GB2442509A - Shrinkable material having a pattern of indicia for assessing the amount of shrinkage - Google Patents

Abstract

A pattern of indicia at a predetermined spacing such as stripes 4, 6 or circles 8 is applied to a shrinkable material which may be formed as a sleeve and is preferably shrunk by the application of heat, e.g. a heat shrink sleeve. Upon shrinking of the material the pattern undergoes change and by assessing the dimensional change in the pattern before and after shrinking it is possible to determine the amount of shrinkage in the material. For example the heat shrink sleeve may be placed around a collection of articles such as electric cables, shrunk in place, and then the pattern assessed to ensure adequate shrinking has occurred. For example the spacing between the stripes or lines 4, 6 may be measured. A non-destructive in-situ visual guide for assessing shrinkage is provided.

Description

HEAT SHRINK MATERIAL

This invention relates to a method of applying a heat shrunk covering, including a step of determining the extent of shrinking of the covering film.

Heat shrink materials are used for a variety of purposes, including encapsulation protection, packaging (in the form of shrink-wrapped packing) and sleeving for cables and similar components. The present invention will be described with particular reference to the sleeving of bundles of wires or cables in electrical wiring harnesses, but it will be appreciated that the principles underlying the present invention can be applied also to other applications in which shrinkable materials and in particular, heat shrink materials are used.

Heat shrink materials commonly comprise stretched polymer films. The stretching causes the long-chain molecules of the polymer to be straightened. When the materials is heated, the polymer molecules relax, causing the material to shrink or "recover".

In some applications, for example when heat shrink sleeving is used on electrical harnesses in, for example, gas turbine engines, it is important for the sleeve to be shrunk evenly. As the sleeve shrinks radially, the wall thickness of the sleeve increases, and consequently, if inadequate shrinkage occurs, the sleeve wall remain thin in some regions. The thinner regions are relatively weak, and may be unable to withstand the stresses that arise as a result of the conditions to which the sleeve is exposed. For example, in an aircraft engine, the sleeve may be exposed to heat and to pressure effects as a result of changes in altitude. The inadequately shrunk regions may burst, leaving cables unsupported and consequently exposed to possible damage.

Furthermore, a failed sleeve causes a very visible quality problem.

Shrink sleeves for electrical wiring harnesses are commonly available in different diameters so that the sleeve can be matched to the number and thickness of wires or cables of the limb of the wiring harness that is to be accommodated. The wall thickness of the material of the unshrunk sleeves tends to be constant over the full range of sleeve diameters. The unshrunk sleeve diameters may typically range from approximately 5 to 50 mm, although sleeves with diameters outside this range are available. The problems arising from inadequate shrinking of the sleeves is more severe as the sleeve diameter increases, because higher pressure differentials arise when shrinking the larger diameter sleeves as they are shrunk over a harness limb of appropriate size, giving rise to increased stresses in the sleeve wall.

The wall thickness of a sleeve that has been shrunk around a harness limb cannot be determined easily without destroying the sleeve. Consequently, it is difficult to establish, by visual inspection, whether or not a sleeve has been shrunk sufficiently over its full extent.

US 4505218 discloses a tamper-indicating seating arrangement which may be applied to an item such as a valve. Two layers of shrinkable tube are applied to the item, the two layers being transparent, but having marking patterns on them. After shrinking, both marking patterns are visible from the exterior, and together form a unique pattern which, in practice, cannot be reproduced if the seal formed by the shrinkable tubes is removed. The shrinkable tubes do not serve as any form of support for the item which they cover, and consequently it is immaterial whether or not all parts of each tube are shrunk to an adequate extent.

According to the present invention there is provided a method of applying a shrunk covering, the method comprising: (i) placing a shrinkable material over an article or a collection of articles, the unshrunk material being provided with a pattern of indicia at predetermined spacings; and (ii) shrinking the material; characterised in that the method further comprises: (iii) determining the extent of shrinking of the material by assessment of a characteristic of the pattern after shrinking.

The characteristic of the pattern which is assessed to determine the extent of shrinking of the material may be a dimension of one or more of the indicia. Alternatively, or in addition, the characteristic may comprise the space between at least two of the indicia.

The characteristic may be assessed by direct dimensional measurement performed by eye.

