GB2281242A

GB2281242A - Rupturable sheet materials

Info

Publication number: GB2281242A
Application number: GB9317927A
Authority: GB
Inventors: Michael W Seitz
Original assignee: ASSI PACKAGING UK Ltd
Current assignee: ASSI PACKAGING UK Ltd
Priority date: 1993-08-26
Filing date: 1993-08-26
Publication date: 1995-03-01
Anticipated expiration: 2013-08-26
Also published as: GB9317927D0; GB2281242B

Abstract

Sheet material 12, 14, 16 is provided with a line of apertures (24, 26 Figs. 4 - 6) or perforations forming a line of weakness along which the material is designed to rupture. In order to compromise the strength of the sheet to a lesser extent, the widths of the apertures decrease with increasing aperture depth. A perforation rule 18 having triangular teeth may be used to form the apertures. Alternative teeth profiles are also disclosed (Figs. 3a - e). The apertures may be used in combination with tear tapes (Figs. 4 - 6). The sheet material may be corrugated. <IMAGE>

Description

RUPTURABLE SHEET MATERIALS This invention concerns sheet materials designed to rupture along a line of weakness, a perforating rule and its method of use in forming such rupturable sheets.

It is well known to form a rupture line as a series of perforations in a sheet of material. One method of forming a line of parallel sided elongate perforations is to use a castellated or rectangular-toothed perforating rule, in either a flat bed or a rotary die cutter.

Where the material is corrugated cardboard forming a tear-open carton, a tear tape may be provided on the side of the sheet facing the perforating rule, this side becoming the inside of the carton. The perforations are made through the tape and cardboard. In use the tape will rupture to form a straight edge aligned with the perforations in the cardboard and which will thereby shear through the cardboard efficiently. Positioning of the perforations relative to the tape is not absolutely critical: they may fall anywhere within that proton of the width of the tape which gives a tear strip of sufficient width not to break in use. Where the perforations are close to an edge of the tape, the entire tape may be pulled through the cardboard without rupturing.

A tear tape of this type is disclosed in our U.K. Patent No.

2233316.

Although the material between such perforations will shear relatively easily and neatly when rupture is wanted, the sheet can be undesirably weakened against other forms of loading. For example the strength of corrugated cardboard in a box compression test may be reduced considerably by the presence of the perforations. A tear tape can in some instances help to reinforce the sheet if present, but significant weakness may still remain.

The present invention aims to compromise the strength of the sheet to a lesser extent than hitherto and accordingly provides a rupture mechanism comprising a line of apertures formed in a sheet of material, wherein the widths of the apertures measured in the direction of the line decrease with increasing aperture depth, and/or the apertures have blind ends. Preferably the decrease in width is substantially linear with variation in aperture depth. For example the apertures may be triangular or trapezoidal in plan (that is, in the sheet cross-section along the line of weakness).

The apertures may extend completely through the thickness of the sheet to form perforations having slot-like mouths at one face of the sheet and point-like mouths at the other sheet face. The pitch of the perforations may be between 1 and 3 times, preferably twice, the length of each slot-like mouth.

Advantageously, a tear tape or cord is fixed to one side of the sheet to coincide substantially with the line of apertures. Where the sheet is corrugated cardboard comprising an inner liner, fluting and an outer liner, applying the tape or cord to the face of the sheet furthest from the perforation rule has the advantage that a particularly clean outer edge to the rupture is formed, as the apertures are then of greatest width in the outer liner.

The invention also provides a perforating rule used in forming a rupture mechanism as described in the preceding three paragraphs comprising tapering teeth pitched at between 1 and 3 times, preferably twice, the thickness of the sheet of material. The teeth are preferably triangular.

The teeth may be centre bevelled, or where a square cut normal to the line of weakness is desired in thicker materials, a double long side bevel may be used. The perforation rule is preferably of thinner material than is conventionally used to form rectangular perforations, e.g. 2 pt or less.

The invention also provides a method of forming a rupture mechanism in a sheet of material comprising the steps of: fixing a tear tape or cord to one surface of the sheet and pressing a perforation rule as described in the preceding two paragraphs through the thickness of the sheet along a line substantially coincident with the tape or cord.

