GB1563324A

GB1563324A - Apparatus for moulding articles from foamable plastics material

Info

Publication number: GB1563324A
Application number: GB41733/76A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1975-10-07
Filing date: 1976-10-07
Publication date: 1980-03-26
Also published as: FR2327067B3; DK447776A; CH619396A5; FR2327067A1; DE2544714A1

Abstract

In the process, foamable plastic granules, which may already be prefoamed, are foamed in a mould (1) under the effect of steam on heated mould walls. The moulding is stabilised by cooling the mould walls and is taken out of the mould. In order to ensure uniform and complete foaming, the steam is passed through the granules. To this end the steam is allowed to flow into the mould (1) via steam inlet openings (12). Since the total cross-section of the steam outlet openings (11) is smaller than that of the steam inlet openings (12), a steam pressure builds up in the mould so that the steam is distributed over the entire mould cavity. This effect is reinforced by a buffer volume of a distribution chamber (9) and/or of a collection chamber (5). Valves (6, 10) can be provided to control the steam flow. Vacuum pumps (13) can also be connected via valves (14) to the mould cavity. <IMAGE>

Description

(54) APPARATUS FOR MOULDING ARTICLES FROM FOAMABLE PLASTICS MATERIAL (71) I, HANS ERLENBACH, a German citizen, of 5429 Lautert, Rhein-Lahn-Kreis, Federal Republic of Germany, do hereby declare the invention for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to apparatus for moulding articles from foamable plastics material, the apparatus including a mould into which the material, preferably granular, is introduced and in which the material, under the action of steam, is moulded by foaming. The mould is connected to means for controlled heating and cooling of its walls and is also connected to, separately from the heating and cooling means, a steam supply system for introducing steam into the mould, the steam supply system including means for guiding the steam, which means is separate from the heating and cooling means.

In known such apparatus (U.S. Patent Specification No. 3,225,126 and German Patent Specification No. 2,237,397), guidance of the steam through the interior of a mould is achieved in that the steam inlet or a number of steam inlets extend through the mould wall, the steam being allowed to pass at the edge of the mould through a narrow gap between the two mould halves or parts. However this method of steam guidance is largely fortuitous. It can be used with a narrow mould cavity, because in such a case on the one hand the heat dissipated by the mould walls can also act on the inner zones of the article being moulded, and on the other the mould wall offer adequate guidance for steam to flow through the cavity. However, when a thicker article is to be produced, all the zones of the mould cavity are no longer adequately supplied with steam, so that the plastics material is not adequately formed in those portions of the article.

The more one tries to increase the output of moulding apparatus and to shorten moulding time, the greater this difficulty becomes.

An object of this invention is to improve the distribution of steam in the mould cavity and to provide for a predetermined controllable distribution of steam in the cavity, so that the introduced steam occupies the entire cavity, the conditions of control and distribution of the steam in the cavity being reproducible, and capable of being adjusted and controlled in predetermined manner.

According to this invention, there is provided apparatus for moulding articles from foamable plastics material, in which a mould, in which the material is foamed under the action of heat and steam, is connected to means for controlled heating and cooling of the mould cavity walls, and is also connected, separately from the heating and cooling means, to a steam supply, wherein the steam supply includes a steam flow path which in addition to the mould cavity comprises at least one steam buffer space which cooperates with at least one steam outlet in the steam flow path downstream of the mould cavity, the internal cross-sectional area of the outlet being adjustably variable.

The path and distribution of the steam in the mould cavity can be reproducibly arranged for optimum effect. By accurate adjustment of the steam outlet downstream of the mould cavity, in conjunction with the buffer steam space, it is possible to create a reproducible dynamic pressure and therefore a predeterminable steam pressure in the mould cavity in the foaming material.

In this way, it is possible reliably to control foaming, moulding and welding, which can be adapted to suit the particular form and wall thickness of the article, and also the characteristics of the material used.

