Injection Molding Technique of PE (Polyethylene)
PE is crystalline material with minimum hygroscopicity, which is not more than 0.01%. Therefore, drying process does not be required before processing. With good chain flexibility, small key intercropping force, low melt viscosity and good liquidity, PE molecular league does not need higher pressure for shaping thin wall and long process product.Scope of PE contractibility rate is large with large shrink value and obvious direction. The shrink value of LDPE is about 1.22%, and HDPE is about 1.5%. Therefore, warpage which transforms easily and cooling conditions of die make great difference to contractibility. So temperature of die should be well controlled and maintain cooling uniformity and stability.
Crystallization capacity of PE is high. Temperature of die has a big effect on crystallization status of mold. Temperature of die is high and melt cools slowly. Crystallinity ofmold part is high, so the strength is also high.
Melting point of PE is not high, but the specific heat capacity is large, so some heat needs to be consumed when plasticized. So in order to improve product efficiency, the plasticizing unit needs large heating power.
Scope of PE softening temperature is quite small and melt is easily oxidized. Therefore, while in the process of shaping up, contact with melt and oxygen should be avoided as far as possible, in order to avoid lowering quality of mold.
Materials of PE product are quite soft and easily demolded, so powerful release can be done when the mold has shallow side groove.
Non-newtonian feature of PE melting is not obvious. Change of rate of shear has a small effect on viscosityand PE melting viscosity is also affected less by temperature.
Cooling speed of PE melting is quite slow, so cooling must be done totally. Mould should have quite good cooling system.
If PE melting adopts direct feed port when injecting, easily increasing stress and creating imbalanced searching and enlargement as well as deformation with clear direction, so feed port parameters choosing should be paid more attention.
Molding temperature of PE is quite wide. In flowing status, a little fluctuation of temperature has no effect on injection moulding.
Heat stability of PE is quite good. There is no obvious decomposition phenomenon under 300 Celsius degrees, making no difference to quality.
Main Conditions of Molding of PE (Polyethylene):
Temperature of charging barrel is mainly related to density degree of PE and flow rate of melt. Besides, it is also related to type and property of injection molding machine and shape of level one mold. As PE is crystalline polymer, so grain must absorb some heat while fusion, so temperature of charging barrel should be 10 Celsius degrees higher than its melting point. As to LDPE, temperature of charging barrel should be controlled within 140-200 Celsius degrees . Temperature ofcharging barrel should be controlled within 220 Celsius degrees. Backstage of charging barrel should get the least value and the front end should get the largest value.
Mold temperature has quite big effect on crystallization condition of mold plastic. The temperature is high, the melt crystallinity is high and strength is high, but the contractibility rate will increase. Generally, mold temperature of LDPE is controlled within 30-45 Celsius degrees, while temperature of HDPE can increase for 10-20 Celsius degrees relatively.
Injection Molding Pressure
Improving injection molding pressure contributes molding filling of melt. As the flowing rate of PE is quite good, so the lower injection pressure should be chosen carefully except product with thin wall and long body. Common injection pressure is 50-100Mpa with simple shape. Large mold plastic can have lower injection pressure, and visa verse.