Method and apparatus for the continuous production of thermoplastic sheets

The invention relates to a process for the continuous production of thermoplastic plastic webs according to the preamble of claim 1 and an apparatus for performing this method according to the preamble of patent claim 24. Such a method and a zugehö membered device are known from DE-A-29922152nd

Semi-finished products made of thermoplastic material, such as sheets, films, tapes, etc., are produced mainly by extrusion. In the extruder die, the macromolecules of the polymer melt direct account of the action of external forces in a preferred direction corresponding to the direction of flow. This orientation will freeze at the rapid cooling after leaving the die, leading to internal stresses of the semifinished product. Furthermore lead such orientations, reinforced by vortices that arise from cross-sectional constrictions in the nozzle, often to melt fracture, wherein the plastic melt then a stick-slip phenomenon emerges from the nozzle, resulting in a rough surface of the semifinished product. Reworking the surface of the extrudate after leaving the die of the extruder is therefore required.

Known production lines for extruded sheets or films made of thermoplastic materials consist of an extruder with nozzle, a subsequent smoothing and calibrating unit, a cooling device, a deduction for the plastic sheet and means for cutting the plastic sheet, stacking the plates, etc. The result of the rough surface of the extrudate required reworking takes place in the calibrating unit. Such a calibrating unit is usually one of several rolls of existing with highly polished surfaces calender. The calender causes cooling of the extrudate while substantially eliminating variations in thickness and a smoothing of the surface of the plastic web. In this case, the calender must be set so that only a smoothing of the surface, but not a deformation of the entire web takes place, since then additional non-symmetrical internal stress states may occur, which results in warping of the web by itself.

A disadvantage of the use of the calender for the smoothing and calibration of the extrudate, therefore, is that so that although a certain smoothing of the surface can be achieved, however, it is not possible to remove the already present in the web by extruding residual stresses. There is even a danger, yet incorporate other additional stresses. In the area of ​​the rollers of the calender takes place a bend of the plastic sheet around these rolls, so that the web must be in the thermoelastic state, thus also the temperature of the rolls must be kept relatively high, so that only a limited cooling takes place in the region of the rollers. Therefore, this cooling takes place mainly during the plastic sheet, the cooling rolls of the calender wraps, is so bent, thus being dimensionally erred for the aspired level, low-stress plastic sheet. The subsequent further cooling in the subsequent cooling unit is then depressurized. But also during the passage of the plastic web through the calender affects only one brief line pressure on the cooling path so that only briefly applied pressure in total throughout the cooling phase of the plastic sheet, and thus the so far achieved properties of the surface of the plastic sheet still far from ideally aspired condition.

From EP-AO 046 526 a method and an apparatus for the continuous production of webs of thermoplastic materials is also known. In this method, the thermoplastic particles are scattered on the early lower belt of a twin piston oscillating press to a raw material layer. This layer of raw material is preheated to the sub-band and then subjected to between the two endless belts of the double piston oscillating press a continuous surface pressure. The printing plates of the double-piston swing press are divided into the inlet region assigned heating zone and the lead-out area allocated cooling zone.

In the double-piston swing press lies on the upper band, a fixed plate on while positioned a vibrating plate on the lower band. Thereby, the printing plates act directly on the press bands to exercise the pressing pressure, so that between the fixed platen and the moving pressure belt, a frictional force occurs. Thus, this device can only with low pressure, operate, otherwise a speedy destruction of the surfaces of the press bands occurring due to the frictional forces. In order to achieve a smooth surface and substantial reduction of internal stresses of the plastic sheet, however, higher pressures are often required, which are not applied with this little operationally reliable device.

Another disadvantage is that fluctuating and undefined contact surfaces are formed by the direct concern of the printing plates on the press bands and by the vibrating plate. This in turn results in a varying, non-uniform surface pressure and to undefined temperature profiles during the cooling of the plastic web, so that, again, no substantial reduction in the residual stresses can be achieved. On the contrary, you get low quality plastic sheets.

In DE-A-2,922,152 a further method and a device for the production of films and sheets of sinterable powders is shown. The device consists of a double band press at a minimum weight sub-band, over which a spreading device for the sinterable powder is arranged. In the double belt press are printing plates which can bear against the press belt directly or where the pressure by means of rotating roller conveyors is transmitted to the press belts, whereby the powder is pressed into a plastic sheet between the press belts. The inlet to the associated printing plates are constructed as heating plates and the outlet associated printing plates as cooling plates.

