3 common defects in polyphenylene sulfide injection molding, how to treat them?

Time:2022-07-27 08:27:30 / Popularity: / Source:

polyphenylene sulfide injection molding 
Polyphenylene sulfide, full name of polyphenylene sulfide, English name polyphenylene Sulfide (PPS for short), is a thermoplastic engineering plastic with a phenyl sulfide group on main chain of molecule.
Polyphenylene sulfide has excellent heat resistance, and long-term use temperature is 200℃; it has good chemical resistance, and has excellent chemical properties similar to polytetrafluoroethylene; it also has special rigidity and good blendability with various fillers and other polymer materials. At present, it is the lowest price among high temperature engineering plastics and can be molded by general thermoplastic processing methods.
Polyphenylene sulfide is a linear polymer compound composed of benzene rings and sulfur atoms alternately arranged. Due to rigid structure of benzene ring and soft thioether bonds, it has excellent heat resistance, flame retardancy, medium resistance and good affinity with other inorganic fillers. However, tensile strength and flexural strength of unmodified polyphenylene sulfide are only moderate, elongation and impact strength are also low.
Therefore, polyphenylene sulfide is often modified by glass fiber and other inorganic fillers to enhance its filling and modification, so that it can further improve its physical and mechanical properties while maintaining heat resistance, flame retardancy and medium resistance.
Now we will analyze and discuss some of more common product defects.

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

Due to good blendability of polyphenylene sulfide and excellent rigidity, rigidity of polyphenylene sulfide is further improved by adding glass fibers. Heat resistance of PPS is very good. Usually, temperature of injection barrel is set at 280-340 ℃, and temperature resistance of products produced is above 200 ℃ for long-term use. Why is there often discoloration when producing some electrical products?
This is because current market competition is fierce. In order to reduce production costs, 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 compounds by themselves. Due to miscellaneous flow of these materials, plasticization requirements are relatively high, process control is relatively difficult, resulting in such problems.
Melting point of PPS (282-285℃) is too different in melting temperature of added material, which causes added material to burn, gasify, carbonize, discolor and turn black, yellow streaks and black spots appear.
In view of above phenomenon, we need to consider and find solutions from following aspects:

1. Raw materials:

If there is always discoloration in production process, it is necessary to first check whether there are problems with materials used, such as whether new materials doped with other materials and foreign objects have quality problems, whether recycled materials used are qualified, whether compounding materials are formulated correctly, etc., then check for other reasons after removing them one by one.

2. In terms of process conditions:

Main consideration is melting temperature. Generally, temperature of barrel should be reduced section by section, especially temperature of the first two sections, and different temperatures should be used for different materials, such as nylon blend modified polyphenylene sulfide; impact strength can be significantly improved. Although melting temperature and thermal decomposition temperature of polyphenylene sulfide and nylon are quite different, affinity is not ideal, but they can produce very good melt mixing effect at higher temperature. Temperature is controlled in sections from barrel to nozzle, which are 260℃, 280℃, 300℃, and 310℃ in sequence.
PC polycarbonate, PPO polyphenylene ether, PTFE polytetrafluoroethylene, PI polyimide and other blended modified polyphenylene sulfide materials of various specifications, heating temperature of barrel is different. Of course, final selection of molding temperature should also comprehensively consider product shape, size, mold structure, product performance requirements and other aspects.
In addition, if screw speed is too fast, back pressure is too high, injection rate is too fast, and nozzle aperture, runner, gate size is too small, etc., melt will generate high shear heat, resulting in melt fracture of PPS, and it is easy to make gas in mold cavity unable to be discharged in time, resulting in partial burns and blackening of product.

3. In terms of materials and operation methods:

If a black spot is found as soon as machine is turned on, it is mostly related to material stored in barrel, so pay attention to operation method. When material stored in barrel is PPS before starting machine, barrel should be cleaned 3 to 4 times with new material at molding temperature (injection empty). If stored materials are other materials, especially materials with poor thermal stability such as PVC, POM, etc., this requires that temperature cannot be raised when starting up, barrel cannot be cleaned with PPS. Only materials with good thermal stability can be used at lower temperatures, such as PS polystyrene, PE polyethylene and other cleaning materials.
After cleaning, raise temperature of barrel to normal processing temperature of PPS, then clean it with PPS material before processing. In process of processing, if it is necessary to temporarily suspend production, temperature of barrel should be lowered to below 280℃ for heat preservation (because melting temperature of PPS is 280℃), so as to avoid discoloration due to decomposition of material after a long time.

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