What Is Insert Molding? Process, Design Tips & Uses

By Owen Lin, Engineer, Tuowei Precision · November 2026

What is insert molding? It is an injection molding process in which a pre-made component, called the insert, is placed in the mold before plastic is injected. The molten plastic flows around the insert and locks it in place, so the part comes out as one piece with the insert permanently built in.

Most inserts are metal: threaded brass nuts, steel bushings, pins, electrical contacts and terminals. The process gives a plastic part metal strength or conductivity exactly where it is needed, without a separate assembly step. This guide explains how the process works, what inserts are used, how the mold holds them, the design rules that prevent loose or cracked inserts, and when adding inserts after molding is the cheaper choice.

Short answer

Short answer: Insert molding places a pre-formed insert, usually metal, into an injection mold cavity and molds plastic around it in one cycle. The plastic encapsulates and mechanically locks the insert, producing a single part with metal threads, contacts or reinforcement inside. It is used for threaded bosses, connectors, sensors, knobs and medical device components.

What Is an Insert in Injection Molding?

An insert is any component placed in the mold before injection so that it ends up inside the finished part. The plastic does the joining, so no screws, glue or press fit are needed afterwards.

Common inserts include:

• Threaded inserts: brass or stainless nuts that give a plastic part durable metal threads.
• Bushings and sleeves: metal wear surfaces for shafts and pivots.
• Pins and studs: locating features or fasteners that stick out of the part.
• Electrical contacts and lead frames: terminals, connector pins and busbars.
• Magnets, sensors and small electronics: sealed inside the plastic for protection.
• Plastic parts: a pre-molded component can also be the insert, which overlaps with overmolding. Our guide to the difference between overmolding and insert molding explains where the line falls.

How Does the Insert Molding Process Work?

The cycle is a standard injection molding cycle with one extra step at the start.

• Prepare the inserts. Inserts are cleaned of oil and, for some materials, preheated so they do not chill the plastic.
• Load the inserts. An operator or robot places each insert onto pins, pockets or magnets in the open mold.
• Close the mold. The tool clamps shut, holding each insert firmly in position.
• Inject the plastic. Molten plastic fills the cavity and flows around the inserts.
• Cool. The plastic shrinks onto the inserts as it solidifies, which tightens the grip.
• Eject and inspect. The part is ejected, then checked for insert position, flash on threads and cracks around the insert.

Manual vs Automated Loading

Loading is where the process differs most from standard molding, and it drives both cost and quality.

• Manual loading suits prototypes and low volumes. It needs no automation, but cycle time depends on the operator, and a missing or tilted insert is a real risk.
• Robotic loading places inserts consistently and keeps the cycle short. The robot and grippers add cost, so it pays off at higher volumes.
• Vertical presses with rotary tables are popular for insert work. Gravity helps inserts stay seated, and one table station is loaded while another is being molded.

Small details matter here. Bowl feeders, insert orientation and part checks with sensors often decide whether a production run holds its scrap rate.

How Does the Mold Hold the Insert?

The insert must not move when plastic hits it at injection pressure. A shifted insert means scrap, and often a damaged mold.

• Core pins: threaded inserts slide onto pins that also keep plastic out of the threads.
• Pockets and recesses: the insert sits in a machined seat in the mold.
• Magnets or vacuum: used for flat or ferrous inserts that cannot sit on a pin.
• Shut-off surfaces: the mold seals against the insert so plastic cannot flash onto exposed areas.

Inserts need tight, consistent positioning in the mold for reliable results, so the inserts themselves must be made to tight, consistent dimensions. An insert that varies by a tenth of a millimeter from batch to batch causes flash or crushed inserts.

Insert Molding Design: How to Design the Part and the Mold

Good insert design is mostly about three things: locking the insert, protecting it from flash, and giving the plastic enough strength around it.

• Choose inserts made for molding. Molded-in inserts have knurls, grooves or undercuts that resist pull-out and rotation. A smooth insert will spin under torque.
• Give the plastic enough wall. Thin plastic around a metal insert can crack as it shrinks onto it. Follow the resin supplier’s guidance on wall thickness around inserts.
• Mind thermal expansion. Metal and plastic expand at very different rates. Brittle or glass-filled resins and amorphous plastics such as PC are most prone to stress cracks around inserts.
• Preheat when needed. A warm insert reduces molded-in stress and helps the plastic fill and weld around it.
• Protect functional surfaces. Threads, contact faces and sealing surfaces need a shut-off or pin so plastic cannot reach them.
• Place gates carefully. Do not aim the melt straight at a small insert or pin, or it may bend or shift.
• Plan inspection. Agree pull-out and torque tests for threaded inserts before production.

Our view: specify inserts that are designed for molding from a reputable supplier, and lock their dimensions on the drawing. Most problems we see come from switching insert suppliers mid-project, not from the molding itself.

Insert Molding vs Post-Mold Inserts

Insert molding is not the only way to put metal threads in plastic. Inserts can also be pressed in after molding with heat (heat staking), ultrasonic vibration or simple press fits.

