Nylon PA66 and PA6 Injection Molding — A Complete Guide for Automotive and Industrial Parts
A Tier 1 automotive engineer is designing an HVAC bracket that must survive 110°C continuous under-dash temperature, 30G vibration, and 2,000-hour thermal cycling without cracking. The material shortlist has one entry: PA66-GF30. But nylon is not one material — it is a family, and choosing between PA66, PA6, PA46, and PA12 is the difference between a part that passes PPAP on the first submission and one that fails validation because someone assumed “nylon is nylon.”
A different engineer — at an industrial power tool manufacturer — is converting a die-cast aluminum gear housing to injection-molded plastic. The target is 40% weight reduction at equivalent stiffness. The obvious candidate is PA66-GF50, but the glass loading that delivers the stiffness also makes the material anisotropic, abrasive, and notch-sensitive. The mold steel that lasts 500,000 shots with unfilled PA66 will be at end of life by 150,000 with PA66-GF50.
Nylons (polyamides, PA) are the most widely used engineering thermoplastics in injection molding — and they come with a set of behaviors that do not exist with ABS, PC, or PP. They absorb moisture from the air and change dimensions. They gain 40–60% of their impact strength after molding, not during it. They process at temperatures that degrade the material if the residence time is too long. And the glass-filled grades that deliver metal-replacement mechanical properties wear out mold steel three times faster than unfilled grades.
This guide covers what engineers and buyers need to know about injection molding nylon: how PA66 differs from PA6, why moisture matters more than any other process variable, when to use glass-filled grades, and what changes in mold design and processing when you move to nylon.
Nylon Is Not One Material
The polyamide family spans a wide range of properties, but for injection molding, four grades dominate:
| Grade | Melt Point | Continuous Use Temp | Moisture Absorption (24h) | Price ($/kg) | Defining Characteristic |
|---|---|---|---|---|---|
| PA66 | 255–265°C | 100–120°C | 1.2–1.6% | $3.00–4.50 | Best strength/stiffness/thermal combination |
| PA6 | 220–225°C | 85–100°C | 1.6–2.0% | $2.80–4.00 | Better surface finish, easier to process |
| PA46 | 290–295°C | 140–160°C | 1.8–2.2% | $8.00–12.00 | High-temperature nylon, competes with PPS |
| PA12 | 175–180°C | 70–85°C | 0.2–0.3% | $7.00–10.00 | Lowest moisture absorption, best dimensional stability |
PA66 and PA6 together account for roughly 85% of all nylon injection molding. PA66 is the default choice for automotive under-hood, structural, and high-temperature applications; PA6 is preferred for consumer products and industrial components where surface finish matters and peak temperatures are lower. PA46 is a specialist for under-hood components that see temperatures above 140°C — thermostat housings, charge-air cooler end caps, and engine oil system components. PA12 is the choice where dimensional stability in humid environments is critical, such as fuel system quick-connectors and pneumatic valve bodies.
The difference between PA66 and PA6 starts with chemistry. PA66 is made from hexamethylene diamine and adipic acid (six carbons each — hence “66”), producing a polymer with a higher density of hydrogen bonds between chains, which directly produces the higher melt point, higher strength, and higher stiffness. PA6 is made from caprolactam (six carbons in the ring — hence “6”), producing a slightly less dense hydrogen-bond network, lower melt point, and marginally lower mechanical properties — but better flow and surface appearance.
In practice, a PA6 part will typically have a glossier surface, fewer visible flow marks, and a 10–15°C lower processing window than an equivalent PA66 part. A PA66 part will have 15–20% higher tensile strength at 100°C and better fatigue resistance. The choice between them is not about which is “better” — it is about whether the application needs the thermal and mechanical margin of PA66 or the processing window and surface finish of PA6.
Moisture — The Defining Variable
No other engineering plastic is as sensitive to moisture as nylon. This is not a processing nuisance — it is a material characteristic that must be designed for and accounted for at every stage from material handling to part dimensioning.
Drying Before Molding
Nylon is hygroscopic. Pellets sitting in an open container at 50% relative humidity will absorb enough moisture within 2–4 hours to be unprocessable. Processing nylon with moisture content above 0.15–0.20% produces:
- Hydrolysis in the barrel — water molecules at 270–290°C react with the amide bonds in the polymer chain, cutting molecular weight and reducing mechanical properties. A 1% reduction in molecular weight from hydrolysis can reduce tensile strength by 3–5%.
- Splay marks and silver streaks — steam erupting from the melt at the flow front produces visible surface defects. On unpainted textured parts, light splay may be acceptable. On painted or Class A surfaces, any splay is rejectable.
