Nylon (PA) Injection Molding: Grades, Properties & Processing

A nylon mold that fails on the first trial usually fails in the steel, not the resin. A cavity compensated for 1.0% shrinkage on a PA66 grade that shrinks 1.8% delivers undersized gears and a steel change order that costs weeks. A gate sized for unfilled nylon erodes within 20,000 shots once the material switches to 30% glass fiber. Both failures are visible on the drawing long before the steel is cut — and both are the reason this guide reads nylon from the mold side, not the chemistry side.

Nylon — polyamide, or PA — is the workhorse engineering thermoplastic behind gears, snap-fit clips, automotive connectors, and under-hood housings. It is a semicrystalline family — PA6, PA66, PA12, PA46, and semi-aromatic grades such as PA6T — whose defining traits for the toolmaker are high shrinkage, aggressive moisture pickup, fast crystallization, and, in glass-filled grades, an appetite for wearing out steel. That combination makes nylon tooling a different discipline from ABS tooling: the shrinkage guess is bigger, the drying gate is non-negotiable, and the gate and steel decisions are made against abrasion.

This guide is written for the people who buy and build the tool: mold buyers, tooling engineers, and manufacturing engineers. It walks from the properties that dictate mold decisions, through the design rules for nylon tooling, into the processing window the mold must survive, and ends with defects traced to their mold-side root causes. Processing numbers follow typical published values for injection-molding grades; your grade datasheet overrides them.

The Snapshot

• Nylon is a family of semicrystalline polyamides; PA6, PA66, PA12, PA46, and PA6T-class grades dominate injection molding.
• Mold shrinkage of unfilled nylon runs 1.0-1.8%, with PA66 grades typically at 1.5-2.0% — two to three times ABS — so cavity compensation is the first number on the drawing.
• PA6 and PA66 absorb 1.5-3.5% moisture at 23°C / 50% RH; the drying gate is <0.2% — typically 80°C for 4-6 hours in a dehumidifying dryer.
• 30% glass fiber cuts flow-direction shrinkage to roughly 0.2-0.5% but makes it anisotropic — flow vs cross-flow differ by up to 2-4x, which is what warps fiber-filled parts.
• HDT at 0.45 MPa runs 180-230°C (PA6/PA66); mold temperature 40-90°C sets crystallinity, surface, and cycle.
• Glass-filled nylon is abrasive: gate inserts, runners, and core steel are wear components, not afterthoughts.
• DieStrike builds nylon-capable tooling to IATF 16949 / ISO 9001 systems, with CMM-verified cavity dimensions on every mold.

What Is Nylon (PA)?

Nylon (PA, polyamide) is a family of semicrystalline thermoplastics whose polymer backbone repeats the amide group — the -CO-NH- linkage — thousands of times per chain. The grade names encode the chemistry: PA6 is polymerized from caprolactam, a single six-carbon ring-opened monomer; PA66 pairs hexamethylenediamine with adipic acid, two monomers that each carry six carbons; PA12 uses laurolactam with twelve carbons; PA46 pairs diaminobutane with adipic acid; and PA6T builds terephthalic acid into the backbone for higher heat resistance. For the mold designer, the family is best read as a ladder of melting points — from 175°C for PA12 up to roughly 310°C for PA6T-class grades — because the melting point sets the mold-temperature window and the cooling load the tool must handle.

The defining chemistry for tooling is hydrogen bonding. Adjacent chains form hydrogen bonds at each amide site, which drives crystallization, stiffness, and a melting point far above commodity plastics. The same bonds attract water: the amide groups that give nylon its mechanical backbone pull moisture out of the air, which is why every nylon datasheet carries a moisture-absorption number and why drying sits at the top of every processing rule. Semicrystalline behavior also means nylon keeps crystallizing after ejection — post-mold shrinkage — so critical dimensions are measured after parts stabilize, not at the press.

Nylon Properties That Drive Mold Design

Material selection and tool design are one decision, not two. The table below maps the nylon properties that matter most to the specific mold decisions they drive. If a datasheet number is missing at RFQ time, this is the list to ask for.

← swipe to scroll →

Values are typical published ranges for injection-molding grades; actual numbers vary by grade and test method.

