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General Purpose Screws
A GP, or general purpose screw, has been the mainstay of the plastic industry, but more prevalent to the injection molding industry. The design has many meanings to many different people, but most will agree that the GP screw does an adequate job with most resin groups but does not reach maximum melting or mixing efficiencies with any group.
General purpose designs (we like to label the design as a single flighted compression screw utilizing a medium compression ratio) vary by manufacturer with feed, transition and metering lengths and depths being very different; thus, different shear rates, heat histories and melt qualities are evident throughout the general purpose screw population.
Spirex has taken a different approach to the GP screw. We have designed very specific feed depths, compression ratios and section lengths. For every screw diameter and L/D, our GP design utilizes compression ratios from 2.8:1 (on small diameter screws) to 2.6:1 (on large diameter screws) which is higher than most of the industry, but we feel that represents the need for better melting through the range of resin types.
The basic inadequacy of the GP design is in its ability to mix or homogenize the melt pool adequately to meet the higher demands of part quality and yields that we have today. The laminar flow path that is developed is not designed to reorient the polymer, which is desirable for achieving better melt quality.
While we at Spirex do not recommend the use of a GP design very often, we still manufacture many for general applications. Rather, we prefer to direct the customer to a low-shear, distributive mixing section (patented Pulsar® or V-Mixer™ designs) with a medium compression ratio that delivers a more homogenized melt pool (see ratings) and can process the same variety of materials.
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