Stretching Device and Method for Stretching a Plastic Film in the Transport Direction Thereof

Abstract

The invention describes a stretching device for stretching a plastic film in its transport direction, having a first roller that can be driven by a first drive and can be rotated at a first rotational speed, and having a second roller that can be driven by a second drive and can be rotated at a second rotational speed, the second rotational speed being greater than the first rotational speed, the second roller being arranged downstream of the first roller in the transport path of the plastic film.

The device is characterized by means of varying the amount of air between the plastic film and flat elements of the first roller and/or the second roller on which the film rests.

Claims

1. A stretching device for stretching a plastic film in its transport direction, having a first roller that can be driven by a first drive and can be rotated at a first rotational speed, and having a second roller that can be driven by a second drive and can be rotated at a second rotational speed, the second rotational speed being greater than the first rotational speed, the second roller being arranged downstream of the first roller in the transport path of the plastic film, characterized by means for changing the amount of air between the plastic film and surface elements of the first roller and/or of the second roller on which the film rests.

2. A stretching device according to claim 1, characterized in that the means for varying the amount of air comprise depressions in the surface of the first roller and/or the second roller, the air being receivable in the depressions.

3. A stretching device according to claim 1, characterized in that the means for varying the amount of air comprised through openings in the surface of the first roller and/or the second roller said through openings providing a fluid-communicating connection between the outer surface and a cavity within the first roller and/or the second roller.

4. A stretching device according to claim 1, characterized in that the means for varying the amount of air comprised through openings in the surface of the first roller and/or the second roller, wherein at least part of the through openings can be provided by a porous material, in particular by a microporous material.

5. A stretching device according to claim 1, characterized in that the means for varying the amount of air comprise through openings in the surface of the first roller and/or the second roller, at least a part of the through openings being provided with a porous material, in particular by a microporous material, the porous material being a sintered material, in particular, a sintered metal.

6. A stretching device according to claim 1, characterized in that the means for varying the amount of air comprised through openings in at least part of a roll shell of the first roll and/or the second roll, at least part of the through openings being formed by bores.

7. A stretching device according to claim 1, characterized in that the first and/or the second roller comprise at least one cavity which can be subjected to an air pressure that is lower than the ambient pressure.

8. A stretching device according to claim 1, characterized in that the first and/or the second roller comprise at least one cavity which is divided into at least two segments in the axial direction and/or in the circumferential direction by means of at least one separating element, wherein at least one segment can be subjected to an air pressure which is reduced or increased in comparison with the ambient pressure.

9. A stretching device according to claim 1, characterized in that the means for changing the amount of air between the plastic film and surface elements of the first roller and/or the second roller on which the film rests comprises at least one suction device with which air can be sucked out of the region in which the plastic film comes into contact with the first and/or the second roller in the transport direction.

10. A stretching device according to claim 1, characterized in that the suction device is divided into sections transversely to the transport direction of the plastic film.

11. A stretching device according to claim 1, characterized in that the means for changing the amount of air between the plastic film and surface elements of the first roller and/or the second roller, on which the film rests, comprises at least one upper-pressure device, with which the surface of the plastic film facing away from the roller can be acted upon with air under upper pressure.

12. A stretching device according to claim 11, characterized in that the means for changing the amount of air between the plastic film and surface elements of the first roller and/or the second roller on which the film rests comprise at least one electrode with which the plastic film can be electrostatically charged upstream or at the line of contact of the plastic film with the roller.

13. A method for stretching a plastic film in its transport direction with a first roller, which is driven by a first drive and rotated at a first rotational speed, and with a second roller, which is driven by a second drive and rotated at a second rotational speed, the second rotational speed being greater than the first rotational speed, the second roller being arranged downstream of the first roller in the transport path of the plastic film, characterized in that the quantity of air present between the plastic film and surface elements of the first roller and/or of the second roller, on which the film rests, is changed by means of changing the quantity of air.

14. A blow film installation comprising at least one extruder for producing a plastic melt, a die head for producing a film tube, a flattening device for converting the film tube into a double-layer plastic film, a driven extraction device for drawing off and further transporting the double-layer plastic film, and a winding device for winding up at least one layer of the double-layer plastic film, characterized by at least one stretching device according to claim 1.

