Page 370 - Injection Molding Advanced Troubleshooting Guide

366 38 Read-through

Case Study: Lifter Detail Deflection
In this case, a filled polypropylene part was showing visually objectionable
read-through at all lifter features. Initial troubleshooting presumed that the
temperature of the lifters was the root cause of the problem. However, there
were two indicators that the temperature of the components was not the
root cause of the read-through. Firstly, the defect was visible on the first
shot with a fully cooled mold and did not get worse, and secondly, changing
mold temperature had no effect.
As a solution the tip of the lifter was supported with a pin behind the lifter
head. This pin kept the tip of the lifter from flexing under cavity pressure
and led to elimination of the read-through on this part.


38.3.2.2„ Mold: Lifter Spot
If a lifter is not spotted such that it is bottomed in the lifter pocket, the lifter will be
forced back by injection pressure, which will lead to reading of the lifter. A lifter
should be well spotted to the bottom in the lifter pocket to minimize the amount of
lifter movement when exposed to pressure from injection.
To check lifter spot the lifter and pocket can be cleaned and blued with die blue to
inspect for contact between the pocket and lifter head.

38.3.2.3„ Mold: Cooling
To limit differential shrink it is important to ensure uniform cooling across a mold.
This often means that mold components such as slides and large lifters may need
improved cooling strategies. To improve cooling of mold components use of alter-
nate materials such as copper alloys can allow greater heat transfer. Whenever
using copper alloys to improve cooling it is critical to get water lines to the compo-
nent. If using a copper alloy without water lines to it the copper alloy will conduct
the heat from the plastic efficiently but if the component is not cooled the result
will be that the component runs hotter than the surrounding mold. Always provide
cooling water to alternate materials such as copper alloys to provide an outlet for
the heat that is transferred from the plastic. There are many ways to sneak small
bubblers into mold details (see Chapter 14 on mold cooling).
Often times it is difficult to determine if a hot lifter, lifter movement, or deflection
is the root cause of read-through. Experience has shown that the more typical
cause is deflection or movement of the lifter rather than temperature. However,
remember that both of these issues must be addressed during mold design.
Mold surface temperatures and part ejection temperatures will help determine
where hot spots are occurring in the mold. If you are experiencing chronic mold
cooling issues, have an experienced tooling person investigate options to creatively