What Is Overmolding? Process, Materials & Applications

By Owen Lin, Engineer, Tuowei Precision · November 2026

When engineers ask what is overmolding, the short version is this: it is an injection molding process that molds one material directly over or around another to make a single, bonded part. The first part, called the substrate, is usually a rigid plastic. The second layer is usually softer, such as a thermoplastic elastomer (TPE) or silicone.

You handle these parts every day. Toothbrush handles, power tool grips, phone cases, sealed connectors and medical device handles all combine a stiff structure with a soft, grippy or sealing surface this way, without glue or assembly. This guide covers how the process works, the two main production methods, which material pairs bond, the design rules that prevent peeling, and when the extra tooling cost pays off.

Short answer

Short answer: Overmolding is a multi-step injection molding process in which a second material is molded onto a previously molded substrate, forming one integrated part. The bond comes from chemical adhesion between compatible materials, mechanical interlocks in the design, or both. It is used for soft-touch grips, seals, vibration damping and protection of electronics.

What Is Overmolding in Injection Molding?

The process is a variation of standard injection molding. Each material is still melted, injected into a steel or aluminum mold, cooled and ejected. The difference is that the mold for the second shot already holds the first part, and the new material flows against that part’s surface.

Three terms come up often and are easy to confuse. Our guide on the difference between overmolding and insert molding compares the two side by side:

• Overmolding is the general term for molding a material over an existing part.
• Two-shot molding (also called 2K or multi-shot molding) is one way to do it, with both shots on one machine in one cycle.
• Insert molding places a pre-made component, usually metal, in the mold and molds plastic around it. Many molders treat it as a related process, but its purpose is usually strength or function rather than feel.

How Does the Overmolding Process Work?

Whatever the method, every job follows the same basic sequence:

• Mold the substrate. The rigid part is injection molded in its own cavity.
• Position the substrate. It moves into the second cavity, either inside the machine or by robot or operator.
• Inject the second material. It fills the space between the substrate and the mold wall. Heat and pressure help it wet and bond to the substrate surface.
• Cool and eject. The combined part cools until both materials are stable, then leaves the mold as one piece.

Two-Shot Molding

In two-shot molding, a press with two injection units makes both shots in one cycle. After the first shot, the mold rotates, or a core moves, so the fresh substrate sits in the second cavity. The soft material goes in while the substrate is still warm, which helps adhesion.

Two-shot tooling is complex and expensive, and it needs a multi-shot press. In return, cycle times are short, alignment between the two materials is very consistent and labor is minimal.

Pick-and-Place (Two-Step) Molding

Here the substrates are molded first, often in a separate tool and machine. A robot or operator then loads each one into a second mold for the second shot.

The tools are simpler and cheaper, and standard single-shot presses can run them. Cycle time and labor cost per part are higher, though. The substrate has also cooled by the time it is covered, which weakens the bond for some material pairs. Preheating or plasma treatment helps.

Our View: Start With Pick-and-Place

Our view: for prototypes and runs up to the low tens of thousands of parts, pick-and-place is usually the right starting point. Move to two-shot when volumes are high enough to pay back the tool, or when bond and alignment need the consistency of a single cycle. The break-even point varies with part size and labor cost, so ask your molder to quote both routes.

Two-Shot vs Pick-and-Place at a Glance

Factor	Two-shot molding	Pick-and-place
Tooling	One complex multi-shot mold	Two simpler molds
Press	Multi-shot press with two injection units	Standard single-shot presses
Cycle time	Short	Longer per part, plus handling time
Bond	Stronger, second shot lands on a warm substrate	Weaker for some pairs, substrate has cooled
Best for	High volumes, consistent bond and alignment	Prototypes and runs up to the low tens of thousands

Which Material Pairs Bond?

Material compatibility is the most important decision in the whole project. If the two materials do not bond chemically, the design has to hold the outer layer mechanically, or it will peel.

