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Additive Feeders Chapter 4: Operation 27 of 50
6. Determine the desired additive feeder setpoint (% of extruder (master) screw RPM).
To determine this, you must know:
• Total extruder feedrate (lbs./hr.).
• The RPM of the extruder screw at that feedrate.
• The letdown ratio of the additive (% or :).
• The additive feedrate required at that letdown ratio (total feedrate X
letdown ratio).
• The additive feedrate RPM needed to achieve this feedrate (as
determined by the calibration procedure).
Extruder Tracking Equation:
Set Point = (rpm of feeder / rpm of extruder) x 100
Example:
Total extruder feedrate = 500 pounds per hour
Extruder screw rpm for 500 lbs/hr = 50 rpm
Additive letdown percentage = 7%
Additive feedrate = 500 X 0.07 = 35 lbs./hr.
Feeder auger rpm needed for 35 lbs./hr. = 44 rpm (as the calibration procedure has
shown)
Assuming this, set point = 44 rpm/50 rpm x 100 = 88 or 88% of the extruder screw speed.