Complete guide to injection molding nylon — melt temperatures, mold temperatures, drying, troubleshooting common defects, and optimization strategies.
Why Nylon Requires Special Processing Attention
Nylon is one of the most challenging materials to injection mold successfully. Its combination of high melt temperature, extreme moisture sensitivity, high melt viscosity, and tendency toward warpage means that small deviations in processing parameters cause large quality variations. A molding machine running polycarbonate without incident may produce entirely rejected parts when switching to unfilled PA66 without adjusting for its specific requirements.
This guide provides the complete parameter framework and troubleshooting methodology for injection molding engineers working with nylon materials.
Core Processing Parameters
**Temperature Settings**:
| Parameter | PA6 | PA66 | PA12 | PA6-GF30 | PA66-GF30 |
|—|—|—|—|—|—|
| Melt Temp (°C) | 230-260 | 270-290 | 240-270 | 240-270 | 280-300 |
| Mold Temp (°C) | 60-80 | 80-100 | 40-60 | 80-100 | 80-110 |
| Nozzle Temp (°C) | 230-260 | 270-290 | 240-270 | 245-275 | 285-300 |
| Front Zone | 230-250 | 265-280 | 235-255 | 235-255 | 270-285 |
| Middle Zone | 235-255 | 270-285 | 240-260 | 240-260 | 275-290 |
| Rear Zone | 230-250 | 265-280 | 235-255 | 235-255 | 270-285 |
**Why higher mold temperature for PA66?**
PA66 has higher melt viscosity and higher crystallinity. Low mold temperature produces parts with low crystallinity that continue to crystallize after ejection, causing warpage and dimensional instability. Higher mold temperature (80-100°C) locks in crystallinity during the cycle, producing dimensionally stable parts.
**Drying: The Non-Negotiable First Step**:
Never mold nylon that hasn’t been properly dried. Required drying conditions:
| Material | Temperature | Hours | Max Moisture |
|—|—|—|—|
| PA6 | 80°C | 4-6h | 0.20% |
| PA66 | 85°C | 4-6h | 0.15% |
| PA12 | 80°C | 3-4h | 0.10% |
| All GF/CF grades | 85°C | 4-6h | 0.12% |
Use desiccant dryers with dew point monitoring. Verify with moisture analyzers for critical parts.
Injection and Pressure Parameters
**Injection Speed and Pressure**:
Nylon’s high melt viscosity requires higher injection pressure than polypropylene or polystyrene:
| Parameter | PA6 | PA66 | PA12 |
|—|—|—|—|
| Injection Pressure | 80-120 MPa | 100-140 MPa | 70-100 MPa |
| Pack Pressure | 60-80 MPa | 70-100 MPa | 50-70 MPa |
| Hold Pressure | 50-70 MPa | 60-90 MPa | 40-60 MPa |
| Back Pressure | 0.3-0.5 MPa | 0.3-0.7 MPa | 0.2-0.4 MPa |
**Injection Speed**: Medium to fast. Too slow causes cold shuts and short shots. Too fast causes jetting and burn marks. Aim for fill time of 1-3 seconds for most parts.
**Pack and Hold**:
Nylon has high thermal contraction during cooling (1.5-2.5% linear shrinkage). Insufficient pack pressure results in sink marks and voids. Recommended pack time: 2-5 seconds depending on wall thickness.
**Shrinkage**:
| Material | Shrinkage Parallel (%) | Shrinkage Perpendicular (%) | Anisotropy |
|—|—|—|—|
| PA6 | 0.8 | 1.4 | 0.6% |
| PA66 | 1.0 | 1.7 | 0.7% |
| PA12 | 0.8 | 1.2 | 0.4% |
| PA6-GF30 | 0.4 | 1.1 | 0.7% |
| PA66-GF30 | 0.5 | 1.2 | 0.7% |
| PA6-CF30 | 0.2 | 0.5 | 0.3% |
GF and CF reinforcement significantly reduce overall shrinkage but increase anisotropy. CF30 has the most balanced shrinkage.
