Page 345 - Injection Molding Advanced Troubleshooting Guide

35.3 Part Sticking on Cover Troubleshooting 341

the depth of the texture may need to be reduced before parts can be successfully
molded without sticking and scuffing.
When it comes to polished surfaces a 320 draw polish can provide a good polished
surface to provide adequate release. There are many cases, especially with softer
materials such as polypropylenes, TPE, or TPU, where an increase in polish level
will lead to more of a vacuum effect between the plastic and the mold surface, lead-
ing to increased sticking.

35.3.2.4„ Mold: Vacuum
On highly polished molds there will be cases where the plastic has an adhesive
force on the steel creating a vacuum against the steel. This is most typical with soft
materials and makes sticking a potentially major concern.

If the part is sticking due to high-polish vacuum issues, the first option to evaluate
is the impact of a less polished surface. There are cases when the part is brought
up to a polish level that is greater than what is required, so if the polish can be re-
duced the part may come free. A second option would be incorporating an air pop-
pet in a hidden location. The air poppet will allow air to flow between the part and
the mold surface breaking the vacuum.
Understand the polish requirements for a given part and determine if that will
work for the chosen material. If molding a material such as TPU a polished surface
will create major sticking problems, but a sand-blasted finish will allow easy re-
moval of the molded part.

35.3.2.5„ Mold: Cavity Balance
A multicavity mold should be balanced within 3% for all cavities. If the mold is
imbalanced between cavities the process that each cavity experiences will be dif-
ferent. Unbalanced molds will have different cavity pressures, which in turn can
lead to different shrink rates and make some cavities stick while others do not.
Run a cavity balance study to determine what the balance of the mold is. It should
be less than 3%. If the mold is showing greater than 3% imbalance, investigation
into the root cause must be undertaken.
For more details see Chapter 12 on cavity balance.

35.3.2.6„ Mold: Buildup
Sometimes buildup from material by products will cause a part to stick. The
buildup may be sticky to the material or actually even create slight undercuts if
thick enough.
Verify that the mold surface is thoroughly cleaned. If buildup is an ongoing issue
the mold should be vented better. Utilize the proper cleaner combination to remove