Polysulfone（PSU) injection

Polysulfone（PSU) injection

Polysulfone (PSU, also known as PSF) is a high-temperature and high-strength thermoplastic engineering plastic. It has high mechanical strength, electrical insulation, thermal deformation temperature and certain chemical corrosion resistance, especially good thermal aging, creep resistance and dimensional stability.

Typical applications: mainly used in electronic and electrical, food and daily necessities, automotive, aviation, medical and general industries;

Make all kinds of contactors, connectors, transformer insulating parts, thyristor caps, insulating sleeves, coil frames, terminals, printed circuit boards, shaft sleeves, covers, TV system parts, capacitor films, brush holders, alkaline battery boxes, wire and cable coatings.

Polysulfone (PSU, also known as PSF) can also be used as protective cover assembly, electric gear, battery cover, internal and external parts of aircraft, external protective cover of spacecraft, shutter of camera, lamp parts and sensors.

Instead of glass and stainless steel for steam dishes, coffee containers, microwave cookers, milk containers, milker components, beverage and food dispensers.

Health and medical devices include surgical plates, sprayers, humidifiers, dental instruments, flow controllers, grooves and laboratory equipment. They can also be used for dental inserting with high bonding strength.

It can also be used as chemical equipment (pump cover, protective layer outside the tower, acid resistant nozzle, pipeline, valve container), food processing equipment, dairy processing equipment and environmental protection infection control equipment.

1、 Injection molding process characteristics / conditions:

① PSU is an amorphous polymer with no obvious melting point. TG is 190 ℃ and the molding temperature is above 280 ℃. The product is transparent.

② The molding characteristics of PSU are similar to those of PC. The flow characteristics of melt are close to Newtonian fluid, and the viscosity of polymer melt is sensitive to temperature. When the melt temperature exceeds 330 ℃, the melt viscosity can decrease by 50% every 30 ℃.

③ Although the melt viscosity of PSU is sensitive to temperature, its viscosity is still very high and its fluidity is poor in the molding process; In addition, the cooling rate of the melt is fast and the molecular chain is rigid. Therefore, the internal stress generated in the molding is difficult to eliminate.

④ Obviously, there is no hydrophilic group in the molecular structure of PSU, the water absorption is small, and the equilibrium water absorption is 0.6%. However, in the molding process, the existence of trace water will lead to melt degradation due to high temperature and strong mechanical force. Therefore, it must be dried before injection molding.

⑤ Too high injection rate will lead to melt fracture of PSU melt, which limits the mold filling rate and makes it difficult to fill the mold.

2、 Requirements for injection molding:

Generally, precision wear-resistant screw barrel group is selected, and the requirements are as follows:

① When processing PSU, the commonly selected screw form is single head, full thread, equidistant and low compression ratio, and its length diameter ratio is between 14 and 20;

② Due to the high melt viscosity of PSU, in order to reduce the load of screw rotation, the injection molding machine is required to have a low gear speed regulation device, which can carry out stepless speed regulation between 15 ~ 45r / min;

③ In order to meet the requirements of PSU forming and processing, the control device for barrel temperature shall have free sample selection and adjustment in the high temperature area, and have stable working performance. The temperature shall be about 400 ℃;

④ Extended nozzles with heating and temperature control devices shall be selected. The hole diameter of the nozzle is greater than 5mm, and casting will not occur due to the high melt viscosity.

3、 Product and mold design:

Pay attention to the following points when designing PSU products and molds:

① The fluidity of PSU melt is poor, and the ratio of melt flow length to wall thickness is only about 80. Therefore, the wall thickness of PSU products shall not be less than 1.5mm, mostly more than 2mm. PSU products are sensitive to notch, so arc transition shall be adopted at right angles or acute angles.

The molding shrinkage of PSU is relatively stable, which is 0.4% ~ 0.8%. The shrinkage in the melt flow direction is basically consistent with that in the vertical direction. The stripping slope should be 50:1.

In order to obtain bright and bright products, the surface roughness of the mold cavity is required to be RA 0.4 or more. In order to facilitate the melt flow, the main flow channel of the die is required to be short and thick, its diameter is at least 1 / 2 of the product thickness, and has a slope of 3 ° ~ 5 °. The section of the shunt channel is preferably circular arc or trapezoid to avoid the existence of bends.

