Mikko Brunner, MD of Renlaw, a leading South African machine knife manufacturer, sheds light on the three most common slitting processes that packaging converters employ.

Companies converting materials such as paper, plastics, foils, laminates, tapes and non-wovens into new packaging products such as plastic bags, toilet paper rolls and reels of tape are challenged by the range of substrate introductions, processing and machine conditions and operator effectiveness.

As all of these factors affect their ability to satisfy customers’ expectations, converters primarily employ razor blade/score slitting, shear slitting or crush-cutting in their slitting and rewinding operations. Their decisions are based on which process is the most fit-for-purpose for the majority of job changeovers, the desired results, and the materials.

1. Razor blade or score slitting

Razor blade slitting makes use of the cheapest consumables, and is the most cost-effective and fastest to set up because a fixed blade is mounted on the machine and cuts the web – either in the gap between two rollers or in the slot of a single, grooved roller.

The limitations of this set-up are that the thin blades can’t cut rigid or abrasive materials – they can only cut soft films such as polyethylene, vinyl, polypropylene and some light gauges of polyester. Razor blades are also unable to cut to close tolerances because they cut in an ‘air-gap’.

2. Shear slitting

In shear slitting, two circular blades cut against each other – effectively shearing the web like a rolling pair of scissors. The blades need to be set with just the right amount of overlap to maximise performance. It is a similar scenario to when an open pair of scissors is pushed along paper, cutting it without the blades moving against each other. The relative angles must be correct for optimal cutting performance, as incorrect angles can result in poor cut quality or even cause the material to tear.

There are two main types of shear slitting. The first (and preferred) type is wrap slitting. Here the web is wrapped around female knives, which are mounted with spacers, to create a solid shaft effect. If the web is not sufficiently supported, it can flex into the gaps between these knives and distort, resulting in irregular slit widths.

With kiss slitting, on the other hand, the web only just touches or ‘kisses’ the outer diameter of the female knife. Ideally, the web should engage the female and male knife simultaneously at a ‘critical point’ to ensure a clean cut. Hitting either the male or the female blade first can result in web rupture. To this end, individually settable knives work better than a series of knives mounted on a single shaft, as the operator can set the critical points of each blade individually.

Kiss slitting set-ups and adjustments are faster than those for wrap slitting because the blades can be loosened, moved and tightened into their new positions individually. When making wrap slitting adjustments, the operator must first disassemble the entire shaft, blades and spacers. Wrap slitting does, however, offer two benefits. The first is an improved cut with tighter tolerances, and the second is less material wrinkling and sagging because the blades and spacers support the web.

3. Crush-cutting

This type of slitting relies on crush-cutting slitter blades that crush the web material to cut it into bands. The blades shouldn’t be sharp because the fragile edges will dull, flare over and chip easily and can even damage the platen roller against which

they cut. The blades and platen rollers must be made with high-quality tool steels, hardened and then ground or polished to fine finishes to increase their sharp-lives.

The blades can also be ground with specific radii, according to the type of material they’ll be cutting (see table below). Although crush-cut systems are quick to set up, they create a fair amount of dust when cutting paper, resulting in a loss of popularity in high-speed printing set-ups.

Another factor to bear in mind is the blade and platen roller’s sensitivity to damage. Problems such as chips often result in incomplete cuts, while tears cause feathering or stringing on the web edges.

Crush-cutting isn’t suited to all applications – it’s most effective when cutting pressure-sensitive tapes and softer materials such as polyethylene. Although it severs the web with harder web materials such as cellophane and acetate, crush-cutting often results in tiny lateral cracks along the web edges.

When choosing the slitting process most suited to specific converting jobs, converters need to analyse the desired results, the materials and frequency of job changeovers. While disposable razor blades are the cheapest option, they don’t suit every application. In addition to representing Lutz disposable razor-blades in South Africa, Renlaw manufactures the full range of slitting and crush-cutting blades and provides a resharpening service. A correctly sharpened blade has around 90% of the sharp-life of a new blade and represents a cost-effective option.