The invention will now be described by way of example, with reference to the draw ings, in which: Fig. 1 is a diagram of one form of steam guidance means in apparatus for moulding fomable thermoplastics material; Fig. 2 is a diagram of another form of steam guidance means in such apparatus; Fig. 3 is a section of one embodiment of a mould in an apparatus for moulding foamable thermoplastics material; Fig. 4 is a plan of the core of the mould of Fig. 3; Fig. 5 is an enlarged section on the plane which is marked III-III in Fig. 4; Fig. 6 is a section of another embodiment of a mould; Fig. 7 is an enlarged section of a modified detail of Fig. 6; Fig. 8 is a diagrammatic section of another embodiment; Fig. 9 is a diagrammatic section of another embodiment; and Fig. 10 shows another embodiment in plan view of the mould core and in two corresponding sections.

Referring to Figs. 1 and 2, steam guidance means of a foam moulding apparatus comprises a steam buffer space 2 outside a mould cavity 1 and also means for matching a steam outlet 3 to a steam inlet 4 and the volume 2.

In Fig. 1 the steam buffer space 2 is a steam collecting chamber 5 which has an outlet valve 6 which, in accordance with the working cycle of the apparatus, is closed either fully or to a predetermined degree while steam is entering the chamber 5 through the inlet 4. As a result, pressure in the steam passing through the mould to foam the plastics material is built up firstly in the chamber 5 and, upstream thereof, through a constantly open steam outlet 7 of the mould cavity, the steam pressure being buffered by the chamber 5 and being of substantially equal magnitude at every outlet 7 of the cavity 1. This steam pressure then builds up backwards inside the cavity 1 or in the material therein, so far as a steam inlet 8.

As indicated by broken lines in the lefthand part of Fig. 1, the steam buffer space may also comprise a steam distributor chamber 9 which communicates with the mould cavity 1 through the constantly open steam inlet 8. The steam inlet 4 of the chamber 9 has a conventional steam inlet valve 10 controlled in accordance with the working cycle of the apparatus.

In Fig. 2, the steam buffer space 2 is constituted by a steam distributor chamber 9, which also serves for matching a steam outlet in the path of steam flow following the mould cavity 1. For this purpose, there is an adjustment to suit steam outlets 11, such that their combined cross-sectional area is smaller than the combined cross-sectional area of steam inlets 12. This ratio of crosssectional areas can be large. For example, the combined cross-sectional area of the inlets 12 may be up to about 50 times the combined cross-sectional area of the outlets 11, but this large ratio is only used in extreme cases. Normally the ratio is up to about 1:20. In this respect, it is mainly the shaping of an article, its wall thickness, the material to be used, and the volume of the steam distributor chamber 9 which are taken into account As seen in the right-hand part of Fig. 2, the buffer volume of steam 2 can additionally comprise one or more steam collecting chambers 5 having a pre-set reduced outlet valve 6 for discharge of steam. The steam inlet 4 has an inlet valve 10 controlled in accordance with the working cycle of the apparatus.

The embodiment shown in Figs. 3 to 5 is a mould for a plate-like article of foamed plastics material and is smooth on one side, or has a gas and liquid-tight film applied on one side.

A mould cover 102 is provided for forming the smooth surface or for receiving the film to be applied to the plate being produced, while a mould core 101 accommodates the means necessary for introducing the foamable material and the steam and guidance means. The cover 102 thus has a single mould wall 104 on which is a heating and cooling chamber 106. The cover includes a projecting peripheral mould wall 103 which merges into an encircling shoulder 105 which projects beyond the mould wall 104. The wall 104 is interrupted only by an inlet 107 for mould removing air, this inlet being a tube in the central zone of the cover 102 and extending in sealing-tight manner through the chamber 106.

The core 101 has a mould wall 108 opposite the mould wall 104. This mould wall 108 is surrounded by a wide U-shaped groove 109 which forms an outer surrounding steam passage 110 sealed by a seal 112 in its outer member 111. At the four corners of the core 101, steam connections, which are merely indicated at 113 in Fig. 4, discharge into the steam passage 110. Formed in the core behind the mould wall 108 is a heating and cooling chamber 114 into which connections, indicated in Fig. 4 at 115a and 115b, discharge. An injector 116 (Fig. 4) for the material to be introduced into the mould extends through the mould wall 108.

