Plastics, hopper feeding systems

Changing the initial melt temperature in either direction may solve the problem. With a barrel and screw, it is important to study the effects of temperature changes. Another approach is to increase the pressure in processes that use process controls. Particle size, melt shape, and the melt delivery system may have to be changed or better controlled. A vacuum hopper feed system may be useful. With screw plasticators, changes in screw design may be helpful. Usually, a vented barrel will solve the problem. 

For use in plastics, the preferred system is masterbatch or compound, as the pigment is a very hard substance with needle-Iike particles that are difficult to incorporate directly into a resin. For masterbatch, the processing temperature should be about 10°C higher than normal and the recommended machine configuration is one with a distributive screw design and twin hoppers, using the first to feed in resin and additives and the second to dose the pigment into the melt. 

For uniform and stable extrusion it is important to check periodically the drive system, the take-up device, and other equipment, and compare it to its original performance. If variations are excessive, all kinds of problems will develop in the extruded product. An elaborate process-control system can help, but it is best to improve stability in all facets of the extrusion line. Some examples of instabilities and problem areas include 1) non-uniform plastics flow in the hopper 2) troublesome bridging, with excessive barrel heat that melts the solidified plastic in the hopper and feed section and stops the plastic flow 3) variations in barrel heat, screw heat, screw speed, the screw power drive, die heat, die head pressure, and the take-up device 4) insufficient melting or mixing capacity 5) insufficient pressure-generating capacity 6) wear or damage of the screw or barrel 7) melt fracture/sharkskin (see Chapter 2), and so on. 

With mirri-brrlk containers, the plastic is suctioned, into the hoppers that feed the machines. This transfer system woiks well with pellets of uniform size, but less so with powders, which tend to stratify. Medium-sized injection molders prefer to use the suction transfer system because it is inexpensive. In contrast, a single silo bulk system feeding one hopper bin costs about 100,000, plus 20,000 for each additional receiver hopper bin. 

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