What problems should be paid attention to when injection molding of polysulfone PSU products

Time:2021-11-21 09:42:18 / Popularity: / Source:

Polysulfone (PSU, also known as PSF) is a high-temperature resistant, high-strength thermoplastic engineering plastic with high mechanical strength, electrical insulation properties, heat distortion temperature and certain chemical resistance, especially heat aging, creep resistance and dimensional stability.
injection molding of polysulfone PSU products 
Typical application range: Mainly used in electronic and electrical, food and daily necessities, automotive, aviation, medical and general industry sectors;
Production of various contactors, connectors, transformer insulators, thyristor caps, insulating sleeves, coil frames, terminals, printed circuit boards, bushings, covers, TV system parts, capacitor films, brush holders, alkaline Storage battery box, wire and cable coating.
Polysulfone (PSU, also known as PSF) can also be used as protective cover components, electric gears, battery covers, aircraft internal and external parts, outer protective covers of aerospace vehicles, camera shutters, lighting components, and sensors.
Instead of glass and stainless steel to make steam dinner plates, coffee containers, microwave cookers, milk containers, milking parts, beverage and food dispensers.
In terms of hygiene and medical equipment, there are surgical trays, sprayers, humidifiers, dental equipment, flow controllers, groovers and laboratory equipment. It can also be used for dental implants with high bonding strength;
It can also be used as chemical equipment (pump cover, tower outer protective layer, acid-resistant nozzle, pipeline, valve container), food processing equipment, dairy product processing equipment, environmental protection and infection control equipment.

1. Injection molding process characteristics/conditions:

① PSU is a non-crystalline polymer with no obvious melting point, Tg is 190℃, and molding temperature is above 280℃. Product is transparent.
②Molding characteristics of PSU are similar to those of PC. Flow characteristics of melt are close to Newtonian fluids, viscosity of polymer melt is more sensitive to temperature. When melt temperature exceeds 330℃, melt viscosity can be reduced by 50% for every 30℃ increase.
③Although melt viscosity of PSU is sensitive to temperature, its viscosity is still very high, its fluidity is poor during molding process; in addition, cooling rate of melt is faster, molecular chain is rigid. Therefore, internal stress generated during molding is difficult to eliminate.
④ Obviously, there is no hydrophilic group in molecular structure of PSU, water absorption is small, and equilibrium water absorption is 0.6%. However, during molding process, presence of trace moisture will cause melt degradation due to high temperature and strong mechanical force. Therefore, it must be dried before injection.
⑤Excessive injection rate will cause melt fracture in PSU melt, which will limit filling rate and make filling difficult.

2. Requirements for injection molding:

Generally, precision and wear-resistant screw barrel group is selected, and requirements are as follows:
①When processing PSU, screw type usually selected is single-head, full-thread, equidistant, low compression ratio, and its length to diameter ratio is between 14 and 20;
②Because of high melt viscosity of PSU, in order to reduce load of screw rotation, injection molding machine is required to have a low-speed speed control device, which can be steplessly adjusted between 15 and 45r/min;
③In order to meet requirements of PSU forming and processing, control device for temperature of barrel should have free selection and adjustment in higher temperature area, have stable working performance, and temperature requirement is about 400 ℃;
④ An extended nozzle equipped with a heating and temperature control device should be selected. Hole diameter of nozzle is greater than 5mm, because melt viscosity is large, there will be no casting.

