Last updated: 2026-09-02 · 5 min read

Structural Foam Molding — How Does It Reduce Weight and Eliminate Sink Marks on Thick Parts?

Structural foam molding injects a gas (usually nitrogen) into the melt to create a micro-cellular core — the result is a part with a solid skin and a foamed interior that is 10–30% lighter than a solid part, with no sink marks on thick sections because the gas pressure compensates for shrinkage during cooling. The process is used for thick-walled parts (6mm and above) where solid injection molding would produce sink marks, excessive weight or impractically long cycle times — typical applications include material handling pallets, equipment housings and automotive structural components.

Cycle time — The total time for one injection molding cycle: injection + packing + cooling + mold open/close + ejection. Typical range: 15–60 seconds. Cycle time determines production rate and cost per part — reducing cycle by 5 seconds saves 10–15% annual production cost. See also related guide.

Key Facts

  • Weight reduction: 10–30% compared to solid molding at the same wall thickness — the foamed core has lower density, while the solid skin maintains surface quality and structural rigidity.
  • Sink mark elimination: the gas pressure inside the part compensates for volumetric shrinkage during cooling — thick sections that would sink in solid molding remain flat in structural foam.
  • Surface finish: structural foam parts have a characteristic swirly or marbled surface texture from the gas cells near the skin — not suitable for cosmetic surfaces without secondary finishing (painting, film application).

    • Cycle time — The total time for one injection molding cycle: injection + packing + cooling + mold open/close + ejection. Typical range: 15–60 seconds. Cycle time determines production rate and cost per part — reducing cycle by 5 seconds saves 10–15% annual production cost. The related guide covers adjacent topics.

    Structural Foam vs. Solid Injection Molding

    Factor Structural Foam Solid Injection
    Weight 10–30% lighter Full density
    Max wall thickness Up to 50mm+ (no sink) 6–10mm (sink marks above)
    Surface finish Swirly/marbled (not cosmetic) Smooth (cosmetic-ready)
    Clamp force Lower (reduced cavity pressure) Higher (full cavity pressure)
    Cycle time Longer (gas must diffuse) Shorter for thin walls

    Design Rules for Structural Foam Parts

  • Minimum wall thickness: 3mm for most resins — below 3mm, the gas cells cannot form properly and the part behaves like a solid injection molded part with no weight benefit.
  • Skin thickness: the solid skin is typically 0.8–1.5mm on each side — thick enough to provide a smooth surface and structural rigidity, thin enough to allow the gas core to expand.
  • Gate size: structural foam requires larger gates than solid molding (typically 3–5mm diameter) to allow the gas-charged melt to flow without freezing off. Pin-point gates are not suitable; edge gates or fan gates are standard.

    • What This Means for Your Quotation

    When the part has thick sections (above 6mm) and cosmetic surface is not required, the quotation should present structural foam as an option — with weight reduction, sink mark elimination and the surface finish trade-off documented. Structural foam molds cost 10–20% more than solid molds (gas injection equipment, larger gates), but the per-part cost may be lower due to reduced material usage and lower clamp force requirements.

    Frequently Asked Questions

    Can structural foam parts be painted or plated?

    Yes — the swirly surface can be covered with paint, but the gas cells near the surface may cause pinholes in the paint film if the skin is too thin. A minimum skin thickness of 1.5mm is recommended for painted parts. Metal plating is not recommended because the gas cells create an uneven surface that does not plate uniformly.

    What resins work with structural foam?

    Most thermoplastic resins can be processed with structural foam — PP, PE, ABS, PC, PA, PPO and PBT are all common. The resin must have sufficient melt strength to contain the gas bubbles without collapsing; very low-viscosity resins (some grades of PP) may require modification. Glass-filled resins work well and add stiffness to the thicker sections.

    How does structural foam compare to gas-assist injection molding?

    Gas-assist injects a single gas channel through a thick section (hollow core along a rib or boss); structural foam disperses gas throughout the entire part (micro-cellular core everywhere). Gas-assist maintains a smooth outer surface and is suitable for cosmetic parts; structural foam produces a marbled surface but handles much thicker sections. Gas-assist is for ribs and bosses; structural foam is for entire thick-walled parts.

    Key Takeaways

    1. Structural foam reduces weight 10–30% and eliminates sink marks on thick sections (6mm+) by creating a solid skin with a micro-cellular gas core — the trade-off is a marbled surface finish unsuitable for cosmetic parts.
    2. Design rules: minimum 3mm wall, 0.8–1.5mm solid skin, larger gates (3–5mm) — the process is for thick, non-cosmetic structural parts where solid molding would sink or weigh too much.
    3. The quotation should present structural foam when walls exceed 6mm and cosmetic finish is not required — with weight savings, surface finish trade-off and per-part cost documented.

    Send aCatMould the part drawing with the wall thickness and cosmetic requirements, and the quotation evaluates structural foam vs. solid injection molding — with weight reduction, surface finish trade-off and per-part cost documented — so the process selection is based on the part’s actual requirements.

    Sources
    Plastics Technology — structural foam molding process and design
    SPE — structural foam and gas-assist molding comparison
    MoldMaking Technology — structural foam mold design

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