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The special properties of nylon make it widely used in automobiles, electronic and electrical equipment, mechanical structure, sports equipment, textile and so on. However, with the miniaturization of automobiles, the high performance of electronic and electrical equipment, and the acceleration of the process of lightweight mechanical equipment, the demand for nylon and its performance are gradually increasing. So the modification of nylon is very important.
pp wax for Modification of nylon
Matters needing attention in nylon modification
1. Setting of barrel temperature
(1) Because nylon is a crystalline polymer, its melting point is obvious. The barrel temperature of nylon resin in injection molding is related to the properties of resin, equipment and shape of product.
(2) Too high material temperature is easy to cause color change, brittleness and silver wire, while too low material temperature makes the material hard and may damage the die and screw.
(3) Generally, the lowest melt temperature of PA6 is 220 ℃ and PA66 is 250 ℃. Due to the poor thermal stability of nylon, it is not suitable to stay in the barrel for a long time at high temperature, so as not to cause discoloration and yellowing of materials. At the same time, due to the good fluidity of nylon, it flows rapidly when the temperature exceeds its melting point.
2. Mold temperature setting
(1) The mold temperature has a certain influence on the crystallinity and molding shrinkage. The mold temperature ranges from 80 ℃ to 120 ℃. High mold temperature, high crystallinity, increased wear resistance, hardness, modulus of elasticity, decreased water absorption, increased molding shrinkage, suitable for thick products;
(2) If the wall thickness is greater than 3 mm, it is recommended to use a low temperature mold with 20 ~ 40 ℃. For glass reinforced materials, the mold temperature should be higher than 80 ℃.
3. Wall thickness of products
The flow length ratio of nylon is 150-200, the wall thickness of the product is not less than 0.8mm, generally 1-3.2mm, and the shrinkage of the product is related to the wall thickness of the product. The thicker the wall thickness is, the greater the shrinkage is.
The overflow value of nylon resin is about 0.03mm, so the exhaust hole groove should be controlled below 0.025.
5. Runner and gate
The hole diameter of gate should not be less than 0.5T (t is the thickness of plastic part). With submerged gate, the minimum diameter of gate should be 0.75mm.
6. Glass fiber filling range
In the process of nylon molding, reducing the mold temperature, increasing the injection pressure and reducing the material temperature will reduce the shrinkage of nylon to a certain extent, increase the internal stress of the product and make it easy to deform. For example, the shrinkage of PA66 is 1.5% ～ 2%, the shrinkage of PA6 is 1% ～ 1.5%, and the shrinkage can be reduced to about 0.3% after adding glass fiber additive.
Practical experience tells us that the more glass fiber is added, the smaller the molding shrinkage of nylon resin. However, if the glass fiber is added too much, it will cause floating fiber on the surface, poor compatibility and other consequences. Generally, the effect of adding 30% is relatively good.
7. Use of recycled materials
It is better not to exceed three times to avoid the product discoloration or sharp decline of physical and mechanical properties. The application amount should be controlled below 25%, too much will cause the fluctuation of process conditions, and the mixture of recycled materials and new materials must be dried.
8. Safety instructions
When the nylon resin is started up, the temperature of the nozzle should be turned on first, and then the temperature should be heated in the feeding barrel. When the nozzle is blocked, never face the spray hole, so as to prevent the sudden release of the melt in the feeding barrel due to pressure accumulation, which may cause danger.
9. Application of release agent
The use of a small amount of mold release agent can sometimes improve and eliminate the bubble and other defects. The release agent of nylon products can be zinc stearate and white oil, or mixed into paste. The amount of release agent must be small and uniform to avoid surface defects. The screw should be emptied when the machine is shut down to prevent the screw from breaking during the next production.
10. Post treatment
(1) The products should be heat treated after forming
Common methods in mineral oil, glycerin, liquid paraffin and other high boiling point liquid, the heat treatment temperature should be 10 ~ 20 ℃ higher than the use temperature, and the treatment time depends on the wall thickness of the product. The thickness below 3 mm is 10 ~ 15 min, the thickness is 3 ~ 6 mm, and the time is 15 ~ 30 min. the product after heat treatment should be slowly cooled to room temperature, so as to prevent the sudden cooling from causing the re generation of stress in the product.
(2) The products should be treated with humidity control after molding
Humidity control is mainly used for products with high humidity. There are two methods: one is boiling water humidity control; The other is the wet process of potassium acetate aqueous solution (the ratio of potassium acetate to water is 1.25:1, boiling point 121 ℃).
Boiling water is simple, as long as the product is placed in the environment of 65% humidity, so that it can reach the equilibrium moisture absorption, but the time is long. The treatment temperature of potassium acetate aqueous solution is 80 ~ 100 ℃, and the treatment time mainly depends on the product wall thickness, when the wall thickness is 1.5mm, about 2h, 3mm, 8h, 6mm, 16 ~ 18h.
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