Device For Drawing Thermoplastic Sheet Material

Abstract

Thermoplastic sheets are monoaxially and/or biaxially stretched by rotating disks between a pair of feed rollers and a pair of draw-off rollers. The circumferential surfaces of the rotating disks are touched by the sheeting edges pressed onto them and maintained in firm engagement by atmospheric pressure. In the case of transverse stretching the rotating disks are arranged in a slanting position with regard to the direction of movement of the sheeting.

Description

The present invention relates to a device for the mono- and biaxial stretching of sheets made from thermoplastic materials, the gist of the invention being the stretching across the width.

In principle, there are two methods that are applied in industry for the stretching across the width of sheets. One method consists in applying the so-called tube stretching process, according to which a sheet extruded in the form of a tube is blown to a larger diameter, which effects a stretching of the sheet across the width. This tube stretching process has the disadvantage, however, that during the flattening of the tube creases may be formed. In order to carry out the tube stretching process, complicated devices as well as a considerable height of building are also required.

The second method is to the so-called flat stretching process. In this process a sheet, which has been prepared in a flat form, for example, by means of a calender, extruder and slot die, is seized at the edges by means of fast gripping elements, for example vise clamps, which run on diverging paths in a transverse stretching frame. These processes require a complicated mechanism and a great overall length of the transverse stretching frame. Since in most cases, in addition to the transverse stretching, a longitudinal stretching is effected, which is carried out in two steps according to present processes, the overall length is again increased. Another disadvantage found in the working with vise-clamps is the fact that extremely exact guides are required. In the course of this process the sheet is subject to breaking; furthermore, a loss of the edges seized by the vise clamps has to be taken into account.

German Pat. No. 851,962 describes a transverse stretching device which also operates by a fast gripping of the sheet edges. In this case the flat sheet is passed over a hot elliptic cylinder surface having slanting, diverging circular sectional areas and is held at its edges by means of two rotating disks, the circumference of which corresponds to that of the circular sectional surfaces, and the planes of which are parallel to these surfaces. The holding means in the form of these disks may be in the form of sprocket wheels, for example, in which case the sheet edges are clamped between the sprockets of the sprocket wheel and the chain. Another variant proposes providing the disks with grooves, in which case the edges are held in the grooves by means of steel wires. This device has the considerable disadvantage that, in particular with a high transverse stretching ratio, the sheeting cannot be held safely, or the seized edges may break, which results in an irregular transverse stetching. The device mentioned above does not ensure a safe working, in particular with respect to the high sheet speed that is required nowadays.

It was necessary, therefore, to develop a technically simple stretching process not showing the disadvantages inherent in the above-mentioned processes and devices.

The present invention provides a process in which rotating disks which are preferably in a slanting position with respect to the direction of movement of the sheet are used as holding means for the sheeting edges, according to which process the sheeting edges are pressed onto the circumferential surface of the disks where the edges touch these surfaces, in a manner as to avoid the formation of air pockets between the sheet and the disk, and in which process the disks are heated to such a temperature that the sheeting edges striking the disks adapt themselves to the surface of the circumferential surfaces of the disks, by which method the sheeting edges are held by the disks during the transverse stretching in a force-locking manner, and in which process the sheet is heated to stretching temperature in known manner and is drawn off in the direction of movement of the sheet.

It is surprising and could not have been foreseen that by the elimination of the air layer normally formed between the sheet and the support where the sheeting touches the disk surfaces, in combination with the adjusting of the sheeting edges to the disk surfaces caused by the heating, the sheeting edges could be seized and held so well during the transverse stretching, the pressure required being provided only by normal air pressure. Since the present invention dispenses with complicated mechanical devices for holding the sheet, a completely trouble-free operation is ensured, in spite of the simple technique of the process of the invention.

According to the process of the invention the total surface of the sheeting edges is pressed steadily and uniformly onto the surfaces of the disks by means of the air pressure. The necessary grip force is thus introduced in an optimum manner into the sheeting edges, and a force-locking connection between the sheeting edges and the disks is obtained. In the case of all other mechanical holding devices known so far, the gripping elements (for example vise clamps, steel wires or steel chains) penetrate to a certain extent into the sheeting edges, thus forming a fast-gripping connection. The required grip force is only led over small proportions of the surface with a correspondingly high pressure into the sheeting edge, which results in the breaking of the sheet, if the pressure is too high.

