Last updated: 2026-09-02 · 5 min read

What Is Mineral-Filled Nylon — and How Does It Change Shrinkage?

Mineral-filled nylon is PA6 or PA66 loaded with talc or calcium carbonate (typically 10–40 percent by weight), and its main gift to a molded part is dimensional stability: shrinkage drops, the flow-to-cross-flow shrinkage difference narrows, and warpage follows it down. The trade is mechanical — impact strength and elongation fall. Choosing mineral fill over unfilled or glass-filled nylon is a flatness-versus-toughness decision, and the shrinkage numbers below are how you make it deliberately.

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. See also related guide.

Key Facts

  • The shrinkage drop is real but secondary — the anisotropy change is what kills warpage. Unfilled PA6 shrinks noticeably more along flow than across it; adding mineral filler both lowers the average shrinkage and, more importantly, pulls the flow and cross-flow values toward each other. Flat parts warp because of the difference, not the average.
  • The mechanism: mineral particles do not shrink and physically restrain the polymer matrix as it contracts. The same logic reduces the coefficient of thermal expansion — which matters in assemblies where a nylon part mates with metal across a temperature range.
  • The costs arrive on the mechanical side: impact strength and elongation drop, weld line strength drops (the filler particles interrupt re-bonding at the knit line), specific gravity rises (more material mass per part), and the melt is slightly more abrasive on tooling — though far less than glass fiber.

    • 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.

    Unfilled vs Mineral vs Glass at a Glance

    Property Trend Unfilled PA6 20–30% Mineral 30% Glass
    Mold shrinkage (typical) 1.2–1.8% 0.8–1.2% 0.3–0.5% flow / 0.7–1.0% cross
    Shrinkage anisotropy moderate low — best flatness high — worst warp driver
    Stiffness baseline up moderately up strongly
    Impact / elongation high down down more
    Weld line strength good reduced reduced more
    Tool abrasion minimal mild severe

    How to Decide: Five Checks Before You Pick the Grade

    1

    Check the flatness budget

    If the drawing calls a flatness tolerance that unfilled nylon has missed for you before, mineral fill is the honest first lever. Pull the flow and cross-flow shrinkage from the grade’s data sheet, ask your molder for the measured shrinkage on similar geometry, and check whether the anisotropy gap closes enough to hold the callout.

    2

    Check the mating materials

    Where the nylon part bolts to aluminum or steel across a temperature range, the lower thermal expansion of a filled grade reduces the fit swing. Note the resin still moves with moisture — nylon absorbs water — so the expansion check pairs with a moisture-conditioning assumption, not just temperature.

    3

    Check the impact requirement

    Mineral fill spends toughness to buy stability. Snap fits, press fits, and any drop-exposure part should re-check impact targets at service temperature — not just room temperature — because the filled grade’s ductility drops exactly where a snap arm needs it.

    4

    Check where the weld lines land

    Filler weakens knit lines. On a load-bearing or sealing part, the DFM review should map every weld line and confirm none sits on the load path; if one must, gating changes or a low-fill-load design fix it cheaper than a resin change later.

    5

    Verify shrinkage at T1 with the real measurement

    Data-sheet shrinkage is a starting assumption, not a promise. Measure the actual part dimensions at T1 against the drawing, back-calculate the real shrinkage per surface, and adjust the steel if the flatness budget demands it — while the correction is still cutting steel, not re-cutting it.

    Processing Notes That Follow the Filler

  • Drying still applies — mineral-filled nylon absorbs moisture (somewhat less per finished part than unfilled, because the matrix is diluted), and wet nylon still degrades the same way.
    Dry to the grade’s specification and keep the hopper covered.
  • Expect slightly higher injection pressures and more care at vents — the filled melt is a little less free-flowing, and a blocked vent shows up faster on a flat cosmetic surface.
  • Plan for density in the costing: filled grades weigh more per part, so the material line in your cost-per-part moves even when the resin price per kilogram looks similar.
  • Check gate and runner sizing against the data sheet’s processing window rather than copying the unfilled tool’s settings — the fill behavior difference is real at thin walls.

    • Frequently Asked Questions

    Does mineral fill reduce water absorption?

    Somewhat — the filler dilutes the nylon matrix, so the finished part absorbs less water in total and swells less.
    It does not turn nylon into a low-moisture resin: conditioning assumptions and wet-service dimensional checks still apply, and the weld-line and impact penalties remain. Treat the moisture improvement as a bonus, not a reason.

    Why is my mineral-filled part heavier than I calculated?

    Density. A 30 percent mineral fill raises specific gravity well above unfilled nylon, so the same part volume carries more mass. Recalculate part weight and per-shot material cost with the filled grade’s specific gravity before you lock the quotation, or the material line will surprise you at the first invoice.

    Can I combine mineral and glass fill?

    Yes — hybrid grades pair a glass fraction for stiffness with a mineral fraction for flatness, and they are common on large flat structural parts. The same rules apply with both penalties present: check weld lines and impact with extra care, and verify shrinkage anisotropy at T1 because the glass component reintroduces directional shrinkage.

    Key Takeaways

    1. Mineral-filled nylon buys dimensional stability: lower shrinkage, a narrower flow-to-cross-flow gap, and visibly better flatness on wide parts.
    2. The trade is mechanical — impact, elongation and weld line strength drop, and part weight rises with density.
    3. Decide with five checks: flatness budget, mating materials, impact at service temperature, weld line placement, and a T1 shrinkage measurement to close the loop.

    Filler selection looks like a resin table decision but behaves like a warpage insurance decision. Bring the flatness callout and the service-temperature impact case into the DFM review, and aCatMould will propose the grade class with the shrinkage anisotropy measured — not assumed — so the flat part you approved on paper is the flat part that comes out of the tool.

    Sources
    Plastics Technology — filled nylon grades and shrinkage
    DuPont — polyamide molding guide
    BASF — ultramide data and processing notes

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