Prototype Overmolding &
Insert Molding

Overmolding and insert molding provide a design engineer with some unique capabilities. Overmolding refers to a single part that has a rigid plastic substrate with a softer elastomer overmold. The elastomer can be either mechanically captured by the substrate or by selecting a material combination that will bond at the interface between substrate and overmold.

It’s common to use overmolding to design parts that are used for sealing. The rigid portion allows assembly and support, while the softer elastomeric portion provides a seal. Grip features are another typical application for overmolding. A toothbrush provides a common example of overmolding as a grip feature. Overmolding also allows the engineer to apply color in unique ways since both the substrate and the overmold can be different colors.

Insert molding involves placing a part that is typically metal, but can be other materials such as ceramic, inside the mold and then molding plastic around the metal to yield a single part. A common application of insert molding is the addition to threaded inserts to the housing which can then be assembled using machine screws.

What is Overmolding?

Overmolding is a two-step process that is also sometimes called two-shot molding. Step 1 involves molding the rigid plastic substrate. Step 2 involves overmolding a softer elastomer material onto the rigid plastic substrate from step 1 to form a single part. In high-volume production, this typically involves a rotating, multi-cavity mold. While one side of the mold is molding the substrates, the other side of the mold is overmolding the substrates from the last substrate cycle. When this cycle ends, the overmolded parts are ejected, the substrates are rotated into position for overmolding while a new set of substrates is molded, and so on.

Prototype overmolding achieves the same result by fabricating two separate molds. The first prototype mold is used for injection molding the rigid substrate part. The second prototype mold is used for overmolding the softer elastomer over the rigid substrate. After setting up the first prototype mold in the molding machine, a quantity of substrates is molded and allowed to cool and shrink. Then the second prototype mold is set up in the molding machine. Rigid substrates are manually placed into the second prototype mold, and the softer elastomer material is overmolded onto the rigid substrate.

A unique form of overmolding known as co-injection molding utilizes a specialized molding machine with two injection barrels. This allows two different plastics to be injected into the mold, allowing for molding a single part containing layers of each plastic. Few applications require co-injection molding. An improved food container requires one plastic for assembly to a lid and a second plastic that functions as an oxygen barrier.

Prototype Overmolding Examples

Overmolding example – Polycarbonate substrate is shown in orange, and flexible TPE overmold is shown in yellow




Overmolding example – A complex micro molded COC substrate is shown in orange, and a flexible TPE overmold is shown in yellow.

Propert Shutoff Design Image
Poor Shutoff Design Image

Overmolding and Insert Molding Design Considerations

The substrate and overmold need to both obey general injection molding rules. Wall thicknesses must be consistent; the draft must be included to allow for part ejection, rib thickness must be approximately 60% of the base wall thickness, etc. However, a few additional molding rules apply to the design of overmolded parts.

When designing an overmolded part, there must be shutoff surfaces which constrain where the overmolded elastomer may flow. In the example shown, the substrate is shown in yellow, and the overmold is shown in orange. The blue highlighted surface is the shutoff surface. When the substrate is placed in the mold for overmolding the elastomer, the steel in the mold contacts this shutoff surface, which creates a seal that prevents the overmolded elastomer from flowing outside the constrained area. An example of poor shutoff design is also shown. There is no shutoff surface. The mold design must allow a small amount of clearance between the outer diameter of the substrate and the mold cavity. The elastomer overmold has the potential to overflow into this gap because there is no shutoff to prevent overflow from occurring.

Arburger Allrounder 370E 600-170 EDrive Injection Molding Machine
Complex Part Design Image - Complex Geometry

Proper Shutoff Design

Improper Shutoff Design

Complex Part Design Image - Complex Geometry

In addition to shutoff surface design considerations, insert molding requires capture features that allow the metal insert to be properly placed and held in the mold during overmolding. In this example, the blade has an internal slot feature that is used to capture the blade in the mold. The second image shows the blade placed in the mold with the two blue highlighted features protruding through the blade to provide capture and alignment of the blade. As shown in the last image, the final part will have slots through the plastic and blade but it is possible to further overmold this part to fill in these slots.

Blade Design Insert Molding

Example of the blade to be insert molded

Blade Part Design for Insert Molding

Example of blade capture features in mold, highlighted in blue.

Blade inside of Blue Housing Insert Molding

Blade is insert molded with a handle.

