Polyphenylene sulfide, the full name of polyphenylene sulfide, the English name polyphenylene Sulfide (abbreviation: PPS), is a thermoplastic engineering plastic with a phenylthio group in the main chain.
Polyphenylene sulfide is excellent in heat resistance, long-term use temperature is 200°C; chemical resistance is good, it has excellent chemical properties similar to PTFE; also has special rigidity, good with various fillers and other polymer materials The blending. At present, it is a kind of high-temperature resistant engineering plastic that has the lowest price and can be formed by general thermoplastic processing methods.
Polyphenylene sulfide is a linear polymer compound in which benzene rings and sulfur atoms are alternately arranged. Since the phenyl ether rings have a rigid thioether bond, they have excellent heat resistance, flame retardancy, and resistance. Medium and good affinity with other inorganic fillers. However, the tensile strength and bending strength of the unmodified polyphenylene sulfide are only moderate, and the elongation and impact strength are also low. Therefore, polyphenylene sulfide often uses glass fibers and other inorganic fillers to enhance the filling and modification, so as to further improve physical and mechanical properties while maintaining heat resistance, flame retardancy, and resistance to media.
Now we analyze and discuss the defects of several common products.
First, the product is low temperature resistance, discoloration, appear black, yellow lines and black spots
Due to the good blending of polyphenylene sulfide and its excellent rigidity, its rigid properties are further improved by adding glass fiber. PPS heat resistance is very good, usually injection cylinder temperature is set at 280 ~ 340 °C, the production of the product temperature is above 200 °C long-term use temperature, why in the production of some electrical products, often appear discoloration phenomenon ?
This is due to the fierce market competition. In order to reduce production costs, some manufacturers use recycled materials (recycled materials) in the production of medium and low-grade products, add some other materials, and add flame-retardants, fillers, and other compounding materials. Due to the complicated flow of these items, the plasticization requirements are relatively high and the control of the process is difficult, resulting in such problems.
PPS melting point (282 ~ 285 °C) added material melting temperature difference is too large, resulting in added material burns, gasification, carbonization, color discoloration, black and yellow spots appear.
In view of the above phenomenon, it is necessary to consider and find solutions from the following aspects:
1. If there has been discoloration in the production process, it is necessary to first check whether there is any problem with the use of materials, such as whether there are quality problems with new materials doped with other materials and foreign materials, whether the used recycled materials are qualified, and whether the compound is formulated Matching method is correct, etc., after exclusion, check other reasons.
2. Process conditions :
The main consideration is the melting temperature. Generally, the cylinder temperature should be reduced segment by segment, especially the temperature of the first two stages, and different temperatures should be used for different materials, such as nylon blend modified polyphenylene sulfide; the impact strength can be significantly improved. Although the melting temperature and the thermal decomposition temperature of polyphenylene sulfide and nylon are very different, the affinity is not ideal, but they can produce a very good melt-mixing effect at higher temperatures. The temperature was controlled from the barrel to the nozzle in sections of 260°C, 280°C, 300°C, and 310°C, respectively.
PC polycarbonate, PPO polyphenylene ether, PTFE polytetrafluoroethylene, PI polyimide, etc. Blend modified polyphenylene sulfide materials of various specifications, barrel heating temperature are different. Of course, the final selection of molding temperature must also consider the aspects of product shape, size, mold structure, and product performance requirements.
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, gate size is too small, etc. will cause the melt to produce high shear heat, causing PPS melt fracture phenomenon, And easy to make the gas in the cavity can not be discharged in time, causing local burns and black products.
3. Materials, methods of operation:
If a black spot is found on the boot, this is mostly related to the stock of the cartridge, so we must pay attention to the method of operation, when the stock before the boot is PPS, use a new material at the molding temperature to clean the barrel 3 ~ 4 times (injection into the air). If the stored material is other materials, especially materials with poor thermal stability, such as PVC, POM, etc., this requires that the temperature cannot be increased during the start-up, and the PPS cannot be used to clean the cartridge. It can only be thermally stabilized at a relatively low temperature. Good material such as PS polystyrene, PE polyethylene and other materials. After cleaning, the temperature of the barrel is raised to the normal processing temperature of the PPS, and then the PPS material is used to clean it before processing. In the process of processing, if it is necessary to suspend the production temporarily, the temperature of the cylinder should be lowered to below 280°C to keep the temperature (because the melting temperature of PPS is 280°C), so as to avoid discoloration due to the decomposition of the material for too long.
