The English name of polyphenylene sulfide (PPS) is a new type of engineering plastic, which was industrialized in the United States in 1973. In recent years, it has developed rapidly and has become the sixth largest engineering plastic and the first largest special engineering plastic after nylon (PA), polycarbonate (PC), polyoxymethylene (POM), thermoplastic polyester (PET and PBT), and polyphenylene oxide (PPO).

PPS is a polymer with a benzene sulfur group on its main chain. Due to its alternating structure of benzene ring and sulfur, the molecular chain has great rigidity and regularity. Therefore, PPS is a crystalline polymer with many excellent properties, such as heat resistance, rigidity, flame retardancy, and electrical insulation. The lone pair electrons on sulfur atoms make PPS have good affinity with glass fibers, inorganic fillers, and metals, making it easy to make various reinforced composites and alloy materials.

1、 Performance of PPS

1. Physical and mechanical properties

PPS is a white, high crystallinity polymer with a density of 1.34. Its mechanical properties are excellent, with superior tensile and bending strength compared to PA, PC, PBT, etc. It has extremely high rigidity and creep resistance, but its brittleness is large, and its notch impact strength is low, lower than PA, PC, PBT, and higher than POM. After adding glass fiber reinforcement, its mechanical properties are better. PPS is an inert substance and non-toxic.

2. Thermal performance

Due to PPS being a crystalline polymer with a maximum crystallinity of 65%, its crystallization temperature is 127 ℃, melting point is 286 ℃, and thermal deformation temperature is 260 ℃. It decomposes in air at temperatures above 430-460 ℃, and its thermal stability far exceeds that of engineering plastics such as PA, PBT, and POM. Its long-term use temperature is the highest in thermoplastic plastics, reaching 220-240 ℃. PPS still has good insulation and flame retardancy, with a critical oxygen index equivalent to PVC, reaching 47%. Without the need for flame retardants, PPS can reach UL94-V0 level.

3. Electrical performance

PPS has a symmetrical molecular structure, no polarity, and low water absorption, so its electrical insulation is very good. Compared with other engineering plastics, its dielectric constant is small, and its arc resistance is equivalent to thermosetting plastics. Under the conditions of high temperature, high humidity, frequency conversion, etc., PPS can maintain excellent electrical insulation. By adding conductive fillers, conductive PPS composite materials can be prepared for anti-static and electromagnetic shielding purposes.

4. Chemical resistance

Due to its high crystallinity, PPS has excellent chemical resistance and is insoluble in any organic solvent below 200 ℃. In addition to strong oxidizing acids, it can withstand various acids, alkalis, and salts. After long-term immersion in various chemicals at high temperatures, it still maintains high strength, second only to polytetrafluoroethylene. PPS also has good weather resistance and radiation resistance.

5. Processing performance

1) Water absorption

The water absorption of PPS is relatively low, only 0.02%.

2) Liquidity

PPS has good flowability and can process thin-walled products. Its flowability varies depending on the variety and specifications of PPS. Linear PPS has higher flowability than branched cross-linked PPS. As the temperature increases, the flowability of the material increases. However, if the temperature is too high or the material stays in the barrel for too long, partial cross-linking of the material may occur, leading to a decrease in flow.

3) Crystallinity

PPS is a crystalline polymer, and its crystallinity varies with the cooling temperature and rate during molding. The faster the cooling rate, the lower its crystallinity. The size of crystallinity has a significant impact on its strength, heat resistance, weather resistance, and dimensional stability. As the crystallinity increases, the thermal deformation temperature of the product increases, resulting in an increase in rigidity, surface finish, surface hard shrinkage rate, and dimensional stability.

4) Thermal stability

PPS undergoes partial oxidation crosslinking reaction at high temperature for a long time, leading to a decrease in material fluidity, darkening of color, and affecting the quality and performance of the product. In addition, PPS has strong adhesion to metals, and it is necessary to prevent materials from solidifying inside the machine barrel.

