PROCESS AND EQUIPMENT TO REGULATE THE THICKNESS OF ORIENTED TUBULAR FILM THAT IS MANUFACTURED IN A FILM BLOWING PROCESS

Abstract

The thickness profile of a film tube manufactured using a film blowing process is regulated using a control system consisting of segmented control zones and at least at two measuring units for the film thickness such that the specified target film thickness profile exhibits deviations from a uniform film thickness in the form of one or two thin spots opposite each other, which serve to compensate for film thickness changes across the film width which occur during the monoaxial orientation in machine direction so that a film web is produced as a result of orientation whose thickness profile has as few deviations as possible from the target thickness across the entire width of the film web, the final film should exhibit a thickness profile where the thickness increase from the middle of the film to the edges is as minor as possible and the flatness of the film is improved.

Claims

1. A method to regulate thickness of an oriented tubular film that is manufactured in a film blowing process, laid flat and repositioned using an oscillating take-off unit, oriented monoaxially in machine direction using an orientation unit (2) and subsequently wound with a winder (3), the method comprising: using a film thickness profile control system that has segmented control zones and at least two measuring units for film thickness to regulate a thickness profile of the tubular film such that the specified target film thickness profile exhibits deviations from a uniform film thickness in the form of one or two thin spots opposite each other, which serve to compensate for film thickness changes across the film width which occur during the monoaxial orientation in the machine direction, so that film is produced as a result of orientation whose transverse thickness profile has as few deviations as possible from the target film thickness across the entire width of the film web, wherein thin spots of a defined geometry are imprinted on the film tube by means of individual selection and/or control of the individual actuators of the segmented control zones, whereby influences of the running direction of the oscillating take-off unit on the thin spot geometry are taken into consideration.

2. The method of claim 1, wherein the geometry of the flanks of the thin spots is different as a function of the running direction of the oscillating take-off unit.

3. The method of claim 2, wherein different functions are integrated into the control algorithm of the control unit for the left-hand and right-hand flank of a thin spot.

4. The method of claim 1, wherein at least an initial actual film thickness profile of the film tube upstream of orientation, preferably around the circumference of the inflated film tube is measured, and at least a second actual film thickness profile of the laid-flat and oriented film tube downstream of orientation and upstream of winding the film web is measured.

5. The method of claim 4, wherein the second actual film thickness profile across the film width is measured at oscillating least one of the single-layer film webs, preferably at both single-layer film webs of the slit-open and laid-flat tubular film web downstream of orientation and upstream of winding the film web.

6. The method of claim 4, wherein the second actual film thickness profile across the width of the edge trims is measured at the single-layer film web downstream of orientation and upstream of winding the film web.

7. The method of claim 1, wherein the extruded film tube is regulated around the circumference to match a specified target film thickness profile by means of a film thickness profile control system.

8. The method of claim 1, wherein the control algorithm to address the individual segmented control zones comprises a superimposition of the basic profile derived from the film thickness profile regulation of the film blowing line the orientation profile from the repositioning of the film tube by the oscillating take-off unit and the influences of the orientation process and under consideration of the geometry of the thin spots caused by the running direction of the oscillating take-off unit, an offset value is superposed which takes the angular misalignment caused by the oscillating take-off unit into account and which tracks the movement of the turning bars and is continually adjusted.

9. The method of claim 8, wherein the reel profile is measured by adding up the film thickness across the winding width derived from measurement of the second actual film thickness profile.

10. The method of claim 8, wherein the reel profile of the reel as it forms is recorded by a camera.

11. The method of claim 1, wherein the edge trim width which will be cut off during the downstream trimming step is derived from measurement of the second actual film thickness profile at the single-layer film web and is then manually or automatically adjusted.

12. A system to implement the method to regulate thickness of an oriented tubular film that is manufactured in a film blowing process of claim 1, the system comprising a film blowing line with an oscillating take-off unit and at least one orientation unit as well as at least one winder and at least one film thickness profile control system.

13. The system of claim 12, wherein the film thickness profile control system comprises segmented control zones, at least two measuring devices to measure the film thickness, one to measure the actual thickness profile of the film tube downstream of the die head and upstream of the orientation unit, preferably around the circumference of the inflated film tube, and at least one to measure the actual thickness profile of the oriented film downstream of the orientation unit, as well as a master control system and evaluation unit.

14. The system of claim 12, wherein the measuring units to measure the actual film thickness profile of the oriented tubular film downstream of the orientation unit and upstream of the winder are installed at least at one of the slit-open and separate single-layer film webs.

15. The system of claim 12, wherein a device to perform the edge trimming is located downstream of the orientation unit and downstream of the measuring device, whereby positioning of the cutting units of the device to perform the edge trimming can be set to manual or automatic.

16. The system of claim 13, wherein the segmented control zones are integrated into a film die head, a stationary or rotating cooling ring, a thickness control unit between film die head and collapsing equipment, or a downstream thickness control unit which moves synchronously with the oscillating take-off unit, or combinations thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0070] Other details, features and advantages of the subject matter of the invention can be found in the claims as well as in the following description of the associated drawings in which a preferred embodiment of the invention is shown by way of example.

