Case Study: Granulator Rotor Knife Upgrade for Automotive Plastic Recycling

In early 2025 our German distributor in Stuttgart called about a recycler processing automotive shredder residue (ASR) — the light fraction left after a car is shredded and the metals are recovered. The customer was getting 3 days from a D2 rotor knife and 5 days from a D2 bed knife, with frequent chipping on the rotor. The line was a 250 kW granulator running 24/7, throughput 1,800 kg/h. Downtime for knife changes was 25 minutes per event, twice per week, costing €900 per change in lost margin. This case study walks through the audit, the trial, the result, and a downstream effect on screen wear that we did not predict.

The result in one line: Replaced D2 with a YG15 rotor + YG10X bed, 4-edge reversible geometry, torque-wrench discipline on the bolts. Rotor life: 3 days → 22 days. Bed life: 5 days → 28 days. Annual savings: €164,000. Screen wear dropped 40 % as a side effect.

The line and the substrate

The customer was buying 8 rotor knives and 5 bed knives per week, with frequent chipping events. The cost of knives was 5 % of revenue, the cost of knife-change downtime was 8 % of revenue. The line was unprofitable at the 3-day mark.

The field audit: what we found

We spent one day on the line with a 10× loupe, a surface-roughness tester, a hardness file, and 5 kg of ASR feedstock. Five findings:

1. The substrate was the dominant variable

ASR is a nightmare substrate for an industrial blade: 30 % glass fibre, 5 % metal fragment, 2 % rubber, 1 % wood, plus the variable plastic mix. The glass fibre is highly abrasive (it accelerates HSS wear by 5–8×), the metal fragments cause impact chipping, the rubber wraps around the rotor, and the wood swells with humidity. A D2 knife is not a serious answer for this substrate.

2. The rotor chipping was impact-driven

Visual on the worn rotor knives showed chips 2–5 mm in size, originating from the leading edge. The chip pattern was consistent with metal-fragment impact. A 5 % contamination rate means roughly 1 metal hit per second at 1,800 kg/h throughput.

3. The D2 bed was wearing faster than the rotor

The D2 rotor at HRC 58 was wearing the D2 bed at HRC 58 at the same rate. A “matched” pair is not always the right answer — the bed sees less impact and could afford to be harder.

4. The 4-edge reversible geometry was correct in concept but wrong in execution

The customer was rotating the rotor knives 90° per edge, as designed. But the rotation was not always even — the customer was rotating “whenever it looked dull,” not on a measured wear threshold. Some edges saw 50 % more use than others.

5. The screen was wearing faster than it should

The 8 mm screen was lasting 14 days, which is short for a 250 kW granulator. The screen wear was concentrated around the rotor arc, suggesting that the rotor knife geometry was not feeding material through the screen evenly.

The trial: three changes, four knives, six weeks

We shipped four trial rotor knives and three trial bed knives to the customer with a written protocol:

All other variables (substrate, throughput, screen, wear strips) were held constant. Six weeks, four rotor trials, three bed trials.

The results

Trial C was the right combination. YG15 (15 % Co) on the rotor gave the impact resistance to survive the metal fragments. YG10X (10 % Co, fine grain) on the bed gave the wear life to match the rotor. The torque-wrench discipline on the bolts eliminated the chipping-from-loose-knife failure mode. The 8-day rotation protocol ensured even use of the four edges, doubling the effective life per knife.

The screen life went from 14 days to 22 days as a side effect. The YG15 rotor’s geometry feeds material through the screen more evenly than the D2 rotor, and the carbide edge stays sharp longer, reducing the “smash” of partially-cut material against the screen.

The economic case

The audit-and-trial cost paid back in the first 2 weeks. The screen life is a downstream benefit that we did not predict at the start of the audit. The customer is now running the Trial C specification as standard.