Here, too, is disadvantageous in that it is in the double belt press shown with applied to the press belts printing plates is a non-reliable, suitable only for low pressure device. Are arranged rotating roller strips between the plates and the press belts, so a lot of effort to control these roller conveyors is necessary so that a complicated, error-prone device results. Because of an undefined and varying contact surfaces to obtain a non-uniform surface pressure and fluctuating, irregular temperature profiles, so that this method and apparatus for the manufacture of qualitatively improved plastic webs is not suitable.

Starting from this prior art the invention has the object of providing a method and apparatus for the continuous production of extruded plastic webs of thermoplastic material, in particular for the further processing into plates or foils, to specify with which both an improved quality of the surface of the plastic web as well as reduced residual stresses thereof can be reached.

This object is mediated by the technical teaching described in the characterizing part of claim 1 and serving for carrying out this process is specified in the characterizing part of patent claim 24th tf--.

The advantages attainable with the invention consist in particular in that the pressure evenly areally without fluctuations throughout the AbkühIdauer on the extruded plastic sheet in the flat position in which also the finished product is intended to apply and acts. This excellent surface of the plastic sheet up results not yet unheard goodness. Since the web is not subjected to bending stress during cooling, but perfectly flat remains, no further internal stresses in the sheet may occur. Rather, it has been shown that the inevitable resulting from extruding residual stresses can be greatly reduced .with the help of the inventive method. The possible with the method and apparatus of the constant maintenance of certain temperature profiles allows to adjust the cooling rate of the plastic sheet with the requirements of the microstructure of the respective thermoplastics. Therefore, the method according to the invention brings about a decisive improvement in the properties of extruded plastic webs of thermoplastic material.

A preferred embodiment of the invention will be described in more detail below. Show it<ul><li>FIG. 1 is a general view of a production plant for the continuous production of a plate-like or sheet-like extrudate</li><li>Fig. 2 shows a section through a double belt press with an upstream extruder,</li><li>FIG. 3 is a printing plate in plan view,</li><li>Fig. 4 is a section, by the pressure plate along the line AA in Fig. 3</li><li>Fig. 5 shows an apparatus for the continuous coating of substrate webs,</li><li>Fig. 6 shows an enlarged detail of the inlet zone in a device for two-sided coating of carrier webs,</li><li>Fig. 7 shows the inlet zone of a double belt press,</li><li>Fig. 8 shows a device for welding a plurality of extruded thermoplastic webs,</li><li>FIG. 9, the inlet zone of a double belt press with heated and Umlenktrommein</li><li>Fig. 10 the inlet zone a double belt press with projecting lower band unit.</li></ul>

In Fig. 1, a production plant for continuous production of thermoplastic sheets and films is shown schematically. It consists of a double belt press 1, an extruder 2 is in front of things, enema. The emerging from the nozzle 3 of the extruder 2 web-shaped plastic melt 4 is introduced according to the inventive method in the double belt press 1 and cooled down in this by the action of surface pressure, that the plastic melt 4 after leaving the double band press 1 to form a solid plastic sheet 5 having a smooth and polished or embossed surface is formed. This plastic sheet 5 passes at a uniform rate to a cross cutting unit 6, where it is divided into plates 7 of any size that are stacked in pallets for transportation. 8 Alternatively, the plastic web 5, if it is a thin, flexible plastic sheet or a film, to be wound onto a roll 12 in a winder unit 11 after leaving the double band press. 1 If the roller 12 is filled, it can be further 5 wound onto a spare roll 13 without interrupting production the plastic web. The entire plant is controlled by a computer in a control cabinet. 9 The data input by the user via the terminal 14th

The Fig. 2 shows the extruder 2 with a downstream double belt press 1 schematically in section. The known as trained extruder 2 has an elongated cylinder 19, in which one of a motor 16 via a reduction 17 driven screw 18 is located. The screw 18 takes in its rear part, in its helical grooves 20, the granules 14 to be processed for the thermoplastic material on which is filled in a hopper 15 of the extruder second These granules are transported with the screw 18 and melted during this transport. Given more heating jackets 21 are around the cylinder 19 placed. At the front end of the screw 18, the plastic melt is homogenized and densified, and exits through the nozzle 23 of the flanged on the front part of the cylinder 19 extruder tool 22nd The nozzle 23 of the extruder tool 22 are the outgoing and warm plastic melt 4, the cross-sectional shape. The nozzle 23 is formed as a slot die, so as to obtain a flat and sheet-shaped plastic melt 4 of substantially rectangular cross section.