Description

[0054] Further advantages, features and details of the invention can be seen in the following description, in which various examples of embodiments are explained in detail with reference to the figures. The features mentioned in the claims and in the description may be essential to the invention individually or in any combination of the features mentioned. Within the scope of the entire disclosure, features and details described in connection with the method according to the invention naturally also apply in connection with the stretching device according to the invention, and vice versa in each case, so that reference is or can always be made mutually with respect to the disclosure concerning the individual aspects of the invention. The individual figures show:

[0055] FIG. 1 Schematic view of a stretching device according to the invention

[0056] FIG. 2 A stretching roller of an embodiment of the invention

[0057] FIG. 3 A stretching roller of a further embodiment of the invention

[0058] FIG. 4 A stretching roller of a further embodiment of the invention

[0059] FIG. 5 An embodiment of the invention with a suction device

[0060] FIG. 6 An embodiment of the invention with an electrode

[0061] FIG. 7 A blow film line according to the invention with a stretching device

[0062] FIG. 8 Another blow film line according to the invention with a stretching device

[0063] FIG. 9 An embodiment of the invention with a nozzle

[0064] FIG. 10 An embodiment of a device according to the invention with a covering device

[0065] FIG. 11 A variant of a covering device

[0066] FIG. 12 An embodiment of a device according to the invention

[0067] FIG. 13 An embodiment of a device according to the invention

[0068] FIG. 1 shows a schematic representation of a stretching device according to the invention. The means for changing the amount of air between the plastic film and surface elements of the first roller and/or the second roller on which the film rests are not visible in this figure and will be explained in more detail in the following figures. The ribbon-shaped plastic film 101 enters the stretching device 100 in the transport direction T. The plastic film first runs onto one or more rollers. The plastic film first runs onto one or more preheating rollers, of which only one preheating roller 102 is shown. A preheating roller has the task of bringing the film to a predefined temperature. For this purpose, a preheating roller is usually temperature-controlled, whereby a temperature-controlled fluid is often introduced into the preheating roller.

[0069] After leaving the preheating roller(s) 102, the film web 101 passes onto a first roller 110, which can generally also be referred to as the first stretching roller 110. This stretching roller is connected to a drive that is not shown, for example, its own electric motor, which drives the roller 110 in rotation at a first rotational speed.

[0070] The first roller 110 is preferably associated with a first feed roller 111, which together with the first roller 110 provides an infeed gap for the film. Preferably, the infeed gap or the running path of film 101 is arranged in such a way that film 101 runs in the infeed gap tangentially to the rollers 110, and 111. The roller nip already serves to minimize the air between the stretching roller 110 and the film.

[0071] The first roller 110 is also associated with a second feed roller 112, which forms an exit nip with the roller 110. The second feed roller 112 can be adjustable in the circumferential direction of the first roller 110. The second feed roller 112 serves to ensure that the film leaves the first roller 110 along a line that runs parallel to the axial direction of the roller 110.

[0072] As seen in the transport direction T of film 101, a second roller 120 is arranged downstream, which can be referred to as the second stretching roller 120. This stretching roller 120 is also connected to a further drive which is not shown, for example, its own electric motor, which drives the roller 120 in rotation at a second rotational speed. The second rotational speed is greater than the first rotational speed, whereby the second stretching roller then has a greater circumferential speed than the first stretching roller. This results in film 101 being stretched between the outlet nip of the first roller 110 and the inlet edge of the second roller 120 in the ratio of the circumferential speeds in its transport direction. The distance between the outlet nip and the inlet edge is often referred to as the stretching nip.

[0073] It is possible that the first roller 110 and the second roller 120 can be moved relative to each other. This allows the stretching gap to be influenced. A change in the stretching gap can influence the properties of the film.

[0074] The second roller 120 is preferably assigned a third feed roller 121, which together with the first roller 120 provides a second infeed gap for the film. Preferably, the infeed nip or the running path of film 101 is arranged in such a way that film 101 runs in the infeed nip tangentially to the rollers 120, and 121. The roller nip already serves to minimize the air between the stretch roller 120 and the film.

[0075] In the case that the drafting gap is small, the third feed roller 121 would collide with the roller 110, so in this case, the third feed roller had to be swung off.

[0076] The second roller 120 is also optionally associated with a fourth feed roller 122, which forms an exit nip with the roller 120. The second feed roller 122 can also be adjustable in the circumferential direction of the first roller 120. The second feed roller 122 serves to ensure that the film leaves the first roller 120 along a line that runs parallel to the axial direction of roller 120.

[0077] In principle, one or more feed rollers can also be dispensed within a stretching device according to the invention. Nevertheless, an infeed nip or outfeed nip can be mentioned. This is to be understood as the line along which the film is applied to the stretching roller or along which the film is released from the stretching roller.

