Metal inserts are especially common. Examples include threaded inserts, bushings, pins, electrical contacts, sleeves, and stamped metal components. By combining metal and plastic in a single molded part, insert molding can reduce secondary assembly while adding features that would be difficult to achieve with plastic alone.

How Does Insert Molding Work?

The basic insert molding process can be divided into several steps:

1. Prepare the Insert

The insert is manufactured before molding begins.

Depending on the product, it may be a:

<!--[if !supportLists]-->· <!--[endif]-->threaded brass insert;

<!--[if !supportLists]-->· <!--[endif]-->steel or stainless steel component;

<!--[if !supportLists]-->· <!--[endif]-->bushing or sleeve;

<!--[if !supportLists]-->· <!--[endif]-->pin;

<!--[if !supportLists]-->· <!--[endif]-->electrical contact;

<!--[if !supportLists]-->· <!--[endif]-->stamped metal part.

The insert geometry is important because the surrounding plastic must hold it securely after molding.

2. Place the Insert in the Mold

The insert is positioned accurately inside the injection mold.

For lower-volume production, inserts may be loaded manually. High-volume production can use robots or automated feeding systems to improve cycle time and positioning consistency. Industrial insert-molding systems can also integrate stamping and molding operations into automated production lines.

3. Close the Mold

The mold closes around the insert and holds it in the required position.

Proper insert location and retention are critical. If an insert moves while the mold closes or while plastic is injected, the finished part may have dimensional problems or the mold itself may be damaged.

4. Inject the Plastic

Molten thermoplastic is injected into the mold cavity and flows around the insert.

Features such as knurls, grooves, holes, undercuts, or other mechanical retention geometry can help lock a metal insert into the plastic after molding.

For metal-to-plastic insert molding, retention is generally achieved mechanically rather than by relying on chemical adhesion between the materials.

5. Cool and Eject the Part

The plastic cools and solidifies around the insert. The mold then opens and the completed insert-molded component is ejected.

The result is one integrated component containing both the molded plastic and the preformed insert.

6. Inspect the Finished Component

Inspection may include:

<!--[if !supportLists]-->· <!--[endif]-->insert position;

<!--[if !supportLists]-->· <!--[endif]-->molding dimensions;

<!--[if !supportLists]-->· <!--[endif]-->thread condition;

<!--[if !supportLists]-->· <!--[endif]-->flash;

<!--[if !supportLists]-->· <!--[endif]-->plastic coverage;

<!--[if !supportLists]-->· <!--[endif]-->cosmetic appearance;

<!--[if !supportLists]-->· <!--[endif]-->pull-out or torque performance when required.

Critical insert features should be clearly identified on the engineering drawing. 

Common Types of Inserts

Insert molding can incorporate many different preformed components.

Threaded Inserts

Threaded metal inserts provide durable internal threads inside plastic parts.

They are particularly useful when screws will be installed and removed repeatedly, because an integrated metal thread can offer better durability than threading directly into many plastics. Protolabs specifically identifies threaded inserts as a way to reinforce fastening capability in molded plastic components.

Bushings and Sleeves

Metal bushings can provide wear-resistant or dimensionally stable surfaces where shafts, pins, or other mating components interact with a plastic housing.

Electrical Contacts

Pins, terminals, contacts, and conductors can be molded directly into electrical or electronic components.

Stamped Metal Components

More complex stamped metal features can also be integrated into molded plastic. Insert molding manufacturers use combinations of precision press working and injection molding for applications such as electrical and automotive components.

What Materials Are Used for Insert Molding?

Insert Materials

Common metal insert materials include:

<!--[if !supportLists]-->· <!--[endif]-->brass;

<!--[if !supportLists]-->· <!--[endif]-->steel;

<!--[if !supportLists]-->· <!--[endif]-->stainless steel;

<!--[if !supportLists]-->· <!--[endif]-->other application-specific metals.

Brass is particularly common for threaded inserts, while steel and stainless steel are also widely available depending on required strength and corrosion resistance.

Plastic Materials

Many injection-molding thermoplastics can be used for insert molding, depending on the application.

