Process · Note 03
Drying is not a precaution for some resins. It is a requirement.
Nylon, polycarbonate, PBT, and PET absorb water from the air. Moulded wet, they do not just cosmetically splay — the polymer chain is cut, and the strength does not come back.
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- Polydyne LLC
Hygroscopic resins take moisture into the pellet itself, not just onto its surface. Heat that pellet to melt temperature with the water still in it, and the water attacks the polymer chain directly. This is hydrolysis, and it is permanent: the melt viscosity drops, the part moulds easily, it looks acceptable, and its impact strength has gone. No downstream process recovers it.
The visible symptoms are the mild ones
Silver streaking radiating from the gate — splay — is the usual first sign, along with bubbles, surface haze, and inconsistent shot weight. Those are the cheap failures, because they get the part rejected at the press. The expensive failure is the batch that looks right, ships, and breaks in service.
| Resin | Dryer temperature | Time | Target moisture |
|---|---|---|---|
| Nylon 6/6 | 175 – 195 °F | 4 – 6 h | Below 0.20 % |
| Polycarbonate | 250 °F | 3 – 4 h | Below 0.02 % |
| PBT | 250 °F | 3 – 4 h | Below 0.04 % |
| PET | 300 – 350 °F | 4 – 6 h | Below 0.005 % |
| ABS | 175 – 195 °F | 2 – 4 h | Below 0.10 % |
Typical drying conditions before moulding. Follow the resin supplier data sheet for the specific grade and dryer.
A hot hopper is not a dryer
Heating alone drives surface moisture off but leaves what is inside the pellet. Removing that needs dry air pushed through the resin bed at a controlled dew point, held for the full residence time. A desiccant dryer running at a dew point around minus 40 °F is the normal arrangement. Resin left open on the floor after drying reabsorbs moisture within hours, so the run is scheduled around the dryer, not the other way round.