Page 88 - Injection Molding Advanced Troubleshooting Guide

72 8 Machine Performance

„ „8.6„ Velocity to Pressure Transfer

A key part of the injection molding cycle that is often overlooked is what occurs
when the machine hits the transfer position. At transfer the machine is changing
from controlling velocity with an abundance of pressure to controlling pressure.
How well the machine makes this transition is critical to part quality. Many defects
can occur due to a poor velocity to pressure transfer including:

ƒ Sink
ƒ Short shots
ƒ Flash
ƒ Gloss variation
ƒ Sticking
ƒ Dimensional issues
ƒ Pin push
The factors that influence velocity to pressure transition the most are:
ƒ Fill only weight:
If the cavity is not filled far enough during first stage the machine will not be
able to reach the set hold pressure effectively. In this case the machine will
typically have a period of undershooting the setpoint, and then overshoot the
setpoint before finally settling in to the set second-stage pressure.
ƒ Second-stage volume settings:
If the volume setting is not providing enough flow to achieve set pressure,
whether it is called hold, pack, or second-stage volume or speed, it is having the
same impact. Note that this is not a setting where one value works for all molds.
See Figure 8.11 and Figure 8.12 for examples of good and bad velocity to pres-
sure transfer.