Plastic Injection Molding Processing Conditions

HDPE processing conditions

Generic Class

HDPE (High Density Polyethylene)

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Typical Applications

Containers in refrigeration units, storage vessels, household goods (kitchenware), seal caps, bases for PET bottles, etc. Major use is in blow-molding applications (packaging applications)

Injection Molding Processing conditions

Drying Not normally necessary if stored properly. Melt Temperature 180 – 280 C (356 – 536 F); for high molecular weigh grades, the suggested melt temperature range is 200 – 250 C (392 – 482 F) Mold Temperature 20 – 95 C (68 – 194 F) (higher temperatures for wall thickness of up to 6 mm; lower temperature for wall thicknesses greater than 6 mm.) The cooling rate should be uniform to minimize shrinkage variations. For optimum cycle times, the cooling channel diameters should be at least 8 mm and must be within a distance of 1.3 d from the mold surface (where “d” is the diameter of the cooling channel). Material Injection Pressure 70 – 105 MPa Injection Speed High injection velocity is recommended; profile injection velocity can be used to reduce warpage in the case of components with a large surface area.

Runners and Gates

Diameters of runners range from 4 – 7.5 mm (typically 6 mm). Runner lengths should be as short as possible. All types of gates may be used. Gate lands should not exceed 0.75 mm in length. Ideally suited for hot runner molds; an insulated hot tip runner is preferred when there are frequent color changes.

Chemical and Physical Properties

High density polyethylene is produced from polymerization of ethylene (lower temperature and pressure conditions are used compared to the production of low density polyethylene). The material is free from branching and this is made possible by the use of stereospecific catalysts. Because of molecular regularity, HDPE has a high level of crystallinity (compared to LDPE).

Higher levels of crystallinity contribute to higher density, tensile strength, heat distortion temperature, viscosity, and chemical resistance. HDPE is more resistant to permeability than LDPE. The impact strength is lower. The properties of HDPE are controlled by the density, and molecular weight distributions. Injection molding grades typically have a narrow molecular weight distribution.

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When the density is 0.91 – 0.925 g/cm^3, the material is known as Type 1; Type 2 materials have densities in the range of 0.926 – 0.94 g/cm^3, and Type 3 materials have densities in the range of 0.94 – 0.965 g/cm^3.

The material flows easily and the MFR ranges from 0.1 – 28. Higher molecular weights (lower MFR grades) have better impact resistance.

Being a semicrystalline material, the molding shrinkage is high (order of 0.015 – 0.04 mm/mm or 1.5 – 4%). This is dependent on the degree of orientation and level of crystallinity in the part (which in turn are dependent on processing conditions and part design).

PE is susceptible to environmental stress cracking, which can be minimized by reducing internal stresses by proper design and using the lowest MFR material at a particular density level. HDPE is soluble in hydrocarbons at temperatures greater than 60 C, but resistance to these materials is greater than that for LDPE.

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