GB2040788A

GB2040788A - Moulding foamed plastics

Info

Publication number: GB2040788A
Application number: GB8001259A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-01-30
Filing date: 1980-01-15
Publication date: 1980-09-03
Also published as: FR2447803A1; SE8000748L; GB2040788B; ES488064A0; CH642903A5; FR2447803B1; ES8100153A1

Abstract

In a method of making moulded objects (62) from foamed plastics, e.g foamed polystyrene, the plastics raw material is introduced in the form of pre-expanded beads containing an expanding agent into a divisible mould (2) and is subjected in this mould to the action of a hot, gaseous foaming agent, which causes the beads to fuse together while further expanding. The thus obtained foam is, before the moulding is removed from the mould, stabilized by forcing through it a stabilizing agent in the form of a gas, e.g. air. The foaming and stabilizing operation can be repeated. A vacuum can also be applied to the moulded object during the working cycle. The relevant mould may possess, instead of the usual nozzles (26) or slits, mould inner walls (28, 30) of porous material, e.g. sintered metal. <IMAGE>

Description

SPECIFICATION A method of making moulded objects from foamed plastics, and a mould This invention relates to a method of making moulded objects from foamed plastics, such as foamed polystyrene, wherein the plastics raw mate rial is introduced in the form of pre-expanded beads containing an expanding agent into a divisible mould and is subjected in this mould to a hot gaseous foaming agent, which induces the beads while further expanding to fuse with one another, whereupon the thus obtained foam is stabilized before the moulded object is removed from the mould.

Usually such a process is carried out by introducing the pre-expanded beads of the relevant raw material, for example by means of an air stream, into a mould, by acting on them there with steam causing the beads to expand further and to fuse with one another, by then cooling the mould with water to stabilize the thus formed foam and finally opening it, in order to eject the thus obtained moulding by means of air or a mechanical ejector. The corresponding mould commonly possesses for this purpose at least one so-called steam chamber, out of which the steam and possibly also the air serving for ejection enters via a plurality of nozzles into the mould cavity. This steam chamber contains moreover a sprinkler tube system, from which the inner mould wall bounding the mould cavity is sprinkled with cooling water. The alternating heating up and cooling down of the mould at each operating cycle naturally leads to high energy consumption.

An attempt has already been made to counteract this, according to French published patent 2,338,124, by transferring the moulded blankwhich has been obtained by steaming the expanded beads into a cold mould for the purpose of stabilizing and calibrating (sizing) it, this cold mould consisting either of the same mould which meantime has been surface-cooled on the side of the mould cavity or, preferably, of a separate mould. Both these methods and also the related equipment are, however, relatively expensive and, if they are operated with surface water cooling, result in wet products.

The task underlying the present invention therefore is so to form a method of the aforementioned type that it can be carried out, with at least approximately equal energy economy, more simply and with virtually any conventional mould and installation, and moreover provides only dry products.

This task is achieved according to the present invention by a method of making moulded objects from foamed plastics material, such as for example foamed polystyrene, wherein the plastics raw material is introduced into a divisible mould in the form of pre-expanded beads containing an expanding agent and is subjected in this mould to the action of a hot, gaseous foaming agent which causes the beads to further expand and to fuse together, whereupon the thus obtained foam is stabilized before the moulded object is removed from the mould, characterized in that the stabilizing of the foam is carried out by forcing a stabilizing agent in the form of a gas through the moulded object.

It has indeed surprisingly been found that the stabilization of the foam by the conventional cooling of the mould wall can, for rather shorter cylce times, be replaced by the forcing of a gas through the moulded object, the mould itself remaining substantially uninfluenced thereby. With the omission of water cooling, almost dryworkpieces are inevitably produced, without special measures in the design of the mould being necessary for this purpose, such as for example have been disclosed by German patent specification 2237 397. Added to this, the mould can be of a relatively inexpensive material, without special corrosion or thermal conducting properties.

Since also deposits originating from water have been eliminated, the usual nozzles or slits in the inner walls of the mould can be replaced by making these inner walls of a porous material, as a result of which the moulded object receives an uninterrupted, uniform surface and moreover the structure of the foam will be more uniform.

The gas used as stabilizing agent will in general be air, sucked in at ambient temperature. Special cooling is not required, since the effect of the stabilizing agent is based decisively upon the cell structure of the previously foamed plastics material being made permeable, so that the expanding and foaming agents contained therein can escape. The pressure of the stabilizing agent, if the wall thickness of the moulded object is to be maintained, preferably should not exceed the pressure of the previously applied foaming agent.

