GB2082496A

GB2082496A - Sheet forming

Info

Publication number: GB2082496A
Application number: GB8118773A
Authority: GB
Original assignee: Bellaplast Maschinenbau GmbH
Current assignee: Bellaplast Maschinenbau GmbH
Priority date: 1980-06-20
Filing date: 1981-06-18
Publication date: 1982-03-10
Also published as: FR2484903B1; GB2082496B; CH651246A5; DE3023086A1; FR2484903A1; SE8103799L

Abstract

A process for producing moulded articles from synthetic thermoplastic material, wherein the latter is plasticised by heating and compressing until it can flow in a highly plastic state, and is extruded in the form of a strip by extrusion through the nozzle, and wherein, for stabilising this strip, both its surfaces are brought into contact with thermally-conducting surfaces which are maintained at a predetermined temperature and both its surface zones are cooled and solidified until self-supporting, and, optionally after surface conditioning effected by heating the surface zone of the strip from within, the strip, thus stabilised, is subjected to thermal deformation to form the moulded articles, characterised in that during stabilisation at least one surface of the strip forced out through the extruder nozzle is passed over at least one roughened cooling surface, in the course of which it is brought into contact only with the peaks 22 formed by the roughness on these cooling surfaces. <IMAGE>

Description

SPECIFICATION A process and apparatus for producing thinwalled moulded articles from synthetic thermoplastic material The invention relates to a process and apparatus for producing moulded articles from synthetic thermoplastic material, wherein the latter is plasticised by heating and compression until it can flow in a highly plastic state, and is extruded in the form of a strip by extrusion through the nozzle, and wherein, for stabilising this strip, both its surfaces are brought into contact with thermally-conducting surfaces which are maintained at a predetermined temperature and both its surface zones are cooled and solidified until self-supporting, and, optionally after surface conditioning effected by heating the surface zone of the strip from within, the strip, thus stabilised, is subjected to thermal deformation to form the moulded articles.

The production of moulded articles from synthetic thermoplastic material by such a process is already known from DE-AS 22 52 219. In this process the extruded mate riai is heated particularly strongly and is highly plasticised, so that during extrusion it flows out through the nozzle as in a casting process. Such an extrusion process with a high material temperature and a high level of plasticising is only possible due to the fact that according to DE-AS 2252219 the extruded strip is picked up immediately after it leaves the wide-slit nozzle, for stabilisation on both its surfaces by smooth stabilising and cooling surfaces maintained at a predetermined temperature, and is thus cooled and solidified in contact with these smooth, thermally-conducting surfaces until it is self-supporting.The defoFmability by thermal deformation of the strip thus cooled and solidified in its surface zones is not affected detrimentally since, in fact, on the way to the thermal deformation process a restoration of or conditioning to a temperature suitable for thermal deformation is brought about in both the surface zones by thermal conduction from the interior of the strip which has remained at the extrusion temperature, if the strip is provided with an opportunity for this to occur between the stabilisation process and the thermal deformation process. However, the disadvantage has been encountered that when extruding relatively thin strips, and particularly when producing very thin-walled moulded articles, the wall of the moulded article shows the formation of bars over the whole of its surface.The invention is therefore based on the problem of providing a process and apparatus for stabilising the strip emerging from the extruder nozzle, wherein the strip to be stabilised, which is in a plastic state, is fixed mechanically on the cooling roller, transverse to its travel direction.

This problem is solved according to the invention in that at least one surface of the strip forced out through the extruder nozzle is passed over at least one roughened cooling surface, in the course of which it is brought into contact only with roughness peaks formed on this cooling surface.