For inlet and discharge of steam, the mould wall 108 has a number of parallel grooves 117 and 118. The grooves 117, for discharge of steam, extend (Fig. 4) only within the surface of the mould wall 108, while the grooves 118, for inlet of steam, extend at one side beyond the surface of the mould wall 108, where they have con nections 119 with the annular passage 110.

The bases of the grooves 117 have outlets 120 extending through the bottom wall of the core 101. As Fig. 4 shows, it is possible also to provide mechanical ejectors 121 laterally of the steam discharge grooves 17.

To form channel-like steam distributor chambers and channel-like steam connector chambers, the grooves 118, 117 are in each case covered with a metal strip 122, 123 forming with the mould wall 108 a lower mould face on the core 101. The strips 123 of the grooves 118 which form the steam distributor chambers have steam inlet apertures 124 (see also Fig. 5) located at predetermined intervals and comprising nozzlelike bores. Similarly, the strips 122 for the grooves 117 have steam outlet apertures 125 which likewise comprise nozzle-like bores.

The combined cross-section of all the steam inlet apertures 124 is greater than the combined cross-section of all steam outlet apertures 125. The ratio of cross-sections is dependent upon for example the thickness of the article to be produced, and the foamable material used. For example, the ratio of the combined cross-sections of the steam outlet apertures may be 20:1. However, in some cases, it may be as much as 50:1.

The steam distributor chambers 118, are arranged alternately and parallel to the steam exhaust chambers 117, and in such as way that a respective steam exhaust chamber 117 extends adjacent each lateral edge of the mould wall 108, since steam flows along this edge from the passage 110 into the mould interior. There are four steam exhaust chambers 117 and three steam distributor chambers 118 between them. The resultant distribution and guidance of steam through the material 127 is seen in Fig. 5.

For filling the mould-with granulated foamable material, the core 101 and cover are brought sufficiently close to each other that the edges of the encircling shoulder 105 just meet the peripheral edge of the mould wall 108, but the seal 112 is not closed. The granulated material is then blown in by compressed air through the injector 116, the air then flowing out past the seal 112, by way of the passage 110. Portions of this filling air can be diverted through the steam inlet apertures 124, the steam distributor chambers 118, and into the passage 110.

At the same time, the filling air can be discharged through the steam outlet apertures 125 into the chambers 117 and thence through a valve controlled according to the working cycle of the apparatus and which is connected to the chambers 117 and which is outwardly open during mould filling. Also the valve which is connected to the steam inlets 113 and which is also controlled according to the working cycle, can, to facilitate discharge of the filling air, be opened to the outside during mould filling process and if a plurality of valves are provided for the steam inlets 113, then they can all be opened to the outside.

When filling is completed, the core 101 and the cover 102 are pressed fully together so that the seal 112 seals off the passage 110 to the outside. All that remains open is the gap 126 around the mould wall 108, connecting the passage 110 with the mould cavity 127. Heating of the mould walls 104 and 108 takes place by means of the heating and cooling chambers 106 and 114. With the valve or valves connected to the steam inlets 113 and controlled in accordance with the operating cycle of the apparatus, the supply of steam to the passage chamber 110 is opened. The steam then passes partially along the periphery of the mould wall 108 through the gap 126 into the mould cavity 127 where it becomes distributed as shown by the arrows in Fig. 5. At the same time, the steam flows out of the passage 110 through the connecting apertures 119 into the steam distributor chambers 118 and through the apertures 124 into the mould cavity. At each aperture 124 the steam is distributed as indicated by the arrows in Fig. 5, all around and into the foamable material. The steam thus introduced into the material then flows through the apertures 125 into the chambers 117, whence they are discharged through the apertures 120, by way of valves (not shown) controlled in accordance with the working cycle of the apparatus. After a given period of time, the supply of steam is shut off. To remove a produced article from the mould, the passage 110, the chambers 118 and 117, and also the air inlet 107 are subjected to compressed air by means of the aforementioned valves controlled in accordance with the working cycle. As the cover and core are moved apart, the article is separated from the mould walls 104 and 108.

