Page 172 - Injection Molding Advanced Troubleshooting Guide

17.3 Brown Streak Troubleshooting 161

17.3.3.5„ Machine: Screw Design
Any given screw geometry is not going to be the correct geometry for every mate-
rial. For materials that are shear sensitive high compression ratios may lead to
degraded material from the excess shear. The compression ratio of the screw is the
difference between the flight depth in the feed zone of the screw and the flight
depth in the metering zone of the screw. Compression ratios for thermoplastic in-
jection molding usually range from 1.5 : 1 to 4.5 : 1 with “GP” screws usually falling
between 2.5 : 1 and 3 : 1. The higher the shear rate the more likely shear-sensitive
materials will degrade. Many amorphous materials will react better with low com-
pression ratios.

The screw L/D can impact material degradation and lead to brown streaking. L/D
refers to the screw flighted length over the diameter of the screw. Larger L/D ratios
can provide more gradual application of shear to the plastic. Although a larger L/D
(greater than 20 : 1) provides a more gradual introduction of shear it can also lead
to longer residence time that could cause brown streaking.

There are a variety of mixing elements available for injection molding screws.
Many of these mixing elements help create melt uniformity and improved color
mixing. There are mixing elements that will lead to high shear rates and may de-
grade the material leading to brown streaks.

Case Study: Brown Streaking from Screw Design

In this case study a machine was running a PC/ABS material with a colorant
concentrate added at 4%. The job was running well but another screw manu-
facturer wanted to evaluate their mixing screw in our process. The original
screw was replaced with the new screw and immediately scrap from brown
streaking started. A variety of process adjustments were made working with
the new screw, but nothing seemed to resolve the problem. After running for
about 2 weeks with excess scrap the screw was finally replaced with the
original screw, which resolved the scrap problem. Examination of the new
screw showed that the root of the screw flights seemed to have gotten very
hot (steel changed color). It appeared that the mixing zone on this screw
was creating way too much shear. The screw designer had some ideas on
changing geometry but they ended up missing their opportunity, because no
one wanted to continue trying to “fix what wasn't broken”, so the original
screw stayed in the machine.


Processors should at least understand what type of screw and what the key num-
bers are for the screw in the machine of choice. Decisions can be made about
whether the screw is suitable for the material to be molded. Keep in mind that the
term “GP” is a relative term and one “GP” screw is not necessarily the same as
another.