GB2052358A

GB2052358A - Producing mouldings from foamable thermoplastics

Info

Publication number: GB2052358A
Application number: GB8017611A
Authority: GB
Original assignee: CIVAG AG
Current assignee: CIVAG AG
Priority date: 1979-05-31
Filing date: 1980-05-29
Publication date: 1981-01-28
Also published as: MA18864A1; IT1140985B; BR8003408A; IL60199A0; JPS5628835A; DE2922314A1; BE883585A; FR2457763A1; FR2457763B3; PT71333A; SE8004057L; NL8003168A; IT8022471A0

Abstract

The present invention relates to a process for the production of moldings from foamable thermoplastics, especially from foamable polystyrene foam, in which pre-foamed plastic granules are post-expanded and sintered together in a mold with the supply of heat, a vacuum being applied to the mold before this is filled with the pre- foamed plastic granules and the mold being pre-warmed from the outside, characterized in that a hot inert gas is passed into the mold during or after introduction of the pre-foamed plastic granules in order to warm the granules.

Description

SPECIFICATION Process and device for the production of mouldings from foamable thermoplastics The invention relates to a process and a device for the production of moldings from foamable thermoplastics, especially from pre-foamed polystyrene foam, for example Styropor or Styrocell, (Registered Trade Marks) with which pre-foamed plastic granules are post-foamed and sintered in a mold with the supply of heat.

With the processes known hitherto, heat is supplied to the mold from the outside, but this is a reasonable possibility only in the case of very thin moldings. Otherwise molds are used which are provided with holes or slits through which the pre-foamed plastic granules cannot pass but the steam can flow into the mold and between the granules.

However, in addition to the difficulty in obtaining moldings of uniform quality, a particular disadvantage proved to be the fact that the moldings released from the mold initially are still very wet and that it requires a considerable drying time to re-extract from the moldings the water which has formed by condensation of the steam.

A further disadvantage of these known processes lies in the relatively long production time and in the necessity for the provision of stationary large steam generators and in the very high energy consumption as a result of the requisite re-cooling of the molds after the production of the moldings, during which considerable amounts of energy are lost.

In order to eliminate these disadvantages, at least in part, provision is made in another known process of the initially mentioned type for the application of a vacuum to the mold before it is filled with the pre-foamed plastic granules and for pre-warming of the mold from the outside, without the heating medium required for this being able to penetrate into the mold. A small amount of steam is passed into the mold during or after the introduction of the granules, the vacuum continuing to be applied to the mold during the entire filling time.

However, even with this process the fact that steam is employed inside the mold and partially condenses and remains occluded within the molding continues to provide a disadvantage, quite apart from the corrosive influences which the steam exerts on the mold.

The object of the invention was, therefore, to provide a process and a device for carrying out a process for the production of moldings from expandable thermoplastic granules which permits the production of flawlessly dry moldings directly after release from the mold and, at the same time, suppresses corrosion of the inside of the mold to a minimum.

This object is achieved according to the invention by a process for the production of moldings from foamable thermoplastics, especially from foamable polystyrene foam, for example Styropor, Styrocell (Registered Trade Mark) or the like, with which pre-foamed plastic granules are post-expanded and sintered together in a mold with the supply of heat, a vacuum being applied to the mold before this is filled with the pre-foamed plastic granules and the mold being pre-warmed from the outside, which process is characterized in that a hot inert gas is passed into the mold during or after introduction of the pre-foamed plastic granules in order to warm the granules.

In addition to the fact that it aids the introduction of the pre-foamed plastic granules, as a result of the suction effect, the pre-evacuation of the mold prior to the introduction of the pre-foamed plastic granules in the main has the advantage that, in conjunction with the continued application of a vacuum during the entire filling time, the hot inert gas which is fed in and serves to heat the granules to the temperature required for post-expanding and for sintering (about 110 to 1 20 C) does not first have to displace the air between the granule grains, so that, with this procedure, the hot inert gas can penetrate more rapidly and more uniformly into all regions of the mold and, thus, foaming and sintering of the granules are more uniform.

