Processing technology characteristics of polyamides

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Processing characteristics of polyamides (1) The rheology of polyamides in the molten state is nearly Newtonian; in other words, the viscosity of the polymer melt is not sensitive to shear rate (pressure), but is primarily dependent on temperature. (2) Polyamides have high water absorption. It is prone to oxidation and discoloration at high temperatures (it may cross-link, leading to a decline in performance). Therefore, the granules must be dried before processing, preferably by vacuum drying, as this method is fast and helps to prevent oxidation. (3) Polyamide is a crystalline polymer with a high melting point and a relatively narrow melting temperature range of about 10°C; hence, during injection molding, the nozzle must be kept warm to avoid blockages. In the molten state it is amorphous; as it cools slightly, crystallization occurs, which has a significant impact on its properties. The cooling rate is related to the mold temperature. Therefore, its injection mold should preferably have heating and cooling devices. (4) The polyamide melt has low viscosity. It has good fluidity, rapid cooling and hardening, allowing for fast molding; it is suitable for producing thin-walled, complex parts with a minimum wall thickness of 0.45 millimeters, but it tends to cause \"slagging\" during injection. Therefore, the nozzle should be of the spring needle valve type to prevent material leakage. At the same time, the mold should be precisely machined to prevent flash, and injection molds should have cold slug wells. The draft angle should be 20–40’ for the cavity and 25–40’ for the core. (5) Polyamides have poor stability in the molten state and are prone to degradation, which reduces their performance (with the exception of nylon 9). Therefore, it is not allowed to remain at high temperatures for too long. Additionally, during injection molding, the barrel temperature should be set slightly above the melting point. (6) Polyamides exhibit a high shrinkage rate after molding, generally ranging from 2 to 3%. When manufacturing parts with precise dimensions, several trial productions are necessary, during which the dimensions of the prototypes are measured to adjust the mold. Additionally, sufficient guarantee is provided for the cooling time.