Die-cutting problems: How to solve and avoid them – Part II
In the concluding part of our series of articles we address a few individual die-cutting challenges which require particularly intensive cooperation between the label manufacturer and the die supplier. At heart, the problems outlined here depend on the adhesive material to be die-cut, or rather on its composition. The face material, the adhesive and in particular the liner material, in themselves and in combination, can make very special demands on the user and the die-cutting tool.
Die-cutting of abrasive facestocks
In the die-cutting process, the flexible die is sometimes exposed to extreme stress. In time the tool will become worn, so that perfect diecuts become difficult or impossible. The wear is due partly to compression of the cutting tip, but mainly to abrasion of the cutting edges. The consequences of wear can be seen in an erosion of the height of the die, an increased cutting edge radius (i.e. rounding of the blade) and gouging of the cutting edges.
Very heavy wear effects arise in the die-cutting of abrasive materials and inks. These include for example, thermosensitive materials, highly pigmented flexographic inks (e.g. opaque white), fluorescent and metallic inks, screen printing inks as well as matt laminations and lacquers. The particles contained in such materials (e.g. silicon dioxide) sometimes act like sandpaper and leave pronounced gouges on the cutting edges.
To a certain degree the user can compensate for wear effects by increasing the cutting pressure or decreasing the clearance (cylinder gap). However, at some stage the point will be reached at which the face material can no longer be cleanly separated and the flexible die must be changed. With particularly abrasive materials sometimes only a few hundred die-cuts can be made with an untreated flexible die.
Maximum operating performance with coatings
Die manufacturers offer various finishing options for increasing the life of a die. Laser hardening has proved effective for stabilising cutters, as its computer-controlled laser process produces a hardness of 66-68 HRC in the tip of the blade. This protects the steel against embrittlement and fractures without impairing the height tolerance and flexibility of the die. There are many surface treatments and coatings in addition to laser hardening that help against abrasive wear.
Galvanic coatings (e.g. with nickel or chrome components) are relatively economical and considerably lengthen the life of flexible dies. Chrome coated flexible dies are used by many converters for the can also be combined with laser hardening (figure 1). However, the comparatively high layer thickness of galvanic coatings (several micrometres) can cause slight rounding of the blade with steep cutting angles. The cutting geometry might therefore need to be adjusted when die-cutting filmic materials.
Considerably thinner coating thicknesses of only around 1 μm can be achieved with DLC (diamond-like carbon) coatings, which nevertheless give the die an extreme hardness. This coating is employed for choice with abrasive filmic materials and for very long runs. Plasma-based processes (e.g. ion implantation) can also be used for hardening the die surface. The comparatively high cost of the nongalvanic processes contrasts with an enormous increase in operating performance which pays off in many cases.
In general no universally valid recommendations can be made for an optimum finishing variant. This decision must be taken by the user in close consultation with the die supplier, who alongside the application parameters will also take cost-benefit aspects into account.
Protection against extreme adhesives
It is not only the facestock material that can degrade the die-cutting quality; the adhesive used can sometimes also lead to considerable impairments in die-cutting. Particular problems are caused, for example, by extreme hotmelt adhesives based on thermoplastic rubber or UV acrylate, as well as multi-layer labels.
When die-cutting such materials and glues, viscous adhesive is deposited on the tools cutting edges, which become completely gummed up in the course of production and make perfect diecutting impossible. Adhesives leaking out of the label edges are a real problem for subsequent processing as well, as they seriously impair the dispensing procedure.
To increase the operating performance of the dies and minimise cleaning times, the use of a nonstick coating is unavoidable (figure 2). The non-stick coatings minimise adhesive and ink residues on the blades as well as on the plate surface, thus ensuring an uninterrupted production process. The flexibility and magnetic adhesion of the die remain completely unimpaired.
An important point when working with non-stick coated dies is that the tools must first be “run in” for a few revolutions to remove the coating from the blade edge. This only guarantees full functioning of the die. Moreover it should be borne in mind when cleaning nonstick coated tools that some cleaning agents may cause the coating to dissolve. In this instance users should always observe the supplier’s recommendations and check the compatibility of their cleaning agents.
Die-cutting on thin PET liners
The trend towards ever thinner PET liner materials is ongoing. While a few years ago the normal thickness was 30-36 μm, now 23 μm has become the standard, and even thinner liners are already being tested. The thin PET liners are usually combined with equally thin upper plastic materials (PE, PP, MDO), although paper print substrates are also possible. The advantages put forward by the manufacturers lie principally in cost efficiency (e.g. fewer roller changes, lower transport and storage costs, recyclability), comparatively higher stability and better printing and processing characteristics.
