Page 174 - Injection Molding Advanced Troubleshooting Guide

17.3 Brown Streak Troubleshooting 163

result in brown streaking. Usually when these types of contamination occur they
will be short lived because it was most likely an isolated case, for example if some-
one spilled something in a gaylord of material.
Material can be contaminated during the grinding process. On jobs where regrind
is allowed precaution must be taken to ensure that the regrind does not become
contaminated with another material. Setting up a robot or sprue picker to automat-
ically drop the runner in the grinder is a good way to help stay consistent when
using regrind. Some shops will not allow an operator to grind any parts so that the
risk of contamination is minimized.
Residue from a darker color will often cause dark colored streaks. Thorough clean-
ing and purging during changeover are the key to success.
For more information see Chapter 10 on purging.

17.3.4.3„ Material: Color Concentrate
Brown streaking can sometimes link back to the color concentrate being used for
the material. If the carrier used for the concentrate cannot withstand the same
process temperatures as the base plastic the result could be brown streaking from
degradation of the carrier resin. It is also possible that the color concentrate is not
mixing well with the material, which may result in streaking that can look like
material degradation. Also verify that the color concentrate is being added at the
correct let down ratio, because too high of a colorant ratio can result in a variety of
appearance issues including streaking.