Summary of related knowledge of nylon (PA)

Nylon (PA) means a plastic composed of a polyamide resin. Such a resin can be obtained by polycondensation of a diamine and a dibasic acid, or by a ring-opening polymerization of a lactam formed by dehydration of an amino acid. Unlike PS, PE, PP, etc., PA does not gradually increase with the increase of the heating temperature. Softening, but softening in a narrow temperature range close to the melting point, the melting point is obvious, melting point: 215-225 ° C. Flow occurs as soon as the temperature is reached. There are many varieties of PA, mainly PA6, PA66, PA610, PA11, PA12, PA1010, PA612, PA46, PA6T, PA9T, MXD-6 aromatic guanamine, etc. PA6, PA66, PA610, PA11, PA12 are most commonly used.

Nylon engineering plastics appear as horny, tough, super bright, white (or milky white) or yellowish, transparent or translucent crystalline resin, which is easily colored into any color. The molecular weight of nylon as an engineering plastic is generally from 1.5 to 30,000. Their density is slightly greater than 1, and the density is 1.14-1.15g/cm3. Tensile strength: > 60.0 MPa. Elongation: > 30%. Bending strength: 90.0MPa. Notched impact strength: (kJ/m2) > 5.

The shrinkage of nylon is 1% to 2%. Pay attention to the dimensional change of moisture absorption after molding. Water absorption rate 100% Relative to moisture absorption, it can absorb 8%. The use temperature can be between -40 and 105 °C. Melting point: 215 to 225 °C. Suitable wall thickness 2～3.5mm. The mechanical properties of PA such as tensile compressive strength change with temperature and moisture absorption, so water is a plasticizer for PA. After adding glass fiber, the tensile strength can be improved. About 2 times, the temperature resistance is also improved accordingly. The wear resistance of PA itself is very high, so it can be operated without lubrication. If you want special lubrication effect, you can add sulfide in PA.

Second, the main advantages of PA performance

1. High mechanical strength, good toughness, high tensile and compressive strength. The specific tensile strength is higher than that of metal, and the compressive strength is comparable to that of metal, but its rigidity is not as good as that of metal. The tensile strength is close to the yield strength and is more than twice as high as ABS. It has strong absorption capacity for impact and stress vibration, and its impact strength is much higher than that of ordinary plastics, and it is superior to acetal resin.

2. The fatigue resistance is outstanding, and the original mechanical strength can be maintained after repeated bending of the workpiece. PA is often used in occasions where the periodic fatigue of the escalator handrails, new bicycle plastic rims, etc. are extremely obvious.

3. High softening point, heat resistance (such as nylon 46, etc., high crystalline nylon has high heat distortion temperature and can be used for a long time at 150 degrees. After PA66 is reinforced by glass fiber, its heat distortion temperature reaches 250 degrees or more).

4. The surface is smooth, the friction coefficient is small, and it is wear-resistant. It is self-lubricating when it is used as a moving mechanical component, and the noise is low. When the friction is not too high, it can be used without a lubricant. If it is necessary to use a lubricant to reduce friction or help heat dissipation, water oil, grease and the like can be selected. Therefore, as a transmission component, its service life is long.

5. Corrosion resistant, very resistant to alkali and most salt liquids, also resistant to weak acids, engine oils, gasoline, aromatic hydrocarbon-resistant compounds and general solvents, inert to aromatic compounds, but not resistant to strong acids and oxidants. It can resist the erosion of gasoline, oil, fat, alcohol, weak cockroaches, etc. and has good anti-aging ability. It can be used as packaging materials for lubricating oil, fuel, etc.

6. Self-extinguishing, non-toxic, odorless, weather resistant, inert to bio-erosion, good antibacterial and anti-mildew ability.

7. Excellent electrical properties. The electrical insulation is good, the volume resistance of nylon is very high, and the breakdown voltage is high. In the dry environment, it can be used as a power frequency insulation material, and has good electrical insulation even in a high humidity environment.

8. The parts are light in weight, easy to dye and easy to form. It can flow quickly due to its low melt viscosity. It is easy to mold, and has a high freezing point after filling, which can be quickly shaped, so the molding cycle is short and the production efficiency is high.

Third, the main shortcomings of PA performance

1. Easy to absorb water. The water absorption is large, and the saturated water can reach more than 3%. The degree of stability and electrical properties are affected to a certain extent, especially the thickening of the thin-walled parts is large; the water absorption also reduces the mechanical strength of the plastic. When selecting materials, the influence of the use environment and the accuracy of matching with other components should be considered. Fiber reinforcement reduces the water absorption of the resin, allowing it to work at high temperatures and high humidity. Nylon has a very good affinity with glass fiber. Commonly used in the production of combs, toothbrushes, clothes hooks, net bag ropes, fruit packaging bags and so on. Non-toxic, but not in contact with acid and alkali for a long time. It is worth noting that after adding glass fiber, the tensile strength of nylon can be increased by about 2 times, and the temperature resistance is also improved accordingly.

