Co-Injection Molding — When Do You Use the Skin-Core Process, What Are the Design Rules, and What Defects Are Unique to This Technology?

Last updated: 2026-09-03 · Reading time: about 6 min

Co-injection fires two materials through one gate into one cavity, in sequence — skin, core, skin — forming a single sandwich wall. Use it when surface and interior need different things: a virgin, UV or colored skin over an economy, regrind or foamed core. Its failure modes are its own — core breakthrough and delamination — and you design them out with the skin ratio, the gate and the material pair.
See also related article.

Key facts

<>

  • One cavity, one gate, three phases: the skin (5–15% of shot) coats the walls, the core (60–80%) fills the middle, a final skin (10–20%) seals the gate — a skin-core-skin sandwich, not two bonded layers.
  • Four reasons to use it: an economy or regrind core (material cost per part drops 15–30%), a property split (UV or color skin over a tough core), recycled content, or a foamed core that is lighter and sink-free.
  • Four defects are unique to the process — core breakthrough, interface delamination, uneven skin and core in the runner — each traceable to the skin ratio, flow length, gate position or material pairing.

    • Shrinkage — The dimensional reduction from mold cavity to cooled part. Amorphous resins: 0.4–0.7% (uniform). Semi-crystalline: 1.0–2.5% (directional). Mold makers apply shrinkage factor when machining cavity. Actual shrinkage varies with wall thickness, gate location and packing pressure. See also related guide.

    When does skin-core earn its place — and how do you set the ratio?

    The sandwich is built in three phases, and each phase is where a defect starts. Understand the sequence and the failure modes make sense.

    1. Skin first — 5–15% of the shot. The virgin skin flows through the gate and coats the cavity walls, setting the visible surface. Starve it and the skin will not the flow end before the core does — that is core breakthrough.
    2. Core second — 60–80%. The core flows down the center, pushing the skin ahead of it, filling the interior with economy resin, regrind or a foamed compound. This is where the material saving or the weight reduction comes from.
    3. Skin last — 10–20%. A final skin shot follows the core to seal the gate and the runner, so no core material shows at the sprue. Skip it and the core freezes into the runner and colors the regrind.

    The skin-to-core ratio is the dial that trades surface coverage against core volume. Set it to the job.

    Application Skin/core ratio Why
    Economy or regrind core 70/30 Maximum core volume for material saving; skin still fully covers
    UV-stable or colored skin 80/20 20% skin is enough to protect and hide the core
    Foamed core 85/15 A solid skin encapsulates the foam — lighter part, no sink
    Property combination 75/25 Tough or rigid core, cosmetic or chemical-resistant skin

    What defects are unique to co-injection — and how do you design them out?

    These four do not occur in single-material molding. Each has one root cause and one fix, and all four are cheaper to design out than to sort at the press.

    Defect Cause Fix
    Core breakthrough at the flow end Flow too long; the core outruns the skin Add a second gate to halve flow length; raise the skin ratio 15%→25%
    Interface delamination Incompatible pair, or the core runs >30°C hotter than the skin Use a bonding pair; pull the melt-temperature gap under 30°C
    Uneven skin distribution Gate at one edge; skin thins toward the far end Move the gate to the center; hold the wall within 15%
    Core in the runner Skin volume too small to fill the runner first Raise skin volume 10%→15% so skin fills the runner before core

    Pair materials that bond. Skin and core must melt within 30°C of each other, adhere at the interface, and shrink within 0.5% or the part warps or peels. Bonding pairs: ABS/ABS, PC/PC, PP/PP, PC/PMMA. Pairs that delaminate: PC/PP, ABS/PA, PC/POM.

    “Co-injection looks simple on paper — two materials, one gate — but it fails in ways a single-material mold never does. The core wants to burst through at the flow end and the interface wants to peel, so we set the skin ratio, the gate and the material pair before we cut steel. We would rather spend a day on the skin-core balance than sort breakthrough parts at the press.”

    — Project Engineer, aCatMould

    Frequently asked questions

  • 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.

    • How is co-injection different from two-shot molding and overmolding?

    Co-injection fires both materials through one gate into one cavity, in sequence — skin, core, skin — so they form a single sandwich wall locked together by the flow, not two bonded layers.

    Two-shot (2K) molding shoots material A, then indexes or rotates the part to a second cavity for material B — two distinct shots, two visible materials. Overmolding lays a second material over a preformed substrate. Co-injection is the only one of the three that hides the core inside a single wall.

    Can co-injection make clear or optical parts?

    Rarely. The skin-core interface carries two refractive indexes, so it scatters light and the part turns translucent rather than clear. That is fine for diffusers and indicator lenses, where you want the light spread evenly.

    True optical co-injection needs a skin and core with matched refractive indexes from the same resin family — a narrow material window. Automotive tail-light lenses use it: a hard, scratch-resistant skin over a tough core, both optically matched.

    How do you validate the skin-core structure?

    Cut a cross-section perpendicular to flow and read the sandwich under a microscope — skin thickness at five points (gate, mid-flow, flow end, two sides), held within ±20% of nominal. This is destructive, so run it on 3–5 parts per mold trial.

    Two non-destructive checks back it up: a color check for core breakthrough (dye the core, then look for it at the flow end and weld lines) and a weight check within ±3% of target — a heavy part means too much core, a light one an over-expanded foam core.

    Key takeaways

    1. Co-injection hides one material inside another through a single gate — skin, core, skin — so a virgin, UV or colored surface can wrap an economy, regrind or foamed core.
    2. Set the skin/core ratio to the job: 70/30 for maximum core saving, 80/20 for UV or color, 85/15 to encapsulate a foam core.
    3. Its unique defects — core breakthrough, delamination, uneven skin, core in the runner — all trace to the skin ratio, flow length, gate position or material pairing, so design them out before T1.

    Co-injection buys a part that looks like its skin and performs like its core. Choose it when the surface has to be virgin, UV-stable or colored while the interior can be economy, recycled or foamed — then set the ratio, pair materials that bond, and design out the four defects before the first shot.

    Sources
    Plastics Technology — co-injection and sandwich molding process
    MoldMaking Technology — skin-core gating and mold design
    Society of Plastics Engineers — multi-material molding resources
    Prospector — resin compatibility and material-pairing data
    MatWeb — resin shrinkage and melt-temperature data

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet