CA2696510A1

CA2696510A1 - Press system

Info

Publication number: CA2696510A1
Application number: CA2696510A
Authority: CA
Inventors: Roland Karlsson; Jerry Stigsson; Anders Paulsson
Original assignee: Diab International Ab; Roland Karlsson; Jerry Stigsson; Anders Paulsson
Current assignee: Diab International AB
Priority date: 2007-08-16
Filing date: 2007-08-16
Publication date: 2009-02-19
Also published as: CN101827695A; BRPI0721934A2; JP2010536603A; EA201000336A1; WO2009022954A1; EP2178686A1; KR20100071989A

Abstract

Press system (10, 11, 12, 13, 14) for production of partially expanded polymer bodies, comprising a mould cavity (20) of variable volume, temperature control means (30), counter 5 pressure means (40) arranged to counteract expansion of the mould cavity during moulding, wherein the pressure applied by the counter pressure means is arranged to increase in response to expansion of the mould cavity. There is also provided a method of moulding a partially expanded polymer body.

Description

PRESS SYSTEM

The field of the invention The present invention relates to a press system, more in detail the invention relates to a press system for production of partially expanded polymer bodies.

Background of the Invention Today, PVC based rigid foam polymer materials are being widely used, mainly as core material in sandwich structures in the naval or aeronautic sector, or as thermal/acoustic insulators in the building sector. In a sandwich structure the core separates two structurally more rigid materials, such as fibre reinforced plastics (FRP), metal or the like. Such sandwich structures have many advantages compared to more traditional single layer structures, such as lower weight, insulation properties etc. Whilst other rigid foam polymer materials, such as foamed polyurethane etc. can be produced using streamlined continuous extrusion methods, the production of PVC based rigid foam polymer materials involves moulding of discrete partially expanded bodies (hereafter referred to as embryo bodies) under high pressure in a press. The embryo bodies are subsequently subjected to a chemical-physical treatment to obtain the rigid foam polymer material.

More in detail, the production process of a PVC based rigid foam polymer material initially involves formation of a plastisol paste consisting of a mixture of powders (PVC and other compounds) and liquid substances (in particular isocyanates). The paste filled in a closed mould cavity and is subjected to a heating and subsequent cooling process under high pressure resulting in a partially expanded embryo body. The embryo body is then further expanded through an additional heat treatment in water and/or a steam oven.
The formation of the final rigid foamed material is a result of a hydrolysis reaction of the isocyanate groups present in the material, with subsequent build up of a polymer which crosslink the chemical structure.

At present, the methods for the production of embryo bodies involves filling the each mould with an excess amount of paste with respect to the polymer content in the finished product.
The excess amount is then allowed to leak out from the mould during the moulding process.
The moulding process comprises heating the plastisol in a closed mould, whereby a high pressure is created by the thermal expansion of the plastisol and the activation of the blowing agent dissolved therein. During this expansion step, the excess amount is allowed to leak out.
The plastisol is kept at elevated temperature a predetermined time to allow the plastisol to gelatinize, where after the mould cavity is cooled to a temperature that is low enough to remove the embryo body from the mould. The excess amount is approximately equal to about 8%, in terms of weight of the product leaving the mould.
The excess paste emerges from the top edge of the mould. Consequently there is a non-recoverable wastage of material, since the PVC gelatinizes and some of the blowing agent substances deteriorate at high temperature.

US 6352421, Olivier Giacoma, filed 2000-02-15, solves the problem of escape of paste from the mould during the heating step by providing a secondary mould compartment into which the excess of paste is fed during the heating step, and from which a small part of the paste is allowed to escape into a perimetral waste-collecting groove. According to the disclosed method, paste is top-filled in the primary mould compartment, during heating the paste expands about 8% and excessive paste is fed to the secondary compartment by connecting grooves. The secondary compartment has a volume that is slightly less than 8%
of the primary compartment. Hence, the amount of waste paste is reduced to about 8% of the volume of the secondary compartment, which is approximately equal to 0.64% of the volume of the primary compartment.
US 2768407, Lindemann, Filed December 5, 1950, relates to production of closed cell cellular bodies from thermo-plastic masses. It is stated that a problem in the prior art is that it is in practice impossible to keep a mould filled with a mass containing an expansion agent completely sealed during the heating stage. A solution to this problem is proposed: by, after the gases have been dissolved under pressure in the mass and the mass has gelatinised completely, expanding the volume of the mould by 1/5 to 2/5 of the original volume. Use of a mould with a moveable die is proposed, and it is stated that it is necessary to apply a high pressure e.g. 150-300 atm (bar) to slow down the decomposition of the expansion agent and to cause the gas to dissolve. It is also stated that suitable thermo-plastics include polyvinyl-chloride.

