Brass Insert Installation — Permanent Threads in Plastic Parts

Heat-installed threaded brass inserts give a plastic part a permanent, reusable metal thread that holds machine-screw torque and survives repeated assembly cycles. The insert is heated, pressed into a moulded pilot hole, and the surrounding plastic flows around the knurls and undercuts as it solidifies. The result is a thread that pulls out harder than a self-tapping screw, does not strip on the second service call, and stays in place across the product's service life.

We install brass, stainless, and aluminium inserts on our benchtop and custom presses, single or multi-up. Common applications include electronic housings that need a serviceable cover, automotive consoles with cosmetic A-surfaces, medical device assemblies that need cleanroom-compatible processing, and any consumer product where a self-tapping screw is failing in the field. The same impulse-heated tooling that runs heat staking on our presses also runs brass insert installation — one machine, two processes, one set of operator skills.

How the Process Works

The insert sits on the press tip with the knurled end facing down toward the moulded pilot hole in the part. The tip heats the insert (impulse-controlled, so it does not soak heat across the whole press), the press head advances, the heated insert melts into the pilot hole, the plastic flows around the knurls and undercuts, and the tip cools and retracts. The insert is now mechanically locked into the plastic by the re-solidified resin. Cycle time is a few seconds per insert; multi-up tooling installs up to eight inserts in one cycle.

Why Heat Installation Beats Press Fit

A pressed-in brass insert relies on interference between the insert knurls and the cold plastic of the pilot hole. That works on benign resins at room temperature for a few cycles. Heat installation melts the plastic and lets it re-solidify around the insert, which means the plastic flows into every knurl and undercut and locks the insert against pull-out and rotational torque. The difference shows up the first time the housing sees a service technician with a power driver, and again the first time it sees a temperature cycle in the field.

Selecting the Right Insert

The right insert depends on the host resin, the screw size, the load case, and how often the joint will be opened and closed. Brass is the standard for most thermoplastics; stainless is the choice for medical, cleanroom, or harsh environment applications; aluminium is occasionally used to save weight. Knurl pattern, undercut depth, head shape, and overall length all affect pull-out and torque performance. We help specify the insert, the boss geometry, the pilot hole diameter, and the install parameters as part of every quote — the insert and the boss have to be designed together.

Related Pages

For the long-form guide on insert selection, design rules, and failure modes see Heat Staking Inserts: The Honest Guide. For impulse heat staking (rivet-style head formation) see Impulse Heat Staking. For the press that runs both processes, see Model BTP Benchtop Press. For multi-station automation, see Custom Automation.