The pre-warming of the mold from the outside to the temperature necessary for post-expansion and for sintering of the pre-foamed plastic particles softened on the outside means that the hot inert gas fed in has to serve only to warm the pre-foamed plastic granules and not also to warm the mold, so that a relatively small amount of hot inert gas is required.

Inert gases which can be employed are all gases which do not, like, for example, steam, tend to condense under the given process conditions. Suitable inert gases therefore include, for example, nitrogen and carbon dioxide. Of course, it would also be possible to use the rare gases as inert gases, but, at least at the present point in time, this will not be done for reasons of economy. However, air which contains water vapor, and preferably has a water vapor content of up to 50%, or contains another ignition-inhibiting gaseous compound, for example carbon tetrachloride or other halogenated hydrocarbons, can also be employed as the inert gas.

Heating of the inert gases is effected in a manner which is known to those skilled in the art.

Preferably, the pre-foamed plastic granules are fed to the mold in pre-metered amounts, which favors the production of moldings with constant characteristics and, moreover, has the additional advantage that the pre-foamed plastic granules can be filled into the mold at several points in the mold at the same time, say by leading several tap lines from the metering device to diverse points in the mold or, alternatively, by the provision of several individual metering devices, the overall capacity of which corresponds to the amount of granules required for the production of the particular molding.

In a development of the invention, the process according to the invention, with which only a relatively small amount of hot inert gas is required, for which reason alone there is already virtually no water of condensation obtained in the mold or in the molding to be produced, so that the molding can be released from the mold dry, can be even further improved by the hot inert gas flowing, at least in part, into the mold at the same time as the latter is filled with the pre-foamed plastic granules, say in such a way that the granules are already pre-heated on entry into the mold, or that the hot inert gas aids the entry of the granules into the mold.

In a particularly advantageous manner, the hot inert gas, which can be supplied in metered amounts via a metering device, can be used as the transport agent for introducing the foam granules into the mold.

In the case of the processes known hitherto for the production of moldings from expandable polystyrene foam, with which processes the mold has been warmed from the outside - without passing steam into the mold - the change between warming the mold and subsequent cooling prior to the release of the molding was generally effected by first warming the mold by means of hot steam and then cooling it by spraying with cold water. With this procedure, however, virtually all of the energy which previously has had to be expanded for warming the molds is lost with the cold water. In addition to this considerable loss in energy, there is the even more serious disadvantage that it was possible to realize installations of this type in a logical manner only as stationary large installations with large heating and cooling units.

Advantageously, therefore, heating and cooling of the mold are effected from the outside by means of a sealed two-cycle heating/cooling system which is independent of the hot inert gas to be fed into the mold, the heating system and the cooling system being coupled to one another in such a way that no great energy losses result, but only the peak values have to be heated up or cooled down again, in that the return from the two systems temporarily still remains coupled with the other system in the particular case, when switching from one system to the other.

For simpler removal of the finished moldings, especially in the case of complicated configurations, vacuum and/or compressed air can be used alternately, and, particularly advantageously, when the two halves of the mold are moved apart, the molding is first kept in one of the two mold halves by means of reduced pressure and released from the other mold half by means of compressed air and is then ejected from the first mold half by switching over from reduced pressure to compressed air in this half.

A further possibility for improving the characteristics of the moldings to be produced can, finally, also be realized by reducing the size of the mold cavity after filling of the mold is complete, by means of shifting a movable mold wall.

In order to carry out the process according to the invention for the production of moldings from expandable foam granules, the mold is provided with slits which are connected to a common suction line, the dimensions of the slits preventing passage of foam granules and the slits being provided in the parting planes of the mold halves and/or in the lower parts of the mold, so that uniform evacuation of the mold and uniform in-flow of the hot inert gas be ween the granule particles inside the mold, in addition to uniformly compact packing of the mold with granules, are ensured.

In order to ensure rapid and sudden evacuation of the mold without the use of suction pumps which are too large, the suction line can advantageously be connected via a valve to a vacuum tank on which the actual suction pump acts, so that, because of the capacity of the vacuum tank, which is large compared with the capacity or capacities of the mold or molds, rapid evacuation is possible, whilst the pauses between the evacuation during the production of one part and the subsequent evacuation during the production of a further part can serve for re-evacuation of the vacuum tank by the suction pump.