Summary of the Invention The object of the invention is to provide a new press system for production of rigid expanded polymer embryo bodies which overcomes the drawbacks of the prior art. This is achieved by the press system and the method as defined in the independent claims.

Like in all materials production processes, important parameters for moulding embryo bodies when producing rigid expanded polymer materials are e.g. materials consumption, energy consumption, work flow and throughput time. The proposed press system is superior to the prior art with respect to at least one of these parameters.

Embodiments of the invention are defined in the dependent claims.
Brief Description of the Drawings The invention will be described in detail below with reference to the drawings, in which:
Figs. 1 a to 1 e schematically show a cross sectional view of a press system for production of partially expanded polymer bodies, in different stages of moulding an embryo body.

Fig. 2 is a schematic diagram of selected process parameters of a mould process in a press system according to the embodiment of figs. l a to 1 d.

Figs. 3a to 3d schematically show another embodiment of the press system.

Fig. 4 is a schematic diagram of selected process parameters of a mould process in a press system according to the embodiment of figs. 3a to 3d.

Figs. 5a to 5c schematically show another embodiment of the press system.

Fig. 6 is a schematic diagram of selected process parameters of a mould process in a press system according to the embodiment of figs. 5a to 5c.

Figs. 7a and 7b schematically show another embodiment of the press system.
Fig. 8 schematically show another embodiment of the press system.
Fig. 9 is a flow chart over a method according to the present invention.
Detailed Description of Preferred Embodiments Figs. 1 a to 1 d schematically show a cross sectional view of a press system 10 according to the present invention in different stages of moulding a partially expanded polymer body.
According to one embodiment, the press system 10 comprises a mould cavity 20 of variable volume, temperature control means 30, counter pressure means 40 arranged to counteract expansion of the mould cavity 20 during moulding, wherein the pressure applied by the counter pressure means 40 is arranged to increase in response to expansion of the mould cavity 20. In the embodiment disclosed in figs. l a to 1 d, the press system 10 further comprises a press arrangement comprising a press base 120 and a press top 50 with a moveable press member 130 urged in the downwards direction by the counter pressure means 40, and a replaceable mould too160 arranged between the press base 120 and the moveable press member 130. The press base 120 and the press top 50 are firmly interconnected by clamps 140 to avoid relative movement.

In the embodiment disclosed in figs. l a to 1 d, the mould cavity 20 is provided as a replaceable mould too160 comprising a first mould member 70 and a second mould member 80. A recess in the first mould member 70 defines a portion of the mould cavity volume 20 and sealing means 90 is fitted in between the mating side flanges 100 and 110 respectively.
As can be seen in the figs. la and lb, the first mould member 70 and the second mould member 80 are moveable with respect to each other during moulding, and the sealing means 90 is arranged to provide an essentially hermetic seal between the first 70 and second 80 mould members during at least a portion of such a volume expansion of the mould cavity 20.

In other embodiments, as will be shown below, the mould cavity 20 of variable volume may be formed as an integrated part of the press system 10, or in other suitable ways.

The temperature control means 30 are provided to effectively control the temperature of the 5 plastisol in the mould cavity 20 during the moulding process. Initially, the formation of the partially expanded polymer body requires heat to activate the blowing agent and to initiate the gelatinization of the plastisol, thereafter when the gelatinization has reached a certain point the heat created by the process exceeds the amount consumed and the plastisol has to be cooled in order to avoid overheating. This will be discussed more in detail below. According to one embodiment, the temperature control means 30 comprises conduits for a heating/cooling media, such as water or the like. Alternatively, the temperature control means may be provided as separate heating and cooling means, e.g. electrical heating means and cooling conduits for cooling media.

