Ribbon screw conveyors are used for several reasons. The two most common are for handling sticky materials and material that tends to segregate during conveying. The proper ribbon screw conveyor design will eliminate the segregation of large from small and light from heavy.
A screw conveys on the periphery of the flight, so considering the % of housing loading and the material flow characteristics you can estimate the amount of material fall back that you will get when using a ribbon screw. This fall back is what will keep the material all blended while it is being conveyed.
Sticky materials tend to stick to a screw at the root of the flight to the pipe and build up from there until the screw is “logged”. This is not necessarily a uniform build up and it only takes one spot to stop the product flow. A ribbon screw feeder would be used for volumetric feeding of sticky materials. These are materials that will have minimal fall back due to the ribbon but need the ribbon design to stop product build up on the screw and subsequent plugging.