Nylon is a widely used engineering plastic in the injection molding industry, featuring excellent toughness, wear resistance and mechanical strength. However, it has an obvious disadvantage of strong hygroscopicity. Nylon pellets tend to absorb moisture from the air during storage, transportation and feeding. If the materials are not fully dried prior to injection molding, the absorbed water will vaporize rapidly under high temperature during plasticization. This will cause a series of product defects such as silver streaks, bubbles, surface pits and cracks, and lead to molecular degradation of the material, which reduces the tensile strength, toughness and fatigue resistance of finished products. Therefore, drying is an indispensable and critical pre-process for nylon injection molding. Standard operating procedures must be strictly implemented to guarantee the appearance quality and structural performance of products.
Standard Drying Parameters for Different Types of NylonNylon covers various categories, and materials with different compositions and modified formulas differ greatly in temperature resistance and water absorption capacity. Classified drying is required instead of unified heating. Pure PA6 absorbs moisture quickly. Its standard drying temperature is controlled between 80℃ and 90℃ with a conventional drying duration of 4 to 6 hours. For materials that have been stored for a long time or severely dampened, the drying time should be extended to more than 8 hours. Pure PA66 has higher crystallinity and molding temperature, which puts forward stricter requirements on drying. The suitable temperature ranges from 90℃ to 100℃, and the standard drying period is 6 to 8 hours to completely remove bound moisture inside pellets.
Glass fiber reinforced nylon is one of the most common modified materials. More internal pores make it easier to retain moisture. The drying temperature is set at 90℃ to 105℃ for 4 to 6 hours. Overheating is prohibited to prevent the coupling agent on glass fiber surface from failing, which would result in brittle products and surface peeling. Flame-retardant and toughened modified nylon has poor heat resistance, so the drying temperature shall be lowered to 75℃ to 85℃ to avoid decomposition and carbonization of functional additives. PA12 has relatively low water absorption, and can meet production requirements after being dried at 80℃ to 85℃ for 3 to 4 hours.
Selection Requirements of Drying EquipmentAppropriate drying equipment shall be selected according to production scenarios to ensure stable drying quality. Dehumidifying dryers are the first choice for nylon processing. They adopt closed circulation of hot air with low dew point, which effectively prevents secondary moisture absorption of materials. This type of equipment is applicable to mass production and manufacturing of high-precision structural parts with uniform and stable drying effect. Vacuum dryers are suitable for small-batch production of high-precision and heat-sensitive nylon products. Under negative pressure environment, moisture evaporates at a low temperature, which maximally protects the inherent properties of materials.
Ordinary hot air ovens are only used for sample making and trial production. Affected by ambient humidity, open hot air cannot thoroughly remove deep-bound moisture from materials, so such equipment is not allowed for mass production. No matter which device is adopted, operators shall clean residual carbonized materials and dust in hoppers, filter screens and air ducts before startup, to prevent impurities from mixing into raw materials and causing product defects.
Key Control Points for On-site Drying ProcessMoisture content is the core index to judge drying quality. The moisture content of ordinary nylon products shall be controlled below 0.2%, while that of high-precision structural parts shall be less than 0.05%. For high-end precision and medical-grade products, the standard is set below 0.02%. The loading capacity of material hoppers shall not exceed 70% of the total volume, and the material layer shall be kept with uniform thickness to ensure hot air penetrates thoroughly and avoid the problem of outer layer dried while inner layer damp.
After drying, materials shall be kept at a constant temperature of 70℃ to 80℃ to prevent secondary moisture absorption when exposed to air. In rainy seasons, coastal areas and other high-humidity environments, dried materials shall not be exposed to the air for more than 30 minutes, and re-drying is required if the time limit is exceeded. Recycled sprue and runner scraps have loose structure and faster water absorption. When mixed with new materials, they need longer drying time, and more frequent sampling inspection shall be arranged.
Common Misunderstandings and Process Taboos
Many quality problems in production result from non-standard drying operations. Some operators adopt high temperature and short-time drying to improve production efficiency. Temperature above 110℃ will cause nylon discoloration, molecular degradation and toughness reduction, leading to brittle finished products. Excessively long drying is another common mistake. Drying for more than 12 hours will accelerate material aging and weaken physical properties, shortening the service life of products.
In addition, storing dried materials in open air is a wrong operation. Heated nylon materials have high activity and absorb moisture rapidly when contacting humid air, making previous drying ineffective. Mixing different types of nylon in one dryer is also forbidden, as mismatched temperature parameters will lead to incomplete drying or material aging.
ConclusionThe stability of nylon injection molding quality is largely determined by standardized drying processes. Due to the high hygroscopicity of nylon, production must follow the principles of low temperature, long-time drying, closed operation and moisture prevention. By setting targeted temperature and duration for different nylon materials, selecting suitable drying equipment, strictly controlling moisture content, loading volume, heat preservation and exposure time, and eliminating irregular operations, enterprises can fundamentally reduce batch defects such as silver streaks, bubbles and cracks. Standardized drying management improves product appearance and yield, stabilizes physical properties of materials, cuts down scrap rate and production cost, and supports efficient and stable operation of injection molding lines.
