Weighbatch make two types of gravimetric extruder controllers. The Gravex range are typical “loss-in-weight” controllers which sit on the inlet to the extruder and measure the mass rate of material going into the extruder throat. The Monex range are less typical and are “gain-in-weight” controllers which monitor the output of a palletising extruder. For more information on the Monex range click here...

An alternative to the Gravex is the ExtruBlend which combines a MiniBlend blender and a Gravex controller into one compact unit. For more information on the ExtruBlend range click here ...

Gravex Controller

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 achieve a set kgs/hr or a set grams/meter

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 loadcells 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.

Interface Modules

In order to operate the Gravex controller needs input from the process: The extruder RPM and the process line speed. In order to control it needs to be able to change the extruder 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 analog voltage input. Extruder and line control can be by motorised potentiometer, digital output, relay output or analog output.

Control Outputs

Fundamental to the design of Gravex is the idea 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.