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What Is Overmolding?

In product design, some of your products may have a soft material (such as TPE, silicone, or rubber) wrapped around a base material (hard plastic or metal). This is done to improve grip, prevent slipping, or enhance appearance. Such products are quite common, like tool handles, toothbrush grips, and phone cases. Their production usually requires a overmolding process.

Overmolding involves using two different plastic materials, injected separately in one injection molding machine. After the first molded in one set of molds, the product is taken out and placed into another set of molds for the second injection molding. Therefore, overmolding typically requires two sets of molds.

Powerful Overmolding Factory

No More Slippery Handles — Bond, Seal, Protect With Overmolding

Sanpin leads the injection molding industry with comprehensive capabilities. Overmolding stands as one of our proven specialties. We solve tough overmolding problems like:

Fixing Production Defects

✕ Delamination: Soft material peels off easily due to insufficient adhesion with hard substrate.
✓ Material Lab: Test 300+ material pairs, deliver compatibility reports.
✕ Flash: Excess material creates rough edges, driving up finishing costs.
✓ Precision Molds: ±0.5°C temperature control eliminates flash.
✕ Sink Marks: Uneven cooling causes surface dents, ruining looks and durability.
✓ Real-time pressure monitoring: Auto-adjust pressure to prevent sink marks.

Addressing Design Pitfalls

✕ Conflicting wall thickness: Over-thick hard layers cause soft material cracks; under-thick layers lack strength.
✓ Fast DFM reviews: Get wall thickness tips within 48 hours.
✕ Visible parting lines: Molten plastic overflow at shut-off surface ruins premium product aesthetics.
✓ Patented mold positioning: <0.02mm error between base and overmold.
✓ Hidden parting lines: Seamless solutions for electronics/medical devices.
Overmolding Process

Sanpin’s expertise in overmolding helped us overcome key manufacturing challenges across multiple product lines. It reduces surgical grip delamination by 30% while surpassing FDA torque standards by 20% due to their TPU-PEEK overmolding technology significantly improved product durability. By implementing real-time pressure control, Sanpin ensured 98.6% dimensional accuracy in 500k overmolded acoustic seals. This delivers unmatched precision. Meanwhile, their 1:1 pilot production for smartwatch components resulted in 99.3% tactile consistency. As a result, our products enhance the user experience. These advancements directly drove 19% growth for medical OEMs and cut electronics returns by 63%.

Author

Elizabeth Johnson

Product Development Manager

Overmolding Medical Handle Overmolding Ejection Process

Gallery

Overmolding Applications And Case Studies

Injection Molding Factories use overmolding in a wide range of industries.

Overmolding Button
Overmolding Tool
Overmolding Part
Overmolding Hardware
Industry Representative Products Key Advantages
Medical Devices Surgical instrument grips, Prosthetic components, Catheter connectors, Syringe plungers Biocompatibility (ISO 10993), Chemical resistance, Enhanced sterilization compliance (FDA/CE)
Automotive Steering wheel grips, Airbag modules, Lightweight seat shells, Battery pack housings 40-50% weight reduction, Vibration damping (SAE J1475), Flame retardancy (UL94 V-0)
Consumer Electronics Phone cases, Earbud charging cases, Game controller grips, Smartwatch bands Anti-slip & shock absorption, IP68 waterproofing, Seamless multi-color aesthetics
Industrial & Tools Power tool handles, Wrench anti-slip sleeves, Precision instrument housings 300% impact resistance improvement, MIL-STD-810G compliance, Ergonomic fatigue reduction

Case Studies:

Challenge

Frequent drops cause TWS earbud cases to crack. This leads to a 23% user complaint rate.

Solution

Hard PC inner shell and TPU shock-absorbing overmold

Surface Treatment

Skin-like matte texture, fingerprint-resistant finish

Results

A Top 3 global smartphone brand reduced after-sales repair costs by 41%

TWS Earbud Case

Watch a Video of Overmolding In Injection Molding Factory

Material Combinations For Overmolding Technology

Overmolding technology is a paradigm of multi-material integrated manufacturing in industrial production, with diverse material pairing options. Sanpin summarizes the material combination strategies for Overmolding as follows:

Functional Synergy

i.e.: Combining polypropylene (PP) with SEBS elastomers leverages molecular chain entanglement to eliminate adhesives, while achieving gamma sterilization resistance (25kGy dose) for medical-grade handles

Performance Enhancement

i.e.: Nylon (PA66)-TPV elastomer composites exhibit exceptional resistance to oil/acid corrosion and ultra-low friction (μ=1.2). It’s ideal for high-voltage battery housings in electric vehicles.

