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PE Injection Moulding

Some characteristics of PE:

PE is a crystalline raw material and has extremely low hygroscopicity of not more than 0.01%, so that it is not required to be dried before processing. The PE molecular chain is flexible, the force between the keys is small, the melt viscosity is low, and the fluidity is excellent. Therefore, the thin wall long process injection moulded part can be formed without too high pressure during moulding. PE has a large shrinkage range, large shrinkage value, and obvious directionality. The LDPE shrinkage is about 1.22%, and the HDPE shrinkage is about 1.5%. Therefore, it is easy to deform and warp. The cooling condition of injection mould has a great influence on the shrinkage rate, so the mould temperature should be controlled well to keep the cooling uniform and stable.
The crystallization ability of PE is high, and the temperature of injection mould has a great influence on the crystallization condition of plastic part. The mould temperature is high, the melt cooling speed is slow, and the plastic part has high crystallinity, so the strength is high accordingly.
The melting point of PE is not high, but the specific heat capacity is large, so it still needs to consume much heat during plasticization. Therefore, the plasticizing device is required to have a large heating power in order to improve injection moulding production rate.
The softening temperature range of PE is small, and the melt is easy to be oxidized. Therefore, the contact between the melt and oxygen should be avoided as much as possible during injection moulding, so as to avoid reduction of the quality of injection moulded parts.
PE plastic injection moulded parts are soft in texture and easy to release, so when the plastic parts have shallow side grooves, they can be strongly demoulded.
The non-Newtonian properties of the PE melt are not obvious, the change of the shear rate has little effect on the viscosity, and the viscosity of the PE melt is less affected by the temperature.
The PE melt has a slower cooling rate, so it must be sufficiently cooled. The mould should have a good cooling system, if the PE melt is fed by direct feed port during injection, it is easy to increase the stress and increase the deformation of the search unevenness and directionality. Therefore, the selection of feed port parameters should be careful.
PE has a relatively wide range moulding temperature, and in the flow state, a slight fluctuation in temperature has no effect on injection moulding.
PE has good thermal stability, and generally has no obvious decomposition phenomenon below 300 degrees, and has no effect on quality.

Main moulding conditions of PE:

Barrel temperature: The temperature of barrel is mainly related to the density of PE and the flow rate of melt. It is also related to the type and performance of the injection moulding machine and the shape of the first-class plastic part. Since PE is a crystalline polymer, the crystal grains absorb a certain amount of heat during melting, so the cylinder temperature should be 10 degrees higher than its melting point. For LDPE, the cylinder temperature is controlled at 140-200 °C, the HDPE barrel temperature is controlled at 220 °C, the temperature of the rear of barrel is taken as the smallest value, and the temperature of the front of the barrel is taken as the largest value.
Mould temperature: The mould temperature has a great influence on the crystallization condition of plastic part. The mould temperature is high, the melt crystallinity is high, and the strength is high, the shrinkage rate will increase accordingly. Generally, the mould temperature of LDPE is controlled at 30 ° C - 45 ° C, and the temperature of HDPE is correspondingly higher by 10-20 ° C.
Injection pressure: Increasing the injection pressure is conducive to the filling of the melt. Since the fluidity of PE is very good, in addition to the thin-walled slender products, the lower injection pressure should be selected well, and the general injection pressure is 50-100 MPa. For the plastic parts with simple shape and large wall thickness, the injection pressure can be lower.