A Comparison of Overmolding Performance Across Different Materials for Automotive Connectors

This article breaks down the core technical requirements of automotive connector overmolding from three dimensions — material compatibility, mold design, and process control — and offers practical optimization directions for industry pain points such as sealing, temperature resistance, and durability. It is intended as a reference for R&D and process engineers.

Automotive connectors are fundamental components of a vehicle's electrical system, and the overmolding process directly affects their sealing performance, insulation reliability, and service life. Different combinations of substrate and overmolding materials differ significantly in compliance, bond strength, and resistance to environmental aging, which in turn determines the end performance of the finished connector. This article analyzes the topic from three dimensions — material compatibility, process control, and quality control — to provide a reference for process selection and quality management in automotive connector overmolding.

[I. Material Compatibility Requirements]

[1. Compliance Requirements]

Overmolding materials for automotive connectors must meet automotive-grade compliance requirements. Different applications impose explicit requirements on environmental, temperature, and flame-retardant performance. Mainstream domestic OEMs currently require compliance with environmental standards such as RoHS 2.0 and REACH SVHC, and some high-voltage connectors for new energy vehicles must additionally meet the VW-1 flame rating.

The baseline compliance characteristics of common overmolding materials differ as follows:

Overmolding Material | Environmental Compliance | Temperature Range | Flame Rating Fumed-type liquid silicone rubber (LSR) | Easily meets RoHS/REACH; no low-molecular-weight extractables | -40°C to 200°C | Up to UL94-V0 Solid silicone rubber (millable) | Requires control of small molecules from the cure system | -30°C to 180°C | Up to UL94-V1 Thermoplastic elastomer (TPE/TPV) | Some grades contain phthalate plasticizers; screening required | -40°C to 120°C | Generally HB/V2

For high-temperature, high-humidity environments such as the engine bay and battery pack, fumed-type LSR is the preferred choice: its long-term aging resistance is more stable, the risk of low-molecular-weight extractables is lower, and it meets OEM requirements for a 15-year service life.

[2. Interface Bonding Compatibility]

Interface bond strength is the core performance metric of automotive connector overmolding. The bonding compatibility between the overmolding material and the connector substrate (copper alloy, PBT, PA66, PPS, etc.) directly determines the sealing reliability of the finished part.

Bonding characteristics vary across substrate-material combinations, and mainstream solutions fall into three categories:

Plastic substrate + LSR: Most engineering plastics (PA, PBT) can achieve direct bonding through mold-surface activation or substrate modification, with bond strengths of 0.8-2.0 MPa — sufficient for standard mating and sealing requirements. For low-surface-energy PPS and LCP, silane coupling-agent pretreatment or an injection-bonding process is needed to raise the strength. Metal substrate + LSR: Copper-alloy substrates usually require primer treatment, achieving bond strengths of 1.2-2.5 MPa. Untreated bare copper generally bonds at less than 0.5 MPa and is prone to delamination and cracking. Plastic substrate + TPE: Thermoplastic elastomers can achieve mechanical interlocking through two-shot injection molding. This suits lower-demand applications, but bond strength is generally below 1.0 MPa and long-term creep risk is higher. For connectors rated IP67 or above for water ingress, interface bond strength must be verified through tensile-shear testing to prevent interfacial gaps — and the leakage and short circuits that follow — over long-term use.

[II. Key Process Considerations]

[1. Mold Structure Design]

Runner and venting design of automotive connector overmolding molds directly affects molding quality. LSR injection overmolding in particular places far higher demands on mold venting than conventional injection molding.

Key mold design points for different overmolding processes:

Liquid silicone overmolding: Use a cold-runner system with runner diameters of 3-6 mm. Gates should preferably be located at thin-wall edges to avoid trapped air. Parting surfaces need vent grooves 0.02-0.05 mm deep, with vent-groove width controlled at 5-10 mm; excess material can be removed in a later trimming operation. Two-shot injection overmolding: After the first shot, retain a 0.1-0.3 mm textured (roughened) surface to improve the mechanical interlock of the second shot. Design locating inserts for the substrate on the fixed-mold side, with positioning tolerance within ±0.02 mm to prevent flash. For multi-cavity connector molds, gate balance across cavities must be maintained — keep runner length variation within 5% — to avoid cavity-to-cavity filling inconsistency that causes bond-strength scatter.

