As release liners become ever thinner, die-cutting equipment manufacturers have to find solutions that are able to meet the requirements of converters. European technical director for RotoMetrics, Bo Meyer, examines the challenges.
As many readers are not familiar with the intricacies of the die-cutting process, I need to define exactly what it entails starting with the very basic elements.
In order to understand the process of cutting, we need to look at the exact physical action that takes place. When slicing bread or meat, you are moving the knife backwards and forwards thereby cutting or tearing the fibers of the material. In our industry, however, it is a completely different game. The die-cutter presses a sharp edge through the material, without any sideward movement or tearing, ie there is only pressure and a sharp edge, which passes through the material. It is more or less like chopping wood with an axe.
Bo Meyer, RotoMetrics
When cutting, you need a proper anvil to form a stable base. In the case of slicing bread, it is the breadboard that performs the function of an anvil; when chopping wood, it is the chopping block on the ground; and when die-cutting labels, it is the release liner which performs this function.
Depending on the label material being cut, the process is different. When cutting paper, the liner material will be compressed so it gets so hard that the top material will be cut. The pressure will normally make the paper burst when the knife is about two thirds into the material. Filmic material, however, will not burst but must be cut all the way through.
Today’s challenges
The thickness of release liners has decreased in recent years, from an average of 67 microns in 1980 to 23 microns today. This development has changed the game for the production of labels and reduced the window of success for die-cutting significantly. The new thinner liners require greater control of the entire die-cutting process and tooling precision must reflect this demand. It might be necessary to change the inventory to tight tolerance magnetic cylinders, tight tolerance solid dies and the new generation of flexible dies.
The market is also moving towards release liners that can be recycled. This will present new compression rates on the liners and hence affect the clearance on the tools. A way of overcoming this could be to install an adjustable anvil to compensate for the new generation of liners.
EXAMPLE
Definition of clearance / drop on a solid die: The distance from the tip of the cutting line to the bearer.
Definition of clearance / drop on a magnetic cylinder: The distance from the surface of the magnetic body to the bearer. The standard is 0.480 measured on the radius.
Compression of the liner
Standard glassine liner
Liner thickness: 53 µm
Compression approx. 15% = 8 µm
Left is 53 µm less 8 µm = 45 µm
Solid die clearance = 0.045 mm
Flexible die: 0.480-0.045 = 0.435 = using 0.440 mm
Compression of a standard PET 23 liner
Liner thickness: 23 µm
Compression approx. 15% = 3 µm
Left is 23 µm less 3 µm = 20 µm
Solid die clearance = 0.020 mm
Flexible die: 0.480-0.020 = 0.46 = using 0,455 mm
Maintenance is critical
The state of the die station is becoming more and more important and care must be taken to maintain overall performance. A strong preventative maintenance schedule is necessary as you address the converting challenges of thin film liners.
The die station is a piece of mechanical equipment that must be kept in top condition in order to convert these new thin liners. Frequent inspection of current cylinder inventory is a must and very smooth anvil surface must be maintained.
Problems to look out for include excessive wear in the die frame slots indicating that the anvil and die are not being held firmly, potentially leading to inconsistent die strike. Bearers on support rolls and magnetic cylinders or solid dies should be inspected for wear that could affect liner impression. If the bearers are not in good condition and well lubricated, they can generate excess heat and thermal expansion.
Hot web leaving the print station brings heat to the cutting tool causing thermal growth, ie expansion, and a rise in temperature by as little as five degrees centigrade can lead to die-cutting issues.
It is worth noting that the thermal conductivity for a PET liner is 10 times higher than for a paper-based liner, and therefore, a PET liner will bring more heat into the die station and chill rollers might be necessary.
Work with your customer
It is important that the converter and the customer approve targeted liner strike prior to job launch. Visual inspection remains important, and surface inspection lights are recommended for confirming acceptable liner impression. It is not possible to use a PET liner without marking it.
A key process at the converter is testing for liner strike control on press. The margin between acceptable or unacceptable die strikes is 2.5 microns out of die tolerance.
The die system supplier should work with the converter to develop the right testing for die-cutting performance. Options include stretching the material using a ‘snap test’ or applying pressure to the die-cut areas to determine the integrity of the liner. Pressure can be applied using a finger.
It is not possible to colour the liner or use a malachite test, the only option is carbon powder, which will make the liner markings black, but the powder is expensive and difficult to work with without coloring everything black.
Solutions
RotoMetrics offer two versions of an adjustable anvil to meet the abovementioned challenges. On the Adjustable Clearance Anvil (ACA) both bearers can be adjusted simultaneously, making it easy to adjust for different liners or wear on the tool. With the Accustrike Adjustable Anvil, the bearers can be adjusted individually, offering the option to adjust different clearance on each side if needed.
A range of flexible dies for the new generation of liners has also been developed. This includes the AccuStar flexible die, which offers the longest life, highest accuracy and most exact tolerances available because they are precision finished and laser hardened to master most applications, such as extensible/filmic substrates and abrasive thermal transfer materials.
The AccuFlex flexible die is designed to provide best-in-class performance when converting materials on thin film liners and challenging materials. It features more acute angles, razor sharp/high precision cutting edges, extreme precision and control of total plate height tolerance and is suitable for cutting against PET 23 or special front materials.
