Volumetric vs. gravimetric feeding
Most feeders may be categorized as volumetric or gravimetric. Volumetric feeders operate by delivering a certain volume of material per unit time and are typically the feeding solution with the lowest capital cost. However, volumetric screw feeders cannot detect or adjust to variations in a material’s bulk density during operation. As a result, these feeders are typically most effective with relatively free-flowing materials that have consistent bulk density, such as pellets, and in applications where a guaranteed feeding accuracy is not crucial to the operation.
During gravimetric feeding, dry bulk material is fed into a process at a constant weight per unit of time. Gravimetric feeding provides better monitoring of the feeding process by providing a feedback loop that measures weight and speed. This helps to determine the actual weight of material being fed on a second-to-second basis.
Once the size and type of feeder is established, a hopper of the appropriate shape and size must be selected to contain the right amount of filler required for the continuous plastics-compounding operation. Hoppers are available in cylindrical, asymmetrical and symmetrical shapes, and in sizes ranging from one liter to several hundred liters.
A feeder hopper is sized based upon the refill requirements of the feeder and the physical space available at the site. A general rule-of-thumb for calculating the appropriate size is 12 hopper refills per hour, with the maximum fill level in the hopper at 80% of the hopper volume.
Precipitated calcium carbonate tends to compact in hoppers and may cause the formation of flow-stopping ratholes and bridges. Flow-aid devices that work by inducing particle-particle vibrations - such as Coperion K-Tron’s ActiFlow® - should be considered to ensure predictable flow by preventing the formation of highly dense material zones inside the hopper. In addition, this approach can help to reduce headroom requirements and eliminate cleaning concerns because there is no need to use mechanical agitators inside the hopper. For extremely cohesive materials alternative mechanical agitators are available.
Feeding devices vary depending on the bulk material to be fed: single screw or Bulk Solids Pump™ (a patented product by Coperion K-Tron) for free-flowing powders and granulates, twin-screw feeders for difficult powders (Fig. 2), vibratory trays for fibers and friable materials.
While a single-screw feeder may work with free-flowing grades of CaCO3, a twin-screw feeder is generally recommended to achieve a reliable result. A variety of screw designs are available according to the flow rate and characteristics of the mineral filler.
Because calcium carbonate is so widely used in plastics-compounding operations, it is important to appreciate how much the success of the overall operation depends on its feeding solution. The correct design of feeding systems for calcium carbonate is not a trivial proposition. There are a number of variables that affect the flow of this often difficult powder, and the return on investment of a plastics-compounding plant is directly impacted by the proper selection of the feeding system.