The shrinkable material may be a material that is shrinkable under the application of heat. For example, it may be a stretched polymer material.

Another aspect of the present invention provides a heat shrinkable material for use in a method as defined above, the material being provided with the pattern of indicia. The material may be formed into a sleeve suitable for application over a bundle of wires or cables forming a limb of a wiring harness.

The pattern of indicia may comprise stripes. If the shrinkable material is in the form of a sleeve, the stripes may extend axial or circumferentially of the sleeve. In one embodiment, the pattern comprises both axially and circumferentially extending stripes.

In an alternative embodiment, the sleeve may be provided with axially extending stripes, with further indicia provided at axial intervals along the sleeve. The further indicia may comprise interruptions in the stripes at predetermined spacings.

The stripe width may be substantially equal to the spacing between adjacent stripes.

The stripe width andlor the spacing between adjacent stripes may be not less than 4mm.

A set of shrinkable sleeves of different diameter may be provided, so that an appropriate sleeve can be selected for application over a particular collection of articles, such as a bundle of wires or cables. In such a set, the stripe width and stripe spacing may be equal over all of the sleeves of the set.

Another aspect of the present invention comprises the use of a pattern of indicia at predetermined spacings on a shrinkable material as an aid to an assessment of the extent of shrinkage of the material.

For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:-Figure 1 shows a first embodiment of a heat-shrinkable sleeve, both before and after shrinking; Figure 2 corresponds to Figure 1, but shows an alternative heat-shrinkable sleeve; Figure 3 represents the sleeves of Figures 1 and 2 shrunk to an acceptable extent; and Figure 4 corresponds to Figure 3 but shows irregular shrinking of the sleeves.

The sleeve 2 shown in Figure 1 is made from a suitable heat-shrinkable polymer, such as a polyolefin. A pattern of indicia is applied to the sleeve by a suitable printing process. In the embodiment of Figure 1, the indicia comprise axial stripes 4 and circumferential stripes 6. The stripes 4 have the same width as each other, and the spacing between adjacent stripes 4 is consistent around the circumference of the sleeve. Similarly, the circumferential stripes 6 have the same width as each other arid adjacent circumferential stripes 6 are spaced regularly in the axial direction. As iflustrated in Figure 1, the widths of the stripes 4 and 6 are somewhat smaller than the spacing between adjacent stripes. However, in other embodiments, the widths of the stripes may be the same as or greater than the spacing between them.

The top part of Figure 1 shows the sleeve in an unshrunk condition. The lower part of the Figure shows the sleeve in the shrunk condition. Although not shown in the drawings, the unshrunk sleeve would normally be fitted over a bundle of wires or cables forming a limb of a wiring harness, and then shrunk by the application of heat in order to hold them together and to protect them from damage. The sleeve may be cut to an appropriate length from a continuous length of sleeving, or provided as pre-cut lengths.

A typical shrink ratio for heat shrink sleeves is 100%, so that a sleeve having a diameter of 50 mm will reduce to a diameter of 25 mm when fully recovered. However, the shrink ratio will depend on various factors, including the overall diameter of the bundle of wires or cables to be accommodated. Consequently, for adequate performance, 80% recovery may be specified or, in some circumstances, recovery down to a minimum of 50% may be acceptable.

As will be appreciated from Figure 1 that, as the sleeve shrinks, the stripes 4, 6 and the spacing between them decreases. Consequently, the stripe widths and spacings can be assessed to determine the extent to which the sleeve has shrunk. In many cases, simple inspection by eye will be sufficient to establish whether or not adequate shrinkage has occurred. Alternatively, a suitable measuring instrument may be used either to assist visual inspection or to provide an automatic read-out of acceptability.

In the context of bundling wires or cables in wiring harnesses, the reduction in diameter caused by shrinkage is usually more important than any reduction in axial length of the sleeve. Consequently, in some circumstances the circumferential stripes 6 can be omitted, so that the pattern of indicia comprises only the axial stripes 4. Nevertheless, even if only the axial stripes 4 are provided, it may be beneficial to include some form of axial marking at predetermined regular intervals along the stripes 4 to assist in determining the axial recovery of the sleeve. This marking may comprise additional printed marks on the sleeve, or it may comprise regular interruptions in the stripes 4.

