Nylon (PA6, PA66 & Glass-Filled) Injection Molding: Properties, Drying & Process Guide

Nylon (polyamide, PA) is the workhorse semi-crystalline engineering thermoplastic for structural brackets, gears, under-hood automotive manifolds, power-tool housings, and electrical connectors that must survive continuous heat, hydrocarbon exposure, and cyclic mechanical load.

However, nylon introduces three physical behaviors that catch buyers off guard if treated like commodity ABS or polypropylene:

1. Reversible Hygroscopic Behavior: Before molding, excess moisture inside the pellets (> 0.20%) causes hydrolytic chain scission and silver splay in the barrel; after molding, the bone-dry part slowly absorbs 1.5% to 2.8% ambient moisture from the air, growing dimensionally by +0.2% to +0.6% while doubling or tripling its impact toughness.
2. Over-Drying Viscosity Spikes: Unlike polycarbonate (which wants 0.00–0.02% moisture), drying nylon too aggressively (< 0.05%) triggers solid-state post-condensation in the hopper, spiking melt viscosity and causing short shots.
3. Severe Anisotropy in Glass-Filled Grades: Adding 30% chopped glass fibers (PA66-GF30) boosts heat deflection temperature to 250 °C and tensile strength to ~200 MPa, but creates a 3:1 shrinkage mismatch between the flow direction (0.2–0.4%) and cross-flow direction (0.8–1.1%).

Use this reference alongside the material selection guide, POM (acetal) guide, and PBT guide to specify the right polyamide grade and tooling standard.

PA6 vs PA66 vs 30% Glass-Filled PA66 vs PA12: Engineering & Processing Matrix

More than 85% of injection-molded polyamide parts fall into four grade families. Their thermal, mechanical, and moisture-absorption gaps determine which grade belongs on your drawing:

Property / Processing Parameter	Unfilled PA6 (Nylon 6)	Unfilled PA66 (Nylon 6/6)	30% Glass-Filled PA66 (PA66-GF30)	Unfilled PA12 (Nylon 12)
Crystalline Melting Point ($T_m$)	220 °C (428 °F)	260 °C (500 °F)	260 °C (500 °F)	178 °C (352 °F)
Melt Temperature Window	240–270 °C (465–520 °F)	275–295 °C (525–565 °F)	285–305 °C (545–580 °F)	210–250 °C (410–480 °F)
Mold Temperature Window	70–90 °C (160–195 °F)	80–100 °C (175–210 °F)	90–120 °C (195–250 °F)	50–80 °C (120–175 °F)
Desiccant Drying (Dew Point ≤ -30 °C)	80 °C (175 °F) for 4–8 h (never exceed 85 °C or pellets oxidize)	80 °C (175 °F) for 4–8 h	80 °C (175 °F) for 4–6 h	80 °C (175 °F) for 3–6 h
Target Moisture Before Molding	0.08% – 0.15% (max 0.20%)	0.08% – 0.15% (max 0.20%)	0.06% – 0.12% (max 0.15%)	< 0.10%
Moisture Absorption (23 °C / 50% RH Air)	2.7% – 3.0% (9.5% saturated in water)	2.3% – 2.6% (8.5% saturated in water)	1.5% – 1.7% (5.5% saturated in water)	0.6% – 0.8% (1.5% saturated in water)
Mold Shrinkage (Flow / Cross-Flow)	1.0% – 1.5% (nearly isotropic)	1.2% – 1.8% (nearly isotropic)	0.2–0.4% (Flow) 0.8–1.1% (Cross-Flow)	0.7% – 1.2% (nearly isotropic)
Tensile Strength (Dry As-Molded / 50% RH)	80 MPa / 45 MPa	85 MPa / 55 MPa	195–210 MPa / 135 MPa	50 MPa / 45 MPa
Heat Deflection Temp (HDT @ 1.8 MPa)	65–75 °C	75–90 °C	245–255 °C	50–55 °C
Primary Buyer Use Case	Tough snap-fit housings, clips, caster wheels, better surface finish than PA66	Higher heat resistance, gears, cable ties, connectors, engine bay brackets	Metal-replacement brackets, power tool bodies, fluid manifolds, high-load mounts	Fuel lines, pneumatic fittings, precision fluid meters where water swell cannot be tolerated

Glass-Filled Nylon Injection Molding (15%–50% GF PA6 & PA66)

Searches for glass-filled nylon injection molding represent one of the highest commercial-intent segments in industrial plastics because PA6-GF30 and PA66-GF30 routinely replace die-cast aluminum and stamped steel brackets at 30–50% lower weight and piece cost.

However, molding glass-reinforced polyamide requires four non-negotiable engineering controls:

1. Hardened SPI Class 102 Tool Steel (50–54 HRC)

Chopped E-glass fibers (10–13 µm diameter) suspended in a fast-moving nylon melt stream erode standard pre-hardened P20 (28–32 HRC) gates, runner turns, and shut-offs in 5,000 to 15,000 shots.

• RFQ Rule: Always specify SPI Class 102 through-hardened H13, S7, or D2 (50–54 HRC) cavity/core inserts and replaceable carbide or nitrided submarine/tab gate inserts, plus a bimetallic screw and barrel on the molding press.

2. Controlling Anisotropic Shrinkage and Warpage

Because glass fibers align parallel to the melt flow vector during filling, 30% GF PA66 shrinks only 0.2%–0.4% along the flow path while shrinking 0.8%–1.1% perpendicular to the flow path.

• On flat plates, long brackets, or circular gears gated from a single side, this 3:1 differential shrinkage twists the part into a saddle or potato-chip warp.
• Remedies: Run a pre-tooling Moldflow fiber-orientation and warpage simulation, use multi-point or diaphragm/ring gating on cylindrical gears, maintain uniform wall thickness (2.0–3.0 mm), or specify a glass-sphere / mineral-glass hybrid grade (e.g., PA66-GF15-M25) when flatness outranks peak tensile strength.

3. Eliminating Surface “Floating Fibers” (Silver Frosting)

If molten glass-filled nylon touches a cold mold wall (< 70 °C), the polymer skin freezes instantly and leaves exposed white glass fibers stranded on the part surface.

• Process Fix: Run a pressurized hot-water or hot-oil thermolator at 95–120 °C (200–250 °F) combined with a high first-stage injection velocity. The hot steel delays skin freeze-off long enough for a resin-rich polymer layer to encapsulate the glass fibers against the cavity wall. If a Class A cosmetic finish is required without painting, specify PA6-GF30 (or a PA6/PA66 copolymer blend) over straight PA66-GF30; PA6 crystallizes more slowly and produces a dramatically cleaner, high-gloss black surface.

4. Generous Gate Dimensions and Shallow Venting (0.010–0.015 mm)

Tiny pin gates shear glass fibers down from ~0.3 mm length to dust, cutting impact strength by 25–40%. Size edge or tab gates to at least 65%–80% of the nominal wall thickness. At the same time, because molten PA66 above 275 °C has water-thin viscosity (~50–100 Pa·s), cut perimeter vents to 0.010–0.015 mm (0.0004–0.0006 in) maximum depth so the low-viscosity melt does not flash into the vent channels.

Understanding Nylon’s Two-Sided Moisture Problem

Buyers must audit moisture at two completely separate stages of the nylon lifecycle:

1. Before Molding (Preventing Hydrolysis vs Over-Drying):
   • Virgin nylon shipped in sealed foil-lined bags arrives pre-dried at ~0.08–0.12% moisture and can go straight into a closed desiccant hopper.
   • If exposed to shop air for more than 2–4 hours, pellets absorb moisture (> 0.25%). In the 280 °C barrel, water steam-cracks the polyamide chains (hydrolysis), producing silver splay marks, nozzle drool, and a permanent loss of molecular weight and impact resistance.
   • Conversely, baking nylon in a hopper at > 90 °C or for > 12 hours drives moisture below 0.05%. At < 0.05% moisture, nylon undergoes solid-state polymerization—its viscosity climbs sharply, cavity fill becomes erratic, and parts turn yellow. Require your molder to verify a 0.08%–0.15% moisture window using a Karl Fischer or loss-on-drying analyzer.
2. After Molding (Conditioning for Impact Toughness & Dimensional Growth):
   • Directly out of the press (“Dry-As-Molded” / DAM), unfilled PA66 has 0.1% moisture, high tensile strength (85 MPa), and low elongation—meaning thin snap-fit fingers or cable ties can snap brashly if assembled in a cold, dry warehouse.
   • Once exposed to normal 23 °C / 50% RH air over several weeks (or accelerated in a 60–80 °C humidity chamber for 8–16 hours), PA66 absorbs ~2.5% moisture. Its glass transition temperature ($T_g$) drops below room temperature, Notched Izod impact strength jumps 3× to 4×, and linear dimensions expand by +0.002 to +0.003 mm/mm (+0.2% to +0.3%) for unfilled PA66 (or +0.6% for unfilled PA6).
   • Drawing Callout Rule: Always state on your 2D drawing whether tight tolerances (±0.05 mm) are inspected “Dry-As-Molded (24–48 hrs post-molding)” or “Conditioned to Equilibrium (ISO 1110)”.

When to Switch from Nylon to POM (Acetal) or PBT

• Switch to POM (Acetal) when the part is a precision gear, valve spool, or pump housing operating in wet, humid, or submerged water environments where nylon’s 2.5–8.5% water absorption would swell the part out of tolerance.
• Switch to PBT (Polybutylene Terephthalate) when the part is an outdoor electrical connector, sensor housing, or coil bobbin that requires stable dielectric insulation and ±0.03 mm pin-grid pitch across 10% to 90% relative humidity swings (PBT absorbs < 0.15% moisture).

Buyer FAQs

What is the difference between Nylon 6 (PA6) and Nylon 66 (PA66) in injection molding?

Nylon 66 (PA66) has a 40 °C higher melting point (260 °C vs 220 °C), higher stiffness and creep resistance at elevated temperatures, slightly lower moisture absorption (2.5% vs 2.8% at 50% RH), and faster crystallization for shorter cycle times—making it the default for under-hood automotive and high-heat electrical parts. Nylon 6 (PA6) processes at lower temperatures (240–270 °C), offers higher impact toughness, costs 10–15% less, and produces a significantly smoother, glossier surface finish in glass-filled grades (PA6-GF30) because its slower crystallization hides glass fibers better than PA66.

What moisture content is required before injection molding nylon?

For unfilled PA6 and PA66, the optimal pre-molding moisture content is 0.08% to 0.15% (0.20% absolute maximum), achieved by drying in a dehumidifying desiccant dryer at 80 °C (175 °F) for 4–8 hours. Unlike polycarbonate or PET, do not over-dry nylon below 0.05% moisture, as bone-dry polyamide undergoes solid-state post-condensation that sharply increases melt viscosity and causes short shots.

Why do glass-filled nylon parts warp and have white surface fibers?

Glass-filled nylon (e.g., PA66-GF30) warps because glass fibers align with the melt flow, restricting shrinkage in the flow direction to 0.2%–0.4% while the cross-flow direction shrinks 0.8%–1.1%. White “floating fibers” appear when the melt freezes prematurely against a cold cavity wall (< 80 °C). Both issues are controlled by running a hot mold (95–120 °C), injecting at high velocity, balancing gate locations via Moldflow simulation, or switching to a PA6-GF30 or glass/mineral hybrid grade.

Do nylon parts grow in size after injection molding?

Yes—as an injection-molded nylon part equilibrates from its bone-dry as-molded state (~0.1% moisture) to standard ambient air (23 °C, 50% RH), unfilled PA6 grows by +0.5% to +0.7%, unfilled PA66 grows by +0.25% to +0.35%, and 30% glass-filled PA66 grows by +0.10% to +0.15%. Engineers designing tight bearing bores or mating housings must either account for hygroscopic growth in CAD or switch to low-absorption PA12, POM, or PBT.

Disclaimer

PlasticsTechnologyAlliance.com is an independent buyer resource. It does not manufacture parts, sell materials, or certify suppliers. Polyamide shrinkage, moisture growth, and processing windows vary by glass loading and impact modifier package; always verify parameters against the resin manufacturer’s grade-specific datasheet.