Vent, Screw and Cooling Designs - Injection Molding

Injection Molding Vent Design

Why Back Pressure is Important in Injection Molding

Back pressure is the pressure on the front of the screw during screw retraction to prepare the next shot. Increasing the back pressure collects more material in front of the screw. If the injection speed strokes are not changed, the part weight will be heavier than before because of the higher compression taking place in the same volume in front of the screw.

Injection Molding Screw Design

Compression ratio = H1/H2

L/D = Flighted Length / Screw Diameter

Reciprocating screw injection molding machines are equipped with a general purpose screw design and are adequate for processing the material

This screw design should have 50% of its length at the feed zone, 25% at the compression zone, and 25% at the metering zone. A screw L/D (length to diameter) ratio between 18:1 and 24:1 and a screw compression ratio between 2:1 and 3:1 are recommended. A floating check ring, rather than a ball check, is also recommended. Nozzles should have a free-flow design, and be as short as possible.

Injection Molding Cooling Design

The cooling channels must be installed correctly. If there are any voids between them, the cooling effect on the part is not balanced. This is why warpage occurs.