Polyamide fiber commonly known as Nylon, the English name Polyamide(PA for short), the density of 1.15g/cm3, is the main chain of the molecule contains repeated amide groups. [NHCO]— A general term for thermoplastic resins, including aliphatic PA, fat — Aromatic PA and aromatic PA. Among them, aliphatic PA has many varieties, large yield and wide application. Its name is determined by the specific number of carbon atoms of the synthetic monomer. It was invented by the famous American chemist Carothers and his research team.

According to the number of carbon atoms of binary amine and dibasic acid, nylon synthesized from two monomers are: 46, 66, 610, 612, 613, 1010, 1313. According to the number of carbon atoms contained in the monomer, nylon is named: 4, 5, 6, 7, 8, 9, 11, 12, 13. Nylon has excellent toughness, self-lubricating, wear resistance, chemical resistance, gas permeability, oil resistance, non-toxic and easy coloring and other advantages, so nylon is widely used in industry. High impact strength (higher than ABS 'POm but lower than PC). Low thermal deformation temperature, large moisture absorption, poor dimensional stability. The most commonly used PA66 has the strongest structure in nylon materials, and PA6 has the best processing properties.

First, nylon process characteristics

1. Rheological properties of nylon: most of nylon is a crystalline resin, when the temperature exceeds its melting point, its melt viscosity is small, melt fluidity is excellent, should prevent the occurrence of overflow. At the same time, due to the fast condensation speed of the melt, the material should be prevented from blocking the nozzle, runner, gate, etc., resulting in insufficient product phenomenon. Mold overflow value 0.03, and melt viscosity is more sensitive to temperature and shear force changes, but more sensitive to temperature, reduce melt viscosity first from the barrel temperature.

2. The water absorption and drying of nylon: the water absorption of nylon is large, and the wet nylon in the molding process, the viscosity is shown to decline sharply and silver wire appears on the surface of mixed bubble products, and the mechanical strength of the resulting products decreases, so the material must be dried before processing.

3. Crystallinity: in addition to transparent nylon, nylon is mostly crystalline polymer, high crystallinity, products tensile strength, wear resistance, hardness, lubrication and other properties have been improved, coefficient of thermal expansion and water absorption tend to decline, but the transparency and impact resistance is unfavorable. Mold temperature has a greater influence on crystallization, mold temperature is high, crystallinity is high, mold temperature bottom crystallinity is low.

4 Shrinkage rate: similar to other crystalline plastics, there is a large shrinkage of nylon resin, the general shrinkage of nylon is the largest relationship with crystallization, when the product crystallinity is large product shrinkage will increase, in the molding process to reduce mold temperature increase injection pressure reduce material temperature will reduce shrinkage, but the product internal stress increase easy deformation. For example, PA66 shrinkage rate 1.5-2% for products without additives, PA6 shrinkage rate between 1% and 1.5%. The addition of glass fiber additives can reduce the shrinkage to 0.3%(but it is slightly higher in the direction perpendicular to the process).

5 molding equipment: nylon molding, mainly pay attention to prevent the "nozzle salivation phenomenon", so the processing of nylon material is generally selected self-locking nozzle. In addition, it is best to choose the injection molding machine with the appropriate plasticizing capacity.

6. Products and mold: the flow length ratio of nylon is between 150-200, the wall thickness of nylon products is not lower than 0.8mm, generally between 1-3.2mm, and the shrinkage of products is related to the wall thickness of products, the thicker the wall thickness, the greater the shrinkage. The overflow edge value of nylon resin is about 0.03mm, so the vent slot should be controlled below 0.025. The product wall is thin and difficult to form or requires high crystallinity of the mold temperature control, requiring products to have a certain degree of flexibility generally use cold water temperature control. Runner and gate gate aperture should not be less than 0.5*t(here t is the thickness of plastic parts). With the diving gate, the minimum diameter of the gate should be 0.75mm

Two, nylon molding process

1. Barrel temperature: Because nylon is a crystalline polymer, the melting point is obvious, and the barrel temperature selected by nylon resin in injection molding is related to the performance of the resin itself, equipment and shape factors of the product. Too high material temperature is easy to make the rubber parts appear color change, brittle quality and silver wire, and too low material temperature makes the material very hard and may damage the mold and screw. Generally, the melt temperature of nylon 6 is 210 бу C, and nylon 66 is 260 бу C. Due to the poor thermal stability of nylon, it is not appropriate to stay in the barrel at high temperature for a long time, so as not to cause the material to discolor and yellow, at the same time because of the good fluidity of nylon, the temperature exceeds its melting point after the rapid flow.

2. Injection pressure: the viscosity of nylon solution is low, the fluidity is good, but the condensation speed is faster, and the problem is easy to occur on the products with complex shape and thin wall thickness, so it still needs a higher injection pressure, especially with glass fiber. Usually the pressure is too high, the product will overflow problem; The pressure is too low, the product will produce ripples, bubbles, obvious fusion marks or insufficient products and other defects, most nylon varieties of injection pressure does not exceed 120MPA, generally in the range of 60-100MPA selection is to meet the requirements of most products, as long as the product does not appear bubbles, dents and other defects, generally do not want to use higher pressure holding, So as not to cause an increase in internal stress of the product.

3 Injection speed: for nylon, the injection speed is good for fast, which can prevent the ripple caused by the cooling speed is too slow and the problem of insufficient filling mold. The effect of fast injection speed on the performance of the product is not outstanding.

4. Mold temperature: mold temperature has a certain influence on crystallinity and molding shrinkage, high mold temperature crystallinity, wear resistance, hardness, elastic modulus increase, water absorption decrease, the molding shrinkage of products increase, suitable for thick products; Low mold temperature low crystallinity, good toughness, high elongation, shrinkage reduction, suitable for thin products with good transparency. If the wall thickness is greater than 3mm, it is recommended to use a low-temperature mold of 20 to 40C. For glass reinforcement materials, the mold temperature should be greater than 80C.

Three, nylon processing precautions

1. The use of recycled materials: it is best not to exceed three times, so as not to cause product discoloration or a sharp decline in mechanical and physical properties, the dosage should be controlled below 25%, too much will cause fluctuations in process conditions, recycled materials and new materials must be dried.

2. Safety information: nylon resin boot should first open the nozzle temperature, and then in the feeding cylinder heating, when the nozzle is blocked, do not face the spray hole, to prevent the solvent in the cylinder due to pressure accumulation and sudden release, dangerous.

3. The use of release agent: The use of a small amount of release agent sometimes has the effect of improving and eliminating bubbles and other defects. The release agent of nylon products can be selected zinc stearate and white oil, etc., can also be mixed into paste use, the amount must be small and uniform when used, so as not to cause surface defects of products.

4. In the shutdown to empty the screw, to prevent the next production, twist off the screw.

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