How do solve the common problems of PPS injection molding?

Polyphenylene sulfide, known as polyphenylene sulfide (PPS), is a thermoplastic engineering plastic with a benzene sulfide group on the main chain of the molecule.

Polyphenylene sulfide has excellent heat resistance, long-term use temperature at 200 ℃; good chemical resistance, with excellent chemical properties similar to polytetrafluoroethylene; also has a special rigidity, good blend-ability with a variety of 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.

Polyphenylene sulfide is a linear polymer compound with an alternating arrangement of a benzene ring and sulfur atoms.

Due to the rigid structure of the benzene ring and soft thioether bond, it has excellent heat resistance, flame retardancy, media resistance, and good affinity with other inorganic fillers.

However, the tensile strength and flexural strength of unmodified polyphenylene sulfide are only medium levels, and the elongation and impact strength are also low.

Therefore, polyphenylene sulfide is often modified with glass fiber and other inorganic filler enhancements, so that it can further improve the physical and mechanical properties while maintaining heat resistance, flame retardancy, and media resistance.

Now on several more common product defects to analyze and discuss.

Common Defect 1

1. A product’s temperature resistance is low, with discoloration, darkening, yellow lines, and black spots.

PPS is very good heat resistance, usually, the injection barrel temperature is set at 280-340 ℃, and the production of products are temperature resistance of 200 ℃ above the long-term use of temperature, but why in the production of some electrical products, there is often discoloration phenomenon?

This is because now the market competition is fierce, to reduce production costs, some manufacturers produce low-grade products using recycled materials (recycled materials), adding some other materials, adding their flame retardants, fillers, and other materials mixed.

Due to the flow of these materials barge, and plasticization requirements being relatively high, the process control is more difficult, resulting in such and such problems.

PPS its melting point (282 ~ 285 ℃) added material melting temperature difference is too large, resulting in the addition of material burns, gasification, carbonization, color discoloration, and blackening, there are yellow lines and black spots phenomenon.

In response to the above phenomenon, the following aspects need to be considered and solutions found.

1. Raw material

If there has been discoloration in the production process, you need to first check whether there is a problem with the material, such as adulteration of other and foreign materials to the new material whether there is a quality problem, the use of recycled material is qualified, with mixed materials whether the formula with the correct method, etc., one by one to exclude, and then check other causes.

2. Process conditions

Mainly consider the melting temperature, generally to reduce the barrel temperature section by section, especially the temperature of the first two sections, and for different materials using different temperatures, such as nylon blended modified polyphenylene sulfide; can significantly improve the impact strength.

Although the melt temperature and thermal decomposition temperature of polyphenylene sulfide and nylon are very different, the affinity is not ideal, but they can produce a very good melt mixing effect at higher temperatures. The temperature is controlled from the barrel to the nozzle in sections, in the order of 260℃, 280℃, 300℃, and 310℃.

PC polycarbonate, PPO polyphenylene ether, PTFE polytetrafluoroethylene, PI polyimide, and other blended modified polyphenylene sulfide materials of various specifications, the barrel heating temperatures are different.

Of course, the final selection of the molding temperature will also take into account the product shape, size, mold structure, product performance requirements, and other aspects.

In addition, if the screw speed is too fast, the back pressure is too high, the injection rate is too fast, as well as the nozzle aperture, sprue, gate size is too small, etc. will make the melt produce high shear heat, resulting in PPS melt rupture phenomenon, and easy to make the gas in the mold cavity can not be discharged promptly, resulting in local burns and blackening of the product.

3. Material, operation method

If a black spot is found once the machine is turned on, it is mostly related to the barrel storage material, so we must pay attention to the operation method and method, when the barrel storage material is PPS before the machine is turned on, we should use the new material to clean the barrel 3~4 times under the molding temperature (for empty injection).

