Page 369 - Injection Molding Advanced Troubleshooting Guide

38.3 Read-through Troubleshooting 365

38.3.1.2„ Molding Process: Mold Temperature
Uneven mold temperatures can lead to a difference in gloss levels on a molded
part. If a mold component is not cooled well enough the results will be differential
shrink. This differential cooling and differential shrink can lead to variation in
gloss levels.
Ensure that the mold surface is running evenly. Surface temperature checks with
a pyrometer will provide information on the actual running temperature of the
mold. Do not rely on thermolator set temperature to determine mold temperature.
Verify the surface temperature and part ejection temperature versus the docu-
mented process.

38.3.2„ Read-through Troubleshooting Mold Issues

The true root cause for read-through is most often a mold-related concern such as:

ƒ Component deflection
ƒ Lifter spot
ƒ Cooling
ƒ Wall stock changes

38.3.2.1„ Mold: Component Deflection

Mold component deflection is one of the most typical reasons for read-through.
When a mold component deflects under molding pressures it will tend to spring
back when the pressure drops, which can result in read-through.
Components that commonly experience deflection are ejector pins, lifter rods,
cores, and ejector plates. The shape of the component will be what is normally seen
in the read-through defect.
When it comes to mold components such as ejector pins, lifter rods, and cores the
longer the unsupported length the more likely the component will bend under
cavity pressure and then flex back against the plastic resulting in read-through on
the part. Always make sure that there is adequate bearing surface and support to
minimize the amount of unsupported length to avoid deflection.
To limit deflection of ejector plates it is necessary to have ejector plate stop buttons
located behind the ejector plate to back up the area of deflection. If only the stan-
dard four-stop buttons are used there is opportunity for the mold components to
flex the ejector plate, which will result in read-through. During mold design utilize
adequate ejector plate stop buttons to limit ejector plate deflection.