The molten plastic material 4 exits as to processing temperature located viscoelastic web from the nozzle 23 of the extruder 2, is taken up by the belts of the double belt press 1 and introduced into the inlet zone 61 of the double belt press the first The continuously operating double belt press 1 is also shown in FIG. 2 in section. It owns four mounted in a press frame Umlenktrommein 24,25, 26, 27. The press frame is omitted from the drawing for clarity. To two of lying on a horizontal plane deflection drums that rotate in the Umlenktrommein 24 and 25 in accordance with arrows, an endless, made of metal press belt 28, 29 is guided, which is tensioned by means of hydraulic cylinders 30th Between the two press belts 28, 29, in the reaction zone 62, which consists of the plastic melt 4 in the drawing, from right to left leading web cooled surface pressure.

The pressure exerted on the plastic melt surface pressure is 4, 29 deposited on printing plates 31, 32 hydraulically or mechanically to the inner sides of the press belts 28 and transferred from there to the plastic melt. 4 For hydraulic pressure transmitting a pressurized settable, fluid pressure medium in the space 33, which is up and down from the pressure plate 31 and the press belt inside 28 and limited to the sides of the seal 34, introduced, so that the space 33 as a pressure pad acts. As fluid pressure medium oil or air is used for example. The structure of the along the edge of the printing plate 31 rectangular closed seal 34 can be seen in Fig. 4. It consists of a U-shaped retaining bar 35 of metal, in which the elastic sealing member 36 is pinched. The U-shaped retaining strip 35 is located in a groove 38 along the edge of the printing plate 31 on the retaining strip 35 is the groove base to an O-ring 37, on the groove base is applied a printing medium, so that the sealing body 36 against the inside of the press belt 28 is pressed and the space 33 to seal the sides.

To the plastic melt 4 to cool to the desired final temperature on leaving the double belt press 1, the pressure plates 31, 32 are maintained at a temperature which is much lower than the final temperature to be reached of the plastic melt. 4 For this purpose, channels 40 31.32 4 are mounted in the pressure plates, as also shown in FIG. It can be seen. These channels 40 are traversed by a cooling liquid which absorbs heat and discharges and thus 31 keeps the target temperature of the printing plate at the desired low level. Of course, the cooling of the printing plate can also electrically, for example by means of Peltier elements.

In the printing plate 31 are located transversely to the direction of the press belt 28 mounted over the width of the printing plate reaching grooves 41, as shown in FIG. 3 and 4 can be seen. In these grooves 41 rod-shaped heat-conducting elements 39 are used. These thermally conductive elements 39 are made of copper and are connected at one end to the pressure plate 31 to form a good Wärmeleitkontaktes and touch the other hand, the inside of the press belt 28 by grinding. The groove bottom of the groove 41 for lies on the heat-conducting member 39 on an elastomeric seal 42 which is acted upon by the groove base with a pressure medium, so that the heat-conducting element 39 always has contact with the press belt 28th

Since there is a thermal gradient between the pressure plate 31 and the plastic melt 4, sets a heat flow from the molten plastic material 4 via the press belt 28 and the heat conducting members 39 in the pressure plate 31, so that the plastic melt during the passage through the double belt press is continuously cooled. The rate of cooling of the molten plastic material 4 depends on the temperature level of the printing plate 31 and the number of heat conducting members 39, as well as their thermal conductivities. The achievable end temperature of the plastic melt 4 can be further than the speed of the press belts 28, rules 29 in the double belt press. Of course, the pressure plate 32 may be also formed as the pressure plate 31 and heat-conducting elements contain 39, in which case the cooling speed is doubled. Other materials and training forms for such heat-conducting elements are described in the published patent application DE-OS 33 25 578, so that there is no longer need to be discussed further.

