Polyphenylene sulfide, the full name of polyphenylene sulfide, the English name of polyphenyleneSulfide, is the main chain of phenylthio thermoplastic engineering plastics.
Polyphenylene sulfide excellent heat resistance, long-term use temperature at 200 ℃; Good chemical resistance, similar to the excellent chemical properties of PTFE; also has a special rigid, good with a variety of fillers and other polymer materials Of the blend.At present, it is the lowest price of high temperature engineering plastics but can be molded by the general method of thermoplastics varieties.
PPS is a linear polymer compound in which benzene rings and sulfur atoms are alternately arranged. Since the rigid structure of the benzene ring is linked by a thioether bond, it has excellent heat resistance, flame retardancy, Medium and good affinity with other inorganic fillers.But unmodified polyphenylene sulfide tensile strength, bending strength is only moderate level, elongation and impact strength is also low.
Therefore, polyphenylene sulfide often use glass fiber and other inorganic fillers to enhance filling modification, making it to maintain the heat resistance, flame retardancy and resistance to the media at the same time, to further improve the physical and mechanical properties.
Here are a few of the more common product defects to be analyzed and discussed.
First, the product low temperature, discoloration, appear black, yellow and black spots
Because of the good blend of polyphenylene sulfide, with excellent rigidity, through the addition of glass fiber, its rigidity can be further improved.PPS heat resistance is very good, usually the injection barrel temperature is set at 280 ~ 340 ℃, the production of Temperature products are above 200 ℃ long-term use temperature, why can produce some electrical products, often appear discoloration phenomenon?
This is because the market is fiercely competitive, in order to reduce production costs, some manufacturers use low-grade products in the production of renewable materials (recycled materials), add some other materials, add their own flame retardant, filler and other compound materials. As these materials Complex flow of goods, plasticizing and relatively high requirements of the process control more difficult, so there is such a problem.
PPS melting point (282 ~ 285 ℃) added material melting temperature difference is too large, resulting in added material burns, gasification, carbonation, color change black, yellow and black spots appear.
In view of the above phenomenon, need to consider and find solutions from the following aspects:
1. Raw materials:
If the production process has been discoloration, we must first check whether there is a problem with the material, such as doped with other materials and foreign matter to the new material for quality problems, the use of recycled materials is qualified, with the compound is formulated with law Correct, etc., one by one, and then check for other reasons.
2. Process conditions:
Mainly consider the melting temperature, generally to gradually reduce the barrel temperature, especially the first two sections of the temperature, but also for different materials using different temperatures, such as nylon blend modified polyphenylene sulfide; can significantly improve the impact strength. Both polyphenylene sulfide and nylon melt temperature and thermal decomposition temperature difference between the poor, the affinity is not ideal, but they at higher temperatures can produce very good melt mixing effect. Temperature from the barrel to the nozzle segmentation control , Followed by 260 ℃, 280 ℃, 300 ℃, 310 ℃.
PC polycarbonate, PPO polyphenylene ether, PTFE polytetrafluoroethylene, PI polyimide and so on all kinds of modified polyphenylene sulfide material, barrel heating temperature are different.Of course, the molding temperature The final selection should also consider the product shape, size, mold structure, product performance requirements and so on.
In addition, if the screw speed too fast, the back pressure is too high, the injection rate is too fast, and the nozzle aperture, runner, gate size is too small will melt high shear heat, resulting in PPS melt fracture phenomenon, And easy to make the cavity of the gas can not be discharged in time, causing the product burns black.
3. Materials, methods of operation:
If the boot is found to have black spots, which are mostly related to the material stored in the barrel, it should be noted that the method of operation, when the material stored in the barrel before power on is PPS, the material should be used at the molding temperature barrel cleaning 3 ~ 4 times (air injection) .If the material stored for other materials, especially the poor thermal stability of materials such as PVC, POM, which requires the boot can not increase the temperature, and can not be cleaned with PPS cartridge, only At lower temperatures with good thermal stability of materials such as PS polystyrene, PE polyethylene and other clean materials.
After cleaning, and then the cylinder temperature rose to PPS normal processing temperature, and then use the PPS material for cleaning before processing .In the process of processing, if the need for temporary suspension of production, the barrel temperature required to be below 280 ℃ Insulation, (due to the melting temperature PPS 280 ℃), so as not to over time material decomposition and discoloration.
Second, the poor surface quality parts, poor finish, glass fiber exposed
Appear above several aspects of the problem, the main reason is the mold temperature is low, resulting in poor quality parts surface.
Since polyphenylene sulfide is a crystalline polymer compound, the temperature of the mold has a significant effect on the performance of the polyphenylene sulfide article.
Parts due to sudden cooling and not fully crystallized, so that its mechanical strength, heat resistance of the material has declined; mold temperature increases, the parts of the resulting crystallinity and rigidity also increased.
Crystallization of PPS injection molded parts is affected by the cooling rate of the melt, which leads to a rapid cooling of the melt and a substantial increase in viscosity of the PPS, resulting in a decrease in mobility of the segments. Crystallization, crystallinity is small; when the melt cooling rate is too slow and the crystallization, the crystallization continues to expand to a large extent, the mold temperature increases, the workpiece crystallinity increased.
1. Under normal circumstances, the mold temperature should be controlled at 120 ℃ or more, why mold heating?
Because the mold temperature of polyphenylene sulfide molding parts at 38 DEG C less than 5% crystallinity, it can be said that although the basic parts of the shape, but the internal structure of the part has not yet reached the requirement to improve the crystallinity of the parts must be post-processed, Heat treatment at 204 ℃ for 30min, the crystallinity can be increased to 60%, so the rules in the injection molding process provides that the process heat treatment: products require processing within 48 hours.