Factor	Insert molding	Post-mold insertion
Pull-out and torque strength	Highest	Good with heat or ultrasonic methods
Sealing around the insert	Fully encapsulated	Possible gaps
Tooling	Mold needs insert seats and shut-offs	Standard mold plus a simple fixture
Cycle time	Longer, due to loading	Molding is faster; insertion is a separate step
Best volume	Medium to high	Prototypes to medium
Flexibility	Insert design is fixed in the tool	Easy to change insert type later

At low volumes, molding a plain boss and adding heat-set inserts afterwards is often cheaper and easier to change. Choose insert molding when the part needs the strongest possible joint, must be sealed around a contact or pin, or will run in volumes where removing a secondary step pays back the tooling.

What Materials Are Used?

Most thermoplastics work well around inserts. Common choices are nylon, PBT, POM (acetal), PC, ABS and PP, plus glass-filled grades for strength. Thermosets such as epoxy and phenolic are used for some electrical parts.

For inserts, brass is the most common because it machines easily, resists corrosion and conducts well. Stainless steel, steel, aluminum and copper alloys are also used. Match the plastic and the insert to the part’s job: a glass-filled nylon with brass inserts suits a structural bracket, while PBT with copper terminals suits a connector. Our plastic injection molding page lists the resins we mold.

Insert Molding Tolerances

Parts with inserts hold tolerances similar to standard molded parts, plus the positional variation of the insert itself. ISO 20457 sets out tolerances for plastic molded parts, while insert positions are best controlled with geometric tolerances under ASME Y14.5. These are the tolerances we work to:

Work type	Typical tolerance
Standard molding with inserts	±0.005 in (±0.127 mm)
High-precision work	±0.002 in (±0.051 mm)
Parts with metal inserts	±0.003 in (±0.076 mm)
Mass production	±0.005 in (±0.127 mm)

What Drives the Cost?

Four things set the price of a part with inserts. The first is the mold: every insert needs a seat, a locating pin or magnet and a shut-off, which adds machining and fitting time to the tool. The second is loading, which adds seconds to every cycle unless a robot does it.

The third is the insert itself. A standard brass threaded insert is cheap, but a custom machined contact or a stamped lead frame can cost more than the plastic around it. The fourth is scrap. When a part fails inspection, both the plastic and the insert are lost.

Keeping Insert Molding Costs Down

The practical lesson is to standardize. Use catalog inserts where you can, keep the number of inserts per part low, and design them so they can only be loaded one way.

Advantages and Disadvantages

Advantages	Disadvantages
Strong metal threads and contacts in plastic parts	Mold needs insert seats, pins and shut-offs
Removes a secondary assembly step	Loading slows the cycle
Insert is sealed and cannot loosen easily	Risk of cracks around inserts in some resins
Lighter than an all-metal part	Insert supply and tolerances must be controlled
Combines metal conductivity with plastic insulation	A defect scraps both the insert and the plastic
Reliable at high volumes with automation	Insert changes after tooling are costly

Common Mistakes in Insert Molding

• Using press-fit inserts in the mold. Inserts made for pressing often lack the knurl depth needed to resist pull-out once molded in.
• No thread protection. Without a core pin or plug, plastic flashes into the threads and every part needs rework.
• Ignoring stress cracking. Brittle resins can crack days after molding, not at the press. Run aging tests on first samples.
• Loose insert tolerances. An insert that is slightly oversize gets crushed by the mold; one that is undersize lets plastic flash.
• Changing insert suppliers. A different knurl or diameter can mean new tool adjustments.
• Forgetting the loading step in the quote. Manual loading adds labor to every part, so check how inserts will be placed.

Frequently Asked Questions

What is a molding insert?

A molding insert is a pre-made component, usually metal, that is placed in an injection mold before plastic is injected. The plastic flows around it and locks it in place. Typical examples are threaded brass nuts, bushings, pins and electrical contacts. The term can also mean a replaceable steel block inside a mold.

What is the difference between insert molding and overmolding?

Insert molding molds plastic around a pre-made insert, usually metal, in one shot to add strength or function. Overmolding molds a second material, often a soft elastomer, over a molded plastic substrate to add grip or sealing. Insert molding needs one mold; overmolding usually needs two shots.

What are the three types of molding?

Lists vary, but the three most commonly cited plastic molding processes are injection molding, blow molding and compression molding. Insert molding and overmolding are variations of injection molding. Rotational molding and thermoforming are also widely used, so the answer depends on which processes a source groups together.

How strong are molded-in inserts?

Molded-in inserts usually give the highest pull-out and torque resistance of any threaded insert method, because the plastic shrinks around knurls and grooves. Actual strength depends on the insert design, the resin and the wall around it, so specify pull-out and torque tests for critical joints.

Can plastic inserts be insert molded?

Yes. A pre-molded plastic part can be placed in a mold and covered with a second plastic, although that is usually called overmolding. The insert must withstand the melt temperature without softening, and the two materials should be compatible or mechanically locked so they stay joined in service.

Starting an Insert Molding Project With Tuowei Precision

At Tuowei Precision, we build insert molds in aluminum for quick-turn prototypes and in hardened steel for production. We manage the full route from insert specification to molded part, so insert dimensions and mold seats are coordinated from the start.

For an accurate quote, send STEP files for the part and the insert, the insert part number or drawing, your resin, annual volume and any pull-out or torque requirements. Our engineers review insert locking, wall thickness around the insert and loading method before tooling. Start on our insert molding services page.

About the Author

Owen Lin is an engineer at Tuowei Precision. He plans tooling and production processes and works with the quality team to keep every batch consistent.