Volumetric feeders distribute powder, granules, and pellets based on volume rather than weight. Accuracies are usually two to five percent of the target but can be less than one percent with easy-to-handle powders. Cohesive and poor flowing materials require additional conditioning to achieve constant flow and batch delivery.
Trantec’s Volumetric powder feeder consist of two or three key parts, depending on the material’s flow characteristics. The feeder body, also known as the conditioning section, (optional) agitator, and auger screw.
Volumetric Feeding process
Material is tipped or conveyed into the conditioning section of the metering feeder hopper; the auger rotates at the required speed, producing a perpendicular motion. The Material penetrates between the rotating auger flights, and is propelled into a tube or trough that can control the material flow direction.
Depending on the material characteristics, the metering feeder may need product conditioning through agitation. An agitator turns 360 degrees, gently stirring and moving the bulk solid, removing any confined air or lumps present in the material. Agitators operate continuously or intermittently and have several geometries to suit the characteristics of the powder, flake or granule being metered.
Delicate and friable materials could bridge unless gently and intermittently activated. In continuous operation, the agitator will help keep fibrous materials loose, or de-aerate and densify fine powders. However, for very free flowing materials, the feeder can be operated without an agitator installed.
Volumetric feeders are manually calibrated to select the correct dosing speed, the rpm varies based on the characteristics of the material. Trantec carries out rigorous product testing to configure your machine to your requirements at the specification stage and these test results act as an on-site guide to the feed rates you can expect.
Screw Feeder Auger
The most critical part of any screw feeding system is the metering screw itself. Located in a trough at the base of the feeder, the flighted auger screw converts the rotary motion into linear motion. The flights of the auger are angled (known as the helix angle) which determines the motions that are imparted into the bulk solid. Variations of the auger, such as diameter, flight pitch and core size play a key role in defining overall accuracy and feed rate of a metering feeder.
Variable pitch and variable core screws
For some products, the most exact metering is achieved using variations in the auger flight pitch. Varying the pitch along the length of the screw can intensify the density of the bulk solid as it moves towards the feeder outlet. Even density equates to more accurate volumetric dosing of the powder.
Incorporating a taper core to the screw produces similar effects. Combining the two elements optimises machine performance.
A final variation is to add an extra flight in between the existing flights of an auger screw. Known as a multistart auger, the effect of the extra flight is to smooth the product flow.
At slow speeds, single flight screws have a “pulsing” effect on the material flow at the outlet of the feeder. The pulse of powder occurs each time a flight rotation passes a point on the feed tube. At higher feed rates, these pulses join up and appear continuous. But at lower feed rates the pulse becomes noticeable and can be problematic. In the past, the only way to address this was to buy another auger or new machine.
However, adding an extra flight at once halves the pulsing effect, on some augers a third or even fourth flight can be added. These multi-start screws effectively extend the dynamic range of the metering feeder and are a specialist area for Trantec volumetric screw feeders.