GB2134839A - Manufacture of moulded articles - Google Patents

Abstract

In a method and apparatus for the manufacture of moulded articles which require the simultaneous application of heat and pressure as part of the manufacturing process use is made of a platen which incorporates electrically energisable windings of a kind which provide a travelling wave field. The field emanates from a plurality of co- extensive pole faces at a face of the platen and is selectively directed at a mould such that the heating effect of the field is concentrated substantially wholly in the mould. The resulting field effect facilitates the efficient use of electrical energy for heating of articles in a mould, and the uniformity of the heating effect permits a higher degree of temperature control and uniformity than hitherto attainable.

Description

SPECIFICATION Manufacture of moulded articles This invention relates to a method of and apparatus for the manufacture of moulded articles of a kind which require the application of heat and pressure as a part of the manufacturing process.

One method which has long been used for the manufacture of articles of curable materials, and in particular precision rubber articles such as rubber O-ring seals, employs a pair of mould halves formed with cavities which, when the pair of halves are assembled, define the required shape of the O-ring seal. Raw material for forming the article is inserted in the cavities either by pressure injection through feed channels in the mould, or by placing the required quantity of material into each cavity before the mould halves are brought together.

The application of heat and pressure to the mould is then effected by positioning the mould halves in a press of a kind having a pair of heated platens which engage surfaces of the respective mould halves.

For long it has been the practice to use platens of a kind having internal cavities through which steam at a high pressure may be fed to effect the required heating. Alternatively, where a steam supply is not readily available, or a more accurate degree of temperature control is required, it has been common to employ induction heating for the platens; this has been achived by employing platens of a ferrous material and each formed with a cavity of a suitable size for location of an induction coil.

The use of induction heating as compared with steam heating can result in a saving of energy costs in some applications, but with continually increasing energy costs further ways have been sought for some time to improve the mould heating efficiency. Also, although induction heating facilitates reasonably good temperature control, it is found that for the manufacture of certain precision components an even greater degree of temperature control would be advantageous.

Some benefit has been achieved by the provision of thermal insulation around the sides of the platens, and also between the platens and the associated press body, but nevertheless there continues to be a substantial loss of energy and inability to achieve a suitably improved accuracy of temperature control.

The present invention seeks to provide a method of and apparatus for the heating of articles during moulding in which still further improvements are obtainable in the reduction of thermal energy loss and accuracy of temperature control.

In accordance with one aspect of the present invention apparatus for the simultaneous application of heat and pressure to an article in a mould comprises press means having a pair of press members movable one relative to the other and between which mould means of a kind comprising electrically conductive material is positioned for the application of pressure thereto, and heating means for the application of heat and pressure to the mould means, said heating means comprising electrical windings which are electrically energisable to act as the primary of a travelling wave field generator, and the secondary of the field generator being comprised substantially entirely by the mould means of electrically conductive material such that the principal heating effect of the primary is concentrated in the mould means.

In accordance with another aspect of the present invention there is provided heating means comprising: a plurality of laminations each extending in a plane which is generally perpendicular to a face of the heating means which in use confronts mould means to subject the mould means to heat and pressure, said laminations being shaped to define at said face a plurality of co-extensive pole faces, and electrical windings associated with successive pole faces and arranged for connection to successive phases of a multiphase power supply thereby to result, when energised, in the generation of a travelling wave field.

In accordance with a further aspect of the present invention a method for the manufacture of a moulded article which requires to be subject to heat and pressure when in a mould comprises providing mould means of a kind comprising electrically conductive material; providing heating means which is electrically energisable to act as the primary of a travelling wave field generator, and arranging the mould means between a pair of relatively movable press members such that, in use, the heating means applies pressure and directs a travelling wave field to the mould means which serves as a short circuited secondary wherein the heating effect of the primary generator field is substantially wholly concentrated.