Substrate	Common outer material	Bond notes
Polypropylene (PP)	SEBS-based TPE	Strong chemical bond with standard grades
ABS, PC, PC/ABS	Modified TPE or TPU	Needs grades formulated to bond to these plastics
Nylon (PA6, PA66)	Nylon-bonding TPE or TPU	Dry the nylon; moisture weakens the bond
PC, PA	Self-bonding liquid silicone (LSR)	Needs self-bonding LSR and a substrate that tolerates the cure heat
Metal	TPE, TPU, rubber or rigid plastic	Usually relies on mechanical locks or primers

The softness of the outer layer is specified as durometer hardness, usually on the Shore A scale for grips and seals. It is measured following ASTM D2240, so state both the scale and the value on the drawing. Two materials with the same Shore value can still feel quite different in the hand, so sample parts are worth the wait.

Check the Bonding Chart, Then Test Real Parts

Material suppliers publish bonding charts, and they are the best first check. Bond strength still varies with grade, molding conditions and color, so test real parts before committing to production.

What Are Overmolded Parts Used For?

Engineers use the process to add a property the substrate cannot provide alone:

• Grip and comfort: soft-touch handles on tools, kitchen utensils and toothbrushes.
• Sealing: integrated gaskets on enclosures and lids, with no separate O-ring to assemble.
• Vibration and shock damping: bumpers, feet and impact zones on devices.
• Electrical insulation: covers over metal contacts and terminals.
• Appearance: two colors or textures in one part, with no paint.
• Part consolidation: one molded part replaces two parts plus adhesive or fasteners.

What Is Overmolding in Electronics?

In electronics, the second material encapsulates components to protect them from water, dust and impact. Cable assemblies are a common example. The plastic molded over a connector both seals the joint and acts as a strain relief, so the cable does not break at the plug.

Covering a populated circuit board is more delicate. Standard injection pressure and temperature can damage components or solder joints, so low-pressure molding with hot-melt polyamides is often used instead. Tell the molder exactly which components sit under the encapsulation and their temperature limits.

Why Use Overmolding for Medical Devices?

Medical devices use it for secure grips on instruments, soft patient-contact surfaces, and seals that keep fluids out of housings. A one-piece part has no glue line and fewer crevices than an assembly, which helps cleaning and sterilization.

The trade-off is qualification. Every material that touches the patient must be a medical grade with the right biocompatibility testing, commonly under the ISO 10993 series, and the bond must survive the chosen sterilization method. Those requirements depend on the device and its market, so confirm them with your regulatory team before selecting materials.

Design Guidelines That Prevent Peeling

Most failures trace back to design, not processing. These rules prevent the common ones:

• Lock the soft layer mechanically. Even with a chemical bond, add through-holes, undercuts or grooves so the material wraps into the substrate.
• Avoid feather edges. Thin edges peel first. End the soft layer in a groove or against a shoulder, so its edge is buried.
• Keep walls uniform. Industry design guides commonly suggest roughly 1 to 3 mm for most TPE grips, but confirm the range for your material. Very thick sections sink and cool slowly.
• Design the shut-off. The substrate must seal tightly against the second mold, or the soft material will flash onto areas meant to stay bare.
• Support the substrate. Pressure from the second shot can deflect a thin substrate. Add ribs or support points in the mold.
• Match temperatures. The substrate must tolerate the second material’s melt temperature without softening or warping.
• Account for two shrink rates. Each material shrinks differently, which can bow long, thin parts.

Tolerances to Expect

Two-material parts hold tolerances similar to standard molded parts, with a little extra variation where the materials meet. ISO 20457 is the international standard for tolerances on plastic molded parts, and it is a useful shared reference for drawings. Typical values for two-material parts:

Type of work	Typical tolerance
Standard two-material parts	±0.005 in (±0.127 mm)
Precision work	±0.002 in (±0.051 mm)
Over metal inserts	±0.005 in (±0.127 mm)
Complex parts	±0.003 in (±0.076 mm)

Soft surfaces compress under a probe or caliper, so agree how they will be measured. Hold tight tolerances on the rigid substrate and keep looser ones on the elastomer.

