The slitting method and knife geometry must be matched to the paper. There is no universal knife that works for all grades. The choice between shear slitting, razor slitting, and score (crush) cutting depends primarily on paper weight and stiffness.
Slitting Method Selection by Paper Type
| Paper Type | GSM Range | Recommended Method | Why |
|---|---|---|---|
| Lightweight kraft | 30–60 | Razor (≤40gsm) or shear (40–60gsm) | Razor produces cleanest edge with minimal dust on thin grades |
| Standard kraft | 60–200 | Shear | Shear provides highest precision and cleanest edge |
| Coated paper | 80–350 | Shear | Shear minimizes coating damage and dust |
| Cup paper | 150–350 | Shear | Consistent edge quality for downstream forming |
| Heavy board | 250–600+ | Shear with deeper overlap | Heavier materials resist cutting and need more engagement |
Razor slitting works best on lightweight paper, typically 40gsm and below. The thin, sharp blade slices through paper fibers without compressing them. For kraft ≤40gsm, razor slitting produces the cleanest edge with minimal dust. Position the blade at a 19–22° angle relative to the web surface. Shallower angles (15–20°) increase blade life; steeper angles (25–30°) improve cut quality but wear the blade faster.
Shear slitting is the standard for most paper grades from 60gsm upward. Two circular knives create a scissor-like action that cuts fibers cleanly rather than tearing them. Shear slitting produces the highest precision edge quality of the three methods.
Score slitting (crush cutting) is generally not recommended for quality-critical paper applications. It produces rougher edges and more dust. However, it remains common in older equipment and certain nonwoven applications.
Knife Geometry Settings by Paper Stiffness
Once the slitting method is selected, knife positioning determines edge quality. The key settings are overlap (vertical engagement) and side clearance.
For shear slitting:
| Paper Type | Overlap (Penetration) | Side Clearance | Notes |
|---|---|---|---|
| Lightweight kraft (30–60gsm) | 0.3–0.5mm | 0.02–0.05mm | Minimal overlap to avoid crushing thin paper |
| Standard paper (60–200gsm) | 1.0–1.5mm | Effectively zero (light contact) | Standard starting range for most grades |
| Heavy paper/board | 1.5–3.0mm | Light contact | Deeper overlap needed to shear through stiffness |
| Coated paper | 1.0–1.5mm | Light contact | Monitor blade wear; coating accelerates dulling |
The rule for overlap is to use the least engagement that produces a complete, clean cut. Every extra tenth of a millimeter is extra contact length, extra friction, and faster edge wear. Too little overlap tears the web rather than shearing it. Too much accelerates knife wear and generates dust from knife bodies rubbing rather than edges cutting.
For side clearance in shear slitting, the target is effectively zero—the male knife is loaded laterally against the female knife so the faces stay in light contact. A measurable gap, even 0.05mm, lets the web fold into the gap instead of shearing, causing wavy or turned-over edges.
For razor slitting on lightweight kraft (≤40gsm):
- Blade extension: 0.5–2.0mm beyond anvil surface
- Holder pressure: 0.4–0.8 bar pneumatic for consistent penetration
- Blade material: Ceramic-coated steel for abrasive kraft grades
For converters who need to validate knife configurations for specific paper grades, reviewing roll converting equipment options helps identify the machine categories best suited to different material profiles.
Post time: Oct-27-2026