Views: 4 Author: Site Editor Publish Time: 2023-12-05 Origin: Site
The special properties of nylon make it widely used in automobiles, electronic and electrical equipment, mechanical structures, sports equipment, textiles, and other fields. But with the miniaturization of automobiles, the high-performance of electronic and electrical equipment, and the acceleration of lightweight mechanical equipment, the demand and performance requirements for nylon are gradually increasing. Therefore, the modification of nylon is particularly important.
Notes on Nylon Modification
1. Barrel temperature setting
(1) Because nylon is a crystalline polymer, its melting point is obvious. The temperature of the barrel selected for nylon resin injection molding is related to the performance of the resin itself, equipment, and shape factors of the product.
(2) Excessive material temperature can easily cause discoloration, brittleness, and silver threads in the rubber parts, while too low material temperature can make the material very hard and potentially damage the mold and screw.
(3) The minimum melt temperature of PA6 is generally 220 ℃, while PA66 is 250 ℃. Due to the poor thermal stability of nylon, it is not advisable to stay in the material barrel at high temperatures for a long time to avoid causing discoloration and yellowing of the material. At the same time, due to the good fluidity of nylon, it flows quickly when the temperature exceeds its melting point.
2. Setting of mold temperature
(1) The mold temperature has a certain impact on crystallinity and molding shrinkage, with a range of mold temperature ranging from 80 to 120 ℃.
Choose high mold temperature, high crystallinity, increased wear resistance, hardness, and elastic modulus, decreased water absorption, and increased molding shrinkage of the product, suitable for thick products;
Choose low mold temperature, low crystallinity, good toughness, high elongation, and reduced shrinkage, suitable for thin products with good transparency.
(2) If the wall thickness is greater than 3mm, it is recommended to use a low-temperature mold at 20-40 ℃. For glass reinforced material molds, the temperature should be greater than 80 ℃.
(3) For molds with thin walls that are difficult to shape or require high crystallinity for heating control, and those that require certain flexibility, cold water temperature control is generally used.
3. Wall thickness of the product
The flow length ratio of nylon is 150-200, and the wall thickness of the product is not less than 0.8mm. It is generally selected between 1-3.2mm, and the shrinkage of the product is related to the wall thickness. The thicker the wall thickness, the greater the shrinkage.
4. Exhaust
The overflow value of nylon resin is about 0.03mm, so the exhaust hole slot should be controlled below 0.025.
5. Runners and Gates
The gate aperture should not be less than 0.5t (t represents the thickness of the plastic part). Using a submerged gate, the minimum diameter of the gate should be 0.75mm.
6. Glass fiber filling range
During the molding process of nylon, reducing the mold temperature, increasing injection pressure, and reducing material temperature will to some extent reduce the shrinkage rate of nylon, making the product more susceptible to deformation due to increased internal stress. For example, the shrinkage rate of PA66 ranges from 1.5% to 2%, and the shrinkage rate of PA6 ranges from 1% to 1.5%. After adding glass fiber additives, the shrinkage rate can be reduced to around 0.3%.
Practical experience tells us that the more glass fiber is added, the smaller the molding shrinkage of nylon resin. However, adding too much fiberglass can cause surface float and poor compatibility, and generally adding 30% is relatively effective.
7. Use of recycled materials
It is best not to exceed three times to avoid discoloration or sharp decline in physical and mechanical properties of the product. The application amount should be controlled below 25%, as excessive use can cause fluctuations in process conditions. The mixture of recycled and new materials must be dried.
8. Safety instructions
When starting nylon resin, the nozzle temperature should be turned on first, and then the feeding tube should be heated. When the nozzle is blocked, it is important to avoid facing the spray hole to prevent the melt inside the tube from suddenly releasing due to pressure accumulation, which may cause danger.