The opening of the valve in the suction line can be effected by means of a trigger actuated by closure of the mold and closing of the valve can be effected by means of an impulse trigger which generates a closing impulse for the valve in the suction line when the filling operation is ended.

One or more metering devices, which are connected to one or more feed channels in the mold, are used for pre-metering the pre-foamed plastic granules supplied from a large storage vessel and are advantageously themselves connectable via a valve to the suction line, in order on the one hand to facilitate the introduction of the granules from the storage vessel into the metering device under the action of suction and, on the other hand, to effect pre-evacuation of the granules in the metering device, so that considerable amounts of air are not again introduced into the mold when the granules are introduced, which air would either necessitate a certain time for evacuation or would make the uniform penetration of the granules in the mold by the hot inert gas more difficult.

The metering device can possess a cylinder with a movable piston and a piston rod which passes through this piston and acts as a closing member for the outlet orifice (which is connected to the inlet orifice or orifices of the mold) and is actuated by a second cylinder, in which case the movability of the piston serves to adjust the metering volume and, moreover, can, of course, also be used for ejection of the granules into the mold.

Particularly advantageously, however, orifices which can be connected to a source of compressed air, but preferably to the source of the hot inert gases, should be provided in the region of the movable piston of the metering device and in particular in the piston itself, in orderto effect the introduction of the granules with the aid of the hot inert gas which is in any case required for warming the granules and thus at the same time to take advantage of the granules already being in the pre-warmed state on entry into the mold. This results not only in a shortening of the cycle time but also in an improvement in the uniformity of the warming of the granule particles inside the mold and thus in the uniformity of the characteristics of the moldings formed.

The device can be provided with a time function element which delays the start of filling of the mold relative to evacuation of the mold, this time function element being so set, in accordance with the particular mold used, that filling of the mold takes place about 0.2 to 20 seconds after the start of evacuation, the length of delay depending on the size of the mold and thus on the speed of evacuation.

Warming of the mold from the outside to the necessary operating temperature - if the granules contained in the mold are also warmed in this way, this makes only a non-uniform and very slow warming and, accordingly, a many times shorter cycle time possible - is effected wth the aid of a two-cycle heating/cooling system which is completely separate from the inside of the mold, is complete within itself and operates with a fluid, preferably liquid, medium, the heating cycle and the cooling cycle being coupled to one another via thermostatcontrolled valves in such a way that after switching the feed to the mold from one cycle to the other cycle, the return from the mold still remains temporarily coupled to the other cycle.In this way, for example, the cooling medium (for example oil or water), which is under the same pressure as the heating medium, does indeed ensure rapid dissipation of the heat, but the hot liquid still enters into the heating cycle until the thermostat closes this via the corresponding valve and opens the return valve to the cooling cycle.Conversely, when the heating cycle is switched on, the heating medium which has been severely cooled by the mold, which in the meantime has become cold, first passes into the cooling cycle at the return and only when it issues from the mold at a higher temperature, as a result of the intermediate warming of the mold, is it fed back to the heating cycle, so that, in this way, only the peak valves have to be heated up or cooled down again and, thus, on the one hand lower costs result due to the avoidance of high heat losses and, on the other hand, the heating/cooling installation can be made considerably smaller.

Particularly advantageously, the two halves of the separable mold can form the cover plates of hollow box-like mold frames for throughput of the heating or cooling liquid and, in order to simplify change of the mold, provision can additionally be made for the connections for the hot inert gas, the suction line and the granule feed to the mold to be constructed as a component, on the machine side, to, in each case, one of the replaceable mold halves in the region of the mold separation. This component on the machine side can then, for example, be connected by simple plug-type connections to corresponding connections on another mold, by which means changing of the mold can be considerably simplified and shortened.

