Overmolding, Insert Molding & 2K Molding Services | Multi-Material Injection Molding China | JBRplas

Multi-Material & Multi-Component Injection Molding

Not every plastic part is made from a single material. Grips need soft-touch surfaces on rigid bodies. Connectors need metal threads encapsulated in plastic. Seals need flexible lips bonded to structural frames. JBRplas provides three multi-material molding processes — overmolding, insert molding, and two-shot (2K) molding — from a single Shenzhen facility.

The right process depends on materials, volumes, and geometry. This page explains the differences, the capabilities, and how to choose.

Overmolding

Overmolding bonds a second material — typically a soft TPE or TPU — onto a previously molded rigid substrate (PC, ABS, PP, PA). The two materials form a chemical or mechanical bond at the interface, producing a single integrated part.

Typical Applications

• Soft-touch grips on power tools, medical devices, and consumer electronics
• Integrated gaskets and seals for IP-rated enclosures (IPX4–IPX7)
• Wearable device bands — rigid structure with skin-contact TPE layer
• Overmolded buttons and keypads — soft actuation surface on rigid housing
• Anti-slip feet and bumpers on industrial equipment housings
• Dental and surgical instrument handles — rigid core + soft grip

Compatible Material Pairs

Overmolding requires chemical compatibility between the substrate and overmold material. Not every TPE bonds to every substrate. The table below lists the pairs JBRplas processes regularly:

Bond strength is validated per production batch using peel testing per ISO 813. For medical applications, only ISO 10993 certified TPE grades are used.

Process Requirements

• Substrate temperature control: The overmold bond relies on the substrate surface being at the correct temperature when the overmold material is injected. The process window is typically 10–15°C — too cold and the bond fails; too hot and the substrate deforms.
• Substrate mold design: The substrate mold must produce parts that fit precisely into the overmold cavity. Dimensional tolerances on the substrate at the overmold interface are held to ±0.05mm.
• Overmold tool design: Shut-off surfaces along the substrate edges must be machined to prevent flash on the overmold boundary. This is manual die-spotting work that requires experienced toolmakers.
• Cycle time: Overmolding adds 15–25 seconds to the total cycle versus single-material molding, depending on part size and material pair.

Insert Molding

Insert molding encapsulates a pre-formed component — typically a metal threaded insert, bushing, electrical contact, or PCB — inside an injection-molded plastic body. The insert is placed into the mold before each shot and the plastic flows around it, locking it in place during solidification.

Typical Applications

• Threaded brass inserts in plastic housings for screw assembly
• Stainless steel bushings in bearing housings and pivot points
• Electrical contacts and terminals in connector bodies
• PCB encapsulation for sensor housings and electronic modules
• Magnet inserts in sensor trigger components
• Metal reinforcement plates in structural plastic brackets

Insert Types and Tolerances

Process Requirements

• Insert positioning: Inserts are placed into the mold manually or via automated pick-and-place. Positioning tolerance is maintained by machined insert pockets in the mold — the insert fits into a precision-machined recess that holds it in place during injection.
• Insert preheating: Metal inserts are preheated to 80–120°C before loading. A cold insert causes the melt to freeze prematurely around it, creating a weak interfacial bond and sink marks on the opposite face.
• Cycle time impact: Manual insert loading adds 10–20 seconds per cycle versus an unloaded mold. Automated loading reduces this to 5–8 seconds, suitable for volumes above 100,000 parts per year.
• Tooling requirements: P20 steel minimum for insert molding due to the wear from metal-on-steel contact during insert placement. H13 recommended for volumes exceeding 100,000 cycles.

Overmolding vs Insert Molding

2K Molding (Two-Shot Molding)

2K molding injects two different materials or colours into the same mold in a single machine cycle. The first material is injected into the primary cavity; the mold then rotates (rotary platen) or the part is transferred (core-back) to a second cavity where the second material is injected. The result is a single integrated part with two materials — produced in one cycle.

Typical Applications

• Two-colour keycaps and buttons (symbol in one colour, body in another)
• Automotive tail light lenses — clear PC lens + black ABS housing in one part
• Toothbrush handles — rigid PP core + soft TPE grip in one shot
• Rotary knobs with soft-touch outer ring and rigid inner hub
• Medical device housings with integrated soft seals

2K vs Overmolding

JBRplas Approach

For programs under 200,000 parts per year, JBRplas recommends the insert-overmold method — mold the substrate, transfer it manually or robotically to the overmold tool, and inject the second material. Process parameters, bond strength, and dimensional results are identical to 2K. The difference is cycle time and labour — 2K is faster but the tooling investment only pays back at high volumes.

For programs above 200,000 parts per year, JBRplas can manufacture dedicated 2K rotary tools with hot runner systems for both materials. We quote both approaches and let the production economics decide.

Choosing the Right Process

Frequently Asked Questions

What is the minimum order quantity for overmolding or insert molding?

No formal MOQ. We run overmolding and insert molding from 500 pieces for prototyping and validation. For insert molding, insert supply is typically the limiting factor — we can source inserts or work with customer-supplied components.

Can you do overmolding with medical-grade materials?

Yes. Our ISO 13485 certification covers overmolding programs. We use ISO 10993 certified TPE grades (Kraiburg THERMOLAST M, etc.) for skin-contact medical applications. Full lot traceability and USP Class VI material certificates are provided as standard for medical programs.

Do you source inserts or do I supply them?

Both. We can source standard threaded inserts (brass, stainless steel) and bushings from our approved supplier network. Custom or proprietary inserts are supplied by the customer. We recommend providing a dimensional specification and tolerance for any customer-supplied inserts.

How do you validate bond strength for overmolded parts?

Peel strength testing per ISO 813 on 3 sample parts from every production batch. Target bond strength is specified at the DFM stage based on the material pair. For medical and automotive programs, additional durability testing (thermal cycling, chemical exposure) is available.

Can you do overmolding at low volume?

Yes. For programs under 100,000 parts, we use the insert-overmold method — mold the substrate on one press, transfer to a second press for overmolding. This avoids the cost of a dedicated 2K rotary tool while producing identical part quality. Learn more →

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