Page 212 - Injection Molding Advanced Troubleshooting Guide

22.3 Color Swirls Troubleshooting 203

This additional restriction can create a situation where the process is actually pres-
sure limited due to the restriction. Another concern with mixing nozzles and mix-
ing tips is the possibility that the mixing elements can trap material, potentially
causing contamination and degradation of material.

Case Study: Screw Design
This case involved a PC/ABS automotive trim piece. The part was molded in
a press with a general-purpose screw and was experiencing nearly 10%
scrap due to color swirls. In an attempt to minimize the color swirl defects
the back pressure of the machine was maximized, screw recovery was maxi-
mized, and a mixing nozzle was used, but all still with ongoing part defects.
A new mixing screw was purchased for the machine and the mold was
sampled. After the screw was replaced the color swirl problem on this mold
was completely eliminated. Back pressure reduced from 300 psi hydraulic
to 100 psi hydraulic and the mixing nozzle was eliminated.


22.3.3.2„ Machine: Inadequate Back Pressure
It is crucial in processing to understand if the molding machine is actually doing
what you are asking it to do. An all too common problem is a machine that is not
actually achieving the set point for pressure. When considering back pressure, a
low actual pressure on a molding machine can translate into poor color mixing and
lead to color swirls. Actual pressure can be verified in a number of ways including
on-press hydraulic gauges, secondary in-line gauges added to test ports, or process
monitoring equipment such as the RJG eDART. If the machine has a back pressure
setting of 150 psi hydraulic pressure but is only delivering 50 psi of actual pres-
sure the machine must be checked for pressure calibration.

Also keep in mind the impact of the intensification ratio if moving a mold from one
machine to another. You cannot expect the same melt quality if you use the same
hydraulic pressure setting on a machine with a lower intensification ratio. The in-
tensification ratio is a critical value to know for every machine to understand the
actual pressure being applied to the plastic.

Example: Intensification Ratio
If machine “A” has a 15:1 intensification ratio a hydraulic back pressure set
point of 100 psi will yield 1500 psi in actual plastic pressure. If this is
compared to a machine that has a 9:1 intensification ratio the actual plastic
pressure will only be 900 psi. This difference will potentially make a signifi-
cant change to the quality of the melt produced by the machine.
