Insert Moulding: When to Mould Around Metal Inserts

Plastic is a poor material for a load-bearing thread. Mould a screw thread straight into a plastic boss and, after a handful of assembly cycles, it strips. Threaded metal inserts solve this — but the question is when to press them in after moulding, and when to mould the plastic around them in the first place. That second option is insert moulding, and for the right part it’s stronger, cheaper and more reliable than the alternatives.

Insert moulding (or “insert molding”, as much of the market spells it) is one of the core injection moulding processes engineers choose between, alongside conventional single-shot, 2K two-shot and over-moulding. This guide explains what it is, how it compares to those neighbouring processes, how inserts get loaded, where it’s used, and what it costs — written for the engineers and procurement managers deciding whether it’s the right route for their part.

What insert moulding is

Insert moulding is injection moulding around a pre-placed component — usually metal — so that the finished plastic part captures and permanently retains that insert. The insert is loaded into the mould cavity before the tool closes; molten plastic is then injected around it; as the plastic cools and shrinks, it grips the insert mechanically, locking it in place.

The result is a single moulded part with the insert fully integrated — a plastic housing with a solid brass threaded bush already in it, a connector with its metal contacts embedded, a knob with a steel shaft moulded through it. There’s no secondary press-fit, glue or ultrasonic step, and the bond is stronger and more consistent than post-moulding assembly because the plastic has flowed into every knurl, groove and undercut on the insert.

Threaded and metal inserts

By far the most common use of insert moulding is putting durable threads into plastic parts. The inserts themselves are engineered for the job:

• Threaded brass inserts: the workhorse. A brass bush with an internal thread and an external knurl or groove pattern that the plastic grips. Gives a metal thread that survives repeated assembly and disassembly.
• Steel inserts and studs: for higher strength, external threads (studs), shafts, or where a fastener needs to project from the part.
• Electrical contacts and terminals: metal pins, blades and lead frames moulded into connectors and switches.
• Other embedded metal: bushings, bearings, magnets, tubes and stiffening plates that need to become part of the moulding.

The insert’s external geometry matters as much as the plastic design. Knurls, hex flats, grooves and undercuts give the polymer something to key into, resisting both pull-out (axial force) and rotation (torque) when a fastener is later driven in. A well-specified insert and a well-designed boss together produce a joint far stronger than a moulded-in plastic thread.

Insert moulding vs over-moulding vs 2K moulding

These three processes all produce a single part made of more than one element, and they’re easy to confuse. The distinction is what’s being combined and how:

• Insert moulding moulds plastic around a pre-made solid insert — usually metal — placed in the tool by hand or robot.
• Over-moulding moulds a second material (often a soft TPE) over an already-moulded plastic substrate — think a soft-touch grip over a rigid handle.
• 2K (two-shot) moulding moulds two plastic materials in one automated cycle on a specialist two-shot machine, with no manual loading between shots.

Process	What’s combined	How it’s loaded	Best for	Relative tool cost	Key trade-off
Insert moulding	Plastic + pre-made (metal) insert	Insert placed in tool (manual or robotic)	Threaded bosses, contacts, embedded metal	$$	Cycle slowed by insert loading; insert cost
Over-moulding	Rigid plastic + softer plastic/TPE	Substrate re-loaded for second shot	Soft-touch grips, seals, sealed assemblies	$$$	Two cycles unless run on a 2K machine; material bond
2K (two-shot)	Two plastics in one cycle	Fully automated, no manual loading	High-volume multi-material or multi-colour parts	$$$$	High tool and machine cost; pays back at volume

Quick decision guide: if you need metal in a plastic part — threads, contacts, shafts — insert moulding is almost always the answer. If you need a soft second plastic over a rigid one at moderate volume, that’s over-moulding. If you need two plastics combined at high volume with the lowest per-part cost, and can justify the tooling, that’s 2K. For a fuller walk-through of all the processes side by side, see our guide to the types of injection moulding.

Automated vs manual insert loading

The defining feature of insert moulding — and its main cost and quality variable — is how the insert gets into the mould. There are two routes.

Manual loading

An operator places each insert into the cavity by hand (often onto core pins) before the tool closes, every cycle. It’s flexible, needs little extra tooling investment, and suits lower volumes, larger inserts, complex loading, and early production. The downsides: it adds labour to every shot, slows the cycle while the press waits for loading, and introduces human variability — a mis-placed or missing insert is a scrapped part or, worse, a damaged tool.

Automated loading

A robot or pick-and-place system feeds and positions the inserts, often from a vibratory bowl feeder, with vision checks to confirm each insert is present and correctly seated. It costs more upfront in automation and integration, but for high volumes it’s faster, more consistent and far less error-prone, and it removes the operator from a hot, repetitive task. Automation also enables shorter, more stable cycle times, which is where the per-part economics turn in its favour.

• Choose manual loading for lower volumes, prototypes, bridge production, large or awkward inserts, and parts still ramping up.
• Choose automated loading for high, steady volumes where cycle time and insert-placement reliability dominate the cost.