Modifications are possible. For example, for production of thicker articles, the chambers 117 may be provided in the cover 10, while the core 101 retains the chambers 118.

Instead of the encircling steam passage 110, it would be possible to provide only two side passages parallel with the chambers 118 along the lateral edges of the mould wall 108. The connection from the steam inlets 113 to the distributor chambers 118 could then be constituted by a channel bore in the core.

In the embodiment shown in Figs. 6 and 7, the mould comprises a core 151 and a cover 152, each having a respective mould wall 153, 154, behind which are respective heating and cooling chambers 155, 156.

These chambers 155 and 156 have in each case supply and dicharge means 1 57a, 1 57b, 158a, 158b. For introduction of steam into the mould cavity 159 or a mould depression 159a formed between the inter-engaging core and cover, there is an encircling passage 160 into which a steam supply connection 161 discharges. The passage 160 is formed by a groove of U-shaped section on the periphery of the cover 152. The open face of this groove is carried up to a surface 162 on the core 151. A seal 164 is carried by the outer member 163 of the U-shaped groove. The inner member 165 of the Ushaped groove has a shoulder 166 which overlaps the core which at the same time constitutes that end of the mould wall for the edge of the article to be produced. Between this shoulder 166 and the member 165, however, an open slot or gap is left all round through which the steam in the passage 160 can enter the mould cavity 159 which is filled with the material to be foamed. The steam thus introduced into the cavity is discharged through an outlet 167 disposed centrally in the core 157 and/or through outlets 168 in the cover. All steam outlets can have an outlet valve controlled in accordance with the working cycle of the apparatus. By control of these valves, it is also possible optionally and according to given conditions regarding mould and materials, to use one or other steam outlet and one or other group of steam outlets.

The steam outlets 167 and 168 can also be used for blowing off the air used for filling the mould, by means of an injector applied at 169. Finally, the steam outlets 167 and 168, by operating their valves, can be used also for introducing mould-removing air, which is then used instead of or in addition to mechanical ejectors such as that indicated at 170.

During filling, it is possible for the core 151 and cover 152 to be maintained at a predetermined distance from each other, so that the inner member 165 has its step 166 meet the core, so that the granular material can no longer pass between these two parts.

The seal 164 then no longer engages the surface 162 of the core 157, so that the filling air blown by the injector into the mould cavity 159 emerges through the slot formed between the inner member 165 and the shoulder 166, past the steam passage 160 and the seal 164. During this time, a steam valve controlled in accordance with the working cycle of the apparatus and arranged upstream of the steam inlet 161 is closed.

By using the central steam outlet 167 and/or the steam outlets 168, guidance of the steam in the interior of the filled mould cavity can be arranged according to requirements. Since the U-shaped groove around the mould cavity 159 has between the inner member 165 and the shoulder 166 a greater area than that of all the steam outlets 167 and 168, pressure is built up in the steam in the mould cavity 159, thus preventing steam flowing uselessly and unhindered through passages forming within the material.

In the detail shown in Fig. 7 (which corresponds to the portion of Fig. 6 adjacent the inlet 161), the U-shaped groove which forms the steam passage 160 has its outer member 163' and its inner member 165' as part of the core. Similarly, the steam intake 161' is provided in the core. This has advantages if the core is stationarily mounted in the apparatus, because then no flexible steam supply lines are required.

Further, in this detail, a plurality of steam inlets 161' may be provided on the core.

The steam passage 160 is in this case closed by an encircling end face 162' extending around the periphery of the cover 152 and as far as the U-shaped groove. Otherwise the construction is as shown in Fig. 6.

Fig. 8 shows an embodiment wherein the core 151 has a plurality of core mould parts 171. These core mould parts 171 then stand like islands on the core wall 172 to form a steam chamber 160 around each mould core part 171 and the whole of the mould.

As indicated in the lower part of Fig. 8, the wall 72 may have a base wall part 173 which, by way of a step 174, merges into the main wall part 175 of the core.