Sleeves 2 may be provided in different diameters, for accommodating differently sized bundles of wires or cables. It is desirable that the widths of the stripes 4 and 6, and the spacing between them, are consistent acrosè the full range of sizes. It is also desirable, to aid visibility, for the pattern to be as large as possible. That is to say, the width of the stripes 4, 6, and the spacings between them, should be sufficiently large so that they can be determined clearly, even after shrinkage of the sleeve.

Taking these factors into account, one possibility is for the smallest unshrunk sleeve size, for example having a diameter of 6 mm and a circumference of approximately mm, to have two stripes 4, each having a width ofapproximately 5 mm, disposed diametrically opposite each other. There would thus be a spacing of approximately 5 mm between the two stripes 4 and both the stripe width and the stripe spacing would reduce to approximately 2.5 mm at 100% recovery.

The same stripe width and spacing would be applied to sleeves of other diameters in the range. For example, a sleeve having a diameter of 50 mm with a circumference of approximately 160 mm, would have 16 axial stripes 4, each with the same width of 5 mm and the same spacing of 5 mm. The larger number of stripes 4 on a larger diameter sleeve provides a higher "resolution", in that the overall total shrinkage of two or more stripes 4 and intervening spaces can be determined. This additional accuracy for larger diameter sleeves is an advantage given the higher stress levels that occur in such sleeves. Alternatively, the smaller diameter sleeve could have only a single stripe so that the 50 mm diameter sleeve may have approximately 8 stripes. In such a case, each stripe would have a width of 10 mm, and the spacing between adjacent stripes would also be 10 mm. Stripes and spacings of large width are easier to measure.

Figure 2 shows an alternative pattern of indicia on the sleeve 2. In Figure 2, the indicia comprise regularly spaced dots 8. The dots are all of the same size and configuration, and the spacings between adjacent dots in the axial and circumferential directions are equal. The dots may comprise simple block circles, but other shapes may enable easier measurement. The dots shown in Figure 2 are each shown as a pair of concentric circles. The region between the two circles may be filled in to provide a single circle with a wide perimeter.

Figure 3 is a diagrammatic side view of the shrunk sleeves as shown in the tower parts of Figures 1 and 2. It will be appreciated that the stripes 4, 6 and the dots 8 retain a consistent size and spacing, indicating that the sleeve has shrunk evenly over the length visible in Figure 3, and around the circumference. By measuring the thickness and spacing of the stripes 4, 6, and the size and spacing of the dots 8, it is possible to establish the % recovery of the shrunk sleeve. By contrast, Figure 4 shows an irregular distribution and size of the stripes 4, 6 and dots 8 along the length of the sleeves, indicating that, white adequate shrinkage has occurred towards the left-hand end of the sleeves 2 as shown, inadequate shrinkage has occurred towards the right-hand end.

While the uneven shrinkage shown in Figure 4 is clearly visible to the eye, it may be desirable to provide a measuring instrument to assist in assessing the extent of shrinkage. If, as mentioned above, the pattern size (in terms of width and spacing of the stripes 4, 6 and the size and spacing of the dots 8) is consistent over a range of sleeve diameters, then a common inspection tool can be provided. The inspection tool may comprise a flexible component which can be wrapped around a harness limb fitted with a shrunk sleeve. The component may be transparent, or may be provided with a window through which the stripes 4, 6 or dots 8 are visible, and graduations, for example at a spacing of 0.5 mm over a 10 mm extent, may be provided. The graduations could be marked as % recovery, rather than actual distance. With such a device, a 10 mm window would reveal two stripes 4 and two spacings of a 100% recovered sleeve having 5 mm stripes.

An alternative measuring device may comprise an instrument using barcode technology, capable of determining the thickness and spacing of a series of adjacent stripes. Such an instrument may be able to provide a direct read out of the % recovery.

In order to make an accurate assessment of % recovery over the full extent of a shrunk sleeve 2, a series of measurements on each sleeve is required. For example, measurements may be taken every 20 mm around the circumference of the sleeve, with a minimum of two readings on smaller diameter sleeves, and measurements may be taken at intervals of 150 mm along the axial length of the sleeve.

