LDPE processing conditions

Generic Class

LDPE (Low Density Polyethylene)

Typical Applications

Closures, bowls, bins, pipe couplings.

Injection Molding Processing Conditions


Not usually necessary 
Melt Temperature  180 - 280 C (355 - 535 F) 

Mold Temperature

20 - 70 C (68 - 158 F) 
For uniform and economic heat removal, it is recommended that the cooling channel diameters be at least 8 mm and the distance from the surface of the mold to the edge of the cooling channel be not more than 1.5 times the diameter of the cooling channel. 

Material Injection Pressure

Up to 150 MPa

Pack Pressure

Up to 75 MPa

Injection Speed

Fast speeds are recommended; profiled speeds can limit warpage problems of large surface area parts. 

Runners and Gates

All conventional types may be used; LDPE is well suited for hot runner molds. Insulated hot tip runners are preferred for frequent color changes.

Chemical and Physical Properties

Low density polyethylene is produced by the polymerization of ethylene at high pressure and temperature. The material is semicrystalline-crystalline. The crystallinity level is low because of chain branching. The material is tough but possesses moderate tensile properties and exhibits creep. However, it has good impact and chemical resistance. It is an easy flow material because of long chain branching.

Commercial materials have densities in the range of 0.91 - 0.94 g/cm^3. LDPE is permeable to gases and vapors. Very close tolerances are not possible with this material and its relatively large coefficient of thermal expansion makes it less suitable for long term applications.

Shrinkage is of the order of 0.02 - 0.05 mm/mm (2 - 5%) when density is between 0.91 - 0.925 g/cm^3. When density is between 0.926 -.04 g/cm^3, the shrinkage is of the order of 1.5 - 4%. Actual shrinkage values are dependent on the molding conditions.

LDPE is resistant to many solvents at room temperatures but aromatic and chlorinated hydrocarbons cause swelling. Like HDPE, it is also susceptible to environmental stress cracking.