The upper part of Fig. 8 shows another arrangement in which the base wall part 173 merges directly into the main wall part 175 and instead of the step 174, it has a closure strip 177 around the part 175 and which forms the end of the cavity. It is also possible for this closure strip, as indicated at 178, to extend as a wall strengthener as far the core wall 172.

In the upper part of Fig. 8 the step is provided by cover wall parts 183, the cover 152 having an encircling wall 181 with a seal 182 in its end face. The wall parts 184 adjacent the wall parts 183 close the encircling steam passage 160 in the core 151, apart from the steam inlet gap between the passage 160 and the cavity 159. In the lower part of Fig. 8, this gap is a slot 179 between the wall 173 and the mould wall part 183 in the region of the step 174.

In the middle part of Fig. 8, the steam inlet is formed by bores 185 provided in the closure strip 177 and into which nozzlelike steam ports 186 are inserted. The upper part of Fig. 8 shows a further alternative form of steam inlet. Here, 2 pair of blind bores 187 are provided in the wall thickening 178 which extends as far as the bottom wall 172, one of the blind bores 187 discharging into the passage 160 and the other into the cavity 159.

Again, nozzle openings 186 are incorporated at the mouth of the bores where they discharge into the cavity 159.

As in the embodiment of Fig. 6, the steam passage 160 is charged through a steam connection 161. As explained in connection with Fig. 6, steam outlets 167 and 168 discharge directly to the outside or by way of a control valve. For easier connection to a control valve, the steam outlets 167 discharge into a steam outlet manifold 188.

A similar arrangement could also be provided on the cover 152, if a common means of controlling the outlets 168 is needed. As in Fig. 6, the steam outlets 167 and 168 can also be controlled as air outlets and as inlets for mould-removing air. The disposition of mechanical ejectors and the injector have for clarity been omitted from Fig. 8.

The heating and cooling chambers 155 and 156 and their inlets and outlets 157a, 157b, 158a, 158b can be constructed similarly to Fig. 6. Also as in Fig. 6, it is possible, for discharge of the air used for filling the mould, to maintain the core and cover at a slight distance apart so that the passage 160 is not effectively sealed by the seal 182, the filling air then passing outwards past the steps 174 or sealing strips 177 or 178 into the passage 160 and thence past the seal 182. The mould parts can be brought towards the steps 174 or the closure strips 177, 178 so that particles of material cannot enter the passage 160. Since when the mould is open the passage 160 is being opened repeatedly, it is in any event safeguarded against deposit of particles.

Fig. 9 shows an embodiment of a mould for an article which has projecting wall parts not only at the periphery but also in its central zones. Therefore, in addition to depressions 1 59a provided for the peripheral walls and formed by meeting parts of the mould wall, there are also depressions 190 in the core 151 or cover 152 for intermediate walls or other projections in the middle zone of the surface of the article.

Steam is introduced into the cavity 159 also at the end faces of such additional depressions 190. Steam passages 191, 192 are connected to the depressions 190, in the core 151 in Fig. 9, and communicate with the passage 160. Such a steam passage 191 is shown in the upper part of Fig. 9 as an example and if formed in that the corresponding depression 190 is of a depth which exceeds the amount required by the shape of the article, there being inserted into this depression 190 a separating element 193 which constitutes the end mould wall for the intermediate wall of the article. Provided at intervals in this separating element are nozzlelike fine bores 194.

Shown in the lower part of Fig. 9 is a variation of the embodiment, with core wall parts 195 (which form the depressions 190) extending beyond the depression 190 and including a channel 192 which extends parallel to the mould depression 190 and communicates with the passage 160. At intervals there are provided in the wall parts 195 bores 196 connecting the mould depression 190 to the channel 192, and provided with a nozzle-like insert 197, so forming the means for supplying steam from the passage 160 through the channel 192, the bores 196 and the inserts 197 to the mould depression 190.

