Multi-shot/Co-injection

Multi-shot/co-injection molding forms a single part from two or more polymers in one tool cycle, creating bonded layers, skins, or soft-touch features.

Overview

Multi-shot/co-injection molding (also called multi-material or 2-shot molding) injects two or more polymers into one mold to create a single integrated part. Common setups include hard/soft combinations (rigid substrate with TPE grip/seal), two-color cosmetic parts, or co-injected “skin/core” structures to tune appearance, stiffness, or cost.

Choose it when the multi-material interface must be repeatable, automated, and high-volume, and when you want a molded-in seal, grip, or color break without secondary assembly. It can improve part integrity and reduce labor, but it drives up tooling complexity (gating, sequencing, tooling shutoffs), press requirements (multi-barrel or rotating platen), and process development time.

Key tradeoffs: material compatibility and adhesion limit options; cosmetic knit lines and material “bleed” can be hard to control; regrind and scrap handling are more constrained; engineering changes are expensive once the tool is built.

Common Materials

  • PC
  • ABS
  • PC/ABS
  • PP
  • TPE
  • PA66

Tolerances

±0.005"

Applications

  • Soft-touch tool grips with rigid cores
  • Gasketed housings with molded-in seals
  • Two-color consumer product bezels and buttons
  • Under-hood automotive seals and caps
  • Medical device handles with overmolded grips
  • Skin-core cosmetic panels using virgin skins and regrind cores

When to Choose Multi-shot/Co-injection

Use multi-shot/co-injection when the part needs two materials or colors in a single automated cycle with consistent alignment and bonding at production volumes. It fits parts where sealing, grip, vibration damping, or a premium cosmetic surface must be built in without adhesives or assembly. Expect best results when the interface geometry is well-controlled and the material pair is proven for adhesion.

vs Standard Injection Molding

Choose multi-shot/co-injection when a single-material part can’t meet functional or cosmetic needs, such as needing a molded-in seal, soft-touch zone, or multi-color features. It avoids secondary assembly and improves repeatability, but increases tooling and process complexity versus standard molding.

vs Overmolding

Choose multi-shot/co-injection when you want both shots controlled in one tool/press sequence with tight registration and high throughput. It typically provides better automation and consistency than handling pre-molded substrates, but requires more specialized tooling and equipment.

vs Insert Molding

Choose multi-shot/co-injection when the “insert” is another polymer shot and you need a continuous polymer-to-polymer interface without placing components. Insert molding is better when you must encapsulate metal, magnets, or electronics; multi-shot is better for all-polymer multi-material features and eliminating insert handling.

vs Compression Molding

Choose multi-shot/co-injection when you need detailed thermoplastic features, tight feature definition, and high-cavity productivity. Compression molding fits thicker sections and simpler geometry; multi-shot/co-injection fits complex interfaces, snap features, and cosmetic class-A surfaces with controlled material placement.

vs Liquid Silicone Rubber (LSR) Molding

Choose multi-shot/co-injection when both materials are thermoplastics and you need weldable, recyclable thermoplastic construction with fast cycle times. LSR is the better fit when you need silicone’s temperature/chemical resistance and long-term sealing, but it changes tool design, processing, and bonding strategy.

Design Considerations

  • Pick material pairs with proven adhesion or design a mechanical interlock (undercuts, through-holes, ribs) where chemical bonding is weak
  • Design shutoffs and interface geometry with generous land widths to prevent material bleed and flash at the boundary
  • Control wall thickness transitions and avoid thick cores to reduce sink/warp amplified by differing shrink rates between materials
  • Specify cosmetic requirements by zone (A/B/C surfaces) and define allowable knit lines and color breaks early to guide gating and sequencing
  • Add robust datum features and alignment geometry if the tool uses a rotating platen/indexing to maintain shot-to-shot registration
  • Call out resin grades, colorants, and regrind allowance explicitly; small changes in melt flow or hardness can shift fill balance and interface location