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Nylon is widely used in industry because of its excellent toughness, self-lubrication, wear resistance, chemical resistance, gas permeability, oil resistance, non-toxicity and easy coloring.

I. PROCESS CHARACTERISTICS OF NYLON

1. Rheological properties of nylon: Most of nylon is crystalline resin. When the temperature exceeds its melting point, its melt viscosity is small and its melt fluidity is excellent. It should be prevented from overflowing. At the same time, due to the rapid condensation speed of melt, it is necessary to prevent material blocking nozzles, runners, gates and other products from insufficient phenomenon. The overflow boundary value of the die is 0.03, and the melt viscosity is more sensitive to temperature and shear stress, but more sensitive to temperature. The first step to reduce the melt viscosity is the barrel temperature.

2. Water absorption and drying of nylon: Nylon has a larger water absorption. In the process of forming, wet nylon shows a sharp decline in viscosity and silver filament on the surface of products mixed with bubbles. The mechanical strength of the products decreases, so the materials must be dried before processing.

3. Crystallinity: Apart from transparent nylon, nylon is mostly crystalline polymer with high crystallinity. Its tensile strength, wear resistance, hardness, lubricity and other properties have been improved. Its thermal expansion coefficient and water absorption tend to decrease, but it is not conducive to transparency and impact resistance. Mold temperature has a great influence on crystallization, with high mould temperature, high crystallinity and low mould temperature bottom crystallinity.

4. Shrinkage: Like other crystalline plastics, nylon resin has a larger shrinkage rate. Generally, the shrinkage of nylon is related to crystallization. When the crystallinity of the product is high, the shrinkage of the product will increase. Reducing the temperature of the mould, increasing the injection pressure and lowering the material temperature will reduce the shrinkage, but the internal stress of the product will increase and be easily deformed. For example, the shrinkage rate of PA66 is 1.5-2% for products without additives. The shrinkage of PA6 ranged from 1% to 1.5%.

5. Molding equipment: When nylon is formed, the main attention should be paid to prevent the "salivation phenomenon of nozzles". Therefore, the self-locking nozzles are generally used in nylon material processing. In addition ** choose injection moulding machine with appropriate large plasticizing capacity.

6. Products and moulds: the length ratio of nylon is 150-200, and the wall thickness of nylon products is not less than 0.8 mm, which is generally chosen between 1-3.2 mm. The shrinkage of products is related to the wall thickness of products, and the thicker the wall thickness, the larger the shrinkage. The overflow value of nylon resin is about 0.03mm, so the outlet slot should be controlled below 0.025. It is difficult to shape the thin wall of the product or to control the temperature of the mould with high crystallinity, which requires the product to have certain flexibility. In general, cold water is used to control the temperature. The diameter of runner and gate should not be less than 0.5*t (where t is the thickness of plastic parts). With submerged gate, the minimum diameter of gate should be 0.75 mm.