Last updated: 2026-09-04 · 5 min read
Mineral-Filled Nylon — How Does It Compare to Glass-Filled Nylon, and When Should You Specify It for Your Injection Molded Part?
Mineral-filled nylon (nylon 6 or 66 with 30–50% talc, mica or wollastonite) is the alternative to glass-filled nylon when you need dimensional stability and a smooth surface — but not maximum stiffness. The key difference is anisotropy: glass fibers orient in the flow direction and cause differential shrinkage (0.2% flow vs. 0.8–1.2% transverse); mineral particles do not orient and shrink equally in all directions (0.3–0.6%). Flat parts that stay flat, with no fiber read-through.
Key Facts
<>
Parting line — The line where the two mold halves (A-side and B-side) meet. Parting line location affects: gate placement, ejector pin marks, witness lines, and cosmetic appearance. A well-designed parting line minimizes visible witness marks. The related guide covers adjacent topics.
How Does Mineral-Filled Nylon Compare to Glass-Filled?
Seven properties, side by side:
| Property | Unfilled Nylon 6 | GF30 Nylon 6 | Mineral-Filled (40%) Nylon 6 |
|---|---|---|---|
| Tensile strength | 80 MPa | 180 MPa | 110 MPa |
| Flexural modulus | 2,400 MPa | 10,000 MPa | 5,500 MPa |
| Shrinkage (flow) | 1.0–1.5% | 0.2–0.4% | 0.3–0.6% |
| Shrinkage (transverse) | 1.0–1.5% | 0.8–1.2% | 0.5–0.8% |
| Warpage tendency | High (isotropic shrinkage) | High (fibers orient) | Low (isotropic shrinkage) |
| Surface appearance | Smooth, glossy | Fiber read-through, rough | Smooth, no fiber read-through |
| Impact strength (notched) | 8 kJ/m² | 10 kJ/m² | 6 kJ/m² |
The key difference is anisotropy. Glass fibers orient in the flow direction during injection — this creates different shrinkage in the flow direction vs. the transverse direction. The differential shrinkage causes warpage on flat or asymmetric parts. Mineral particles do not orient — they shrink equally in all directions, producing flat, dimensionally stable parts without warpage.
Mineral-filled nylon fills the gap between unfilled nylon (too weak, too much shrinkage) and glass-filled nylon (too much warpage, poor surface). It is not the right choice when maximum stiffness or tensile strength is required — glass-filled nylon is 2× stiffer and 1.5× stronger. For a structural component (load-bearing bracket, gear, bearing cage), glass-filled is the correct choice.
When Should You Specify Mineral-Filled Nylon?
Four use cases, each tied to a specific property:
- Flat parts where warpage is unacceptable. Housings, covers, brackets with large flat surfaces. Glass-filled nylon warps because fibers orient in the flow direction; mineral-filled stays flat. For a 200mm flat bracket: glass-filled PA6 warps 1.5–3mm; mineral-filled PA6 warps 0.2–0.5mm.
- Cosmetic surfaces where fiber read-through is not acceptable. Glass fibers show through the molded surface as visible lines, especially on thin walls and glossy finishes. Mineral-filled nylon has a smooth, uniform surface — no fiber read-through, even on walls down to 1.5mm.
- Dimensional stability across humidity changes. Mineral-filled nylon absorbs less moisture than glass-filled — the mineral filler does not absorb water like the nylon matrix. Dimensional change from dry-as-molded to conditioned (50% RH): 0.3% vs. 0.8% for glass-filled.
- Weight-sensitive parts. The weight difference is small (2–5%), but mineral-filled nylon has better flow than glass-filled at equivalent filler loading, allowing thinner walls. The real saving comes from the ability to reduce wall thickness without sacrificing flatness.
| Parameter | Setting | Notes |
|---|---|---|
| Melt temperature | 250–290°C | Higher than unfilled — mineral filler increases viscosity |
| Mold temperature | 80–120°C | Higher temp improves surface finish and weld line strength |
| Drying | 80°C, 4 hours (max 0.1% moisture) | Mineral filler does not absorb moisture, but the nylon matrix does |
| Injection speed | Medium to high | Less shear-sensitive than glass-filled |
| Screw speed | Low to medium (50–100 rpm) | High screw speed degrades the nylon matrix |
The most critical requirement is drying. Wet nylon produces splay marks and loss of mechanical properties. The mineral filler itself does not absorb moisture, but the nylon surrounding it does. Dry to 0.1% moisture before molding. The melt temperature must be high enough to flow through the mineral filler but not above 310°C where the nylon degrades.
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.
Mineral-filled nylon fills the gap: more stiffness than unfilled nylon, less warpage than glass-filled. The correct choice when flatness and surface quality matter more than maximum strength.
Frequently Asked Questions
Can you combine mineral filler and glass fiber in the same nylon grade?
Yes — hybrid grades exist (e.g., 20% glass + 20% mineral) that combine glass stiffness with mineral dimensional stability. The mineral particles disrupt fiber orientation, reducing warpage; the glass fibers carry the load, maintaining stiffness. Used for automotive structural parts (engine covers, transmission components) where both stiffness and flatness are required.
The hybrid grade costs 10–20% more than mineral-filled alone but 10–15% less than glass-filled at equivalent total filler loading. The decision: if the part needs more stiffness than mineral-filled alone provides but less warpage than glass-filled alone produces, the hybrid is the middle ground.
Does mineral-filled nylon weld as well as unfilled nylon?
Mineral-filled nylon has a weaker weld line — the mineral particles accumulate at the weld line, creating a filler concentration with less nylon matrix to bond the two flow fronts. Weld line strength is typically 60–75% of bulk strength, compared to 80–90% for unfilled nylon.
To strengthen the weld: increase mold temperature (100–120°C) to keep the melt fluid longer at the weld; increase injection speed to improve bonding pressure; add overflow wells past the weld line to move the mineral concentration away from the functional area. If the weld is in a high-stress area, redesign the gate location to move it to a lower-stress zone.
What are the most common mineral fillers and how do they differ?
Three fillers, each with a distinct shape and property contribution. Talc (magnesium silicate): the most widely used; platelet shape reinforces in two dimensions, improves stiffness and provides the best surface finish. Mica (potassium aluminum silicate): larger platelet shape, provides the highest stiffness and best dimensional stability — used for automotive under-hood parts where heat resistance and flatness are critical.
Wollastonite (calcium silicate): needle-like shape that bridges cracks and absorbs impact energy — provides the best impact strength among mineral fillers. Used for parts requiring a balance of stiffness and toughness (sporting goods, power tool housings). The choice among them depends on which property the part needs most: surface finish (talc), flatness (mica), or impact (wollastonite).
Key Takeaways
<>
Mineral-filled nylon fills the gap between unfilled nylon (too much shrinkage, too much warpage) and glass-filled nylon (too much anisotropy, poor surface).
If your part is flat, dimensionally critical and has a cosmetic surface — but does not require the maximum stiffness of glass fiber — mineral-filled nylon is the correct material choice. Specify it during the DFM phase, and verify the weld line location with mold flow analysis before cutting steel.
Sources
<>
Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet