Screw Specification

The general purpose screw is the most common screw found in our industry because it can be used to process many different plastics materials. Other, more specialised screw designs exist to run special materials such as vinyl or thermosetting materials.

Figure 6. General Purpose Screw

A term used to specify screws is the length over the diameter ratio or “L over D”(L/D). This ratio will give you an idea of the amount of mixing that will take place. The higher the ratio- the more mixing.

The compression ratio is the ratio of the depth or volume of material in the metering section to the depth or volume of material in the feed section.

This gives an indication of how much the material is compressing. The more compression- the more sheer (heat).

General purpose screws typically have:
· L/D ratios of 16:1 to 25:1
· Compression ratios of 2:1 to 3.5:1

Plastics pellets enter the screw and barrel assembly through the hopper and enter the feed zone where the flights of the screw are the deepest.