Typical applications

Insert moulding shows up anywhere a plastic part needs the strength, conductivity or precision of metal at specific points:

• Threaded bosses: the classic case — brass inserts in enclosures, housings and casings that will be assembled and serviced with metal screws.
• Electronics and connectors: metal contacts, pins, terminals and lead frames moulded into connectors, switches, sensors and plugs, where position and electrical integrity are critical.
• Automotive components: embedded studs, brackets, bushings and sensor housings that must survive vibration and temperature.
• Medical devices: metal blades, cannulae and precision components integrated into moulded plastic handles and bodies.
• Handles, knobs and tools: steel shafts and drive features moulded into plastic grips.
• Consumer and industrial products: magnets, bearings and reinforcement plates captured in moulded parts.

DFM and cost considerations

Insert moulding adds a metal component, a loading step and some specific design risks to a standard moulding. Get ahead of them at the design stage and the process is reliable and economical; ignore them and you’ll see loose inserts, flash, sink and scrap. The key points:

• Design the insert to be gripped. Specify knurls, grooves or hex features so the plastic resists both pull-out and rotation. Match the retention to the torque and load the joint will actually see.
• Keep enough plastic around the insert. Too thin a wall around a metal insert causes sink, weak grip and cracking; the boss needs adequate, uniform material around the bush. This is core injection moulding DFM.
• Locate and retain the insert positively in the tool. It must seat on a core pin or feature that holds it precisely and stops plastic flowing where it shouldn’t — under the insert or into a thread.
• Protect the threads. The tool must shut off against the insert so molten plastic doesn’t flood the internal thread; a masked or shouldered core pin does this.
• Mind the thermal mismatch. Metal and plastic shrink at very different rates; the surrounding plastic must be designed and processed to grip without cracking as it cools. Material choice matters — check it against your material selection.
• Account for the full cost. Insert moulding carries the insert’s piece-price, the loading labour or automation, and a slower cycle than a plain moulding. It’s still usually cheaper and more reliable than moulding a part and then pressing or ultrasonically installing inserts as a secondary operation — because it’s one operation, not two, with a stronger joint.

How Sino approaches insert moulding

Sino has run insert moulding for OEMs across electronics, automotive and industrial products for over 20 years, from our UK-managed, ISO 9001:2015-certified factory in Shenzhen — with a mould repair and support facility in Querétaro, Mexico for customers nearshoring into the Americas. Because we design the tool, source and inspect the inserts, mould the part and measure it under one roof, insert placement, shut-off and retention are engineered together rather than bolted on.

That in-house control is where insert moulding succeeds or fails: a tool that locates the insert precisely and shuts off cleanly against it, matched to the right plastic and process, gives a repeatable joint shot after shot. We’ll advise honestly on whether your part is better served by insert moulding, by pressing inserts in afterwards, or by over-moulding — and, for high volumes, whether automated loading pays back. You can see where it sits among the other processes in our complete guide to injection moulding.

Frequently asked questions

What is insert moulding?

Insert moulding (or insert molding) is injection moulding plastic around a pre-placed component — usually a metal insert such as a threaded brass bush, an electrical contact or a shaft. The insert is loaded into the mould before the tool closes, and the plastic is injected around it, capturing it permanently as the material cools and shrinks. The result is a single part with the metal fully integrated.

What’s the difference between insert moulding and over-moulding?

Insert moulding moulds plastic around a pre-made solid insert, almost always metal — for threads, contacts or shafts. Over-moulding moulds a second, usually softer plastic (such as a TPE) over an already-moulded plastic substrate — for soft-touch grips, seals and sealed assemblies. In short: insert moulding adds metal; over-moulding adds a second plastic.

Why use a metal insert instead of moulding the thread in plastic?

A moulded plastic thread wears and strips under repeated assembly, and it can’t take much torque or load. A metal insert gives a durable thread that survives many assembly and disassembly cycles and handles far higher loads. For anything that will be serviced, or that carries a real fastening load, a threaded metal insert is the reliable choice.

Is insert moulding more expensive than standard injection moulding?

Per part, yes — it adds the insert’s cost, the loading step (labour or automation) and a slower cycle. But compared with moulding a part and then installing inserts as a separate secondary operation, insert moulding is usually cheaper overall and produces a stronger, more consistent joint, because it’s a single integrated operation rather than two.

The bottom line

Insert moulding is the right call whenever a plastic part needs metal built into it — durable threads, electrical contacts, shafts or embedded hardware — and you want that metal captured reliably in a single operation. Choose it over over-moulding when you’re adding metal rather than a soft second plastic, and over post-moulding insert installation when you want a stronger joint with fewer steps. Get the insert retention, shut-off and surrounding wall right at the design stage, and it’s one of the most dependable processes in moulding. Send us your part or drawing and we’ll tell you honestly whether insert moulding is the right route — and how to design it so the inserts stay put.