[0078] Further stretching rollers can be provided, in particular with one or two feed rollers in each case, whereby two stretching rollers arranged directly one after the other are driven in each case in such a way that the stretching roller arranged downstream in each case has a higher circumferential speed than the preceding stretching roller.

[0079] Downstream of the stretching rollers 110, and 120, a cooling roller 130 is arranged, with which film 101 can be cooled again so that the new molecular orientation within the film resulting from the stretching solidifies.

[0080] With reference to FIG. 2, an embodiment of the stretching device according to the invention is explained below. A perspective view of a stretching roller is shown. In the example shown, it is the stretching roller 120, which comprises a roller shell 140 and end faces, of which the end face 141 is visible. The stretching roller has a shaft, and an axle or axle stubs. Of the aforementioned components, only part 142 projecting from the end face 141 is visible. The roller shell 140 now includes depressions 143, which in the present example is shown as a circumferential groove. Further examples of depressions have been given earlier in the description of the invention. The characteristic of depressions is that they only extend into the roll shell, but do not extend through the roll shell 140 into the interior space of the roll defined by the roll shell and the end faces.

[0081] FIG. 3 is similar to FIG. 2, but here the roller 120 comprises through openings 150. The exact design of the through openings has already been described above. The through openings extend into the interior of the roller and thus form a fluid-communicating connection between the interior and the surroundings of the roller 120. Instead of through openings, openings can also protrude from the surface, which only comprises cavities that are open to the outside, for example, the open area of porous material.

[0082] The interior of the roller 120 can optionally be pressurized with negative pressure. For this purpose, the axis, the shaft, or the stub shaft can be provided with fluid lines. In the case of a shaft, fluid lines comprise a rotary union. The fluid line of the axis, shaft, or stub shaft is connected to another fluid line, such as a hose 151, which leads to a vacuum source.

[0083] FIG. 4 shows further possible features of a roller 120 which has through openings. Here, the interior space, which is now visible due to the missing representation of the roller shell, can be divided into several individual spaces with partition walls, in the present example two partition walls 160, 161. This makes it possible to apply different air pressures to the different individual chambers. In the present case with two individual rooms, one of them can, for example, be subjected to negative pressure, while in the second individual room no upper or lower pressure can be applied, so the ambient pressure prevails here.

[0084] FIG. 5 shows an example of an embodiment in which a suction device 165, which extends in the transverse direction with respect to the transport direction of film 101, is arranged upstream of the stretching roller, here in particular the stretching roller 120. This suction device comprises a closed box 166 with one or more suction openings 167. In particular, the box can be subjected to a negative pressure so that air from the inlet gap enters the suction device through the suction openings in the direction of the arrow L and is discharged.

[0085] FIG. 6 shows a further example of a stretching device according to the invention, in which an electrode 170 is arranged upstream of the inlet gap of a stretching roller, in the example shown the stretching roller 120. This electrode can be subjected to an electrical potential that differs from the electrical potential of film 101. This results in an electrical charge of the film, which leads to the film being additionally held on the stretching roller with an electrical force.

[0086] FIG. 7 shows apparatus 1 for the production of a film tube, namely a blown film line, which initially comprises at least one extruder 2, with which, for example, plastic present in granular form can be plasticized. Via pipe 3, the plastic melt thus produced is fed to an extrusion tool 4, which can also be referred to as a nozzle head, from which this melt is transferred into a film tube 6, so that this melt stream can be drawn out of an annular gap 5, which is not visible in this figure, in the drawing-off direction z. Now there is a film tube 6 which has not yet solidified. This is inflated from the inside by a slight upper pressure so that it has a larger diameter inside the optional calibration device 7. The film tube is solidified by a tempering device 8, which is often also called a cooling ring because of its ring-like design enclosing the film tube.

[0087] After passing through the calibrating device, film tube 6 enters the active area of a flat laying device 9, in which the circular film tube is converted into an elliptical cross-section with increasing eccentricity until it finally forms a double-layered plastic film in the area of influence of the take-off rollers 10, which are joined together at their sides.

[0088] The flattening device is rotatably arranged, whereby the axis of rotation is essentially aligned with the hose axis 11, which is indicated in FIG. 1 by a dashed line. The rotatability of the flattening device is indicated by arrow 12.

[0089] FIG. 7 also shows a reversing device 15, which has the task of guiding the flattened film tube from the flattening device to the stationary roller 16 without causing damage.

[0090] Downstream of the reversing device 15, a stretching device 100 according to the invention is now arranged, which has already been explained in connection with FIGS. 1 to 6 and the further description. The stretching device shown in FIG. 7 corresponds to the one shown in FIG. 1. It should also be noted that a cutting device can be positioned upstream of the stretching device 100, with which one or two edges of the fold can be cut open or cut off. In addition, a separating device can be provided with which the double-layer plastic film can be divided into one or more single-layer films.

