Raw material drying guide: which material, why and how to dry it

Moisture is a silent enemy of quality in injection molding. Raw material absorbs moisture from the air (hygroscopic) and, once it reaches high temperature in the barrel, that moisture either turns to vapor or chemically cuts the polymer chain.

What does moisture do?

• Silver streaks (splay): Silvery streaks on the part surface in the flow direction.
• Loss of strength: In materials like PC, PET and PA, water cuts the chain at high temperature (hydrolysis); the part becomes brittle. This can be invisible — the surface comes out fine, but impact strength drops.
• Filling inconsistency: Viscosity changes, and parts differ from cycle to cycle.

Representative drying values

The values below are typical application ranges. Check the manufacturer's data sheet for the exact value; it varies by grade and filler content.

PP, PE and most POM grades generally don't need drying; but check anyway if the environment is very humid or the container was left open.

Choosing a dryer

• Use desiccant air: dew point is typically -30 °C to -40 °C. Hot-air ovens can fall short in a humid environment, especially for PA and PET.
• Count the drying time from the moment the material reaches temperature, not from the moment it enters the drying hopper.
• The line from the dryer to the machine should be short and insulated; dried material left exposed picks up moisture within minutes (especially PA).

Quick checks on the shop floor

• Do an air shot: foaming or bubbly melt is a sign of moisture.
• When in doubt, use a moisture analyzer; the target moisture for PET and PA is generally very low (check the value on the data sheet).
• If you use regrind, dry that too; fine dust absorbs moisture faster than virgin material.

Note: For materials requiring continuous drying, it's also good practice to purge the material inside the screw after a machine stop; material left waiting degrades in the heat.

See the calculation tool for cooling and cycle calculations; drying is a condition outside the tool's scope but directly affects the result.