Overmolding Material Selection

When researching elastomers for overmolding applications, it’s important to first identify any mechanical, thermal and environmental requirements the overmolded elastomer needs to meet. For example, the overmolded elastomer may have a mechanical requirement of providing a seal. The design engineer would then want to specify overmolded elastomer materials with a low compression set, so the seal is properly maintained over time. If a low compression set material isn’t specified correctly, the seal compression will be reduced over time, and the pressure that the seal may hold is also reduced.  Most elastomers have a relatively high compression set so properly selecting the elastomer to be used for sealing application is critical.

The next consideration for material selection is whether the overmolded elastomer is to be bonded or mechanically captured to the substrate. A bonded overmold means that a rigid substrate material and an elastomer overmold are selected in a combination that provides a bond. The most common rigid substrate materials used to bond elastomer overmolds are polypropylene, polycarbonate, and ABS. Less common rigid substrate materials used for bonding are acrylic, nylon, PET, polystyrene, polyethylene, and PBT. 

Once the design engineer has determined which rigid substrate material to use, they will then search for an elastomer grade that will bond to it. For example, assume the design engineer selects polycarbonate as the substrate. When searching for a compatible overmolded elastomer, it’s helpful to use a material search engine such as The material search engine allows the design engineer to search for compatible grades while also searching for a wide variety of material properties. For example, when searching for an elastomer that will bond to polycarbonate with a durometer of 40-50 shore A, one of the candidates presented is Avient Versaflex OM 1245X-1. A review of the specification sheet for this material confirms that it is intended for applications requiring bonding to polycarbonate and has a durometer of 46 shore A, so this grade of elastomer would be a good fit for the application. 

Overmolding Design Image on White Background

Example of bonded overmold.

Overmolding Design Image White BG
Overmolding Design Image from the Side

Example of mechanically captured overmold.

What is Injection Molding?

Insert molding is an injection molded process utilized when the design engineer needs to design a plastic part that has one or more integrated metal components. The insert molded component is typically metal but can be other materials such as ceramics. The key criteria is that the inserted material must be able to withstand the heat and high pressure of the plastic during molding. One typical example of insert molding is the use of threaded inserts. Threads can be molded directly into the plastic, and for many applications, this has sufficient strength. However, in applications where the strength of molded threads is not sufficient, then metal threaded inserts may be insert molded into the plastic part to provide much stronger threads. 

Fabricated Mold Image on White Background

Example of threaded insert

Fabricating molds for insert molding creates a unique challenge because the metal insert is held firmly in place by the mold until the part is ejected from the mold. Part shrinkage starts immediately upon completion of plastic injection while the part is still within the closed mold. This means that the metal insert fixed in place by the mold will act to partially restrict the shrinkage of the plastic while the mold is closed. After ejection, normal part shrinkage resumes, but most of the part shrinkage has already occurred. Therefore, if there is a tight tolerance for the location of the metal insert within the part, it takes experienced molders to assist the design engineer to ensure that the final position of the metal insert meets requirements.

Blade insert molding example blue handle

Blade insert molding example

A blade shown in gray that is insert molded with a blue polycarbonate slide.
Needle insert molding example blue handle

Needle insert molding example

A needle is insert molded with a blue polypropylene hub

Protoshop Prototype Overmolding and Insert Molding

Overmolding allows design engineers flexibility to achieve multiple functional requirements within a single part that would be difficult to meet without both a rigid substrate and an overmolded elastomer. Challenges include the importance of material selection to ensure proper bonding of the elastomer overmold to the substrate and including proper shutoffs in the substrate to constrain the elastomer during overmolding. Insert molding provides design engineers with the ability to integrate metal parts with plastic. Capturing the metal insert in the mold and adjusting for partially constrained part shrinkage requires experienced mold fabricators.

Protoshop has successfully built prototype molds for hundreds of challenging overmolded and insert molded parts. Allow us to lend our extensive expertise to review your part design. We will identify any opportunities for improvement so your overmolded and insert molded parts meet functional requirements the first time. At Protoshop, we offer much more than an automated review of your part model. One of our experienced engineers will perform a thorough review of your design and will meet with you online to review a report of our observations and answer any questions you may have. Our personal approach to assisting our customers is unique among prototype molding service providers. It is a critical service we offer as it better ensures the success of every prototype mold we build.