The heating means may be comprised by at least one of said press members or a mould may embody said heating and mould means. However to facilitate ease of mould handling and any servicing necessary to the heating means it is preferred thåt the heating means is releasably securable to a press member in such manner that in use it lies between a mould and a press member to confront a surface of the mould for the application of heat and pressure thereto.

Preferably a layer of electrically non-conductive thermal insulation material is provided between the heating means and mould means to minimise heat loss from the mould to the heating means.

Conveniently the thermal insulation material is secured to the heating means. Preferably a thin metal plate, e.g. of stainless steel, is provided to cover the exposed surface of the thermal insulation material to protect the latter from the wear which otherwise might result from direct contact with the mould means.

Pì-eferably the heating means comprises a plurality of co-planar and co-extensive pole faces, said faces being arranged at that face of the heating means which is intended to confront a mould means so that, in use, the field generated by the heating means is selectively directioned to the mould means. The windings associated with successive pole faces preferably are connected to respective phases of a multi-phase power supply thereby to result in the generation of a travelling wave field. The co-extensive poles may be parallel with each other or, for example, may be arranged in a concentric manner.

Preferably the heating means is comprised by a press platen, which may be one of a pair of platens, operable to apply pressure to the mould means when positioned between the relatively movable press members. The heating means, irrespective of whether it is comprised by a press platen, may be integral with or detachably secured to a press member, or may rest unsecured on a press member.

Ferrous metals are particularly suitable for forming the mould means in view of their general electrical properties and strength but other materials such as aluminium may be used especially if any pressure which the plate is required to exert on the mould means is not so great as would cause distortion of a mould means of aluminium or similar strength material.

The present invention requires the provision of a travelling wave field generator in, for example, only one of the pair of platens or equivalent press members which conventionally are used to sandwich and compress a mould means, though if desired for additional heating two heating means may be provided and arranged to confront opposite surfaces of the mould means. To minimise heat losses where only one heating means is provided the platen or equivalent part of the press body opposite the generator should be thermally insulated from the mould means, e.g. by being provided with a layer of electrically nonconductive thermal insulation material and a cover plate as above-described in respect of the heating means.

The heating means comprising the primary of the travelling wave field generator preferably is of a laminated ferritic construction thereby to minimise heat generation within itself.

To assist further in reduction of thermal losses, the sides of the mould means may be provided with electrically non-conductive thermal insulation material.

To assist in achieving substantial uniformity of temperature throughout the mould means, and avoidance of localised hot spots, the electrical windings of the heating means may be arranged to result in a non-uniform distribution of heat input to the mould means in such a manner as to compensate for a non-uniform distribution of heat loss therefrom.

The provision of a non-uniform distribution of heat input is particularly desirable for mould means which, because of considerations such as those of space, is not provided with thermal insulation at surfaces which in use of the mould means normally are exposed to the ambient air temperature. However, a non-uniform distribution of heat input may be advantageous also for mould means provided with thermal insulation so as to permit a greater degree of temperature uniformity to be achieved.

For the manufacture of some moulded articles it may be preferable to subject some regions of an article to a higher curing temperature than other regions. The desired variation of temperature is unlikely to correspond with that due to thermal loss from a uniformly heated mould means but may be achieved by the above-referred feature of a non-uniform distribution of heat input.

A non-uniform distribution of heat input may be obtained by, for example, a non-uniform spacing of the successive pole faces of the heating means, or a variation of the number or size of wires in the windings associated with successive pole faces. In addition, winding channels which define therebetween the pole faces that assist in creating the travelling wave field may be provided with slots at right angles thereto so that the wires in the winding channels can be diverted thereby to control the field at right angles to the direction of the field due to the main windings.

