Threaded Inserts | Molded-In Inserts

Overview

Molded-in inserts are designed to maximize their outer surface area to increase the pull-out and torque-out strength in a plastic part. While some parts simply have a diamond knurl covering the entire length, a flat groove or undercut around the outer diameter can be used to create a valley between two knurled regions that further increases pull-out strength. Blind options are commonly used in holes or bosses in larger molded parts so that the liquid plastic doesn’t infiltrate the threads during solidification, but thin boards can still use through-threaded inserts so long as the part is as tall or taller than the panel thickness. These parts are typically cut from round bar stock of brass, aluminum, or stainless steel depending on the electronic or corrosive requirements of the application.

Installation

Molded-in inserts involve the most costly method for putting metal threads in plastic due to the required tooling associated with every new part but result in the best performance when comparing pull-out and torque-out resistance. A die used for injection molding is fitted with pins that meet the minor diameter of the insert. These pins precisely and reliably place the inserts within the volume of the part, whether flush with a flat surface or at the end of a pylon. After solidification, the pins slip out of the inserts as the die is disassembled, leaving permanent, strong threads. This method is ideal for thermosetting epoxies that can’t be re-melted for heat staking and are too stiff to adequately secure a press-in insert.

Demonstration of pin-placed, threaded inserts being molded into a plastic assembly.

Watertight electrical box made from threaded inserts molded in plastic.