It should be possible, as desired, to connect a source of compressed gas or a suction pump for releasing the finished molding from the mold to slits in the two mold halves, and these slits can advantageously be the slits already required for evacuation of the mold and preferably should be arranged on the base of recesses in the mold halves. The connecting of a source of compressed gas or of a suction pump can advantageously be controlled by means of a cycle control device, which operates in such a way that when the two halves of the mold move apart a reduced pressure is first applied to the slits in one mold half, in order to retain the molding in this mold half as a result of the reduced pressure, whilst, at the same time, ejection of the molding from the other mold half is effected by supplying compressed air to the slits of this mold half.After the mold halves have been moved apart, the connection of the slits of the first mold half to the suction line is interrupted and, instead of this, compressed air is likewise applied to these slits, in order to now eject the molding from the first mold half. Of course, ejection can also be effected with the aid of mechanical ejectors, these being used in place of the said combined vacuum/compressed air release or as an aid to this.

A further improvement in the quality of the moldings produced in a device according to the invention results if the back wall of at least one mold half is made movable, in order to enable postcompaction to be carried out in the mold after filling with the pre-foamed plastic granules is complete, it being possible for this movable back wall for compaction at the same time also to be used as a mechanical ejector.

Further advantages, characteristics and details of the invention can be seen from the description of an illustrative embodiment given below and also with the aid of the drawing.

The mold 1 consists of two halves 2 and 3, which are screwed onto mold frame parts 4 and 5 with the insertion of sealing rings which are not shown, so that heating liquid or cooling liquid can be passed through the cavities 6 and 7 which are thus formed.

Channels 9, which are connected to a common vacuum line 8, are provided in the mold and the slits of these channels, which open into the interior space 10 of the mold, are so fine that no material can issue from them. These slits can be located either on the circumference of the mold 1 or on its surface, that is to say in particular in lower parts of the mold. The vacuum line 8 is connected via a valve 9a to a vacuum tank 11, to which a vacuum is continuously applied by means of a suction pump 11 a.The control technique for evacuation of the mold is such that, after closure of the mold, the valve 9a is opened via a time function element 13, either by means of a pneumatic trigger 12 or by means of an electrohydraulic trigger 12, which can be located either in the knee joint system of the machine or in the closing system of the machine, and the connection between the interior space 10 of the mold 1 and the vacuum tank 11 is thus produced.

The valve 9a (valves 47 and 48 are initially always open) remains open until the mold is completely filled, and closure of the valve 9a can be effected by the same control technique - pneumatically or hydraulically - either after filling of the mold is complete via an impulse trigger 14, or via a time relay 15. Evacuation is effected in a time sequence, so that it takes place about 0.2 to 20 seconds before filling of the mold is started, it being possible to adjust the time depending on the size of the mold, using the time function element 13.

The pre-foamed plastic graules are fed to the mold 1 via one or more metering devices 16, which, depending on the volume and the desired characteristics of the molding to be produced, introduce a predetermined, measured amount of plastic granules into the mold. Of course, it would also be possible simultaneously to feed the granules into the mold at different points in the mold using only one metering device 16 - this system deviating from that in the illustrative embodiment shown.

The metering device 16 contains a movable piston 17, the position of which determines the volume of the measurement space 18, a piston rod 19 passing through this piston 17 and closing the feed opening 20 after filling of the mold is complete. The measurement space 18 is pre-evacuated via a suction line 22 which contains a valve 21 and is connected to the suction line 8 or the vacuum tank 11,so that the pre-foamed plastic granules are sucked from a storage vessel 23 via a valve 24 into the measurement space 18 of the metering device 16. At this point in time, the mold is closed by the piston 25, to which pressure is applied and which is connected to the piston rod 19, and the pressure valve 27 is opened.If the pressure valve 27 is closed and, instead of this, the pressure valve 26 is opened, the piston 25 draws the piston rod 19 back and thus exposes the feed opening 20.