The process may advantageously be applied to all thermoplastic materials, for example, polyvinyl chloride, polyolefins, or polyesters. It has become evident in practice that the process permits working at higher temperatures than had been possible with processes known so far. This fact often represents an advantage, because of the particular mechanical properties of the sheet to be obtained in this way, for example, with regard to shrinkage. The transverse stretching can be effected particularly well, for example, with (rigid) polyvinyl chloride sheets in the plastic range. A transverse stretching of flat sheets within the temperature range specified above has not been possible so far, as the damaging or deformation of the sheeting edge cannot be avoided, when the sheeting edges are seized in a fast-gripping manner by means of vise clamps, chains or steel wires, which finally results in the tearing of the sheet. With the stretching process of the invention, however, the edges remain without fault; only the edges in touch with the disk surfaces keep their original strength, as they are exempt from the stretching.

The process of the invention is normally applied to transverse stretching. In some cases, in particular those in which a high transverse stretching ratio is to be obtained, the process is to be applied several times. For this purpose the process is repeated with an increasing transverse stretching ratio, i.e. the devices described in the following are connected in series, the corresponding angular positions of the disks to the direction of movement of the sheet being maintained.

Although the process is particularly applicable to the transverse stretching of sheetings, a longitudinal stretching may be combined with it at the same time, which may be done in various ways. The longitudinal stretching may be effected prior to the transverse stretching process, for example, by means of known measures, whereby a sheet is obtained which is stretched longitudinally and across the width. The longitudinal stretching may also be effected following the transverse stretching. It depends on the machine aggregate, which is possibly present, as well as on the type of thermoplastic material used on the one hand, and on the desired sheet properties on the other hand, whether the longitudinal stretching is carried out first or the transverse stretching. However, a simultaneous biaxial stretching is also possible, in which case the pair of rollers 6,7 drawing off the sheeting has a higher speed than the pair of feed rollers 2,3, depending on the longitudinal stretching ratio desired.

As has been described above, the longitudinal stretching can be effected by known measures, in which case there will be a neck formation across the width of the sheeting, if there is no gripping of the sheeting edges. However, the process of the invention may also be applied advantageously to the longitudinal stretching optionally desired. In this case the disks 4 which are disposed at an angle to the direction of movement of the sheeting, so as to be adjustable, are adjusted parallel to the direction of movement of the sheeting, as shown in phantom outline in FIG. 1, and the process is effected in analogous manner to the method described in the case of the transverse stretching.

The sheeting edges can be pressed onto the disks - where they touch them - by various methods, as long as the presence of an air layer between the sheet and the disks is prevented. As pressing means, free running or driven pressing rolls have, for example, proved useful. It is also possible to press the sheeting edges onto the disks by means of a directed gas stream, for example by means of compressed air, via a so-called air knife 12 as shown in FIG. 2A. The heating of the sheet to stretching temperature, while the process is being carried out, is effected by known measures, for example, by means of heating gases and/or electrical equipment through pipes or heating elements 9 shown in FIGS. 2A and 6. It is advantageously effected by means of a series of heating elements of the same kind which are arranged parallel to one another, but which can be adjusted separately. The heating may be effected, for example, by hot air which is blown through a bundle of parallel tubes into the room formed by the disks and the sheet. If necessary, the other side of the sheet may additionally be blown with hot air in the same manner, in which case a semicircular plate or a similar object has to be provided, by which means the external stream of hot air is made to follow the sheet. Another possibility to heat the transverse stretching zone can be realized by electric radiators comprising element 14 shown in FIG. 2B, which have been designed accordingly, and which are adjusted in their form or their arrangement to the sheet. The heating of the disks, too, may be effected according to several methods, for example, by means of heated gases or liquids, or by way of electrically operated devices.

The process of the invention is normally carried out without supporting the sheeting, so that the sheet is stretched in a curved surface. Owing to the stretching tension, the sheet surface has the shape of a saddle, and the middle of the sheeting (shown in FIG. 2 by a broken line having the reference numeral 8) moves along a different path than the edges. As a result, the stretched sheet is thicker in the middle by about 10 to 20 percent. This undesired thickness distribution can be avoided, for example, by the production of an opposite thickness profile in the manufacture of the unstretched sheet.

