WO1981001117A1

WO1981001117A1 - Improvements relating to the moulding of thermosetting moulding material

Info

Publication number: WO1981001117A1
Application number: PCT/GB1979/000172
Authority: WO
Inventors: R Pointer
Original assignee: Nat Plastics Ltd; R Pointer
Priority date: 1979-10-26
Filing date: 1979-10-26
Publication date: 1981-04-30
Also published as: EP0041944A1

Abstract

Thermosetting moulding material in loose granular or powder form is pre-heated before moulding in an electromagnetic induction oven (5) in a container (2) of moisture-permeable, thermally-insulating non-dielectric material, preferably of "rigid" expanded polystyrene of a density of about 25 grams/litre. The invention includes apparatus for charging the container (2) with moulding material and for moving it to an oven (5) and from the oven to discharge the material into a mould (7).

Description

Improvements relating to the moulding of thermosetting moulding material

In the moulding of thermosetting moulding materials a common preliminary step is to pre-heat the moudling material, usually in powder or granular form, to a tempera¬ ture as near as practicable to the moulding temperature so as to soften the material before it is placed in the mould, making it flow more readily in the mould and so to reduce the duration of the moulding process.

To facilitate the handling and pre-heating of the mould¬ ing material it is common to form the material under a low degree of compression at a relatively low temperature into ^■pellets' of such size that one or more of them comprises the appropriate quantity of material for one moulding. The pellet or pellets is/are usually preheated by electro- •magnetic induction, for example by use of a radio frequency oven to soften them, the softened pellets being transferred to the oven and thence to the mould by hand.

Whilst this procedure is satisfactory in many cases it does not provide the desirable uniformity of heating with all moulding materials and has other disadvantages. Recently attempts have been made to avoid the pelleting step by preheating loose moulding powder or granules. An appropriate quantity of powder or granules has been placed in a container such as a polyethylene or cardboard box for preheating in the induction oven, the box being removed from the oven by hand and the contents then being tipped into the mould. These attempts have not been entirely successful in that uniform heating is still difficult to attain, since the boxes retain moisture given off by the moulding material during the pre-heating operation and this retained moisture absorbs radio frequency energy giving rise to excessive localised heating of the moulding material near the walls of the box and/or of the walls of the box them¬ selves. Excessive localised heating of the moulding mater¬ ial results in low quality mouldings whereas localised heating of the walls of the box is likely to cause damage to the box. Furthermore, moisture collected on the walls of the box may be tipped into the mould with the mouldin -material to give rise to bubbles in the final moulding.

According to the invention in a moulding process lo thermosetting moulding material in powder or granular fo is pre-heated in an electromagnetic oven in a container o moisture-permeable, thermally insulating, non-dielectric material.

By a non-dielectric material we mean a material that is not substantially heated by electromagnetic induction effects when placed in the electromagnetic field of an in duction oven. Such materials are identified by low diele tric constants and small loss angles, thus having low los factors. (The loss factor is the multiple of the dielectr constant and the dissipation factor, the tangent of the l angle).

The other specified properties of the material of th container are important for the following reasons:

Moisture-permeability enables moisture present in th moulding material to be dissipated through the walls of t container as the material is heated. If all the moisture not removed from the moulding material and the container during the pre-heating it can give rise to the problems m tioned above.

Thermal insulation assists in the rapid and uniform heating of the charge of moulding material, providing bet flow of the material in the mould than has been obtained with other pre-heating methods and thus leading to the pr duction of mouldings with reduced internal stresses than has hitherto usually been the case. It is desirable to avoid substantial compression of the charge of moulding material before it is placed in th mould. During pre-heating, the volume of the charge tends to increase and compression can be prevented by using a r ilient container (which will permit the charge to expand whereas the previously-mentioned polyethylene or cardboar boxes tended to cause the granules of the charge to be co pressed together so that the material did not flow readil in the mould) or by a suitable shaping of the material-

OKPI containing space in the container.