Shrinkage and dimensional stability. Unreinforced nylon shrinks 1.0-1.8%, with PA66 grades typically at the high end (1.5-2.0%) — two to three times the shrinkage of ABS or polycarbonate. Because the resin is semicrystalline, shrinkage is larger and less forgiving than amorphous materials, and it continues after ejection as the part crystallizes further. Cavities are compensated zone by zone — thick sections shrink more than thin ones — then tuned on the first molding trial, and critical dimensions are validated after parts stabilize.

Moisture. PA6 and PA66 pick up 1.5-3.5% moisture at ambient conditions; submerged in liquid water, saturation runs about 8-9%. Absorbed water plasticizes the polymer: it breaks hydrogen bonds, lowers the glass transition from roughly 50-70°C toward room temperature, and can cut stiffness by 20-30% between the dry-as-molded state and 50% RH equilibrium. Dimensions grow roughly 0.2-0.3% linearly for each 1% of absorbed water. For the tool, the practical consequence is that a part's dimensions at the press are not the dimensions in service — tolerances must be specified at a moisture condition, and parts conditioned or tested accordingly.

Heat resistance. An HDT at 0.45 MPa of roughly 180-190°C for PA6 and 220-230°C for PA66, far above ABS, is why nylon owns under-hood and hot environments. For the tool, the HDT sets expectations, not exotic steel: a 40-90°C mold runs on standard prehardened inserts for unfilled grades, and the cooling layout — not the steel — decides whether the part reaches the crystallinity the datasheet promises.

Wear and glass reinforcement. Glass fiber is what most structural nylon parts actually mold with. 30% GF lifts tensile strength to roughly 140-190 MPa and modulus to about 9-10 GPa, but the fibers are abrasive and orient with flow. Flow-direction shrinkage drops to 0.2-0.5% while cross-flow shrinkage stays at 0.6-1.0% — the gap is what warps fiber-filled parts — and the fiber-laden melt erodes gates, runners, and core edges. Both facts are mold-design inputs: anisotropic compensation and wear-resistant steel.

Electrical behavior. Dry nylon is a serviceable insulator with volume resistivity near 10^14-10^15 Ω·cm. Absorbed moisture degrades insulation resistance and dielectric strength, which is why connector housings specify moisture-conditioned electrical testing per the application standard. For the tool, that means surface finish and gate placement — the decisions that control molded-in stress — matter more than the datasheet's dry values, because a stress concentration at a weld line shows up early in a humid environment.

Mold Design Considerations for Nylon

Nylon is demanding on the machine and demanding on the steel. The rules below are the ones that matter when the tool is being designed for a nylon part — especially a glass-filled one.

Shrinkage compensation. Compensate cavities for the grade's shrinkage — 1.0-1.8% unfilled, and per-direction for glass-filled compounds — split by wall-thickness zone. Because nylon keeps crystallizing after ejection, the compensation also has to account for post-mold shrinkage on critical dimensions. The five shrinkage compensation rules apply directly to nylon parts and cover the zone-by-zone method, including the anisotropic case.

Wall thickness and ribs. Nominal walls of 1.0-3.0 mm are the working range for most nylon parts; thin-wall housings push to 0.6-0.8 mm with glass-filled grades. Ribs should run 50-60% of the nominal wall thickness at the base — thicker ribs guarantee sink marks on the opposite surface, and with high shrinkage that sink shows faster than it would in ABS. Radius rib bases at 0.25-0.5 mm and keep wall transitions gradual, so the anisotropic shrinkage of fiber-filled grades does not turn a step into a warp initiator.

Gate design. Unfilled nylon fills through small gates; glass-filled compounds need larger, fully rounded gates with generous radii so fibers survive the gate without breaking — and without eroding it. Gate placement decides weld-line location: for gears, keep weld lines off loaded tooth flanks; for housings, off cosmetic faces. The gate design best practices guide covers placement and sizing in detail, including the fiber-filled case.

Runner system. Nylon freezes fast, so the runner decision is a cycle decision. Cold runners work, but the sprue and runner freeze quickly and regrind rates climb; hot runners are standard for high-volume nylon tools — automotive connectors above all — and the hot runner vs cold runner comparison covers when each pays off. Keep the manifold temperature balanced across cavities: nylon's crystallinity and dimensions are sensitive to melt-temperature differences between cavities.