- Brittleness from degradation — a nylon part molded with wet material may look acceptable but fail impact testing because the polymer chains have been shortened by hydrolysis during processing.
Drying requirements: 80°C for 4–6 hours in a desiccant dryer with a dew point below −20°C. A hot-air hopper dryer is not adequate — it cannot achieve the dew point needed to drive moisture content below 0.15%. The dried material must be conveyed to the press hopper in dry air or consumed quickly. Nylon pellets left in an open press hopper for two hours on a humid day will have re-absorbed enough moisture to cause processing problems.
Moisture Conditioning After Molding
This is the part that surprises engineers who are new to nylon: a nylon part fresh out of the mold is not at its final properties. Dry-as-molded nylon (DAM) PA66 is strong but brittle — the notched Izod impact strength is typically 3–5 kJ/m². After absorbing moisture from the air to equilibrium (typically 1.5–2.5% water content by weight at 50% RH), the same part has an impact strength of 8–15 kJ/m² — a gain of 50–60%.
The mechanism: water molecules act as plasticizers, inserting themselves between the polymer chains at the amide-bond sites. This allows chain segments to move more freely, increasing ductility and impact resistance. The trade-off is that tensile strength and modulus drop by 10–20% as the material conditions from DAM to equilibrium moisture content (EMC).
The practical implications:
- Dimensional change: a PA66 part typically grows by 0.5–0.8% in linear dimensions as it absorbs moisture from DAM to EMC. A 100 mm dimension becomes 100.5–100.8 mm. If the drawing tolerance is ±0.2 mm on that dimension, the moisture-driven dimensional change alone consumes the entire tolerance band. Tolerances on nylon parts must be specified at conditioned equilibrium, not DAM.
- Time to equilibrium: a 2 mm wall PA66 part reaches near-equilibrium moisture content in approximately 40–60 days at 23°C / 50% RH. A 4 mm wall part takes 120–180 days. Accelerated conditioning — immersing parts in 80°C water for 1–4 hours or in 23°C water for 24–48 hours — achieves the same effect in a production-compatible timeframe.
- Mechanical testing at the wrong condition: an impact test performed on DAM nylon will show unacceptably low values and fail a specification that was written for conditioned nylon. An impact test performed immediately after accelerated conditioning will show higher values than field-equilibrated parts. The correct procedure is to test at equilibrium per ISO 1110 or ASTM D570 — or to agree with the customer on a specific conditioning protocol and correlation data.
Moisture-related surface defects such as splay and silver streaks are covered in detail in the defect diagnosis guide. For nylon specifically, inadequate drying is the single most common root cause of processing problems — it accounts for roughly 60% of all nylon processing issues in our experience.
Glass-Filled Nylon — The Metal Replacement Material
Adding glass fibre to nylon transforms it. For a detailed treatment of glass fibre reinforcement across all resin types, see the glass-filled plastics guide. This section focuses on what is specific to glass-filled nylon.
| Property | PA66 Unfilled | PA66-GF30 | PA66-GF50 | PA6 Unfilled | PA6-GF30 |
|---|---|---|---|---|---|
| Tensile strength | 80–85 MPa | 170–190 MPa | 210–230 MPa | 75–80 MPa | 150–170 MPa |
| Flexural modulus | 2.8–3.0 GPa | 8.0–9.5 GPa | 14–16 GPa | 2.6–2.8 GPa | 7.5–8.5 GPa |
| HDT @ 1.8 MPa | 70–75°C | 245–255°C | 250–260°C | 60–65°C | 200–210°C |
| Notched Izod (DAM) | 4–5 kJ/m² | 8–10 kJ/m² | 10–12 kJ/m² | 5–6 kJ/m² | 9–11 kJ/m² |
| Mold shrinkage | 1.2–1.8% | 0.3–0.7% | 0.1–0.4% | 1.0–1.5% | 0.2–0.6% |
| Density | 1.14 g/cm³ | 1.35–1.38 g/cm³ | 1.55–1.58 g/cm³ | 1.13 g/cm³ | 1.35–1.37 g/cm³ |
Glass-filled nylon is the most common material for metal-to-plastic conversion projects — replacing die-cast aluminum, stamped steel, and machined brass with injection-molded plastic. The combination of a PA66 base resin (for thermal capability) with 30–50% glass fibre loading (for stiffness and strength) produces parts that can carry structural loads in automotive, industrial, and consumer applications.
