What Is Nylon Injection Molding and How Does It Work?

What Is Nylon Moulage par injection?

Nylon injection molding is a manufacturing process in which polyamide¹ thermoplastique² resin is melted, injected into a steel mold under pressures of 750–1,250 bar, and cooled into precision parts with tensile strength typically ranging from 60 to 170 MPa depending on grade and reinforcement.

Nylon — commercially known as polyamide (PA) — was the world’s first synthetic engineering thermoplastic, introduced by DuPont in 1935. Today it remains one of the most widely molded engineering resins, valued for its exceptional fatigue resistance, self-lubricating surface, and cost-effective performance in structural applications.

The defining characteristic of nylon is its semi-crystalline molecular structure: polymer chains pack into ordered crystalline regions during cooling, which gives nylon its high stiffness and strength compared to amorphous resins like ABS or PC. However, the same crystallinity causes relatively high and variable shrinkage — the primary challenge in nylon part design.

In our factory, we process nylon on standard reciprocating screw injection molding machines with vented barrels and dehumidifying dryers. The key upstream step — drying — is non-negotiable: nylon is highly ³ and must arrive at the machine barrel with moisture content below 0.2% by weight. Skip the drying step and you will see splay marks, bubbles, and mechanical properties that fall 20–30% short of material datasheet values.

Nylon’s toughness, chemical resistance, and dimensional stability under load make it the go-to choice for gears, bearing cages, electrical connectors, cable ties, and under-hood automotive components — applications where metals are too heavy and standard commodity plastics lack the strength.

Compared with other engineering resins, nylon offers an exceptional strength-to-cost ratio. PA6 pellets trade at roughly one-third the cost of PEEK and one-half the cost of PPS, while delivering tensile strength, fatigue resistance, and chemical compatibility that satisfy the majority of structural plastic applications in the 80–130°C operating range.

What Are the Types of Nylon Used in Injection Molding?

PA6, PA66, and PA12 cover more than 90% of injection-molded nylon applications; the right grade depends on operating temperature, moisture exposure, and required mechanical performance.

PA6 (polycaprolactam) is the most economical grade and the easiest to process because its lower melt temperature reduces barrel wear and cycle time. PA66 (polyhexamethylene adipamide) has a higher heat deflection temperature — 90°C versus 65°C for PA6 at 1.8 MPa — making it preferred for engine compartment parts that see sustained thermal loads.

PA12 occupies a specialty niche: its very low moisture absorption (0.25% versus 2.5–3.5% for PA6) makes it the standard for fluid-handling tubing, fuel lines, and pneumatic hoses. When dimensional stability in humid environments is critical, PA12 outperforms PA6 and PA66 by a wide margin despite its lower stiffness.

Glass-fiber-reinforced grades (GF15, GF30, GF50) multiply tensile strength and dramatically reduce creep — but they introduce anisotropic shrinkage: flow-direction shrinkage can be 0.2–0.5% while transverse shrinkage remains 0.8–1.5%. Running analyse du flux des moules⁴ before cutting steel is mandatory for glass-filled nylon parts with tight tolerances.

What Are the Nylon Injection Molding Process Parameters?

Nylon melt temperature should be set between 230°C and 295°C depending on grade, with barrel zones increasing from rear to front — rear zone 10–20°C below mid, nozzle 5–10°C above front — to ensure homogeneous melt and prevent cold slugs.

The table below summarizes the key process window for the most common nylon grades. These are starting-point values; actual optimization should be guided by part geometry, wall thickness, and runner system design. Process windows are intentionally conservative — we recommend running mold trials before committing to high-production settings.

Mold temperature has a significant impact on crystallinity and surface finish. For unreinforced PA6, a mold temperature of 60–80°C gives a good balance of cycle time and part quality. Dropping mold temperature below 40°C to speed up cycle time reduces surface crystallinity, which actually lowers fatigue resistance and can create internal stresses that cause long-term dimensional creep.

For glass-filled grades, we recommend mold temperature of 80–100°C. Hotter molds allow glass fibers to reorient more freely and reduce the fiber-knit appearance at weld lines. In our factory, we use heated mold temperature controllers with ±2°C precision for glass-filled nylon parts — not oil at the press.

