Polyethylene PE injection molding process

PE is a crystalline raw material, and its hygroscopicity is extremely small, not exceeding 0.01%, so no drying treatment is required before processing. The PE molecular link chain has good flexibility, small inter-bonding force, low melt viscosity, and excellent fluidity, so 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 direction 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 influence on the shrinkage rate. Therefore, the temperature of the mold should be controlled to maintain uniform and stable cooling.

The crystallization ability of PE is high, and the temperature of the mold has a great influence on the crystallization state of the plastic parts. High mold temperature, slow melt cooling, high crystallinity of plastic parts and high strength.

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 plasticizer needs to have a larger heating power in order to improve production efficiency.

The softening temperature range of PE is small, and the melt is easy to oxidize. Therefore, in the molding process, contact between the melt and oxygen should be avoided as much as possible to avoid reducing the quality of the plastic parts.

The PE parts are soft in texture and easy to demold, so the plastic parts can be demolded strongly when they have shallow grooves.

The non-Newtonian property of PE melt is not obvious, the change of shear rate has less effect on viscosity, and the viscosity of PE melt is 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 through the direct feed port during injection, it is easy to increase the stress and produce non-uniform search and significantly increased deformation in the direction. Therefore, the feed port parameters should be selected.

The molding temperature of PE is relatively wide. In the flow state, a little temperature fluctuation has no effect on injection molding.

The thermal stability of PE is good, generally there is no obvious decomposition phenomenon below 300 degrees, and there is no effect on the quality.

The main molding conditions of PE

Barrel temperature: The barrel temperature is mainly related to the density of PE and the melt flow rate, in addition to the type and performance of the injection molding machine and the shape of the first-class plastic parts. Since PE is a crystalline polymer, the crystal grains must absorb a certain amount of heat during melting, so the barrel temperature should be 10 degrees higher than its melting point. For LDPE, the temperature of the barrel is controlled at 140-200 ° C, the temperature of the barrel of HDPE is controlled at 220 ° C, the minimum at the rear of the barrel and the maximum at the front.

Mold temperature: The mold temperature has a great influence on the crystallization of the plastic parts. The mold temperature is high, the melt crystallinity is high, the strength is high, but the shrinkage rate will also increase. Usually the mold temperature of LDPE is controlled at 30 ℃ -45 ℃, and the temperature of HDPE is correspondingly higher by 10-20 ℃.

Injection pressure: Increasing the injection pressure is conducive to the filling of the melt. Due to the good fluidity of PE, in addition to thin-walled and slender products, the injection pressure should be well selected. The general 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.