3. Product and mold design:

Pay attention to following points when designing PSU products and molds:
①Fluidity of PSU melt is poor, ratio of melt flow length to wall thickness is only about 80. Therefore, wall thickness of PSU products should not be less than 1.5mm, mostly above 2mm. PSU products are more sensitive to notches, therefore, arc transitions should be used at right or acute angles.
Molding shrinkage rate of PSU is relatively stable, ranging from 0.4% to 0.8%, and melt flow direction is basically same as shrinkage in vertical direction. Draft angle should be 50:1.
In order to obtain a clean and bright product, surface roughness of mold cavity is required to be Ra 0.4 or more. In order to facilitate flow of melt, main runner of mold is required to be short and thick, with a diameter of at least 1/2 of thickness of product, and an inclination of 3° to 5°. The cross section of runner is preferably circular or trapezoidal to avoid existence of bends.
②Form of gate can be determined by product. But size should be as large as possible, straight part of gate should be as short as possible, its length can be controlled between 0.5 and 1.0mm. Position of feed inlet should be set at thick wall.
③Set enough cold slug holes at the end of main runner. Since PSU products, especially thin-walled products, require a higher injection pressure and a faster injection rate during molding, in order to discharge air in molding compound in time, a good vent or groove should be provided. Depth of these vent holes or grooves should be controlled below 0.08mm.
④Setting of mold temperature should be beneficial to improve fluidity of PSU melt during film filling. Mold temperature can be as high as 140℃ or more (at least 120℃ or more). Drying process parameters: hot air circulation drying: 120~140℃, 4~6h, material layer thickness is 20mm; negative pressure boiling drying: 130℃, 0.5~1.0h.

4. Injection molding process parameters:

①Cylinder temperature

Increasing barrel temperature is beneficial to reduce viscosity of melt, but an excessively high barrel temperature not only reduces many properties (such as impact strength), but also discolors and decomposes plastic.
Usually PSU melt decomposes at about 400℃, but in actual injection molding, decomposition occurs when barrel temperature reaches 320℃ or higher. When temperature exceeds 360℃, decomposition is obvious.
There are many factors that affect temperature setting of PSU barrel, but main factors are melt viscosity and product wall thickness. When wall thickness is less than 5mm, barrel temperature can be higher, reaching above 300℃, some grades of PSU plastic can reach above 320℃, and special grade of PSU plastic can reach above 350℃. When wall thickness of product is above 5mm, temperature can be controlled between 280 and 300℃.

②Injection pressure

Although viscosity of PSU melt is sensitive to shear rate, due to poor fluidity of PSU melt, a higher injection pressure must be used.
Higher injection pressure can increase density of product and reduce molding shrinkage, but it places higher requirements on mold. Generally, it is selected above 100 MPa, and sometimes it can reach 140 MPa.

③Injection rate

Generally, increase in injection rate is conducive to filling of melt. But for PSU, due to faster cooling rate, if injection rate is not properly controlled, it will also cause melt fracture. Therefore, in addition to product thickness of about 2mm, material has difficulty in filling mold and a higher mold filling rate is required. Generally, a small and low injection rate is appropriate.

④Screw speed

Since melt viscosity of PSU is relatively high, it is better to lower screw speed, generally 15-45 r/min.

⑤Mold temperature

Since Tg of PSU is as high as 190℃, higher mold temperatures can be used but the highest temperature does not exceed 160℃. Choice of mold temperature often varies with thickness of product. When product wall thickness is 2~5mm, mold temperature can be controlled between 10~120℃. When wall thickness exceeds 5mm or less than 2mm, mold temperature can be as high as 140~150℃.

⑥ Molding cycle

PSU has better thermal stability at normal processing temperature. It can stay in barrel for a long time at 320℃. Therefore, setting of molding cycle is only considered from perspective of production efficiency.

5. Matters needing attention:

① Molding temperature of PSU is relatively high. For this reason, inside of barrel must be cleaned before injection.
② PSU products should not contain metal inserts. Due to difference in coefficient of thermal expansion, stress concentration problems are likely to occur around insert. Therefore, try to use inserts as little as possible, or preheat inserts during molding. Generally preheat to 200℃.
③ PSU is an amorphous polymer, and product is transparent. Therefore, it is necessary to use as little or no release agent as possible during molding;
④Post-processing of products
Due to the greater rigidity of PSU molecular chain, internal stress of product is greater, and it is necessary to perform annealing treatment on it. General principle of heat treatment: in hot air circulating air box, temperature is 10-20℃ below its Tg, and time is 2-4h.

Go To Top