The disposition of steam outlet orifices 167, 168 is similar to that in the embodiments of Figs. 6 and 8. The steam inlet 161' is similar to that described in connection with Fig. 7. The heating and cooling chambers 155 and 156 and their connections 157a, 157b, 158a, 158b are likewise similar to those in Figs. 6 and 8. The means of controlling the mould for dissipation of filling air and for intake of mould-removing air can likewise be as in Figs. 6 and 8. Escape of particles of material through the steam outlets 167 and 168 is prevented in that steam outlet orifices 189, similar to the steam inlet apertures in the mould walls, have a plurality of small apertures, or are in the form of a mesh. The combined cross-section of all steam inlet apertures can be related to the combined cross-section of all steam outlet apertures. For example, the combined cross-section of all the steam inlet apertures can be substantially greater, for example in the ratio of 20:1, than the combined crosssection of all the steam outlet apertures.

By choice of this ratio, it is possible according to the shape of the mould, the wall thickness, the material and so on, to pre-set a required build-up of pressure of steam introduced into the interior of the mould according to the relevant ratio. The ratio can be made larger, for example up to a ratio of 50:1, if special requirements make it necessary.

Fig. 10 shows an embodiment which, for a mould for a relatively complicated article, reproduces the design shown in Fig. 9. In this embodiment the core 151 has longitudinally and transversely extending bores 192 in the core walls 195, which communicate with the mould cavity through connecting bores with nozzles 197. The encircling steam passage has four steam inlets 161', see the plan view in Fig. 10. All other parts correspond to those explained in connection with Figs. 6 and 10, and carry the same reference numerals.

WHAT I CLAIM IS: 1. An apparatus for moulding articles from foamable plastics material, in which a mould, in which the material is foamed under the action of heat and steam, is connected to means for controlled heating and cooling of the mould cavity walls, and is also connected, separately from the heating and cooling means, to a steam supply,

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. they discharge into the cavity 159. As in the embodiment of Fig. 6, the steam passage 160 is charged through a steam connection 161. As explained in connection with Fig. 6, steam outlets 167 and 168 discharge directly to the outside or by way of a control valve. For easier connection to a control valve, the steam outlets 167 discharge into a steam outlet manifold 188. A similar arrangement could also be provided on the cover 152, if a common means of controlling the outlets 168 is needed. As in Fig. 6, the steam outlets 167 and 168 can also be controlled as air outlets and as inlets for mould-removing air. The disposition of mechanical ejectors and the injector have for clarity been omitted from Fig. 8. The heating and cooling chambers 155 and 156 and their inlets and outlets 157a, 157b, 158a, 158b can be constructed similarly to Fig. 6. Also as in Fig. 6, it is possible, for discharge of the air used for filling the mould, to maintain the core and cover at a slight distance apart so that the passage 160 is not effectively sealed by the seal 182, the filling air then passing outwards past the steps 174 or sealing strips 177 or 178 into the passage 160 and thence past the seal 182. The mould parts can be brought towards the steps 174 or the closure strips 177, 178 so that particles of material cannot enter the passage 160. Since when the mould is open the passage 160 is being opened repeatedly, it is in any event safeguarded against deposit of particles. Fig. 9 shows an embodiment of a mould for an article which has projecting wall parts not only at the periphery but also in its central zones. Therefore, in addition to depressions 1 59a provided for the peripheral walls and formed by meeting parts of the mould wall, there are also depressions 190 in the core 151 or cover 152 for intermediate walls or other projections in the middle zone of the surface of the article. Steam is introduced into the cavity 159 also at the end faces of such additional depressions 190. Steam passages 191, 192 are connected to the depressions 190, in the core 151 in Fig. 9, and communicate with the passage 160. Such a steam passage 191 is shown in the upper part of Fig. 9 as an example and if formed in that the corresponding depression 190 is of a depth which exceeds the amount required by the shape of the article, there being inserted into this depression 190 a separating element 193 which constitutes the end mould wall for the intermediate wall of the article. Provided at intervals in this separating element are nozzlelike fine bores 194. Shown in the lower part of Fig. 9 is a variation of the embodiment, with core wall parts 195 (which form the depressions 190) extending beyond the depression 190 and including a channel 192 which extends parallel to the mould depression 190 and communicates with the passage 160. At intervals there are provided in the wall parts 195 bores 196 connecting the mould depression 190 to the channel 192, and provided with a nozzle-like insert 197, so forming the means for supplying steam from the passage 160 through the channel 192, the bores 196 and the inserts 197 to the mould depression 190. The disposition of steam outlet orifices 167, 168 is similar to that in the embodiments of Figs. 6 and 8. The steam inlet 161' is similar to that described in connection with Fig. 7. The heating and cooling chambers 155 and 156 and their connections 157a, 157b, 158a, 158b are likewise similar to those in Figs. 6 and 8. The means of controlling the mould for dissipation of filling air and for intake of mould-removing air can likewise be as in Figs. 6 and 8. Escape of particles of material through the steam outlets 167 and 168 is prevented in that steam outlet orifices 189, similar to the steam inlet apertures in the mould walls, have a plurality of small apertures, or are in the form of a mesh. The combined cross-section of all steam inlet apertures can be related to the combined cross-section of all steam outlet apertures. For example, the combined cross-section of all the steam inlet apertures can be substantially greater, for example in the ratio of 20:1, than the combined crosssection of all the steam outlet apertures. By choice of this ratio, it is possible according to the shape of the mould, the wall thickness, the material and so on, to pre-set a required build-up of pressure of steam introduced into the interior of the mould according to the relevant ratio. The ratio can be made larger, for example up to a ratio of 50:1, if special requirements make it necessary. Fig. 10 shows an embodiment which, for a mould for a relatively complicated article, reproduces the design shown in Fig. 9. In this embodiment the core 151 has longitudinally and transversely extending bores 192 in the core walls 195, which communicate with the mould cavity through connecting bores with nozzles 197. The encircling steam passage has four steam inlets 161', see the plan view in Fig. 10. All other parts correspond to those explained in connection with Figs. 6 and 10, and carry the same reference numerals. WHAT I CLAIM IS:

