GB2057342A - Moulds - Google Patents

Abstract

The present invention relates to a single-split metal mould for the production of mouldings from pre-foamed plastic granules, especially expandable polystyrene foam granules, comprising two double mould shells having metal walls (15, 16) which fit together to define therebetween a moulding cavity (11) the thickness of the mould cavity walls being not more than 7 mm. and preferably being between 2 and 3 mm. <IMAGE>

Description

SPECIFICATION Single-split metal mould for the production of mouldings from foamable thermoplastic granules The present invention relates to a single-split metal mould for the production of mouldings from foamable thermoplastic granules, preferably of expandable polystyrene foam.

A large number of processes and devices for the production of mouldings for expandable polystyrene foam (EPS) are known in which pre-formed plastic granules are post-foamed, and sintered, in a mould with the supply of heat.

With these known processes, either the mould only is warmed from the outside, this being possible only in cases where very thin mouldings are to be produced, or moulds are employed which are provided with holes or slits through which steam or hot water used for warming the mould, but not the pre-foamed plastic granules themselves, can penetrate and get in between the granules. Furthermore, processes and devices of the aforementioned type are known, in which a vacuum is applied to the mould before it is filled with the pre-foamed plastic granules and the mould is pre-heated from the outside, with the heating medium necessary for doing this being able to penetrate into the mould. A small amount of steam is passed in during or after the introduction of the granules into the mould and a vacuum continues to be applied to the mould during the entire filling time.In this case, the mould again has holes or slits, but these open into special vacuum or steam feed lines.

Moulds are also known which have heating and cooling chambers behind the actual mould walls, but these are of relatively large construction and thus have the disadvantage that the replacement of heating medium by cooling medium and vice versa is both time-consuming and it also gives rise to considerable energy losses.

Moulds made of aluminium, which either have been cast and then subsequently machined or have been produced purely by machining from a solid block of material are normally used for all of the processes and moulds described above. The wall thickness of these moulds has to be between 8 mm and 14 mm, by reason of their manufacturing techniques, and the holes or slits required in the walls have to be drilled or milled subsequently at the required locations.

Apart from the fact that the manufacturing techniques used to date especially with regard to the subsequent machining which is necessary, are relatively involved and do not result in absolutely identical moulds, the subsequent machining also necessarily leaves relatively rough surfaces, which give rise to poor release qualities for the mould.

Furthermore, the wall thicknesses of 8 mm to 14 mm necessitated by the mould manufacturing process have proved disadvantageous because of their poor erthermoconductivity. Moreover, moulds of this type made of aluminium become very severely corroded by the water and steam supplied to them, in use, so that subsequent machining is frequently necessary to restore the mould to its original condition.

It is an object of the invention, therefore, to provide a mould for the production of mouldings from pre-foamed plastic granules, especially of expandable polystyrene foam, which overcomes or substantially reduces the disadvantages described above, that is to say which can be manufactured relatively easily, which is readily reproducable with smooth, non-oxidising surfaces and which has betterthermoconductivity properties.

According to the invention, there is provided a single-split metal mould for the production of mouldings from pre-foamed plastic granules, especially from expandable polystyrene foam the mould comprising two shells having metal walls which fit together to definetherebetween a moulding cavity, the thickness of the moulding cavity walls being not more than 7 mm.

Preferably, the moulds are manufactured by an electroforming process, by spraying metals by a coating process (for example flamespraying) or by pressing or drawing corresponding metal sheets and subsequently joining the two shells.

If an electroforming process is used, a shell is first electroformed by electrodeposition on a model. If the mould is to have openings in it to the moulding cavity, all of these orifices are then covered and shielded by wax. After the shell is finished, the size of the desired moulding cavity with all its connections is fashioned by applying a wax film. Inner strength results from the construction of the connecting orifices and this strength can be increased by further stays or webs. The technique of the electroforming process is known, so that it will not be described in more detail here.

Those walls of the shells which face towards the internal moulding cavity preferably have a wall thickness of 2 to 3 mm and consists in particular of copper or nickel or of alloys thereof with other metals.

Depending on the metal or metal alloy from which the mould is made, it can be advantageous additionally to apply to the surface of the mould, especially the internal mould cavity surfaces thereof, an antioxidation protective coating or a protective coating which has an even greater hardness and/or smoothness than the mould itself, by the electroforming process or by the C.F.S. process (closed filed sputtering process). In the case of a copper mould, this additional protective coating can be, for example, of nickel or silver. Preferably, a nickel coating is subsequently applied to the surface of the mould by electroplating and this coating prevents corrosion in the mould cavity and at the same time produces a surface of optimum smoothness.

With the aid of the C.F.S. process, any metal can be applied by spraying in a closed field. The C.F.S.

process is fully described by G. B. Sugden of The General Engineering Co. (Radcliffe) Ltd., in the journal "Technik und Anlagen" so no further description thereof will be given here.

In order to increase the stability of the double shell of the mould, stays, webs or other connections having a reinforcing effect can be fitted between the shell walls, in addition to the connections which pass through the double shell.

It is particularly advantageous thatthe construction of the particular holes or slits which are used either for applying a vacuum to the mould cavity or as inlets for the heating medium, which can be either steam or a hot inert gas, can be effected at the time the moulds are manufactured, for example by the electroforming process.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawing which shows a mould 1 having an internal moulding cavity 11, and a steam distribution chamber 12 and a heating and cooling chamber 13 provided in respective double skinned mould shells. The internal space 11 which defines the moulding cavity as well as the steam distribution chamber 12 and the heating and cooling chamber 13 are formed by shell walls 14,15, 16 and 17, which are produced in known manner by an electroforming process in which a bath model is suspended in a copper bath and the desired wall thickness is formed thereon after a corresponding dwell time. All functional connections are incorporated in this shell by covering.