What the customer changed in the SOP

Three SOPs were updated:

1. Knife spec. “Rotor knife, 200 × 40 × 12 mm, YG15 tungsten carbide, 4-edge reversible, 0.25 mm chamfer. Bed knife, 300 × 40 × 20 mm, YG10X tungsten carbide, 2-edge, 0.15 mm chamfer. Diamond-wheel re-grind, 0.002 mm infeed, flood coolant. Mill certificate with cobalt content and ISO 513 classification required.”
2. Bolting SOP. “All rotor and bed knife bolts torqued to 120 Nm with a calibrated torque wrench, every knife change and every rotation. Calibrated annually.”
3. Rotation SOP. “Rotor knife rotated every 8 days, regardless of visual. The 4 edges are used in order: 1-2-3-4-1-2-3-4. Rotation date stamped on the knife hub.”

The new SOPs are now applied to all 5 granulator lines at the recycler.

Lessons learned

1. ASR is a carbide substrate, not a tool-steel substrate. Glass fibre + metal fragments + abrasive fillers will burn through D2 in days. YG15 is the only sensible answer.
2. Rotor and bed do not have to be the same material. A tougher rotor (YG15) + a wear-resistant bed (YG10X) is a better combination than both at the same grade.
3. Torque discipline is non-negotiable on carbide. A loose carbide knife shatters. A torque wrench is the cheapest insurance.
4. Rotation protocol matters. A 4-edge reversible knife that is rotated “when it looks dull” is a 2.5-edge knife. A measured schedule doubles the effective life.
5. Downstream effects matter. The screen life improvement was a 40 % bonus that we did not predict. The new rotor geometry fed material through the screen more evenly.

What this means for similar recyclers

The same pattern reproduces on most ASR, WEEE, and heavy-contamination recycling lines:

• ASR (mixed plastic + glass + metal): YG15 rotor + YG10X bed
• WEEE (mixed plastic + metal + glass fibre): YG15 rotor + YG10X bed
• Heavy-contamination film recycling: YG10X rotor + YG8 bed
• Clean film recycling: M2 HSS rotor + D2 bed (carbide is overkill)
• Glass-filled polymer (PA66 30 % GF): YG8 bed + M2 HSS rotor

The decision rule: the rotor takes the impact, the bed takes the wear. Match the materials to the dominant failure mode on each.

Granulator knife selection in 60 seconds

If you have a granulator line and the knives are wearing too fast:

1. Quantify the substrate. Polymer family, filler content (glass, mineral), contamination profile (metal, sand, foreign polymer).
2. Quantify the throughput. kg/h, rotor speed, screen size.
3. Identify the dominant failure mode. Rotor chipping = impact, bed edge wear = abrasion, screen wear = knife geometry.
4. Pick the rotor steel first. YG15 for impact, YG10X for moderate impact, M2 HSS for clean cuts.
5. Pick the bed steel second. YG10X for wear, YG8 for moderate wear, D2 for clean cuts.
6. Specify the geometry. 0.20–0.30 mm chamfer on the rotor, 0.10–0.20 mm on the bed.
7. Specify the bolting. Torque wrench, calibrated, specific value.
8. Specify the rotation. Measured schedule, not visual.

For a written granulator audit on your line, send the substrate, the throughput, the rotor speed, the current knife spec and the current service life to engineering@kaipu-industrial.com or use the request-a-quote form. A typical audit takes 1 day on site, returns a written diagnosis and a trial protocol, and ships the trial knives within 3 weeks. ROI is typically inside 6 months on lines with > €50k/year knife spend.

For the broader granulator selection guidance, see How to choose a granulator knife and the granulator section in The KAIPU 5-Factor Blade Selection Framework.

About the author

KAIPU Engineering is the technical team at KAIPU Industrial Blades, in operation since 1998. ISO 9001:2015 certified. The team ships to converters, recyclers and OEMs across four continents, with active distribution in Germany, Poland, Italy, Turkey, India, Vietnam and Brazil, and a dedicated recycling-industry cell for ASR, WEEE and heavy-contamination applications.