[0091] Arrow 17 indicates that this film tube, after passing through the stretching device 100, is guided for further processing, which is not specified in more detail here.

[0092] FIG. 8 shows a further embodiment of a blown film line according to the invention, in which the stretching device 100 is now arranged between the removal rollers 10 and the reversing device 15. It should be noted that the rollers within the stretching device are now arranged in such a sequence that the plastic film can be transported from the bottom to the top.

[0093] FIG. 9 shows a further example of a stretching device according to the invention, in which at least one nozzle 180 is arranged upstream of the inlet gap of a stretching roller, in the example shown the stretching roller 120. Through this nozzle, the film can impinge on the side facing away from the roller with a pressurized fluid 181, in particular compressed air. This ensures that the film is printed with a force against the roller 120 so that less air is drawn into the area between the film and the roller 120 by the movement of the film.

[0094] Numerous possible features of the invention are indicated in the description and in the claims. Even if a free combinability of different features is not always mentioned, this is to be regarded as also disclosed, provided that no contradictions are caused.

[0095] FIG. 10 shows a further embodiment of a device according to the invention, in which a covering device 185 is shown on one of the stretching rollers. Such a covering device can also be provided on any other stretching roller. With this covering device 185, the angular area of the stretching roller that is not wrapped by the film can be covered, so that a vacuum that is applied to areas of the roller surface and/or in its interior is not disturbed and is maintained as far as possible. The covering device can interact with the stretching roller without contact. The covering device can, for example, be a bent sheet. The covering device can extend in the axial direction over almost the entire length of the roller. Seals can be provided at the lateral edges of the cover device, which additionally seal the area between the roller and the cover device to further improve the reduction of false air. The cover device can be formed from several cover elements that can be moved against each other. This makes it possible to optimally adjust the cover element even with different film widths and with different wrap angles of the film.

[0096] FIG. 11 shows a variant of a covering device, whereby the covering device comprises at least one circulating belt 186, which is also guided by the stretching roller. At least two deflection rollers 187 are provided for guiding and/or driving the circulating belt. The deflection rollers can be displaceable in the circumferential direction and/or in the radial direction of the stretching roller.

[0097] To minimize the effect of false air intake described in connection with FIGS. 10 and 11, which is to be avoided, it can be provided according to the embodiment example of FIG. 12 that the fourth feed roller 122 is dimensioned and/or positioned in such a way that the smallest possible area is created which is not wrapped by the film. Since this area that is not wrapped depends on a number of parameters, it is not possible to make any specific statements about the position and dimension of the roller 122. It is possible for a skilled person to determine such details.

[0098] To minimize the effect of false air intake described in connection with FIGS. 10 and 11, which is to be avoided, a first guide roller 190 and a second guide roller 191 can also be provided, as shown in FIG. 13. Each of the guide rollers can be in contact with the film. The first guide roller is arranged upstream of the stretching roller in question, and the second guide roller is arranged downstream of the stretching roller. The first guide roller may be identical to the second feed roller 112. A sealing element 192 is arranged between both guide rollers 190 and 191, which preferably contacts the circumferential surfaces of the guide rollers. The resulting cavity consisting of the stretching roller, the film, the guide rollers, and the sealing element is closed, in particular, by seals arranged on the front side of the cavity, but not shown.

TABLE-US-00001 List of references 100 Stretching device 101 Plastic film 102 Preheating roller 110 First roller 111 First feed roller 112 Second feed roller 120 Second stretching roller 121 Third feed roller 122 Second feed roller 130 Cooling roller 140 Roll shell 141 Front face 142 Part protruding from the face 141 143 Depressions 150 Through opening 151 Tube 160 Partition wall 161 Partition wall 165 Suction device 166 Closed box 167 Suction opening 170 Electrode 180 Duse 181 Fluid 185 Covering device 186 Circulating belt 187 Deflection rollers 190 First guide roller 191 Second guide roller 192 Sealing element 1 Device for the production of a film tube 2 Extruder 3 Line 4 Extrusion tool 5 Not visible annular gap 6 Not yet solidified film tube 7 Optional calibration device 8 Tempering device 9 Flattening device 10 Removal rollers 11 Tube axis 12 Arrow to illustrate the rotatability of the flattening device 13 14 15 Reversing device 16 Stationary roller 17 Arrow indicating further processing T Transport direction Z Removal direction