I. PEEK Material Properties
PEEK is a semi-crystalline thermoplastic polymer with a glass transition temperature of about 143°C and a melting point of about 343°C. PEEK has excellent mechanical properties, with tensile strengths of 90-100 MPa and flexural strengths of 140-160 MPa. At the same time, PEEK also has good chemical resistance, and is stable in most strong acids, bases and organic solvents. In addition, PEEK is flame retardant. In addition, PEEK has excellent flame retardant properties, with an ultimate oxygen index of more than 35%.
Material Preparation
Drying
PEEK material has strong water absorption, in the injection molding must be sufficiently dry before processing, in order to avoid moisture in the injection molding process caused by bubbles, silver wire and other defects. It is generally recommended to dry at 150 ℃ for 3-4 hours, after drying the moisture content should be controlled below 0.02%.
Material Storage
PEEK materials should be stored in a dry, ventilated and cool environment, avoiding direct sunlight and humidity. At the same time, should pay attention to different batches of materials stored separately, so as not to affect the quality and stability of injection molded products.
Third, the choice of injection molding equipment
Injection molding machine
Due to the high processing temperature of PEEK, the barrel and screw of the injection molding machine need to have good high temperature resistance. General use of special high-temperature injection molding machine, its barrel and screw are usually made of special alloy materials, able to withstand high temperatures of more than 400 ℃.
Injection unit
Injection unit injection pressure and injection speed should be able to meet the requirements of PEEK injection molding. The injection pressure is generally between 100-150 MPa, and the injection speed is adjusted according to the structure and size of the product.
Heating system
The heating system should be able to uniformly and stably raise the barrel temperature to the set value, and maintain good temperature control accuracy. Generally, electric heating or oil heating is adopted to ensure the accuracy and stability of the temperature of each section of the barrel.
Fourth, mold design
Mold materials
Due to the high temperature and high pressure in the PEEK injection molding process, the mold material should have good heat resistance, wear resistance and corrosion resistance. Commonly used mold materials are H13 hot work mold steel, S136 stainless steel and so on.
Mold structure
The structural design of the mold should take into account the shrinkage and fluidity of PEEK, the shrinkage of PEEK is large, generally between 1.2% -2.0%, so in the design of the mold should be reserved enough shrinkage allowance. At the same time, in order to ensure that PEEK can fill the mold cavity smoothly, the mold gate, runner and exhaust system should be designed reasonably.
Cooling System
Effective cooling system is essential to control the dimensional accuracy of PEEK injection molded products and shorten the molding cycle. Cooling channels should be evenly distributed to ensure uniform temperature in all parts of the mold.
V. Process parameter setting
Cylinder temperature
The barrel temperature of PEEK is generally divided into three control sections, namely, charging section, compression section and metering section. The temperature of the charging section is generally between 320-340℃, the temperature of the compression section is between 360-380℃, and the temperature of the metering section is between 380-400℃. Specific temperature settings should be adjusted according to the performance of the injection molding machine and the structure of the product.
Mold temperature
Mold temperature has an important impact on the performance and appearance of PEEK injection molded products. General mold temperature control between 160-200 ℃, for the complex structure, wall thickness of the product, the mold temperature can be increased appropriately.
Injection pressure and speed
The selection of injection pressure and speed should be adjusted according to the structure and size of the product. General injection pressure between 100-150 MPa, injection speed between 30-80 mm / s. For thin-walled products, a higher injection pressure should be used. For thin-walled products, higher injection speeds and pressures should be used to ensure the filling integrity of the product.
Holding pressure and time
The holding pressure and time are set to compensate for the shrinkage of the product during the cooling process and to ensure the dimensional accuracy of the product. The holding pressure is generally 50%-80% of the injection pressure, and the holding time is adjusted according to the thickness and size of the product, generally between 5-20s.
Cooling time
The length of cooling time depends on the thickness of the product, the temperature of the mold and the thermal properties of the plastic. Generally, the cooling time is between 20-60s, for products with large wall thickness, the cooling time should be extended appropriately.
VI. Common Problems and Solutions
Air bubble
Bubble is one of the common defects in the PEEK injection molding process, the main reason is that the material drying is not sufficient, the injection speed is too fast or poor mold exhaust. Solution measures include: strengthen the material drying process, reduce the injection speed, optimize the mold exhaust system.
Silver filaments
Silver threads are usually formed when moisture or volatiles in the material vaporize during the injection process. The solution is to ensure adequate drying of the material, increase the barrel temperature and reduce the injection speed.
Warp Deformation
Warp deformation is mainly caused by uneven mold temperature, insufficient cooling time or unreasonable product structure design. It can be solved by optimizing the cooling system of the mold, prolonging the cooling time and improving the product structure design.
Poor surface roughness
Poor surface roughness may be caused by wear and tear of the mold surface, low injection speed or high barrel temperature. Solutions include: repairing or replacing the mold, increasing the injection speed, and reducing the barrel temperature.
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