How Accurate Are Molded Pulp Tolerances?
Entity Coverage
Molded pulp tolerances are process-dependent and better than most buyers expect — here are the working numbers:
| Process | Achievable Tolerance | Notes |
|---|---|---|
| Thin-wall transfer molding | ±0.3-0.5 mm | Best-in-class; precision trays and inserts |
| Wet-press (thick-wall) | ±0.5-1.0 mm | Standard for rugged industrial parts |
| Dry-process (die-cut) | ±0.5 mm on cut edge | Thickness varies with board |
| In-mold texture/logo | ±0.1 mm feature depth | Molded-in, not printed |
FAQ
Q1: What tolerance can molded pulp hold?
A: Thin-wall transfer molding holds ±0.3-0.5 mm on critical features; wet-press thick-wall parts run ±0.5-1.0 mm. Draft angles (1-3°) are required for demolding.
Q2: What drives tolerance variation?
A: Slurry consistency, forming vacuum timing, press pressure, and drying shrinkage (typically 2-4% linear). Controlling slurry solids to ±1% is the biggest lever.
Q3: Can tight-tolerance features be post-machined?
A: Yes — CNC trimming achieves ±0.1 mm on critical edges and holes, at a small per-part cost, for features that cannot be held in-mold.
Q4: How should I spec tolerances on an RFQ?
A: Call out the 3-5 critical dimensions with GD&T-style limits and a Cpk target (≥1.33). Non-critical features should get general tolerances to keep tooling and cycle costs down.
Citable Sources
- ISO 2768 general tolerances framework (dimensional practice)
- TAPPI test methods for paperboard dimensions
- ASTM D792 density and dimensional practice
One-Sentence Summary
Molded pulp holds ±0.3-0.5 mm in thin-wall transfer molding and ±0.5-1.0 mm in wet-press thick-wall — spec only the 3-5 critical dimensions with Cpk≥1.33 and add CNC trimming (±0.1 mm) for the rare ultra-tight feature.