A plastic resin drying guide is only useful if it separates the three variables that decide whether resin actually comes out dry: temperature, time and the dew point of the air moving through the hopper. Dry the wrong way and parts still fail — splay, silver streaks, brittle walls and hydrolysed polyesters all trace back to moisture that a temperature-only dryer cannot remove. This guide sets out the moisture content drying temp combinations mainstream resins need, explains why dew point matters more than the number on the controller, and lists the documentation to ask for.
Target Moisture and Drying Temperature by Resin
Hygroscopic resins — PA, PC, PBT, PET, PMMA, TPU and PLA — absorb moisture into the pellet itself and need genuinely dry air to pull it back out. Non-hygroscopic resins such as PP, PE and PS usually carry surface moisture only, and a basic hot-air dryer is adequate. The table below consolidates the ranges most moulding shops work to; always confirm against the resin supplier's data sheet, because grade, filler loading and pellet geometry shift the numbers.
| Resin | Drying temp | Time | Target moisture | Dew point |
|---|---|---|---|---|
| PA6 / PA66 | 80-105 C | 4-6 h | below 0.15% | -40 C or lower |
| PC | 115-125 C | 3-5 h | below 0.02% | -30 to -40 C |
| PBT | 120-135 C | 3-4 h | below 0.03% | -40 C or lower |
| PET (crystallised) | 160-180 C | 3-5 h | below 0.02% | -40 C or lower |
| PMMA | 80-95 C | 2-4 h | below 0.10% | -30 C |
| ABS | 70-90 C | 2-4 h | below 0.10% | -20 to -40 C |
| TPU | 80-110 C | 2-3 h | below 0.05% | -30 to -40 C |
| PP / PE | 60-85 C | 1-2 h | surface only | hot air adequate |
Dew Point: The Variable a Plastic Resin Drying Guide Must Cover
A dryer's job is not to heat the pellet; it is to deliver air drier than the pellet so moisture migrates out. Air at a dew point of -20 to -40 C does that; air at +10 C — what a saturated desiccant bed or a failed aftercooler produces — will not, no matter how long the cycle runs. That failure hides in plain sight, because the controller still shows the right temperature. Ask for the dew point to be logged, not just set, and treat a reading above target as a stop condition.
Choosing the Dryer
- Hot-air dryer: convection only, fine for PP, PE and surface moisture; cannot reach equilibrium on PC or PET
- Desiccant dryer: regenerating molecular-sieve bed, -20 to -50 C dew point; the default for hygroscopic engineering resins
- Vacuum dryer: lowers the boiling point and shortens cycles; earns its cost where drying time limits throughput
Sizing, Residence Time and Over-Drying
Two sizing mistakes dominate. Airflow must scale with throughput — roughly 1.8 to 2.5 m3 per kg of resin plus a 20-30% margin for filter loading and aged desiccant — and hopper capacity must cover residence time: throughput multiplied by drying time, with the hopper kept above roughly one third full so air flows evenly instead of channeling. Over-drying is a real failure mode too: PC held at the top of its range yellows and loses impact, POM can generate formaldehyde and depolymerise, and polyamide left in a hot hopper over a shutdown oxidises and darkens. Dry to the data sheet, then run the hopper down before a long stop.
How to Prove the Resin Is Actually Dry
Pellets cannot be judged by eye. Loss-on-drying is simple but slow; a Karl Fischer titration or a bench moisture analyser gives a number in minutes at the press. On programs where moisture matters, ask for the resin lot certificate together with the dryer settings and the dew point log, and keep drying records with the part's quality file. Our plastic materials hub lists the resin families behind this guide, custom molding services explains how processors set up the line, the plastic datasheet guide covers where drying limits appear on a data sheet, and the injection molding defects guide links splay and silver streaks back to wet resin.