With the arrangement shown, the introduction of the measured amount of pre-foamed plastic granules from the measurement space 18 of the metering device 16 into the mold is effected with the aid of the hot inert gas which is required for heating the granules to the operating temperature and which, at least in part, flows via a valve, which is not shown, and a steam line, which is not shown, into the piston 17, which is of hollow construction, and issues under pressure through orifices, which again are not shown, in the front face of this piston. In addition to the fact that the hot inert gas which is in any case required is advantageously used at the same time for introduction of the granules into the mold 1,this process also has the further advantage that the granules are already in a very uniformly pre-heated state on entering into the mold.

The amount of hot inert gas, including the losses which also arise, for example, due to the continuous evacuation of the interior space 10 of the mold 1, is measured precisely so that it suffices to warm the particular amount of granules used to a temperature at which a post-expansion and a sintering-together of the softened surface layers of the granule particles can take place. A particular advantage of the premetering of both the granules and the hot inert gas lies in the fact that it is possible to effect filling of the mold at several points in the mold atthe same time.

If only a portion of the requisite amount of hot inert gas is required for introducing the granules from the metering device 16 into the mold 1, or if the introduction of the granules is to be effected in another way, the hot inert gas can also be passed into the mold at several points, say in such a way that the hot inert gas aids the entry of the granules into the mold.

In order to obtain the necessary operating temperature in the mold 1,so that this does not also need to be warmed by the hot inert gas, which is intended to serve exclusivelyforwarming the granules, a sealed heating/cooling cycle is used which consists of a heating cycle 30 and a cooling cycle 31, which is coupled to the heating cycle and the operating medium of which can be either water under high pressure or oil. The heating cycle 30 contains a heating device 32, a delivery pump 33, an inlet valve 34 and a return valve 35, whilst the cooling cycle has a cooling unit 36, a delivery pump 37 and also corresponding inlet and return valves 38 and 39. The inlet valves 34 and 38 are controlled by means of a relay 40 and the return valves 35 and 39 are controlled by means of a thermostat 41.

After the release of a finished molding from the mold, the cavities 6 and 7 behind the mold haves 2 and 3 are already filled with heating liquid by triggering via the relay 40, heating liquid penetrating into the two cavities 6 and 7 at the same time through the opened valve 35. The inlet valve 38 of the cooling cycle 31 is closed during this operation.

After the the foaming operation has taken place, the inlet valve 34 of the heating cycle 30 is closed and, instead of this, the inlet valve 38 of the cooling cycle is opened, so that cooled medium flows in from the cooling cycle and cooling of the mold thus starts.

The cooling medium, which is under the same pressure as the heating medium, ensures rapid dissipation of the heat and, according to the invention, as a result of the coupling of the two systems 30 and 31, the hot liquid continues to flow into the heating cycle, that is to say the return valve 35 of the heating cycle still remains open although the inlet valve of the cooling cycle has already been opened, until the thermostat 41 closes the valve 35 and, instead of this, opens the return valve 39 of the cooling cycle. In this way, the liquid, which is still hot and is expressed by the cooling medium from the cavities 6 and 7, is returned to the heating cycle 30 and the medium passes into the cooling cycle 31 only when the temperature of the issuing liquid falls below a certain value as a result of the cooling of the mold 1.When the mold 1 is opened and the mold release operation is effected, reversal of the inlet valves 34 and 38 again takes place and this time the return valve 39 of the cooling cycle still remains open for some time until, as a result of the increasing heating of the mold, the medium issuing from the cavities 6 and 7 has become so hot that the thermostat 41 again switches over and closes the valve 39 and, instead of this, opens the valve 35.

As a result of this type of heating, the energy loss is not very high, since only the peak values have to be heated up or cooled down again. For several machines it is possible to use either individual cycles or central cycles for several machines.

As can be seen from the figure, the feed for the material and for the hot inert gas can be effected via a component 42 in the region of the mold separation.

With the embodiment shown, the roar wall of the mold half 3 is preferably constructed as a movable plate 43, in order to enable compaction to be carried out after filling of the mold 1 with the pre-foamed plastic granules is complete. For this purpose, this plate 43, is movable from the outside by means of rods 44, which pass, with seals, both through the mold half 3 and through the mold frame 5. This movable plate 43, which forms the base of the mold half 3, can at the same time be used as a mechanical ejector.