Examples include:

<!--[if !supportLists]-->· <!--[endif]-->ABS;

<!--[if !supportLists]-->· <!--[endif]-->polypropylene (PP);

<!--[if !supportLists]-->· <!--[endif]-->polycarbonate (PC);

<!--[if !supportLists]-->· <!--[endif]-->acetal/POM;

<!--[if !supportLists]-->· <!--[endif]-->nylon;

<!--[if !supportLists]-->· <!--[endif]-->PBT and PPA;

<!--[if !supportLists]-->· <!--[endif]-->PPS;

<!--[if !supportLists]-->· <!--[endif]-->PEI;

<!--[if !supportLists]-->· <!--[endif]-->PEEK;

<!--[if !supportLists]-->· <!--[endif]-->TPU and other engineering polymers.

Material selection should consider mechanical strength, temperature, chemical exposure, shrinkage, electrical requirements, appearance, and the stresses created around the insert during molding and service. Commercial insert-molding capabilities cover a broad range of commodity and engineering resins.

Advantages of Insert Molding

Reduced Assembly

One of the biggest advantages is the ability to combine previously separate components during molding.

A threaded insert, electrical contact, or bushing does not necessarily need to be installed as a separate downstream assembly operation.

This can reduce part count and simplify manufacturing.

Stronger Fastening Features

Metal threaded inserts can provide durable fastening points in plastic components, especially where repeated assembly and disassembly are expected.

Integration of Different Material Properties

Insert molding allows engineers to use plastic where low weight, insulation, shape complexity, or moldability is useful while retaining metal where strength, wear resistance, conductivity, or threaded fastening is required.

Consistent Insert Position

When the mold and process are properly designed, inserts can be located accurately relative to molded plastic features.

This can be useful for electrical connectors, housings, mounting features, and precision mechanical assemblies.

Potential for Automated Production

Insert loading and molding can be integrated into automated production systems, especially at higher volumes. Some specialized production lines combine press forming, injection molding, and downstream operations.

Limitations and Challenges of Insert Molding

Insert molding also creates additional manufacturing considerations compared with standard injection molding.

More Complex Tooling

The mold must locate and retain the insert while allowing the plastic to flow around it.

This often makes mold design more complex than a comparable plastic-only component.

Insert Positioning

The insert must remain correctly located during mold closing and injection.

Variation in insert dimensions or poor retention can lead to misalignment, flash, incomplete encapsulation, or mold damage.

Knit Lines

As molten plastic flows around an insert, separate flow fronts may meet again on the opposite side. This can create a knit or weld line.

Depending on part geometry and loading, the area around the insert may require additional wall thickness, ribs, or other design changes. Protolabs specifically identifies knit lines around inserts as a design consideration.

Cycle Time

Manual insert loading adds time to each molding cycle. Automation can reduce this issue but adds equipment and process complexity.

Higher Upfront Tooling Requirements

Insert molding is generally most economical when the integrated design justifies the additional mold and process complexity.

For very small quantities or frequently changing designs, separately installing an insert after molding may sometimes be more practical.

Insert Molding vs. Overmolding

Insert molding and overmolding are related processes, but they are not exactly the same.

In insert molding, a premanufactured component—commonly a metal insert—is placed into the mold and plastic is molded around it.

In overmolding, an additional material is molded over an existing substrate, often to combine two plastics or add an elastomeric layer to a rigid component. Typical examples include soft grips, seals, handles, and multi-material housings.

A simple way to distinguish them is:

The terminology can overlap in industry, but the engineering intent is usually different.

Insert Molding vs. Heat-Set Inserts

Insert molding should also be distinguished from installing threaded inserts after a plastic part has already been molded.

With insert molding, the metal insert is placed inside the mold before plastic injection.

With a heat-set insert, the plastic component is molded first and the threaded insert is subsequently pressed into the plastic using heat.

Insert molding integrates the insert during the molding cycle, while heat-set installation is a secondary manufacturing operation.

The better solution depends on production volume, tooling, insert geometry, material, required strength, assembly requirements, and cost.

