Selecting Injection Molding Screws

Using the right injection molding screws is crucial to making quality parts consistently and with maximum production output.
To select the right screw, details of the particular part to be moulded must be known (that is, part material, weight, size and wall thickness). The basic mould design is also important so that flow length from gate, shot weight and runner system are all taken into consideration.
Choosing a screw without knowledge of the parts is like buying a car without any preference for performance and handling requirements.
The basic design of any screw has 3 zones along its length:
1. Feed zone2. Transition zone3. Metering zone
The feed zone conveys the solid plastic pellets which are fed from the hopper to the transition zone where they are compressed by a change in screw geometry. This compression forces the pellets to melt through the action of pushing up against each other. This is called shearing. The metering zone then conveys the melt to the front of the screw ready for injection into the mould cavity.
In the transition zone the material is compressed by the change in the depth of the screw channels from the feed zone to the metering zone. The ratio of the change in depth is called the compression ratio and

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