Custom Overmolding Mold Manufacturing & Molding Services
- Combine structural strength with soft-touch, sealing, or grip functionality in a single part
- Eliminate secondary assembly — reduce cost and potential failure points
- Achieve multi-material performance that bonding or fastening cannot match
From mold design through production — KTM manages your overmolding project under one roof.
An NDA can be signed if needed before the quotation.
Understanding the Process
What is Overmolding?
Overmolding is a multi-step injection molding process where a second material — typically a soft thermoplastic elastomer (TPE), thermoplastic polyurethane (TPU), or liquid silicone rubber (LSR) — is molded directly over a pre-formed rigid substrate.
The substrate is molded first in one mold, then placed into a second mold where the overmold material is injected over it to form a single, integrated part. The bond between the two materials is achieved through chemical affinity between compatible polymers, mechanical interlocking through designed-in features, or both. The result is a composite part that combines structural rigidity from the substrate with functional properties from the overmold layer — soft-touch grip, vibration dampening, sealing, electrical insulation, slip resistance, or improved aesthetics — without adhesives, fasteners, or secondary assembly. While roughly 80% of overmolding projects involve soft-over-hard combinations (such as TPE over ABS), overmolding also applies to hard-over-hard combinations — for example, PC over ABS for structural or aesthetic reasons, or PC over PMMA for optical clarity combined with impact resistance.
How the Overmolding Process Works
Stage 1 — Substrate Molding
The cooled substrate is manually or robotically loaded into the second mold cavity, which is mounted on a second injection press. The overmold material (TPE, TPU, LSR, or a second rigid plastic) is then injected over, around, or through the substrate, forming the final bonded part.
PROCESS COMPARISON
Overmolding vs. Insert Molding vs. Two-Shot Molding
Understanding when to use each process — and which mold you need.
Overmolding
Two-mold process
Insert Molding
Pre-placed inserts
Two-Shot Molding
Rotary process
Many projects combine multiple processes. A handheld electronic device, for example, may use insert molding to embed brass contacts into a PA housing, then overmolding to apply a TPE grip surface to the same housing.
KTM builds molds for all three processes — insert molding, overmolding, and two-shot molding — under one roof. We evaluate your part design and recommend the most cost-effective approach.
Have an Overmolding Project in Mind?
Send us your 3D files or drawings. Our engineering team will review your design and respond with a DFM analysis — typically within 24 hours.
Engineering Excellence
Overmolding Mold Design & Engineering
The mold determines bond quality, dimensional accuracy, cycle time, and part consistency. Below are the critical design factors our engineering team addresses on every overmolding project.
MATERIAL EXPERTISE
Material Compatibility for Overmolding
Material selection determines whether the overmold bonds to the substrate reliably — or delaminates in service. Not all polymer combinations are compatible, and even within compatible families, bond strength varies by resin grade.
Chemical Bonding
The overmold material partially melts into the substrate surface at the molecular level during injection. This requires compatible polymer families and correct processing temperatures. It produces the strongest bond.
Mechanical Interlocking
The overmold material flows through holes, around undercuts, or into textured features on the substrate — physically locking in place after cooling. Necessary when chemical compatibility is insufficient (metals, POM, PBT).
Bond Failure Prevention
Substrate surface texture directly affects bond strength — a smoother surface produces stronger chemical bonding with TPE/TPU. Heavy textures reduce adhesion. We compensate with mechanical interlock features when texture is required.
Soft-Over-Hard Combinations (Most Common — ~80% of projects)
Hard-Over-Hard Combinations (~20% of projects)
Factors That Cause Bond Failure — and How We Prevent Them
Bond failure (delamination) is the most common quality issue in overmolded parts. Based on our production experience, the root causes trace to these factors:
Incompatible material grades
Even within compatible families, not all grades bond equally. We verify with the resin supplier's overmolding compatibility data for your specific grades.
Substrate surface contamination
Mold release agents, dust, or moisture on the substrate surface prevent adhesion. Substrates should be overmolded as soon as possible after forming.
Incorrect mold temperature
A substrate surface that is too cold prevents the overmold material from wetting and bonding. Each material pair has an optimal mold temperature window — we establish this during trials and lock it into the production process sheet.
Insufficient injection pressure or speed
The overmold material must contact the substrate surface with enough energy to initiate bonding.
Substrate surface finish too rough
For TPE and TPU overmolding, a smoother substrate surface produces stronger chemical bonding. If the substrate has a heavy texture (such as MT or VDI finishes), adhesion decreases — the deeper the texture, the weaker the bond. We compensate with mechanical interlock features when texture is required.
No mechanical interlock features
For any material combination with marginal chemical compatibility, designed-in undercuts, through-holes, or channels provide critical bond reinforcement.
We address every one of these factors during DFM review and validate bond strength with pull or peel testing during mold trials before approving production.
Explore further → For specialized overmolding applications — including silicone (LSR) overmolding, rubber overmolding, and PCB overmolding for electronics encapsulation — material selection and mold design require additional considerations. We cover these topics in detail in our technical guides.
INDUSTRIES WE SERVE
Overmolding Applications
Delivering precision overmolding solutions across diverse industries
Device housings, diagnostic equipment grips, sealing components, and surgical instrument handles with biocompatible TPE or LSR overmolds.
Full material traceability and quality documentation provided
Interior soft-touch trim, waterproof sensor housing seals, HVAC control knobs, grip handles with vibration dampening, and acoustic insulation components.
DFM analysis and CMM inspection included with every project
Overmolded cable assemblies with strain relief, connector housings with seals, POS terminal enclosures, and handheld device grips with soft-touch finishes.
40+ presses in house from 90T to 600T for scalable production
Power tool and hand tool grips, irrigation connectors, marine hardware with weather seals, and equipment control knobs with anti-slip overmolds.
In-house mold design, CNC machining, and overmolding production
Working on a Similar Project?
Send your 3D files or drawings — our engineering team will review your design and respond within 24 hours.
Success Story
TWS Earbuds Charging Case
Premium soft-touch overmolding for consumer electronics
Project Results
Premium soft-touch TPU overmold on PC substrate
Consistent matte texture across all units
Perfect color matching to brand specification
"KTM delivered exceptional quality on our TWS case project. The soft-touch finish exceeded our expectations and they maintained consistent quality across 500K+ units. Their engineering team solved our initial bonding challenges quickly."
Our Capabilities
Production Capabilities
State-of-the-art facilities equipped for precision overmolding production
Precision Mold Making
In-house tooling: Complete mold design and manufacturing
Tolerance: +/-0.02mm precision capability
Materials: P20, H13, S136, NAK80 tool steels
Quality Assurance
CMM inspection: 3D coordinate measuring
100% inspection: Critical dimension verification
Documentation: PPAP, FMEA, SPC available
FAQ
Common questions about our overmolding services
Get Started
Request a Quote
Share your project details and our engineering team will provide a comprehensive quote within 24-48 hours.
Project Details
Ready to Start Your Overmolding Project?
Get a free quote within 24-48 hours. Our engineering team will review your requirements and provide DFM feedback.