[2. Molding Parameter Control]

Molding temperature, injection pressure, and cure time are the three core parameters of automotive connector overmolding, and the process windows differ significantly between material systems.

Typical parameter ranges for mainstream materials:

Overmolding Material | Material Temperature | Mold Temperature | Injection Pressure | Cure / Cooling Time Addition-cure LSR | 15-25°C (A/B material tanks) | 150-180°C | 50-100 MPa | 30-120 s Solid silicone rubber | Room-temperature feeding | 160-190°C | 30-80 MPa | 120-300 s TPE/TPV | 180-220°C | 30-60°C | 80-150 MPa | 20-60 s (cooling)

For addition-cure LSR, mold temperature fluctuation must be held within ±5°C. Too low a temperature leads to incomplete cure and reduced bond strength; too high a temperature causes premature curing and short shots. Injection speed should follow a "slow-fast-slow" profile: fast filling to limit premature curing, then low-speed packing to reduce internal stress.

[III. Quality Control Essentials]

[1. Common Defect Prevention]

The most common defects in automotive connector overmolding are bubbles, flash, and interfacial delamination. Each has distinct causes and preventive measures.

Prevention and control points by defect type:

Bubbles: Mainly caused by inadequate mold venting, excessive material moisture, or a mismatch between cure speed and injection speed. Prevention: keep LSR raw-material moisture strictly below 0.1%; optimize parting-surface vent-groove design; adjust the cure system to match injection speed and avoid premature scorch. Flash: Mainly caused by excessive parting-surface clearance, excessive injection pressure, or insufficient clamping force. Prevention: hold parting-surface machining accuracy within 0.01 mm; calculate clamping force from the cavity projected area, at no less than 8 tonnes per square centimeter of projected area; optimize gate location and size to reduce over-packing. Interfacial delamination: Mainly caused by substrate surface contamination, inadequate pretreatment, or a mismatched bonding system. Prevention: complete overmolding within 24 hours of substrate molding to avoid surface contamination with oil and dust; low-surface-energy substrates must receive plasma pretreatment or primer; run material bonding-compatibility tests in advance to select a matching overmolding material grade. [2. Finished-Part Performance Testing]

Overmolded automotive connectors must pass full performance testing before entering volume production. Test items are divided into standard and application-specific categories to satisfy different automotive requirements.

Standard mandatory items include: interface bond tensile-shear strength, dimensional tolerance, water-sealing performance, and visual defects. IP water-ingress testing must be performed per IEC 60529; high-voltage connectors must meet at least IP67, and low-voltage connectors no less than IP65.

Application-specific items are determined by the use case and include: thermal cycling (-40°C to 125°C, no delamination or cracking after 1,000 cycles), heat aging (hardness change within ±10 Shore A after 1,000 hours at 150°C), flame retardancy (per UL94), and salt-spray corrosion testing (per GB/T 10125, no corrosion or delamination after 96 hours). All test records must be retained to satisfy OEM traceability requirements.

[IV. Summary and Outlook]

Performance differences in automotive connector overmolding ultimately stem from material compatibility and process-control precision. Each material combination has its strengths: liquid silicone offers advantages in temperature resistance, aging resistance, and bonding stability, making it suitable for mid- to high-end automotive-grade connectors; thermoplastic elastomers offer high molding efficiency and low cost, fitting low-voltage, less demanding applications.

As new energy vehicles push requirements for connector sealing reliability and high-voltage insulation ever higher, the share of addition-cure liquid silicone overmolding will continue to grow. Future industry development will focus on three directions — primer-free bonding technology, integrated injection molding processes, and inline defect inspection — further improving the reliability and production efficiency of automotive connector overmolding to meet the industry's demands for cost reduction and quality upgrading.