A moulded container having a material-containing space which represents a semi-ellipsoid is particularly suitable. We have found that the material having the best co - bination of these properties is the so-called "rigid" expanded polystyrene of density of 25 grams/litre. This material can readily be fabricated into containers by known methods (e.g. by moulding) and it has the further advantage that it can provide a smooth textured internal surface to the container from which the granules of the pre-heated charge are readily released. We have found that any parti¬ cles of moulding material which stick to the walls of the container and which become incorporated with a new charge when the container is next filled tend to be heated much more rapidly than the new charge and give rise to tempera¬ ture variations across the charge and, sometimes, damage to the container.

Preferably the internal volume of the container is such as to be substantially filled by the proposed charge of moulding material, whereby if the induction oven is arranged with its electrodes spaced apart a distance minimally in excess of the height or length of the container, the inter- electrode space is substantially filled by at least one wall of the container and a substantially uniformly thick layer of moulding material. Minimum radiation loss and improved uniformity of heating result. However, for some large cav¬ ities it can be desirable to load the pre-heated moulding material separately into different parts of the cavity and in this case the moulding material for the cavity will be preheated in two or more containers each having an internal volume such that it is substantially filled by the requisite quantity of moulding material.

The container may have a removable lid of moisture- permeable, thermally insulating, non-dielectric material which is kept closed during the preheating step of the moulding process.

Use of the method enables the charge of moulding mater¬ ial to be heated more economically, more rapidly and more uniformly than has hitherto been the case. Because the heating is more rapid and uniform it is possible to heat charge to a higher temperature than usual without risk of the material beginning to polymerise and set before it is placed in the mould. Consequently the material flows mor readily in the mould and less heating is needed during th moulding cycle, which can thus be reduced in duration. I one example a moulding requiring 283.5 gms of phenolic moulding material was previously produced by forming the material into two equal pellets for pre-heating in a radi frequency electromagnetic induction oven. The pellets we placed in the oven and transferred from the oven to the mould by hand. Normal production was 31.25 mouldings per hour. By eliminating the pelleting step and weighing the moulding material into a rigid expanded polystyrene box

(whose walls were moisture-permeable, thermally-insulatin and non-dielectric) which was placed in the oven and then discharged into the mould by hand, it was possible to in¬ crease the production rate to 53.7 mouldings per hour using the same operator. The shortened moulding cycle necessitated the introduction of a separate fettling oper tion instead of this being incorporated in the work load the mould operator, but despite this the production costs were reduced by approximately 25 per cent and the quality of the mouldings was improved.

The container may be simply a box, to be discharged into the mould by inversion, or it can have an open botto closed by a separate plate. The plate can be a metallic baseplate which will rest upon the bottom electrode of an oven having horizontal electrodes, becoming electrically a part of the bottom electrode, so that the moulding mat ial effectively fills the space within the container between the electrodes. After pre-heating, the container with its baseplate can be transferred to a position above the mould cavity and the baseplate then withdravm to dis¬ charge the moulding material into the cavity.

The invention includes apparatus for use in associat with moulding equipment for thermosetting moulding materi f -⁰ including a pre-heating electromagnetic induction oven and a mould having a moulding cavity for receiving pre-heated moulding material from the oven, which apparatus is charac¬ terised by a plurality of containers of moisture-permeable, thermally-insulating, non-dielectric material each capable of receiving a charge of the moulding material sufficient for the cavity of the mould or a part thereof, means for supplying the charges to the containers and trans¬ fer means for moving charged containers one-by-one to the oven and for transferring the pre-heated charges from the containers to the moulding cavity.

The automatic apparatus may be simply a conventional automaton or robot device programmed to perform manipula¬ tions of the kind previously mentioned being performed by an operator. Such an automaton or robot may be linked to the moulding equipment so that the pre-heating of one charge is completed just as the mould is opened and cleared of the previous moulding so that maximum use of the mould is obtained. One form of automatic apparatus is illustrated in the accompanying drawing, the sole Figure of which is a dia¬ grammatic side elevation of the apparatus.