Second, the surface quality of the parts is not good, poor smoothness, glass fiber exposed
There are several problems in the above aspects, the main reason being that the temperature of the mold is too low, resulting in poor surface quality of the parts.
Since polyphenylene sulfide is a crystalline polymer compound, mold temperature has a significant effect on the performance of polyphenylene sulfide parts.
Due to sudden cooling, parts are not crystallized sufficiently, so that their mechanical strength and heat resistance of the material are reduced;
As the mold temperature increases, the crystallinity of the part increases and the rigidity increases.
The crystallization conditions of PPS injection molded parts are affected by the cooling rate of the melt. The rapid cooling rate of the melt and the large increase in viscosity of PPS lead to a decrease in the mobility of the segments, and the extreme reduction of the chain rearrangement into the crystal lattice. The crystallization is limited or even does not crystallize well, and the crystallinity is small; when the melt cooling rate If you are slow, you will have no time to crystallize, crystals will continue to expand to a great extent, mold temperature will increase, and the crystallinity of parts will increase.
1. Under normal circumstances, the mold temperature should be controlled above 120 °C. Why should the mold be heated?
Since the crystallinity of the polyphenylene sulfide parts molded at a mold temperature of 38°C is less than 5%, it can be said that although the parts are basically formed, the internal structure of the parts has not yet reached the requirements. In order to improve the crystallinity of the parts, post-treatment must be performed. Heat treatment at 204°C for 30 minutes can increase the crystallinity to 60%. Therefore, it is stipulated in the injection molding process that the procedure heat treatment requires that the products be processed within 48 hours. Put the product into the oven, its thickness is not more than three times that of the product, and it rises to 200°C within one hour at room temperature. After two hours of storage, when the parts are relatively large, the time is delayed backwards. The power supply is cut off with the oven cooling. Room temperature, remove the product. However, it should be noted that the mold temperature is lower than the molded product.
The surface finish is not good enough to form matt parts that require less surface finish and can basically meet the requirements. (Note: For parts with high surface finish, mold temperature should be controlled above 120°C.)
2 mold cavity surface finish is not high, but also part of the surface finish is not high
Parts with high surface finish are required. The mold cavity is polished, plated and then polished until it meets the requirements for use.
Third, the phenomenon of cracking parts
This is mainly due to the internal stress of the product.
Internal stress refers to the stress caused by improper molding, temperature change, etc., in the case of no external force, and the essence is that the high elastic deformation of plastic molecules is frozen in the product. The internal stress of plastic products can affect the mechanical properties and performance of the products, such as warping, deformation and even small cracks: internal stress can also make the injection molded products show higher mechanical properties in the flow direction, and perpendicular to the direction of flow The strength is lower, making the product performance non-uniform, thereby affecting the use of the product. In particular, cracking will be accelerated when the product is exposed to heat or contact with special solvents.
The internal stress of PPS products is caused by the orientation stress and temperature stress, and is sometimes related to improper demoulding.
1. Orientation stress
The internal macromolecules in the injection products are prone to internal stress and cause stress concentration.
The melt (material) is rapidly cooled during injection molding, the melt viscosity is higher at lower temperatures, and the oriented molecules are not sufficiently relaxed. The resulting internal stress has an effect on the mechanical properties and dimensional stability of the part. Therefore, the melt temperature (barrel temperature) has the greatest effect on the orientation stress. Increasing the melt temperature (the barrel temperature) decreases the melt viscosity, and thus the shear stress and the orientation stress decrease.
1 The relaxation of the orientation stress at high melt temperature (barrel temperature) is greater, but as the viscosity decreases, the pressure transmitted from the injector screw to the mold cavity increases, which may increase the shear rate, leading to the orientation stress. Increase.