9. Use of release agent
The use of a small amount of release agent can sometimes improve and eliminate defects such as bubbles. The release agent for nylon products can be zinc stearate and white oil, or it can be mixed into a paste and used in a small and uniform amount to avoid surface defects of the product.
Empty the screw during shutdown to prevent it from being twisted and broken during the next production.
10. Post processing
(1) After the product is formed, it needs to undergo heat treatment
The commonly used method is in high boiling liquids such as mineral oil, glycerol, and liquid paraffin. The heat treatment temperature should be 10-20 ℃ higher than the use temperature, and the treatment time varies depending on the wall thickness of the product. For thicknesses below 3mm, it takes 10-15 minutes, and for thicknesses between 3-6mm, it takes 15-30 minutes. After heat treatment, the product should be slowly cooled to room temperature to prevent stress regeneration in the product caused by sudden cooling.
(2) After the product is formed, it needs to undergo humidity adjustment treatment
Humidity control treatment is mainly carried out on products with high environmental humidity, and there are two methods: one is boiling water humidity control; The second is the wet process of potassium acetate aqueous solution (the ratio of potassium acetate to water is 1.25:1, and the boiling point is 121 ℃).
Boiling water humidification is simple, as long as the product is placed in an environment with a humidity of 65% to achieve equilibrium moisture absorption, but the time is longer. The treatment temperature of potassium acetate aqueous solution humidification is 80-100 ℃, and the treatment time mainly depends on the wall thickness of the product. When the wall thickness is 1.5mm, it is about 2 hours, 3mm is about 8 hours, and 6mm is 16-18 hours.
Example Analysis
1. Glass fiber reinforced nylon
Adding 30% glass fiber to nylon significantly improves its mechanical properties, dimensional stability, heat resistance, and aging resistance, with a fatigue strength 2.5 times that of unreinforced nylon. The molding process of glass fiber reinforced PA is roughly the same as that of unreinforced PA, but due to poor flow compared to before reinforcement, the injection pressure and injection speed should be appropriately increased, and the barrel temperature should be increased by 10 to 40 ℃.
Due to the orientation of glass fibers along the flow direction during the injection molding process, the mechanical properties and shrinkage rate increase in the orientation direction, leading to deformation and warping of the product. Therefore, when designing the mold, the position and shape of the gate should be reasonable, and the temperature of the mold can be increased in the process. After the product is taken out, it should be slowly cooled in hot water. In addition, the larger the proportion of fiberglass added, the greater the wear on the plasticizing components of the injection molding machine. It is best to use bimetallic screws and machine barrels.
2. Flame retardant nylon
Due to the addition of flame retardants to nylon, most flame retardants are prone to decomposition at high temperatures, releasing acidic substances that have a corrosive effect on metals. Therefore, plasticizing components (screws, rubber heads, rubber rings, rubber washers, flanges, etc.) need to be plated with hard chromium. In terms of technology, try to control the temperature of the machine barrel as much as possible and the injection speed not too fast to avoid discoloration and mechanical performance degradation of the product caused by the decomposition of the rubber material due to high temperature.
3. Transparent nylon
It has good tensile strength, impact resistance, rigidity, wear resistance, chemical resistance, surface hardness, and other properties. It has a high transmittance, similar to optical glass, and a processing temperature of 300-315 ℃. When forming and processing, it is necessary to strictly control the barrel temperature. If the melt temperature is too high, it will cause discoloration of the product due to degradation, and if the temperature is too low, it will affect the transparency of the product due to poor plasticization. Try to keep the mold temperature as low as possible, as high mold temperatures can reduce the transparency of the product due to crystallization.
4. Weather resistant nylon
Adding additives such as carbon black that absorb ultraviolet rays to nylon greatly enhances its self-lubrication and metal wear, which can affect the cutting and wear of machine parts during molding and processing.
Therefore, it is necessary to use a combination of screws, barrels, rubber heads, rubber rings, and rubber washers with strong feeding capacity and high wear resistance.
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