An intermediate opening of the mould can be carried out, but usually will be superfluous. The compressing of the consequently expanded foam material which occurs as the mould is re-closed generally leads to a somewhat softer product in the direction of this compression, while the permeability is nevertheless improved by the intermediate expansion. A product which is completely free of condensed water can also be obtained if a hot gas, such as hot air, is used as foaming agent instead of steam.

The gaseous stabilizing agent, generally air, can also be used simultaneously as an ejector agent, for which purpose the mould only needs to be opened just before the supply of coolant has been terminated.

Because of their special method of manufacture, the products of th is invention, by comparison with corresponding, conventionally produced products, in general display a more beady structured surface, which gives rise to a characteristic, striking appearance. A certain smoothing of the surface can, if required, be obtained by repeating the foaming and stabilizing operation within the same working cycle.

To obtain a similarly smooth and closed surface as is usual with products made by the conventional foaming methods using water cooling of the mould, a vacuum can be applied to the moulding during the stabilizing phase. In addition to the smoothing of the surface, this also gives rise to a product which is denser to the passage of a gas or liquid, and which to this extent displays a further similarity to the products produced in the conventional manner.

An embodiment of a method of making a moulded object and a suitable mould will now be described, by way of example only, with reference to the accompanying drawing which shows diagrammatic alloy a closed mould, including the essential feed and discharge lines together with associated control devices.

The mould 2 illustrated here consists of the two mutually separable mould parts 4 and 6, which enclose between them the mould cavity 8. Each mould part is composed basically of two elements 10 and 12, 14 and 16 respectively, which are sealingly connected together by means of bolts and sealing inserts or the like, not shown here, and contain between them respective steam chambers 18,20. The cavity8 is in communication via an injector 22 with a supply line 24 for the introduction of the bead-shaped plastics raw material by means of an air current. In addition, each of the two steam chambers 18 and 20 is connected with the mould cavity via a plurality of nozzles 26, distributed as uniformly as possible, which provide passage for a gaseous medium, such as steam or air, but prevent the plastics material from penetrating ino the steam chambers. Instead of the nozzles 26, the elements 12 and 14 may also possess mould inner walls 28,30 respectively of a porous material, such as sintered metal.

The steam chamber 18 possesses steam inlet and air inlet openings 32,34 respectively, of which the steam inlet opening 32 is in communication via a cyclically controlled valve 36 with the steam line 38.

The air inlet opening 34 can be connected as desired via cyclically controlled two-way valve 40 either with the air feed line 42 or with atmosphere. The steam chamber 20 likewise possesses steam and air inlet openings 44, 46 respectively, of which the steam inlet opening 44 is in communication via a cyclically controlled valve 48 with the steam feed line 38 and the air inlet opening 46 can be connected as desired via a cyclically controlled two-way valve 50 either with the air feed line 42 or with the atmosphere.

All the parts so far described are conventional.

Missing however are the conventional water spray tubes in the interior of the steam chambers 18 and 20 or water ducts in the inner walls of the mould. In addition, each of the two two-way valves 40 and 50 possesses in the advantageous form of embodiment of the invention described here a blocking position, and at the two lengths of pipe connecting the air inlet opening 34 to the two-way valve 40 and the air inlet opening 46 to the two-way valve 50 respectively, there are branch lines 52, 54 respectively, which lead via cyclically controlled valves 56, 58 respectively and a following line 60 to a vacuum source.

To produce a moulding, such as the illustrated moulding 62, the following procedure may now be used: With the mould closed or substantially closed, the plastics raw material in the form of preshaped beads is blown in by air through the injector 22 into the mould cavity 8, the air finding its escape either via the nozzles 26 or the like and the steam chambers or via a gap between the two mould parts 4 and 6.

Steam is then introduced via the valve 36 and slightly later also via the valve 48 as foaming agent into the steam chambers, this steam passing via the nozzles 26 or the like into the mould cavity 8 and causing the bead-shaped plastics raw material situated there to expand further and fuse together, whereupon the steam, with the valves 36 and 48 again closed, can be discharged into the atmosphere via one or both of the two-way valves 40 and 50.

These process steps are also conventional. By contrast to the conventional procedure, however, air is now blown through the moulded object via one of the steam chambers, for example 18,forth purpose of stabilizing the moulded object, this air then escaping into the other steam chamber, for example 20. The valve 50 of this second steam chamber can be so controlled that it does not allow the air to escape, if indeed at all via this route, until after a certain time, with the result that a back-pressure at first builds up. In order to prevent the moulded object from becoming detached from the inner walls of the mould, the air pressure which becomes established in the mould cavity 8 preferably should not exceed the pressure of the previously introduced foaming agent.