The trade-offs that matter in mold design and processing:
Mold wear. Glass fibre is abrasive. It is essentially short glass rods (200–400 μm long, 10–14 μm diameter) flowing through the mold at 200–500 mm/s under 80–120 MPa pressure. The glass erodes the steel at every point of flow direction change — gates, parting line edges, and rib corners.
A standard P20 mold running unfilled PA66 can reliably produce 500,000+ shots with normal maintenance. The same P20 mold running PA66-GF30 will show visible gate wear by 100,000 shots and parting line erosion by 200,000. For glass-filled nylon production volumes above 100,000 shots per year, the mold steel should be upgraded to H13 (48–52 HRC) for cavity and core inserts, with the gate insert in hardened H13 or carbide. The incremental steel cost is recovered through reduced tool maintenance and longer service intervals.
Warpage from anisotropic shrinkage. Glass fibres orient in the direction of flow during cavity filling. Shrinkage in the flow direction is constrained by the fibres (0.1–0.3%) while shrinkage in the transverse direction is 3–5× higher (0.5–1.0%). A part with a simple end-gated rectangular geometry will warp into a saddle shape — the edges parallel to flow bow outward from lower shrinkage; the edges perpendicular to flow bow inward from higher shrinkage.
Mold flow analysis with fibre orientation prediction is the standard tool for predicting and compensating for this behavior. For complex geometries with multiple flow paths, the fibre orientation field — and therefore the shrinkage field — varies across the part, and mold compensation by trial and error is rarely successful. Simulation with validated fibre orientation models is the cost-effective path.
Notch sensitivity. The glass fibres in a PA66-GF30 part create stress concentrations at every fibre end within the polymer matrix. At a sharp internal corner — the root of a rib, the base of a boss, the edge of a hole — the geometric stress concentration and the fibre-end stress concentrations superimpose. The result: a glass-filled nylon part is more sensitive to sharp corners than an unfilled nylon part, not less. The standard DFM rule — minimum 0.5 mm internal corner radius — is even more important for glass-filled grades. A 0.2 mm radius that works in unfilled PA66 will produce a crack initiation site in PA66-GF30.
Processing Parameters
Barrel Temperature Profile
Nylon has a sharp melting point — unlike amorphous materials such as ABS and PC that soften gradually over a temperature range, nylon transitions from solid to liquid within a 10–15°C window. This means the barrel temperature profile matters more.
| Zone | PA66 Unfilled | PA66-GF30 | PA6 Unfilled | PA6-GF30 |
|---|---|---|---|---|
| Feed (rear) | 260–270°C | 265–275°C | 230–240°C | 235–245°C |
| Compression | 270–280°C | 275–285°C | 240–250°C | 245–255°C |
| Metering (front) | 275–290°C | 280–295°C | 245–260°C | 250–265°C |
| Nozzle | 270–285°C | 275–290°C | 240–255°C | 245–260°C |
The practical window is tighter than the datasheet suggests. At the low end, unmelted or partially melted granules produce visible surface defects and inconsistent part weight. At the high end, residence times above 8–10 minutes at 290°C+ cause thermal degradation — the polymer chains break, producing yellowing (visible), reduced viscosity (detectable by part weight drift), and loss of mechanical properties (measurable but not visible).
The correct practice is to run at the lowest barrel temperature that produces complete melting with acceptable melt viscosity — typically 5–10°C above the minimum that fills the cavity without short shots — and to size the shot weight so that the barrel residence time is under 5 minutes. A machine with a shot capacity more than 3× the part-plus-runner weight will have excessive residence time and risks thermal degradation of nylon.
Mold Temperature
Nylon mold temperature controls surface finish, crystallinity, and dimensional stability:
| Application | Recommended Mold Temp | Why |
|---|---|---|
| Unfilled PA6 — cosmetic parts | 80–90°C | Higher gloss, fewer flow marks |
| Unfilled PA66 — structural parts | 80–100°C | Balance of crystallinity and cycle time |
| PA66-GF30 — automotive | 90–110°C | Maximum crystallinity for thermal stability |
| PA6-GF30 — industrial | 85–100°C | Good surface, controlled shrinkage |
A mold temperature below 60°C produces a nylon part with an amorphous skin layer that has not fully crystallized. This layer is 0.1–0.5 mm thick and has different shrinkage behavior, chemical resistance, and appearance than the crystalline core. When the part is subsequently exposed to temperature in service — the 80°C under-hood environment for an automotive bracket, or the 60°C interior of a power tool housing — this amorphous skin layer undergoes post-molding crystallization, producing dimensional change, surface blush, and possible warpage.
