Three common defects in polyphenylene sulfide injection molding, how to prescribe right medicine

Time:2021-09-18 12:31:42 / Popularity: / Source:

Polyphenylene sulfide is a thermoplastic engineering plastic with a phenylsulfide group on main chain of molecule.
Polyphenylene sulfide has excellent heat resistance, long-term use temperature is 200 ℃; good chemical resistance, with excellent chemical properties similar to polytetrafluoroethylene; also has special rigidity, good compatibility with various fillers and other polymer materials. At present, it is the lowest price among high temperature resistant engineering plastics and can be molded by general thermoplastic processing methods.
defects in polyphenylene sulfide injection molding 
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 bond, it has excellent heat resistance, flame resistance, 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 used glass fiber and other inorganic fillers to enhance filling modification, so that it can further improve physical and mechanical properties while maintaining heat resistance, flame retardancy and media resistance.
Now we will analyze and discuss several common product defects.

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

Because polyphenylene sulfide has good blending properties and excellent rigidity, its rigidity is further improved by adding glass fiber. Heat resistance of PPS is very good. Usually injection barrel temperature is set at 280~340℃, temperature resistance of products produced is above 200℃ for long-term use. But why do some electrical products often appear discoloration? ?
This is because market is now fiercely competitive. In order to reduce production costs, some manufacturers use recycled materials when producing mid-to-low-end products, add some other materials, add flame retardants, fillers and other blended materials by themselves. Because these materials are mixed, plasticization requirements are relatively high, process control is more difficult, so there are problems of this kind.
Melting point of PPS (282~285℃) is too different for melting temperature of added materials, causing burns, vaporization, carbonization, color change and blackening of added materials, yellow streaks and black spots appear.
In view of above phenomenon, it is necessary to consider and find solutions from following aspects:

1. Raw materials:

If there is discoloration during production process, first check whether there are any problems with materials, such as whether new materials mixed with other materials and foreign matter have quality problems, whether recycled materials used are qualified, whether compounding materials are formulated correctly, etc., after eliminating one by one, check again other reasons.

2. Process conditions:

Melting temperature is mainly considered. Generally, temperature of barrel should be reduced step by step, especially temperature of the first two sections, different temperatures are used for different materials, such as nylon blend modified polyphenylene sulfide, which can significantly improve impact strength. Although melting temperature and thermal decomposition temperature of polyphenylene sulfide and nylon are very different, affinity is not ideal, they can produce very good melting and mixing effects at higher temperatures. 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, etc. are blended and modified polyphenylene sulfide materials of various specifications, heating temperature of barrel is different. Of course, final selection of molding temperature must also consider product shape, size, mold structure, product performance requirements and other aspects.
defects in polyphenylene sulfide injection molding 
In addition, if screw speed is too fast, back pressure is too high, injection rate is too fast, nozzle diameter, runner, gate size is too small, etc., high shear heat will be generated in melt, which will cause melt fracture in the PPS, it is easy to cause gas in cavity to not be discharged in time, causing partial burns and blackening of product.

3. In terms of materials and operating methods:

If a black spot is found as soon as machine is turned on, it is mostly related to material storage in barrel, so you must pay attention to operation method. When material stored in barrel is PPS before starting up, clean barrel 3 to 4 times (air injection) with new material at molding temperature. If stored materials are other materials, especially materials with poor thermal stability such as PVC, POM, etc., this requires that temperature should not be raised when machine is turned on, barrel cannot be cleaned with PPS, and only materials with good thermal stability, such as PS polystyrene, PE polyethylene, etc., can be used at lower temperatures.
After cleaning, raise barrel temperature to normal processing temperature of PPS, then clean it with PPS material before processing. In the process of processing, if production needs to be temporarily suspended, temperature of barrel must be lowered to below 280℃ for heat preservation (because melting temperature of PPS is 280℃), so as to prevent material from decomposing and discoloring for too long.

2. Surface quality of parts is poor, finish is poor, and glass fiber is exposed

Main reason for above problems is that mold temperature is too low, resulting in poor surface quality of parts.
Since polyphenylene sulfide is a crystalline polymer compound, mold temperature has a significant effect on performance of polyphenylene sulfide parts.
Part cannot be fully crystallized due to sudden cooling, so that its mechanical strength and heat resistance of material are reduced; mold temperature increases, crystallinity of part increases, and rigidity also increases.
Crystallization condition of PPS injection molded parts is affected by cooling rate of melt. Melt cooling rate is fast, PPS viscosity increases greatly, which leads to decrease of segment mobility, extreme event of segment rearrangement and jumping into crystal lattice becomes smaller, crystallization is limited or even cannot crystallize well, and crystallinity is small; when melt cooling rate is slow, there is time to crystallize. Crystallization continues to expand to a large extent, mold temperature increases, and crystallinity of part increases.

1. Under normal circumstances, mold temperature should be controlled above 120 ℃, why do you need to heat mold?

Because crystallinity of polyphenylene sulfide parts molded at 38℃ is less than 5%, it can be said that although parts are basically formed, internal structure of parts has not yet reached requirements. In order to improve crystallinity of parts, post-treatment must be carried out. Heat treatment at 204℃ for 30min, crystallinity can be increased to 60%, so in injection molding process regulations, process heat treatment: products require processing within 48 hours.
Put product in an oven, thickness of which is not more than three times that of product, within 1 hour, room temperature will rise to 200℃. After keeping it for 2 hours, (when parts are relatively large, time will be delayed) cut off power and cool down to room temperature in oven, then take out product. But it should be explained that mold temperature is low and molded product.
Surface finish is not good enough, and matte finish can basically meet requirements for parts that require low surface finish. (Note: For parts that require high surface finish, mold temperature should be controlled above 120℃.

