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Performance Analysis of High Density Polyethylene HDPE
Chemical and physical properties:
The high crystallinity of PE-HD results in its high density, tensile strength, high temperature distortion temperature, viscosity and chemical stability.
PE-HD has stronger resistance to permeation than PE-LD. PE-HD has a low impact strength. The characteristics of PH-HD are mainly controlled by density and molecular weight distribution. The PE-HD molecular weight distribution suitable for injection molding is very narrow. For the density of 0.91 - 0.925g / cm3, we call it the first type of PE-HD; for the density of 0.926 - 0.94g / cm3, called the second type of PE-HD; for the density of 0.94 - 0.965g / cm3 It is called the third type PE-HD.
The material has good flow characteristics and an MFR between 0.1 and 28. The higher the molecular weight, the worse the flow characteristics of PH-LD, but the better the impact strength.
PE-LD is a semi-crystalline material with a high shrinkage after molding, between 1.5% and 4%.
PE-HD is prone to environmental stress cracking. Cracking can be mitigated by using materials with very low flow characteristics to reduce internal stresses. PE-HD is easily dissolved in hydrocarbon solvents when the temperature is higher than 60C, but its solubility resistance is better than PE-LD.
Injection molding process conditions:
Drying: No drying is required if stored properly.
Melting temperature: 220-260C. For larger molecules, the recommended melting temperature range is between 200 and 250 °C.
Mold temperature: 50-95C. Plastic parts with a wall thickness of 6 mm or less should use a higher mold temperature, and plastic parts with a wall thickness of 6 mm or more use a lower mold temperature. The plastic part cooling temperature should be uniform to reduce the difference in shrinkage. For optimum machining cycle time, the cooling tunnel diameter should be no less than 8 mm and the distance from the mold surface should be within 1.3 d (where "d" is the diameter of the cooling tunnel).
Injection pressure: 700-1050 bar.
Injection speed: High speed injection is recommended.
Runners and gates: The runner diameter is between 4 and 7.5 mm and the runner length should be as short as possible. Various types of gates can be used, and the gate length should not exceed 0.75mm, which is especially suitable for hot runner molds.