The function of the screw is to convey, melt, meter, and inject a thermoplastic material. The general purpose screw that is contained in the barrel is rated by L/D ratio (ratio of flighted length of screw to the outside diameter of the screw) and Compression ratio (ratio of the depth of a flight in the feed section divided by the depth of a flight in the metering zone).
The L/D ratio indicates the relative length of the screw and the compression ratio indicates the amount the screw compresses the plastic. Screw flights are the ridges that protrude from the central shaft (root) of the screw. A general purpose screw is comprised of three sections:
- feed
- transition or compression
- metering
Feed Section:
Section closest to the hopper. It makes up 50% of the flighted screw length. The flights in this section are deep and uniform. The purpose of the feed section is to compact and convey material forward to the other sections.
Transition or Compression Section:
This section is located in the middle of the screw. It makes up 25% of the flighted screw length. The flights in this section make a transition from deep flights toward the hopper to shallow flights toward the nozzle. The transition or compression sections purpose is to melt the material through shear. Shear is frictional heat generated by the plastic layers moving in relation to each other between the screw and barrel. The shear generated in this section provides 70-80% of the heat required to melt the plastic.
Metering Section:
This section is located closest to the nozzle. It makes up 25% of the flighted screw length. The flights in this section are shallow and uniform. The metering sections purpose is to provide a homogenous melt and to deliver this melt in front of the screw.
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