Overmolding

Process Overview

Multi-Material Composite Molding

Overmolding bonds silicone to a substrate — typically a plastic component, metal insert, or rigid plastic housing. The bond is achieved either in-mold (the substrate is placed in the mold before silicone injection or compression) or through post-mold assembly using adhesive or thermal methods.

The commercial case is straightforward: clients who previously needed separate vendors for plastic injection, metal fabrication, silicone molding, and assembly can consolidate this into a single SIKING programme. What costs 5–6 vendors, SIKING delivers from one partner — eliminating inter-vendor logistics, quality escapes at handoffs, and the project management overhead of coordinating multiple suppliers.

In-mold forming: insert placed in cavity, silicone compressed or injected around it — permanent mechanical bond
Adhesive assembly: silicone bonded to plastic or metal using industrial adhesive or double-sided tape
Thermal / ultrasonic welding: fuse materials using high temperature and pressure or ultrasonic energy
Substrates: plastic (PC, ABS, PP, Nylon), metal (aluminium, steel, brass)
Plastic injection moulding in-house: 80–400T capacity for the plastic component

3

Bonding methods

1

Partner (not 5–6 vendors)

In-house

Plastic injection for substrate

Overmolding Assembly

Implementation Methods

Three Bonding Methods — Matched to Application

SIKING selects the bonding method based on your material combination, bond strength requirement, and production volume.

In-Mold Forming

The substrate (metal insert or plastic component) is placed in the mold cavity. Silicone is then compression- or injection-molded directly around it. The result is a permanent mechanical bond — not a glued joint. No adhesive failure mode. No delamination risk from adhesive degradation.

Best for: high-bond-strength requirements, sealed electronics, waterproof assemblies.

Adhesive Assembly

Silicone bonded to the substrate using industrial adhesive or double-sided tape. Suitable for configurations where in-mold forming is not feasible — large substrates, post-assembly configurations, or where the substrate is sourced separately. Adhesive selection matched to substrate and service environment.

Best for: ergonomic grips, covers, flexible sealing elements bonded to rigid housings.

Thermal / Ultrasonic Welding

High temperature or ultrasonic energy fuses the materials at the interface. Creates a permanent joint without adhesive. Ultrasonic welding is fast and consistent on compatible material combinations. Thermal welding is used where ultrasonic is not geometrically feasible.

Best for: high-volume assembly, plastic-to-plastic bonds, encapsulated assemblies.

Commercial Case

The Multi-Vendor Problem, Solved

What the multi-vendor model costs you

—Separate vendor qualifications for plastic injection, metal fabrication, silicone molding, secondary processing, and assembly
—Quality escapes at every vendor handoff — and unclear liability when they happen
—Logistics costs and lead time stacking across multiple supply links
—Engineering coordination overhead to align specifications across vendors

What SIKING consolidation delivers

✓Single qualification, single purchase order, single point of accountability
✓Plastic injection in-house (80–400T) — substrate made on-site, not sourced externally
✓No hidden handoffs; disclosed qualified partners where required
✓Measurably lower total landed cost compared to multi-vendor coordination

Process Flow

In-Mold Overmolding — Step by Step

1

Substrate Preparation

Plastic or metal substrate produced in-house or incoming inspection if sourced. Surface treatment as required (primers, activation).

2

Insert Placement

Substrate positioned in mold cavity. Fixture checks alignment before press close. Position confirmed per drawing.

3

Overmold

Silicone molded around insert via compression or transfer. Material bonds to substrate. Bonding method selected per application.

4

Bond Verification

Pull test or peel test on first-article and in-process samples. Adhesion confirmed before production continues.

5

Assembly & Inspection

Secondary processing if specified (printing, coating). Final dimensional and functional inspection. Pack and ship.

Best Suited For

Sealed electronics requiring silicone gasket bonded to plastic housing
Ergonomic grips — silicone overmolded onto rigid plastic handles or tools
Waterproof assemblies — IP-rated seals integrated into a composite part
Phone side buttons, medical device buttons with tactile silicone surfaces
P+R (plastic + rubber) composite keypads
Multi-material assemblies currently requiring 3+ separate vendors

Not Suited For

Simple single-material silicone parts with no substrate — compression or LSR is more cost-effective
Applications requiring post-bond disassembly — in-mold bonds are permanent
Very large substrate components that exceed in-house plastic injection capacity (80–400T) — qualified partner extension required
Applications where silicone-to-substrate bond is not a design requirement — evaluate if a simpler assembly method achieves the same function

Common Questions

Overmolding — FAQ

What substrates can SIKING overmold silicone onto?

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Confirmed substrates: PC, ABS, PP, Nylon (hard plastics), TPU/TPE (soft plastics), aluminium, steel, and brass. Silicone-to-plastic adhesion depends on the specific plastic grade and surface preparation — some plastics require primer or plasma treatment for reliable bond strength. This is evaluated during DFM review and validated in the first-article run.

Can SIKING produce the plastic substrate in-house, or do I need to supply it?

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SIKING operates plastic injection molding in-house at 80–400T capacity. For the majority of composite part programmes, the plastic substrate is produced on-site — same facility, same quality system, no external supply chain risk. For parts requiring larger plastic tonnage (beyond 400T), SIKING uses a disclosed qualified partner extension. You can also supply your own substrate — incoming inspection protocol will be applied.

How do you validate bond strength?

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Bond validation is performed via pull test or peel test on first-article samples, with acceptance criteria defined per the customer's functional requirement or applicable standard. In-process sampling at defined intervals during production runs maintains bond quality monitoring. If your application has a specific bond strength requirement (e.g., N/cm peel force), specify this in the RFQ and it will be built into the first-article inspection plan.