Another method of balancing the thickness consists in a different heating of the sheet in a transverse direction to the direction of movement of the sheeting, or in a forced prevention of the formation of the "saddle surface", by means of supporting the sheet. This can be effected, for example, by means of supporting rolls 10 (FIG. 3) or a supporting surface 11 (FIG. 4). In this connection, the support devices are required to have a friction that is as small as possible, with regard to the sheet, in order to avoid damage to the sheet. The friction can be further reduced by producing an artificial air cushion 11a between the sheeting and the support 11, by introducing air into support 11 through conduit 11b and causing it to flow out through perforations 11c, in which case the blown-in supporting air may serve to heat or slightly heat the sheet at the same time. A thickness balancing may also be effected by a simultaneous or subsequent longitudinal stretching of the sheet. In this process, the sheet is advantageously influenced by means of suitable variable heating means, for example hot air, in a way that it passes the zone in which it is expanded at a decreasing temperature. This is, for example, provided by having rollers 10 shown in FIG. 3 maintained at decreasing temperatures in the direction of movement of sheet material 1. In the same manner, a thermofixation of sheets can be effected, while maintaining the sheet dimensions. It is known, indeed, that the neck formation in the course of the longitudinal stretching can be avoided by stretching the sheeting with a very narrow gap between the rollers 2,3 and 6, 7. Apart from the fact that a minor neck formation cannot be avoided, there is also the disadvantage that a balancing of the thickness, which is desirable in particular in the case of extruded flat sheets, is not possible.

With high stretching temperatures and/or in the case of stretching very thin sheets it is more suitable, however, in spite of the more complicated machinery, to provide the disks with a drive of their own, such as motors 16 shown in FIG. 5, by which means the course of the longitudinal stretching before and/or behind the disks can be regulated.

Another subject of the invention is a device for the performance of the process. In its most simple design for the transverse stretching or simultaneous transverse - longitudinal stretching, it consists of at least one heated pair of feed rollers, which heats the unstretched sheet and feeds it into the device, of at least one heating device for maintaining the temperature within the stretching zone, of at least one pair of rotating disks, which are arranged in a diverging direction from the draw-off direction of the sheeting, and of a pair of draw-off rollers, which have a greater speed than the pair of feed rollers, where a longitudinal stretching is effected at the same time; furthermore, the device of the invention is characterized by the fact that in those places where the sheeting edges touch the disks, there are other devices which press the sheeting edges onto the circumferential surfaces of the disks, while preventing the formation of air pockets, and that there are means for heating the disks.

If a very strong transverse stretching is required, two or more devices are connected in series, such as those shown in FIG. 6 according to the stretching ratio desired, in which case the divergency of the disks from the draw-off direction of the sheeting may show a variable angle.

If in addition to the transverse stretching a longitudinal stretching is intended, the longitudinal stretching may be effected, with the device of the invention, by simple variation of the number of revolutions either in the gap between the pair of feed rollers and the disks, or between the disks and the pair of draw-off rollers; however, if only a longitudinal stretching is desired, without any transverse stretching, the process is to be applied in analogous manner, while the disks are brought ina parallel position.

In order to obtain a good adhesion of the sheeting edges to the circumferential surfaces of the disks, the latter have advantageously been subjected to a special treatment. By special treatment, there is to be understood that they are ground and polished or provided with a ground and polished coating of another material, for example a chromium coating. It has proved to be advantageous in special cases to give the disks the form of a truncated cone or to give them a rounded-off form. The stretching device can be used for different widths of the unstretched sheets and/or it may be used universally for different transverse stretching ratios, if its inclination, such as angle A shown in FIG. 6, and/or the distance of the disks 4 are adjustable along line 18 also shown in FIG. 6. For this purpose, use is made of known devices, for example, toothed rims for adjustment, an infinitely variable adjustment being preferred.

Besides the fact that only little space is needed and that complicated holding means for the sheeting edge are no longer required, the novel system has the further advantage that the increase in the width is continuous and without jump, since the edges of the sheeting describe a pure sine curve during the transverse stretching.