Embodiments of the invention and details of the prior art apparatus will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a side view of a conventional single daylight induction heated press; Figure 2 is an exploded perspective view of a conventional induction heated platen; Figure 3 is a side view of a conventional double daylight induction heated press; Figure 4 is a side view of a single daylight press in accordance with the present invention; Figure 5 is a perspective view of part of the heating means of Figure 4; Figure 6 is similar to Figure 5 but showing schematically the winding pattern; Figure 7 is a perspective view of part of the heating means; Figures 8a and 8b show in cross-section details of part of the heating means construction; ; Figures 9a and 9b show in cross-section details of temperature sensors for sensing the mould temperature; Figure 10 is a side view of a double daylight press in accordance with the present invention; Figure 11 is an exploded perspective view of a mould and heating means of another embodiment of the invention; Figure 11 a shows in detail a part of Figure 1 1, and Figure 12 shows in cross-section a press in combination with the mould and heating means of Figure 11.

A conventional single daylight up-stroking type moulding press as shown in Figure 1 comprises a fixed press head 10 and a lower press table 11 movable vertically relative to the press head. The press head has secured thereto a layer of electrically non-conductive thermal insulation material 12 and a heated top pressure platen 13, and the press table has secured thereto a similar layer of insulation 12 and a heated bottom platen 14. The arrangement is such that the two platens face one another and are thermally insulated from the head 10 and table 1 1 of the press body.

Mould means comprising a two-part mould 15 for containing the material which is to be subject to the combined action of heat and pressure is arranged to be slidable laterally, in a closed position, to between the platens 13, 14. The press table 11 is then operable to urge the mould upwards against the top platen 13 and maintain said condition for the time required to treat the material in the mould.

The construction of each conventional press platen 13, 14 is shown in more detail in Figure 2.

Each platen comprises a pair of thick ferrous metal plates 1 6, 1 7 typically formed of steel. One plate, 17, is formed with an annular cavity for location of a toroidal induction coil 1 8. The windings of the coil 18 extend generally in the circumferential direction of the coil. Saod coil acts in use as the primary of a transformer type induction heater, with the surrounding ferrous metal plates 16, 17 of the platen serving as a short circuited secondary which therefore becomes heated in consequence of electrical energisation of the coil by alternating current of a suitable magnitude and frequency.

The supply of electrical energy to the coil is controlled in part by a temperature control sensor 1 9 positioned in a platen near to that platen surface which contacts the mould.

Where it is required to use one press body simultaneously to heat and compress the material in two moulds, a third heated platen 20 is provided, as shown in Figure 3, the press then being known as a double daylight type press. The third platen incorporates a temperature control sensor and induction heater similar to those of the other two platens 13, 14, but does not, of course, require to be thermally insulated from the press body.

A single daylight type press in accordance with the present invention is shown in Figure 4 and comprises a press head 10 and vertically movable press table 11.

In contrast to the conventional single daylight press incorporating two heated platens, in the press shown in Figure 4 there is provided only one heating means, referred to as a heat generator 21, which although serving to effect heating of the mould is in itself substantially unheated. The construction of the generator 21 is described in more detail below.

The facing surfaces of the press head and the heat generator are each covered first with a layer of electrically non-conductive thermal insulation 22 and then with a thin aluminium wear plate 23.

Each wear plate 23 is arranged to be readily removable for replacement if worn or damaged to such an extent that the thermal insulation material is no longer protected from direct contact with the mould 15.

The heat generator 21 (see Figure 5) comprises a plurality of iron laminations 27 which, when assembled by bolts 43 extending therethrough form a series of straight and parallel pole faces 24.

Prior to assembly the laminations are coated with a varnish to reduce eddy-current effects.

The resulting slots between the pole faces 24 are lined (see Figure 8a) with empire tape and leatheroid or like materials, such as are conventionally used in the winding or electrical motors, to provide electrical insulation between the laminations and enamel coated copper wires 25 (see Figure 8b) which are then wound around the pole faces. The layout of the copper windings 25 is shown schematically in Figure 6, and the assembly of wound laminations is subsequently encapsulated in epoxy resin. The resulting construction of the generator 21, with an electrical connection box 28 secured thereto, is shown in perspective in Figure 7. The connection box 28 facilitates connection of the windings to an industrial three phase supply, the standard 50 c.p.s. supply being suitable.The connections are made such that the windings of successive pole faces are connected respectively to the successive phases of the supply.

