PA Injection Molding Guidelines: Principles, Properties, and Processing

Polyamide, or PA, is widely called nylon. It's a key type of engineering plastic. Its molecular chain has repeating amide groups (–NHCO–) that define its core structure.

These highly polar amide groups form strong hydrogen bonds. The bonds give PA great mechanical strength, wear resistance, self-lubrication, and chemical resistance.

Common PA types include PA6, PA66, and PA12, named by monomer carbon atoms. PA is semi-crystalline, but its biggest molding challenge is high moisture absorption. Strict moisture control ensures successful molding.

Material Characteristics of Polyamide (PA)

High Moisture Absorption and Hydrolysis Risk

Moisture sensitivity is the biggest challenge in PA injection molding. Amide groups pull in water easily. PA6 can absorb over 9.5% water when saturated. Moisture in the melt breaks it down at high heat, causing brittle parts, bubbles, and surface flaws.

Absorbed water softens PA, making it tougher but also larger in size. You must dry raw materials fully before molding. Custom Machined Plastic Part designs also need to account for size changes from water absorption later.

Pronounced Crystallization Behavior

PA crystallinity, crystallization rate, and crystal structure are highly influenced by processing conditions—especially cooling rate.

• Fast cooling (low mold temperature): Produces fine, imperfect crystals, improving toughness and transparency (for thin parts), but results in higher and uneven shrinkage and poorer dimensional stability.
• Slow cooling (high mold temperature): Promotes more complete crystallization, increasing rigidity, hardness, dimensional stability, and heat resistance, though excessive crystallinity may cause brittleness.

Narrow Melting Window and Thermal Oxidation Risk

PA has a relatively narrow melting temperature range (PA6 ≈ 220 °C, PA66 ≈ 260 °C). Processing temperatures must be high enough to ensure flow but carefully controlled to prevent overheating. Prolonged exposure to high temperatures in air can cause thermal oxidation, resulting in yellowing and property degradation.

Fast Solidification and High Flowability

PA melts have low viscosity and excellent flowability, but once the temperature drops below the melting point, viscosity rises rapidly due to crystallization. This requires fast injection speeds and sufficient holding pressure to compensate for shrinkage.

Key Process Parameters for PA Injection Molding

Material Pre-Drying (Critical Step)

Drying is mandatory and must be strictly controlled.

Equipment: Use a dehumidifying dryer with a recommended dew point below –40 °C.

Drying conditions:

• PA6: 80–100 °C for 4–6 hours (do not exceed 110 °C to avoid oxidation).
• PA66: 100–120 °C for 4–8 hours.

Glass-fiber reinforced grades: Extend drying time by 1–2 hours.

Moisture content requirements:

• < 0.15% (reinforced grades)
• < 0.10% (unreinforced grades)
• < 0.05% for high-performance applications

After drying, material should be used immediately. During machine shutdown, purge the barrel with a stable polymer (e.g., HDPE) or reduce barrel temperature below 160 °C to prevent moisture uptake and degradation.

Melt Temperature Control

During the process of customizing the injection molding of plastic parts, temperatures must remain within a narrow window to ensure flow while avoiding degradation.

General rule: 20–50 °C above melting point; never exceed 300 °C.

PA6: Recommended melt temperature 230–260 °C; Rear zone: 210–230 °C | Front zone: 240–260 °C

PA66: Recommended melt temperature 260–290 °C; Rear zone: 240–260 °C | Front zone: 270–290 °C

Nozzle temperature: 5–10 °C lower than the front zone, but above the melting point to prevent freezing.

Mold Temperature Optimization

Many custom injection molding manufacturers believe that mold temperature is the most effective tool for controlling the crystallinity of PA products, thereby determining their final performance characteristics.

• Typical range: 60–120 °C
• High mold temperature (80–120 °C): Improves dimensional stability, rigidity, heat resistance, and surface gloss. Promotes uniform shrinkage and reduces internal stress. Essential for thick-wall, unreinforced PA parts.
• Low mold temperature (60–80 °C): Shortens cycle time and increases toughness but leads to higher shrinkage, poorer dimensional accuracy, and possible post-crystallization deformation.
• For glass-fiber reinforced PA, higher mold temperatures (80–100 °C) are recommended to reduce fiber exposure, surface roughness, and warpage.

Injection Speed, Pressure, and Holding

• Injection speed: High speed is recommended to fully fill the cavity and improve weld line strength. It also promotes uniform glass-fiber distribution in reinforced grades.
• Injection pressure: Typically 60–100 MPa; reinforced grades require higher pressure.
• Holding pressure & time:Extremely important due to PA's high crystallization shrinkage. Holding pressure should be 50–80% of injection pressure and maintained until gate freeze-off. Insufficient holding causes sink marks, voids, and undersized parts.

Back Pressure and Screw Speed

• Screw speed: Medium-low, typically 40–70 rpm, to avoid excessive shear heating.
• Back pressure: Low back pressure (5–15 bar) is sufficient to homogenize the melt and remove trapped air. Excessive back pressure increases shear heat and should be avoided.

Mold Design and Auxiliary Considerations

• Ventilation: Adequate venting is essential due to fast filling speeds and possible moisture vapor release. Recommended vent depth: 0.015–0.025 mm.
• Gating system: Short and thick runners with sufficiently large cold-slug wells. Slightly larger gate sizes help reduce shear heating and improve packing.
• Cooling system: Must be uniform and efficient to maintain stable mold temperatures, especially for high-temperature molding.

Common Defects and Solutions

Silver Streaks, Bubbles, Rough Surface

Most commonly caused by insufficient drying (over 90% of cases). Verify drying conditions immediately. Excessive melt temperature may also contribute.

Dimensional Instability

The instability in the dimensions of Custom Injection Molded Plastic Parts is mainly caused by moisture absorption and post-crystallization. Use thoroughly dried material and maintain mold temperatures above 80 °C to stabilize crystallization.

Excessive Brittleness

May result from over-drying (oxidation), excessive melt temperature (thermal degradation), or low mold temperature causing high internal stress. Adjust drying and temperature settings accordingly.

Sink Marks and Voids

Increase holding pressure and holding time, raise mold temperature to delay skin solidification, and optimize gate location near thick sections.

Glass Fiber Exposure (Fiber Float)

Increase mold and melt temperatures, raise injection speed, improve screw mixing efficiency, and select appropriate reinforced PA grades.

Conclusion

Choosing the right custom injection molding manufacturers for your nylon products is crucial. LVMA boasts a complete injection molding supply chain, offering a variety of injection molding services, such as two-color injection molding and insert molding. LVMA's team will use their expertise to advise you and answer any questions you may have.