Some plastics, such as PA, PC, PMMA, PBT, PET, polysulfone (PSF), polyphenylene ether (PPO), etc., are easy to absorb moisture due to their macromolecules containing hydrophilic groups. If moisture content exceeds allowable value, defects such as silver threads, bubbles, shrinkage holes, etc. will appear on the surface of product, and in severe cases, raw materials will be degraded, affecting appearance and internal quality of product. Therefore, these plastic raw materials must be dried before molding.

 

For non-hygroscopic plastics such as PE, PP, PS, PVC, POM, etc., if packaging is tight, storage and transportation are good, pre-drying is generally not required. Some resins are not hygroscopic by themselves, but some hygroscopic additives are added to make the entire raw material system hygroscopic. This raw material must also be dried before molding.
There are many drying methods for plastic raw materials, such as hot air circulation drying, infrared heating drying, vacuum heating drying, fluidized bed drying and air drying, etc., which should be selected according to plastic properties, production batches and molding conditions. Plastics for small batch production are mostly dried by hot air circulation oven and infrared heating; plastics for mass production are usually dried by boiling and air drying; plastics (such as PA) that are easily oxidized and discolored by heat for a long time at high temperature and plastics that are sensitive to water (such as PC) should be dried in a vacuum oven.
When using hot air circulation drying, factors affecting drying effect are drying temperature, drying time and material layer thickness. Generally speaking, the higher temperature, the faster low molecules and water volatilize, but it cannot exceed softening temperature or melting point of plastic, otherwise material will easily become soft and agglomerated, making feeding difficult; drying temperature should not be too low, otherwise it will not be easy to remove moisture. A longer drying time is beneficial to improve drying effect, but it is easy to cause energy waste, and it will also cause decomposition and discoloration of plastics with poor thermal stability; if time is too short, moisture content cannot meet requirements. Thickness of material layer should not be too large when drying. If material layer is too thick, due to poor thermal conductivity of plastic, drying effect of surface layer and center layer will be different under same drying conditions. Thickness of material layer is 20-50mm.
Allowable water content of some plastics is shown in Table 1, and drying conditions of commonly used plastics are shown in Table 2.
Table 1 Allowable water content of some plastics during molding

Table 2 Drying conditions of commonly used plastics

Sometimes it is necessary to quickly know whether moisture content in raw material meets requirements. Before injecting product, it is generally necessary to observe whether surface of injected strip is smooth by air injection method. If it is smooth, moisture content in raw material is considered to meet requirements.

 

It must be noted that dried raw materials should be used immediately. If they are not used temporarily, they should be sealed and stored to avoid moisture absorption again. Dried resin that has not been used for a long time should be re-dried before use. Some plastics that are particularly moisture-sensitive at molding temperatures should also consider sealing or heating hoppers during molding.