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The GravEx is a gravimetric extrusion control system which utilises the loss in weight principle to control and maintain a consistent weight or thickness of your extruded product. It will maintain your product weigh to better than +/- 0.5% of target.
It will automatically react to variations in material bulk density, extruder back pressure, ambient conditions and any other influence to extruder output stability to maintain your product consistency.
The GravEx throughput controller sits between the material storage hopper and the extruder throat. It monitors the loss in weight of the material in its weigh chamber and from this calculates the mass throughput rate of the extruder. In Auto Control mode GravEx can control the extruder or the process line speed to maintain a set output or a set product weight per unit length.
When the weight of material in the weigh chamber falls below a preset value a slide valve opens and refills the chamber from the material storage hopper.
To prevent damage to the load cells the GravEx has a protective outer hopper made from powder coated zinc plated steel. The inner weigh chamber and the inlet slide valve are made from 304 grade stainless steel. The control electronics are mounted in a cabinet attached to the outer hopper.
In order to operate the GravEx controller needs input from the process: The extruder screw speed and the process line speed. In order to control it needs to be able to change the extruder screw or process line speed. There are many ways this can be done depending on the control wiring for the main system.
Rather than run potentially hazardous signals from the main electrical cabinet to the GravEx controller, a set of interface modules are installed in the main electrical cabinet and a low voltage communications cable run to the Gravex controller.
All control signals are optically isolated. Speed input modules can use digital pulses or analogue voltage input. Extruder and line control can be by motorised potentiometer, digital output, relay output or analogue output.
Fundamental to the design of GravEx is the philosophy that when not in Auto Control mode, the extruder or line speed control should revert to the normal operator control. To best achieve this GravEx uses control signals to increase or decrease the speed rather than setting an absolute speed using an analog reference output. The preferred control outputs are motorised potentiometer (for older lines) or direct ramp up/down digital inputs to the line PLC (for newer lines). Analog control output is the least desirable option because it almost impossible to avoid a step change of speed when changing from Auto to Manual control mode.
Weighbatch Ltd, 167 Ellis St, PO Box 5421 Frankton, Hamilton, New Zealand, Ph + 64 (0)7 847 0230, Fax + 64 (0)7 847 2919, firstname.lastname@example.org
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