A device for the stretching of plastic sheets constructed in accordance with the invention is illustrated diagrammatically by way of example in the accompanying drawings of which:

FIG. 1 is a partially schematic plan view of a drawing apparatus for plastic sheet material, which is one embodiment of this invention;

FIG. 2 is an end view in elevation of the apparatus shown in FIG. 1;

FIG. 2A is another end view in elevation of another embodiment of the invention shown in FIGS. 1 and 2;

FIG. 2B is still another end view in elevation of still another embodiment of the invention shown in FIGS. 1 and 2;

FIG. 3 is an end view in elevation of a further embodiment of the invention shown in FIGS. 1 and 2;

FIG. 4 is a further end view in elevation of a further embodiment of the invention shown in FIGS. 1 and 2;

FIG. 5 is another partially schematic plan view of a still further embodiment of the invention shown in FIGS. 1 and 2; and

FIG. 6 is a plan view of still a further series embodiment of the invention shown in FIGS. 1 and 2.

The sheet 1, which has been pre-heated in known manner, is pressed against the driven pre-heating roller 3 by means of a roller 2 which has an elastic surface, for example rubber or silicon rubber; subsequently the sheeting edges touch the two heated disks 4. The two rubber rollers 5 then press the sheeting edges onto the circumferential surfaces of the disks 4 in such a way that no air layer can be formed between the edges and the circumferential surfaces. Thus, and be means of the heating of the disks, the sheeting edges adapt themselves to the surface of the circumferential surfaces, so that a fast grip during the transverse stretching process is effected. The end of the stretching device is formed by the pair of draw-off rollers 6,7, which are driven, and which have a greater speed than the pair of feed rollers 2,3, of a longitudinal stretching is to be effected simultaneously. The broken line 8 shown in the side view indicates that the middle of a sheeting that is not supported during the stretching process moves along a different path than the edges. The heating elements are in the form of parallel tubes 9, through which heating gas is blown in. As can be seen from FIG. 1, the disks 4 in this case are in the form of truncated cones. In the variant according to FIG. 5, the circumferential surfaces of the disks are rounded off. In connection with the constructional measures not shown in these figures which serve for the infinitely variable adjustment of the angle of the disk axes, as well as of the distance of the disk pairs from each other, the device of the invention may be adapted most easily to any transverse stretching ratio and any width of the unstretched sheet.

FIGS. 3 and 4 show the supporting means for the sheeting in the form of rolls 10 and/or a supporting plate 11.

As can be seen easily, the device may be designed in many possible ways.

Claims

What is claimed is:

1. A device for stretching solid thermoplastic sheet material being fed from feed rollers to takeup rollers comprising a pair of rotatably mounted disks disposed after said feed rollers in contact with the edges of said sheet material being fed by said feed rollers, heating means arranged to apply heat to the edges of said sheet material contacting said disks, said disks being disposed at a divergent angle with respect to the edges of said sheet material being fed from said supply rollers, and pressure applying means disposed on the opposite sides of said sheet material from said disks and adjacent said disks for applying pressure to said edges of said sheet material to urge them in forceful contact with the circumferential surfaces of said disks whereby the formation of air pockets between said edges and said disks is prevented and said edges are maintained in firm engagement with said circumferential surfaces of said disks by atmospheric pressure to laterally stretch said sheet of thermoplastic material in accordance with the divergent angle of said disks without using any mechanical means gripping over said edges of said sheet.

2. A device as claimed in claim 1, wherein several stretching devices are connected in series.

3. A device as claimed in claim 1, wherein the disks are in the form of a truncated cone.

4. A device as claimed in claim 1, wherein the circumferential surfaces of the disks are rounded off.

5. A device as claimed in claim 1 wherein longitudinally adjustable means support said rotatably mounted disks whereby the distance of the disks from each other can be adjusted.

6. A device as claimed in claim 1 wherein angularly adjustable means support said rotatably mounted disks whereby the divergent angle of said rotatably mounted disks is variable.

7. A device as claimed in claim 1 wherein said heating means comprises a source of heated gas directed at said thermoplastic sheet material while it is engaged between said pressure means and said disks.

8. A device as claimed in claim 1 wherein said pressure means have an elastic pressure surface.

9. A device as claimed in claim 1, wherein supporting means for the sheet material are arranged between the disks.

10. A device as claimed in claim 9, wherein the supporting means are rolls.

11. A device as claimed in claim 9 wherein said supporting means comprises a curved plate.