- Molding and processing properties of polyethylene PE
PE is a crystalline raw material with very little hygroscopicity, no more than 0.01%, so there is no need for drying before processing.
PE molecular chains have good flexibility, small forces between bonds, low melt viscosity, and excellent fluidity. Therefore, thin-walled long-flow products can be formed without too high pressure during molding.
The shrinkage range of PE is large, the shrinkage value is large, and the directionality is obvious. The shrinkage rate of LDPE is about 1.22%, and the shrinkage rate of HDPE is about 1.5%. Therefore, it is easy to deform and warp, and the cooling conditions of the mold have a great impact on the shrinkage rate. Therefore, the mold temperature should be controlled to keep the cooling uniform and stable.
PE has high crystallization ability, and the temperature of the mold has a great influence on the crystallization status of plastic parts. The mold temperature is high, the melt cools slowly, the crystallinity of the plastic part is high, and the strength is high.
The melting point of PE is not high, but the specific heat capacity is large, so it still needs to consume more heat during plasticization, so the plasticizing device is required to have large heating power in order to improve production efficiency.
The softening temperature range of PE is small, and the melt is easily oxidized. Therefore, contact between the melt and oxygen should be avoided as much as possible during the molding process to avoid reducing the quality of the plastic parts. injection molding
PE parts are soft in texture and easy to demould, so when plastic parts have shallow side grooves, they can be demoulded forcefully.
The non-Newtonian properties of PE melt are not obvious, changes in shear rate have little effect on viscosity, and the viscosity of PE melt is also less affected by temperature.
The cooling rate of PE melt is slow, so it must be fully cooled. The mold should have a better cooling system.
If the PE melt is fed directly through the feed port during injection, it is easy to increase the stress and cause uneven search and obvious directional increase in deformation. Therefore, attention should be paid to selecting the feed port parameters.
The molding temperature of PE is relatively wide. In the flowing state, slight fluctuations in temperature have no effect on injection molding.
PE has good thermal stability. Generally, there is no obvious decomposition phenomenon below 300 degrees, and it has no impact on quality.
Barrel temperature: The barrel temperature is mainly related to the density of PE and the melt flow rate. It is also related to the type and performance of the injection molding machine and the shape of the primary plastic part. Since PE is a crystalline polymer, the crystal grains must absorb a certain amount of heat when melting, so the barrel temperature should be 10 degrees higher than its melting point. For LDPE, the barrel temperature is controlled at 140-200°C, and for HDPE, the barrel temperature is controlled at 220°C. The minimum value is taken at the rear of the barrel and the maximum value is taken at the front end.
Mold temperature: Mold temperature has a great influence on the crystallization state of plastic parts. High mold temperature results in high melt crystallinity and high strength, but the shrinkage rate will also increase. Usually the mold temperature of LDPE is controlled at 30℃-45℃, while the temperature of HDPE is correspondingly higher by 10-20℃.
Injection pressure: Increasing the injection pressure is beneficial to the filling of the molten material. Since PE has good fluidity, except for thin-walled and slender products, a lower injection pressure should be carefully selected. Generally, the injection pressure is 50-100MPa. The shape is simple. For larger plastic parts behind the wall, the injection pressure can be lower, and vice versa.