Views: 75 Author: Site Editor Publish Time: 2021-02-25 Origin: Site
As a leading plastic injection molding manufacturer in China, Goodtech often uses polyethylene (PE) materials to make products during injection molding. Now I will introduce the process of PE injection molding.
Polyethylene is a crystalline raw material with extremely poor moisture absorption, no more than 0.01%, and no drying is required before processing. Polyethylene molecular chain has good elasticity, small bond force, low melt viscosity, and excellent fluidity. Therefore, thin-walled long-flow products can be formed without too much pressure during molding. Polyethylene has a large shrinkage range, a large shrinkage value, and an obvious direction. LDPE and HDPE shrink by about 1.22% and 1.5%, respectively. Therefore, it is easy to deform and warp, and the mold cooling conditions have a greater impact on the shrinkage. Therefore, the mold temperature should be controlled to maintain uniform and stable cooling.
Polyethylene has a strong crystallization ability, and mold temperature has a great influence on the crystalline state of plastic parts. Mold temperature is high, melt cooling is slow, plastic parts have good crystallinity and high strength.
The melting point of polyethylene is not high, but the specific heat capacity is large, so it still needs to consume more heat when plasticizing, so the plasticizing device is required to have a larger heating power to improve production efficiency. The softening temperature range of polyethylene is small, and the melt is easy to oxidize. Therefore, contact between the melt and oxygen should be avoided as much as possible during the molding process to avoid reducing the quality of plastic parts.
Polyethylene products are soft and easy to demold, so when the product has shallow grooves, it can be demolded strongly. The non-Newtonian properties of the polyethylene melt are not obvious, the shear rate change has a small effect on the viscosity, and the temperature has a small effect on the melt viscosity. The polyethylene melt cools slowly, so it must be cooled sufficiently. Has a good mold cooling system.
If the polyethylene melt is injected directly into the feed port, the stress should be increased, resulting in uneven shrinkage, and it has obvious directionality to increase deformation, so attention should be paid to the selection of feed port parameters. Polyethylene has a wide molding temperature, and a slight temperature fluctuation does not affect injection molding in a fluid state. Polyethylene has good thermal stability, generally, there is no obvious decomposition phenomenon below 300 degrees, and has no effect on product quality.
Polyethylene molding conditions
Cylinder temperature: The temperature of the cylinder is mainly related to the density of PE and the flow rate of the melt. It is also related to the model and performance of the plastic injection molding machine and the shape of the first-grade plastic part. Polyethylene is a crystalline polymer, the crystal grains will absorb a certain amount of heat when melted, so the barrel temperature should be above 10 degrees. For LDPE, the barrel temperature is controlled at 140-200°C, the HDPE barrel temperature is controlled at 220°C, the rear barrel temperature is controlled at 220°C, and the front end temperature is controlled at 220°C.
Mold temperature: Mold temperature has a great influence on the crystalline state of plastic parts. High mold temperature, high melt crystallinity, high strength, but increased shrinkage. In general, the mold temperature of LDPE is controlled at 30°C-45°C, while the temperature of HDPE mold is increased by 10-20°C.
Injection pressure: Increasing the injection pressure is beneficial to the filling of the melt. Because PE has good fluidity, in addition to thin-walled materials, a lower injection pressure should be selected, generally 50-100 MPa. With a simple appearance, the wall of the plastic injection molding parts is larger and the injection pressure can be lower, and vice versa.