Polyphenylene sulfide, the full name of polyphenylene sulfide, English name polyphenyleneSulfide, is a thermoplastic engineering plastic with phenylthio on the molecular 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, it also has special rigidity, good with various fillers and other polymer materials. The blendability. At present, it is the lowest price in high temperature engineering plastics and can be molded 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, bending strength of unmodified polyphenylene sulfide is only moderate, and the elongation and impact strength are also low.
Therefore, polyphenylene sulfide often uses glass fiber and other inorganic fillers to enhance the filling and modification, so as to further improve the physical and mechanical properties while maintaining heat resistance, flame retardancy and resistance to the medium.
Now we analyze and discuss several common defects.
First, the product has low temperature resistance, discoloration, blackening, yellow and black spots
Because PPS has good blending properties and excellent rigidity, its rigid properties are further improved by adding glass fiber. The heat resistance of PPS is very good. Usually, the temperature of the injection cylinder is set at 280~340°C. Product temperature is above 200°C for long-term use. Why does it often cause discoloration when producing some electrical products?
This is due to the fierce competition in the market. 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 by themselves. The flow of materials is complicated, the plasticization requirements are relatively high, and the control of the process is more 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, yellow lines and black spots appear.
In view of the above phenomenon, it is necessary to consider and find solutions from the following aspects:
1. Raw materials:
If there has been a 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 compounding materials are formulated or not. 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; this can significantly improve the impact strength. The melting temperature and thermal decomposition temperature of both polyphenylene sulfide and nylon are very different, and the affinity is not ideal, but they can produce very good melt-mixing effect at higher temperatures. Temperature control from the barrel to the nozzle , followed by 260 °C, 280 °C, 300 °C, 310 °C.
PC polycarbonate, PPO polyphenylene oxide, PTFE polytetrafluoroethylene, PI polyimide, etc. Blended modified polyphenylene sulfide materials of various specifications, the barrel heating temperature varies. Of course, molding temperature The final selection must also consider the shape of the product, 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, gate size are 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 startup, most of this is related to the stock of the cartridge. Therefore, it is necessary to pay attention to the method of operation. When the material before the boot is stored as PPS, use a new material to clean the barrel 3 at the molding temperature. 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 starting, and the barrel cannot be cleaned with PPS. At the lower temperature with a good thermal stability of materials 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 for cleaning. In the process of processing, if the production needs to be suspended temporarily, the temperature of the barrel needs to be lowered to below 280°C. Insulation (due to the melting temperature of PPS 280 °C), so as not to discolor the material for a long time.
Second, the surface quality of the parts is not good, the finish is poor, and the glass fiber is exposed
There are several problems in the above aspects. The main reason is that the temperature of the mold is too low and the surface quality of the parts is not good.
As polyphenylene sulfide is a crystalline polymer compound, mold temperature has a significant effect on the performance of polyphenylene sulfide parts.
Due to sudden cooling, the parts are not crystallized sufficiently, so that the mechanical strength and the heat resistance of the materials are reduced. When the mold temperature is increased, the crystallinity of the parts is improved, and the rigidity is also improved at the same time.
The crystallization conditions of PPS injection molded parts are affected by the cooling rate of the melt. The melt cooling speed is fast, the viscosity of PPS increases greatly and the segmental mobility decreases, the extremes of the segment rearrangement jump into the lattice become smaller, and the crystallization is limited or even impossible. Crystallization is good, crystallinity is small; when the melt cooling rate is slow, it is too late to crystallize, the crystals are continuously expanded to a large extent, the mold temperature is increased, and the crystallinity of the parts is improved.
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 met the requirements. To improve the crystallinity of the parts, post-treatment must be performed. Heat treatment at 204 °C for 30 minutes, its crystallinity can be increased to 60%, so in the injection molding process specification, the process heat treatment: Product requirements within 48 hours of processing.
Put the product into the oven, its thickness does not exceed three times that of the product, and rises to 200°C within 1 hour at room temperature. After 2 hours of holding, (When the part is relatively large, the time is delayed backwards). Cut off the power and cool it with the oven. At room temperature, take out the product. But to illustrate the mold temperature is low molding products.
The surface finish is not good enough to form matt parts that require low surface finish. Basically it can meet the requirements. (Note: For parts with high surface finish, mold temperature should be controlled above 120°C.)
2. The mold cavity surface finish is not high, and it is also one of the reasons that the surface finish of the component is not high.
The parts with high surface finish are required. The mold cavity is polished, plated and 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 absence of external force. 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: The internal stress can also make the injected products show higher mechanical properties in the flow direction, and perpendicular to the flow direction. The strength is lower, which makes the product performance non-uniform, thus affecting the use of the product. Especially when the product is exposed to heat or contact with special solvents will accelerate cracking.
The internal stress of PPS products is caused by the orientation stress and temperature stress, and sometimes it is related to improper demoulding.
1. Orientation stress
After injection of macromolecules in the interior of the product, internal stress is likely to occur, causing stress concentration.
The melt (material) is rapidly cooled during injection molding, and the melt viscosity is relatively high at a relatively low temperature, and the orientated molecules are not sufficiently relaxed. The resulting internal stress has an effect on the mechanical properties and dimensional stability of the part. The body temperature (barrel temperature) has the greatest influence on the orientation stress. Increasing the melt temperature (barrel temperature) decreases the melt viscosity, and thus the shear stress and orientation stress decrease.
1 The degree of relaxation of the orientation stress is high at high melt temperatures (barrel temperatures), but as the viscosity decreases, the pressure delivered by the injection machine screw into the mold cavity increases, which may increase the shear rate, resulting in an orientation stress. Increase.
2 If the pressure holding time is too long, 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 has an effect on 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 orientation molecules have enough time to return. Random state.
4 If the mold temperature is high, the melt cools slowly, which can reduce the orientation stress.
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 quickly, thus creating a stress that is unevenly distributed within the product volume.
1 Since the specific heat capacity of PPS is larger, the thermal conductivity is smaller, and the surface layer of the product is cooled much faster than the inner layer. The solidified shell layer formed on the surface of the product will hinder the free shrinkage of the interior during continuous cooling. As a result, tensile stress occurs inside the product. In the outer layer, compressive stress occurs.
2 The greater the stress generated by the shrinkage of thermoplastics, the lower the stress generated by the compaction of the material in the mold, ie, the short holding time and the low pressure holding pressure can reduce the stress to a good degree.
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, the greater the orientation stress and the 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, so 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 injection molding process constraints, it is impossible to completely avoid internal stress, can only minimize 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 should 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 temperature of the mold 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 plastic molecules to be arranged in an orderly manner and the orientation stress of the products to become large. 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 compensating pressure action, 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 the temperature, pressure, etc., but it is better to use variable speed injection, that is, fast filling, change to low speed when the cavity is full, and the speed of injection is fast on one hand. , reduce weld marks, on the other hand low-speed pressure can reduce the molecular orientation.
4 Design gate location. Generally, the gate location should be set at the thick-walled part of the product. The flat product should use flat gates and fan-shaped gates; the PPS material should not adapt to the submerged point gate. The ejector should be designed to be large. Surface metal area ejection; demoulding slope should 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.
6 After the mold is ejected, 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 is to make the chain and chain section in the plastic molecule have certain activities. The ability to freeze the elastic deformation is relaxed and the oriented molecules return to a random state.