1. An apparatus for moulding articles from foamable plastics material, in which a mould, in which the material is foamed under the action of heat and steam, is connected to means for controlled heating and cooling of the mould cavity walls, and is also connected, separately from the heating and cooling means, to a steam supply,

wherein the steam supply includes a steam flow path which in addition to the mould cavity comprises at least one steam buffer space which co-operates with at least one steam outlet in the steam flow path downstream of the mould cavity, the internal cross-sectional area of the outlet being adjustably variable.

2. An apparatus according to claim 1, wherein at least one wall of the mould cavity has steam outlet orifices whose total cross-sectional area is matched to the volume of the mould cavity to produce a build-up of steam in the buffer steam space and in a steam inlet to the mould.

3. An apparatus according to claim 2, wherein the steam inlet comprises a plurality of steam inlet apertures in the mould.

4. An apparatus according to claim 3, wherein the steam inlet and outlet apertures each comprise a plurality of small apertures, or a mesh.

5. An apparatus according to claim 3 or claim 4, wherein the total cross-sectional area of all the steam inlet apertures is greater than that of all the steam outlet apertures.

6. An apparatus according to claim 5, wherein the total cross-sectional area of all the steam inlet apertures is in a ratio of up to 50:1 to that of all the steam outlet apertures.

7. An apparatus according to any of claims 2 to 6, wherein the steam buffer space comprises at least one steam collecting chamber disposed behind a wall of the mould cavity and in constant communication with the mould cavity through steam outlet orifices, the volume of the steam collecting chamber being matched to the crosssectional area of the steam inlet or the steam inlet apertures.

8. An apparatus according to any of claims 2 to 6, wherein the steam buffer space includes at least one steam distributor chamber in constant communication with the mould cavity through steam inlet apertures.

9. An apparatus according to claim 7 and 8, wherein the buffer steam space includes steam distributor chambers and steam collecting chambers in the form of passages at the back of the cavity wall or walls, the steam inlet and outlet apertures being ports in the wall adjacent the passages.

10. An apparatus according to claim 9, wherein the steam distributor and collecting chambers are grooves in the cavity wall covered by strips which have ports and which close the wall surface.