If the stored material is other materials, especially poor thermal stability of materials such as PVC, POM, etc., which requires that the temperature can not be raised at the start of the machine, and can not be used to clean the barrel of PPS, only at a lower temperature with the good thermal stability of materials such as PS polystyrene, PE polyethylene, and other clean materials.

After cleaning, and then the barrel temperature to the normal processing temperature of PPS, and then cleaned with PPS material, before processing. In the process, if you need to temporarily suspend production, the barrel temperature should be lowered to 280 ℃ below the insulation, (because the melting temperature of PPS is 280 ℃), so as not to take too long for material decomposition and discoloration.

Common Defect 2

The surface quality of the parts is not good, poor finish, and glass fiber exposed.

The main reason for the above problems is that the mold temperature is low, resulting in the poor surface quality of the parts.

Since polyphenylene sulfide is a crystalline polymer compound, mold temperature affects the performance of polyphenylene sulfide parts.

Parts are due to sudden cooling and are not fully crystallized, so their mechanical strength and heat resistance of the material is reduced; mold temperature increases and the crystallinity of the parts improve the rigidity at the same time.

The crystallization of PPS injection molded parts is affected by the cooling rate of the melt. Fast melt cooling rate, PPS viscosity increased significantly resulting in reduced chain segment activity, chain segment rearrangement jump into the lattice of the pole encounter becomes smaller, crystallization is limited or even can not be well crystallized, crystallinity is small; when the melt cooling rate is slow to crystallize, crystallization in a large extent continuously expanded, the mold temperature increases, the crystallinity of the parts subsequently increased.

1. Under normal circumstances, the mold temperature should be controlled at 120 ℃ or more, why should the mold be heating?

Because the mold temperature at 38 ℃ molding of polyphenylene sulfide parts crystallinity is less than 5%, it can be said that although the parts are basically formed, but the internal structure of the parts has not yet reached the requirements.

To improve the crystallinity of the parts, we must carry out post-treatment, heat treatment at 204 ℃ for 30 min, the crystallinity can be increased to 60%, so in the injection molding process rules, the process heat treatment: products require treatment within 48 hours.

Put the product into the oven, its thickness does not exceed three times the product, in 1-hour room temperature to 200 ℃, keep 2 hours, (parts are relatively large, the time is delayed backward) cut off the power with the oven cooling to stop room temperature, remove the product. But to illustrate the mold temperature is low for injection molding products.

The surface finish is not good enough to form a matte to the requirements of the surface finish is not high parts, basically to meet the requirements. (Note: the requirement for parts with high surface finish, and mold temperature should all be controlled at 120 ℃ or more.)

2. Mold cavity surface finish is not high, but also the surface finish of the parts is not high one of the reasons

Requires high surface finish parts, mold cavity polishing, plating branding, and then polishing until the use of the requirements.

Common Defect 3

The parts cracking and other phenomena

This is mainly due to the existence of internal stress within the product.

Internal stress, refers to the absence of external forces, plastic internal stress due to improper molding, temperature changes, and other reasons, the essence of the plastic molecules is high elastic deformation is frozen in the product and the formation.

The internal stress of plastic products can affect the mechanical properties of the product and the use of performance, such as warping, deformation and even small cracks: internal stress will also make injection molding products in the flow direction shows high mechanical properties.

While the strength perpendicular to the flow direction is low, so that the product performance is not uniform, thus affecting the use of products. Especially when the product is subjected to heat or contact with special solvents will accelerate the cracking.

The internal stress of PPS products is caused by orientation stress and temperature stress and is sometimes improperly related to mold release.

1. Orientation stress

Injection products are prone to internal stress after the orientation of macromolecules, resulting in stress concentration.

In the injection molding melt (material) rapidly cooled, the melt viscosity is higher at lower temperatures, and the molecules of orientation can not be fully relaxed, so the resulting internal stress on the mechanical properties of the parts and dimensional stability have an impact.

