GB2194753A

GB2194753A - Treating heat shrinkable material

Info

Publication number: GB2194753A
Application number: GB08703018A
Authority: GB
Inventors: Tiziano Scapoli
Original assignee: WR Grace and Co
Current assignee: WR Grace and Co
Priority date: 1987-02-10
Filing date: 1987-02-10
Publication date: 1988-03-16
Also published as: GB8703018D0

Abstract

Control of the shrink potential of heat-shrinkable tubing is effected by inflating the tubing between a movable pair of pinch rolls (5) and a static pair of outlet pinch rolls (7), heating a shrink-controlling encircling collar (6a, 6b) disposed between the inlet and outlet pinch rolls (5 and 7) in order to control the degree of partial shrink of the film passing over the bubble, followed by cooling. Adjustment of the pressure in the bubble is effected by moving the pinch rolls (5) towards and away from the shrink-controlling circling members (6a, 6b), in response to the sensor (3) detecting changes in width of the flat-folded heat-shrinkable tubing (1) coming from a supply roll (2) to be partially shrunk. <IMAGE>

Description

SPECIFICATION Improvements relating to heat shrinkable materials The present invention relates to the production and/or use of heat shrinkable tubular plastics film, and provides a means for effecting controlled shrinking of the film before use, for example in order to adjust the shrink potential of the film to a desired uniform value or to adjust the diameter which the tubular film has in its inflated state.

It is known to provide heat-shrinkable plastics film in the form of tubing which has been inflated while heated to its orientation temperature, thereby allowing the expansion of the film to occur with the film retaining a memory of its pre-expansion configuration. Subsequent application of heat will release the shrink energy stored in the film, causing the film to shrink back towards its starting configuration.

It may be desirable later to modify the shrink potential of the film, or the diameter it has in the inflated state, thereby enabling a uniform stock of a relatively high shrink material to be converted into various different batches of different shrink potential. Furthermore, if any variations in the shrink potential occur cyclically during the manufacturing process, it may be desirable to even out these variations to provide a much more uniform shrink potential along a continuous length of heat-shrinkable tubing.

One end use for heat-shrinkable plastics tubing which must have a constant "inflated'' diameter and/or a constant shrink potential is in the packaging field where, for example, the tubing may be welded transversely at regular intervals and severed to define end-sealed plastics bags of heat-shrinkable nature. Such bags can be used for enclosing a product under vacuum or with a slight residue of an inert or other conditioning gas, and subsequently heat shrunk to provide a tidy finished package.

We now provide apparatus for effecting controlled shrinking of heat-shrinkable tubing comprising means for inflating such tubing and passing it through an encircling member which contacts the exterior of the film to define a reduced diameter configuration for the film bubble, means for confining the inflated bubble to either side of said encircling member, means for applying shrinking heat to the bubble approaching or contacting said encircling member, and means for subsequently applying cooling to terminate the shrink process when a desired degree of shrinkage has been attained.

Subsequently the film can be rolled flat between pinch rolls and can be stored in any suitable form, for example by being reeled up.

Advantageously the encircling member is of tapering form to accept the inflated bubble before shrinkage and to progressively reduce the available space for accommodating the exterior of the bubble during the partial shrinking operation which takes place up to the application of shrink-arresting cooling.

Particularly conveniently, means may be provided for adjusting the length of the bubble approaching said encircling member in response to variations in the width of the tubing before inflation.

A further aspect of the present invention provides a method of adjusting the shrink potential of heat-shrinkable tubing, or the diameter it has in the inflated state, comprising taking flat-folded shrinkable tubing, inflating it to form a bubble, confining the external periphery of the bubble while applying heat to the bubble to cause it to undergo partial shrink, and applying cooling to arrest the shrink at a desired value.

Advantageously the length of the inflated bubble may be adjusted in order to compensate for variations in the width of the starting flat-folded tubular film material.

In order that the present invention may more readily be understood the following description is given, merely by way of example, with reference to the accompanying drawings in which the sole Figure is a side elevational, partly schematic, view of apparatus for adjusting the shrink potential of flat-folded heatshrinkable tubular film, in accordance with the present invention.

In the drawing, a continuous length of flatfolded heat-shrinkable tubing 1 is paid out from a supply roll 2 and passes a sensor 3 on its way to a static pair of pinch rolls 4 The sensor 3 measures the width of the flatfolded film and provides a signal indicative of variation in the width of that film.

The flat-folded film 1 then passes to a movable pair of pinch rolls 5 which define the start of a bubble la in the film.

The periphery of the bubble reduces as the inflated tubing passes a shrink-controlling encircling member 6a, 6b from which the inflated bubble emerges with a reduced periphery as a finished bubble 1b to pass between a further static pair of pinch rolls 7 from which it emerges as flat-folded finished tubing 1c.

In the illustrated embodiment of the present invention the encircling shrink-controlling member is in the form of a convergent conical collar 6a which includes a surrounding heating jacket 8 for a heating medium to maintain the temperature of the conical collar at a desired value to prompt the desired partial shrinking of the film 1 passing therethrough.

A downstream portion 6b of the shrink-controlling collar is of generally cylindrical form and is cooled by coolant in coils 9.

The shrink-controlling member 6a, 6b is supported at 10.

The temperature of the heating medium flowing around the convergent part 6a of the collar is appropriate to achieve limited shrink to reduce the periphery of the bubble down from the dimensions of the pre-shrinking or inlet bubble 1a down to the corresponding di mensions of the post-shrinking or outlet bub ble portion ib. The reduction in diameter of the conical collar 6a is consistent with the mean amount of shrink expected.

It may be that the width of the tubing on supply roll 2 varies as a result of manufactur ing tolerances, and this could result in the infl ated bubble 1a varying in pressure, since the volume of air within the entire bubble trapped between the pinch rolls 5 and the the outlet pinch rolls 7 is substantially constant.

