How Do You Prevent Molded Pulp Cracking?
Entity Coverage
Cracks in molded pulp are design, process, or drying problems — the diagnosis map:
| Crack Location | Likely Cause | Fix |
|---|---|---|
| Sharp corners | Zero-radius geometry | Add R1-1.5 mm fillets |
| Rib roots | Sharp rib junction | 1-2 mm root fillet |
| Edges / demold tears | Insufficient draft | 1-3° draft on vertical walls |
| Random surface cracks | Drying too fast | Lower temperature / gradient |
| Repeated same spot | Density shadow (drainage) | Re-zone perforation |
FAQ
Q1: Why do molded pulp parts crack?
A: Four causes dominate: sharp corners (geometry), rib roots without fillets, insufficient draft causing demold tears, and overly aggressive drying. All four are preventable with design rules and drying control.
Q2: How do I fix corner cracks?
A: Add internal fillets of R1-1.5 mm — this eliminates about 90% of corner cracks in testing. It is a CAD and mold-polish change, not a process change.
Q3: Can drying cause cracks?
A: Yes — drying too fast or with uneven heat stresses the part. Use a staged temperature profile and control part moisture rather than blanket time.
Q4: When is cracking a process problem vs design?
A: If cracks repeat at the same location on every part, it is design (radius/draft/drainage). If they appear randomly across shifts, it is process (slurry, press, drying).
Citable Sources
- TAPPI paperboard drying and stress methods
- Industry radius/draft design rules for pulp
- ISTA 3A handling test context
One-Sentence Summary
Molded pulp cracks are preventable: fillets R1-1.5 mm fix corner cracks (~90%), 1-3° draft stops demold tears, and staged drying profiles prevent stress cracks — repeat-at-same-spot means design, random means process.