common defects analysis and solutions of PPS injection molded products

Polyphenylene sulfide (PPS) is a kind of thermoplastic engineering plastics with phenylene sulfide group in the main chain

polyphenylene sulfide has excellent heat resistance and long-term service temperature of 200 ℃; Good resistance to chemicals, with excellent chemical properties similar to polytetrafluoroethylene; 1t also has special rigidity and good blending with various fillers and other polymer materials. At present, it is the lowest price in high temperature engineering plastics and can be molded by general thermoplastic processing methods

polyphenylene sulfide (PPS) is a linear polymer compound composed of benzene ring and sulfur atom alternately. Due to the rigid structure of benzene ring and the connection of soft sulfide bond, PPS has excellent heat resistance, flame retardancy, medium resistance and good affinity with other inorganic fillers. However, the tensile strength and bending strength of unmodified PPS are only in the middle level, and the elongation and impact strength are also low. Therefore, glass fiber and other inorganic fillers are often used to enhance the filling modification of PPS, so as to further improve the physical and mechanical properties while maintaining the heat resistance, flame retardancy and medium resistance

several common product defects are analyzed and discussed

1. The product has low temperature resistance, discoloration, blackening, yellow lines and black spots

due to its good blending property and excellent rigidity, the rigidity is further improved by adding glass fiber. The heat resistance of PPS is very good. Generally, the temperature of injection barrel is set at 280 ~ 340 ℃, and the temperature resistance of products produced is above 200 ℃ for long-term use. But why does discoloration often occur in the production of some electrical products

this is because the market competition is fierce now. 1n order to reduce the production cost, some manufacturers use recycled materials (recycled materials) when producing medium and low-grade products, add some other materials, and add flame retardants, fillers and other mixing materials. Because of the complex flow of these materials, and the high requirements of plasticization, it is difficult to control the process, so there are such and such problems

the melting point of PPS (282-285 ℃) is too different from the melting temperature of the added materials, resulting in burn, gasification, carbonization, color change, blackening, yellow lines and black spots< 1n view of the above phenomenon, we need to consider and find solutions from the following aspects: 1. 1f there is discoloration phenomenon in the production process, we need to first check whether there are problems with the materials, such as whether there are quality problems with the new materials doped with other materials and foreign matters, whether the recycled materials are qualified, and whether the formula of the mixing materials is correct, After eliminating one by one, check other reasons2. Process conditions: the melting temperature is mainly considered. Generally, the barrel temperature should be reduced step by step, especially the temperature of the first two stages, and different temperatures should be used for different materials, such as nylon blending modified polyphenylene sulfide; The impact strength can be improved significantly. Although the melting temperature and thermal decomposition temperature of PPS and nylon are quite different, and their affinity is not ideal, they can produce very good mixing effect at higher temperature. The temperature from barrel to nozzle is controlled by stages, which are 260 ℃, 280 ℃, 300 ℃ and 310 ℃ respectively PC polycarbonate, PPO polyphenylene oxide, PTFE polytetrafluoroethylene, P1 polyimide and other blending modified polyphenylene sulfide materials have different barrel heating temperatures. Of course, the final selection of molding temperature should also consider the shape, size, mold structure, product performance requirements and so on in addition, if the screw speed is too fast, the back pressure is too high, the injection rate is too fast, and the nozzle aperture, runner and gate size are too small, the melt will produce high shear heat, which will cause the PPS melt fracture, and the gas in the mold cavity can not be discharged in time, which will cause the local burn and blackening of the product 3. Material usage and operation method: if black spots are found as soon as the machine is started, it is mostly related to the storage of material in the barrel. Therefore, attention should be paid to the operation method. When the material stored in the barrel is PPS before the machine is started, the barrel should be cleaned 3-4 times with new material at the molding temperature (air injection). 1f the stored materials are other materials, especially those with poor thermal stability, such as PVC, POM, etc., it is required that the temperature should not be raised when the machine is started up, and PPS should not be used to clean the barrel, and only materials with good thermal stability, such as PS, PE, etc., can be used for cleaning at a lower temperature. After cleaning, raise the barrel temperature to PPS normal processing temperature, and then clean it with PPS material before processing. 1n the process, if the production needs to be suspended temporarily, the barrel temperature should be reduced to below 280 ℃ for heat preservation (because the melting temperature of PPS is 280 ℃), so as to avoid discoloration due to decomposition of materials for a long time< Second, the surface quality of the parts is poor, the finish is poor, and the glass fiber is exposed as polyphenylene sulfide (PPS) is a crystalline polymer compound, mold temperature has a significant effect on the properties of PPS parts the mechanical strength and the heat resistance of the material decrease because of the insufficient crystallization due to sudden cooling with the increase of mold temperature, the crystallinity and rigidity of the parts increase the crystallization of PPS injection molded parts is affected by the melt cooling rate. With the rapid cooling rate of the melt, the increase of PPS viscosity leads to the decrease of the activity of the chain segments, the decrease of the probability of the chain segment rearrangement jumping into the lattice, the limitation of the crystallization and even the low crystallinity; When the melt cooling rate is slow, there is time for crystallization, and the crystallization expands to a large extent. With the increase of mold temperature, the crystallinity of the product increases 1. Under normal circumstances, the mold temperature should be controlled above 120 ℃. Why should the mold be heated because the crystallinity of PPS parts formed at 38 ℃ is less than 5%, it can be said that although the parts are basically formed, the internal structure of the parts has not yet met the requirements. 1n order to improve the crystallinity of parts, it is necessary to carry out post-treatment. After heat treatment at 204 ℃ for 30 min, the crystallinity can be increased to 60%. Therefore, in the injection molding process rules, the process heat treatment: the products are required to be treated within 48 hours. Put the product into the oven, the thickness of which is not more than three times that of the product. Within one hour, the room temperature rises to 200 ℃ and remains for two hours (when the parts are relatively large, the time is delayed). Cut off the power supply and cool with the oven until the room temperature, and take out the product. But to explain the mold temperature is low, forming products the surface finish is not good enough to form sub gloss. The parts with low surface finish can basically meet the requirements(Note: for parts with high surface finish, the mold temperature should be controlled above 120 ℃)

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