TAILIEUCHUNG - Plastic Product Material and Process Selection Handbook Part 16

Tham khảo tài liệu 'plastic product material and process selection handbook part 16', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 16 Other processes 509 Because the pressures of injection are low at approximately 25 to 50 psi 172 to 345 kPa very fragile inserts can be molded and mold wear is at a minimum. Some formulations for LIM also may be molded at temperatures as low as 200F 93C which permit the encapsulation of some heat-sensitive electronic components that do not lend themselves to encapsulation at conventional transfer molding temperatures of 300F 149C or higher. Vacuum Assisted LIM The vacuum assisted liquid molding process has been used for the manufacture of large composite parts. In this process a preform is placed in an open mold and a plastic vacuum bag placed on top of the mold. A vacuum is created in the mold using a vacuum pump. A resin source is connected to the mold. As vacuum is drawn through the mold resin infuses into the preform. Application includes the fabrication of large products with complex geometry such as panels of allcomposite buses railroad cars and vehicle components. Impregnation This method has been popular impregnating liquid plastic in products such as electrical coils and transformers. The liquid plastic is forced by pressure vacuum or their combination into the interstices of the component. A related process is trickle impregnation. It uses reactive polymerizable plastics with a low viscosity first catalyzing them followed with dripping them onto a transformer coil or similar device with small openings Chapter 1 . Capillary action draws the liquid into the openings at a rate slow enough to allow escape of the air displaced by the liquid. When the device is fully impregnated exposing it to heat cures the plastic system. Chemical etching This is the exposure of certain plastic surfaces to a solution of reactive chemical compounds. Solutions are oxidizing chemicals such as sulfuric and chromic acids or metallic sodium in naphthalene and tetra-hydydrofuran solutions. Such solutions are highly corrosive thus require special handling and disposal procedures.

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