2 The pressure holding time is too long and the orientation stress increases. Increasing the pressure of the injection machine will also increase the orientation stress due to the increase of shear stress and shear rate.
3 The thickness of injection molded products also affects the internal stress, and the orientation stress decreases with the increase of the thickness of injection molded products. Because the thick-walled injection molded parts cool slowly, the melt cools in the mold cavity, the relaxation time is long, and the oriented molecules have enough time to return. Random state.
4 If the mold temperature is high, the melt cools slowly and the orientation stress decreases.
2. Temperature stress
The temperature difference between the melt temperature (material temperature) and the mold temperature during plastic injection is large, which causes the melt close to the mold wall to cool more rapidly, thus creating a stress that is unevenly distributed within the product volume.
1 Since PPS has a large specific heat capacity and a small thermal conductivity, the surface layer of the product cools much faster than the inner layer, and the solidified shell layer formed on the surface of the product will hinder free shrinkage when the interior continues cooling. As a result, tensile stress occurs inside the product. In the outer layer, compressive stress occurs.
2 The greater the stress caused by the shrinkage of thermoplastics, the lower the stress of the material due to compaction in the mold, ie the short holding time and the low pressure holding pressure, can reduce the stress to a good extent.
3 The shape and size of the product also have a great influence on the internal stress. The greater the surface area to volume ratio of the product, the faster the surface is cooled and the greater the orientation stress and temperature stress.
4 The orientation stress is mainly generated in the surface layer of the product. Therefore, it can be considered that the orientation stress should increase as the ratio of the surface of the product to its volume increases.
5 If the product thickness is not uniform or the product has metal inserts, it is easy to produce the orientation stress. Therefore, the insert and the gate should be set at the thick wall of the product.
Through the above several aspects of analysis, due to the structural characteristics of plastics and the limitations of injection molding process conditions, it is impossible to completely avoid internal stress, can only reduce the internal stress or try to make the internal stress distribution within the product.
The method is:
1 The injection temperature has a great influence on the internal stress of the product. Therefore, the temperature of the cylinder must be properly increased to ensure that the material is well plasticized, the components are uniform to reduce the shrinkage rate, and the internal stress is reduced; the mold temperature is increased, and the product is cooled slowly. Relax orientation molecules to reduce internal stress.
2 Excessively high holding pressure for too long time pressure can make the orientation of plastic molecules large and produce large shear force, which causes the orderly arrangement of plastic molecules and the product orientation stress. Therefore, the lower injection pressure should be used as far as possible; The pressure holding time is too long, the pressure in the mold is increased due to the pressure compensating effect, the melt material has a higher squeezing effect, the degree of molecular orientation is increased, and the internal stress of the product is large, so the holding time should not be too long.
3 The injection rate has a much smaller effect on the internal stress of injection molded parts than temperature, pressure and other factors, but it is better to use variable speed injection, ie fast filling, when the cavity is full, then use low speed, and the speed change injection is fast on the one hand. , reduce weld marks, on the other hand low-speed pressure can reduce the molecular orientation.
4 Design gate location. In general, the gate location should be set at the thick-walled part of the product. Flat products should use flat gates, fan-shaped gates; PPS materials are not suitable for the potential point gate. The ejector device should be designed to eject the large metal area; the draft must be large.
5 When the product has metal inserts, the inserts must be pre-heated (generally heated to about 200°C) to prevent internal stress caused by inconsistent expansion coefficient between the metal material and the plastic material, and transitions need to use arc transitions.
6 After the moulding, the product should be post-treated within 24 hours to eliminate the internal stress. The heat treatment temperature is about 200°C and the holding time is 2 to 3 hours. The essence of this is that the segments and chains in the plastic molecule have a certain mobility, the elastic deformation of the freeze is relaxed, and the oriented molecules return to the random state. (Author: Shaanxi Huada Technology Co., Ltd. Ran Wenqing)
WENZHOU DIYE VALVE&FITTINGS CO.,LTD , https://www.diye-valve.com