Three common defects in polyphenylene sulfide injection molding, how to prescribe the right medicine?
Polyphenylene sulfide, the full name polyphenylene sulfide, the English name polyphenyleneSulfide (referred to as PPS), is a thermoplastic engineering plastic with a phenylthio group in the main chain of the molecule.
Polyphenylene sulfide is excellent in heat resistance, long-term use temperature is 200 Â° C; good chemical resistance, excellent chemical properties similar to polytetrafluoroethylene; also has special rigidity, good with various fillers and other polymer materials Blending. At present, it is the lowest price of 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, and the thioether bonds of the rigid structure of the benzene ring are linked to each other to have excellent heat resistance, flame retardancy, and resistance. Medium and good affinity with other inorganic fillers. However, the tensile strength and flexural strength of 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 modification, thereby further improving physical and mechanical properties while maintaining heat resistance, flame retardancy and medium resistance.
Several common product defects are now analyzed and discussed.
First, the product has low temperature resistance, discoloration, blackening, yellow lines and black spots
Since polyphenylene sulfide has good blendability and excellent rigidity, its rigid properties are further improved by the addition of glass fibers. PPS has good heat resistance. Usually, the temperature of the injection cylinder is set at 280-340 Â°C. The temperature resistance of the products produced is above 200 Â°C. Why can it often cause discoloration when producing some electrical products? ?
This is because the market is fiercely competitive. In 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 their own mixed materials such as flame retardants and fillers. Due to the flow of these materials, the plasticization requirements are relatively high, and the process control is difficult, which causes such problems.
The melting temperature of the material added by PPS (282-285 Â°C) is too different, resulting in burnt, gasification, carbonization, color discoloration and blackening of the added material, and yellow streaks and black spots appear.
In view of the above phenomena, we need to consider and find solutions from the following aspects:
1. Raw materials:
If there is always discoloration in the production process, it is necessary to first check whether there is any problem with the materials, such as whether there are quality problems with new materials doped with other materials and foreign materials, whether the recycled materials used are qualified, and whether the compounding formula is formulated. Wait correctly, and then check for other reasons.
2. Process conditions:
Mainly considering the melting temperature, it is generally necessary to reduce the barrel temperature step by step, especially the temperature of the first two sections, and use different temperatures for different materials, such as nylon blend modified polyphenylene sulfide; can significantly improve the impact strength. Although the melting temperature and thermal decomposition temperature of polyphenylene sulfide and nylon are very different, the affinity is not ideal, but they can produce very good melt-mixing effect at higher temperatures. The temperature is controlled from the barrel to the nozzle section, which are 260 Â° C, 280 Â° C, 300 Â° C, and 310 Â° C.
PC polycarbonate, PPO polyphenylene ether, PTFE polytetrafluoroethylene, PI polyimide, etc. Blend modified polyphenylene sulfide materials of various specifications, the heating temperature of the barrel is different. Of course, the final selection of the molding temperature should also consider the shape, size, mold structure, and product performance requirements of the product.
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, sprue, gate size is too small, etc., the melt will have high shear heat, causing melt fracture of PPS. Moreover, it is easy to cause the gas in the cavity to be discharged in time, causing local burns and blackening of the product.
3. Materials and operation methods:
If it is turned on, it will be found to have black spots. This is mostly related to the storage of the barrel. Therefore, it is necessary to pay attention to the operation method. When the material stored in the cylinder is PPS before starting, the new material should be used to clean the barrel at the molding temperature. 4 times (air injection). If the stored materials are other materials, especially those with poor thermal stability, such as PVC, POM, etc., this requires that the temperature cannot be raised at the time of starting, and the barrel cannot be cleaned with PPS, and can only be stabilized at a lower temperature. Good materials such as PS polystyrene, PE polyethylene and other clear materials.
After cleaning, the temperature of the barrel is raised to the normal processing temperature of PPS, and then washed with PPS material before processing. In the process of processing, if the production needs to be temporarily suspended, the temperature of the cylinder should be lowered to 280 Â° C or less (because the melting temperature of PPS is 280 Â° C), so as to avoid disintegration and discoloration of materials over time.
Second, the surface quality of the parts is not good, the smoothness is poor, and the glass fiber is exposed.
The main reason for the above problems is that the mold temperature is low, resulting in poor surface quality of the parts.
Since polyphenylene sulfide is a crystalline polymer compound, the mold temperature has a significant effect on the properties of the polyphenylene sulfide article.
The workpiece is not crystallized due to sudden cooling, so that the mechanical strength and the heat resistance of the material are lowered. The mold temperature is increased, the crystallinity of the workpiece is increased, and the rigidity is also improved.
The crystallization condition of the PPS injection molded part is affected by the melt cooling rate. The melt cooling rate is fast, the PPS viscosity is greatly increased, the chain activity is reduced, the segment rearrangement jumps into the crystal lattice, the crystallization is limited or even crystallized, and the crystallinity is small; when the melt cooling rate is low When it is slow, it can be crystallized, and the crystallization is greatly expanded, the mold temperature is increased, and the crystallinity of the workpiece is increased.
1. Under normal circumstances, the mold temperature should be controlled above 120 Â°C, why should the mold be heated?
Because the mold temperature of the polyphenylene sulfide molded at 38 Â° 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. In order to improve the crystallinity of the part, post-treatment must be carried out. After heat treatment at 204 Â° C for 30 min, the crystallinity can be increased to 60%. Therefore, in the details of the injection molding process, the process heat treatment: the product is required to be processed within 48 hours.
Put the product into the oven, the thickness of which is not more than three times that of the product, and the temperature rises to 200 Â°C within 1 hour. After 2 hours, when the parts are relatively large, the time is delayed, the power is cut off with the oven. The product was taken out at room temperature. However, it is necessary to describe a product in which the mold temperature is low.