The product into the oven, the thickness of not more than three times the product within 1 hour at room temperature rise 200 ℃, hold for 2 hours, (parts larger, the time delay back) cut off the power supply with the oven cooling only Room temperature, remove the product, but to illustrate the mold temperature is low molded products.
Surface finish is not good enough, the formation of matte surface finish requirements of the parts is not high, the basic can meet the requirements. (Note: Requirements of parts surface finish, the mold temperature should be controlled at 120 ℃ above.)
2. Mold cavity surface finish is not high, but also the parts surface finish is not high one of the reasons
Requires high surface finish parts, mold cavity polishing, plating and then polishing until the use requirements.
Third, parts cracking and so on
This is mainly due to the internal stress within the product.
Internal stress, in the absence of external force, the internal plastic due to improper molding, temperature changes and other causes of stress, its essence is the high-elastic deformation of plastic molecules are frozen in the product formed.
Plastic products, internal stress can affect the mechanical properties and performance of products, such as warping, deformation and even small cracks: internal stress will make the injection molded products in the flow direction shows high mechanical properties, and perpendicular to the flow direction The lower the strength, the product performance is not uniform, thus affecting the use of products, especially when the product is heated or contact with special solvents will accelerate cracking.
Internal stress of PPS products is caused by orientation stress and temperature stress, and sometimes it is also related to improper release.
1. Oriented stress
Injecting internal macromolecule orientation easily leads to internal stress, resulting in stress concentration.
Melt (material) in the injection molding rapid cooling, the melt viscosity at higher temperatures, the orientation of the molecules can not be fully relaxed, the resulting internal stress on the mechanical properties and dimensional stability of the parts have affected. Therefore, the melting Body temperature (cylinder temperature) has the greatest effect on the orientation stress. Increasing the melt temperature (cylinder temperature) decreases the viscosity of the melt and decreases the shear stress and the orientation stress.
① At high melt temperature (barrel temperature), the relaxation of orientation stress is larger, but when the viscosity decreases, the pressure delivered by the screw of the injection machine to the mold cavity increases, which may increase the shear rate and lead to the orientation stress Increase.
② dwell time is too long, increasing the orientation stress: Increasing the pressure of the injection machine will also be due to the increase of shear stress and shear rate of orientation stress increases.
The thickness of the injection molded products also has an impact on the internal stress. The orientation stress decreases with the increase of the thickness of the injection molded products. Because the thick-walled injection-molded parts are cooled slowly, the melt is cooled in the mold cavity, the relaxation time is longer and the orientation molecules have enough time to return Random state.
④ If the mold temperature is high, the melt slow cooling, orientation stress can be reduced.
2. Temperature stress
The large difference in temperature between the melt temperature (mold temperature) and the mold temperature during injection molding allows the melt near the mold wall to cool more rapidly, resulting in an uneven distribution of stresses within the product volume.
①PPS has a large specific heat capacity and a small thermal conductivity, so the surface layer of the product is cooled much faster than the inner layer. The solidified shell formed on the surface of the product hinders the free shrinkage during the continuous cooling of the inner part. As a result, tensile stress is generated inside the product, In the outer layer there is compressive stress.
② Thermoplastic shrinkage by the greater the stress, the material in the mold due to the compaction effect of the lower stress, that dwell time is short, packing pressure is low, can well reduce the stress.
③ product shape and size of the internal stress also has a great impact on product surface area and volume ratio, the faster the surface cooling, orientation stress, the greater the temperature stress.
The orientation stress is mainly generated in the surface layer of the product, therefore, it can be considered that the orientation stress should increase with the increase of the volume ratio of the product surface.
⑤ If the product thickness uneven or metal inserts with products are prone to orientation stress, so the insert and gate should be set at the thick-walled products.
Through the analysis of the above aspects, due to the structural characteristics of plastic and injection molding process conditions, it is impossible to completely avoid the internal stress, only to minimize internal stress or try to make the internal stress distribution within the product.
The method is:
① injection temperature has a great impact on the size of the stress within the product, it is necessary to properly increase the temperature of the barrel to ensure good plasticization of the material, so that the composition is uniform to reduce shrinkage and reduce internal stress; increase the mold temperature, the product cooling slowly, To relax the orientation of molecules, reduce internal stress.
② too high pressure holding time is too long, the pressure can make the plastic molecular orientation increases and produce greater shear force, the plastic molecules arranged in order, product orientation stress increases, therefore, to try to use a lower injection pressure; if Dwell time is too long, the mold pressure due to the role of pressure increase, the melt produced by the higher squeeze effect, the degree of molecular orientation increased, so that products within the stress increase, so dwell time should not be too long.
③ injection rate of stress on the injection molding parts than temperature, pressure and other factors is much smaller, but the best variable speed injection, fast filling, when the cavity is filled with low speed, variable speed injection on the one hand the filling process fast , Reduce the weld marks, on the other hand low-speed packing can reduce the molecular orientation.
④ design gate location, in general, the gate position should be set in the thick-walled products, flat products to use flat gate, fan gate; PPS material does not meet the potential point gate. Top device should be designed to large Top metal surface ejection; stripping slope to be large.
⑤ When the product with a metal insert, the insert to be preheated (generally require heating 200 ℃ or so), in order to prevent inconsistencies between the linear expansion coefficient of the metal material and the plastic material to produce internal stress, the transition at the need arc transition.
⑥ after the mold products to be processed within 24 hours to eliminate stress, heat treatment temperature is about 200 ℃, holding time of 2 to 3 hours. Its essence is to make the plastic molecules in the chain, the link has some activity Ability to freeze the elastic deformation is relaxed, the orientation of the molecules back to the random state.