In order to compensate for any such variations, and thereby to keep the pressure of the gas bubble within the film to. a substantially constant value, the inlet pinch roll pair 5 defining the start of the inflated bubble portion 1a is movable towards and away from the shrinkcontrolling collar 6a, 6b under the influence of the tubing width variation sensor. Thus, for an extreme reduction of tubing width from a maximum value to a minimum value, the movable pinch roll pair 5 would be contrplled to move away from the shrink-controlling collar from the solid line position to the broken line position shown in the drawing. This movement is actuated by a ram 11 under the influence of a controller 12 connected to the tubing width sensor 3.

It will of course be appreciated that, whereas the width of the starting film material 1 may vary considerably as a result of manufacturing tolerances, the ability of the apparatus illustrated in the drawing to maintain a substantially constant pressure in the inlet bubble portion 1a and consequently a substantially uniform pressure in the outlet bubble portion 1 b which is dimensionally controlled by the diameter of the cylindrical collar portion 6b, the finished flat-folded film 1c has much greater uniformity of dimension than the starting material 1 coming from the tubing roll 2.

As indicated above, the method and apparatus in accordance with the present invention can be used for reducing the shrink potential from a maximum value of manufactured film on the tubing roll 2 to a desired value for a particular Application. This allows the conversion of a single type of starting stock tubing to be adjusted to several possible desired dimensions and shrink potentials.

Alternatively, as just described, the apparatus and method in accordance with the present invention enable the uniformity of dimension and shrink potential of heat shrinkable plastics #tubing to be improved during partial shrinking of the tubing.

In the above description, no detailed mention has been made of the method of starting the process, i.e. of inflating the bubble la, 1b.

Such bubble inflation is well known in the art of bubble orientation of heat-shrinkable plastics film and will be well within the ability of the skilled reader. For example, the stationary outlet pinch rolls 7 may be separated while an inflation nozzle is passed between them and into the interior of the film 1 to allow suffici ent inflation gas to be introduced into the bubble 1a, 1b. Then, once the inflation nozzle (not shown) is removed and the outlet pinch rolls 7 have been closed together, the bubble will be self-sustaining through the sealing effect of the inlet pinch rolls 5 and the outlet pinch rolls 7 confining the inflation gas to the bubble la, 1 b.

If, over the long term, the gas is lost from the bubble, then to some extent this can be accommodated by variation of the position of the movable pinch roll pair 5. Alternatively, the long term loss of gas may be adjusted by movement of the outlet pinch rolls 7, thereby maintaining the maximum scope for compensating movement of the inlet pinch rolls 5 under the control of the sensor 3.

Although the illustrated embodiment uses a heating jacket 8 to heat the convergent shrinkcontrolling collar 6a, using for example hot water or steam as the heating medium, the heating may instead be derived by using electric heating.

Claims

1. Apparatus for effecting controlled shrinking of heat-shrinkable tubing comprising means for inflating such tubing and passing it through an encircling member which contacts the exterior of the film to define a reduced diameter configuration for the film bubble, means for confining the inflated bubble to either side of said encircling member, means for applying shrinking heat to the bubble approaching or contacting said encircling member, and means for subsequently applying cooling to terminate the shrink process when a desired degree of shrinkage has been attained.

2. Apparatus according to claim 1, wherein the encircling shrink-controlling member is heated to apply the shrinking heat to the inflated film.

3. Apparatus according to claim 1 or claim 2, wherein said shrink-controlling encircling member has a cooled portion to define the finished dimensions of the partially shrunk, inflated film.

4. Apparatus according to either of claims 2 and 3, and including temperature control means associated with said shrink-controlling encircling member.

5. Apparatus according to any one of the preceding claims, wherein the shrink-controlling encircling member includes a first convergent portion defining the reduction in dimensions of an inflated film passing through the member, and a second cylindrical portion defining the finished dimensions of the partially shrunk but still inflated film.

6. Apparatus according to any one of the preceding claims, wherein said bubble-confining means comprise pinch rolls to either side of said shrink-controlling encircling member, means being provided for moving at least one of said sets of bubble-confining pinch rolls relative to the shrink-controlling encircling member.

7. Apparatus according to claim 6, wherein the bubble-confining pinch rolls at the upstream side of said shrink-controlling encircling member are movable; and including means responsive to the width of flat-folded tubing being supplied to said upstream pinch rolls, effective to control the spacing of said upstream pinch rolls from the shrink-controlling encircling member for controlling the inflation pressure within the bubble to improve the consistency of the pressure with time.

8. Apparatus according to any one of the preceding claims, and including means for releasably holding said shrink-controlling encircling member in position, to allow one such encircling member to be replaced by another of different dimensions, for changing the degree of partial shrink effected by the apparatus.

9. A method of adjusting the shrink potential of heat-shrinkable tubing, or the diameter it has in the inflated state, comprising taking flat-folded shrinkable tubing, inflating it to form a bubble, confining the external periphery of the bubble while applying heat to the bubble to cause it to undergo partial shrink, and applying cooling to arrest the shrink at a desired value.

10. A method according to claim 9, and including the step of varying the pressure within the inflated bubble, for controlling the degree of partial shrinkage in response to variations in width of the starting flat-folded heat shrinkable tubing.

11. A method according to claim 9 or 10, and including the step of varying the degree of partial shrinkage effected during the method, for changing the shrink potential of the finished partially shrunk film.

12. Apparatus for controlling the shrink potential of heat-shrinkable tubular film material, substantially as herein before described with reference to and as illustrated in the accompanying drawings.

13. A method of controlling the shrink potential of heat-shrinkable tubing, substantially as hereinbefore described with reference to the accompanying drawing.