Instead of the hydraulic pressure transmission in the laminator 1 can also be a mechanical pressure transfer can be provided, whose training will be illustrated in FIG. 2 by the pressure plate 32. Between the pressure plate 32 and the press belt 29 inside fixed, consisting of metal rollers 43 are mounted. With the aid of hydraulic cylinders 44, the pressure plate 32 and thus the rollers 43 is positioned against the inside of the press belt 29 so that thereby the desired pressure is exerted on the plastic web.

The pressure plate 32 is also, as already described above, cooled. This is a heat flow of the plastic melt 4 on the press belt 29 and the rollers 43 in the pressure plate 32 and the plastic melt 4 is cooled to the desired temperature. If not sufficient, the size of the heat flux over the rollers 43, as can additional heat conductive elements 39 which are formed as described above, are mounted between the rollers. If desired, in an inventive device can also be combined with the mechanical transmission of the hydraulic pressure with the use of heat conductive members.

The plastic melt 4 is placed so after leaving the extruder 2 to a press belt 29 or 28 of the laminator in the field of front idlers 24, 25 and conveyed by the press belt in the double belt press, or right in the middle of the gap between the two idler pulleys 24,25 in inlet zone 61 is extruded, so that the plastic melt 4 of the two press belts 28, 29 is detected simultaneously. While the resin melt 4 is continuously drawn through the double belt press 1, so that the double band press 1 also acts as a take-off device for the extrudate, it is cooled under the simultaneous action of surface pressure. The applied surface pressure acts as a calibration which imparts the thermoplastic resin sheet has a certain thickness and a certain surface structure. By uniformly and areally acting during cooling pressure is obtained, as has been found, advantageously, a correction of existing orientations of the plastic molecules and the related, originating from the extrusion residual stresses, so that the process of this invention extruded plastic sheets or films of yet not unknown quality can be produced.

The surface structure of the double belt press 1 leaving the plastic sheet 5 hangs within wide limits on the surfaces of the pressing .r bands 28, 29th Possess the press belts have a smooth surface, so the) surface derme plastic sheet becomes smoother, more polished look, - that is, the plastic sheet 5 is in the double belt: 'press 1 burnished. This effect can be enhanced by providing the surface of the pressing belts with a high-gloss, electrodeposited chromium layer. Possess the press belts a structured surface, for example knobs, corrugations, ornaments, random structures, etc., so the plastic sheet 5 is replaced by an embossed into the surface reflection of this pattern. It is also possible to structure the carrier web, which consists for example of a metal foil with a textured surface to leave so converge in front of the laminator with the plastic melt, the non-patterned surface of the structure support web rests on a press belt of the laminator and the structured surface of the structure support web to the plastic melt, in which case the structuring of the carrier structure impresses under the surface pressure in the double-belt press in the plastic web. On leaving the double band press in the guide drums 26 and 27 of the structure support is then separated from the plastic sheet. 5

Some thermoplastics may require cooling in several stages. For this purpose, the pressure plates 31, 32 of the double belt press 1 are divided into a plurality of consecutive areas that are cooled to the required different setpoint temperatures. Each of these areas is provided with heat-conducting elements 39, so that the reaction zone 62 has decreasing temperatures. The plastic melt is then cooled 4 successively to the various temperatures during passage through the reaction zone 62 in the double belt press the first If required by the plastic melt, a region can also between any two areas in the printing plate, which are provided with heat-conducting elements may be provided with no heat-conducting elements. In the reaction zone 62 is followed by a cooling section in each case a distance, where the intermediate temperature is maintained, so that takes place a step-like cooling of the plastic melt. With this configuration, the cooling rate can be optimally adapted to the due to the microstructure of the polymer melt needs. However, it should be emphasized as essential to the invention that in the reaction zone by cooling under surface pressure on any final temperature.