As a starting point, the knowledge that the above-mentioned forming of bars can be traced back clearly to the smooth construction of the cooling surfaces used in the stabilisation of the extruded strip was used. Due to the smoothly polished nature of the cooling surfaces the plastic film emerging from the wide-walled nozzle cannot grip on this smooth surface in order to counteract the shrinkage stresses which arise transverse to the travel direction during cooling. These shrinkage stresses cause the formation of wrinkles inside the strip, leading to the above-mentioned formation of bars.A further disadvantage is occasioned by the smoothly polished construction of the surfaces of the cooling rollers in that they encourage the monomer vapours which arise to form bubbles between the extruded strip and the surface of the cooling roller and thus similarly contribute to impairing the quality of the wall of the moulded article.

It was found surprisingly that cooling rollers with a roughened surface provide a fixing for the extruded plastic film during stabilisation, due to the roughness peaks. This fixing transverse to the travel direction of the strip which is to be stabilised is achieved by the plastic film engaging appropriately with the roughened surface of a cooling roller and thus counteracting the shrinkage stresses during stabilisation. The superficial impression of the roughened roller on the strip which is to be stabilised is elminated in the thermal deformation device, due to the plastic core in the strip which is to be stabilised. A functional coaction is involved, namely, an apparently hydraulic effect by the plastic material of the core on the casing-type surface zones.This causes a very exact, particularly firm moulding into the surface of the die and thereby eliminates the impression of the roughness on the roughened roller. In an equally advantageous manner, the rough surface of the roller can accommodate the monomer vapours which form during processing so as to counteract the formation of bubbles during stabilisation. Due to the advantageous punctiform contact of the extruded strip, the cooling of the strip is also carried out less harshly, which again has a positive effect on its ability to be processed further.

In a particularly advantageous way, both the surfaces of the strip forced out through the extruder nozzle can be passed over at least one roughened cooling surface and brought into contact only with the roughness peaks formed on these cooling surfaces.

In a further advantageous step of the process, following on after the first stabilisation process on roughened cooling surfaces, a second stabilisation process can be carried out using smooth cooling surfaces. In a further advantageous way, when producing moulded articles from, for example, polystyrene, the extrusion of the strip is carried out at approximately 240 C, and the first stabilisation process is carried out at approximately 210 to 230 C.

The process according to the invention may be carried out with apparatus which is equipped with at least one extruder press which is suitable for taking thermoplastic material in granular form and compressing and heating it continuously until it is able to flow, an extruder nozzle adjoining the extruder press or extruder presses and constructed as a wide-slit nozzle and equipped with devices for controlling the temperature, an arrangement for stabilising the surface of the strip emerging from the extruder nozzle, adjoining the extruder nozzle, a thermal deformation device for forming moulded articles in the strip by deep drawing or stamping, and devices for cutting the moulded articles out of the strip.

Such apparatus is characterised according to the invention in that the stabilisation arrangement is equipped with a roughened roller or a plurality of roughened rollers disposed one behind the other, for the first stabilising process of the strip extruded through the extruder nozzle, on at least one of its surfaces.

In another advantageous embodiment, adjoining the roughened roller or rollers in the stabilising arrangement, at least one or a plurality of smoothly polished rollers may be provided for a second stabilising process.

In another possible embodiment, each of the roughened rollers disposed one behind the other in the travel direction of the strip has a lesser depth of roughness than the one before it, and the last roller, disposed at the outlet from the stabilising arrangement, corresponds substantially to a smoothly polished roller.

The roughened roller may, for example, have a depth of roughness lying between 30 and 70 ym, and preferably 50 pm. However, the roughness depth on the roller or rollers can be adapted in each case to the material which is to be processed and the material of the roller. It is particularly advantageous for the roughened cylindrical circumferential surface of the roller or rollers to be surface hardened. Due to the good thermal conductivity of aluminium it is advantageous for the cylindrical circumferential surface of the rollers to be made of aluminium. With rollers with aluminium surfaces, anodising is a possible form of such surface hardening, for example.

The roughening of the circumferential surface of the roller may be effected in many different ways: for example, mechanically, e.g. by sand blasting, and also in other ways such as, for instance, etching, erosion, metal spraying and the like. In the event that the roughness peaks produced by foe roughening process are too pronounced, they can easily be flattened, for example, by using a finegrain emery cloth.