The surface finish is not good enough, and the formation of matt parts that require a low surface finish can basically meet the requirements. (Note: If the surface finish of the parts is required to be high, the mold temperature should be controlled above 120 Â°C.)
2. The surface finish of the mold cavity is not high, which is one of the reasons for the low surface finish of the parts.
Parts with high surface finish are required, and the cavity of the mold is polished, galvanized and polished until the requirements for use are met.
Third, the phenomenon of cracking of parts, etc.
This is mainly due to the internal stress inside the product.
Internal stress refers to the stress generated inside the plastic due to improper molding, temperature change, etc. in the absence of external force. The essence is that the high elastic deformation of the plastic molecules is frozen in the product.
The internal stress of plastic products can affect the mechanical properties and performance of the product, such as warping, deformation and even small cracks: the internal stress also causes the injection molded article to exhibit high mechanical properties in the flow direction, and perpendicular to the flow direction. The strength is lower, which makes the product performance uneven, thereby affecting the use of the product. Especially when the product is heated or in contact with a special solvent, it will accelerate cracking.
The internal stress of PPS articles is caused by orientation stress and temperature stress, and sometimes also with improper demolding.
After the macromolecule orientation inside the injection product, internal stress is easily generated, resulting in 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, and the internal stress thus generated has an effect on the mechanical properties and dimensional stability of the part. Therefore, the melt temperature (cylinder temperature) has the greatest influence on the orientation stress, and the melt temperature (cylinder temperature) is lowered to decrease the melt viscosity, and thus the shear stress and the orientation stress are lowered.
1 The degree of relaxation of the orientation stress is high at high melt temperature (barrel temperature), but when the viscosity is lowered, the pressure transmitted by the injection machine screw to the mold cavity increases, which may increase the shear rate, resulting in orientation stress. Increase.
2 The holding time is too long, and the orientation stress is increased: increasing the pressure of the injection machine also increases the orientation stress due to the increase of shear stress and shear rate.
3 The thickness of the injection molded product also has an effect on the internal stress. The orientation stress decreases with the increase of the thickness of the injection molded product, because the thick-walled injection molded part cools slowly, the melt cools in the cavity, the relaxation time is longer, and the orientation molecules have sufficient time to return. Random state.
4 If the mold temperature is high, the melt is cooled slowly, which reduces the orientation stress.
2. Temperature stress
The temperature difference between the melt temperature (material temperature) and the mold temperature during injection of the plastic is large, which causes the melt near the mold wall to cool more rapidly, thereby causing uneven distribution of stress within the volume of the product.
1 Since the specific heat capacity of PPS is large, the thermal conductivity is small, and the surface layer of the product is cooled much faster than the inner layer. The solidified shell formed on the surface of the product will hinder the free shrinkage when the interior continues to cool. As a result, tensile stress is generated inside the product. In the outer layer, compressive stress is generated.
2 The greater the stress generated by the shrinkage of the thermoplastic, the lower the stress caused by the compaction of the material in the mold, that is, the dwell time is short, the holding pressure is low, and the tensile stress can be well reduced.
3 The shape and size of the product also have a great influence on the internal stress. The greater the ratio of surface area to volume of the product, the faster the surface cooling, and the greater the orientation stress and temperature stress.
4 The orientation stress is mainly generated in the surface layer of the article, and therefore, it can be considered that the orientation stress should increase as the ratio of the surface of the article to its volume increases.
5 If the thickness of the product is not uniform or the metal insert of the product is easy to produce orientation stress, the insert and the gate should be placed at the thick wall of the product.
Through the analysis of the above aspects, due to the structural characteristics of the plastic and the limitations of the injection molding process conditions, it is impossible to completely avoid the internal stress. Only the internal stress should be minimized or the internal stress should be distributed evenly within the product.
The method is:
1 The injection temperature has a great influence on the internal stress of the product. Therefore, it is necessary to appropriately increase the temperature of the cylinder to ensure that the material is plasticized well, so that the composition is uniform to reduce the shrinkage rate and reduce the internal stress; the temperature of the mold is increased, and the product is cooled slowly. To relax the oriented molecules, reduce the internal stress.
2 If the pressure is too high, the pressure will be too long to cause the plastic molecules to have a large orientation and produce a large shear force, so that the plastic molecules are arranged in an orderly manner, and the orientation stress of the product is large. Therefore, a lower injection pressure should be used as much as possible; The holding time is too long, the pressure in the mold is increased by the pressure-reducing effect, the melt has a high crushing effect, the degree of molecular orientation is increased, and the internal stress of the product is increased, so the holding time should not be too long.
3 The injection rate has much less influence on the internal stress of the injection molded part than the temperature and pressure. However, it is better to use variable speed injection, that is, fast filling. When the cavity is full, the low speed is used. To reduce the weld mark, on the other hand, low pressure holding pressure can reduce the molecular orientation.
4 Design the gate position. Generally, the gate position should be set at the thick wall of the product. Flat products should use flat gates and fan-shaped gates; PPS materials are not suitable for latent gates. The ejector unit should be designed to eject the large-faced metal area; the draft angle should be large.
5 When the product has a metal insert, the insert needs to be preheated (generally required to heat about 200 Â°C) to prevent the metal material from inconsistent with the plastic material line expansion coefficient to generate internal stress, and the transition needs to use a circular arc transition.
6 After the mold is released, the product should be post-treated within 24 hours to eliminate the internal stress. The heat treatment temperature is about 200 Â°C, and the heat preservation time is 2 to 3 hours. The essence is that the segments and links in the plastic molecule have a certain mobility, the elastic deformation of the freeze is relaxed, and the oriented molecules return to a random state.
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