It is also within the scope of the invention 24 and 25 heat the inlet side Umlenktrommein an expedient temperature. For example, such a temperature between the final temperature of the plastic sheet 5 lying on leaving the double belt press 1 and the temperature of the plastic melt leaving the extruder at 4 second 25 As shown in FIG. 9, have the guide drums 24, in the cylinder casing 64 across the width of Umlenktrommein 24, 25 reaching channels 63. These channels 63 are traversed by a thermal oil that gives off heat to the tail pulleys from which the press belts 28 , 29 are also heated by heat conduction. The press belts 24 and 25 also have then this elevated temperature and the resin melt 4 then remains in the inlet zone 61 of the double belt press 1 in the viscoelastic state. This can advantageously be the residual stresses of the plastic sheet 5 further reduce and increase their dimensional accuracy. In the field of printing plates 31, 32, the plastic sheet is then cooled along with the press belts 28, 29th It may also be advisable to divide the printing plates in a heating and cooling area, said the inlet side guide drums 24, 25 facing the area is heated to a purposeful temperature and thus the plastic melt 4 further heated on the wärmelei border elements. 39 This can replace the heating of the inlet-side idler pulleys or support the same happened. This is particularly important in the following exemplary embodiment where a plurality of extruded thermoplastic webs are welded together. Of the outlet side Umlenktrommein 26, 27 facing the area of ​​the reaction zone 62 is then again, as explained above, designed as a cooling zone.

When extruding the plastic melt 4, as shown in Fig. 7, to operate with little excess so that forms in front of the idler pulleys 24 and 25, a bead 53 of pent plastic melt at the inlet of the double belt press. This will advantageously prevents air from entering the reaction zone 62 with the melt and creates an erroneous plastic sheet with pores. Advantageously, the extrusion of the quantity of the plastic melt 4 in dependence on the size of the bead can be controlled 53rd This uniform conditions are achieved during the entire processing time. In order to be able to already expel dissolved air by extruding the plastic melt to the inlet side guide drums 24, 25 to one another to be adjustable in the direction of arrow 54, causing an additional calendering of plastic melt 4 and leakage of air against the flow direction of the plastic melt. The calendering of the plastic melt 4 in the inlet zone 61 by controlling the roll gap can be important in order to compensate a volume contraction of the cooling melt and so to ensure the dimensional stability of the plastic sheet. 5 The formation of the inlet side Umlenktrommein a double belt press for calendering is known per se and can be carried out according to the published patent application DE-OS 34 02 751st

Another operating according to the inventive method, apparatus used for the production of plastic-coated carrier webs is shown in Fig. 5. This apparatus has an extruder 2 with a slot die 23 and a double belt press the first front of the double belt press 1 is 25, a roller 46 mounted near the deflection drum, which is located at a known, in the drawing unwinding unit not shown. From the roll 46 is a carrier web 47, which may consist for example of a paper, metal or fabric, unwound and brought together in the inlet zone 61 of the double belt press 1 with the plastic melt from the extruder 4 second It is also possible that the carrier sheet 47 consists of a scattered fibrous web. If necessary, the carrier web 47 between the roller 46 and the inlet into the double belt press by means of an infrared lamp heater or hot air may be preheated. During the passage of plastic melt 4 on the carrier web 47 through the reaction zone 62, the plastic melt is cooled under areal pressure 4 and goes to the carrier web 47, a composite one. After leaving the double belt press 1, the coated backing web 48 may be on a roll in a winding unit, as shown in Fig. 1, is wound.

The production of a two-sided plastic coated carrier web is shown in Fig. 6 where the inlet zone 61 of the double belt press 1 is shown enlarged. In the vicinity of the upper inlet drum 24, an upper extruder 49 is attached, the 50 extrudes a molten plastic material on the press belt 28th This hot plastic melt 50 is adhered to the press belt 28 and is transported by it in the double belt press. 1 A lower extruder 45 is mounted in the vicinity of the lower inlet drum 25, and extruded a molten plastic material 51 on the press belt 29 so that this plastic melt 51 is likewise transported into the double belt press the first A drawn from a roller support web 52 runs between the two plastic masses 50 and 51 at the double belt press 1 and is 51 so merged into the inlet zone 61 with the plastic melt 50 that 52 is a plastic melt located on any surface of the carrier web. In the reaction zone 62 of the double belt press 1, the double-coated carrier web is then cooled down under areal pressure.

