Now let's look at the inside of an extruder to see how it works. The plastic raw material in this hopper travels by gravity to the feedthroat and drops onto the rotating screw. The rotation of the screw conveys the plastic forward through the heated barrel. As the plastic is conveyed forward along the screw, the channel depth decreases, forcing the plastic through a smaller area. The combination of compression and screw rotation causes friction which generates heat. This is called shear heating. This heat, along with the heat from the barrel heating system, melts the plastic. By the time the plastic is conveyed to the end of the screw, it should be well mixed and at a proper temperature and pressure needed by the die. The plastic is called a melt at this stage. Beyond the screw, there can be a screen pack. The screen pack is a set of wire mesh filters that the plastic must flow through. The screens are typically from 20 to 100 mesh, which indicates the wires per inch. The screen pack is supported by a breaker plate, which is a sturdy metal plate with many holes for the plastic to pass through. The screen pack filters contaminants from the melt. The screen pack must be set in place with a coarsest screen next to the breaker plate, so each screen can support a finer one. Otherwise, the pressure difference may break the fine wires, contaminating the product and possibly doing permanent damage to the die.