The perforation rule may be pressed towards the tear tape or cord from the opposite side of the sheet, advantageously with the tips of the teeth being pressed through both surfaces of the sheet.

Illustrative embodiments of the invention are described below with reference to the drawings, wherein: Fig. 1 schematically shows on an enlarged scale part of a preferred perforating rule and an associated sheet of corrugated cardboard being cut; Figs. 2a and 2b show alternative perforating rule tooth edge profiles; Figs. 3a-3e show alternative perforating rule tooth shapes; Fig. 4 is a perspective view from the direction of its outer liner of a sheet of corrugated cardboard provided with a rupture mechanism according to a first embodiment of the invention; Fig. 5 is a perspective view from the direction of its inner liner of a second embodiment of the invention incorporating a tear tape; and Fig. 6 is a view similar to Fig. 5 showing a third embodiment of the invention, incorporating two tapes.

Fig. 1 shows in section a sheet of corrugated cardboard, comprising an outer liner 12, an inner liner 14 and fluting 16. The perforating rule 18 includes a regular series of generally triangular teeth 20, having flattened tips 22 designed to pass through both faces of the cardboard sheet, to produce point-like holes in the inner liner 14. If desired, sharp points may be used in place of the flattened tooth tips, producing smaller point-like apertures. The teeth pass through the outer liner 12 to form slot-like holes. Where the teeth pass through the fluting 16 it is correspondingly cut, the width of the aperture so formed decreasing proportionally from the outer liner 12 to the inner liner 14. As illustrated, the sheet thickness t is 3.0 mm, the tooth pitch p 6.0 mm, the tooth flank angle a 45.2- and the tooth tip width x 0.4 mm.

Figures 2a and 2b show alternative tooth edge profiles: the centre bevelled profile of Figure 2a being suitable for general cutting purposes and the double long side bevelled profile of Figure 2b being preferred with thicker materials to produce a square cut finish.

The alternative tooth shapes of Figs. 3a-3e are as follows, the dotted lines representing the material faces when the cutting rule is pressed fully into the sheet: Fig. 3a: similar to Fig. 1 but with a larger tooth flank angle. The flank angle and perforation pitch are chosen on the basis of the thickness of the sheet, the desired ratio of cut to uncut material in the plane of the perforations, and the desired ratio of maximum to minimum aperture width.

Fig. 3b: similar to Fig. 3a but with deeper teeth to produce slot-like aperture mouths at the face of the material nearest the rule and shorter slot-like aperture mouths at the opposite face.

Fig. 3c: a tooth having concave flanks which again produces a slot-like aperture mouth in one material face and a point-like aperture mouth in the other, but which severs less material than a triangular tooth of similar root dimension and therefore weakens the sheet to a lesser extent.

Fig. 3d: similar to Fig. 3c but with deeper teeth to produce short slot-like aperture mouths in place of the point-like aperture mouths.

Fig. 3e: a short tooth profile which does not perforate the sheet, instead producing blind apertures.

Fig. 4 is a schematic illustration of a corrugated cardboard sheet having apertures formed by the perforating rule of Fig. 1. A complete aperture is shown at 24 whereas the aperture at 26 is shown in half section. The apertures 24, 26 have slot-like mouths 28, 30 at the outer liner 12 and point-like mouths 32, 34 at the inner liner 14. The positions reached by the tooth flanks when pressed fully home are denoted by the dotted lines 36. Since the apertures 24, 26 occupy only a relatively small proportion of the sheet cross-section along the rupture line, the strength properties of the sheet are not compromised as greatly as with conventional perforations.

As shown in Fig. 5, the rupture mechanism may include a tear tape 38 applied to the inner liner 14. Tape 38 is of the multifilament or oriented polymer (e.g. polypropylene) type, comprising longitudinally arranged polymer chains or fibres of relatively high longitudinal tensile strength, but relatively weak transversely. At one end (not shown) part of the tape 38 lying to one side or the other of the line of apertures 24, 26 is provided with a pull tab, by which it may be drawn through the sheet in the conventional way. The point-like mouths 32, 34 of apertures 24, 26 lie within that half of the width of the tape 38 lying furthest from the pull tab to produce a tear strip strong enough not to break readily in use.The slight weakening caused by the presence of the apertures 24, 26 in the tape 38, in combination with the guiding effect of the remainder of the apertures 24, 26 that lie within the sheet, causes the tape 32 to separate neatly along its length. The presence of the slot-like aperture mouths 38, 30 causes the outer liner 12 to rupture cleanly.

