Nylon (PA) Injection Molding: A Buyer's Guide, Including Glass-Filled Grades

Nylon (polyamide, PA) is a primary engineering resin for parts that require strength, wear resistance, and the ability to survive heat and chemicals, such as gears, bearings, brackets, and under-hood components. It is frequently reinforced with glass fiber to significantly increase strength and stiffness. This guide covers nylon from a buyer’s perspective, as part of the material selection guide.

Nylon Characteristics

Nylon encompasses a family of engineering thermoplastics, with PA6 and PA66 being the most common. Known for mechanical strength, toughness, wear resistance, and chemical and heat resistance, it often appears under trade names like Ultramid and Zytel.

Buyers select nylon for its specific functional capabilities. Its high strength and toughness handle structural loads. Excellent wear and abrasion resistance suit moving parts like gears and bushings. Good heat and chemical resistance allow it to tolerate under-hood environments, fuels, oils, and solvents. When glass-filled grades (e.g., 30% glass) are used, stiffness and strength increase dramatically.

Essential Planning Constraints

Nylon programs must account for two characteristic behaviors:

Moisture absorption: Nylon is highly hygroscopic. It absorbs water from the air during processing and in service. Undried nylon causes splay and degradation during molding. In service, absorbed moisture can slightly alter dimensions and properties.

Warpage in glass-filled grades: Glass fiber boosts strength but shrinks differently along the fiber flow versus across it. This anisotropic shrinkage makes glass-filled nylon prone to warpage, requiring careful design and tooling consideration for flat or precise parts.

Processing Realities

Nylon’s processing window hinges on its moisture sensitivity. SABIC’s processing guide for LNP compounds categorizes all nylon grades (6, 6/6, 6/10, 6/12, 11, 12) as requiring drying prior to molding. The baseline is typically 3–4 hours at a dew point of 0 °F (−18 °C) or lower.

Unlike commodity resins where drying might just prevent cosmetic surface blemishes, molding wet nylon at melt temperature attacks the polymer chain. The part may look acceptable visually but will be measurably weaker. When qualifying a molder for a nylon program, ask what dew point their dryer holds and if they measure residual moisture, rather than just asking if they dry the material.

Illustrative ranges (unfilled PA):

• Drying: ~80 °C for 4 h or more (critical).
• Melt temperature: ~230–290 °C.
• Mold temperature: ~40–90 °C.
• Mold shrinkage: ~0.8–1.5%+ (glass fill lowers this but adds directionality). (Confirm actual settings with the specific resin datasheet.)

The Impact of Glass Fill

Adding glass fiber fundamentally changes dimensional behavior. Glass-fiber-reinforced compounds typically shrink one-third to one-half as much as unreinforced resin, and they shrink anisotropically.

Switching from an unfilled nylon to a 30% glass-filled grade on an existing tool will result in a part that is physically larger than intended and distorted in a different direction. Proving shrinkage on a prototype tool is often necessary for complex parts before committing to a production mold.

Comparing Nylon

Against acetal/POM, nylon is generally tougher, more heat-resistant, and accepts glass reinforcement better. However, acetal absorbs less moisture and is more dimensionally stable and lower-friction. Match the choice to the specific load, wear, heat, and dimensional requirements detailed in the material selection guide.

Buyer FAQs

What is nylon used for in injection molding?

Nylon is specified for strong, wear-resistant functional parts, including gears, bearings, bushings, automotive under-hood components, power-tool parts, and structural brackets. Glass-filled nylon is used in high-load structural roles and can sometimes replace metal components.

Why does nylon absorb moisture, and does it matter?

Nylon is inherently hygroscopic. Undried nylon degrades during molding, leading to weak parts and surface splay. In service, absorbed moisture can slightly change part dimensions and mechanical properties, requiring tolerance adjustments.

Why is glass-filled nylon so common?

Adding glass fiber (often 30%) dramatically increases nylon’s strength, stiffness, and heat resistance. The trade-off is anisotropic shrinkage, which makes the part prone to warpage and makes the material abrasive to tooling.

Nylon vs acetal (POM), which is better for gears?

Nylon is tougher and handles higher heat, making it suitable for high-load or hot applications. Acetal offers superior dimensional stability, lower friction, and less moisture sensitivity, often making it better for precision, low-friction gears.

Disclaimer

PlasticsTechnologyAlliance.com is an independent buyer resource. It does not manufacture parts, sell materials, or certify suppliers. Material behavior is grade-specific. Confirm properties and suitability against the manufacturer’s datasheet and your supplier’s engineering review.