Common Problems With Threaded Inserts in Plastic

A plastic molding company’s ability is measured by how they manage the most common manufacturing issues in their industry. V1 Plastic employs teams of highly qualified specialists capable of handling even the most complex problems, let alone the more common threaded insert issues. Allow V1 Plastic to walk you through these challenges and demonstrate our understanding of how to solve them:

What Causes Plastic Cracking Around Injection-Molded Threaded Inserts?

Cracking in the surrounding plastic is one of the most typical problems that bothers plastic injection-molded threaded inserts. Plastic cracking is bad for the manufacturing process because it jeopardizes the threaded insert’s ability to stay in the product and securely hold a mating component. So, what is causing this injection molding issue? Cracking in threaded inserts is always caused by molded-in tension during the injection molding process.

Inserts in Plastic
Inserts in Plastic

The plastic will cool and shrink around the threaded insert since it was molten during the injection molding process. Stress happens when the natural shrinking of the plastic is restricted by the brass insert. If the stress is too strong for the plastic’s strength, a fracture will form to release the load. When determining the suitability of insert molding threaded inserts, material shrinkage must be considered.

There are also instances where a part is good after molding but develops a crack while out in the field and exposed to chemicals. The explanation for this could be that some chemicals diminish what is known as a material’s “critical crack propagation length.” Some substances may decrease a plastic’s ability to manage molded-in stress at sub-microscopic levels. When this happens, a crack forms and grows long enough to be seen.

What Causes Tightness in a Screw Insert?

Molded-in stress caused by the shrinking of the surrounding plastic squeezes the insert. If the stress and pressure are high enough, the insert may be able to squeeze inside slightly. If the insert is already on the small side of the tolerance range and the screw is on the large side, resistance occurs, and the insert may feel tight.

Threaded Insert Quality Is Affected by Injection Molding Leakage

When you insert an insert into the mold, molten plastic must flow around it. Additionally, a pin passes through the center of the insert to ready it for the molding process. This pin-to-insert relationship must be correct. If the insert is too tight, it will not fit onto the pin, and you will be unable to manufacture the product. If the insert is too loose, there will frequently be plastic leakage in the tiny gap between the insert and the central pin. This can cause the plastic to clog the threads. The two most popular methods are to source inserts with tighter tolerance ranges and an array of pins.


Inserts are frequently produced in copious quantities. In rare situations, a bag or box of inserts may contain minor amounts of thread-cutting shavings. Not every shaving falls to the bottom of the bag. Many will cling to the insert. If no action is taken to remove this debris, it will shake loose during the injection molding process and migrate to the end product’s surface. If this is discovered, it raises scrap rates or sends contaminated parts. As a result, V1 Plastic is pleased to keep its facility meticulously clean, ensuring that every job is completed to high quality standards. 

Scroll to Top