The functional connections include fill orifices 18 for fillers 8, suction nozzles 4 for applying a vacuum to the mould cavity 11, small holes (or slots) 5 for the admission of steam to the mould cavity 11 and two connections 6, 2 for the steam which is used for heating and sintering. It will be appreciated that the feed lines and outlet lines themselves provide strengthening for the mould shell but additional webs 20 may also be provided. However, other connections which have a reinforcing effect and which connect the walls to one another can be used instead of the webs 20.

An important feature of the illustrated mould is that the steam holes 5 are required only in one half of the mould the other half being constructed as a closed mould half. It is however possible for this mould half to have small holes in it as well if it is desired to suck steam through the material to be foamed.

After the shell which determines the shape to be moulded has been finished it is complete in that it includes all the necessary orifices in it.

Due to the use of a special material which is applied to the mould shell and is not attacked by the electroforming bath, any desired mould design can be produced.

After this material has been applied to the outside of the mould shell, the model is again suspended in the baths. After a corresponding dwell time in the baths, a further shell 6 - 10 mm thick, as required, forms on the rear side and, thus, a cavity forms behind the actual shell wall 15 and 16 which determines the shape to be produced by the mould.

As a result of this design possibility, minimum working spaces, directly suited to the mould size, are obtained for the steam and for the heating and cooling medium.

Filling of the mould is effected by sucking the material with a vacuum via the filler 8 into the moulding cavity 11, at the same time, the steam distribution chamber 12 is filled with steam and, by means of the holes 5 located in the shell wall 14, steam likewise enters into the moulding cavity 11.

After filling of the mould is complete, water is admitted into the heating and cooling chamber 13 and the moulding is thus cooled.

In place of the large working spaces hitherto located behind the mould part determining the shape to be moulded, very small working spaces are now provided, so that the amount of steam present in these spaces can be minimal compared with the known processes. Moreover, the mould of the invention still allows any desired temperature to be transmitted through the shell, which, in turn, is so designed that it can absorb the desired pressures which are set up.

The number of small holes 5 and the size of the vacuum connection 7, the steam chamber 12 and also the fill injectors 8 are designed to suit the particular size of the mouding cavity 11.

The mould of the invention is particularly suitable for use with expandable polystyrene foam beads marketed underthetrademark"STYROPOR" by Badische Anilin -und SodafabrikA.G.

Since the fundamental procedure for making the moulds of the invention corresponds generally to thatforthe manufacture of known moulds made of aluminium, a detailed description thereof has not been given.

The advantages of the illustrated preferred mould of the invention are as follows: (i) shell wall thicknesses of between preferably 2 and 3 mm can be obtained and, for example, copper can be used in place of aluminium, as a result of which a higherthermoconductivity is obtained and, thus, a higher temperature can be transmitted within a short time, as a result of which, in turn, shorter cycle times are achieved, (ii) the working space behind the walls defining the moulding cavity can be kept very small, due to the design of the mould walls in the form of a double shell, as a result of which the heating medium and cooling medium can be exchanged considerably more rapidly and, additionally, the heat losses which arise are smaller, (iii) vacuum and steam holes and reinforcing stays and other inserts can be incorporated during manufacture of the mould, (iv) absolutely pressure-tight chambers are obtained for the heating and cooling media for the steam, since these chambers are virtually in "one piece" and (v) the mould surface is smooth and can be protected against oxidation or hardened or smoothed by means of a further coating, as a result of which, inter alia, better release of the product from the mould is possible and the mouldings produced using the mould of the invention have a better surface quality with regard to smoothness and, where appropriate, hardness. The fact that a harder surface of the moulding is obtained is to be attributed to the possibility of transmitting higher temperatures from the mould to the moulding.

Claims (9)

1. A single-split metal mould for the production of mouldings from pre-foamed plastic granules, especially from expandable polystyrene foam, the mould comprising two shells having metal walls which fit together to define therebetween a moulding cavity, the thickness of the moulding cavity walls being not more than 7 mm.

2. A mould as claimed in claim 1, wherein the moulding cavity shell walls have a thickness of between 2 and 3 mm.

3. A mould as claimed in claim 1 or claim 2, wherein a steam distribution chamber is formed in the one mould shell behind its moulding cavity wall, a heating and cooling chamber being formed in the other shell behind its moulding cavity wall, the spacing between the shell walls which form the walls of the mould being a distance which corresponds to the desired volume for the steam distribution chamber and for the heating and cooling chamber.

4. A mould as claimed in any one of claims 1 to 3, wherein the mould shells are made of copper or nickel or of an alloy thereof with other metals.

5. A mould as claimed in any one of claims 1 to 4, wherein each mould half has a moulding surface on its metal wall to which is applied an additional anti-oxidation protective layer by an electro-forming process or a closed filed sputtering process.

6. A mould as claimed in any one of claims 1 to 5, wherein each mould half has formed integrally therein feed lines and outlet line which act as strengthening members and also webs between the walls which form the shell.

7. A mould as claimed in any one of claims 1 to 6, wherein the mould shells are manufactured by an electroforming process, by spraying metals or by pressing or drawing corresponding metal sheets and, if necessary, subsequently joining and/or reinforcing the shells by an electroforming process.

8. A mould as claimed in any one of the preceding claims wherein one or both of the mould shell walls defining part or all of the moulding cavity is or are formed with slots or apertures therein for the admission or removal of steam to or from the moulding cavity.

9. A single split metal mould substantially as herein described and illustrated in the accompanying drawing.