Drying plastics
Plastic materials contain hygroscopic groups such as amide, ester, ether, and cyanide in their molecular structure, giving them a tendency to absorb moisture. This moisture absorption results in varying degrees of water content. When the moisture exceeds a certain level, defects such as silver streaks, shrinkage pores, and bubbles will appear in the product, and the material will also degrade.
Plastics that easily absorb moisture include PA, PC, PMMA, PET, PSF (PSU), PPO, and ABS. In principle, these materials should be dried before molding. The drying conditions vary depending on the plastic; Table 2-1 shows the drying conditions for common plastics.
There are many methods for drying plastics, such as circulating hot air drying, infrared heating drying, vacuum heating drying, and airflow drying.
It should be noted that the dried material should be protected from re-absorbing moisture. Table 2-2 shows the permissible moisture content of common plastics before molding.
Preheating of inserts
Because plastics and metals have significantly different thermal properties-plastics have lower thermal conductivity, a higher coefficient of linear expansion, and a larger molding shrinkage rate, while metals have a smaller shrinkage rate-plastic products with metal inserts are prone to cracking around the inserts, resulting in lower product strength.
To address this issue, the thickness of the plastic around the insert should be increased during the design of plastic products. Preheating the metal insert during processing reduces the temperature difference between the molten plastic and the metal insert, slowing down the cooling of the plastic around the insert and ensuring more uniform shrinkage, thus preventing excessive internal stress around the insert.
The preheating of inserts depends on the properties of the plastic, the size and type of the insert. For plastics with rigid molecular chains, such as PC, PS, PSF, and PPO, preheating is necessary when inserts are present. However, for plastics with flexible molecular chains and small inserts, preheating may not be required.
The general preheating temperature for inserts is 110–130℃, but for materials like aluminum and copper, it can be increased to 150℃.
Selection of release agent
For certain plastic products with complex release structures, a release agent needs to be sprayed onto the mold core during injection molding to facilitate the smooth ejection of the plastic product from the mold core.
Traditional release agents include zinc stearate, white oil, and silicone oil. Zinc stearate can be used on most plastics except polyamides. White oil is more effective as a release agent for polyamides. Silicone oil is effective but inconvenient to use.