Venting. Vent depths of 0.02-0.05 mm are the typical guideline for unfilled nylon — thinner than ABS, because the flash risk with a low-viscosity melt is higher. Even at 0.2% moisture, the flashed steam must escape or it burns the part at the flow front. Glass-filled grades can vent slightly deeper at the parting line but need careful land widths, and deep, blind pockets may need vacuum venting.

Draft and ejection. Minimum draft of 0.5-1° per side on untextured walls; textured surfaces need 1-2° per degree of texture depth. Nylon sticks to a hot mold, so ejection needs more attention than it does for ABS: adequate draft, balanced cooling so the part releases evenly, and enough ejector area for thin connector walls. The draft angle mistakes list is worth a pass before the steel is cut, because a draft correction after hardening is expensive.

Cooling. Mold temperature is 40-90°C for typical PA6/PA66 tooling (80-120°C for PA46), and the cooling layout decides the cycle — nylon's cycle is cooling-bound because the part must crystallize in the mold. Uniform cavity-to-core temperature matters more than raw speed: uneven cooling produces uneven crystallinity, which shows up as warpage and dimension drift. The cooling channel design tips cover the layout rules for uniform mold temperature.

Mold steel. Unfilled nylon runs on P20 or 718-class prehardened steel for most production volumes. Glass-filled nylon is a different conversation: the fibers are abrasive, so gates, runners, and high-flow surfaces need hardened steel or wear-resistant inserts — H13-class cores, D2 or powder-metal gate inserts — and high-cavity automotive tools are often nitrided. Replaceable gate inserts turn gate wear into a service item instead of a mold rebuild. Medical and food-contact nylon tools move to S136 or 420 stainless, following the same logic as our medical device molding work.

Injection Molding Nylon: The Processing Window

Nylon is highly moldable once the resin is dry — and the drying gate is not optional. The window below is the standard starting point for PA6 and PA66 (typical published practice); your grade datasheet overrides it.

Drying: below 0.2% moisture, 80°C for 4-6 hours. PA6 and PA66 need a dehumidifying dryer — not a hot-air dryer — because the desiccant bed pulls moisture out of the air the resin would otherwise re-absorb. PA12 needs less: 70-80°C for 2-4 hours is common; PA46 demands around 0.10% or below. Dried resin must stay dry: a hopper dryer or sealed feed is standard, because a few open-hopper hours undo the cycle. Over-drying is a smaller risk but real — extended time at high temperature oxidizes the resin and yellows the part.

Melt temperature: 230-290°C for the PA6/PA66 band. PA6 processes at 230-270°C, PA66 at 260-290°C, PA12 at 190-230°C, and PA46 around 290-320°C (typical ranges). Stay inside the window: too cold gives poor flow and weld lines, too hot degrades the polymer and risks hydrolysis-driven brittleness. Extended residence at the top of the window scissors the chains — brittle parts at the press usually trace back to the drying log or the barrel profile, not the mold.

Mold temperature: 40-90°C. The low end gives shorter cycles but lower crystallinity, a poorer surface finish, and less dimensional stability. The high end — up to 90°C, and 80-120°C for PA46 — grows larger crystals, improves dimensional stability, and produces a glossier surface. The mold-temperature decision is a cycle-time versus part-quality trade, and it belongs in the mold qualification, not improvised on the floor.

Pressures and speeds. Typical injection pressure runs 70-140 MPa, with screw back pressure of 0.5-2 MPa to stabilize the melt. Nylon is low-viscosity, so fill can be fast — but fast fill on glass-filled grades raises fiber breakage and gate wear, so speeds are tuned against both. Purge PA with a low-cost resin such as PE or PMMA when changing materials.

Shrinkage: 1.0-1.8% unfilled, anisotropic with glass. The number moves with mold temperature, hold pressure, and wall thickness — and the part keeps moving after ejection as crystallization continues. Unreinforced PA66 typically lands at 1.5-2.0%; 30% glass fiber cuts flow-direction shrinkage to roughly 0.2-0.5%, with cross-flow two to four times higher. Cavities are compensated with the datasheet range, then tuned at trial, and critical dimensions are validated after stabilization.

Venting. Vents of 0.02-0.05 mm depth are the typical guideline for unfilled nylon. Deeper vents flash; shallower vents trap air and steam, causing burn marks at the end of fill. Nylon's low viscosity makes vent discipline more important than it is for ABS.

