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Material Drying Guide for Injection Molding

Improper drying is the single most common cause of injection molding defects — silver streaks, brittleness, splay, and poor surface finish are almost always wet material.

Why Drying Matters

Hygroscopic plastics absorb moisture from air. At melt temperatures (200-350°C), moisture turns to steam, creating voids and mechanical weakness. A nylon part molded with undried material can lose 30-50% of impact strength.

Drying Parameters

Material	Temp (°C)	Time (hr)	Max H₂O	Notes
ABS	80-85	2-4	0.02%	Required
PA6	75-85	4-6	0.02%	Hygroscopic — dry immediately before molding
PA66	80-90	4-6	0.02%	More sensitive than PA6
PC	120	3-4	0.01%	Critical — wet PC becomes brittle
PMMA	80-90	3-4	0.02%	Moisture causes bubbles in clear parts
POM	100-110	2-3	0.01%	Overdrying can degrade
PET	135-160	4-6	0.002%	Requires crystallizing dryer
PP	80-90	1-2	0.02%	Non-hygroscopic

Buyer's Tip: Look at the dryer setup. A hot air dryer (hopper + heater) cannot achieve the -40°C dew point required for PA and PC. If you see a simple hot air dryer and your part is made from PA or PC, you almost certainly have hidden moisture defects. Check the drying temperature display — some factories set the value but never verify actual temperature at machine throat.

How to Verify

Request the drying log (time-stamped records)
Check dryer dew point: -20°C to -40°C
Karl Fischer moisture test before molding (< 0.02%)
First-shot visual: silver streaks = moisture
Mechanical test: compare impact strength

What This Means for Your Project: Add drying spec to your purchase order: 'Material must be dried in a dehumidifying dryer to <0.02% moisture (verified by Karl Fischer) before molding.' For PC requiring high impact, threshold is 0.01%. For PET, 0.002% requires a crystallizing dryer.

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