Therefore, the melt temperature (barrel temperature) has the greatest effect on the orientation stress, and increasing the melt temperature (barrel temperature) decreases the viscosity of the melt, thus reducing the shear stress and orientation stress.

1. The relaxation of orientation stress is greater at high melt temperature (barrel temperature), but when the viscosity decreases, the pressure transferred from the injector screw to the mold cavity increases, which may increase the shear rate and lead to an increase in orientation stress.

2. The holding time is too long, and the orientation stress increases: increasing the injection machine pressure will also cause an increase in orientation stress due to the increase in shear stress and shear rate.

3. The thickness of the injection molded product also affects the internal stress, orientation stress decreases with the increase of the thickness of the injection product, because the thick-walled injection molded parts cool slowly, the melt in the mold cavity cooling, and relaxation time is longer, orientation molecules have sufficient time to return to the random state.

4. If the mold temperature is high, the melt cools slowly, which can make the orientation stress decrease.

2. Temperature stress

Plastic in injection molding melt temperature (material temperature) and the temperature difference between the mold temperature make the melt near the mold wall cool more rapidly, thus creating a non-uniform distribution of stress in the volume of the product.

1. Due to the larger specific heat capacity of PPS, thermal conductivity is small, the surface layer of the product than the inner layer cools much faster, and the product surface formed by the solidification shell layer will prevent the internal cooling of the free shrinkage, the result of the product caused by internal tensile stress, while in the outer layer is generated by compressive stress.

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, that is, the shorter the holding time, low pressure, can be well reduced the stress.

3. The shape and size of the product also have a great influence on internal stress. The larger the ratio of surface area to volume of the product, the faster the surface cooling, and the larger the orientation stress and temperature stress.

4. Therefore, it can be assumed that the orientation stress should increase with the increase of the ratio of surface to volume of the product.

5. If the thickness of the product is not uniform or the product has metal inserts, it is easy to produce orientation stress, so the inserts and gates should be set at the thick wall of the product.

Through the above aspects of analysis, due to the structural characteristics of plastic and injection molding process conditions, completely avoiding internal stress is not possible, can only minimize the internal stress or try to make the internal stress in the product distribution uniform.

The solutions are:

1. The injection temperature has a great influence on the size of the internal stress of the product, so we should appropriately increase the barrel temperature to ensure that the material is well plasticized so that the components are uniform to reduce the shrinkage rate and reduce the internal stress; increase the mold temperature, so that the product cools slowly to relax the orientation molecules and reduce the internal stress.

2. Too high pressure holding time is too long pressure can make the plastic molecular orientation effect increase and produce larger shear force so that the plastic molecules are arranged in an orderly manner and the product orientation stress increases.

3. Therefore, we should try to use a lower injection pressure; if the holding time is too long, the pressure in the mold due to the complementary pressure effect and increases, the melt produces a higher extrusion effect, the molecular orientation degree increases, so that the product internal stress increased, so the holding time should not, Therefore, the holding time should not be too long.

However, it is better to use variable speed injection, i.e. fast mold filling, and low speed when the mold cavity is full. On the one hand, the fast injection mold filling process can reduce fusion marks; on the other hand, low-speed holding pressure can reduce molecular orientation.

4. Generally speaking, the gate position should be set at the thick wall of the product, flat products should use a flat gate and fan gate; PPS material is not suitable for a submerged point gate. The ejector should be designed to eject a large metal area; the slope of the molding should be large.

5. When the product has a metal insert, the insert should be preheated (generally heated about 200℃) to prevent the internal stress caused by the inconsistency of the expansion coefficient between metal and plastic materials, and the transition should be made by the arc.

6. Out of the mold products to be post-treatment within 24 hours to eliminate internal stress, heat treatment temperature of about 200 ℃, insulation time of 2 ~ 3 hours. The essence is to make the chain segments and links in the plastic molecules have certain activity, the frozen elastic deformation is relaxed, and the oriented molecules return to the random state.