Common Defects and Solutions
**Bubbles and Voids**
Causes: Moisture, excessive melt temperature, inadequate pack pressure
Solutions: Verify drying (Karl Fischer <0.15%), reduce melt temp 10°C, increase pack/hold pressure
**Silver Streaks / Flow Lines**
Causes: Moisture vapor flashing off during injection, contamination
Solutions: Improve drying (dew point < -40°C), purge barrel thoroughly, check for resin contamination
**Short Shots**
Causes: Insufficient injection pressure/speed, cold mold, high melt viscosity
Solutions: Increase injection pressure 10-20%, raise mold temperature, raise melt temperature slightly (but not above recommended max)
**Sink Marks**
Causes: Insufficient pack/hold pressure or time, thick sections, high local shrinkage
Solutions: Increase pack pressure 20%, extend hold time, reduce wall thickness variation, add ribs with generous radii
**Warpage**
Causes: Differential shrinkage from orientation, temperature gradients, uneven cooling
Solutions: Increase mold temperature, use balanced gating, counter-gate thick parts, anneal after molding
**Jetting**
Causes: Injection speed too fast, cold mold, gate too small
Solutions: Reduce injection speed at start of fill, increase mold temperature, enlarge gate diameter
**Flash**
Causes: Injection pressure too high, mold wear, insufficient clamp force
Solutions: Reduce injection pressure, check mold for damage, verify clamp force (minimum 3-5 tons/in² projected area)
Cycle Time Optimization
Nylon cycle time is dominated by cooling requirements:
**Cooling Time Estimates**:
| Material | Cooling Time Factor (sec/mm wall) | Notes |
|—|—|—|
| PA6 | 0.8-1.0 | Faster than PA66 due to lower mold temp |
| PA66 | 1.0-1.3 | Higher mold temp = longer cooling |
| PA12 | 0.5-0.7 | Lowest mold temp requirement |
| PA6-GF30 | 1.0-1.4 | GF increases thermal conductivity slightly |
| PA66-GF30 | 1.2-1.6 | High mold temp extends cycle |
**Cooling System Design**:
Baffle and bubbler cooling channels are essential for thick-section nylon parts. Design cooling to maintain uniform mold surface temperature within ±5°C. Hot runner systems significantly reduce scrap and improve consistency for PA66.
**Cycle Time Reduction Strategies**:
1. **Reduce wall thickness** where possible — thinner walls cool exponentially faster
2. **Optimize mold temperature**: Use the minimum mold temperature that produces acceptable crystallinity. Even 5°C reduction can save 10-20 seconds/cycle.
3. **Increase cooling water flow**: Maintain ΔT of 5-8°C across cooling channels
4. **Use conformal cooling** for complex geometries — 3D-printed steel inserts enable near-net-shape cooling channels
5. **Annealing vs. longer cycle**: For parts requiring high dimensional stability, 1-hour annealing post-molding may allow shorter cycles vs. achieving crystallinity in-mold
Material Changeover and Purging
**Purging Nylon from the Barrel**:
When switching from another material to nylon:
1. Purge with PA6 or PA66 at normal processing temperature
2. Use glass-filled purge compounds for heavily contaminated barrels
3. For color changes: purge with natural material, then new color
**Purging TO another material from nylon**:
Nylon leaves carbonized residue at high temperatures. Best approach:
1. Drop melt temperature to 250°C
2. Purge with polycarbonate or acrylic (good solvents for carbon deposits)
3. Increase temperature and purge with target material
4. Inspect screw and barrel for degradation if nylon was processed at temperatures above recommended max
**Shelf Life and Storage**:
Nylon resin has limited shelf life when exposed to humidity:
– **Sealed packaging**: 1+ years if unopened, stored at <30°C
- **Opened bags**: Process within 48-72 hours or re-dry
- **Regrind**: Can be reused at 10-30% loadings (never 100% regrind — property degradation)
- **Moisture-damaged resin**: Will produce steam bubbles during molding — always dry before use regardless of apparent dryness
Whether you need technical guidance on selecting the right nylon grade for your specific application, or want to discuss pricing and supply options for PA6-CF, PA66-GF, or standard nylon materials, our engineering team is ready to help. Nylonplastic.com supplies industrial-grade nylon materials to manufacturers in North America, Europe, and Southeast Asia.
Get a Free Material Consultation →
Contact our technical team for nylon grade recommendations, pricing for bulk orders, or samples for testing. We supply PA6-CF carbon fiber reinforced nylon in 1.75mm and 2.85mm diameters, plus full range of PA6, PA66, PA12, and GF-reinforced grades.
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**FAQs**
**Q: What is the difference between PA6 and PA66?**
A: PA66 (nylon 66) has a higher melting point (265°C vs. 225°C) and better chemical resistance than PA6. PA6 offers better impact resistance and is more cost-effective. PA66 is preferred for high-temperature and under-hood automotive applications; PA6 is common for general engineering.
**Q: How much does glass fiber reinforcement improve nylon?**
A: Adding 30% glass fiber increases tensile strength by 100-120% (from ~80 MPa to ~170 MPa) and flexural modulus by 250-300% (from ~2.8 GPa to ~9 GPa). However, it also reduces impact resistance and increases warpage.
**Q: What is carbon fiber reinforced nylon used for?**
A: Carbon fiber reinforced nylon is used for structural components requiring high stiffness-to-weight ratio, ESD-sensitive applications (electronics packaging, fuel systems), and precision parts requiring dimensional stability. nylonplastic.com supplies PA6-CF for FDM 3D printing and injection molding applications.
**Q: How do I prevent moisture absorption problems in nylon parts?**
A: Dry nylon resin to below 0.2% moisture content before molding (4-6 hours at 80-85°C in desiccant dryer). For dimensional-critical parts, anneal after molding (1-2 hours at 120-130°C) to stabilize crystallinity. Use glass or carbon fiber reinforcement to reduce moisture-induced dimensional change by 70-80%.
**Q: Can nylon be used for food contact applications?**
A: Yes. Both PA6 and PA66 have FDA 21 CFR §177.1500 compliance for food contact. EU Regulation 10/2011 compliance requires specific compound selection with documented SML testing. Always verify specific grade compliance with your supplier.