Injection speed should be moderate: nylon’s low melt viscosity means it fills quickly. Excessive injection speed generates frictional heat that can degrade the polymer and produce discoloration or gas burns at the end of fill. We typically set injection speed at 60–80% of machine maximum for nylon, then fine-tune based on fill balance across multi-cavity tools.

Screw back pressure for nylon should be kept low — 5–15 bar for unreinforced grades, up to 20 bar for glass-filled — since nylon’s low viscosity means excessive back pressure increases residence time without improving melt quality. Extended residence time at barrel temperature accelerates hydrolytic chain scission and reduces molecular weight in the finished part.

What Are the Drying Requirements and Moisture Control?

Nylon must be dried at 80–90°C for 4–8 hours in a dehumidifying hopper dryer to reduce moisture below 0.2% by weight; failure to dry results in hydrolytic degradation of the polymer chain during processing, causing reduced molecular weight, splay, bubbles, and mechanical property losses of 20–30%.

Nylon is one of the most hygroscopic engineering resins in common use. PA6 at equilibrium in ambient conditions (50% RH, 23°C) holds 2.5–3.5% moisture by weight — and each absorbed water molecule attacks the amide bond at barrel temperatures, breaking polymer chains and permanently reducing molecular weight. Unlike ABS or PP where moisture causes only surface splay, wet nylon undergoes irreversible molecular degradation.

The minimum drying specification is: dehumidifying dryer with dew point below −30°C, temperature 80°C, airflow ≥1 m³/hr per kg/hr throughput, duration 4–6 hours for PA6/PA66, 3–4 hours for PA12. A standard hot-air oven is not sufficient for nylon — you need a desiccant dehumidifying system to reach dew points low enough to pull the last percentage points of moisture.

Drying Monitoring and Process Control

In production, we monitor moisture with a Karl Fischer titrator before first shot and whenever material is changed. If moisture exceeds 0.3%, we extend drying by one hour and re-test. Once material is in the heated hopper, it can absorb moisture from compressed air in the machine’s plasticating zone — so we also ensure the purge guard seals properly and the screw is never left idle with nylon in the barrel above 200°C.

Avoiding Over-Drying and Proper Storage

Over-drying is also a concern: PA6 held at 90°C for more than 12 hours begins to show thermally oxidized yellowish color and slight embrittlement. PA12, with its lower moisture absorption, needs shorter drying time. Operators sometimes set a blanket 8-hour cycle for all nylon — this risks damaging PA12. Best practice is to set grade-specific drying recipes in the dryer controller.

Storage after drying is equally important. Dried nylon pellets exposed to ambient air re-absorb moisture within 30 minutes; we transfer pellets directly from the dryer hopper through a sealed conveying line to the machine barrel. For smaller batch runs, we use sealed moisture-proof bags and re-dry if the bag has been open for more than 2 hours.

What Are Common Defects in Nylon Injection Molding and Prevention?

Nylon’s most frequent injection molding defects are warping (caused by shrinkage asymmetry), splay/silver streaks (from moisture or degraded material), and sink marks (from insufficient holding pressure or thick sections); each has a specific root cause and process-level remedy.

Warping is the defect we fight most in nylon, especially with thin flat parts like cover plates and housings. Nylon’s shrinkage of 1.0–2.5% is 3–5× higher than PC and inherently more variable because the crystallization front does not freeze simultaneously across all wall sections. In our factory, we address this with conformal cooling channels to equalize temperature across the tool and by specifying ribs rather than uniform thick sections for structural parts.

Splay and silver streaks are almost always a moisture problem. When we see splay in a production run, the first action is always to pull a sample from the dryer hopper and measure moisture — not to adjust the machine. Nine times out of ten, the dryer has malfunctioned, a desiccant bead is saturated, or someone opened the hopper lid during a shift change.

Weld lines in nylon are stronger than in many resins (nylon’s low viscosity allows good knit-line fusion), but they remain a weak point in glass-fiber-reinforced grades where fibers align parallel to the weld surface. For structural parts with weld lines, we specify weld-line tensile strength at 60–70% of base material strength and position gates to push weld lines away from high-stress areas.

Chemical Resistance and Post-Mold Treatment

Chemical resistance is another factor in defect prevention: nylon’s resistance to oils, greases, and aliphatic hydrocarbons is excellent, but it swells in strong acids and is attacked by phenols. Parts designed for chemical exposure should be tested with the actual service fluid before finalizing wall thickness, as even 0.5% swell can close press-fit interfaces and jam mechanical assemblies.

