GB2030923A

GB2030923A - Apparatus for bi-directionally stretching a plastics foil-web

Info

Publication number: GB2030923A
Application number: GB7929273A
Authority: GB
Original assignee: Lindauer Dornier GmbH
Current assignee: Lindauer Dornier GmbH
Priority date: 1978-09-23
Filing date: 1979-08-22
Publication date: 1980-04-16
Also published as: FR2436665A1; DE2841510B1; DE2841510C2; JPS5542893A; FR2436665B1; GB2030923B; IT7968850A0

Abstract

Apparatus for bi-directionally stretching a plastics foil web (Fig. 1) has two sets of clips (10) respectively gripping a web (2) at its opposite side edges, for stretching the web and transporting it through the apparatus. Each set of clips forms a chain-like series which is extensible (a, a1, a2, a3) substantially in the direction of web transport. Each clip (10) is connected to its adjacent clips by extensible connecting means, preferably in the form of steel bands or cable (15) mounted on the clip and connected to an adjacent clip, the band or cable being wound on a drum on the clip which drum is rotatable by drive means (11, 17, 18). The length of the band is thus variable so as to cause clips to move apart or to move together. <IMAGE>

Description

SPECIFICATION Apparatus for bi-directionally stretching a plastics foil web This invention relates to apparatus for bidirectionally stretching a plastics foil web. Such apparatus is known, having two sets of clips respectively gripping the opposite edges of the web as it is moved by the clips through the apparatus and stretched. Each set of clips forms a chain-like series and the clips are carried by rollers running in guide tracks; the two sets of clips are symmetrical with respect to the centre line of the web. Each set passes round guiding wheels at the ends of the stretching path, one wheel being also a drive wheel.

During stretching the distance between adjacent clips at an edge of the web must be increased to accommodate the stretching, and devices are known to produce this increase in clip spacing. German Specification No. 1,111,812 discloses the use of two guide tracks for each set of clips, so that each of every two adjacent clips has a first guide roller in a first track and also so that the two adjacent clips share a single second guide roller in a second track alongside the first track. The two first rollers are connected by inextensible links to the second roller. The spacing between the two tracks decreases in the direction of web travel, as the web is widened under stretching. Since the links are inextensible, the two first rollers, connected by their links to the single second roller, are forced apart, so that the clip spacing increases as required.German Specification No. 1,259,558 discloses another arrangement, in which adjacent clips are connected by springs which stretch in the direction of clip movement.

One disadvantage of the known apparatus is that precise adjustment of web stretching cannot be achieved due to the differing and relatively complicated structure and (apart from spring breakages which can occur) also due to the differing spring forces produced by the springs which connect the clips. Since the spring forces are not constant, there is no possibility of varying the stretching ratio as required in bi-directional stretching. Another disadvantage is that there is no possibility of shrinking the treated web in the fixing zone of the treatment area.

It is an object of the invention to provide an apparatus by which a bi-directional and variable stretching and shrinking ratio can be adjusted during operation.

According to this invention there is provided apparatus for bi-directionally stretching a plastics foil web, the apparatus having two sets of clips respectively gripping the opposite edges of the web for stretching it and transporting it through the apparatus, each said set forming a chain-like series which is extensible substantially in the direction of web transport, wherein each clip is connected to adjacent clips by extensible connecting means and the extensible connecting means of each clip is caused to become extended at a requisite location, so as to move the clips apart, by drive means connectible to the clip at such location.

An advantage of the invention is that the stretching ratios during the stretching operation and at the same time the shrinking ratios in the fixing zone can be precisely adjusted.

Due to the continuous increase in the size or length of the web in accordance with a given stretching ratio in the stretching zone, the clips at the edges of the web must follow the speed of stretching or elongation in the direction of displacement. Thus while the clips are still close together in the heating zone of the apparatus, they must continuously and progressively move apart as they travel through the stretching zone. Thus the distance between them must continuously increase until the end of the stretching zone is reached. The control required for this may be infinitely variable and may be provided by a worm drive at each clip. This worm drive includes a worm wheel driven by a sprocket wheel connected to it and in engagement with a drive chain.Each worm drive also includes a worm gear rigidly connected to a drum which has one end of a band attached to it, which band is wound or unwound on the drum to a greater or less extent, according to the required longitudinal stretching ratio. The other end of the band is fixed to an adjacent clip. The several bands in this way connect the clips together to form a chain-like series of clips which is extensible substantially in the direction of web transport.

When the web of foil has been stretched as required in the stretching zone and has passed over or through the fixing zone, it shrinks by a given amount during fixing. To compensate for this shrinkage and to keep the web stretched until the end of fixing and until the web leaves the clips, the distances between the individual clips must be corrected, i.e. they must be reduced. This is again effected by control chains, sprocket wheels, worm drives, drum and bands winding on the drums. The control chains are therefore arranged separately from each other in the several treatment zones and driven according to the speeds required. Thus there is only one control chain in the stretching zone, one in the fixing zone, and none in the heating zone.For returning the clips to their original position, a control chain is required only along the return path of the set of clips and is arranged upstream of the driving and deflecting wheel for the set of clips.

Embodiments of the invention will now be described by way of example, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a top plan of an apparatus for bidirectional stretching of a web of foil; Figure 2 is a vertical, transverse section through a guide track with a clip, a worm wheel, a worm gear, a drum, a clip-connecting band, a sprocket wheel and a drive chain; Figure 3 is a top plan of the clip of Figure 2; and Figure 4 is a top plan of part of a guide track with clips connected by bands.

Referring to the drawings, Figure 1, apparatus for bi-directional stretching of a web of foil comprises two guide tracks 1, 1' arranged symmetrically to the centre line m of a web 2.

