Some defects arrive before the material ever reaches the screw. With hygroscopic polymers — polyamides and TPU in particular — moisture is the classic invisible culprit: it does not show in the pellet, but it shows in the part.
The problem
Splay and silver streaks, bubbles and voids, dull or cloudy surfaces, reduced mechanical strength, unstable processing. All of these can come from a material that was perfectly good — but carried too much moisture into the machine.
Why it happens
PA and TPU are hygroscopic: they pull water out of the surrounding air — even through many packaging types — and faster than people expect. During processing that absorbed water does real damage. At melt temperature, moisture drives hydrolysis: it chemically cuts the polymer chains, permanently lowering molecular weight and mechanical properties. "The bag was closed" is not a drying process, and an opened bag left on the shop floor overnight can be enough to cause a problem.
How to solve it
Treat drying as a controlled process, not a formality:
- Residual moisture — dry to the level the polymer actually needs (typically well below 0.2% for PA — follow the grade's datasheet).
- Dryer type & temperature — use a proper desiccant dryer for hygroscopic materials, at the specified temperature.
- Dew point — check it — a dryer that is not reaching dew point is not drying.
- Drying time — give the material the full specified time; shortcuts do not work.
- Handling — reseal, do not leave material open to the air, and manage how long it sits after drying.
Key takeaway
Before you reject a PA or TPU lot for streaks, bubbles or weak parts, rule out moisture. A good material dried badly will fail like a bad material — and the fix is in the dryer, not the purchase order.
Dry it right, and a lot that looked "defective" often turns out to be exactly what you needed.
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