The temperature of the mould is sensed by a temperature control sensor 26 located between the thermal insulation layer 22 on the heat generator 21 and the associated wear plate 23 in good thermal contact with the latter. Alternative constructions of suitable temperature sensors are shown in detail in Figures 9a and 96.

In Figure 9a there is shown a temperature sensor comprising a thermocouple probe 30 which terminates in a copper contact pad 31 resiliently mounted by means of a spring washer 32 which acts between the pad and the thermal insulation material 22. The insulation material is suitably recessed with a shoulder 33 for retention of the pad against the action of the spring washer.

In the construction shown in Figure 9b a curved copper strip 34 is used to combine the functions of the copper contact pad and spring washer of the construction shown in Figure 9a. A thermocouple 35 is soldered to the copper strip for sensing the temperature thereof.

The orientation of the temperature sensors of Figures 9a and 9b when in situ is such that the pad 31 of Figure 9a or the strip 34 of Figure 9b bears against a wear plate 23.

In a second embodiment of the invention, illustrated in Figure 10, there is provided a double daylight type press for the application of heat and pressure to two moulds simultaneously. The press is constructed substantially similar to that shown in Figure 4 except that attached to the press head 10 there is provided a second heat generator 40 for heating an upper mould 15a.

A second temperature sensing unit 26 is located between the layer of the thermal insulation and wear plate of the second generator 40 for the purpose of sensing the temperature of the upper mould 15a. Under some conditions the second, upper mould 15a may be positioned directly on the lower mould 15b but alternatively a plain steel carrier plate 41 (see Figure 10) may be suspended from the press head 10 via hanging bars 42 for the purpose of supporting the upper mould. Thus the lower mould may be removed independently of the upper mould.

Because of the electromagnetic clamping effect experienced by the mould when the heat generator is energised, it is necessary to remove power from the generator when the press is open to enable mould withdrawal. This is achived by providing a pressure sensing device in the hydraulic feed pipe to the press actuating cylinder to monitor the pressure build-up when the press is closed and switch the power on to energise the generator(s) and then maintain the energised condition whilst the pressurised condition prevails. As the pressure reduces to allow the press to open the generator(s) are therefore automatically de-energised.

This feature has a further benefit in that total energy consumption is reduced by ensuring that the generators are inactive whilst the moulds are removed from the press.

For the manufacture of annular shaped articles, especially relatively large articles such as tyres, considerations such as those of thermal capacity of the mould lead to the use of moulds of an annular shape. In this case the heat generator also may be of an annular shape.

Figures 11, a and 12 show the use of an annular shaped heat generator 50 arranged to lie on a press platen 51 and in turn support an annular type mould 52.

In this case the generator 50 is formed of laminations each lying in planes which are parallel with and contain the major axis of the generator.

An end surface 53 of the generator is formed with a plurality of concentric pole faces defined by a plurality of circumferentially extending grooves 54 for the electrical winding. In order for the windings to extend between the successive grooves and thereby result in the required travelling wave the pole faces should be circumferentially discontinuous. That is, radially extending grooves are additionally provided to permit the windings to extend between successive circumferential grooves.

For applications of the invention in which the windings of the heating means are connectable to a three phase supply they may be selectively connectable either in star or delta formation.

When connected in delta formation the rate of energy input and thus rate of application of heat to the mould is greater than when the electrical windings are connected in star formation.

Preferably the electrical windings are connected in delta formation during the period of initial heating of the mould means from ambient temperature, and as the required mould temperature is approached the electrical windings are switched to a star connection. There is thereby achieved a rapid rate of energy input and thus heating of the mould towards the required temperature whilst the lower rate of energy input subsequent to switching to star connection provides a greater measure of control of the mould temperature and assists in avoiding scorching of the material in the mould.

