A part that snaps, cracks at a corner or shatters when dropped usually points to one of a handful of causes. Some are process problems, some are material problems, and the fix is different for each.
Causes and fixes
| Cause | What’s happening | Fix |
|---|---|---|
| Molding wet PC, PET or PBT | Moisture breaks the polymer chains at melt temperature (hydrolysis). The part looks fine but has lost its toughness. | Dry properly before molding. See the drying guide. |
| Dry-as-molded nylon | Nylon is at its most brittle straight out of the press and gets tougher as it absorbs moisture from the air. | Condition parts, or move to an impact-modified nylon. |
| Too much regrind | Each trip through the barrel shortens the polymer chains. | Cut regrind percentage and control how it’s blended back. |
| Overheating | High melt temperature or long residence time degrades the resin. | Lower melt temperature; match barrel size to shot size. |
| Cold service temperature | Polypropylene homopolymer and GPPS turn brittle in the cold. | Switch to impact copolymer PP, or HIPS instead of GPPS. |
| Chemical contact under stress | Oils, cleaners or solvents attack stressed areas (environmental stress cracking). | See stress cracking. |
| Sharp corners and molded-in stress | Stress concentrates at sharp inside corners and near gates. | Add radii; adjust packing and cooling. |
| The grade just isn’t tough enough | The material is right for stiffness or cost but not impact. | Move to an impact-modified or higher-molecular-weight (lower-MFI) grade. |
Tougher options, by material
- Polypropylene: impact copolymer instead of homopolymer. Polypropylene
- Polystyrene: HIPS instead of GPPS. GPPS vs. HIPS
- Nylon: impact-modified grades. Nylon
- ABS / PC: higher-impact grades, or PC/ABS. PC/ABS
- Any resin: a lower-MFI grade usually means more toughness. MFI explained