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Plastic Material PE-HD
Plastic Material PE-HD

 
PE-HD (High Density Polyethylene)


Applications:


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

Injection Moulding Processing Conditions:

DryingGNot normally necessary if stored properly.

Melt TemperatureG220-260 J (for high molecular weight resins, the suggested melt temperature range is 200-250 J).

Mould TemperatureG50-95 J (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 minimise 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 mould surface (where D is the diameter of the cooling channel).

Injection PressureG70-150 MPa.

Pack PressureGUp to 75 MPa (holding time is directly proportional to gate size and part cross-sectional area).


Injection SpeedGFast injection speeds are recommended; profiled speeds 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 moulds; an insulated hot tip runner is preferred when there are frequent colour changes.

Chemical and Physical Properties

(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, PE-HD has a high level of crystallinity (compared to PE-LD).
Higher levels of crystallinity contribute to higher density, tensile strength, heat distortion temperature, viscosity, and chemical resistance. PE-HD is more resistant to permeability than PE-LD. The impact strength is lower. The properties of PE-HD are controlled by the density, and molecular weight distributions. Injection moulding grades typically have a narrow molecular weight distribution.

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 moulding shrinkages are 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 minimised by reducing internal stresses by proper design and using the lowest MFR material at a particular density level. PE-HD is soluble in hydrocarbons at temperatures greater than 60oC, but resistance to these materials is greater than that for PE-LD.