Important Insert Molding Design Considerations

Successful insert molding begins with good part and tooling design.

Important factors include:

<!--[if !supportLists]-->· <!--[endif]-->insert location and orientation;

<!--[if !supportLists]-->· <!--[endif]-->insert dimensional consistency;

<!--[if !supportLists]-->· <!--[endif]-->mechanical retention features;

<!--[if !supportLists]-->· <!--[endif]-->plastic wall thickness around the insert;

<!--[if !supportLists]-->· <!--[endif]-->ribs and supporting geometry;

<!--[if !supportLists]-->· <!--[endif]-->material shrinkage;

<!--[if !supportLists]-->· <!--[endif]-->draft angles;

<!--[if !supportLists]-->· <!--[endif]-->gate location and plastic flow;

<!--[if !supportLists]-->· <!--[endif]-->knit-line location;

<!--[if !supportLists]-->· <!--[endif]-->mold access for insert loading;

<!--[if !supportLists]-->· <!--[endif]-->required thread and insert tolerances.

Standard injection-molding principles such as suitable draft and consistent wall thickness still apply. The insert introduces additional concerns involving retention, molding flow and positioning.

Designers should also avoid unnecessarily thin plastic sections around metal inserts. Adequate surrounding material helps support the insert and distribute loads into the molded component.

Common Applications of Insert Molding

Insert-molded components are used in many products and industries.

Typical applications include:

<!--[if !supportLists]-->· <!--[endif]-->electrical connectors;

<!--[if !supportLists]-->· <!--[endif]-->electronic housings;

<!--[if !supportLists]-->· <!--[endif]-->automotive components;

<!--[if !supportLists]-->· <!--[endif]-->sensors;

<!--[if !supportLists]-->· <!--[endif]-->knobs and handles;

<!--[if !supportLists]-->· <!--[endif]-->medical-device components;

<!--[if !supportLists]-->· <!--[endif]-->threaded plastic housings;

<!--[if !supportLists]-->· <!--[endif]-->gears and mechanical assemblies;

<!--[if !supportLists]-->· <!--[endif]-->power-tool components;

<!--[if !supportLists]-->· <!--[endif]-->industrial equipment.

Insert molding is especially useful when a product requires the geometry and low weight of molded plastic together with specific functional properties provided by a metal component. Industrial insert-molding suppliers use the process across electronics, automotive, medical, appliance, and other precision applications. 

Is Insert Molding Right for Your Part?

Insert molding is worth considering when a design contains components that would otherwise need to be assembled into the plastic part after molding.

It can be particularly effective for designs requiring:

<!--[if !supportLists]-->· <!--[endif]-->reusable metal threads;

<!--[if !supportLists]-->· <!--[endif]-->integrated electrical contacts;

<!--[if !supportLists]-->· <!--[endif]-->wear-resistant bushings;

<!--[if !supportLists]-->· <!--[endif]-->precisely located metal components;

<!--[if !supportLists]-->· <!--[endif]-->reduced assembly operations;

<!--[if !supportLists]-->· <!--[endif]-->compact multi-material structures.

However, the decision should consider production quantity, tooling cost, insert tolerances, molding material, geometry, mechanical loading, and the required level of automation. 

For custom injection-molded parts, Rapid-Model can evaluate the CAD model, drawings, insert requirements, plastic material, tolerances, and production quantity to determine an appropriate manufacturing approach.

Providing the insert CAD file together with the molded-part model and critical dimensions makes DFM evaluation and quotation more efficient.

Conclusion

Insert molding is an injection molding process that places a premanufactured component inside a mold before plastic is injected around it.

The process is commonly used to integrate threaded inserts, bushings, sleeves, electrical contacts, and other metal components directly into plastic parts.

Its main benefits include reduced assembly, durable fastening features, integration of different material properties, and efficient production of complex multi-material components.

Good results, however, depend on insert positioning, mechanical retention, plastic flow, wall thickness, material selection, mold design, and dimensional control. Considering these factors during the design stage can significantly improve the manufacturability and reliability of an insert-molded component.