The release of a finished molding from the mold is effected by producing a reduced pressure behind the molding in the mold half 3 via the suction line 8 (plate 43 is provided with bores for this purpose), so thatwhen the mold 1 is moved apart the molding is retained particularly firmly in the mold half 3 by this reduced pressure. At the same time, when the mold 1 is moved apart, compressed air is injected behind the molding into the mold half 2, by means of the compressed air line 46, which contains a valve 45.

For this purpose, a shut-off valve 47 to the suction line 8 is closed. After the release of the molding from the mold half 2 has been effected in this way, either compressed air is likewise blown in between the base of the mold half 3 and the molding (compressed air line 49 with valve 50) after closing a shut-off valve 48 to the suction line 8, or the plate 43 which serves for post-compaction of the molding after the# swelling operation has gone to completion is actuated as a mechanical ejector.

Claims

1. Procossfortho production of moldings from foamable thermoplastics, especially from foamable polystyrene foam, for example Styropor, Styrocell, (Registered Trade Mark) or the like, with which pre-foamed plastic granules are post-expanded and sintered together in a mold with the supply of heat, a vacuum being applied to the mold before this is filled with the pre-foamed plastic granules and the mold being pre-warmed from the outside, characterized in that a hot inert gas is passed into the mold during or after introduction of the pre-foamed plastic granules in order to warm the granules.

2. Process according to Claim 1, characterized in that the hot inert gas, at least in part, is passed into the mold at the same time as the mold is filled with the pre-foamed plastic granules.

3. Process according to Claim 1 or 2, characterized in that the pre-foamed plastic granules and the hot inert gas are fed to the mold at several points at the same time.

4. Process according to one or more of Claims 1 to 3, characterized in that the hot inert gas is introduced together with the pre-foamed plastic granules, the hot inert gas coming into contact with the pre-foamed plastic granules only on entry into the mold or even beforehand.

5. Process according to one or more of Claims 1 to 4, characterized in that a vacuum continues to be applied to the mold during the entire period over which the plastic granules are introduced.

6. Process according to one or more of Claims 1 to 5, characterized in that the hot inert gas is used not only for warming the pre-foamed plastic granules in the mold but is additionally used as a transport agent when introducing the pre-foamed plastic granules into the mold.

7. Process according to one or more of Claims 1 to 6, characterized in that the hot inert gas is supplied in measured amounts via a metering device.

8. Process according to one or more of Claims 1 to 7, characterized in that the pre-foamed plastic granules are introduced into the mold in premetered amounts.

9. Process according to one or more of Claims 1 to 8, characterized in that the mold is warmed and cooled from the outside via a sealed two-cycle heating/cooling system which is separate from the inside of the mold.

10. Process according to one of Claims 1 to 9, characterized in that after filling of the mold with pre-foamed plastic granules is complete, compaction of the moldings is effected by moving a movable mold wall.

11. Device for carrying out the process according to one or more of Claims 1 to 10, characterized in that the mold (1) is provided with slits (9), which are connected to a common suction line (8) and have dimensions which prevent passage of pre-foamed plastic granules, and with feed lines for the hot inert gases.

12. Device according to Claim 11, characterized by at least one metering device (16) which can be connected via a valve (21) to the suction line.

13. Device according to Claim 12, characterized in that the metering device (16) has a cylinder with a movable piston (17) and a piston rod (19), which passes through the piston, acts as a closing member for the outlet orifice (20) and can be actuated by a second cylinder.

14. Device according to one of Claims 11 to 13, characterized in that orifices which can be connected to a source of hot inert gas are provided in the region of the movable piston (17) of the metering device (16) and in particular in the piston (17) itself.

15. Device according to one of Claims 11 to 14, characterized in that the suction line (8) also serves, after corresponding rearrangement, if desired as a compressed gas line for releasing the finished molding from the mold.

16. Process according to claim 1 for the production of mouldings from foamable thermoplastics, substantially as hereinbefore described and exemplified.

17. Device according to claim 7, substantially as hereinbefore described and exemplified and with reference to the accompanying drawings.