CUT-OFF SERIES #3: Clean cut edges when slitting

A clean cut edge is the most important quality feature when slitting vinyl film on a roll. Frayed edges, crushed rolls, dust or adhesive residues lead to complaints and waste material. The good news is that, with a proper understanding of a few basic principles, these problems can be reliably avoided in day-to-day practice.

In the third part of our slitting series, we look at seven parameters that play a decisive role in determining whether a cut edge frays, a roll becomes crushed, or dust and adhesive residues form. For each point, we provide a specific recommendation based on our many years’ experience with NEPATA slitting machines.

A higher rotational speed is not automatically the better choice. The faster the roller turns, the more frequently the blade passes over areas that have already been cut. This increases friction and, consequently, heat build-up. The result: the blade repeatedly scrapes along the material that has already been cut, which tends to produce more dust and fraying.

Conversely, the slower the roller turns, the lower the motor’s available torque. Beyond a certain point, the blade can no longer fully penetrate particularly tough materials.

Our recommendation: With NEPATA slitting machines, a setting of 50 to 70 per cent of the maximum speed has proven to be a good compromise.

Naturally, slitting requires a certain amount of force. However, the slitter should under no circumstances be pressed down too hard. Excessive pressure squashes the material and, in extreme cases, can even bend the blade – both of which encourage fraying. If, on the other hand, the slitting lever is operated too timidly, the blade may wobble on contact with the roller and damage the material.

Our recommendation: Apply even, moderate pressure whilst maintaining full control over the slitting lever. What is required is a steady, sure hand – not maximum downward pressure. If the cut becomes sluggish, the rotational speed should be increased slightly to provide more torque. For many materials, such as flock film, flex film or laminating film, a medium, brisk speed is recommended; magnetic foils or reflective adhesive films, on the other hand, are usually best processed at a slower speed. The optimum setting is determined through testing and experience.

As a general rule, it is advisable to cut with a rotating blade that turns passively with the material. This results in less friction and therefore less heat build-up in the material, and the cutting force is distributed evenly across the entire circumference of the blade.

However, some materials, such as foam adhesive films, cannot be cut cleanly with a rotating blade. In such cases, the blade on NEPATA slitting machines can be locked in a stationary position. The stationary blade increases the cutting force within the system but generates significantly more friction. The resulting heat can cause the adhesive to smear.

Our recommendation: With small diameters, this effect is negligible. Large-format tapes that require a stationary blade should remain the exception when using this cutting system. Do not carry out too many cuts in quick succession. You can clean the blade in between cuts by cutting a less demanding vinyl film or an empty paper core. In addition, the blade position should be changed regularly to ensure that cuts are not always made at the same point.

For many media, lubricating the blade is a crucial factor. It reduces friction and thus heat build-up, and significantly less adhesive residue is deposited on the blade. As well as oil-based lubricants, water-based variants are also available. These are suitable where there are concerns that silicone might impair the adhesive strength of the vinyl film.

With the automatic pad lubrication system developed by NEPATA, even very small quantities of lubricant are sufficient. In our day-to-day practice, we have not yet observed any adverse effects on the material.

Our recommendation: Use lubricant wherever it does not damage the material.

Every cut raises the temperature of the blade. If a large number of cuts are made in quick succession, the blade can become so hot that the quality of the cut suffers: adhesive increasingly oozes out from the cut edges and, in extreme cases, individual layers may fuse together.

This effect occurs more quickly with tough materials and, in particular, when the blade is stationary. In exceptional cases, the blade may even become warped. However, we are talking here of hundreds of cuts made in rapid succession, such as those required when producing very narrow strips.

Our recommendation: Allow the blade a short break after several cuts so that it can cool down sufficiently.

The higher the ambient or material temperature, the more difficult it is to perform slitting on most vinyl films. This is particularly true of self-adhesive films: adhesives change their viscosity when heated and smear more easily. This is particularly relevant in non-air-conditioned workshops, where temperatures can easily reach 35 degrees Celsius in summer.

Our recommendation: at high temperatures, reduce the speed slightly and always lubricate the blade carefully.

The winding tension of the material has a major influence on the result. Loosely wound rolls with loose, slipping layers are generally much more difficult to slit than tightly wound or linerless materials. When cut, loosely wound rolls tend to undulate, causing the layers to shift relative to one another – fraying is then virtually inevitable. However, rolls that are wound too tightly are also problematic: the roll becomes so compact that the individual layers can hardly be cut through.

Our recommendation: If poorly wound material is fed into the machine, it should be rewound professionally before slitting. Rewinding on a NEPATA machine under controlled web tension ensures the ideal winding tension and thus clean slitter results.

In addition to the parameters described here, there are other factors that influence whether a cut edge will be clean. If you wish to train your staff to become true slitter professionals, we offer practical training on our slitting machines and Converting Centres fitted with slitting units. Our video ‘Slitting like a pro’ gives you a first impression.

Further articles in our ‘Slitter’ series: