Polymers can be divided into two groups by their performance of moisture absorbing, including Hygroscopic Polymers and Non-hygroscopic Polymers.
A. Hygroscopic Polymers
These kinds of plastic material absorb moisture internally through the air, the polymers themselves have strong sensitivity to water, and the absorption performance of different plastic material are different, if the moisture content is higher, the influence to property is also higher (For absorption rate of various plastic material, please take references of Plastic Material: Moisture Absorption), so we can say that Hygroscopic Polymers is necessary to be predried before production, or the moisture may cause below injection defeat to plastic article:
1. Welding line(parting line)
2. The impact/tensile strength of finished product may be weakened
3. Silver line, air bubble, lack of gloss
4. Unstable size of molded article
5. Crack or break
Some hygroscopic resins including: ABS、PA、PBT、PC、PC/ABS、PC/PBT、PEI、PET、PETG、PMMA、PPE、SA, etc.
Here are some tips for drying plastic material:
1. If the weather is warm and dry, it requires less drying.
2. Once the bags or containers of plastic material opened, it is necessary to promptly reseal them after using, and store in warm dry area to prevent the absorbing of moisture.
3. Unopened material (even though in sealed bag) supplied by supplier should be regarded as wet material, thus drying is necessary.
4. Drying time and temperature can be determined by experience, or use the suggested drying parameter by material supplier, or you can take references of this article: Plastic Material: Drying Temperature & Processing Time
B. Non-hygroscopic Polymers
The polymers themselves don't absorb moisture, they are not sensitive to water, but there might be adherent moisture on the surface of plastic material, which require less drying or no need to dry before production if the materials are stored appropriately.
Some non-hygroscopic resins including: PE、PP、PS、PVC、PPS, etc.