5) Contractility

For crystalline plastics, during molding and processing, the crystallinity increases with the increase of mold temperature, and as the crystallinity increases, the shrinkage rate also increases. Therefore, the shrinkage rate increases with the mold temperature. Usually, the shrinkage rate of PPS is small, but the shrinkage rate perpendicular to the flow direction is greater than 2-4 times that of the flow direction. The thickness, shape, and injection speed of the product also have an impact on the shrinkage rate.

6) Secondary processing

PPS products can be machined by cutting and tapping, and processed by ultrasonic welding and adhesive bonding.

2、 Forming equipment and molds

1. Selection of injection molding machines

1) Locking force

The flowability of PPS is similar to that of ABS and POM. When calculating the locking force, the factor of flowability should be considered. The material flow characteristic system K value can be selected, which can be the same as ABS and POM in the fourth group.

2) Injection volume

Choose the appropriate injection amount based on the weight of the product, and it is not advisable to use an injection molding machine with an injection amount exceeding too much to prevent the rubber material from staying in the barrel for too long and causing oxidation crosslinking reaction or even solidification, which can cause discoloration of the product and affect its performance and quality.

3) Screw, barrel

According to the characteristics of PPS, PPS is not corrosive. Therefore, for general PPS materials, ordinary screws can be used. However, due to the high processing temperature and the melting and heat absorption of crystals, the power of the barrel heater must be sufficient. In most cases, PPS incorporates glass fibers to enhance its various properties. These materials cause significant wear on the barrel and screw, and wear-resistant barrel screws, such as plated hard chromium screws or bimetallic screws, must be used.

4) Shooting nozzle

Adopting an open nozzle with an inner diameter φ 3-6mm, it is recommended that the heater of the nozzle be individually controlled to make the molding process more convenient. If some products require prevention of salivation, a self-locking nozzle can be used, but cleaning is more difficult.

2. Product Modeling and Mold Design

1) Product design

PPS is a crystalline polymer, and changes in the corners and wall thickness of the product can cause stress concentration, leading to stress cracking, size changes, strength reduction, and other issues. When designing the product, the corners should be curved transition and uneven thickness should be avoided as much as possible. Generally, the thickness of the product should not exceed 7-10mm. A reasonable design of reinforcing ribs can increase the rigidity of the product, prevent warping, eliminate residual stress, and improve flowability. Increasing the number of ribs has a better effect than increasing the height and thickness of the ribs. Usually, the selection of ribs is half of the product thickness and rounded transition, with a demolding angle of 2-3 ° for the ribs.

2) Mold material

The processing temperature of PPS is relatively high, with rubber material temperature ranging from 290 to 330 ℃ and mold temperature ranging from 130 to 180 ℃. In most cases, PPS is reinforced with glass fiber, containing a large amount of glass fiber, and wear issues must be considered. Therefore, the mold material should be based on 140 to 150 ℃, and suitable alloy materials should be selected, such as SK, SKD, SKH, etc., with a hardness of HRC60-65 and surface electroplating.

3) Runner

The shape of the flow channel should be circular or trapezoidal, with a taper of 2-3 °. A cold material well should be set at the end of the flow channel.

4) Sprue

The gate can be needle type, disc type, fan type, spoke type, side entry type, etc. It is not suitable to use latent gate because PPS products have high hardness and low shrinkage rate. The gate should be set at the thickest section of the product, with a diameter of no less than 0.6mm. For reinforced PPS, the direction of strength and shrinkage due to crystal orientation varies. Therefore, when designing the shape and position of the gate, it is necessary to consider the size changes and strength changes caused by the flow direction of the rubber material, as well as the strength decrease caused by the fusion line. Vertically, the fusion line should appear in areas without load or with small and inconspicuous loads.