Injection Speed and Pressure
Nylon has a sharp melting point and low melt viscosity at processing temperature. This combination creates two processing behaviors that are different from amorphous materials:
Fast injection is usually correct. Nylon melt solidifies rapidly on contact with the mold wall because the temperature drop from 280°C to 100°C crosses the crystallization temperature almost immediately. A slow injection speed produces hesitation marks, flow lines, and incomplete filling in thin ribs and bosses. Fast injection — fill times of 0.5–1.5 seconds for most parts — is the standard approach.
Holding pressure must compensate for high mold shrinkage. Nylon’s mold shrinkage of 1.2–1.8% (unfilled) is 2–3× higher than ABS (0.4–0.7%). The holding pressure phase must pack enough additional material into the cavity during cooling to compensate for this shrinkage. Inadequate holding pressure produces sink marks, internal voids, and dimensional variation. The holding pressure for unfilled PA66 is typically 60–80% of the injection pressure, maintained until the gate freezes.
The injection molding defects guide covers the diagnosis and correction of specific nylon-related defects — sink marks, weld lines, and warpage — in detail.
Mold Design Considerations for Nylon
Gating
Nylon’s low melt viscosity means it flows through small gates more easily than PC or ABS. Gate diameters of 0.8–1.5 mm are typical for unfilled nylon; 1.0–2.0 mm for glass-filled grades. The gate should be positioned so that the melt flows from thick to thin sections — nylon solidifies rapidly, and a flow path that goes from a thin rib (0.8 mm) to a thick boss (3 mm) will produce a hesitation mark at the transition.
Submarine (tunnel) gates work well with unfilled nylon because the flexible part can be separated from the runner by ejection without leaving a large gate vestige. For glass-filled nylon, submarine gates wear faster because the glass fibres abrade the gate land — the gate insert should be a replaceable hardened component.
Venting
Nylon produces more gas during molding than most engineering plastics — residual moisture that was not fully removed by drying, low-molecular-weight fractions volatilizing at melt temperature, and decomposition byproducts if residence time is excessive. Vent depth of 0.015–0.02 mm and width of 3–6 mm at the last point to fill and along weld line locations is standard for nylon. Burn marks and gas traps caused by inadequate venting are among the most common defects in nylon molding.
Shrinkage Allowance
Nylon’s high mold shrinkage — 1.2–1.8% for unfilled PA66, 0.3–0.7% for PA66-GF30 — means the mold cavity must be cut larger than the part drawing dimensions by the expected shrinkage factor. But shrinkage is not isotropic or constant. It varies with:
- Wall thickness (thicker = more shrinkage)
- Flow direction (lower shrinkage parallel to flow in glass-filled grades)
- Mold temperature (higher mold temp = more crystallization = more shrinkage)
- Holding pressure (higher pressure = less shrinkage)
A mold flow analysis that predicts shrinkage distribution across the part geometry is the standard method for determining cavity dimensions. A single “shrinkage factor” applied uniformly to the entire cavity is adequate for simple geometries with uniform wall thickness; for complex parts with varying wall thickness and multiple flow paths, it is a source of dimensional error.
Applications
Automotive
PA66-GF30 is the default material for automotive under-hood and structural components. An HVAC bracket made from PA66-GF30 replaces a multi-piece stamped steel assembly at 60% weight reduction and comparable stiffness. The material’s 245°C HDT at 1.8 MPa provides margin above the 110°C peak under-dash temperature, and the 30% glass fibre loading delivers the creep resistance needed for a clamped joint that must maintain torque over 100,000 thermal cycles from −40°C to +110°C.
Other automotive nylon applications: engine covers (PA66-GF30, combining structural stiffness with NVH damping), intake manifolds (PA66-GF30, replacing welded aluminum at 50% weight reduction), radiator end tanks (PA66-GF30, surviving 135°C coolant at 1.5 bar pressure with 4,000-hour durability), and charge-air ducts (PA6-GF30 for interior surface finish on visible engine-bay components).
Automotive PPAP requirements add an additional layer of documentation and process control for nylon parts — material certification per IMDS, dimensional capability (Cpk ≥ 1.67 on all SC/CC characteristics), and full process validation per the customer’s specific PPAP level.
Industrial Equipment
PA6 is common for power tool housings, pneumatic valve bodies, and industrial equipment covers. The combination of impact resistance (especially after moisture conditioning), good surface finish, and cost-effectiveness ($2.80–4.00/kg) makes it competitive with PC+ABS for industrial enclosures that do not require the flame-retardant properties of PC.