In the embodiment disclosed in figs. l a to 1 d, the mould cavity 20 is formed to produce embryo bodies of flat rectangular panel shape that in later stages of the process are further expanded and cured to form panels of rigid polymer foam material with excellent mechanical properties. Depending on the application of the finished rigid foam material, the mould cavity may be of different shapes, such as spherical, tubular, cylindrical etc.
According to the disclosed embodiment, each one of the first and second mould members 70 and 80 of the replaceable mould too160, comprises a major wall 140 and 150 respectively, parallel with and arranged to be adjacent the respective first and second press members 120 and 130 respectively. The first mould member 70 comprises an essentially perpendicular side flange 100 that circumscribes the major wall 140, and the second mould member comprises a corresponding side flange 110 that mates with the side flange 100, defining a narrow gap there between, wherein the sealing means 90 is fitted. As mentioned above, the replaceable mould too160 may be integrated with the press 10, by forming the first mould member 70 and the press base 120 as a press mould base and by forming the second mould member 80 and the moveable press member 130 as a moveable press mould member.

In figs, 1 a to 1 d the counter pressure means 40 are formed by an arrangement of spiral springs 45 that urge the moveable press member 130 in the direction opposite the expansion direction of the mould cavity 20. In the disclosed embodiment, the expansion direction is upwards but the press system may also be designed so that the expansion direction is downwards sideways or anything there between. A counter pressure means 40 in the form of a spring arrangement, will apply a counter pressure that increases as the expansion commences, but which cannot be actively controlled during a step of moulding. The counter pressure means 40 may be arranged so that the pressure applied increases essentially linearly or exponentially in response to expansion of the mould. The counter pressure means 40 may be arranged so that the pressure applied increases stepwise at one or more points of expansion.
Further, the counter pressure means 40 may be arranged to provide any suitable combination of linear, exponential or stepwise increase of the applied pressure. Hence, the expansion of and the pressure in the mould cavity can be controlled accordingly.

Other types of passive counter pressure means 40 include any types of arrangements that applies an increasing non controllable force on the moveable member 130.
According to another embodiment, the counter pressure means 40 may be comprised of an arrangement that allow the force applied on the moveable press member 130 to be controlled according to a predetermined scheme during the expansion process. One example of such a counter pressure means 40 of controllable type is a hydraulic press system, wherein the applied pressure can be controlled by rising or lowering the hydraulic pressure in the system. Such a hydraulic press system may be controlled by a pressure controlled relief valve according to the description below.

As mentioned above, the mould cavity 20 is filled with plastisol in fig. la.
In order to avoid surface defects on the embryo body, it is of great importance that essentially all air is evacuated from the mould cavity 20 before the moulding cycle is initiated. In order to achieve evacuation of air trapped between the mould members 100 and 110 when the mould cavity 20 is filled with plastisol, the second mould member 110 (being the top one) in the disclosed embodiment is provided with a small evacuation opening 160. The evacuation opening 160 is formed to allow air to pass, but to prevent plastisol to escape from the mould cavity 20.
According to one embodiment, the evacuation opening 160 is so small that the plastisol itself closes the opening due to the high viscosity, whereby only a small amount of plastisol is allowed to leak out of the mould cavity 20. However other types of self closing evacuation openings may be used, such as valve type openings, wherein the plastisol act on a valve body to close the opening. In order to facilitate removal of the embryo body from the mould members 100 and 110, the evacuation opening 160 is formed to avoid that the gelatinized embryo body get stuck therein. One way to avoid this is to make the evacuation opening 160 of conical shape with the broad end open to the mould cavity 20 and a small top opening open to the outside of the mould too160. As is shown herein, the mould too160 is adapted to be used in a press system with parallel press planes, whereby the small top opening is covered by a press plane 130 and the open area is further reduced.

Fig. 2 is a schematic diagram of some process parameters of a mould process in a press system 10 according to the embodiment of figs. la to ld. The mould process of fig. 2 comprises heating the plastisol, T control, in a closed mould cavity 20, whereby a high pressure P is created by the thermal expansion of the plastisol and the activation of the blowing agent dissolved therein. By selecting suitable characteristics for the counter pressure means 40, the pressure P in the mould cavity 20 will, as a result of the heating T control, exceed the pressure applied by the counter pressure means 40, whereby the volume V of the mould cavity 20 will increase. The plastisol is kept at elevated temperature a predetermined time to allow the plastisol to gelatinize, where after the mould cavity 20 is cooled to a temperature that is low enough to allow removal of the embryo body 170 from the mould cavity 20. As is disclosed above, the gelatinization process produces heat, and must be cooled in order to avoid over heating. In fig. 2 it is indicated that the pressure P
continues to rise during a short period after the T control has been switched to cooling, which is a result of the heat produced by the gelatinization process. The point of maximum pressure is indicated by the dotted line in fig. 2. For the same reason the mould cavity 20 volumes will continue to increase until the pressure has reached its maximum. Once the cooling process has reached the point of maximum pressure, the process of cooling leads to a reduction of the volume of the mould cavity 20, mainly corresponding to the resulting negative thermal expansion of the embryo body 170. Fig. lb shows the press system 10 when the volume of the mould cavity 20 has reached its maximum volume defined by the characteristics of the counter pressure means 40 the type of plastisol mix and the process parameters that are used.
Typically, the volume expansion corresponds to 5 to 20 percent or more compared to the filling volume.