② The form of gate can be determined according to the product. However, the size shall be as large as possible, the straight part of the gate shall be as short as possible, and its length can be controlled between 0.5 ~ 1.0mm. The position of the feed inlet shall be set at the thick wall.

③ Sufficient cold material pockets shall be set at the end of the mainstream. Because PSU products, especially thin-walled products, need higher injection pressure and faster injection rate in molding, good exhaust holes or grooves should be set in order to discharge the air in the mold glue in time. The depth of these vents or slots shall be controlled below 0.08mm.

④ The setting of mold temperature should be beneficial to improve the fluidity of PSU melt during film filling. The mold temperature can be above 140 ℃ (at least above 120 ℃). Drying process: hot air circulation drying: 120 ~ 140 ℃, 4 ~ 6h, material layer thickness of 20mm; Negative pressure boiling drying: 130 ℃, 0.5 ~ 1.0h.

4、 Injection molding process parameters:

① Barrel temperature

Increasing the barrel temperature is conducive to reducing the viscosity of the melt, but too high barrel temperature not only reduces many properties (such as impact strength), but also causes the discoloration and decomposition of plastics.

Usually, PSU melt decomposes at about 400 ℃, but in actual injection molding, decomposition occurs when the barrel temperature is more than 320 ℃. When the temperature exceeds 360 ℃, the decomposition is obvious.

There are many factors affecting the temperature setting of PSU barrel, but the main factors are melt viscosity and product wall thickness. When the wall thickness is less than 5mm, the barrel temperature can be higher, which can reach more than 300 ℃, some grades of PSU plastics can reach more than 320 ℃, and special grades of PSU plastics can reach more than 350 ℃. When the wall thickness of the product is more than 5mm, the temperature can be controlled between 280 ~ 300 ℃.

② Injection pressure

Although the viscosity of PSU melt is sensitive to shear rate, due to the poor flow work of PSU melt, higher injection pressure must be used.

Higher injection pressure can increase the density of products and reduce the molding shrinkage, but it puts forward higher requirements for molds. Generally, it is more than 100 MPa, and sometimes it can reach 140 MPa.

③ Injection rate

Generally, the increase of injection rate is conducive to the filling of melt. However, for PSU, due to the fast cooling rate, melt fracture will also be caused if the injection rate is not controlled properly. Therefore, except that the thickness of the product is about 2mm, the material filling is difficult, and a higher filling rate is required, it is generally appropriate to use small and low injection rate.

④ Screw speed

Due to the high melt viscosity of PSU, it is better to require a slightly lower screw speed, generally 15 ~ 45 R / min.

⑤ Mold temperature

Since the Tg of PSU is as high as 190 ℃, a higher die temperature can be adopted, but the maximum temperature does not exceed 160 ℃. The choice of mold temperature often varies with the thickness of the product. When the wall thickness of the product is 2 ~ 5mm, the mold temperature can be controlled between 10 ~ 120 ℃. When the wall thickness is more than 5mm or less than 2mm, the mold temperature can be as high as 140 ~ 150 ℃.

⑥ Forming cycle

PSU has good thermal stability at normal processing temperature. At 320 ℃, it can stay in the barrel for a long time. Therefore, the setting of forming cycle is only considered from the perspective of production efficiency.

5、 Precautions:

① The molding temperature of PSU is high. Therefore, it is required to clean the barrel before injection molding.

② PSU products should not contain metal inserts. Due to the difference of thermal expansion coefficient, stress concentration is easy to occur around the insert. Use as few inserts as possible or preheat the inserts during molding. Generally preheat to 200 ℃.

③ PSU is an amorphous polymer and the product is transparent. Therefore, mold release agent should be used as little or no as possible during molding;

④ Post treatment of products

Due to the high rigidity of PSU molecular chain, the internal stress of the product is large, so it is necessary to anneal it. General principle of heat treatment: in the hot air circulation bellows, it shall be 10 ~ 20 ℃ below Tg for 2 ~ 4H.