface remains at a substantially lower temperature to that of the inner surface. Thus while the temperature at the inner surface of the heating device may be around 300°C, the temperature at the outer covering 20 of the heating device is reduced to around 75°C and is therefore not excessively dangerous to touch. Preferably the material of the thermally insulating layer 18 comprises compressed mineral, ceramic or glass fibres, for example a material marketed by Carborundum Ltd. under the trade name Durablanket: the thermally insulating layer 18 typically has a thickness of typically 25mm. The inner covering 16 must be formed from a heat resistant material, preferably able to withstand continuous temperatures of around 500°C or more, although typically the heating device will be run at around 300°C. A suitable material from which the heat-resistant inner covering 16 may be formed comprises a fabric woven from mineral or ceramic fibres, the fabric being marketed by 3M under the trade name Nextel. The material of the outer covering 20 comprises a woven fibreglass fabric having its outer surface impregnated with PTFE: accordingly, if any plastics from the moulding machine should spill on this surface, it will not adhere permanently but can be wiped off. Thus the heating device of the present invention applies heat directly to the feed barrel, for example, but prevents a substantial proportion of that heat from escaping such that the outer surface of the device does not become excessively hot. Although the heating device of Figures 2 to 4 has been described for use on the feed barrel of a plastics injection moulding machine, it may be used on other apparatus requiring heating or maintaining hot, e.g. plastics extrusion machines including dies, or pipes. As shown in Figure 5, the heating device may be in a flat shape. In this example, the device comprises a heating element 60 of the same construction as shown in Figure 4, but laid flat, a heat barrier 62 of heat-resistant material, for example paper, an inner covering 64 of heat-resistant material, a layer 66 of thermally insulating material, and an outer covering 68. The outer covering 68, which comprises a sheet of PTFE impregnated material, and the inner covering 64 are again stitched together such that the thermally insulating layer 64 is enclosed between the two. The heating element 60 is again prevented from being separated from the other layers forming the device by the wires (not shown) which extend from the electrical resistance heating wires of the heating element 60 through all of the other layers and are brought out to a terminal box 70. The heating devices of Figureε 2 to 4 may comprise a current senεor having a light emitter, to indicate whether the electrical heating element iε functioning. Thiε current εensor is shown at 52 in Figure 2 and its light emitter ( a light emitting diode) at 54. The current senεor may be a CR-45 sensor from CR Magnetics Inc. of Fenton MO, USA. The current senεor compriεes a ring-shaped plastics housing as shown and one of the electrical wires from the heating element pasεes through the centre of thiε ring to a reεpective terminal 51 of the terminal box 50: the electrical wire iε inductively coupled to the current sensor and so long as current is passing through this wire, the light emitter 54 of the sensor is illuminated. The current sensor may be positioned within the terminal box 50, as shown in Figure 2, with the light emitter projecting from or visible at a wall of the terminal box. Alternatively, as εhown in Figure 5, the current εenεor may be mounted under the outer covering of the heating device, with the light emitter 54 projecting therefrom. In any event, the current sensor iε on the lower-temperature side of the thermally insulating layer 18 or 66 of the heating device, and therefore not subjected to excessive heat. The current sensor enables operating personnel to see readily whether the heating element of the heating device iε functioning correctly or not, depending whether the light emitter iε illuminated or not. The current εensor therefore provides a very quick indication if a heating element of any heating device is faulty. However, provided that the heating element is operating correctly, the current senεor does not give any indication that anything else may be wrong. Thus an additional or alternative feature of the heating devices of Figures 2 to 4 and 5 is the provision of a thermosenεitive device (not εhown) which iε fixed to the outer εurface of the heating device. The thermoεenεitive element may comprise a strap which is εewn onto the outer covering on the device and is fitted with a visual indicator which, for example, may turn green if the temperature of the outer surface of the heating device drops below a certain threshold level and which turns red if the temperature rises above a certain threshold level. Thuε it is posεible to monitor the whole heating device in order to enεure that it iε operating correctly. In order for εuch thermosensitive elements to be employed, it iε necessary for the temperature of the outer εurface of the heating device to be relatively low, otherwise the element would be damaged. Thus the heating device of the present invention haε the advantage that a thermoεensitive device can be used to monitor the correct operation of the device. Another advantage of the heating device according to the present invention is that, due to the reduced temperature on the outer surface thereof, a plastics terminal box may be used inεtead of one formed of metal. Thiε is inherently safer because plastic iε not as dangerous to the touch as metal and it is alεo poεεible to work on the box while the machine iε running. Preferably a thermoεensitive strip is sewn or otherwiεe fixed onto the outer surface of the outer cover 11, arranged to indicate (by a change of colour) if the outer temperature of the device is above an upper threshold, or below a lower threshold, indicating some fault in the running of the apparatus.

Claims: Claims 1) An electrical heating device compriεing an electrical heating element having a first surface for application directly to an object to be heated, and a thermally insulating layer which is enclosed within a cover which includes a sheet of heat-resistant material disposed over a second opposite surface of said electrical heating element. 2) An electrical heating device as claimed in claim 1, wherein the device iε in wrap-around form, 3) An electrical heating device as claimed in claim 2, wherein a sheet or one or more straps of flexible material is provided between the heating element and the thermally insulating layer, the sheet or one or more straps being provided with clamping means which bridge the gap between the two endε of the wrap-around heating device. 4) An electrical device aε claimed in claim 3, wherein the sheet or one or more straps are formed of metal. 5) An electrical heating device as claimed in any preceding claim, wherein a heat barrier is disposed adjacent the heating element. 6) An electrical heating device as claimed in claim 5, wherein the heat barrier is formed of heat-insulating paper or ceramics. 7) An electrical heating device as claimed in any preceding claim, wherein the cover includes a sheet of plastics-impregnated fabric disposed over the layer of thermally insulating material. 8) An electrical heating device as claimed in claim 7, wherein the sheet of plasticε-impregnated fabric compriεeε a woven fibreglass fabric having its outer surface impregnated with PTFE. 9) An electrical heating device as claimed in any preceding claim, wherein the sheet of heat-resistant material comprises a fabric woven from mineral or ceramic fibres. 10) An electrical heating device as claimed in any preceding claim, wherein the thermally insulating layer is formed of compressed mineral, ceramic or glass fibres. 11) An electrical heating device as claimed in any preceding claim, wherein the thermally insulating layer has a thickness of between 20 and 30mm. 12) An electrical heating device as claimed in claim 11, wherein the thermally insulating layer has thickness of 25mm. 13) An electrical heating device as claimed in any preceding claim, wherein the heating element comprises a planar array of electrically insulating brickε through which electrical reεistant heating wires run. 14) An electrical heating device aε claimed in claim 13, wherein the electrically inεulating brickε interfit with each other εo as to enable relative flexing. 15) An electrical heating device aε claimed in any preceding claim, wherein a thermosenεitive element is fitted on the outer εurface of the device and indicateε when the latter εurface rises above and/or drops below a predetermined threshold temperature. 16) An electrical heating device as claimed in claim 15, wherein the thermosenεitive element compriεeε a thermoεenεitive strap which is εewn onto the outer surface of the heating device.

Inventors: SCHEERES DAVID JOHANNES, SCHEERES, DAVID, JOHANNES
Applicants: EXOTHERM PROD LTD, SCHEERES DAVID JOHANNES, EXOTHERM PRODUCTS LIMITED, SCHEERES, DAVID, JOHANNES
Application year: 1996
Main classification: H05B3
IPC codes: B29C45, B29C47, H05B3