Fig. 1 c shows a step of unlocking the press system according to this embodiment, wherein the elastic properties of the partially expanded polymer body 170 is utilized to release the interconnection clamp members 140. A compression force exceeding the final pressure in the mould cavity 20 is applied on the press top 50, whereby the counter pressure means 40 and the embryo body 170 are compressed so that the clamps 140 can be withdrawn to unlock the press system 10.
Fig. ld shows the press system 10 when the press top 50 and the second mould member 110 are lifted off the first mould member 100 and the press base 120, whereby the compressed embryo body 170 is starting to pop out from the mould 60 by the internal expansion forces, and fig. l e shows the relaxed embryo body after it has popped out from the first mould member 100. In the Figs. lb to ld, the resulting relative movement of the first and second mould members, 100 and 110 respectively, is exaggerated for illustrative purposes, whereby excess material 180 formed between the upper surface of the side wall 80 and the major wall 70 of the second mould member represent an significant volume of waste material that has to be removed. However, in production scale mould tools 60, the excess material 180 will be less than the previously accepted leakage volume of 8 %. The evacuation opening 160 produces a nipple 190 on the gelatinized embryo body, which is removed together with the excess material 180.

The mould members 70, 80, the press base 120 and the moveable press member 130 may be comprised of any suitable rigid material with reasonable thermal conductivity.
They may e.g be comprised of a metal such as aluminium, stainless steel or the like.
Alternatively, or in combination they may be comprised of a composite material, such as fiber reinforced plastics.
Due to the high pressure in the mould during the mould process; up to and exceeding 200 atm, all parts of the press system must be designed accordingly.
As disclosed above, the expansion of the plastisol during the moulding process is between 5 and 20% and during this process it is important that the high pressure is preserved in the mould cavity 20. However under certain circumstances, the pressure build up in the mould cavity 20 can reach extreme levels due to improper plastisol mix or overfilling. According to one embodiment, the mould is designed so that the moveable mould member 110 provides an essentially hermetic sealing effect at an increase of mould cavity 20 volume of a predetermined value between 6 and 20% with respect to a filling volume, where after the sealing effect is arranged to be reduced to avoid overpressure in the mould cavity. According to an alternative embodiment, the sealing effect is gradually reduced.
Moreover, one of the mould members may be designed to provide an adjustable expansion limit.

Figs. 3a to 3d show an embodiment of a press system, wherein, the press base 121 and the press top 51 are attached by a hinge arrangement 200 along one side and by interlocking means 210 on the opposite side. In the disclosed embodiment, the interlocking means 210 is shown as a rotary lock mechanism, but it may be any suitable interlock mechanism. In the embodiment of figs. 3a to 3d, the counter pressure means 40 is comprised of compression springs 220 and secondary compression members 230. The counter pressure means according to this embodiment is arranged to allow an initial expansion of the mould cavity 20 under a counter pressure from the compression springs 220, followed by a secondary expansion under an elevated counter pressure in form of the combined force from the compression springs 220 and the compression members 230. In figs. 3a to 3d the counter pressure means 40 are shown as a combination of compression springs 220 and compression members 230 in the form of solid members of a flexible material, e.g. rubber or the like, as an illustration that the counter pressure means 40 may formed by any combination of means capable of applying a counter pressure that increases in response to expansion of the mould cavity. Fig. 4 is a schematic diagram of some process parameters of a mould process in a press system 10 according to the embodiment of figs. 3a to 3d. In fig. 4 the point when the expansion of the mould cavity 20 has reached the secondary expansion is indicated by the left dashed line. As is indicated by fig. 4, the increase in pressure P during the secondary expansion results in reduced expansion rate of the volume V.

Fig. 3c illustrates the step of unlocking the press system 11, wherein a force is applied on the left hand side of the press top 51 to compress the embryo body 170 to allow unlocking of the rotary locking mechanism 210 where after the press system can be flipped open as is shown in fig. 3d.