Although reference has been made specifically to switching between star and delta connections it is to be understood that where the electrical windings of the heating means are intended for connection to a multi-phase supply other than of three phases other patterns for connection of the windings may be employed so as to provide at least two rates of energy input depending on the switching arrangement. In general the windings should be connected for maximum rate of energy input during initial warm up of the mould means, and switched to a connection arrangement which gives a lower rate of energy input when the mould means is at substantially the required temperature.

Although the invention has been described specifically in respect of the manufacture of articels which are removed from the mould subsequent to the required application of heat and pressure, in other embodiments of the invention the mould, or a part thereof, which is subject to heat and pressure may form part of the finished article. Thus a metallic element which defines at least part of a mould means may be bonded to the curable material during the application of heat and pressure so as to result in a composite type article.

For the manufacture of an elongate composite article such as a strip comprising a layer of cured material and one or two face layers of metallic material bonded thereto, the metallic material and uncured curable material may be progressed in a stepwise manner through the press means and heating means so that successive lengths are cured in sequence. Thos surfaces of the curable material not covered by metallic material which is to form part of the finished composite article preferably are contacted by re-usable mould means whilst subject to the simultaneous compression and heating effects of the press means and heating means respectively.

Additionally mould means of a re-usable type may be provided to support the metallic material which forms part of the finished article, especially where said metallic material is relatively thin or not of a shape complementary to that of one of the confronting surfaces of the press means and heating means.

From the foregoing it will be apparent that the use in accordance with the invention of heating means comprising a travelling wave field generator results in particularly significant advantages.

Firstly because the heat generator containing the electrical winding is not directly self-heated to a substantial extent there is a saving in total electrical energy consumption and a reduction of heat losses.

Furthermore the consequential decrease in the thermal response time, for a given electrical energy input to the winding, results in the mould temperature increasing more rapidly than in a conventional press in which the platen additionally requires to be heated. Thus apart from saving of energy a single press unit can readily be used to effect cure of a greater number of articles per working shaft than hitherto, or the same number of articles can be processed using fewer presses.

Particularly in contrast to hitherto used heating means of the kind employing an induction winding, the use of means which serves as a travelling wave field generator readily permits selective directioning of the heating effect so that it may be concentrated substantially wholly within the mould means and provide a substantially uniform heating effect throughout the mould means.

In particular, the use of co-extensive pole faces, whether parallel or concentric, gives an important practical effect when the pole faces are connected to the respective phases of a multiphase supply.

By virtue of the travelling wave nature of the resulting field, and its uniform strength as considered in the direction of the length of each longitudinally extensive pole face there results the normally desired substantially uniform dissipation of energy throughout the mould means. However, if a non-uniform dissipation of energy is required in the mould means, for example to compensate for thermal loss from the mould means or to give a desired variation of temperature within the mould means, the required effect may be achieved readily by appropriate selective arrangement of the electrical windings of the heating means.

Other notable benefits derived from the invention are that in contrast to a conventional single daylight press having two platens each with induction coil windings, a press in accordance with the present invention requires only a single heat generator. Accordingly in some cases it is possible to convert single daylight presses into double daylight presses where the spacing between the press head and press table although insufficient to accommodate three conventional induction heated platens is adequate to locate two heat generators and thermal insulation layers.

It will be further observed that more accurate sensing of the mould temperature is possible by means of temperature sensors positioned adjacent or close to the mould means because the mould means is not contacted by platens at a higher temperature than the mould. In consequence the heating means can more readily be controlled to provide the required temperature within the mould means.

Claims (27)

1. Apparatus for the simultaneous application of heat and pressure to an article in a mould comprising:~ press means having a pair of press members movable one relative to the other and between which mould means of a kind comprising electrically conductive material is positioned for the application of pressure thereto, and heating means for the application of heat and pressure to the mould means, said heating means comprising electrical windings which are electrically energisable to act as the primary of a travelling wave field generator, and the secondary of the field generator being comprised substantially entirely by said mould means of electrically conductive material such that the principal heating effect of the primary is concentrated in the mould means.

