Page 155 - Injection Molding Advanced Troubleshooting Guide

16.3 Blush Troubleshooting 143

advantage both to maintain a more consistent viscosity as well as minimizing cycle
time. Avoid processing around tooling issues!
Improving blush whether it is at the gate or downstream from the gate (such as a
wall stock change) often will come down to adjustments in velocity settings. Nor-
mally slowing the velocity will improve the blush. Sometimes creating a velocity
profile to allow a slow fill through the area of concern will be the best solution.
Remember that when adding velocity profiles, the level of complexity in the pro-
cess just increased, which will make it more difficult to repeat.
When making process adjustments for velocity settings it is often advantageous to
make a large change to the set point to see if it will have an effect. Often times a
change of 50% to the initial velocity will be appropriate to determine impact. There
will be cases where the velocity that eliminates the blush will only be a small frac-
tion of what the desired fill velocity is for the mold.
If the blush is appearing at a downstream location such as a wall stock change it
may be necessary to profile the velocity through this area of the part. To determine
where to set the profile it will be important to run short shots to determine the
machine profile settings to allow the velocity to be adjusted at the right point of the
shot. The following sequence describes how to find this point:
1. Set pack and hold pressure to zero or the minimum setting for the machine.
2. Adjust the machine’s transfer position to a larger set point.
3. Examine the part produced; it should be a short shot that is shorter than the fill
only part (95–98%).
4. If the short shot is not at the point where the problem is occurring continue to
complete steps 2 and 3 until the short shots reach the point where the problem
occurs.
5. The setting for the adjusted transfer position will be the setting that needs to be
used for the velocity profile change. Enter this position as the point where the
velocity profile slows down.
6. If possible increase the velocity after the area of concern by adding a step in the
profile where the velocity is changed back to a faster setting.
7. Reset the original transfer position and check a fill only shot (95–98% full), and
keep in mind that the slower velocity may result in a shorter fill only shot. If
required, adjust the transfer position to give the desired fill only shot.
8. Put pack and hold pressure back to the set point.
9. Verify the parts are better. It may take some fine tuning of the profile set points
as well the velocity through the problem area to resolve the issue.