Figs. 5a to 5c show an embodiment of a press system 12, wherein the mould cavity 20 is integrated with the press base 122 and the moveable press member 132. By integrating the mould cavity 20 in the press system 12, the handling of the separate mould tools 60 is omitted. In the disclosed embodiment, the press base 122 and the moveable press member 132 are illustrated as a plunger type arrangement wherein the mould cavity 20 is formed by a recess in the press base 122 and the press member 132 is formed as a mating plunger defining the upper wall of the mould cavity 20. As in the above embodiments, there is provided a sealing member between the press base 122 and the press member 132 to achieve an essentially hermetic seal there between.

In the disclosed embodiment, the press member 132 is arranged to be moveable between a lower position set by a lower shoulder 260 and an upper position set by expansion termination means 250 hindering further movement of the press member 132 in the expansion direction.
Thus, in the disclosed embodiment, the final predetermined mould cavity volume is set by the 10 position of the expansion termination means 250, and the resulting peak pressure depends on the volume of plastisol filled into the mould. Thus, the volume of the mould cavity 20 is defined by the expansion termination means 250, which corresponds to a volume expansion of 5 to 20 percent or more compared to the filling volume depending on the type of plastisol mix and the process parameters that are used.
Fig. 6 is a schematic diagram of some process parameters of a mould process in a press system 12 according to the embodiment of figs. 5a to 5c. Like in the embodiment of figs. 3a to 3d, the counter pressure means 40 of the press system 12 are arranged to allow an initial expansion of the mould cavity 20 under a counter pressure from the compression springs 240, followed by a secondary expansion under an elevated counter pressure caused by the expansion termination means 250. In this illustration, the compression springs 240 are arranged to provide an essentially linear increasing counter pressure, followed by a counter pressure peak, caused by the terminated volume expansion.

Therefore, in order to be able to produce partially expanded polymer bodies with the same characteristics, it is of great importance that the same volume ofplastisol is filled into the mould for all such bodies. Partially expanded polymer bodies with different characteristics can thus be achieved by altering the volume of plastisol filled into the form and/or by altering the final mould cavity volume by changing the position of the expansion termination means 250. According to one embodiment, the expansion termination means 250, and thus also the final mould cavity volume, are adjustable.

Figs 7a and 7b show still another embodiment of a press system 13, wherein the mould cavity 20 is at least partly defined by a flexible wall member 300 separating the plastisol in the mould cavity 20 from the counter pressure means 40 in the form of a hydraulic pressure fluid 310 applying the desired counter pressure over the membrane surface. Like in the embodiment of figs. 5a to 5d, this embodiment comprises expansion termination means 320 comprised of a rigid termination member or a rigid wall on the pressure fluid side of the flexible wall. In the disclosed embodiment, the press system 13 is comprised of a press top 53 and a press base 123 that are firmly interconnected by clamps 140. The press top 53 comprises, temperature control means 30, the flexible wall member 300 and hydraulic conduits 350 connecting the hydraulic fluid side of the flexible wall member 300 with a source of hydraulic pressure 330 illustrated by an arrow. The pressure applied by the hydraulic pressure fluid 310 is supplied and controlled by the source of hydraulic pressure 330. The press base comprises a lower section of the mould cavity 20 and control means 30.

As is shown in fig. 7b, high pressure in the plastisol forces the pressure fluid from the fluid side of the flexible wall (via conduits to a reservoir or the like) until the flexible wall abuts the expansion termination means 320 defining the final mould cavity volume. This, embodiment makes it possible to produce partially expanded polymer bodies of complex shapes in net shape, that in later stages are further expanded and cured to rigid foamed polymer objects.
According to one embodiment, disclosed in fig. 8, the counter pressure applied by the hydraulic pressure fluid 310 is controlled by a pressure controlled relief valve 340. The pressure controlled relief 340 valve is arranged to open when a predefined or controllable pressure threshold is reached, and the hydraulic fluid from the valve is fed to a reservoir 350.
The pressure threshold may be a static pressure, or it may be controllable in order for the pressure in the mould cavity to be controlled according to a predetermined scheme.
There is also provided a method of moulding a partially expanded polymer body in accordance with the above embodiments, comprising the steps:
preparing a plastisol comprising a blowing agent, filling the plastisol into a mould cavity of variable volume heating the plastisol to activate the blowing agent allowing the mould cavity to expand under a counter pressure arranged to increase in response to expansion of the mould, preserving the plastisol at elevated temperature a predetermined time to allow the plastisol to gelatinize and transform into a partially expanded polymer body, cooling the partially expanded polymer body, opening the mould cavity, and removing the partially expanded polymer body.
The above method may be performed with the press system according to the present invention, but it may also be performed in large facility press system, using mould tools of variable volume. The counter pressure may either be actively controlled by e.g. a hydraulic pressure system, or passively controlled by selection of counter pressure members with predetermined characteristics.

