Page 96 - Injection Molding Advanced Troubleshooting Guide

9 Drying

„ „9.1„ Introduction

Removal of moisture prior to processing is absolutely critical for molding success.
Moisture in plastic pellets will turn to gas when subjected to molding tempera-
tures. This gas will be contained in the plastic melt until the plastic enters the
mold where the depressurization on the melt stream will allow the gas bubbles to
reach the surface of the mold, leaving behind the streak of splay. Hygroscopic
materials such as ABS, polycarbonate, nylon, TPU, polyesters, cellulosics, or
PC/ABS absorb moisture from the environment and require drying.
Some additives including fillers and impact modifiers can result in non-hygro-
scopic materials needing to be dried. There will be cases when non-hygroscopic
materials are literally soaking wet (for example from a roof leak), and in these
cases the material will need to be dried prior to molding.
Material suppliers will provide a recommended drying temperature and time for
hygroscopic materials. It is critical to follow these drying specifications to ensure
that the material is dry enough to successfully process (see below for drying re-
quirements).

„ „9.2„ Keys to Drying

Successful drying requires the following:

ƒ Correct temperature
ƒ Dry air
ƒ Air flow
ƒ Time under the above conditions