The Designer's Edge: Plastic Part Sticking, Part 2

 

Oftentimes a sticking issue is easily resolved, but there are always challenging ones. When you are investigating a sticking issue, the first step is to remove anything obvious from the list of possible root causes, including burrs, damage and unpolished areas. If the issue persists, your next step depends on which side of the tool the issue resides. If the part is sticking to the cavity (nonmoving half), add undercuts to the core (moving half).

Remember that some materials will stick to uncooled areas or details (such as GF nylons) from a lack of cooling in the tool. Other materials release easily with an increase in temperatures like that of polypropylene. Still others will run varying water temperatures to address sticking issues. You also need to be careful with running different water temperatures, especially on large tooling. The difference in thermo-expansion can cause wear adn other unwanted issues.

And the same is true with a tool's surface finish. Some materials will stick to highly polished surfaces and prefer a paper draw polish. For others, the shinier the better. I've had numerous instances where removing a high polish and moving to a draw polish was like turning on a light switch, eliminating the sticking issue. Again, this is material specific. Some materials prefer a higher polish and others, such as TPE or TPU (rubber) prefer a textured or blasted surface for release.  It all depends on the durameter of the TPE/TPU. Some prefer a glass bead finish versus an aluminum oxide finish. On parts that require high polished surfaces, coatings can help to reduce sticking or drag marks.

I hope sharing these variables will help you diagnose future sticking problems. Later this month, I will share how to hold the part to the core (moving half) without aggressive undercuts.