Twenty Kinds of Plastics and Their Properties—(9) PE-HD
9. PE-HD high density polyethylene
Typical application range:
Refrigerator container, storage container, household kitchen utensil, seal cover and so on.
Injection molding process conditions:
Drying process: no drying is needed if the storage is properly stored.
Melting temperature: 220~260 °C. For large molecular materials, it is suggested that the melting temperature range mbetween 200~250 °C.
Mold temperature: 50~95 °C. High mould temperature should be used for plastic parts with thick 6mm wall below, and lower mould temperature is used for plastic parts above 6mm wall thickness. The cooling temperature of the plastic parts should be uniform to reduce the difference in the shrinkage rate. For the optimal processing cycle time, the cooling chamber diameter should be no less than 8mm, and the distance from the die surface should be within 1.3d (here, “d” is the diameter of the cooling chamber.)
Injection pressure: 700~1050bar.
Injection speed: it is recommended to use high speed injection.
Runner and gate: the channel diameter is between 4 and 7.5mm, and the length of the channel should be as short as possible. All types of gates can be used, and the length of the gate should not exceed 0.75mm. It is especially suitable for using hot runner mold.
Chemical and physical properties:
The high crystallinity of PE -HD leads to its high density, tensile strength, high temperature distortion temperature, viscosity, and chemical stability. PE-HD has stronger anti permeability than PE-LD. The impact strength of PE-HD is low. The characteristics of PH-HD are mainly controlled by the distribution of density and molecular weight. The PE- HD molecular weight distribution suitable for injection mold is very narrow. For density 0.91~ 0.925g/cm3, we call it the first type PE-HD; for density 0.926~ 0.94g/cm3, it is called the second type PE-HD; for density 0.94~ 0.965g/cm3, it is called the third type PE-HD.
The flow characteristics of the material are very good, MFR is between 0.1 and 28. The higher the molecular weight, the worse the flow characteristics of PH-LD, but better impact strength. PE-LD is a semi crystalline material, and the shrinkage rate is high between 1.5% and 4% after forming. The phenomenon of environmental stress cracking is easy to occur in PE-HD. It is possible to reduce the internal stress by using materials with very low flow characteristics to reduce the cracking. PE-HD is easy to dissolve in a hydrocarbon solvent when the temperature is higher than 60, but its solubility is better than that of PE-LD.