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What are the precautions for injection molded nylon plastic parts
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Nylon plastic parts have many uses, good toughness and wear resistance. For example, there are many nylon wheels on common conveyors, industrial nylon tubes, nylon gears, etc. What are the precautions for re- injection nylon parts? Let's talk with you today.
Polyamide fiber is commonly known as nylon (Nylon), English name Polyamide (abbreviated as PA), density 1.15g / cm3, is a general term for thermoplastic resins containing repeating amide groups— [NHCO] — on the main chain of the molecule, including aliphatic PA, fat— Aromatic PA and aromatic PA. Among them, there are many types of aliphatic PAs, large yields and wide applications, and their names are determined by the specific number of carbon atoms in the synthetic monomers. Invented by the famous American chemist Carothers and his research team.
According to the number of carbon atoms of diamines and dibasic acids, nylons synthesized from two monomers are: 46, 66, 610, 612, 613, 1010, 1313. Named according to the number of carbon atoms contained in the monomer: nylon 4, 5, 6, 7, 8, 9, 11, 12, 13 Nylon has the advantages of excellent toughness, self-lubrication, abrasion resistance, chemical resistance, gas permeability, oil resistance, non-toxicity and easy coloring, so nylon is widely used in industry. High impact strength (higher than ABS` POM but lower than PC). Low heat distortion temperature, high hygroscopicity, and poor dimensional stability. The most commonly used PA66 has the strongest structure among nylon materials, and PA6 has the best processing properties.
First, the technical 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 prevent the occurrence of overflow. At the same time, due to the rapid condensation of the melt, materials should be prevented from blocking the nozzles, runners, gates, etc. due to insufficient products. The mold overflow value is 0.03, and the melt viscosity is more sensitive to changes in temperature and shear force, but more sensitive to temperature. To reduce the melt viscosity, start with the barrel temperature.
2. Water absorption and drying of nylon: Nylon has a large water absorption. During the molding of wet nylon, the viscosity decreases sharply and silver threads appear on the surface of the mixed bubble product. The mechanical strength of the obtained product decreases, so the material must be processed before processing. dry.
3. Crystallinity: Except for transparent nylon, nylon is mostly a crystalline polymer with high crystallinity. The tensile strength, abrasion resistance, hardness, and lubricity of the product have been improved. The coefficient of thermal expansion and water absorption tend to decrease. , But it is not good for transparency and impact resistance. The mold temperature has a greater effect on crystallization. The higher the mold temperature is, the higher the crystallinity is, and the lower the mold temperature is, the lower the crystallinity is.
4. Shrinkage: Similar to other crystalline plastics, nylon resins have a large shrinkage problem. Generally, the shrinkage of nylon has the greatest relationship with crystallization. When the crystallinity of the product is large, the shrinkage of the product will increase, and the mold temperature will be reduced during the molding process. Increasing the injection pressure and reducing the material temperature will reduce the shrinkage, but the internal stress of the product will increase and deform easily. For example, the shrinkage of PA66 is 1.5-2%. For products without additives, the shrinkage of PA6 is between 1% and 1.5%. Adding glass fiber additives can reduce the shrinkage to 0.3% (but the direction perpendicular to the process is slightly higher).
5. Molding equipment: When nylon is molded, the main attention should be paid to preventing the "drooling phenomenon of nozzles". Therefore, self-locking nozzles are generally used for the processing of nylon materials. In addition, it is best to choose an injection molding machine with a relatively large plasticizing capacity.
6. Products and molds: The flow length ratio of nylon is between 150-200, and the wall thickness of nylon products is not less than 0.8mm. Generally, the choice is between 1-3.2mm, and the shrinkage of products is related to the wall thickness of the products. The thicker the thicker the greater the shrinkage. The overflow value of nylon resin is about 0.03mm, so the vent hole groove should be controlled below 0.025. Products with thin walls that are difficult to form or require high crystallinity mold heating control, cold water temperature control is generally used for products that require certain flexibility. The diameter of runner and gate should not be less than 0.5 * t (where t is the thickness of the plastic part). With submerged gate, the minimum diameter of the gate should be 0.75mm
Second, the molding process of nylon
1. Barrel temperature: Because nylon is a crystalline polymer, its melting point is obvious. The barrel temperature selected for nylon resins during injection molding is related to the properties of the resin itself, equipment, and shape of the product. Too high material temperature is easy to cause discoloration, brittleness and silver wire of rubber parts, while too low material temperature makes the material very hard and may damage the mold and screw. Generally, the minimum melt temperature of nylon 6 is 210 ° C, and nylon 66 is 260 ° C. Due to the poor thermal stability of nylon, it is not appropriate to stay in the barrel for a long time at high temperature to avoid discoloring and yellowing of the material. At the same time, because nylon has good fluidity, it will flow quickly after the temperature exceeds its melting point.
2. Injection pressure: Nylon solution has low viscosity and good fluidity, but has a fast condensation speed, which is prone to insufficient problems on products with complex shapes and thin walls. Therefore, higher injection pressure is still needed, especially when added. glassfiber. Generally, the pressure is too high, and the product will have flashing problems; if the pressure is too low, the product will have defects such as ripples, bubbles, obvious weld marks, or insufficient products. The injection pressure of most nylon varieties does not exceed 120MPA, generally 60-100MPA The selection within the range is to meet the requirements of most products. As long as the product does not show defects such as bubbles and dents, it is generally not desirable to use a higher holding pressure to avoid increasing the internal stress of the product.
3. Injection speed: For nylon, the injection speed is fast, which can prevent ripples caused by too slow cooling speed and insufficient filling. The effect of fast injection speed on the performance of the product is not significant.
4. Mold temperature: Mold temperature has a certain effect on crystallinity and molding shrinkage. High mold temperature has high crystallinity, increased abrasion resistance, hardness, elastic modulus, decreased water absorption, increased molding shrinkage of products, suitable for thick Products; low mold temperature, low crystallinity, good toughness, high elongation and reduced shrinkage, 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 ~ 40C. For glass reinforced materials the mold temperature should be greater than 80C.
Third, nylon processing matters needing attention
1. The use of recycled materials: it is best not to exceed three times, so as not to cause discoloration of the product or a sharp decline in mechanical and physical properties. The application amount should be controlled below 25%. Too much will cause fluctuations in process conditions. The mixing of recycled materials with new materials must be Allow to dry.
2. Safety instructions: When the nylon resin is turned on, the nozzle temperature should be turned on first, and then the barrel should be heated. When the nozzle is blocked, do not face the spray hole to prevent the solution in the barrel from being suddenly released due to pressure accumulation. Danger.
3. Use of release agent: Using a small amount of release agent can sometimes improve and eliminate defects such as bubbles. The release agent for nylon products can be selected from zinc stearate and white oil, and can also be mixed into a paste. The amount must be small and uniform during use to avoid causing surface defects on the product.
4. Empty the screw during shutdown to prevent the screw from being broken during the next production.