Last updated: 2026-09-04 · 5 min read

Co-Injection Molding — How Does the Process Work, What Are the Design Rules for Sandwich Parts, and When Does It Save Money Over Single-Material Molding?

Co-injection molding (sandwich molding) injects two materials in sequence — a skin layer (10–30% of wall) followed by a core layer (70–90%) — producing a part with a virgin or engineering surface over a lower-cost, recycled or foamed interior. Three configurations: sequential injection, simultaneous injection, foam-core sandwich. Mold cost adds $8,000–20,000; material cost drops 30–35% per part when the core is cheaper than the skin.

Key Facts

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  • Skin layer: 10–30% of total wall per side. Core layer: 70–90%. Skin viscosity must be equal to or higher than core viscosity (otherwise core breakthrough). Processing temperatures within 20°C of each other. Minimum total wall: 2.5 mm.
  • Three configurations: sequential injection (skin first, then core pushes it forward), simultaneous injection (both through co-injection nozzle, more uniform skin), foam-core sandwich (blowing agent in core, 10–30% density reduction, 10–25% weight savings).
  • Mold cost increase: $8,000–20,000 for dual-channel hot runner + sequence valve. Payback: if core material is 50% cheaper and 70% of wall volume, material cost per part drops 30–35%. Payback volume: 50,000–120,000 pieces.

    • Runner system — The channel network that delivers melt from the machine nozzle to each cavity. Cold runner: solidifies between shots, produces runner waste. Hot runner: stays molten, eliminates runner waste but adds $15k–$40k to mold cost. The related guide covers adjacent topics.

    How Does the Co-Injection Process Work — and What Are the Three Common Configurations?

    1. Sequential injection (most common). The skin material is injected first, filling 40–60% of the cavity. Then the core material is injected behind the skin, pushing the skin forward to complete the fill. The result is a part with skin on the outside and core on the inside. Skin thickness is controlled by the ratio of skin volume to total volume — typically 15–25% of the wall thickness on each side.
    2. Simultaneous injection. Both skin and core materials are injected at the same time through a co-injection nozzle that combines the two streams. The skin flows along the mold wall; the core flows in the center. This produces a more uniform skin thickness but requires precise flow rate control and a specialized co-injection nozzle ($15,000–30,000).
    3. Foam-core co-injection (structural foam sandwich). The core material contains a chemical blowing agent that creates a micro-cellular foam core (density reduction 10–30%) behind a solid skin. The result is a lightweight part with a solid cosmetic surface and a foamed interior — reducing weight 10–25% while maintaining the surface quality of the solid skin material.

    The choice depends on the objective. Sequential injection is the simplest and most widely used — it works on a standard two-shot machine with a co-injection hot runner. Simultaneous injection is specified when skin uniformity is critical (cosmetic surfaces where core breakthrough would be visible). Foam-core is used for weight reduction in automotive and industrial parts.

    What Are the Five Design Rules — and Which Material Combinations Work?

    Five rules that determine whether co-injection is feasible:

    Rule Requirement Reason
    Material compatibility Skin and core must bond — same polymer family or compatible adhesion If materials do not bond, skin delaminates from core
    Melt viscosity ratio Skin viscosity ≥ core viscosity If core is more viscous, it breaks through the skin (core breakthrough)
    Processing temperature Skin and core within 20°C of each other Large difference causes flow instability at the interface
    Minimum wall thickness 2.5 mm total (1 mm skin + 1.5 mm core) Below 2.5 mm, skin layer is too thin to maintain continuity
    Skin thickness ratio 10–30% of total wall per side Below 10%, core breaks through; above 30%, cost advantage is lost

    Common material combinations that work: PC skin + PC/ABS core (cost reduction), ABS skin + recycled ABS core (sustainability), PA66-GF30 skin + PA66 core (structural), PP skin + PP + talc core (stiffness). Incompatible combinations that do not work: PC skin + PP core (no adhesion), POM skin + PA core (different processing temperatures).

    The most important rule is the viscosity ratio. If the core material has a higher viscosity than the skin, the core pushes through the skin during injection and appears on the cosmetic surface — a defect called core breakthrough. The fix is to select a core grade with lower viscosity or increase the skin ratio.

    Cavity — The hollow space inside the mold that forms the part shape. Multi-cavity molds produce multiple parts per shot. Cavity count determines production output but also mold cost and complexity. Each cavity must be balanced for uniform fill. The related guide covers adjacent topics.

    Co-injection trades a higher mold cost ($8,000–20,000 for the dual-channel hot runner) for a lower per-part material cost. The payback volume is 50,000–120,000 pieces — below that, single-material molding is more cost-effective. Above that, the material savings accumulate over every shot.

  • Key parameter identified — critical variable confirmed against spec.
  • Tolerance within achievable range — commercial ±0.15mm or tighter as needed.
  • Material matched to application — resin grade selected for end-use environment.
  • Process window validated at T1 — parameters locked after first trial.
  • Inspection method defined — CMM, visual, or functional test specified.

    • Frequently Asked Questions

    How much does co-injection molding add to the mold cost — and when does it pay back?

    Co-injection adds $8,000–20,000 to the mold cost for the specialized hot runner system (dual-channel manifold, sequence valve and nozzle). The payback comes from material cost savings: if the core material is 50% less expensive than the skin and the core is 70% of the wall volume, the material cost per part drops by 30–35%.

    For a part that costs $0.50 in material with single-material molding, the co-injection material cost drops to $0.33 — saving $0.17 per part. The payback volume is 50,000–120,000 pieces ($8,000 ÷ $0.17 ≈ 47,000 shots).

    What are the most common defects in co-injection molding?

    Three defects. Core breakthrough: the core material breaks through the skin surface — caused by core viscosity being higher than skin viscosity or the skin layer being too thin. Fix: increase the skin ratio or reduce core viscosity.

    Skin-core interface delamination: the skin separates from the core — caused by incompatible materials or insufficient bonding temperature. Fix: select compatible materials or increase the processing temperature. Uneven skin distribution: the skin is thicker on one side — caused by asymmetric flow paths or unbalanced runner. Fix: redesign the runner or adjust the injection sequence.

    Can you use recycled material as the core?

    Yes — co-injection is the most effective way to use recycled material while maintaining a virgin cosmetic surface. The recycled material (regrind or post-consumer recycled) is used as the core layer, hidden behind a virgin skin. The recycled material must be the same polymer family as the skin, ground to a consistent particle size (below 5 mm), and the recycled content should not exceed 50% of the core layer to maintain structural properties.

    This approach is used in automotive interiors (recycled PA66 core behind virgin PA66-GF30 skin) and consumer electronics (recycled PC core behind virgin PC skin).

    Key Takeaways

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  • Co-injection injects two materials: skin (10–30% of wall) + core (70–90%) in one shot.
  • Three configurations: sequential, simultaneous, foam-core sandwich.
  • Five design rules: materials must bond, skin viscosity ≥ core, temperatures within 20°C, min wall 2.5 mm, skin ratio 10–30%.
  • Mold cost adds $8K–20K; material cost drops 30–35% per part.
    Payback: 50K–120K pieces.

    • If your part could benefit from a lower-cost core material, recycled content or a foam core, send the drawing and the current material specification for a co-injection feasibility review. The review checks material compatibility, estimates the material cost savings and calculates the payback volume for your program.

    Sources: Osswald, Injection Molding Handbook · Society of Plastics Engineers (SPE) · ASTM International — Plastic Testing Standards · LKM Mold Base Standards · HASCO Hot Runner Systems

    — Arthur, Tooling Engineer at aCatMould