A Full Service Custom Plastic Injection Molding Company
ISO 13485:2016

Insert Molding

PEEK insert molding

Part insert molded with PEEK

Insert Molding Process

Insert Molding Process

Insert injection molding involves encapsulating a previously fabricated component in molten resin to produce a finished part. The inserted component is most commonly a simple object, such as a knife blade or surgical tube, but in some cases, inserts can be as complex as a battery or motor. Plastic insert molding is ideal for improving the strength and reliability of a component and can help save on cost and production time by eliminating the need for secondary operations such as soldering, connectors and adhesives. 

What Makes Insert Molding Different from Injection Molding and Overmolding?

Insert injection molding is often compared to other molding methods like injection molding and overmolding, but there are some differences between each of the processes. Injection molding involves injecting one or more molten plastic resins into a mold to create a single finished part.

Overmolding is a two-step process where a plastic layer is molded over and around a previously molded part to produce a single component; an adhesive is sometimes required to create a permanent bond. Insert molding is similar to overmolding in that it involves joining separate parts to form a single component. The difference, however, is that plastic insert molding is a single-step process where the preformed insert and molten resin are placed in a mold together to combine two parts.

How do you support the insert in insert molding?

The inserts used for plastic insert molding are typically made from brass, steel or stainless steel and have a threaded surface to provide better adhesion to the plastic. Injection molding inserts and are placed in the mold either by hand or by machine with the mold in a vertical position. This positioning allows gravity to keep the insert in place as the mold is closed. As the molten plastic resin is slowly poured into mold, it helps hold the insert in a fixed position.

Plastic Insert Molding Produces Uniform Parts with Tight Tolerances  

Insert molding (or “insert moulding”) is just one of the many specialized plastic injection molding services Stack Plastics offers. We utilize highly-advanced vertical molding machines that are specifically designed for insert injection molding. The insert molding process requires very tight tolerances to ensure correct formation of the plastic parts—misalignment of even a fraction of a millimeter can lead to complete failure of the process.

Our state-of-the-art plastic insert molding equipment can meet even the most exacting specifications. To guarantee that the metal insert parts match your specifications we’ve forged sound working relationships with a number of vendors who supply us with high quality metal insert components. We will also work with customer-supplied inserts, as your project requires.

Our plastic insert molding services are incredibly flexible and customizable, allowing us to create the ideal solution for your project's unique needs, no matter the requirements or application. We work with a variety of insert molding materials to give your project the exact properties it requires. From low volume prototypes to full production runs, Stack Plastics has the right experience and equipment to make your project a success.

Contact us to learn more about plastic insert molding or to request a quote your plastic molding project.

Injection Molding Threads for Screw Thread Applications

Though it’s not technically insert molding, molding threads into plastic parts is a common production option that we provide. Internal or external threads can be molded directly into the part profile. Molding threads into your part makes it easy to attach other components via screw threads.

Threads in all sizes and types (fine, coarse, etc.) can be injection molded. Some of the most commonly specified threads for molded plastic components include tapered pipe threads, buttress threads, and American Standard machine screw threads (60° sharp threads).

There are a number of important factors to consider when designing threads for injection molded plastic parts, including thread profile width, thread radius, thread cross section, location, and spacing, among others. Whatever your design needs may be, we have the molding capabilities to create threads that meet your requirements.

Plastic Insert Molding Materials

We use a variety of polymers in our insert molding processes to meet your application requirements. Each material contains unique properties and should be selected based on the operating conditions and performance requirements of your finished component.


Thermoplastics are an environmentally- friendly insert molding material that can be reheated and reused. They are often chemical resistant and provide various options for sheen, variance and other properties. A few of the thermoplastic materials we offer for the insert moulding process include:

  • Nylon: a strong plastic material that is resistant to chemicals and abrasion. It is suitable for high temperature applications and can replace metal in certain applications.
  • Polyethylene: includes several different grades of lightweight thermoplastic polymer providing chemical resistance, high toughness, impermeability and excellent dielectric properties. Examples of polyethylene include LDPE, HDPE and MDPE plastics.
  • Polystyrene: a clear, lightweight plastic with a low melting point and resistance to acids and bases. It is a common insert molding material that is used for applications ranging from consumer products to medical devices.  


Thermoset insert molding materials are similar to thermoplastics, with the exception that they cannot be melted and reused after they are cured.

  • Epoxy: high performance polymers that become permanently hard after curing and provide excellent adhesion and heat and chemical resistance.
  • Phenolic: provides high chemical resistance to organic halogenated solvents, and many have UL 94V 0 approval.  


Elastomers include natural and synthetic polymers such as rubber that are known for their elastic properties. Elastomers are recyclable and commonly used for home and automotive applications.

  • Natural rubber: flexible and durable plastic commonly used in automotive applications.
  • Polyurethane: provides excellent wear and abrasion resistance and is highly elastic and impact resistant. This durable material maintains its properties in both high and low temperatures.

Our insert molding machines and state-of-the-art equipment enable us to quickly adapt to the near-constant advancements our industry experiences. Advances in polymer technologies open up new avenues for our business each and every year. We welcome these advances, as we strive to provide our clients with the most current, cutting edge polymer technology.

Industries & Applications for Insert Molding

No matter the function of your injection molded products, whether it be rigorous outdoor use, applications in the consumer environment, or parts on vehicles that leave the earth's atmosphere, our quality has been tested in every possible environment and we proudly stand upon the back of 70 years of exceptional service.

We provide plastic insert molding services for clients in nearly every industry, including:

Contact Us for Plastic Insert Molding & More

We have the insert molding experience and expertise to deliver the complex parts your project requires. Request a quote today or contact Stack Plastics for full-service injection molding for your unique application.