Key Points of Drying Process for PA6 and PA66 Before Injection Molding
PA6 and PA66 are common polyamide engineering plastics with strong hygroscopicity. Insufficient drying before injection molding can easily cause defects such as silver streaks, bubbles, reduced mechanical strength, and dimensional instability. Proper drying control is critical to ensuring product appearance, performance, production stability, and defect reduction.
Core Significance of Pre-Drying
The molecular structures of PA6 and PA66 contain polar amide groups, which absorb moisture from the air rapidly even during short-term storage. Moisture inside the barrel causes resin hydrolysis at high temperatures, reducing molecular weight and weakening mechanical properties while generating gases that create appearance defects.
Inadequate drying leads to silver streaks, bubbles, flow marks, and short shots in finished products. Material strength and toughness decline, making parts prone to breakage and shortening service life. Dimensional stability worsens, making it difficult to meet tolerances for precision components. In severe cases, internal voids cause high scrap rates.
Therefore, PA6 and PA66 must undergo standardized and thorough drying before processing, and re-absorption of moisture after drying must be avoided.
Core Drying Parameter Control
Drying TemperatureTemperature directly affects drying efficiency and material stability. Excessively high temperatures cause yellowing and sticking; low temperatures result in incomplete drying.
PA6: Standard drying temperature 80–90°C
PA66: Standard drying temperature 90–100°C
Glass-fiber reinforced or flame-retardant grades: May increase by 5–10°C, but not exceed 110°C
Practical tips: Use actual hopper internal temperature instead of only display temperature. Avoid rapid heating; use gradual temperature rise to prevent surface-dry and interior-wet conditions.
Drying TimeAdjust based on initial moisture content and material bed thickness.
New material, well-sealed: PA6 approx. 4–6 hours; PA66 approx. 6–8 hours
Opened or visibly moist material: Extend to 8–12 hours
Recycled or crushed material: Extend appropriately and increase inspection frequency
Practical tips: Hopper loading should not exceed 70%. Avoid overly thick material layers to ensure hot air penetration. For continuous production, use online dehumidifying dryers to prevent material aging from prolonged high-temperature baking.
Drying Air Speed and Dew PointStandard hot-air drying is affected by ambient humidity. Dehumidifying dryers are recommended for high-demand products.
Recommended dew point: −20°C to −40°C
Moderate air volume for uniform hot-air circulation and to avoid dead zones
Increase drying intensity in high-humidity environments (rainy seasons, coastal areas)
Moisture Content StandardsDrying qualification is determined by moisture content testing.
Standard products: ≤ 0.05%
Precision structural parts: ≤ 0.03%
Food, medical, and appearance parts: ≤ 0.02%
Moisture analyzers are recommended for each batch; do not rely solely on time.
Drying Equipment Selection and Operation
Common Drying Methods
Hot-air hopper dryer: Low cost, suitable for standard products, affected by ambient humidity.
Dehumidifying dryer: Stable dew point, uniform drying, ideal for continuous and precision production.
Vacuum dryer: Low temperature, fast, low yellowing risk, suitable for high-end transparent or high-appearance parts.
On-Site Operation GuidelinesDry material promptly after opening to avoid prolonged exposure. Insulate drying hoppers to reduce heat loss and condensation. Transfer dried material directly to the molding machine; limit exposure to 1–2 hours. Clean hoppers and pipelines regularly to prevent buildup, caking, and contamination. Thoroughly clean systems when switching materials to avoid mixing.
Common Drying Issues and Solutions
Insufficient DryingSymptoms: Silver streaks, bubbles, poor strengthCauses: Low temperature, insufficient time, thick material layer, high humiditySolutions: Raise temperature, extend time, reduce loading, switch to dehumidifying drying
Material Yellowing and EmbrittlementCauses: Excessively high temperature, prolonged drying, high oxygen in hot airSolutions: Lower temperature, shorten baking time, use closed dehumidification systems
Rapid Moisture ReabsorptionCauses: High workshop humidity, uninsulated pipelines, long transfer timeSolutions: Seal containers, use insulated hoppers, shorten transfer processes
Uneven DryingCauses: Poor air distribution, bridging, cakingSolutions: Stir regularly, optimize loading, check for blocked air outlets
Summary and Production Management
Drying of PA6 and PA66 is easily overlooked but highly influential. Temperature, time, air speed, dew point, and moisture content must be controlled together. Manufacturers should establish standardized drying procedures with fixed parameters, timed testing, and dedicated management to reduce moisture-related defects.
Flexible adjustment based on product grade, material condition, and climate, combined with dehumidifying equipment and moisture testing, significantly improves consistency, lowers reject rates, and enables stable, efficient mass production.
