Page 454 - Injection Molding Advanced Troubleshooting Guide

454 45 Voids

High mold temperatures can lead to increased levels of shrink in the molded part.
The increase in shrink can result in voids forming internally in the part. Mold tem-
perature can be a tradeoff between increased pressure drop due to a cool mold and
increased shrink rate in a hot mold. Balancing the heat input against the heat ex-
traction during the molding process is a key step to producing quality parts.
Part ejection temperature and mold surface temperature will provide key infor ma-
tion on whether the mold temperature is running correctly. These are details that
should be documented during process development to provide troubleshooting in-
formation. If the mold surface or part ejection temperature is running at the wrong
temperature verify the following:
ƒ Thermolator set point and actual water temperature
ƒ Flow rate of coolant
ƒ Correct water circuit layout
Proper cooling is critical to providing a repeatable part. Ensure that the process
conditions match the established process.

45.3.1.4„ Molding Process: High Melt Temperature

A higher melt temperature will lead to a longer time to cool the part. This increased
cooling time can create a void when the outer wall of the part freezes well against
the mold but the interior plastic continues to cool and shrink. Depending on how
resilient the frozen wall is will impact whether the part forms a sink or a void. If
the frozen wall is stiff and will not move due to the internal stresses created by
differential cooling the plastic may form a void.
Check the process melt temperature against the documented process. If the melt
temperature is high investigate the potential causes of the increased melt tempera-
ture. Many factors can impact melt temperature including:
ƒ Barrel temperatures
ƒ Screw recovery rate
ƒ Back pressure
ƒ Screw design
All of the above factors should be compared to the intended process settings to
validate that the process is running correctly. If anything is not running correctly
make the required adjustments to bring the process back to the intended settings.

45.3.2„ Voids Troubleshooting Mold Issues

Mold-related causes of voids may include:
ƒ Cooling