Page 371 - Injection Molding Advanced Troubleshooting Guide

38.3 Read-through Troubleshooting 367

add cooling to the mold. Adding cooling after the mold has been built will be much
more expensive than if the concern was addressed during mold design, but it may
be the only option to eliminate a production problem.

38.3.2.4„ Mold: Wall Stock Changes
One of the fundamental design rules is to maintain nominal wall stock. If there is a
change in wall stock the plastic will be packed and cooled at a different rate, which
will result in differential shrinkage. During part design all efforts should be made
to limit areas of varied wall stock. Thick areas of the part should be cored out to
maintain nominal wall stock. If there is a spot on a part that cannot be cored out
try to design in a gradual transition that will help limit the chances of read through
that comes with abrupt wall stock transitions.
If a nominal wall stock change must occur on a part another factor that can help is
designing in means to hide the read-through on the class “A” surface. Ways to hide
read-through include design features that disguise the read-through such as style
lines. Another way to minimize read-though concerns is use of a textured mold
surface.

38.3.3„ Read-through Troubleshooting Machine Issues

Machine performance should be verified (see Chapter 8).

38.3.4„ Read-through Troubleshooting Material Issues

Read-through is a problem that can occur across all different types of materials.
Viscosity is the main impact that material variation has on read-through. If mate-
rial viscosity is varying over time, whether due to incoming material variation or
moisture content, the cavity pressure will also vary, which leads to problems with
variation in read-through.