what’s PE material?
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 link is flexible, the force between the keys is small, the melt viscosity is low, and the fluidity is excellent, so that a thin-walled long process product can be formed without too high pressure during molding.
PE material 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, and the mold cooling condition has a great influence on the shrinkage rate, so the mold temperature should be controlled to keep the cooling uniform and stable.
The crystallization ability of PE material is high, and the temperature of the mold has a great influence on the crystallization state of the plastic part. The mold temperature is high, the melt cooling is slow, the plastic part has high crystallinity 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. Therefore, the plasticizing device is required to have a large heating power in order to improve production efficiency.
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 in the molding process, so as to avoid the quality of the plastic part.
PE parts are soft in texture and easy to release, so when the plastic parts have shallow side grooves, they can be strongly demolded.
The non-Newtonian properties of PE melt are not obvious, the change of shear rate has little effect on viscosity, and the viscosity of PE melt is less affected by temperature.
The PE melt has a slower cooling rate and must therefore be cooled sufficiently. The mold should have a better cooling system.
If the PE melt is fed with a direct feed port during injection, it is easy to increase the stress and increase the deformation due to uneven search and directionality. Therefore, the feed port parameters should be selected.
PE has a relatively high molding temperature, and in the flow state, a slight fluctuation in temperature has no effect on injection molding.
PE has good thermal stability, and generally has no obvious decomposition phenomenon below 300 degrees, and has no effect on quality.
Injection molding technology of PE material
Barrel temperature: The barrel temperature is mainly related to the density of PE and the flow rate of the melt material. It is also related to the type and performance of the injection molding 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 barrel temperature is controlled at 140-200 ° C. the HDPE barrel temperature is controlled at 220 ° C, the minimum is taken at the rear of the barrel, and the front end takes the maximum value.
Mold temperature: The mold temperature has a great influence on the crystallization condition of the plastic part. The mold temperature is high, the melt crystallinity is high, and the strength is high, but the shrinkage rate also increases.
Generally, the mold temperature of the LDPE is controlled at 30 ° C – 45 ° C, and the temperature of the HDPE is correspondingly higher by 10-20 ° C.
Injection pressure: increasing the injection pressure is conducive to the filling of the melt material. Since the fluidity of PE is very good, in addition to the thin-walled slender products, the lower injection pressure should be selected well.
the general injection pressure is 50-100 MPa. The shape is simple. For larger plastic parts behind the wall, the injection pressure can be lower, and vice versa.