Technology already exists for marking heat shrinking sleeves, for example to apply data identifying the cable, or bundle of cables, to which the sleeve is fitted. Known techniques employ inks that fade in service. In many circumstances, the application of the stripes 4, 6 and dots 8 by such known techniques would not be a disadvantage, since the patterns applied to the sleeves 2 are used primarily to determine shrinkage of the sleeve immediately after application, so subsequent fading would not cause a problem.

Nevertheless, high temperatures inks are available that would remain visible for extended times, even under severe operating conditions with regard to temperature.

The use of such inks to provide the stripes 4, 6 and the dots 8 may be beneficial since they would enable determination of further recovery, or stretching, of the sleeves as a result of high temperature exposure, for example in an engine bay.

The sleeves 2 may be printed with the patterns of stripes 4, 6 and dots 8 in a variety of ways. It is known for heat shrinking sleeves to be supplied in the form of a ladder, with the cut sleeves in flattened form extending between parallel supports which can be adapted to enable the "ladder" to be fed through a printer in order to print identifying data. Printing of the patterns on both sides of the flattened sleeves would therefore be possible. Although this might affect the arrangement of stripes 4 and dots 8 over the surface of the flattened sleeve, it is likely that this effect would be relatively minor and would not affect the subsequent determination of % recovery.

Alternatively, the stripes 4, 6 and dots 8 may be printed continuously onto the sleeving as it is formed.

Claims (19)

S -10- CLAIMS

1 A method of applying a shrunk covering (2), the method comprising: (i) placing a shrinkable material over an article or a collection of articles, the unshrunk material being provided with a pattern of indicia (4, 6, 8) at predetermined spacings; and (ii) shrinking the material; characterised in that the method further comprises: (iii) determining the extent of shrinking of the material by assessment of a characteristic of the pattern after shrinking.

2 A method as claimed in claim 1, characterised in that the assessed characteristic comprises a dimension of at least one of the indicia (4, 6, 8).

3 A method as claimed in claim I or 2, characterised in that the assessed characteristic comprises the spacing between at least two of the indicia (4, 6, 8).

4 A method as claimed in any one of the preceding claims, characterised in that the characteristic is assessed by direct dimensional measurement.

A method as claimed in any one of the preceding claims, charactensed in that the shrinking of the material is achieved by heating the material.

6 A shrinkable material for use in a method in accordance with any one of the preceding claims, the material being provided with the pattern of indicia (4, 6, 8) at predetermined spacings.

7 Use of a pattern of indicia (4, 6, 8) at predetermined spacings on a shrinkable material as an aid to an assessment of the extent of shrinkage of the material.

8 A shrinkable material or use as claimed in claim 6 or 7, characterised in that the material is heat-shrinkable.

9 A shrinkable material or use as claimed in any one of claims 6 to 8, characterised in that the material is formed as a sleeve (2).

A shrinkable material or use as claimed in any one of claims 6 to 9, characterised in that the indicia comprise stripes (4, 6).

11 A shrinkable material or use as claimed in claim 10 when appendant to claim 9, characterised in that the stripes (4) extend axially of the sleeve.

12 A shrinkable material or use as claimed in claim 11, characterised in that the sleeve is provided with indicia (6) which are spaced apart axially of the sleeve (2).

13 A shrinkable material or use as claimed in claim 12, charactensed in that the axially spaced indicia are provided by interruption of the stripes (4) at predetermined intervals.

14 A shrinkable material or use as claimed in any one of claims 10 to 13, characterised in that the width of each stripe (4) is substantially equal to the spacing between adjacent stripes (4).

A shrinkable material or use as claimed in claim 14, characterised in that the width of each stripe (4) and the spacing between adjacent stripes is not less than 4mm.

16 A set of shrinkable sleeves of different unshrunk diameter, each sleeve being in accordance with any one of claims 11 to 15, characterised in that the width of the stripes (4) and the spacing between adjacent stripes (4) are equal over all of the sleeves (2) of the set.

17 A method substantially as hereinbefore described and/or as shown in the acccmpanying drawings.

18 A shrinkable material or use of a pattern of indicia substantially as hereinbefore described and/or as shown in the accompanying drawings.

19 A set of shrinkable sleeves substantially as hereinbefore described and/or as shown in the accompanying drawings.