Cutter Compactor Rotor & Stator Knives

Use the figure in your machine manual — it is machine- and material-specific, and typically falls somewhere in the 0.2–0.5 mm range. The more useful question is whether the gap stays where you set it. If the blade mounting face is not flat, or if you have fitted a new rotor knife against a worn stator, the gap will vary along the cutting length regardless of how carefully it was dialled in. Set the gap with feeler gauges at both ends and the centre of the blade, not at a single point, and re-check it after the first few hours of running once everything has seated and reached working temperature.

That is the classic signature of an over-wide cutting gap, usually from worn blades. When the gap opens, material is kneaded rather than cut, so frictional heat rises past the intended window, film smears into lumps, and bulk density becomes erratic — which the extruder sees as a fluctuating feed. You will typically also see power consumption up and throughput down at the same time. Check the gap and the blade edges before adjusting temperature setpoints or feed rate, because those are compensations for the real problem rather than fixes.

You can, but it is usually a false economy. The two wear to different geometries — the rotor knife loses its edge while the stator tends to wear a radius and hollow where the material stream concentrates. A new rotor knife running against a worn stator carries the entire cutting load across an uneven gap and will dull far faster than its rated life, so you end up back at the same shutdown sooner. Replacing both together costs more per changeover and less per tonne processed.

Match the alloy to your feedstock. D2 at HRC 58–61 is the economical default for clean LDPE, LLDPE and PP film, woven bags and pre-consumer scrap. Move to M2 HSS at HRC 62–64 for PET flake and for printed or dusty post-consumer film — M2 holds its edge at the elevated temperature inside a working compactor, where D2 starts to soften. Reserve carbide-tipped blades for glass-fibre reinforced compounds, mineral-filled material and high-ash streams; they last dramatically longer on those feeds but cost more and tolerate impact less well, so they are the wrong choice if your feed contains occasional metal.

Three to five regrinds is typical for D2 and M2 blades before the blade reaches its minimum usable height and the gap can no longer be adjusted to compensate. Each regrind removes material from the cutting face, so the blade sits progressively lower in its holder — most machines allow this to be taken up with shims up to a point. Carbide-tipped blades are re-ground on the carbide only and generally allow fewer cycles. We mark the minimum height on the manufacturing drawing so your maintenance team knows when a blade is finished rather than guessing.