PA66-GF30 and PA66-GF50 are used for gear housings, pump impellers, bearing cages, and structural brackets that replace die-cast aluminum or zinc at significant weight reduction. A PA66-GF50 gear housing for a professional-grade angle grinder carries the armature bearing load, maintains 0.05 mm bearing-to-gear center distance tolerance after moisture conditioning, and survives a 2-meter drop test onto concrete at −20°C — a demanding set of requirements that only glass-filled nylon can meet.
Consumer Products
PA6 and unfilled PA66 are used for zipper components, cable ties, sports equipment buckles, and kitchen appliance mechanisms — applications where the combination of toughness, wear resistance, and low friction makes nylon the material of choice. Nylon’s self-lubricating properties (coefficient of friction 0.2–0.3 against steel, dry) make it well-suited for sliding and rotating components without external lubrication.
Nylon vs POM vs PBT — Quick Selection Guide
When an engineer specifies nylon for a part, one question that should be asked is: “why nylon, and not POM or PBT?” Three engineering plastics overlap significantly in application space, and the wrong choice among them leads to field failures.
| Criterion | PA66 | POM (Acetal) | PBT |
|---|---|---|---|
| Tensile strength | 80–85 MPa | 60–70 MPa | 55–60 MPa |
| Stiffness | 2.8–3.0 GPa | 2.6–2.8 GPa | 2.3–2.5 GPa |
| Impact (conditioned) | 8–15 kJ/m² | 5–7 kJ/m² | 3–5 kJ/m² |
| Wear / friction | Good | Excellent | Fair |
| Moisture sensitivity | High | Very Low | Low |
| Chemical resistance | Good (except acids) | Good (except strong acids) | Excellent |
| Dimensional stability | Poor (moisture-driven) | Good | Excellent |
| Max continuous temp | 100–120°C | 85–95°C | 120–140°C |
Choose nylon when the part needs impact resistance, fatigue resistance, and thermal capability — and when post-molding moisture conditioning is part of the production process. Typical nylon applications: automotive under-hood, structural brackets, gears with shock loading, power tool housings.
Choose POM when the part needs low friction, wear resistance, and dimensional stability in dry environments — gear trains, bearings, and sliding mechanisms that must maintain precise dimensions without moisture-driven growth. POM’s 0.2–0.3% moisture absorption (vs nylon’s 1.5–2.5%) means a gear molded in POM will not change center distance due to humidity.
Choose PBT when the part needs electrical insulation, dimensional stability, and resistance to automotive fluids — connector bodies, sensor housings, and relay components. PBT’s 0.1% moisture absorption and fast crystallization make it the standard material for precision electronic connectors where nylon’s dimensional instability from moisture uptake is disqualifying.
For a detailed material-to-material comparison framework, see the material selection guide. For PP vs ABS specifically, see the PP vs ABS comparison.
Quality Control
Nylon parts require several QC checks beyond standard dimensional inspection:
- Moisture content of incoming material — Karl Fischer titration or loss-on-drying measurement on each material lot before processing. Moisture above 0.20% means the material must be re-dried before use.
- Part weight — a sensitive indicator of material degradation during processing. A downward drift in part weight over a production run, with no change in machine parameters, indicates that the melt viscosity is decreasing due to thermal degradation of the nylon. The barrel temperature should be checked and the residence time reduced.
- Conditioned vs DAM mechanicals — impact testing must specify the conditioning state. A notched Izod value of 4 kJ/m² (DAM) for PA66 that the customer’s specification calls out at 10 kJ/m² (conditioned) is not a material failure — it is a testing protocol error.
- Dimensional measurement after conditioning — critical dimensions should be measured after accelerated conditioning (80°C water, 2 hours for 2 mm wall) to verify that the part will meet drawing tolerances in its equilibrated field condition.
Nylon is the workhorse of engineering thermoplastics for a reason: it offers a combination of strength, thermal capability, impact resistance, and cost per kilogram that has no competitor in its price range. The trade-offs — moisture sensitivity, dimensional change, and the processing discipline required by glass-filled grades — are manageable by engineers who understand the material rather than treating it as “stronger ABS.”
A nylon part designed with moisture conditioning in the dimensional tolerance analysis, molded in H13 steel if glass-filled, dried to below 0.15% moisture before processing, and conditioned to equilibrium before assembly is a predictable, reliable engineering component. A nylon part designed without understanding moisture, molded in P20 steel with 30% glass fibre, and assembled at DAM dimensions will be a warranty claim waiting to happen.