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AIM Processing Small Plastic Parts Blog

Why Did That Happen? Common Problems With Threaded Inserts in Plastic

[fa icon="calendar"] Jun 20, 2022 9:52:51 AM / by Jon Gelston

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The measure of a plastic molding company’s abilities begins with how they tackle the manufacturing challenges that are most common in their industry. At AIM, we employ teams of highly skilled specialists who are able to tackle even the most complex issues, let alone the more common threaded insert problems. Allow AIM to walk you through these issues and demonstrate our knowledge of how to remedy each situation:

What Causes Cracking in the Plastic around Injection Molded Threaded Inserts?

One of the most common issues which plague plastic injection molded threaded inserts is cracking in the surrounding plastic. Plastic cracking is detrimental to the manufacturing process because it compromises the threaded insert’s ability to remain in the part and securely hold a mating component. So what causes this injection molding problem? In almost every instance of cracking in threaded inserts, the issue is caused by molded-in stress during the injection molding process.

Because the plastic is molten during the injection molding process, it will cool and shrink around the threaded insert. When the natural shrinking of the plastic is constrained by the brass insert, stress develops. If the stress is too great for the strength of the plastic, a crack will develop to relieve the stress. Considering the shrink of a material is important to determining the suitability of insert molding threaded inserts.

There are also cases when a part is fine after molding, but the crack develops when out in the field and potentially exposed to a chemical. The reason for this may be that some chemicals may reduce what can be known as the “critical crack propagation length” in a material. At sub-microscopic levels, exposure to some chemicals may reduce a plastic’s ability to manage molded-in stress. When this happens, the crack develops and lengthens long enough to become visible.

What Causes an Insert to Feels Tight on a Screw?

The molded-in stress from shrinkage of the surrounding plastic acts to squeeze the insert. If the stress & pressure are high enough, the insert can squeeze inward a tiny little bit. If the insert was already on the small side of the tolerance range and the screw is on the larger side, then a level of resistance develops and the insert can feel tight.

Injection Molding Leakage Affects Threaded Insert Quality

When you place an insert into the mold, the process requires molten plastic to flow around it. In addition, there is typically a pin that runs through the center of the insert to prepare it for the molding process. This pin and insert relationship needs to be right. Too tight and the insert won’t go onto the pin and you can’t make the product. Too loose and there will often be leakage of plastic in the tiny gap between the insert and the center pin. This can clog up the threads with plastic. Sourcing inserts with tighter tolerance ranges and an array of pins are the two common solutions.


Inserts are often manufactured in significant volume. In some cases, a bag or box of inserts may have small amounts of shavings from the thread-cutting process. Not all shavings drop nicely to the bottom of the bag. Many will remain loosely attached to the insert. If steps are not taken to remove this debris, it will likely shake loose during the injection molding process and migrate to the surface of the finished product. That increases scrap rates if caught or sends contaminated parts if not. Because of this, AIM is proud to maintain a painstakingly clean facility, ensuring that every project is conducted to high quality standards.

Topics: Plastic Injection Molding, insert molding, Plastic Types, Quality, Plastic vs. metal, Choosing the right plastic injection molder

Jon Gelston

Written by Jon Gelston