2. Apparatus according to Claim 1 wherein the electrical windings of the heating means are electrically energisable to create a travelling wave field which moves in a direction generally transverse relative to the direction of application of pressure.

3. Apparatus according to Claim 1 or Claim 2 wherein the heating means comprises a plurality of co-extensive pole faces.

4. Apparatus according to Claim 3 wherein the pole faces are arranged in a concentric manner.

5. Apparatus according to Claim 3 wherein the pole faces are parallel one with the other.

6. Apparatus according to any one of Claims 3 to 5 wherein the windings associated with successive pole faces are connected to respective phases of a multi-phase power supply.

7. Apparatus according to any one of Claims 3 to 6 wherein the pole faces are co-planar.

8. Apparatus according to any one of the preceding claims wherein the mould means is integral with the heating means.

9. Apparatus according to any one of Claims 1 to 7 wherein in use the heating means lies between the mould means and a press member to confront a surface of the mould means for the application of heat and pressure thereto.

10. Apparatus according to Claim 9 wherein only one surface region of the mould means is confronted by heating means.

11. Apparatus according to any one of Claims 1 to 7 wherein the heating means is comprised by a press platen which incorporates the electrical windings of a travelling wave field generator.

12. Apparatus according to any one of Claims 1 to 7 wherein the heating means is detachably securable to the press means.

13. Apparatus according to any one of Claims 1 to 7 wherein the heating means is integral with a press member.

14. Apparatus according to any one of the preceding claims wherein a layer of electrically non-conductive thermal insulation material is provided between the heating means and the mould means.

1 5. Apparatus according to Claim 14 wherein said thermal insulation material is secured to the heating means.

16. Apparatus according to Claim 15 wherein the thermal insulation material is provided with a wear resisting cover layer.

17. Apparatus according to any one of the preceding claims wherein the heating means is of a laminated construction in which the laminations each lie in planes parallel with the direction of the compressive force exerted by the press means on the mould means.

18. Apparatus according to any one of the preceding claims wherein in use the heating means provides a non-uniform distribution of heat input to the mould means.

19. Apparatus according to Claim 1 and constructed and arranged substantially as hereinbefore described with reference to the accompanying drawings.

20. Heating means for use in apparatus according to any one of Claims 1 to 7 and comprising:- a plurality of laminations each extending in a respective plane which is generally perpendicular to a face of the heating means which in use confronts mould means to subject the mould means to heat and pressure, said laminations being shaped to define at said face a plurality of co-extensive pole faces, and electrical windings associated with successive pole faces and arranged for connection to successive phases of a multiphase power supply thereby to result, when energised, in the generation of a travelling wave field.

21. Heating means according to Claim 20 wherein a layer of electrically non-conductive thermal insulation material is provided on said face which in use confronts the mould means.

22. Heating means according to Claim 20 or Claim 21 wherein said face is covered by a layer of wear resisting material for direct contact with said mould means.

23. Heating means according to any one of Claims 20 to 22 and comprising a temperature sensor.

24. Heating means according to Claim 23 when dependent on Claim 21 wherein the temperature sensor is located in the layer of thermal insulation material.

25. Heating means constructed and arranged substantially as hereinbefore described with reference to the accompanying drawings.

26. Method for the manufacture of a moulded article which requires to be subject to heat and pressure when in a mould comprising: providing mould means of a kind comprising electrically conductive material, providing heating means which is electrically energisable to act as the primary of a travelling wave field generator, and arranging the mould means between a pair of relatively movable press members such that, in use, the heating means applies pressure and directs a travelling wave field to the mould means which serves as a short-circuited secondary wherein the heating effect of the primary generator field is substantially wholly concentrated.

27. Method according to Claim 26 and substantially as hereinbefore described with reference to the accompanying drawings.