Common Nylon Applications

Nylon earns its mold orders where the part carries load, runs hot, or rubs against something. Automotive under-hood parts — connector housings, engine covers, fuel-line clips — are the highest-volume nylon application, and their tooling follows the same logic as our automotive injection molding work: high-cavitation tools, hardened gates for glass-filled grades, and moisture-conditioned dimensional checks. Gears and bushings exploit nylon's wear resistance; gear tooling is gated so no weld line crosses a loaded tooth flank. Electrical housings and connector bodies use flame-retardant and glass-filled grades for stiffness and creep resistance, the space covered on our consumer electronics page. Glass-filled structural parts — power tool housings, pump housings, EV battery frames — push the tool toward wear-resistant steel and replaceable gate inserts, and the EV and energy page covers the battery-side story. Where the part needs transparency or a higher service temperature instead, the polycarbonate guide is the usual upgrade.

When the application and the resin are locked, the mold decides the outcome — DieStrike's mold making line cuts and validates nylon tooling with hardened inserts, balanced gates, and moisture-conditioned first articles.

Nylon Defects: Mold-Side Root Causes and Fixes

Most nylon rejects trace to a mold design decision, not a bad shot — moisture defects included, because the tool's venting and gating decide how the resin's moisture problem shows up in the part. The table below maps each common defect to its mold-side root cause and the tooling fix.

← swipe to scroll →

Warpage in glass-filled nylon is the defect that separates mold engineers from material engineers. Differential shrinkage between flow and cross-flow directions bends the part, especially with asymmetric gating or thin walls. The remedies are design-level — balanced gates, uniform walls, symmetric fiber paths — and the thin-wall warpage guide is the practical playbook for the thin-wall cases.

Moisture splay deserves its own line because it is the nylon defect that starts outside the tool: silvery streaks mean the resin was wet. The fix is drying discipline — 80°C for 4-6 hours to below 0.2% — but the mold decides how bad it looks, because venting and gate design control where the flashed steam escapes. Splay that reappears mid-run points at a leaking hopper dryer, not at the mold.

Brittle parts are usually hydrolyzed parts. Wet resin, an over-hot barrel, or long residence scissors the chains, and the part snaps instead of flexing. A molded PA66 clip that cracks like glass is a drying-log problem, not a design problem — the tool simply delivers the verdict.

Nylon vs ABS: Which Material Fits Your Part?

Nylon and ABS are the two most-asked-about engineering thermoplastics, and the comparison is not close on paper. Nylon wins on strength, heat, wear, and chemical resistance. ABS wins on price, surface aesthetics, and dimensional stability. The table below summarizes the typical published differences.

← swipe to scroll →

Typical published values; verify with the specific grade datasheet.

Choose nylon when the part carries load, runs hot, rubs against something, or lives near fuel and oil: gears, bushings, clips, under-hood housings, connectors. Choose ABS for cosmetic enclosures, humidity-sensitive housings, or low-cost parts that never see real heat. ABS molds with far less drying drama and holds tolerance better out of the box.

The honest shortcut: if the part needs to be strong and stay strong in an engine bay, it is nylon. If it needs to look good, stay cheap, and hold a dimension, it is ABS. If it needs both, the usual answer is glass-filled nylon with a painted finish — which reintroduces the drying, shrinkage, and wear rules above. The ABS material guide walks the ABS side of the comparison in the same mold-side format.

Frequently Asked Questions

Q1. What is nylon?

Nylon is the common name for synthetic polyamides — thermoplastics whose polymer backbone repeats the amide group (-CO-NH-). Engineering grades such as PA6, PA66, PA12, PA46, and PA6T are semicrystalline and injection moldable. DuPont developed nylon in the 1930s and commercialized it in 1938.

Q2. What are the properties of nylon that matter for molding?

Nylon combines high strength — 70-90 MPa tensile for unfilled PA66 — with excellent wear resistance and low friction. It resists oils and fuels, with heat deflection near 180-230°C at 0.45 MPa. Its defining weakness is moisture absorption: PA6 and PA66 equilibrate at 1.5-3.5% moisture, which softens, swells, and de-rates the material. For the mold, the numbers that matter are shrinkage (1.0-1.8%), moisture-driven venting requirements, and glass-fiber abrasion.