Referring to the drawing, a belt 1 of woven polyester fabric, which is a non-dielectric material, is arranged to carry a plurality of spaced-apart containers 2, each made of "rigid" expanded polystyrene of a density of 25 grams/ litre between three stations which are shown at A, B and C in the drawing. At station A, at the left-hand side of the drawing, the container 2 is supported on an automatic weighing machine 3 below the discharge outlet of a hopper 4 containing particulate moulding material. The weighing machine 3 is linked to a discharge control device -4a of the hopper -4 so that following correct positioning of the con¬ tainer 2 on the weighing machine 3 discharge of the moulding material is initiated and then terminated when the required weight of material for the mould cavity has been received in the container 2. In the second station B the charged container 2 is positioned between the plates of a radio- frequency induction oven 5. After pre-heating of the ch in the oven 5, the container 2 is advanced to the third station C where it can be gripped by a swinging transfer arm 6. When a mould 7 is opened and ready to receive the charge, a chute 8 is advanced from the chain line positio to the full line position as the transfer arm 6 swings to lift the container 2 from the belt 1 and discharge its contents into the chute and thence to a cavity 9 of the mould 7. The arm 6 returns the container 2 to the belt, chute is withdrawn and the mould can be closed. Movement of the apparatus are controlled in sequence with the moul ing operation so that the next container 2 with its pre¬ heated charge is presented at the third station C ready f gripping by the swinging arm 6 as soon as the mould 7 ope and is cleared of the previous moulding.

By virtue of the improved pre-heating of moulding ma ial achieved by the method of the invention, not only is the moulding time reduced, but the quality of the mouldin is improved so that mouldings of comparable strength to those produced with prior art pre-heating techniques can be produced with less moulding material than has been dee necessary heretofore.

Claims

1. A moulding process for thermosetting moulding mat¬ erial in which loose thermosetting moulding material in granular or powder form is pre-heated in a container (2) in an electromagnetic induction oven (5) and then fed to a moulding cavity (9) where it is shaped under heat and press¬ ure, characterised in that the pre-heating is effected in a container (2) of moisture-permeable, thermally-insulating, non-dielectric material.

2. A moulding process according to claim 1 in which the container is of moulded expanded polystyrene.

3. A moulding process according to claim 1 or claim 2 in which the container has an open bottom closed by a separ¬ ate plate, e.g. of metallic material.

4. A moulding process according to any preceding claim inwhich the container is substantially filled by the charge of moulding material.

5. A moulding process according to any preceding claim in which the container has a removable lid of moisture- permeable, thermally-insulated, non-dielectric material.

6. A thermosetting moulding produced by the moulding process as claimed in any preceding claim.

7. Apparatus for use in association with moulding equipment for thermosetting moulding materials including a pre-heating electromagnetic induction oven (5) and a mould (7) having a moulding cavity (9) for receiving pre-heated moulding material from the oven (5) characterised by a plur¬ ality of containers (2) of moisture-permeable, thermally- insulating, non-dielectric material each capable of re¬ ceiving a charge of the moulding material sufficient for the cavity (9) of the mould (7) or a part thereof, means (4, 4a.) for supplying the charges to the containers (2) and transfer means (1, 6, 8) for moving charged containers one-by-one to the oven (5) and for transferring the pre¬ heated charges from the containers (2) to the moulding cavity (9).

8. Apparatus according to claim 7 including control means to sequence the filling, heating and emptying of the containers with the operation of the mould (7).

9. Apparatus according to claim 7 or claim 8, in which the transfer means includes a swingable transfer arm (6) and a movable chute (8), the arm (6) removing a container (2) from an endless belt (1) to tip its content into the chute (8) as the latter feeds the discharged mat ial into the moulding cavity (9) of the mould (7).

10. A thermosetting moulding when produced in the apparatus claimed in any of claims 7, 8 and 9.