Traditionally cartons and similar packages have been made from paperboard or similar paper based materials. However, there is now a move towards the use of plastic sheet materials in such cartons.

Paperboard cartons typically comprise a series of crease lines which allow the sheet material to be erected into the finished shape. In traditional paperboard cartons, these fold lines are created using a rule which deforms the sheet material to form the crease line. The creased blank is then erected, either manually or by machine, into the carton. Such methods have not, however, been successfully employed on plastic sheet materials to date. It is more usual in plastic sheet materials to form a hinge line ultrasonically, or by heat creasing. However, these are expensive and complicated procedures.

Accordingly, the present invention seeks to provide a simplified method for forming crease lines in sheet plastics material.

From a first aspect, therefore, the present invention provides a method of cold forming a crease line in a sheet plastics material comprising the steps of: a) positioning the sheet material over a planar platen surface;

b) forming the crease line by bringing a rule into engagement with the plastic sheet material and deforming the sheet material between the end of the rule and the platen surface, the rule having a main body portion and a tip portion, the tip portion having a width of less than 60% of the main body portion.

The invention also extends to creasing apparatus for creasing a plastics sheet material, comprising a planar platen surface and a rule, which in use is brought into engagement with the plastic sheet material to deform the sheet material between the end of the rule and the platen surface, the rule having a main body portion and a tip portion, the tip portion having a width of less than 60% of the main body portion.

It has been found that by using a rule having a reduced width tip, a relatively high force can be applied to the rule without it buckling or deforming while at the same time a satisfactory deforming force can be applied to the plastics material to neck out the plastics material to form a very satisfactory crease line.

Preferably the width of the tip is 40%-60% of the width of the main body of the rule, most preferably about 50%. In a preferred embodiment, a so-called lpt rule (having a main body width of 0.7mm) is adapted to the invention by narrowing its tip to pt (0.35mm) The tip region may be square, but preferably it is rounded. It has been found that by using a round ended rule, there is an inverse linear relationship between the final crease strength and the force applied to the rule. This allows the crease strength to be very accurately set in the creased sheet. Thus it is easy to produce a crease having a relatively low crease strength, which means that the plastic sheet may be formed more easily into a desired shape.

The tip may be circularly rounded, or shaped as an

ellipse or some other curved surface.

From a further aspect, the invention therefore provides a method of cold forming a crease line in a sheet plastics material comprising the steps of: a) positioning the sheet material over a planar platen surface; b) forming the crease line by bringing a rule into engagement with the plastic sheet material and deforming the sheet material between the end of the rule and the platen surface, the rule having a main body portion and a tip portion, the tip portion having a rounded end.

The tip region preferably tapers from the main body portion of the rule, preferably symmetrical.

From a further broad aspect, the invention provides a method of cold forming a crease line in a sheet plastics material comprising the steps of: a) positioning the sheet material over a planar platen surface; b) forming the crease line by bringing a rule into engagement with the plastic sheet material and deforming the sheet material between the end of the rule and the platen surface, the rule having a main body portion and a tip portion, the tip portion tapering from the main body portion to a rounded end.

In the preferred embodiment, the rule tapers from the main body portion to a circularly rounded tip portion whose diameter is 40-60% and most preferably 50% that of the width of the main body portion.

The rule is preferably pushed into the plastics sheet to a depth of between 50% and 900 of the sheet thickness, more preferably over 75% of the sheet thickness.

Preferably the plastics sheet material is 300 to 800 microns thick, and the thickness of the material is reduced to between 100 and 150 microns in the crease.

The plastics material is preferably creased in traditional paperboard creasing machinery. This machinery comprises a platen against which is pressed a die board. The die board mounts cutting blades and creasing rules such that when the board is pressed against the platen, the blades cut through the material in the desired pattern and the creasing rule presses into the material by a predetermined amount to form the creases. The back surface of die board is acted on by a press to effect the cutting and creasing.

Preferably, so-called patching tape is applied to the back surface of the die board so that the cutting and creasing force is properly distributed across the board, thereby achieving the desired depth of cut or crease. This tape is applied to selected areas of the die board behind the respective cutting and creasing rules.

From a further broad aspect, the invention provides a method of creasing a plastics sheet material in an apparatus comprising a platen, a die board, and a press acting on the die board, the die board mounting a creasing rule, wherein a force compensation member is applied between the die board and the press in the region of the creasing rule.

The patching tape applied may be at least 40 microns thick, in some cases over 70 microns thick.

It has been found, however, that when creasing plastic sheet material, the depth of crease is much more critical than in paper creasing, and greater control of the creasing force applied is needed. In a preferred embodiment, patching tape of less than 25 microns, more preferably 20 microns or less may be used. In this way a much greater level of control can be exercised over the creasing force applied leading to a more consistent crease strength.

