Injection Molding Nylon: Processing Parameters and Troubleshooting

Why Nylon Requires Special Processing Attention

Nylon is one of the most demanding engineering plastics to injection mold successfully. Its combination of high melt viscosity, moisture sensitivity, and crystallinity kinetics makes it fundamentally different from polypropylene or polystyrene.

The three biggest challenges in nylon injection molding:

• Moisture sensitivity: Nylon must be dried to below 0.2% moisture before molding. Even 0.1% excess moisture causes splay, surface roughness, and part brittleness.
• High melt viscosity: Nylon melts flow poorly at low shear rates, requiring high injection speeds and pressure to fill thin-walled sections.
• Warpage and shrinkage: High mold shrinkage (1.0-1.5% for PA6, 1.3-1.8% for PA66) combined with anisotropic shrinkage causes warpage in unsymmetric parts.

Mastering these three factors — drying, processing temperature, and mold design — accounts for 90% of successful nylon molding outcomes.

Core Processing Parameters

Temperature settings are the foundation of nylon injection molding. The correct melt temperature ensures proper polymer chain entanglement while avoiding thermal degradation. Here are the recommended settings by nylon grade:

Injection and Pressure Parameters

Beyond temperature, injection pressure and speed are critical for filling nylon molds properly. Nylon’s high viscosity requires higher pressures than polypropylene, but excessive pressure causes flash and mold wear.

Common Defects and Solutions

Understanding the root causes of nylon defects allows targeted solutions rather than guesswork:

• Splay (silver streaks): Caused by moisture or volatile contaminants. Solution: Dry material thoroughly, purge with nylon-specific purge compound, check vent zones.
• Weld lines: Occur where two flow fronts meet. Solution: Increase melt temperature, raise injection speed, add gas vents at weld line locations.
• Short shots: Caused by insufficient flow length or low pressure. Solution: Increase injection speed and pressure, raise melt temperature, redesign gate location.
• Warping: Results from uneven shrinkage. Solution: Increase mold temperature, balance wall thickness, add ribs for stiffness, use symmetric gating.
• Bubble formation: Indicates moisture, air entrapment, or material degradation. Solution: Verify drying, lower back pressure, check for insufficient venting.

Cycle Time Optimization

Nylon cycle time is dominated by cooling requirements. Since nylon molds at elevated temperatures (60-110°C vs 20-40°C for PP), significantly more cooling time is needed. Optimizing cooling is the most effective way to reduce cycle time for nylon parts.

• Mold cooling design: Baffle and bubbler cooling channels positioned 1.5x the cavity depth from the parting surface
• Cooling water temperature: Use chilled water (8-15°C) for large or thick nylon parts to reduce cycle time by 15-30%
• Ejection temperature: Parts should reach 60-80°C at ejection to minimize warpage
• Hot runner optimization: Nylon requires precise hot runner temperature control (within ±2°C) to prevent material degradation in the runner

Material Changeover and Purging

When switching from nylon to another material (or vice versa), proper purging prevents contamination and equipment damage. Nylon leaves significant residue in the barrel due to its adhesive nature.

• Purging from nylon: Use PA-specific purge compound or polypropylene as a purging agent at processing temperatures 20°C above nylon melt temperature
• Purging to nylon: Ensure barrel is completely clean of previous material; purge with nylon-compatible intermediate material
• Color change: Perform two full barrel purges with natural material before adding color concentrate

KSAN offers technical on-site support for nylon processing optimization and troubleshooting for customers running trial production batches.

Frequently Asked Questions

What causes silver streaks (splay) in nylon injection molding?

Silver streaks (splay): moisture vaporizing during injection. Cause: insufficient drying. PA6 dry at 80°C for 4-6h, PA66 at 100°C for 4-6h. Also caused by: excessive moisture in compressed air (install air dryers), mold outgassing at high mold temperatures, contamination in material.

How do I prevent warpage in flat nylon parts?

Warpage in flat parts: caused by differential shrinkage across thickness. Prevention: use balanced fill pattern, increase pack/hold pressure to pack out shrinkage, cool mold uniformly (not just at corners), ensure adequate mold rigidity. Design with ribs and gussets rather than thick flat sections.

What is the recommended mold temperature for nylon?

PA6: mold temperature 80-100°C (lower causes surface crystallinity variations). PA66: mold temperature 100-130°C (lower causes white streaking). Higher mold temperature improves surface appearance and reduces internal stress. For glass-filled grades: increase mold temperature by 10-20°C.

What causes short shots in nylon injection molding?

Short shots: insufficient fill. Causes: low melt temperature (increases viscosity), moisture (causes vapor blocking), inadequate injection pressure/speed, too-small gates, high aspect ratio runners. Fix: increase melt temp by 10-20°C, increase injection pressure, check moisture content, enlarge gates.

How do I reduce cycle time for nylon injection molding?

Cooling: 50-70% of cycle time. Strategies: conformal cooling channels (3D printed mold inserts reduce 20-40%), adequate cooling channel diameter, balanced cooling. Pack/hold time: minimize while maintaining part quality. Hot runners eliminate cold runner waste. Automated ejection reduces handling time.