Viewed in the direction of travel of the web (see the arrow on line m), the two guide tracks extend parallel at the web intake and heating zones 3, 4 to the beginning of a stretching zone 5, and then diverge in the latter zone. Beginning at the transition from the stretching zone 5, the guide tracks 1, 1' again run parallel or slightly converge (see dotted lines) in a fixing zone 6, until the exit -3' of the web 2 from the apparatus. A set of clips 9, 10 is supported and driven in the guide track 1 and at the intake and exit 3, 3' by driving and deflecting wheels 7, 8. The clips are close together or at varying distances apart. In the heating zone 4, the clips 10 are close together.The distance between the clips which at this stage are close together must increase because of the continuous increase in length in which the web 2 is subjected during its passage through the stretching zone 5, the extent of which increase depends on the stretching ratio, and the need to avoid damage to the foil. The increase in distance (a, ar, a2 and a3) is controlled by externally driven control chains 11, 17, 18 (drive means not shown) arranged beside each guide track 1 or 1' and engaging a sprocket wheel 12 on each clip 10. As will be described below, a worm wheel 13 is connected to the wheel 12 to drive a worm gear 21 and a drum 14 rotatable about an axis 16 (Figs. 2 and 3), a band 15 being attached to the drum. When a required maximum stretching ratio has been reached and stretching is thus completed, the web 2 travels into the fixing zone 6.The distances between adjacent clips 10 are at this stage still at the maximum. In view of the shrinkage produced in the web 2 during fixing, these distances must be adapted to the amount of shrinkage if the stretched web 2 is not to be damaged. This adjustment is effected by a second externally driven control chain 17 alongside the guide track 1 1 or 1' and independent of the first chain 1 1. Each control chain 17 similarly engages the sprocket wheels 12 on the clips 10, to drive worm wheels 13, worm gears 21, drums 14, and bands 15. A further control chain 18 is arranged near the driving wheel 7 for the purpose of returning the clips to the minimum spacing as they arrive at the wheel 7.

Referring to Figures 2 and 3, a clip 10 is supported and guided in a guide track 1(1') by upper and lower pairs of horizontal rollers 19, 1 9' and upper and lower vertical rollers 20, 20'. The self-locking worm wheel 13 in the lower part of the clip 10 is rotated by the sprocket wheel 12. A drum 14 is rigidly attached to a worm gear 21 meshing with the worm wheel 13 and rotating about the vertical axis 16 of the drum. One end of a band or cable 15 is attached to the drum 14, runs round the drum (Figs. 3 and 4), and is connected to the next clip 10. The sprocket wheel 12, which is mounted on a shaft 22 and is at one end of the worm wheel 13, engages by means of its toothed rim 23 with a control chain 11, 17 or 18 which is mounted above it and is driven by sprocket wheels 24.

Figure 3 shows a clip 10 supported in a guide track 1 (1'). The band 15, fixed at one end to the drum 14, extends towards the next clip 10 in one direction (see Fig. 4), while the band 15 which is fixed to clip 10 extends towards the next drum 14 in the other direction.

Figure 4 shows part of a guide track 1(1') having clips 10 arranged side by side at varying distances (a, a1, a2, a3) and flexibly joined by bands 15. The distances between the clips 10 moving in the direction of transport (see the arrow at the top left of the Figure) assume values ranging from a to a3 according to the given stretching ratio and the resulting speed of the web 2. Thus the bands 1 5 are wound or unwound to varying extents. This control is effected by the sprocket wheels 12 in engagement with the control chains 11, 1 7, 1 8.

Although bands 15 have been described above, cables may be used instead. The bands may be spring steel bands. The cables may be of steel. The bands or cables may be of a stretch resistant plastics material.

Claims

1. Apparatus for bi-directionally stretching a plastics foil web, the apparatus having two sets of clips respectively gripping the opposite edges of the web for stretching it and transporting it through the apparatus, each said set forming a chain-like series which is extensible substantially in the direction of web transport, wherein each clip is connected to adjacent clips by extensible connecting means and the extensible connecting means of each clip is caused to become extended at a requisite location, so as to move the clips apart, by drive means connectible to the clip at such location.

2. Apparatus according to claim 1 wherein the extensible connecting means of each clip comprises a band or cable mounted on the clip and connected to an adjacent clip, the band or cable being wound on a drum on the clip which drum is rotatable by the drive means.

3. Apparatus according to claim 2 wherein the drive means comprises a chain engageable with a sprocket on the clip, the sprocket being connected to the drum for rotation thereof.

4. Apparatus according to claim 3 wherein the sprocket rotates a worm wheel and the latter rotates a worm gear connected to the drum.

5. Apparatus according to any preceding claim having a drive means for each set of clips in a stretching zone of the apparatus and a further drive means for each set of clips at a location at the inlet of the transport path of the apparatus.

6. Apparatus according to claim 3 wherein the speed of unwinding of the bands or cables in a stretching zone of the apparatus corresponds to the stretching ratio required in the web and to its speed of transport in that zone.

7. Apparatus according to claim 6 wherein the bands or cables are at their maximum length at the end of the stretching zone.

8. Apparatus according to claim 6 or claim 7 wherein the bands or cables are at their minimum length at the inlet of the transport path of the apparatus.

9. Apparatus according to claim 2 wherein the extensible connecting means is a spring steel band.

10. Apparatus according to claim 2 wherein the extensible connecting means is a steel cable.

1 Apparatus according to claim 9 or claim 10 wherein the band or cable is of a steel weave material.

12. Apparatus according to claim 2 wherein the band or cable is of stretch-resistant plastics material.

13. Apparatus according to claim 3 wherein the chain speed is infinitely adjustably variable.

14. Apparatus for bi-directionally stretching a plastics foil web, constructed and arranged substantially as herein described and shown in the drawings.