The force compensation member may be in the form of a tape, such as a non metallised plastics foil material.

Preferably the material is in the form of a tape.

In a typical creasing operation more than one crease will be formed, and the force compensation material may be applied in a suitable pattern to achieve the appropriate creasing force distribution.

It has been found that relatively soft plastic material such as polypropylene can be creased very effectively using a process as discussed above.

By using the invention, a creased sheet is produced whose crease have a relatively low, and consistent, crease strength, typically in the range 400-900 mN.

This makes such sheets particularly suitable for use in automatic erection apparatus which requires blanks with relatively low crease strengths to ensure well defined corners and flat panels.

From a further aspect, therefore, the invention extends to a method of erecting, preferably machine erecting, a carton from a plastics sheet material wherein the blank for forming the carton is creased in accordance with the invention.

The invention also extends to a plastics sheet creased in accordance with the invention and a carton erected from such a sheet.

Some embodiments of the invention will now be described by way of example only, with reference to the examining drawings in which : Figure 1 shows apparatus for creasing plastic sheet material in accordance with the first embodiment of the invention; Figure 2 shows the profile of the crease formed by the apparatus of Figure 1; Figure 3 shows, schematically, apparatus for forming plastic sheet in accordance with the invention; and Figure 4 shows a detail of the apparatus of Figure 3.

With reference to Figure 1, creasing apparatus

comprises a platen 2 and a steel rule 4. The platen 2 has a planar upper surface 6 while the rule 4 has a main body portion 8 and a tip portion 10. The tip portion 10 of the rule 4 is rounded, as shown.

More particularly, the tip portion 10 is in the shape of a circular arc 12 which tapers downwardly from the main body portion 8 of the rule 2 through sides 14.

The sides 14 blend tangentially into the circular arc 12.

In this particular embodiment, the main body portion 8 of the rule has a width W1 of 0.7 mm while the diameter D1 of the tip portion 10 is 0.35 mm. The total length L of the tip region 10 is 2.5 mm. The overall length of the rule 4 is 23.65 mm.

The rule 2 is used to form a crease line 16 in a sheet 18 of a plastics material such as polypropylene.

Typically the thickness of the plastic sheet 18 is 300 to 800 microns.

In order to produce the crease 16, the rule 4 is pressed downwardly into the sheet material 18. As shown in Figure 3, one or more creasing rules 4 are mounted in a die board 20 together with one or more cutting blades 22 which extend slightly below the rules 4. A creasing and cutting force is applied to the die board by a press 24.

The force with which the rule is applied is chosen so as to give a desired crease strength in the crease 14. It has been found that by using a rounded rule tip 8 the strength of the crease 14 is almost precisely inversely proportional to the force applied. Thus a wide range of crease strength can be easily achieved.

Typically the rule 4 will be pushed into the sheet to between 50% and 90%, most preferably over 75%, of the thickness of the sheet 16. This can be set by an appropriate offset of the rule 4 with respect to the cutting blades 22 or some other reference member which will engage the surface 5 of the platen 2. Preferably

the material in the crease is compressed so as to leave a depth Du ouf material between 100 and 150 microns.

In order to correctly distribute the creasing force over the creasing rule, it may be necessary to apply a force correction tape or the like to the back surface 26 of the die board 20.

As shown in Figure 4, the back surface 26 of the die board is marked up to show the outline 28 and creases 30 in the blank being cut and creased.

Typically this is done by marking the outline 28 and creases 30 on a sheet of paper which is stuck to the back surface 26, aligned with the respective cutting blades 22 and creasing rules 4.

Strips of force correcting tape 32 are selectively placed over the crease lines 30, where needed, in order that the creasing force and depth is consistent over the blank. Generally, correction tape 32 will need to be applied to the majority or all of the crease lines 30.

In areas where the tape 32 is applied, a greater force will be produced.

The correction tape 32 is typically a dense but tearable paper or a non-metallised plastics foil and at least some of the tape used is preferably less than 25 microns thick, more preferably less than 20 microns.

Thicker tape may be used for a coarser adjustment, if required.

Using the above described techniques, it has been found that plastics sheets can be produced to have crease strengths of 400 to 900 mN compared to 1500 or more mN of earlier creased plastics products. For the purposes of this application, the crease strength is that measured in a Messmer crease tester when a standard 38mm wide segment is folded through an angle of 60° with the force acting at a distance of 10 mm from the crease.

Such equipment and measurements are standard in the industry and need not be explained in detail here.

A sheet creased in accordance with the invention

can be erected into a suitably shaped carton or merely just a folded component. Due to the relatively low crease strengths obtained using the method and apparatus of the present invention, the creased sheet is particularly suitable for erection into cartons using automatic machinery, producing cartons with crisp corners and flat panels.