In extreme cases, the Rat Hole can collapse resulting in material flushing out of the discharge of the feeder.
The key to feeding cohesive materials in a screw feeder is with the use of Flow Aids. One of the recommended types of flow aids is called Mechanical Agitation.
Mechanical Agitation when used properly will condition the material to a uniform bulk density and eliminate bridging/ratholing resulting in the material filling the flights of the feed screw to nearly 100%. This consistent filling of feed screw flights results in the best accuracies.
There are two primary mechanical agitation systems available – External and Internal. Both have their advantages and disadvantages.
External Agitation typically consists of external paddles agitating a flexible hopper on the inside of the feeder. The advantage is no physical contact with the material by the paddles. This also helps blended materials from becoming desegregated and fragile materials from being degraded.
The disadvantage of external paddle agitation is the flexible hopper is often not compatible with certain types of materials, material temperatures, or chemical cleaners which can leach the plasticizers out of the flexible hopper making it brittle.
Internal Agitation consists of either a horizontal or vertical agitator that gently breaks up bridging and ratholing of materials that are a bit more cohesive than others. It is important to note that Internal Agitation is not meant to mix or stir materials as the agitator typically only spins at a few rpm. Internal Agitation is more aggressive than External Agitation as the material comes into physical contact with the agitator. This could be a disadvantage should the material be adhesive and friable. Internal Agitation will also improve the filling of feed screw flights with lower bulk density materials. Conversely, higher bulk density materials may not be able to be fed due to the amount of torque on the agitation motor. Very cohesive types of materials could even turn as a “slug” inside of the feeder.
The best way to tell what type of agitation to use is by speaking with a sales engineer at a company that manufactures dry material feeders. They can make a recommendation based on the material properties and the specific application. Material testing is always the best way to confirm which type of agitation to use. Other than the transportation cost of the material(s) to and from the feeder company’s test lab, material testing is typically free of charge. You are also encouraged to witness the testing of the material as well.