2. Surface finish of mold cavity is not high, which is one of reasons why surface finish of parts is not high.

For parts that require high surface finish, mold cavity is polished, plated, then polished until it meets requirements for use.

3. Part cracking and other phenomena

This is mainly due to internal stress inside product.
Internal stress refers to stress generated inside plastic due to improper molding, temperature change, etc., in the absence of external force. Its essence is formed by high elastic deformation of plastic molecules frozen in product.
defects in polyphenylene sulfide injection molding 
Internal stress of plastic products can affect mechanical properties and performance of products, such as warping, deformation and even small cracks: internal stress will also cause injection molded products to show higher mechanical properties in direction of flow, while strength perpendicular to direction of flow is lower, making product performance uneven, thereby affecting use of product. Especially when product is heated or in contact with special solvents, it will accelerate cracking.
Internal stress of PPS products is caused by orientation stress and temperature stress, and sometimes also related to improper demolding.

1. Orientation stress

Orientation of macromolecules inside injection product is easy to produce internal stress, causing stress concentration.
During injection molding, melt (material) cools rapidly, viscosity of melt is higher at a lower temperature, oriented molecules cannot be fully relaxed. Internal stress generated in this way has an impact on mechanical properties and dimensional stability of part. Therefore, melt temperature (barrel temperature) has the greatest influence on orientation stress. Increasing melt temperature (barrel temperature) will reduce melt viscosity, so shear stress and orientation stress are reduced.
① Under high melt temperature (barrel temperature), degree of relaxation of orientation stress is greater, but when viscosity decreases, pressure transmitted by injection machine screw to mold cavity increases, which may increase shear rate, resulting in orientation stress Increase.
② If holding time is too long, orientation stress will increase: Increasing pressure of injection machine will also increase orientation stress due to increase in shear stress and shear rate.
③ Thickness of injection molded product also has an effect on internal stress. Orientation stress decreases with increase of thickness of injection molded product. Because thick-walled injection molded part cools slowly, melt cools in cavity and relaxes in cavity for a long time, oriented molecules have enough time to return to a random state.
④ If mold temperature is high, melt will cool slowly, which can reduce orientation stress.

2. Temperature stress

There is a large temperature difference between melt temperature (material temperature) and mold temperature of plastic during injection molding, which makes melt near mold wall cool more quickly, resulting in uneven stress distribution in product volume.
① Due to large specific heat capacity and low thermal conductivity of PPS, surface layer of product cools much faster than inner layer. Solidified shell layer formed on the surface of product will hinder free shrinkage of internal cooling. As a result, tensile stress is generated inside product and compressive stress is generated in outer layer.
② The greater stress caused by shrinkage of thermoplastic, the lower stress generated by compaction of material in mold, that is, holding time is short and holding pressure is low, which can reduce directional stress well.
③ Shape and size of product also have a great influence on internal stress. The greater ratio of surface area to volume of product, the faster surface cooling, the greater orientation stress and temperature stress.
④ Orientation stress is mainly generated in surface layer of product. Therefore, it can be considered that orientation stress should increase as ratio of surface of product to its volume increases.
⑤ If thickness of product is uneven or product has metal inserts, it is easy to cause orientation stress, so inserts and gates should be set at thick wall of product.
Through analysis of above aspects, due to structural characteristics of plastic and limitations of injection molding process conditions, it is impossible to completely avoid internal stress. The only way to reduce internal stress or make internal stress evenly distributed in product.
Method is:
① Injection temperature has a great influence on internal stress of product, so it is necessary to increase barrel temperature appropriately to ensure good plasticization of material, make components uniform to reduce shrinkage rate and reduce internal stress; increase mold temperature to make product cool slowly to relax oriented molecules and reduce internal stress.
② Excessively high holding time and too long pressure can increase orientation of plastic molecules and generate greater shearing force, which will make plastic molecules orderly align and increase orientation stress of product. Therefore, try to use a lower injection pressure; if holding time is too long, pressure in mold will increase due to pressure compensation effect, melt will have a higher squeezing effect, degree of molecular orientation will increase, which will increase internal stress of product. Therefore, holding time should not be too long.
③ Influence of injection rate on internal stress of injection molded parts is much smaller than temperature, pressure and other factors. However, it is best to use variable speed injection, that is, fast filling. When cavity is full, switch to low speed. On the one hand, variable-speed injection has a faster mold filling process and reduces weld marks. On the other hand, low-speed holding pressure can reduce molecular orientation.
④ Design gate location. Generally speaking, gate location should be set at thick wall of product. Flat products should use flat gates and fan gates; PPS materials are not suitable for latent point gates. Ejection device should be designed to eject a large metal area; ejection slope should be large.
⑤ When product has metal inserts, inserts need to be heated in advance (generally required to be heated at about 200℃) to prevent inconsistency of linear expansion coefficient of metal material and plastic material from causing internal stress, transition must be transitioned by arc.
⑥ Product should be post-processed within 24 hours after mold is released to eliminate internal stress. Heat treatment temperature is about 200 ℃, and holding time is 2 to 3 hours. Its essence is to make chain segments and chain links in plastic molecules have a certain mobility, frozen elastic deformation is relaxed, and oriented molecules return to a random state.

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