Injection molding

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The process used to form the plastic substrates of DVDs and many other plastic items. Raw plastic is fed into the machine in the form of small granules or pellets. The plastic enters a barrel where a screw moves the plastic forward and applies an increasing amount of pressure while heating and melting the plastic. The screw turns on each backstroke, pushing molten plastic in front of it. When the mold closes, the screw pushes forward during the injection stroke, forcing liquid plastic through a nozzle into the mold cavity. With DVD molding, the cavity is 120 mm. in diameter, and only 0.6 mm thick. On one side of the cavity, the mold holds a metal stamper, which has the pattern of pits (for pre-recorded discs), or grooves (for recordable media). The mold stays closed for a few seconds, while the plastic cools and hardens. The plastic enters the mold at around 350 degrees Celsius, but the mold is a large block of metal with water cooling, held at a temperature of roughly 95 degrees Celsius. The mold opens and the disc substrate is removed by a robotic take-out arm. The small center section of the molded substrate is discarded, leaving a 15 mm diameter center hole.

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