Sustainable Innovation

i.e.: Bio-based TPE fused with PLA substrates delivers fully biodegradable solutions. By cutting carbon emissions by 50% means it meets EN 13432 industrial compostability requirements—revolutionizing eco-packaging systems.

Overmolding Material Combinations
Substrate Overmold Typical Applications Certification Standards
PP TPE/SEBS Medical handles, tool grips ISO 10993, FDA 21 CFR
ABS TPU Phone cases, smartwatch bands IEC 60529, RoHS
PC Liquid Silicone Rubber (LSR) Automotive transparent buttons, baby pacifiers IATF 16949, LFGB
Nylon (PA6/PA66) TPV Automotive battery housings, industrial gears UL94 V-0, MIL-STD-810G
PBT Glass-fiber reinforced TPE Power tool handles, connectors EN 61340, IEC 60695
PEEK Fluorocarbon Rubber (FKM) Aerospace sensors, medical implants ISO 13485, AS9100
Metal Inserts TPE/TPU Power tool handles, medical devices ISO 2768, ASTM D638
POM Thermoplastic Elastomer (TPE) Gear damping sleeves, precision instrument housings DIN 54800
PPS Silicone Rubber (VMQ) High-temperature sensors, engine components SAE J2460, AMS 3217
PLA Bio-based TPE Eco-friendly packaging, biodegradable products EN 13432, ASTM D6400

Overmolding Vs. Insert Molding Vs. Two-Color Molding

Overmolding involves injecting one material onto an existing substrate (such as hard plastic or metal). This process is completed in separate steps to form a protective or functional layer. In insert molding, the inserts such as metal or electronic components are placed into the injection mold first. Then, plastic is injected to secure and integrate the inserts. In two-shot molding, a rotating mold is used to inject two different materials in the same cycle.

Dimension Overmolding Insert Molding Two-Color Molding
Process Steps Two-step injection: Hard core → Soft overmold Pre-insert placement → Single-shot encapsulation Dual-shot injection (rotating/sliding mold)
Material Combination Soft-hard bonding (e.g., PP+TPE) Plastic + metal/insert (e.g., PA66+copper) Dual-color plastics (e.g., ABS+PC)
Equipment Requirements Standard machine + compatible mold Standard machine + robot/manual insert placement Dedicated two-color machine + rotating mold (±0.02mm)
Material Interaction Chemical bonding (thermodynamic compatibility) Mechanical locking (no chemical bonding) Physical layering (melting point synchronization)
Structural Difference Layered encapsulation (functional integration) Insert core (composite reinforcement) Seamless layering (aesthetic fusion)
Mold Cost $$ $ $$$
Production Cycle 30-60 sec (requires cooling between layers) 45-90 sec (manual insert placement) 20-40 sec (automated dual-shot)
Typical Products Tool handles, medical instruments Automotive connectors, medical electrodes Dual-color taillights, translucent keyboard keys

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FAQ

First, you need to confirm the material combination and structural design of the product. For overmolding, we need to know the material type of the base part, the type of overmolding material required, the wall thickness of the product, and the functional requirements (anti-slip, shock absorption, etc.). You can provide 3D drawings or samples, and we will conduct a feasibility analysis and provide a DFM report within 48 hours.
Delamination is usually caused by insufficient material compatibility or surface contamination. We will first test the material pair compatibility in the laboratory, then adjust the mold temperature and injection pressure, and use plasma surface treatment to improve the adhesion between the two materials. These measures can solve the delamination problem in small-batch production in 1-3 days.
For tiny overmolding products, we use high-precision molds with a temperature control system of ±0.5°C to ensure consistent molding. We also use real-time pressure monitoring and automatic adjustment to control the injection volume, and use CMM for full inspection of key dimensions to ensure that the dimensional error is within ±0.02mm.
Our overmolding production line has a flexible scheduling system. For small and medium-sized orders, we can complete the production of 10,000-50,000 pieces within 7-15 days. For sudden order increases, we can adjust the production plan and add production shifts to meet your delivery requirements. We will confirm the production capacity and delivery time with you before each order.
We provide small-batch trial production services for overmolding. We can use rapid prototype molds or simplified mold structures to complete the trial production of 50-500 pieces, which can reduce the mold cost and cycle. At the same time, we will provide a complete trial production report to verify the material compatibility, dimensional accuracy and functional performance of the product.
We use color matching equipment to analyze the color samples provided by customers, and formulate color formulas for each material. For multi-material overmolding products, we will adjust the color difference of different materials to ensure visual consistency. Our color difference standard is ΔE ≤ 2, which meets the requirements of most consumer electronics and automotive products.