Polyphenylene sulfide, also known as polyphenylene sulfide (PPS), is a thermoplastic engineering plastic with phenylthio groups in its main chain.
Polyphenylene sulfide offers excellent heat resistance, with a long-term operating temperature of 200℃. It also exhibits excellent chemical resistance, with chemical properties similar to those of polytetrafluoroethylene. It also possesses exceptional rigidity and good compatibility with various fillers and other polymer materials. Currently, it is the most affordable high-temperature engineering plastic and can be molded using standard thermoplastic processing methods.
Polyphenylene sulfide is a linear polymer compound composed of alternating benzene rings and sulfur atoms. Rigid structure of benzene rings, linked by soft sulfide bonds, provides excellent heat resistance, flame retardancy, media resistance, and good compatibility with other inorganic fillers. However, unmodified polyphenylene sulfide exhibits only moderate tensile and flexural strengths, its elongation and impact strength are also low.
Therefore, polyphenylene sulfide is often reinforced and modified with glass fiber and other inorganic fillers to further enhance its physical and mechanical properties while maintaining heat resistance, flame retardancy, and media resistance.

 

We will now analyze and discuss several common product defects.

Because polyphenylene sulfide has good blending properties and excellent rigidity, addition of glass fiber further enhances its rigidity. PPS has excellent heat resistance, with injection barrel temperature typically set at 280-340℃. Resulting products are rated for long-term use above 200℃. However, why is discoloration common in production of some electrical products?
This is due to fierce market competition. To reduce production costs, some manufacturers use recycled materials (recycled materials) when producing mid- and low-end products, adding other materials and custom-mixing them with flame retardants, fillers, and other additives. Due to complex flow characteristics of these materials and high plasticization requirements, process control is difficult, leading to various problems.
Melting point of PPS (282-285℃) differs significantly from melting point of added materials. This can cause added materials to burn, vaporize, carbonize, discolor, and darken, resulting in yellow streaks and black spots.

 

To address these issues, consider following aspects and seek solutions:

If discoloration persists during production process, first investigate raw materials for possible issues. For example, consider quality of new materials mixed with other materials or foreign matter, quality of recycled materials, and correct formulation of compounding process. After eliminating these issues, investigate other possible causes.

Melt temperature should be primary consideration. Generally, barrel temperature should be gradually reduced, especially in the first two stages. Different temperatures should be used for different materials. For example, blending nylon with modified polyphenylene sulfide can significantly improve impact strength. Although melting and thermal decomposition temperatures of polyphenylene sulfide and nylon differ significantly, resulting in less than ideal compatibility, they can produce excellent melt mixing at higher temperatures. Temperature is controlled in stages from barrel to nozzle, in order of 260℃, 280℃, 300℃, and 310℃.
Barrel heating temperature varies for various materials, including PC, PPO, PTFE, and PI, as well as for modified polyphenylene sulfide blends. Of course, final selection of molding temperature must also take into account factors such as product shape, size, mold structure, and performance requirements.
In addition, excessive screw speed, back pressure, injection rate, and small nozzle aperture, runner, or gate dimensions can generate high shear heat in melt, leading to melt fracture in PPS. This can also prevent timely exhaust of gases within mold cavity, resulting in localized burns and blackening of product.

If black spots are noticed immediately after starting machine, this is likely related to material in barrel. Therefore, careful operating procedures are essential. If barrel is loaded with PPS before starting machine, purge barrel three to four times with fresh material at molding temperature (injection into air). If loaded material is another material, especially one with poor thermal stability such as PVC or POM, machine temperature must not be raised during startup, and PPS should not be used to purge barrel. Instead, purge barrel at a lower temperature with a thermally stable material such as PS or PE.
After purging, raise barrel temperature to normal PPS processing temperature and purge it with PPS material before resuming processing. If production needs to be temporarily suspended during processing, lower barrel temperature to below 280℃ (as PPS melts at 280℃) to prevent material decomposition and discoloration over time.

Main cause of above problems is low mold temperature, resulting in poor part surface quality.
Since polyphenylene sulfide is a crystalline polymer, mold temperature significantly affects performance of polyphenylene sulfide parts.
Sudden cooling prevents part from fully crystallizing, resulting in a decrease in mechanical strength and heat resistance. Increasing mold temperature improves crystallinity and rigidity of part.
Crystallization of PPS injection molded parts is affected by melt cooling rate. Rapid melt cooling significantly increases PPS viscosity, reducing chain mobility, reducing number of opportunities for chain segments to rearrange and enter crystal lattice. This limits crystallization and even prevents it from fully crystallizing, resulting in low crystallinity. Slow melt cooling allows for more time for crystallization, which continues to expand significantly. As mold temperature increases, crystallinity of part improves.

 

1. Under normal circumstances, mold temperature should be controlled above 120℃. Why is mold heated?
Because polyphenylene sulfide parts molded at a mold temperature of 38℃ have a crystallinity of less than 5%, while part is essentially formed, internal structure does not meet requirements. To improve crystallinity of part, post-treatment is necessary. Heat treatment at 204℃ for 30 minutes can increase crystallinity to 60%. Therefore, injection molding process specifications stipulate that heat treatment process must be completed within 48 hours.
Place part in an oven with a thickness no more than three times thickness of part. Raise room temperature to 200℃ within one hour. Hold for two hours (for larger parts, temperature may be extended). Disconnect power and allow oven to cool to room temperature before removing part. However, it should be noted that parts molded at a low mold temperature may have a matte finish.
Surface finish is not good enough, resulting in a matte finish. For parts with less demanding surface finishes, this generally meets the requirements. (Note: For parts requiring a high surface finish, mold temperature should be controlled above 120℃.)
2. Poor mold cavity surface finish is also a contributing factor to poor surface finish.
For parts requiring a high surface finish, mold cavity should be polished, plated, then polished again until it meets required performance.

