Injection molding process of polyethylene PE

PE is a crystalline raw material, with very little moisture absorption, not more than 0.01%, so there is no need to dry before processing. PE molecular chain is flexible, the inter-bonding force is small, the melt viscosity is low, the fluidity is excellent, so the molding can be formed without too high pressure to form thin-walled long flow products.

PE shrinkage rate range, shrinkage value, directional obvious, LDPE shrinkage rate of 1.22% or so, HDPE shrinkage rate of 1.5% or so. Therefore, it is easy to deformation warpage, mold cooling conditions have a great impact on the shrinkage rate, so the mold temperature should be controlled to maintain uniform and stable cooling.

PE's crystallization ability is high, the temperature of the mold has a very large impact on the crystallization condition of the plastic parts. Mold temperature is high, the melt cools slowly, the crystallinity of the plastic part is high, the strength is also high.

PE's melting point is not high, but the specific heat capacity is larger, so the plasticization still needs to consume more heat, so the plasticizing device is required to have a larger heating power, to improve production efficiency.

PE's softening temperature range is small, and the melt is easy to oxidation, so in the molding process should be as far as possible to avoid contact between the melt and oxygen, so as not to reduce the quality of plastic parts.

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

PE melt non-Newtonian is not obvious, the change in shear rate has less effect on the viscosity, PE melt viscosity 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 PE melt is injected with direct inlet feed, it is easy to increase the stress and produce uneven search and directional increase in deformation obviously, so attention should be paid to the selection of inlet parameters.

PE's molding temperature is wide, in the flow state, the temperature of a little fluctuation on injection molding has no effect.

PE's thermal stability is good, generally below 300 degrees no obvious decomposition phenomenon, there is little impact on the quality.

PE's main molding conditions

Barrel temperature: The barrel temperature is mainly related to the density of PE and the size of the melt flow rate, in addition to the type and performance of the injection molding machine, the shape of the first-level plastic parts. As PE is a crystalline polymer, the grains should absorb certain 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℃, and the barrel temperature of HDPE is controlled at 220℃, with the rear of the barrel taking the minimum value and the front taking the maximum value.

Mold temperature: mold temperature has a greater impact on the crystallization of plastic parts, high mold temperature, high crystallinity of the melt, high strength, but shrinkage will also increase. Usually, LDPE mold temperature control at 30 ℃ ~ 45 ℃, and HDPE temperature is correspondingly then 10 ~ 20 ℃ higher.

Injection pressure: Improving the injection pressure is conducive to melt filling the mold, due to the good fluidity of PE, so in addition to thin-walled slender products, should be fine to choose a lower injection pressure, the general injection pressure of 50 ~ 100MPa. simple shape. After the wall of larger plastic parts, injection pressure can be lower, and vice versa is high.