As mentioned above, the counter pressure may increase linearly or exponentially during the expansion, or it may be varied according to a predetermined scheme during the expansion.
According to one embodiment, the counter pressure increases stepwise.
According to one embodiment, the method comprises the step of:

terminating the expansion of the mould cavity after a predetermined volume expansion.
By terminating the expansion at the appropriate volume pressure in the mould cavity is allowed build up, and a desired high pressure is achieved.

Claims

1. Press system (10, 11, 12, 13, 14) for production of partially expanded polymer bodies, comprising a mould cavity (20) of variable volume, temperature control means (30), counter pressure means (40) arranged to counteract expansion of the mould cavity during moulding, wherein the pressure applied by the counter pressure means is arranged to increase in response to expansion of the mould cavity.

2. Press system according to claim 1 wherein the pressure applied by the counter pressure means increases essentially linearly in response to expansion of the mould.

3. Press system according to claim 1 wherein the pressure applied by the counter pressure means increases exponentially in response to expansion of the mould.

4. Press system according to claim 1 wherein the pressure applied by the counter pressure means increases stepwise at one or more points of expansion.

5. Press system according to claim 1 comprising expansion termination means (250,320) arranged to terminate expansion of the mould cavity at a predetermined mould cavity volume.

6. Press system according to the preceding claim wherein the mould cavity is defined by rigid walls, at least a section of a wall is moveable, the counter pressure means counteract movement of the moveable wall section in the expansion direction, and wherein the expansion termination means is comprised of one or more stop member (250) that prevent further movement of the moveable wall section (150, 132) in the expansion direction.

7. Press system according to claim 1 wherein the mould cavity is defined by rigid walls, at least a section of a wall is moveable, the counter pressure means counteract movement of the moveable wall section in the expansion direction.

8. Press system according to claim 1 wherein the counter pressure means comprises one or more compression springs (45, 220, 240).

9. Press system according to claim 1 wherein the counter pressure means is a hydraulic press system (310).

10. Press system according to the preceding claim wherein the counter pressure applied by the hydraulic pressure fluid is controlled by a pressure controlled relief valve (340).

11. Press system according to claim 5 wherein the mould cavity is at least partly defined by a flexible wall member (300) separating the mould cavity from a hydraulic pressure fluid applying the counter pressure, and wherein the expansion termination means (320) is comprised of a rigid termination member or a rigid wall on the fluid side of the flexible wall.

12. Press system according to claim 1 wherein the mould cavity is defined by a replaceable mould tool (60).

13. Press system according to the preceding claim wherein the replaceable mould tool is comprised of first and second mould members (70, 80) that are linearly moveable with respect to each other, and the counter pressure means are arranged to act on the first and second mould members in the direction of mutual movement.

14. Press system according to claim 1 comprising a press top (50, 51, 52, 53) with a moveable press member (130, 132, 300) and a press base (120,121, 122, 123) that can be interlocked in a press configuration.

15 15. Press system according to the preceding claim wherein the press top and the press base are interlocked by one or more clamp members (140).

16. Press system according to claim 14 wherein the press top (51) and the press base (121) are hinged along one side (200), and are interlocked by interlocking means (210) on the opposite side.

17. Method of moulding a partially expanded polymer body comprising the steps:

preparing a plastisol comprising a blowing agent, filling the plastisol into a mould cavity of variable volume heating the plastisol to activate the blowing agent allowing the mould cavity to expand under a counter pressure arranged to increase in response to expansion of the mould, preserving the plastisol at elevated temperature a predetermined time to allow the plastisol to gelatinize and transform into a partially expanded polymer body, cooling the partially expanded polymer body opening the mould cavity, and removing the partially expanded polymer body.

18. Method of moulding according to the preceding claim wherein the counter pressure is varied according to a predetermined scheme during the expansion.

19. Method of moulding according to anyone of the two preceding claims comprising the step of terminating the expansion of the mould cavity after a predetermined volume expansion.