Given the numerous advantages it is no wonder that more and more users are opting for thin PET liners. Nevertheless they present a real challenge in the case of diecutting. To understand this better, let us look once more at the principles of rotative die-cutting. The cutting tool should cut through the face material and the adhesive layer, without damaging the liner material.
In contrast to paper, which ruptures relatively quickly when compressed, synthetic film materials must be fully penetrated by the cutter. A standard glassine liner can be compressed and absorbs some of the pressure of the penetrating cutter, making it less sensitive to perforation. The PET liner on the other hand is much thinner, has only a minimal silicon layer and is practically incompressible. This makes it particularly sensitive to damage from the cutter, although the material itself is relatively stable. Liner damage is of course absolutely to be avoided, so as to prevent web breaks during matrix removal or the label dispensing process.
Basic requirements and optimum die characteristics
What can a user now do to make die-cutting on thin PET liners function smoothly? In principle, the basic requirements for successful diecutting must be fulfilled: an immaculate, stable die-cutting unit as well as a high-precision cutting tool precisely adjusted to the material. For thin liners, these requirements apply to an even greater extent. Since literally every micrometre counts, it is particularly important that the die-cutting unit and the cylinders be flawless and clean.
The magnet and anvil cylinders must be manufactured with minimum tolerances of 3 μm and be perfectly concentric and parallel, given that the tolerances of the individual components accumulate. The die-cutter and cylinders, including their races and bearings, must be regularly cleaned, serviced and gauged.
In close consultation with the customer, the die manufacturer must adjust the flexible die as precisely as possible to the material to be die-cut, keeping within a height tolerance of two micrometres (figure 4). For conventional paper liners the compression of the liner must also be accounted for in determining the die height. Thus, for example, given a cylinder gap of 480 μm and a liner thickness of 55 μm, a die height of 440 μm is used (gap 480 μm – liner 55 μm + compression compensation 15 μm).
With a thin PET liner on the other hand, given the lack of compression just the liner thickness needs to be deducted from the gap, so that for example a die height of 457 μm results for a 23 μm PET liner.
Not only the height of the flexible die must be exactly right, however; the shape and quality of the cutting edge also play a major part. The cutting tool must cut through the face material very cleanly, which requires a relatively steep cutting angle and in some cases a special blade geometry. Moreover, the edges of the blade must be very smooth, since even the smallest irregularities can lead to unclean diecuts.
Furthermore, the tool should always be hardened so as to cut through the soft upper material cleanly and guarantee sufficient wear protection.
Further supporting measures
In principle, it also makes sense to use an adjustable anvil roller with which the cylinder gap can be variably adjusted in the smallest increments during production, ideally separately on drive and operator sides. This means tolerances in the system and wear on the flexible die can be compensated for within certain limits.
Especially with rectangular label shapes, the vertical displacement of the panels also brings great advantages, as realised for example in the Wink ProShift system. The staggered arrangement of the cavities minimises the proportion of horizontal lines being die-cut simultaneously and thus distributes the cutting pressure more evenly. In this way, reduced cutting pressure allows very even die-cuts to be achieved which in the area of the longitudinal lines also stress the thin PET liner less than with a conventional arrangement of labels.
Die-cut quality control
Given the high requirements described, it almost goes without saying that the results of the die-cutting should be regularly monitored during the production process. An ink test, as used with paper liners, cannot meaningfully be carried out with PET liners since the material is not absorbent. The only option left is very careful visual checking of the material, in which liner markings can be discerned by means of light reflections against a dark background.
It is important to note that even a perfect die-cut will almost always create an impression on the PET liner, because the upper film material must be completely cut through. As long as this marking is only minimally discernible and also occurs evenly over the whole grid, it is a sign of a successful die-cut. Excessively deep cuts can be discerned by a comparatively strong reflection and a tangible impression of the cut outline, sometimes also leaving discernible kinks in the liner material (figure 6). Such defects will almost inevitably lead to web breaks, either during label production or in the dispensing process.
In addition to the visual inspection, a “snap test” is also carried out in practice to check the stability of the liner. For this test, once matrix and labels have been removed, individual strips of the liner material are jerked apart. Clear test criteria are lacking, however, so that the decision on the die-cutting quality ultimately lies within the user’s margin of discretion. The die manufacturers will nevertheless be happy to advise on assessment of results.
Conclusion: The principles of rotary die-cutting
In our series of articles we have described the most important principles of rotary die-cutting. The problems described represent just a snapshot of the challenges that arise in practice, and which given the continuous developments in the industry (e.g. faster speeds, even thinner materials) will certainly not diminish.
Therefore, alongside further technological advances in the manufacturing of flexible dies, intensive cooperation between label manufacturers and suppliers of cutting dies, machinery and materials is absolutely mandatory to allow a future smooth die-cutting process that can meet increasing quality and efficiency requirements.