Then, with the air pressure still existing, the mould may be opened, causing the formed moulding 62 to be ejected out of and away from the mould parts 4 and 6. If the air pressure at one of the mould parts has already been reduced, a renewed, short air pulse can again be applied from there.

With mouldings of relatively large wall thickness or those with widely differing wall thicknesses, it may be recommended that the mould shall be temporarily opened somewhat before or during the air cooling, as a result of which the permeability to air of the moulding is improved.

Since the moulded object 62 produced according to the present invention does not come into contact with any cooling water, it also leaves the mould almost dry. In order to eliminate the last traces of humidity in the form of condensed water, however, a hot gas, such as hot air, may be used instead of the conventional steam as the foaming agent, especially where such a hot gas is in any case available.

It has proved in many cases to be advantageous to pass the stabilizing agent through the moulding alternately from the two opposite sides, which can be achieved by alternately applying air to and venting the two steam chambers 18 and 20 shown here. A certain smoothing of the surface of the moulded object can be achieved if desired by the steaming and stabilizing operations being repeated within the same working cycle, if necessary with somewhat different temperatures and pressures and/or cycle times. Moreover, with the valves 40 and 50 closed, vacuum can be applied to the mould cavity via the valves 56 and 58 and the two steam chambers 18 and 20, causing the previously fused together beads of the plastics foam to expand still further before the mould is finally opened.

If the steaming and stabilizing operation is repeated within the same working cycle, the application of vacuum by the valves 56 and 58 can also be repeated, being then carried out preferably at the end of each individual stabilizing operation.

Claims

1. A method of making moulded objects from foamed plastics material, such as for example foamed polystyrene, wherein the plastics raw material is introduced into a divisible mould in the form of pre-expanded beads containing an expanding agent and is subjected in this mould to the action of a hot, gaseous foaming agent which causes the beads to further expand and to fuse together, whereupon the thus obtained foam is stabilized before the moulded object is removed from the mould, characterized in that the stabilizing of the foam is carried out by forcing a stabilizing agent in the form of a gas through the moulded object.

2. A method according to Claim 1, in which the pressure of the applied stabilizing agent does not exceed that of the previously applied foaming agent.

3. A method according to Claim 2, in which the outflow of the stabilizing agent is controlled in such a manner that during the course of the stabilizing phase a counter-pressure of at least approximately the magnitude of the pressure of the applied stabilizing agent becomes established.

4. A method according to any of the preceding Claims, in which the stabilizing agent is applied with a lower temperature than that of the previously applied foaming agent.

5. A method according to Claim 4, in which the stabilizing agent is applied at substantially ambient temperature.

6. A method according to any of the preceding Claims, in which the stabilizing agent is air.

7. A method according to any of the preceding Claims, in which the feeding of the stabilizing agent is carried out alternately from different sides of the moulded object.

8. A method according to any of the preceding Claims, in which the mould is briefly temporariiy opened before or during the stabilizing phase.

9. A method according to any of the preceding Claims, in which the foaming agent is steam.

10. A method according to any of Claims 1 to 8, in which the foaming agent is a hot gas, preferably hot air.

11. A method according to any of the preceding Claims, in which the foaming and stabilizing operation is repeated within the same working cylce, if necessary with differing temperatures, pressures and/or cycle times.

12. A method according to any of the preceding Claims, characterized inthatthe stabilizing agent is also used as ejecting agent, by which the finished moulding is ejected from the mould.

13. A method according to Claim 12, in which, for the purpose of ejection, the mould is opened while the stabilizing agent is still being suppiied.

14. A method according to any of the preceding Claims, in which a vacuum is applied to the moulded object during the course of the stabilizing phase.

15. A method according to Claim 14, in which the vacuum is applied at the end of the stabilizing phase.

16. A method according to Claims 11 and 14, characterized in that the vacuum also is applied repeatedly.

17. A method according to Claim 16, in which the vacuum is applied at the end of each separate stabilizing operation.

18. A method of making moulded objects according to Claim 1 and substantially as described herein.

19. A mould for carrying out the method according to any of the preceding Claims, consisting of at least two mutually separable mould parts, of which at least one contains a steam chamber in communication with the mould cavity via limited passage cross-sections in an inner wall of the mould, characterized in thatthe mould innerwall is of a porous material, for example porous sintered metal.

20. A mould for carrying out the method of any of Claims 1 to 18 and substantially as described herein with reference to or as illustrated in the accompanying drawing.

21. A moulded object made by the method of any of Claims 1 to 18 or in a mould according to Claim 19 or Claim 20.