Q3. What is the nylon melting point?

It depends on the grade. PA12 melts near 175-180°C, PA6 near 215-225°C, PA66 near 255-265°C, PA46 near 290-295°C, and PA6T-class grades around 310°C (typical values). Melt processing temperatures for PA6/PA66 run 230-290°C.

Q4. Is nylon waterproof?

No. Nylon is hygroscopic: it absorbs water rather than repelling it. PA6 and PA66 equilibrate at 1.5-3.5% moisture in air and reach roughly 8-9% at saturation in liquid water. Absorbed water plasticizes the polymer, lowers stiffness and glass transition, and grows dimensions by roughly 0.2-0.3% per 1% absorbed moisture — which is why nylon tolerances must be specified at a moisture condition.

Q5. How much does nylon shrink in injection molding?

Unreinforced nylon shrinks 1.0-1.8%, with PA66 grades typically at 1.5-2.0%. Glass fiber cuts that to roughly 0.2-0.5% in the flow direction at 30% loading, with cross-flow shrinkage two to four times higher. The flow-versus-cross-flow gap is the main cause of warpage in glass-filled nylon parts, and it must be compensated per direction in the cavity.

Q6. Why does glass-filled nylon warp?

Because shrinkage is anisotropic. Glass fibers align with flow and restrict contraction along their length, so flow-direction shrinkage drops far below cross-flow shrinkage. Asymmetric gating, uneven walls, and local fiber-orientation changes turn that difference into warpage. The fixes are design-level: balanced gates, uniform walls, and symmetric fiber paths — not a bigger press.

Q7. Nylon vs ABS: which is stronger?

Nylon is substantially stronger and stiffer: unfilled PA66 runs 70-90 MPa tensile versus 35-50 MPa for ABS, with far higher heat deflection. ABS wins on cost, surface finish, and dimensional stability because it absorbs almost no moisture and shrinks less. Use nylon for load, heat, and wear; use ABS for cosmetic, low-cost enclosures.

Q8. Do you have to dry nylon before injection molding?

Yes, unconditionally. PA6 and PA66 need drying to below 0.2% moisture — typically 80°C for 4-6 hours in a dehumidifying dryer. Keep dried resin in a hopper dryer. PA12 needs less drying; PA46 demands more, with recommended limits around 0.10% or below. Wet resin surfaces in the part as splay, burn marks, and brittleness.

Q9. What is glass-filled nylon used for?

Glass-filled nylon — typically 30% GF PA66 — is used where stiffness and heat resistance beat cost: automotive connectors, power tool housings, pump housings, structural clips, and gears. The glass raises tensile strength to roughly 140-190 MPa but adds anisotropic shrinkage and abrasive wear on gates and runners, so gating, compensation, and steel selection must be designed for it.

Q10. Is nylon flammable?

Unfilled nylon is combustible. Standard grades typically carry a UL 94 HB rating — slow horizontal burning. Flame-retardant grades achieve V-2 or V-0 ratings, usually with halogen-free additive systems, at some cost in mechanical properties. Flammability is a rated material property per UL 94, not a guess.

The Bottom Line

Nylon is a high-performance engineering material with one dominant failure mode — moisture — and one dominant tooling challenge — shrinkage. Dry the resin below 0.2%, hold the melt window, set the mold temperature deliberately, and compensate the cavity for 1.0-1.8% shrinkage (per direction for glass-filled grades). Do that, and nylon delivers strength, wear life, and chemical resistance that commodity plastics cannot approach. Skip the drying step or guess the shrinkage, and the same part fails in weeks.

Grade selection ranks service temperature, then moisture exposure, then price: PA66 suits the default structural part, PA6 cost and surface, PA12 dimensional stability in humidity, and PA46 and PA6T heat. Glass-filled compounds trade uniform shrinkage for stiffness — the gain is real, and the anisotropy and abrasion are the price. Put the shrinkage compensation, gate wear, and venting decisions on paper before the steel is cut: a DFM pass up front catches most of them, and choosing a mold partner who has cut nylon tooling before closes the rest. Nylon is not a forgiving material; it is a predictable one, once the moisture, shrinkage, and wear rules are treated as laws rather than suggestions.