11. An apparatus according to claim 9 or claim 10, comprising two substantially plate-like flat, opposed, cavity walls wherein one or each wall is provided in alternating sequence with steam distributor and collecting chambers.

12. An apparatus according to claim 9 or claim 10, comprising two substantially plate-like flat, opposed, cavity walls wherein steam distributor chambers are provided in one wall and steam collecting chambers in the other.

13. An apparatus according to claim 11 or claim 12, wherein the steam distributor and collecting chambers are parallel.

14. An apparatus according to claim 13, wherein the steam distributor and collecting chambers are parallel to two opposite side edges of the cavity wall, an additional steam distributor chamber being formed by the two mould parts which have their outer edges fitted sealingly against each other, each distributor chamber communicating with the mould cavity by a slot formed between overlapping portions of the two mould parts.

15. An apparatus according to claim 14, wherein the additional steam distributor chamber being continuous in the region of the periphery of the mould and communicating with the mould cavity all round via the slot, the other steam distributor chambers communicating with the continuous chamber.

16. An apparatus according to claim 1, wherein the steam buffer space includes a steam collecting chamber or chambers which has or have an outlet for the steam to be discharged, the cross-sectional area of such outlet or outlets being adjustably variable in accordance with the cross-sectional area of the steam inlet to the mould cavity so as to produce a dynamic steam pressure.

17. An apparatus acocrding to claim 16, wherein the or each outlet is controlled by a valve which is kept for a time at least partially closed in order to produce a dynamic steam pressure during supply of steam to the mould cavity.

18. An apparatus according to claim 8, wherein the steam distributor chamber includes a chamber in the peripheral zone of the mould and wherein mould cavity depressions have steam inlet connections from the steam distributor chamber to the mould cavity, at least one steam outlet from the mould cavity being provided in a surface of the mould cavity.

19. An apparatus according to claim 18, wherein the peripheral zone chamber is disposed in the region of the plane of separation of the mould parts and is formed between the parts.

20. An apparatus according to claim 19, wherein parts of the steam distributor chamber disposed in middle zone of the mould are formed by passages communicating with the peripheral zone chamber and disposed in mould cavity depressions.

21. An apparatus according to claim 20, wherein the cavity depressions are recessed to a depth exceeding that necessitated by the mould shape so as to form channel-like parts of the steam distributor chamber which communicate with the peripheral zone chamber, separating wall elements being inserted in these depressions and having steam ports, so as to delineate the steam distributor chamber in respect of the mould cavity.

22. An apparatus according to claim 20, wherein the mould wall parts which form the depressions extend beyond the depressions and comprise first bores which communicate with the peripheral zone chamber and connecting bores which lead from the first bores to the mould cavity.

23. An apparatus according to claim 21, wherein the connections between the steam distributor chamber and the mould cavity are at least partly formed by slots formed between the mould wall parts which overlap at the ends of the depressions.

24. An apparatus according to claim 23, wherein the connections between the steam distributor chamber and the mould cavity are at least partly formed by inlet apertures in the end faces of the depressions.

25. An apparatus according to claim 8, wherein the steam collecting chamber includes a chamber in the peripheral zone of the mould and wherein mould cavity depressions have steam outlet connections from the cavity to the steam collecting chamber.

at least one steam inlet aperture to the mould cavity being provided in a surface of the mould cavity.

26. An apparatus according to any preceding claim, wherein the buffer steam space functions also as a receiving and distributing chamber for compressed air.

27. An apparatus according to any preceding claim, wherein the steam outlet or outlets is or are, by way of valves, exposed to compressed air.

28. An apparatus according to any preceding claim, wherein at least one chamber comprising the buffer steam space and/or the steam inlet or steam outlet of the mould can be connected to a vacuum generator.

29. An apparatus according to claim 28, wherein the vacuum generator is connected through a controlled valve to at least one chamber comprising the buffer steam space and/or to the steam inlet and/or to the steam outlet of the mould.

30. An apparatus according to claim 29, wherein a buffer vacuum space is disposed between the vacuum generator and the relevant valve.

31. An apparatus for moulding articles from foamable plastics material, substantially as herein described with reference to the drawings.