Other Advantages of Using a Melt Pump

The prime reason for adding a melt pump to an extrusion system is to improve the throughput stability - from between 5% t0 8% typical for an extruder used alone to the 0.1% of the typical melt pump with differential pressure control. There are, however, other major advantages in using a melt pump at the output of the extruder: The output can almost always be increased - typically by 15 t0 25%. The throughput curve of an extruder used alone as shown in Fig. 6.2 illustrates that the screw type pump in an extruder is a very inefficient pump. If there is any significant pressure at the die, the output of an extruder used alone is reduced by a significant amount, up t0 45% if the back pressure is 4000 psi. But when a melt pump is used, the extruder only has to generate enough pressure to fill the teeth of the gear pump. The gear pump will generate the pressure required to push the plastic through the die.

The pressure required to push the plastic through the die is now being generated by an efficient pump, the melt pump, rather than by the very inefficient screw type pump. Even though there are now two motors instead ofjust one, the total power consumption is almost always less. The melt temperature of the plastic is almost always significantly lowered. Reducing the totalpower consumed also reduces the excess thermal energyimparted to the plastic. This can be a significant advantage for heat sensitive plastics, such as PVC. It also reduces the downstream cooling required, since there is less heat to be removed. In a blown film application, for example, lower melt temperature means higher throughput for a given height of cooling tower.

Much higher die pressures can be achieved, where they are required The melt pump is simply a far more efficient pump. So, dodt be a Luddite: use a melt pump wherever high throughput stability and close tolerances are required!

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