How Does Overmolding Work?

Overmolding starts with a molded substrate. A second material is then molded over selected areas of that part.

A common combination is a rigid plastic substrate with a softer TPE or TPU layer. This construction is often seen on tool handles, sports-equipment components, buttons, and handheld products.

The second material can change the grip, add a non-slip surface, protect selected edges, or create a second color without attaching a separate part afterward.

Overmolding is commonly used where a rigid base and a softer surface are needed in the same part.

Overmolding vs. Insert Molding

The two processes are sometimes discussed together, but they solve different problems.

Both processes can also be used in the same product. A metal insert may be molded into the rigid base first, followed by a soft overmold on the outside.

Material Bonding Is Usually the First Thing to Check

Overmolding is often described simply as “hard plastic plus soft plastic,” but not every hard and soft material combination bonds properly.

Some resin combinations provide useful adhesion by themselves. Others require a TPE grade specifically designed to bond to the selected substrate. In some products, mechanical interlocking is also added through holes, grooves, undercuts, or wraparound features.

Material Pairing

The substrate and overmold resin should be checked by complete grade. Different TPE formulations are designed to bond to different plastics.

Mechanical Interlocking

Where chemical adhesion is limited, holes, slots, undercuts, or wraparound geometry can help hold the second material mechanically.

Surface Condition

Oil, mold release, dust, and contamination on the substrate may reduce bond strength. Storage and handling between the first and second molding operations also matter.

The Second Shot Can Affect the First Part

The overmold material needs enough temperature and pressure to fill the cavity, but those same conditions can deform the first-shot substrate.

Thin walls, long parts, or areas with limited support are particularly sensitive. After the second shot, the substrate may show distortion, sink marks, or dimensional change even though it was acceptable after the first molding operation.

For this reason, dimensions should be checked again after overmolding rather than relying only on first-shot inspection data.

Common Materials Used in Overmolding

These are general material directions only. For tooling work, the complete resin grades and the supplier’s adhesion recommendations should be reviewed.

What Does Overmolding Actually Add to a Product?

Waterproofing, shock resistance, wear resistance, and chemical resistance still depend on the complete part design and the specified materials. Overmolding by itself does not guarantee those properties.

Typical Overmolding Applications

• Sports equipment: pull tabs, grips, handles, and operating parts.
• Hand tools: screwdriver, plier, and power-tool grips.
• Electronics: buttons, frames, protective areas, and handheld housings.
• Automotive: operating buttons, grip areas, and selected interior components.
• Consumer products: handles, non-slip bases, and frequently touched components.

A rigid substrate and softer overmold can be combined where both structure and grip are needed.

What Needs to Be Decided Before Mold Design?

1. Overmold Area: Define exactly where the second material begins and ends, especially if the transition line is on a cosmetic surface.
2. First-Shot Positioning: The substrate must be held securely in the second mold so injection pressure does not move it.
3. Gate Location: The second material must reach the entire overmold area without creating unacceptable gate marks or flow defects.
4. Venting: Poor venting may cause trapped air, burns, short shots, or incomplete filling.
5. Mechanical Locking: If material adhesion alone is not sufficient, the part should provide holes, grooves, wraparound edges, or other retention features.
6. Part Release: Soft materials can stretch or stick during ejection, so draft, mold surface, and ejection method need to be considered.

What Should Be Checked During an Overmolding Trial?

Appearance is only the first check. Depending on how the product is used, pull, peel, twist, repeated-operation, or environmental testing may also be required.

• Check whether the overmold edge lifts or develops a visible gap.
• Check the soft material for short shots, flash, or flow marks.
• Measure whether the substrate changes shape after the second shot.
• Pull or flex the interface to see whether the layers separate easily.
• Confirm final dimensions and actual assembly after overmolding.

Is Overmolding the Same as Two-Shot Molding?

Not exactly. Two-shot molding usually uses a rotary mold, index system, or dual-barrel molding machine to complete both materials within one automated molding cycle.

Overmolding can be produced with two-shot equipment, but it can also be done by molding the substrate first and then transferring that part into another mold for the second injection.

The better process depends on production volume, tooling cost, part geometry, and the level of automation required.

PowerTech Mold Overmolding Capability

For an overmolding project, the first review normally covers the substrate material, overmold area, second-shot positioning, and the way the two materials will be retained.

According to the current equipment information in this page, the factory operates 24 horizontal injection molding machines from 50 to 400 tons, allowing equipment selection according to mold size, part weight, and second-shot requirements.

Before tooling starts, it is also useful to compare overmolding with two-shot molding, insert molding, or conventional secondary assembly. The lowest-risk process depends on the actual part rather than the process name.

Further Reading

• Insert Molding Guide
• Thermosetting Plastics: Properties and Applications
• Common Injection Molding Materials

Frequently Asked Questions