Post-mold moisture conditioning is recommended for structural nylon parts. Immersing freshly molded PA6 parts in 80°C water for 2–4 hours (DAM-to-conditioned cycle) relieves molding stresses and pre-saturates the part to its service-environment moisture level — eliminating the dimensional change that would otherwise occur gradually in the field over the first 3–6 months of use.

What Are the Nylon Applications by Industry?

Nylon’s combination of mechanical strength, fatigue resistance, chemical compatibility, and cost effectiveness makes it the dominant engineering resin in automotive under-hood components, electrical connectors, industrial gears, and consumer goods requiring load-bearing plastic parts.

Automotive accounts for roughly 40% of engineering nylon consumption. Under-hood applications — intake manifolds, air ducts, cooling fans, cable ties, and transmission housings — demand the sustained heat resistance of PA66-GF30, which retains 50% of its room-temperature strength at 130°C. Structural exterior parts like door handles and mirror brackets use unreinforced PA6 for its toughness and UV-stabilized surface quality.

Electrical, Industrial, and Consumer Applications

Electrical and electronics is the second-largest end market. Nylon 66 is the standard material for connector housings, terminal blocks, relay bases, and circuit breaker bodies. Its UL94 V-2 rating (unreinforced) and V-0 at 0.4 mm with flame-retardant additives make it widely accepted in safety-certified assemblies. Glass-filled grades are used for precision connector housings where dimensional stability through reflow soldering temperatures is required.

Industrial machinery applications leverage nylon’s self-lubricating properties: PA6 and PA66 gears, bushings, cam followers, and conveyor chain links operate with no external lubrication at moderate loads, reducing maintenance costs significantly versus metal alternatives. In our factory, we regularly mold PA6 gears with module 1–4 in cavities of 4–16, held to AGMA quality 8 tolerances (±0.025 mm pitch diameter).

Consumer and sporting goods represent a growing segment: ski bindings, bicycle components, power tool housings, and appliance components all use nylon for its combination of high strength-to-weight ratio, impact resistance, and the ability to achieve Class A surface finishes with proper mold polish and processing conditions.

What Are the Design Guidelines for Nylon Injection Molded Parts?

Optimal wall thickness for nylon injection molded parts is 1.5–3.5 mm; thinner walls may cause short shots and excessive fiber orientation in glass-filled grades, while thicker walls extend cycle time and create sink marks over internal ribs.

Nylon’s high shrinkage demands that wall thickness variation be kept below 3:1 across any section. Where thick sections are needed for strength, add hollow structures or ribs rather than solid walls. A 3 mm rib at 60% of wall thickness (1.8 mm) provides nearly equivalent stiffness with far less shrinkage-driven warping than a 3 mm uniform wall extending from a 2 mm section.

Draft angles for nylon should be 0.5–1.0° minimum on side walls, increasing to 1.5–2.0° for textured or matte surfaces. Nylon’s semi-crystalline nature means it can grip polished steel surfaces more aggressively than amorphous resins at certain mold temperatures — inadequate draft leads to drag marks and dimensional error even when ejection force is sufficient.

Gate, Rib, and Draft Angle Guidelines

Gate location is critical for managing weld lines and shrinkagee direction. For glass-filled nylon, we use conception de moules d'injection⁵ simulation to optimize gate position to align fibers in the primary load direction. Edge gates work well for flat parts; pin gates or sub gates are preferred for cosmetic surfaces where gate vestige must be minimized. In our experience, a center gate on a circular nylon gear consistently outperforms a side gate in terms of shrinkage uniformity and runout under 0.05 mm.

Rib design matters especially for nylon: rib thickness should not exceed 50–60% of the adjoining wall to prevent sink marks. Rib height should be ≤3× wall thickness and draft angle ≥0.5° per side. Use fillets at rib bases (radius ≥0.5 mm) to reduce stress concentration — nylon’s notch sensitivity means a sharp internal corner can reduce impact strength by 30–50%.

What Are the Most Common Questions About Nylon Injection Molding?

What is the difference between PA6 and PA66 for injection molding?