If the rollers in the stabilising arrangements are connected to a cooling medium system which can be regulated, then the variations in the cooling effect occasioned by the surface roughness can be very easily accommodated by setting the roller temperature accordingly.

Another possibility for accommodating the variations in the cooling effect of the rollers occasioned by the surface roughness is afforded by controlling the rotary speed of the rollers in the stabilising arrangement appropriately, if the strip feed and with it the stabilising arrangement as well as connected to a drive which can be regulated, preferably smoothly.

Embodiment examples of the invention are described in more detail in the following, with reference to the Drawings.

Figure 1 is a diagram showing the sequence of the process for producing thinwalled moulded articles from synthetic thermoplastic material.

Figure 2 is a stabilising arrangement for the extruded strip of synthetic material.

Figure 3 shows another embodiment of a stabilising arrangement.

Figure 4 is a greatly enlarged partial section through a roughened roller in the stabilising arrangement.

The process system shown in Fig. 1, for producing moulded articles from synthetic thermoplastic material, comprises at least one extruder press 15 which delivers the synthetic thermoplastic material to an extruder nozzle 12 which is in the form of a widvslit nozzle with devices for regulating the temperature.

Immediately after leaving the nozzle the strip 13 emerging from the extruder nozzle 12 is supplied to a stabilising arrangement 10 which will be described in detail later. Adjoining the stabilising arrangement 10 there is a device 16 for converting the continuous ad- ,' vance feed of the strip to stepwise motion.

The strip 13 of synthetic material which is now moved on stepwise is deformed in the thermal deformation device by deepdrawing, for example. The moulded articles thus formed in the strip 13 of synthetic material are cut out of the strip 13 of material in a stamping device 19. The rest of the strip is conducted into a collecting station 20 which may comprise a grinding device. The granular material yielded by the grinding device can then be supplied via a dosing device 21 back to the extruder press 15.

Fig. 2 shows an-'embodirneht'of the stabilis- ing arrangement 10 which '#flises'#' the sur face of the strip emerging from the extruder nozzle 12, by cooling it. For this, the strip 13 which is to be stabilised passes through an arrangement of cooling rollers 11, 14, the cooling roller 11 being roughened on its cylindrical circumferential surface and the cooling roller 14 being smoothly polished on its cylindrical circumferential surface. The roller 11 with a roughened surface fixes the strip 13 emerging from the extruder nozzle 12 and still in a highly plastic state, via its flattened roughness peaks 22, as can be seen more clearly in Fig. 4.These roughness peaks 22 catch in the plasticised strip 13 in such a way that it is held in its transverse direction so as to counteract any shrinkage stresses which may arise, and which would cause a corresponding formation of wrinkles. The roughened roller 11 which is made, for example, of aluminium may have a roughness depth lying between 30 and 70 #m. The cylindrical circumferential surface of the roller 11 which is made, for example, of aluminium, is roughened by means of sand-blasting, and after the sand-blasting process the roughness peaks created thereby are flattened slightly by mechanical means, e.g. with a fine-grain emery cloth, so that it can subsequently be anodised.

As can be seen in Fig. 2, a smoothly polished roller 14 is arranged so that it can be displaced in a guideway 23 through approximately 90 around the centre-point of the roughened cooling roller 11 which lies opposite it. It will be appreciated from the arrangement of the roller 14' shown in dashes and the path shown in dashes of the strip 13 that the distance over which the strip 13 passes round the cooling rollers 11 and 14 and thus the length of the contact surface between the cooling rollers 11 and 14 and the strip 13 is varied by displacing the smoothly polished roller 14'. A smoothly polished adjoining roller 14 like this should be provided if the roughened roller 11 has a considerable depth of roughness. This adjoining, smoothly polished roller 14 substantially eliminates the impression of the rough surface on the roller 11.Adjoining the stabilising arrangement 10 there is the device 16 which converts the continuous smooth movement of the strip 13 into a stepwise motion.

