Threaded Inserts: Heat-Set, Press-Fit & Helical for Stronger Threads

Threads cut directly into plastic strip easily, and threads tapped into soft metals like aluminium can gall or pull out under repeated assembly. When a part has to be bolted together many times, or when it is moulded in plastic, a threaded insert gives it a durable metal thread that far outlasts the parent material. Choosing the right insert — and designing the hole for it — is a small decision that prevents a common and frustrating failure.

This guide explains the main families of threaded inserts, when to use each, and the hole and boss design rules that let them hold. It is written for design engineers specifying inserts in machined metal, sheet metal and moulded plastic parts.

Why Use a Threaded Insert at All?

Inserts solve two problems. In plastics, moulded or machined threads are weak, wear quickly and cannot take much torque; a metal insert provides strong, repeatable threads for screws that will be removed and refitted. In soft metals and thin sections, inserts add strength, repair a stripped thread, or let a thin wall accept a fastener it could not otherwise hold. The result is a joint that survives many assembly cycles without stripping.

The Main Types of Threaded Insert

Heat-set inserts

The standard choice for thermoplastic parts. A heated tip (soldering-iron style or ultrasonic) melts the surrounding plastic so the insert's external knurls and undercuts sink in; as the plastic re-solidifies it grips the insert firmly. Heat-set inserts give excellent pull-out and torque in plastics and are ideal for 3D-printed and injection-moulded enclosures.

Press-fit inserts

Pressed cold into a straight hole, relying on knurls or a slight interference to hold. Simpler than heat-set but with lower pull-out strength; suited to lighter-duty plastic and soft-metal applications where no heat equipment is available.

Self-tapping inserts

These have an external thread that cuts into the hole wall as the insert is driven in, plus an internal machine thread for the screw. They work well in aluminium, castings and rigid plastics, and are a common field repair for a stripped thread.

Helical (wire) inserts

A precision coil of diamond-section wire screwed into a specially tapped hole, forming a smooth, hard, accurate internal thread. Common in aerospace and aluminium structures, helical inserts spread load over more threads and resist the galling and stripping that plague tapped aluminium. They also repair damaged threads to the original size.

Key-locking inserts

A solid externally-threaded bushing that is screwed in and then locked against rotation by driving small keys into the parent material. They deliver the highest pull-out and torque resistance and resist loosening under vibration, making them the choice for safety-critical and high-load metal joints.

Designing the Hole and Boss

An insert only performs if the hole and surrounding material are designed for it. The exact dimensions come from the insert manufacturer's datasheet, but the principles are consistent.

• Use the specified hole size. Heat-set and press-fit inserts need a hole slightly smaller than the insert's knurl diameter so material can grip; self-tapping and helical inserts need a specific pilot or tap-drill size. Follow the datasheet — guessing causes loose or split bosses.
• Give the boss enough wall. In plastics, the boss outer diameter should be roughly twice the insert diameter so it does not crack as the insert is set. Thin walls split.
• Make the hole deep enough. The hole should be a little deeper than the insert so displaced material and the screw tip have somewhere to go, and so the insert seats flush or slightly below the surface.
• Add a lead-in chamfer. A small chamfer at the hole mouth helps the insert start straight and square.
• Mind screw engagement. Engage the screw at least one thread diameter into the insert for full strength, and avoid bottoming the screw against the blind end.

Insert vs. Tapped Hole: When to Choose Which

Not every hole needs an insert. A tapped hole is fine when the part is a strong metal and will be assembled only a few times. Reach for an insert when the part is plastic, when the joint will be opened and closed repeatedly, when the parent metal is soft or thin, or when vibration and high load demand maximum thread strength. In aluminium parts that see frequent service, a helical or key-locking insert often pays for itself by preventing a stripped thread that would scrap the part — a consideration that fits the broader cost logic in our cost-reduction guide and complements good thread design.

Getting It Right the First Time

Threaded inserts are a small detail with an outsized effect on a product's durability. The two mistakes to avoid are choosing the wrong type for the material — for example a press-fit insert where a moulded plastic boss really needs a heat-set one — and sizing the hole or boss without the manufacturer's data. Get those right and the joint will outlast many assembly cycles.

MechPart Pro machines and moulds parts with threaded inserts installed to specification across plastics and metals, and our engineers will advise on insert type, hole size and boss design as part of our free design-for-manufacturability review. Share your model and the fastener you plan to use, and we will confirm the insert and hole that give you a strong, lasting thread. It pairs naturally with the material choices in our materials guide.