It can be seen from figure 5 that the lines 36 denoting the penetration limit of the teeth 20 of the perforating rule are imaginary. The teeth 20 form cuts such as 39, 41 in the fluting.

To promote rupture of the tape 38, or when using corrugated sheet forming machinery in which the perforating and printing equipment lie on opposite sides of the web, it is also possible to form the apertures from the inner liner 14 (that is through the tape 38 first) through to the outer liner 12. However, this produces a less satisfactory edge to the outer liner 12 following rupture, as only the pointlike aperture mouths 32, 34 lie in the outer liner 12.

The rupture mechanism shown in Fig. 6 has the apertures oriented with their slot like mouths 28, 30 at the inner liner 14 and tear tape 38. To produce a neat edge to the severed outer liner 12, a reinforcing tape 40 is provided, having one edge aligned with the point-like aperture mouths 32, 34 as shown, so as to co-operate with the upper part of the tear tape 38 and sever the sheet with a scissors action.

Alternatively, if tape 40 is of multifilament or oriented polymer type, the point-like aperture mouths can lie substantially anywhere within its width. Tape 40 may be fixed to the outside of the outer liner 12 as shown or can be fixed between the outer liner 12 and the fluting 16.

In a box compression test for corrugated cardboard cases of the following specification, the reduction in strength caused by the use of perforations as shown in figure 5 was found to be about half of the corresponding reduction caused by perforations formed using a standard castellated perforation rule.

Case construction : 0201 FEFCO standard Case size (mm) : length 290 x width 140 x height 193 Board grade : 190 g,m'2 white top test / 112 g,m'2 "B" flute / 175 g.m 2 test liner Location of : 45 mm from bottom of case perforation line Loading axis : Vertical

Claims

CLAIMS 1. A rupture mechanism comprising a line of apertures formed in a sheet of material, wherein the widths of the apertures measured in the direction of the line decrease with increasing aperture depth, and/or the apertures have blind ends.
2. A mechanism as claimed in claim 1 wherein the decrease in width is substantially linear with variation in aperture depth.
3. A mechanism as claimed in claim 1 or 2 wherein the apertures form perforations having slot-like mouths at one face of the sheet and point-like mouths at the other face of the sheet.
4. A mechanism according to claim 3 wherein the pitch of the perforations is between 1 and 3 times, preferably twice, the length of each slot-like mouth.
5. A mechanism according to any preceding claim wherein a tear tape or cord is fixed to one face of the sheet to coincide substantially with the line of apertures.
6. A mechanism as claimed in claim 5 wherein the apertures are of greatest width at the face of the sheet opposite to the tear tape or cord.
7. A perforating rule used in forming a rupture mechanism according to any preceding claim, comprising tapering teeth pitched at between 1 and 3 times, preferably twice, the thickness of the sheet of material.
8. A perforating rule as claimed in claim 7 wherein the teeth are substantially triangular.
9. A perforating rule as claimed in claim 7 or 8 of 2 pt or less thickness.
10. A perforating rule as claimed in claim 7, 8 or 9 wherein the teeth are centre bevelled.
11. A perforating rule as claimed in claim 7, 8 or 9 wherein the teeth are double long side bevelled.
12. A method of forming a rupture mechanism in a sheet of material comprising the steps of: fixing a tear tape or cord to one surface of the sheet and pressing a perforation rule as claimed in any of claims 6 to 9 through the thickness of the sheet along a line substantially coincident with the tape or cord.
13. A method as claimed in claim 12 wherein the perforation rule is pressed towards the tear tape or cord from the opposite side of the sheet.
14. A method as claimed in claim 12 or 13 wherein the tips of the teeth are pressed through both surfaces of the sheet.
15. A rupture mechanism substantially as described with reference to or as shown in Figs. 3a-e, 4, 5 and 6 of the drawings.
16. A perforation rule substantially as described with reference to or as shown in Figs. 1, 2a, 2b and 3a-e of the drawings.
17. A method of forming a rupture mechanism in a sheet of material, substantially as described.