Advantages and Disadvantages

Advantages	Disadvantages
Combines rigid and soft properties in one part	Two molds or a complex two-shot tool raise upfront cost
Removes assembly, glue and fasteners	Not every material pair bonds
Strong, permanent bond when designed well	Longer development and sampling time
Built-in seals and grips	A defect scraps both materials, not one
Two colors or textures without painting	Harder to recycle because materials are joined
Better vibration damping and drop protection	Design changes after tooling are costly

Is Overmolding Expensive?

It costs more upfront than single-material molding, because it needs either two molds or one complex multi-shot mold. Per-part cost depends on the route. Two-shot parts are cheap once tooled, while pick-and-place adds handling time to every part.

The comparison that matters is against the assembly it replaces. If the soft layer removes a separate gasket, an adhesive step or a snap-fit cap, savings in labor and part count can outweigh the tool cost at moderate volumes. If the part only needs a soft surface for looks, a soft-touch paint or a separate grip sleeve may be cheaper.

Common Mistakes to Avoid

• Choosing materials by data sheet alone. Bond strength is grade-specific. Ask for real bond tests on molded samples.
• Leaving the edge exposed. A visible thin edge is where peeling starts.
• Skipping substrate drying. Moist nylon or PC bonds poorly and can bubble.
• Over-tolerancing the soft material. Elastomers move under measurement, so tight tolerances there create false rejects.
• Ignoring color. Some pigments reduce adhesion, so test the actual color rather than a natural sample.
• Freezing the tool too early. Bond and fit issues often appear only in first samples, so plan a sampling round.

Frequently Asked Questions

What does overmolding mean?

Overmolding means molding one material over another to create a single part. A rigid plastic or metal substrate is placed in a mold, and a second material, often a soft TPE or silicone, is injected around it. The two bond chemically, mechanically or both, so no assembly is needed afterwards.

How strong is the bond between the two materials?

It depends on the material pair, grade, color and molding conditions. Compatible pairs such as PP with SEBS-based TPE can bond so well that the elastomer tears before it separates. Incompatible pairs barely stick, so they need mechanical locks. Always test peel strength on real molded samples before production.

What is the difference between overmolding and two-shot molding?

Overmolding is the general process of molding one material over another. Two-shot molding is one way to do it, with both materials molded in a single cycle on one machine using a rotating or shuttling mold. Pick-and-place is the other common method, using two separate molding steps.

Which materials are used for the soft layer?

Thermoplastic elastomers are the most common, followed by TPU and liquid silicone rubber. TPE grades are formulated to bond with specific substrates such as PP, ABS, PC or nylon. Silicone needs self-bonding grades and heat-tolerant substrates. Rigid plastics are sometimes molded over other plastics for color or structure.

What is the average lifespan of a plastic injection mold?

Lifespan depends on mold steel, part material and maintenance. Aluminum prototype tools may make thousands of parts, while hardened steel production molds can make hundreds of thousands to over a million. Molds for glass-filled or abrasive materials wear faster. Confirm the expected tool life with your molder when you quote.

Planning an Overmolding Project With Tuowei Precision

At Tuowei Precision, we mold plastic over plastic, rubber over plastic and plastic or rubber over metal, from prototypes to production. Our quality system is certified to ISO 9001:2015 and ISO 14001:2015. Where a part needs metal threads rather than a soft layer, our insert molding service covers it.

For a useful quote, send STEP files for both the substrate and the outer layer, your preferred materials and hardness, colors, annual volume and any sealing or bond-strength requirements. Our engineers check material compatibility, shut-offs and mechanical locks before tooling. You can start on our custom overmolding page.

About the Author

Owen Lin is an engineer at Tuowei Precision. He plans tooling and production processes and works with the quality team to keep every batch consistent.

This article is general engineering information. Medical device material and regulatory requirements vary by device and market; confirm them with qualified regulatory professionals.