This is primarily due to internal stress within product.
Internal stress refers to stress generated within plastic due to improper molding, temperature fluctuations, and other factors in absence of external forces. Essentially, it results from high elastic deformation of plastic molecules being frozen within product.
Internal stress in plastic products can affect mechanical properties and performance of product, causing warping, deformation, and even fine cracks. Internal stress can also cause injection molded parts to exhibit higher mechanical properties in direction of flow, while exhibiting lower strength perpendicular to flow direction. This can lead to uneven performance and compromised product performance. Cracking is particularly accelerated when product is heated or comes into contact with certain solvents.
Internal stress in PPS products is caused by orientation stress and temperature stress, and is sometimes also related to improper demolding.

Oriented macromolecules within injection molding products are prone to internal stress, leading to stress concentration.
During injection molding, melt (material) cools rapidly. At lower temperatures, melt viscosity is higher, preventing oriented molecules from fully relaxing. Resulting internal stress affects mechanical properties and dimensional stability of part. Therefore, melt temperature (barrel temperature) has the greatest impact on orientation stress. Increasing melt temperature (barrel temperature) decreases melt viscosity, thereby reducing shear stress and orientation stress.
① At high melt temperature (barrel temperature), degree of relaxation of orientation stress is greater. However, when viscosity decreases, pressure transmitted to mold cavity by injection molding machine screw increases, which may increase shear rate and lead to increased orientation stress.
② Excessive holding time increases orientation stress: Increasing injection molding machine pressure also increases orientation stress due to increased shear stress and shear rate.
③ Thickness of molded part also affects internal stress. Orientation stress decreases with increasing thickness. This is because thick-walled parts cool slowly, melt takes longer to cool and relax within mold cavity, giving oriented molecules ample time to return to their random state.
④ If mold temperature is high and melt cools slowly, orientation stress can be reduced.

During injection molding, temperature difference between melt temperature (material temperature) and mold temperature is large. This causes melt near mold wall to cool more rapidly, resulting in unevenly distributed stress within part.
① Because PPS has a large specific heat capacity and low thermal conductivity, surface layer of part cools much faster than inner layer. Solidified shell formed on the surface of part hinders free contraction of interior during further cooling, resulting in tensile stress within part and compressive stress in outer layer.
② The greater stress generated by shrinkage in thermoplastics, the lower stress generated by compaction within mold. This means that shorter holding times and lower holding pressures can effectively reduce orientation stress.
③ Shape and size of product also significantly influence internal stress. The larger surface area to volume ratio of product, the faster surface cools, the greater orientation and temperature stresses.
④ Orientation stress primarily occurs within surface layer of product. Therefore, it can be assumed that orientation stress increases with surface-to-volume ratio of product.
⑤ Uneven product thickness or metal inserts are prone to orientation stress, so inserts and gates should be located near thick wall of product.
Based on above analysis, due to structural characteristics of plastics and limitations of injection molding process conditions, it is impossible to completely avoid internal stress. It can only be minimized or distributed as evenly as possible within product.

 

Following methods are available:
① Injection temperature significantly affects magnitude of internal stress in product. Therefore, barrel temperature should be appropriately increased to ensure good plasticization of material and uniform composition to reduce shrinkage and internal stress. Mold temperature should also be increased to slow cooling of product to relax oriented molecules and reduce internal stress.
② Excessively high holding pressure for too long can increase orientation of plastic molecules, generating significant shear forces that align molecules and increase orientation stress in part. Therefore, a low injection pressure should be used. If holding pressure is too long, mold pressure increases due to backfill effect, causing a higher extrusion effect on melt, increasing degree of molecular orientation and increasing internal stress in part. Therefore, holding pressure should not be too long.
③ Impact of injection rate on internal stress in injection molded parts is much smaller than factors such as temperature and pressure. However, it is best to use variable speed injection, meaning rapid mold filling, followed by a slow speed once mold cavity is full. Variable speed injection speeds up filling process and reduces weld marks, while low holding pressure reduces molecular orientation.
④ Design gate location. Generally, gate should be located near thick wall of part. Flat or fan gates are recommended for flat parts. PPS is not suitable for latent point gates. Ejector should be designed to eject over a large metal surface area, and draft angle should be large.
⑤ When product incorporates metal inserts, inserts must be preheated (generally to around 200℃) to prevent internal stress caused by mismatched linear expansion coefficients between metal and plastic materials. A circular arc should be used at transition point.
⑥ After demolding, product must be post-processed within 24 hours to eliminate internal stress. Heat treatment temperature is approximately 200℃, and holding time is 2-3 hours. This process essentially allows chain segments and links within plastic molecules to have some mobility, allowing any frozen elastic deformation to relax and allowing oriented molecules to return to a random state.