PA6 (polycaprolactam) has a melt point of 215–225°C and is processed at 230–260°C; PA66 (polyhexamethylene adipamide) melts at 255–265°C and requires 260–290°C barrel temperatures. PA66 has a higher heat deflection temperature (90°C versus 65°C at 1.8 MPa) and better retention of mechanical properties at elevated temperature, making it preferred for under-hood automotive applications. PA6 is easier to process, lower cost, and sufficient for most structural ambient-temperature applications. Both grades require similar drying protocols (80°C, 4–6 hours) and show similar shrinkage behavior in the 1.0–2.5% range.

How long should nylon be dried before injection molding?

PA6 and PA66 require drying at 80°C for 4–6 hours in a dehumidifying dryer with dew point below −30°C, reducing moisture below 0.2% by weight. PA12, with lower equilibrium moisture (0.25%), can be dried in 3–4 hours at 85°C. Material that has been exposed to ambient humidity for more than 8 hours after drying should be re-dried. Hot-air ovens are not suitable — only desiccant dehumidifying systems achieve the required low dew point. Over-drying PA6 beyond 12 hours at 90°C risks thermal oxidation and slight yellowing.

What causes warping in nylon injection molded parts?

Nylon warping is primarily caused by asymmetric shrinkage: differential cooling rates between thick and thin sections, imbalanced runner systems, or non-uniform mold temperature create internal stresses that distort the part after ejection. Glass-fiber reinforcement amplifies this because flow-direction shrinkage (0.2–0.5%) differs significantly from transverse shrinkage (0.8–1.5%), creating a strong tendency for flat panels to bow in the transverse direction.

Prevention involves maintaining uniform wall thickness (≤3:1 ratio), using balanced cooling channels to target ±5°C temperature uniformity across the tool, avoiding asymmetric runner systems, and running mold flow analysis to predict warpage before steel is cut. We also use ejection simulation to identify regions where differential cooling creates bending moments that cause distortion after the part leaves the tool.

Can nylon be injection molded with glass fiber reinforcement?

Yes — PA6-GF30 and PA66-GF30 are among the most widely molded engineering materials. Glass fiber at 30 wt% increases tensile strength from ~80 MPa to ~170 MPa and dramatically reduces creep, but requires higher processing temperatures (240–295°C barrel), higher injection pressure (900–1,300 bar), and mold temperature of 80–100°C. The mold must use H13 or equivalent hardened tool steel (≥HRC 50) in wear-critical areas due to glass fiber abrasivity. Venting must be generous because glass-filled nylons degas more aggressively. Gate and runner diameter should be 20–30% larger than for unfilled grades to reduce shear-induced fiber breakage.

What mold material is best for nylon injection molding?

For unfilled nylon (PA6, PA66, PA12), P20 pre-hardened steel is suitable for moderate production runs up to 200,000 shots. For glass-filled grades, H13 tool steel hardened to HRC 48–52 is recommended due to abrasive wear from glass fibers — using P20 for glass-filled nylon typically results in cavity erosion within 50,000 shots. For high-volume production exceeding 1 million shots, S136 or 2316 stainless is preferred in the gate and runner system where wear is highest. All mold surfaces should have at least 0.5° draft and be polished to SPI A2 or better for cosmetic parts.

What is the shrinkage rate of nylon 6 and nylon 66?

PA6 shrinkage is 1.0–2.0% in flow direction and 1.2–2.5% transverse; PA66 shrinks 1.5–2.5% in flow and 1.8–3.0% transverse. Glass fiber reduces shrinkage significantly: PA6-GF30 shows 0.2–0.5% in flow direction and 0.8–1.5% transverse. Moisture absorption after molding also causes post-mold dimensional change: PA6 absorbs up to 2.5% moisture at 50% RH, expanding by approximately 0.7% in linear dimension over 24 hours. Parts with tight dimensional tolerances should be measured after conditioning to 50% RH for 48 hours, not immediately after molding.

What industries use nylon injection molded parts?

Automotive is the largest consumer — PA66-GF30 dominates under-hood structural parts (air intake manifolds, radiator end tanks, cooling fan blades). Electrical and electronics use PA66 extensively for connector housings, terminal blocks, and relay bases due to its UL94 rating and dimensional stability. Industrial machinery uses PA6 for self-lubricating gears, bearings, and conveyor components. Consumer goods and sporting equipment (ski bindings, power tool housings) use PA6 for its toughness and surface quality. Medical device housings use medical-grade nylon with biocompatibility certifications.