2. Lightfastness is poor. In the long-term high temperature environment, it will oxidize with oxygen in the air. At the beginning, the color will be brown, and the surface will be broken and cracked.

3. Injection molding technology is more stringent: the presence of trace moisture will cause great damage to the molding quality; the dimensional stability of the product is difficult to control due to thermal expansion; the presence of sharp corners in the product will lead to stress concentration and reduce mechanical strength; wall thickness If it is uneven, it will cause distortion and deformation of the workpiece; the precision of the equipment during processing is high.

4. It will absorb water and alcohol and swell. It is not resistant to strong acids and oxidants. It can not be used as acid-resistant materials.

Fourth, use

Nylon is mainly used in the automotive industry, electrical and electronic industry, transportation industry, machinery manufacturing industry, wire and cable communication industry, film and daily necessities. Manufacturing various bearings, gears, circular gears, cams, oil pipelines, oil reservoirs, protective covers, Support frame, wheel cover, deflector, fan, air filter housing, radiator water chamber, brake tube, engine cover, door handle. Bearing, gear, pulley pump impeller, blade, high pressure seal, pad, lining Sets, oil pipes, ropes, drive belts, grinding wheel adhesives, battery boxes, electrical coils, cable joints, various rollers, pulleys, fan blades, thrusters, nuts, oil-resistant gaskets, oil-resistant containers, casings, hoses, cable jackets , pulley sleeves, bullhead planer sliders, electromagnetic distribution valve seats, cold equipment, gaskets, various oil pipes, transmission belts for automobiles and tractors, zero fog materials for textile machinery industry equipment, and daily necessities and packaging films. There are also considerable yields for packaging tapes, food films (high temperature films for cooked foods, and low temperature films for refreshing beverages).

5. Forming and post-processing

In addition to transparent nylon, other nylons are crystalline plastics with a high melting point, a narrow melting temperature range, and poor thermal stability. PA is more hygroscopic, ** nylon in the molding process, the performance of the sharp drop in viscosity and the presence of silver wire on the surface of the bubble product, the mechanical strength of the resulting product is reduced, so the material must be dried before processing, can be dried at 80 ~ 110 ° C For 6 hours, the water content of the nylon 6 and nylon 66 is 0.1%, the nylon 11 is 0.15%, the nylon 610 is 0.1% to 0.15%, and the maximum is 0.2%. Note that PA plastics tend to cause discoloration when dried above 90 °C.

PA has good fluidity and is easy to overflow. It is advisable to use self-locking nozzles and should be heated. At the same time, due to the fast condensation speed of the solution, it should prevent the material from clogging nozzles, runners, gates, etc., causing insufficient products. The mold overflow value is 0.03, and the melt viscosity is sensitive to changes in temperature and shear force, but is more sensitive to temperature. Lower the melt viscosity first from the barrel temperature. The molding shrinkage range and shrinkage rate are large, the directivity is obvious, and shrinkage and deformation are likely to occur. The use of PA recycled materials should not exceed three times, so as to avoid discoloration of the products or a sharp drop in mechanical physical properties. The application amount should be controlled below 25%. Too much will cause fluctuations in process conditions. The mixed materials must be dried when mixed with new materials. .

When starting the machine, the nozzle temperature should be turned on first, then the barrel should be warmed. When the nozzle is blocked, avoid facing the nozzle hole to prevent the solution in the barrel from being suddenly released due to pressure accumulation, which is dangerous. Empty the screw during shutdown to prevent the screw from breaking during the next production. The use of a small amount of release agent sometimes has an effect of improving and eliminating defects such as bubbles. The release agent of the nylon product may be selected from zinc stearate and white oil, or may be mixed into a paste, and the amount of the release agent may be small and uniform during use to avoid surface defects of the product.

The post-treatment of the nylon article is to prevent and eliminate residual stress in the article or dimensional changes caused by moisture absorption. The post-treatment methods include heat treatment and humidity control.

a). Common methods of heat treatment In mineral oil, glycerin, liquid paraffin and other high boiling liquids, the heat treatment temperature should be higher than the use temperature of 10 ~ 20 ° C, the treatment time varies depending on the wall thickness of the product, the thickness is less than 3mm for 10 ~ 15 minutes The thickness is 3 to 6 mm for 15 to 30 minutes. The heat-treated product should be slowly cooled to room temperature to prevent quenching and cause stress re-generation in the product.

b). Humidity control The humidity control treatment is mainly carried out on products with high environmental humidity. There are two methods: one boiling water conditioning method, and the potassium diacetate aqueous solution humidity control method (the ratio of potassium acetate to water is 1.25:1, boiling point 121 ° C), the boiling water conditioning method is simple, as long as the product is placed in a humidity of 65% environment, so that it can reach the equilibrium moisture absorption, but the time is longer, and the potassium acetate aqueous solution is humidified The treatment temperature is 80-100 ° C potassium acetate aqueous solution humidity control method, the treatment time mainly depends on the wall thickness of the product, when the wall thickness is 1.5mm, about 2 hours, 3mm is 8 hours, 6mm is 16-18 hours.