Fig. 3 shows a modified embodiment of the stabilising arrangement 10. Again, a strip 13 emerging from the extruder nozzle 12 is passed over a plurality of roughened rollers 11 disposed one after the other, in order to effect stabilisation, the depth of the roughness on these rollers being varied stepwise so that the last roller arranged at the outlet from the stabilising arrangement 10 has the depth of roughness of a smoothly polished roller 14.

The rollers 11 shown in Fig. 3, which can be displaced as shown by the double arrows 24 and 25 between the position 11' shown in dashes and the position drawn fully, are used to vary the angle which the strip 1 3 passes through round the rollers 11 and 14 respectively. In this way the length of the contact surfaces for the strip 13 is adjusted so that it is always possible to effect stabilisation according to the thickness of the strip 13. In another embodiment which is not shown, the stabilising arrangement 10 consists only of uniformly roughened rollers 11 which are connected to a cooling medium system which can be regulated, as are the embodiments shown in Figs. 2 and 3. The rollers 11 and 14 in this case are cooled according to the degree of stabilisation required. Due to the punctiform contact between the strip 13 of synthetic material and the surface of the roughened roller 11, the cooling of the surface of the strip 13 is prevented from being too harsh. This gentler cooling of the strip 13 enables high shrinkage stresses due to sudden cooling to be avoided.

Claims

1. A process for producing moulded articles from synthetic thermoplastic material, wherein the latter is plasticised by heating and compression until it can flow in a highly plastic state, and is extruded in the form of a strip by extrusion through the nozzle, and wherein, for stabilising this strip, both its surfaces are brought into contact with thermally-conducting surfaces which are maintained at a predetermined temperature and both its surface zones are cooled and solidified until self-supporting, and, optionally after surface conditioning effected by heating the surface zone of the strip from within, the strip, thus stabilised, is subjected to thermal deformation to form the moulded articles, characterised in that during stabilisation at least one surface of the strip forced out through the extruder nozzle is passed over at least one roughened cooling surface, in the course of which it is brought into contact only with the peaks formed by the roughness on these cooling surfaces.

2. A process according to Claim 1, characterised in that both the surfaces of the strip forced out through the extruder nozzle are passed over at least one roughened cooling surface, in the course of which they are brought into contact only with the roughness peaks formed on these cooling surfaces.

3. A process according to Claim 1, characterised in that following on after the first stabilisation process on a roughened cooling surface, a second stabilisation process is carried out using smooth cooling surfaces.

4. Apparatus for carrying out the process according to one of Claim 1 to 3, with at least one extruder press which is suitable for taking thermoplastic material in granular form and compressing and heating it continuously until it can flow, an extruder nozzle adjoining the extruder press or presses which is made as a wide slit nozzle and is equipped with devices to control the temperature, an arrangement for stabilising the surface of the strip emerging from the extruder nozzle, adjoining the extruder nozzle, a thermal deformation device for forming moulded articles in the strip by deep-drawing or stamping, and devices for cutting the moulded articles out of the strip, characterised in that the stabilising arrangement (10) is equipped with a roughened roller (11) or a plurality of roughened rollers disposed one after the other, for the first stabilising process of the strip (13) extruded through the extruder nozzle (12), on at least one of its surfaces.

5. Apparatus according to Claim 4, characterised in that adjoining the roughened roller (11) or rollers in the stabilising arrangement (10) there is at least one smoothly polished roller or a plurality of smoothly polished rollers (14), for the second stabilisation process.

6. Apparatus according to Claim 4, characterised in that each of the roughened rollers (11) disposed one after the other in the travel direction of the strip (13) has a lesser depth of roughness than the roller before it, and the last roller (14) disposed at the outlet from the stabilisation arrangement (10) corresponds substantially to a smooth, polished roller (14).

7. Apparatus according to one of Claims 4 to 6, characterised in that the rough cylindrical circumferential surface of the rollers (11, 14) is surface hardened.