Polyamide (nylon) is a crystalline plastic with a high and obvious melting point, a narrow melting temperature range, but high melt fluidity; volume change of raw material before and after melting is large, melt has poor thermal stability and is easy to decompose; product shrinkage rate after injection molding is large, but it has good wear resistance, flexibility and cold resistance.
Polyamide (nylon) injection molding can be done with a screw injection molding machine or a plunger injection molding machine.

 

Polyamide (nylon) commonly used varieties include PA6, PA66, PA610 and PA1010, etc., which have hydrophilic genes (amide groups) in their molecular structures and are easy to absorb moisture. Moisture in raw materials has a great influence on performance of resin and quality of product. Therefore, resin must be dried before injection molding. Since this resin is easily oxidized and discolored in high-temperature air, a vacuum oven should be used for its drying process. Drying process conditions are: vacuum degree 9.3*105Pa, temperature 90～110℃, material layer thickness should be less than 25mm, and drying time is 8～12h. If a general drying oven is used, temperature is 80～90℃ and time is 24h to prevent resin from oxidizing at high temperature. Moisture content of polyamide after drying should be less than 0.1%.
Floating dyeing method can be used for raw material color matching. After stirring and mixing evenly, it can be put into production.

Plasticizing temperature of polyamide resin in barrel will vary for different varieties and equipment: barrel temperature of screw injection molding machine is 10～30℃ higher than melting point of resin, and barrel temperature of plunger injection molding machine is 30～50℃ higher than melting point of resin. Note that process temperature fluctuation should be small and strictly controlled within required temperature range. Barrel temperature for plasticizing injection of different types of polyamide can be selected according to numerical reference in Table 6-11.
A screw with a higher compression ratio and a check ring on head should be selected to reduce injection leakage. To prevent casting phenomenon after nozzle injection, pay attention to use of a self-locking nozzle.

Injection pressure selection of different types of polyamide melts is also different. Specific injection pressure value can be selected according to numerical reference in Table 6-11.

Table 6-11 Several polyester amine injection molding process parameters

Temperature of molding mold has a great influence on physical and mechanical properties of polyamide products. When mold temperature is high, crystallinity of product is high, and its various properties increase accordingly; if mold temperature is low, various performance indicators also decrease. In order to obtain better comprehensive product performance, mold temperature should be appropriate, that is, neither too high nor too low. Such a more appropriate product cooling rate can reduce internal stress of product.
Commonly used temperature range of molding mold is 40~1OO℃. Large-walled products should be heated at a higher temperature, and vice versa, mold temperature should be lower.

Polyamide products are generally used as engineering parts, and structural dimensional accuracy of products is required to be high. Due to hygroscopic expansion and shrinkage of polyamide after drying, it is difficult to ensure structural dimensional accuracy of products. Therefore, molded products should be treated by heat treatment annealing to eliminate internal stress of products to reduce deformation of products. Treatment method is: put product in a medium oil or liquid paraffin at 100-120℃, heat it for 0.5h, and then slowly cool it to room temperature.

 

Polyamide is a hygroscopic plastic. Water can play a role similar to plasticizer in polyamide plastic, making product have better toughness, impact strength and tensile strength. After product is demolded, shape and structure of product are unstable due to its hygroscopic expansion. If product is subjected to humidity adjustment after demolding, moisture absorption requirements of product can be accelerated (in air with a relative humidity of 65%, moisture absorption should reach 4%), so that product can be stable in structure and size during application. Humidity control method can refer to data in Table 6-10.
Table 6-10 Heat treatment conditions for plastic products