5) Demolding

In order to ensure the surface smoothness and ease of demolding of the product, the mold cavity, flow channel, and other parts should have a high smoothness and demolding angle, which is generally 1-3 °. The design of the diameter, quantity, and position of the demolding ejector pins is also important. It depends on the shape of the product to ensure that the ejection force is balanced and stable. Otherwise, it may cause local residual stress in the product, leading to problems such as product cracks and uneven dimensional accuracy.

6) Exhaust

The presence of bubbles in PPS products can significantly reduce their performance. Due to poor exhaust, the original burning, discoloration, and surface roughness can affect the appearance and quality of the product. The mold must be reasonably equipped with exhaust slots or holes, and the gap between the parting surface and the demolding ejector pin can also be used for exhaust.

7) Temperature control of molds

Due to the molding process of PPS, the mold temperature is required to be between 130-180 ℃, which generally requires heating. The accuracy and uniformity of the mold temperature affect the performance and dimensional accuracy of the product; According to factors such as the size of the mold, product shape, production capacity, and temperature control equipment, electric heating and oil circulation heating temperature control can be selected. Electric heating temperature control is convenient, economical, and has fast temperature rise, but the temperature uniformity and accuracy are poor. The temperature uniformity and accuracy of the oil circulation heating temperature control for mold temperature are high, and products with high requirements for performance and dimensional accuracy can choose this method. To ensure the uniformity of mold temperature and prevent heat dissipation, asbestos sheets can be installed on the side of the mold and between the mold and the injection molding machine template for insulation.

3、 Forming process

1. Drying of raw materials

Because PPS has low water absorption, it can be dried before processing. But if the raw materials are placed in an open environment with high temperature and humidity for a long time, they need to be dried at a temperature of 150-160 ℃ for 2-3 hours.

2. Forming temperature

The molding temperature of PPS is relatively high, with a barrel temperature between 280 and 360 ℃. While ensuring smooth mold filling and product quality, the barrel temperature should not be too high as much as possible to avoid oxidation, crosslinking, or even solidification of the adhesive inside the barrel. The temperature of the nozzle should be controlled separately to maintain good fluidity of the adhesive without clogging or salivation.

3. Injection pressure

The injection pressure is between 50-200MPa. In order to ensure the mechanical properties and dimensional accuracy of the product, it is better to use a higher injection force, especially for complex and thin-walled parts.

4. Injection speed

The injection speed is fast, and the surface smoothness and strength of the product are also high. However, excessive injection speed can cause the product to warp, burn, and other phenomena. Generally, a moderate injection speed is used.

5. Screw speed

The screw speed can be between 50-200 rpm, and higher speeds can be used for high-speed forming. At this time, the temperature rise of the rubber material is large due to the high shear rate, and the barrel temperature must be lowered.

6. Holding pressure

In order to prevent the generation of internal stress, the storage pressure should be as low as possible to meet the requirements of shrinkage, and the product should not have any depressions.

7. Back pressure

A certain back pressure is beneficial for the exhaust and mixing of the adhesive material, in order to obtain products with stable quality. Generally, the back pressure is between 0.5 and 1.5MPa. For materials with poor drying, the back pressure can be higher.

8. Mold temperature

PPS is a crystalline polymer, and the mold temperature directly affects the crystallinity of the product, resulting in different performance and dimensional accuracy. Different mold temperatures cause significant differences in crystallization, such as 8% at 94 ℃ and 48% at 150 ℃. The mold is high, and the crystallinity of the products obtained is also high. Its mechanical properties are good, but there are problems such as large shrinkage, depression, warpage, and poor impact strength. The mold temperature is low, the crystallinity of the products is low, the shrinkage is small, and the size reproducibility is good (the replication degree of the product size and the mold size), the impact strength is increased, and the ultrasonic welding property is improved, but the mechanical properties are poor. Therefore, the temperature of the forming mold should be determined based on the requirements of the product.

