Material Selection: Choose compatible materials for the substrate and overmold to ensure proper bonding and adhesion. Typically, a rigid substrate material is used with a soft or flexible overmold material, such as TPE or rubber. Consider the properties required for each material and their compatibility.

Part Design: Design the substrate part with features that facilitate overmolding. For example, include appropriate undercuts, ribs, or recesses to allow the overmold material to mechanically lock onto the substrate and improve adhesion.

Wall Thickness: Maintain consistent wall thickness throughout the part to ensure proper flow of both the substrate and overmold materials during the molding process. Varying wall thickness can lead to uneven shrinkage and poor adhesion.

Draft Angles: Incorporate draft angles in the design to facilitate easy removal of the part from the mold. Draft angles also aid in smooth flow of materials and reduce the risk of material entrapment.

Bonding Surfaces: Design the bonding surfaces of the substrate part to provide a large contact area for improved adhesion between the materials. Roughening or incorporating texture on the bonding surfaces can enhance the bond strength.

Overmold Material Placement: Consider the placement and thickness of the overmold material carefully. Determine the areas that require the soft touch, grip, or specific functionality provided by the overmold, and design the part accordingly.

Tooling: Work closely with tooling engineers to ensure that the mold design accommodates the overmolding process. Consider factors such as part ejection, material flow, gate placement, and cooling channels.

Prototyping and Testing: Validate the design through prototyping and testing. This helps identify any issues related to material compatibility, adhesion, shrinkage, or functionality before moving to full-scale production.

By following these guidelines, you can optimize the design for overmolding and achieve a successful integration of different materials, resulting in a functional and aesthetically appealing product. Collaboration with experienced molders and engineers is crucial to ensure the feasibility and manufacturability of the design.

With a manual operation, the substrate is injection molded via a traditional injection molding process. The substrates are then hand-loaded into another mold for the overmolding operation. The operator will also remove the finished part from the mold, provide an inspection, and package the parts. Manual overmolding is the most common procedure for low to mid-volume overmolding.

The advantage of manual overmolding is simplicity. The tooling required for the substrate and overmold is simple (when compared to two-shot tooling), and there is no secondary equipment required. The downside is the labor allocation and potential for inconsistent cycle times created by operators. As annual order volumes increase, the labor involved can become significant enough to justify investment in automation systems or a two-shot molding operation.

More advanced injection molding machines can run two different polymers at the same time. Two-shot molding uses a complicated mold and robotics to mold the substrate on one side and then transfer it to the other half. This happens on every cycle. The process requires a more advanced injection molding machine and mold, but it may yield a lower piece price compared to manual operations.
Deciding between the two molding operations is done on a case by case basis. There is no universal rule as to when an automated overmolding operation is justified. To provide the end-user with the best option, injection molders will look at several variables, such as: labor allocation, annual order quantities, budget, material usage, cycle time, etc.

Plastic Overmolding are regularly used for a wide variety of applications, including:
● Automotive
● Appliance
● Controls
● Medical devices
● Electronic devices
● And more