The apparatus shown in Fig. 2 is also suitable for incorporation of fillers in the plastics sheet 5, such as glass or metal fibers. For this purpose, mounted between the extruder die 22 and the inlet-side return pulley 24, a dispersing device for the fibers. This scatterer scatters the corresponding fibers in the plastic melt 4, the then applicable sink into the melt that they are surrounded by the thermoplastic plastic composition. The plastic melt 4 with the embedded fibers is then, as described above, cooled in the double belt press 1 to a filler-reinforced plastic sheet. 5

A further apparatus for carrying out the inventive method is shown in Fig. 8. This device is used for welding of individual thermoplastic plastic sheets or films into a compact plastic sheet. The individual plastic sheets may be made of conventional extruded and calendered thermoplastic films which were stored until further processing on rollers. Before the double band press 1 unwinding 55, 56 are erected, in which rollers 57 and 58 are located on thermoplastic films. These individual thermoplastic sheets are drawn off the rolls 57, 58, joined in front of the double belt press 1 to form a layered web 59, and introduced into the double belt press the first For each roll 57, 58 is a spare roll 57a, 58a are provided so that at the end of a roll, the production can be maintained by threading the respective spare part without interruption. If necessary, before the double belt press 1 a Infrarotstrahlerhei tion for pre-heating of the laminated web 59 or the individual thermoplastic films be appropriate.

To weld the individual thermoplastic sheets together, they must first be heated in the inlet zone applicable until they are in the viscoelastic state. For this, the inlet-side deflection drums of the double belt press 1 as described above with reference to Fig. 9, heated, so that the individual films are heated when in contact with the press belts 28, 29. If this is not sufficient, so the inlet side portion of the reaction zone can also be heated by means of heat-conducting elements of the printing plate forth, as described in detail above. Once the films have the viscoelastic state corresponding temperature, welding each slide under the surface pressure in the reaction zone with each other to form a compact web.

During the further passage through the reaction zone of the double band press 1 this layered, welded sheet 59 is cooled in accordance with the inventive method in the reaction zone under surface pressure. For this, the pressure plate in the double belt press is in turn provided with heat conducting elements shown in FIGS. 3 and 4 and at least of the outlet side Umlenktrommein area facing the pressure plate is cooled. If necessary, the pressure plate again be divided into separate areas with different temperatures or allow a stepped cooling. At the outlet of the double band press 1 to obtain a cooled, compact plastic web 60 of thermoplastic material, which may be as shown in Fig. 1, wound or divided into individual plates.

Also possible is a calendering the laminated sheet 59 by means of the inlet-side deflection drums, desired according to the FIG. 7 .. If can with each Therme-_': Plast sheets to reinforce a tissue sheet of glass cloth, textile fabric, etc. run in order to lead to a reinforced thermoplastic sheet 60th Although the individual thermoplastic films or sheets on the: are extruded and calendered conventionally rollers 57, 58, it should be emphasized that the compact plastics web 60 in which the films are welded to one another, the inventive benefits, such as reduced residual stress, a press polished surface, good dimensional stability, etc. has.

In Fig. 10 an embodiment of the inventive method is shown to be processed in which extruded thermoplastic flakes to a plastic web. The lower band unit 65 of the double belt press 1 is slightly forward, so that the lower press belt 66 slightly projects out of the inlet zone 61st Shortly before the inlet zone 61, over the press belt 66, a scattering device 67 is mounted, the extruded thermoplastic flakes 68 interspersed on the lower press belt 66 to form a nonwoven. This fleece is transported by the press belt 66 in the inlet zone 61, where they are compressed and heated to the extent as already explained above, that it forms a cohesive viscoelastic plastic web 69th Subsequently during the further passage through the double belt press 1, the plastic sheet 69 is then cooled down under areal pressure. It is here again calendering the scattered web by means of the inlet-side deflection drum 24 possible. If desired, a nonwoven fabric can run into the double belt press 1, and the extruded thermoplastic flakes or powder can be sprinkled into this nonwoven fabric as described before the inlet zone 61st The already mentioned advantages according to the invention for the cooled plastic sheet are also available in this embodiment again.

In the above description, various embodiments of devices for carrying out the inventive method are described. It is important in all of these embodiments, that the thermoplastic material is cooled under pressure surface, to which the inventive apparatuses are equipped with a double band press with heat-conducting elements in the plate.