[0071] The drawings show:

[0072] FIG. 1: Film blowing line with downstream orientation unit in which the invention-design process to regulate the film thickness is applied.

[0073] FIG. 2: Target film thickness profile of a film tube with two thin spots of asymmetric thin spot geometry.

DETAILED DESCRIPTION

[0074] FIG. 1 shows a film blowing line (1) with downstream orientation unit (2) and two winders (3). A dosing unit (4) supplies the resin granules to the extruder (5) where they are melted, homogenised and then routed to the die head (6). The production of multi-layer film necessitates the use of several extruders, with the number of extruders corresponding to the number of film layers. The die head (6) has an annular die through which the extruded melt exits. The cooling air needed to inflate the tubular film (7) is supplied through the die head (6). The tubular film (7) is cooled from the outside by means of cooling air exiting a cooling ring (15) and is then routed through a calibration basket (8). Once the plastic has set, the film tube (7) is laid flat with the collapsing boards (9) and taken off and repositioned continuously with the oscillating take-off unit (10). The film is then oriented monoaxially in machine direction with the orientation unit (2).

[0075] The film tube (7) is slit open on both sides upstream of the orientation unit (2) in a pre-take-off unit (11) and is then routed to the orientation unit (2) as a double-layer film web.

[0076] Alternatively, an annealing unit or temperature control unit can be combined with the pre-take-off unit (11) (not shown here). After orientation, both superposed single-film webs are separated from each other and routed to a winder (3) in each case. After the edges of the film are trimmed in the winders (3), it is wound to film reels or rolls.

[0077] The film blowing line (1) is equipped with a film thickness profile regulation system. It consists of segmented control zones and at least two measuring test points (12) and (13) to measure the actual film thickness profile.

[0078] To permit regulation of the film thickness profile, it is necessary to measure the actual film profile—at least at two points. The actual film thickness profile around the circumference of the film tube (7) is measured between the film die head (6) or the segmented control system in the cooling ring (15) and the collapsing boards (9) or oscillating take-off unit (10) by the measuring device (12). The measuring device (12) used to measure the actual film thickness profile of the inflated film tube (7) is installed to rotate around the film tube (7) preferably at a constant height above the die head (6).

[0079] The actual film thickness profile of each of the two oriented single-layer film webs is measured by the measuring unit (13) across their width between the orientation unit (2) and the winder (3) subsequent to the webs being separated.

[0080] The complete film blowing process is regulated by the system control unit, especially the drives, the cooling air, the segmented control zones—which are integrated into the cooling ring (15), the die head (6) or into a separate thickness regulation—as well as the take-off speed of the tubular film (7).

[0081] The actual film thickness profiles that are measured with the measuring units (12) and (13) are routed to the system control unit and after a target-actual comparison, are transmitted to the segmented control zone in the form of signals.

[0082] Alternatively, the orientation unit (2) can also be integrated into the oscillating take-off unit downstream of the collapsing boards (9) and upstream of the turning bars (16); this possibility is not shown in the figures.

[0083] The double-layer film web is slit open either before or after orientation. Downstream of the oscillating take-off unit (10), the film tube (7) is separated into two single-layer film webs and each single-layer film web is routed to a winder (3). The first measuring device (12) is installed upstream of the collapsing equipment (9) and measures the actual film thickness profile around the circumference of the film tube (7). The second measuring units (13) are located downstream of the oscillating take-off unit (10) after the laid-flat and oriented film tube (7) has been cut open into single layers and upstream of the winder (3), and measure the actual film thickness profile across the width of each single-layer film web. Alternatively, the actual film thickness profile can be measured downstream of orientation by one of the measuring units (13) at the single-layer film web and by the measuring unit (14) at the double-layer film web.

[0084] The edges are trimmed before the film webs are wound in the winder (3).

[0085] FIG. 2 shows a faulty actual film thickness profile with two thin spots (18) which are to be regulated in accordance with the invention-design process. The actuators in the control zone which imprint the opposite thin spots (18) onto the film tube (7) are addressed symmetrically. The running direction of the oscillating take-off unit (10) is taken into account thereby in that in running direction of the oscillating take-off unit, a flat flank (19) is imprinted onto the thin spot (18) because the movement is into the cold film, and in that contrary to the running direction of the oscillating take-off unit, a steep flank (20) is imprinted onto the thin spot (18) because the movement is into the warm film.

[0086] The film tube is cut open at the two positions (17), and then the two flat film webs are routed to the orientation unit (2) or are routed in each case to a separate orientation unit. The film is then measured, the edges trimmed and the film wound in the film winder (3).

REFERENCE NUMERALS

[0087] 1 Blown Film Line [0088] 2 Orientation system [0089] 3 Winder [0090] 4 Dosing unit [0091] 5 Extruder [0092] 6 Film die head [0093] 7 Tubular film [0094] 8 Calibration basket [0095] 9 Collapsing frame [0096] 10 Oscillating take-off unit [0097] 11 Pre-take-off unit [0098] 12 Thickness measurement unit [0099] 13 Thickness measurement unit [0100] 14 Thickness measurement unit [0101] 15 Cooling ring [0102] 16 Turning bar [0103] 17 Interface [0104] 18 Thin spot [0105] 19 Flat flank of thin spot [0106] 20 Steep flank of thin spot