9. Forming cycle

Due to PPS being a crystalline polymer, in order to obtain good mechanical properties, it is necessary to adopt higher mold temperature and longer cooling time to improve crystallinity. For thick walled products, injection time, pressure holding time, and cooling time should be longer than those for thin-walled products. In order to prevent cooling shrinkage and internal stress generation, it is also necessary to extend the holding time appropriately for complex and thin-walled products.

10. Post processing of products

In order to improve the performance of the product, eliminate internal stress, and maintain the performance and dimensional stability of the product, the product can be placed in an oven for annealing treatment. The conditions are usually 200-240 ℃, 2-4 hours, depending on the thickness and purpose of the product. Thick products require longer annealing time, and the annealing treatment causes the product to recrystallize and increase its crystallinity, thereby improving its performance. However, it also leads to shrinkage and changes in the size of the product, This factor should be considered when designing the mold.

11. Use of recycled materials

Recycled materials can be mixed with new materials for use, and the proportion depends on the performance requirements of the product. If the proportion of recycled materials is large, the performance of the product will decrease, generally not exceeding 30%, to avoid a significant decrease in product performance and quality.

12. Shutdown processing

Due to the strong adhesion of PPS to metals, after processing, the PPS material adhered to the machine barrel and screw must be cleaned out before shutdown. Otherwise, the adhesive material will firmly adhere to the screw after cooling, affecting the next production or other materials. High density polyethylene (HDPE) can usually be used for cleaning; To shorten the time, glass fiber reinforced PC material can also be used first, and then HDPE material can be used for cleaning, which has a better cleaning effect but higher cost. During the production process, it is necessary to interrupt the molding process for a period of time. All rubber materials inside the machine barrel must be discharged and the temperature of the machine barrel must be reduced to below 270 ℃. Because the molten material will decompose and release toxic gases when heated for a long time under high temperature conditions, the production site must pay attention to ventilation.

4、 Common defects and solutions

1. Insufficient filling

reason:

1) Insufficient injection pressure; 2) Slow injection speed; 3) Low melting temperature; 4) Poor exhaust; 5) The gate is too small; 6) Wear on the rubber ring.

resolvent:

1) Increase injection pressure; 2) Increase injection speed; 3) Raise the barrel temperature; 4) Add vent holes to the unfilled areas; 5) Expanding the size of the gate or shortening the distance of the gate runner; 6) Check the wear of the rubber ring and replace it.

2. Matte surface

reason:

1) Insufficient product density; 2) Slow filling speed; 3) Low mold temperature; 4) Poor exhaust.

resolvent:

1) Increase the amount of melted adhesive and increase the injection pressure; 2) Increase the barrel temperature and injection speed; 3) Raise the mold temperature; 4) Fully exhaust.

3. Warping

reason:

1) Uneven cooling of the product; 2) Uneven wall thickness of the product; 3) Overfilling; 4) The injection speed is too fast.

resolvent:

1) Adjust the temperature control of the mold to ensure even cooling; 2) The design of the product should strive to ensure uniform wall thickness; 3) Reduce injection pressure and holding pressure; 4) Reduce injection speed.

4. Shrinkage, depression

reason:

1) Insufficient product density; 2) The molten material contains gas; 3) The wall thickness of the product is too thick; 4) Large thermal shrinkage.

resolvent:

1) Increase the amount of melted adhesive, increase injection pressure, and extend injection time; 2) Fully dry the material; 3) The thickness of the product should not exceed 7-10mm; 4) Reduce the barrel temperature and mold temperature.

5. Internal cracks

reason:

1) Product cooling too fast 2) Residual stress

resolvent:

1) Raise the mold temperature, immerse the product in hot water or slowly cool it in an oven after removal; 2) Reduce injection speed and increase mold temperature.

6. Scorch

reason:

1) Poor exhaust; 2) The temperature of the molten material is too high.

resolvent:

1) Increase exhaust holes; 2) Reduce the barrel temperature and injection speed; 3) Increase the gate.

7. Difficulty in demolding and bursting during ejection

reason: