Core flow hoppers

Figure 8.1. Hopper types based on flow patterns (a) Mass flow hopper, (b) Core (funnel) flow hopper.

Tablet machines and capsule filling machines store the powder to be processed in a hopper above the machine. It is important that the powder flows from the hopper to the filling station of the machine at an appropriate rate and without segregation occurring. There are two types of flow that can occur from a powder hopper core flow and mass flow (Figure 11.1).

Whether core flow or mass flow is achieved is dependent on the design of the hopper (geometry and wall material) and the flow properties of the powder. For most pharmaceutical applications, the hopper design for a particular machine will be fixed thus, it is incumbent on the formulator to ensure that mass flow is achieved by modification of the powder properties. 

The funnel flow channel essentially consists of two component parts (i) the drained repose or unconfined surface layer, and (ii) the core flow channel confined by the static bed. Should a hopper with this pattern of discharge be refilled before it has emptied, some of the original material... 

Figure 10.1 Mass flow and core flow in hoppers (a) core flow (b) mass flow...

Core flow. This occurs when the powder flows towards the outlet of a silo in a channel formed within the powder itself [Figure 10.1(a) . We will not concern ourselves with core flow silo design. Figure 10.3 (a-d) shows sketches taken from a sequence of photographs of a hopper operating in core flow. Note the regions of... 

Figure 10.3 Sequence of sketches taken from photographs showing a core flow pattern as a hopper empties. (The black bands are layers of coloured tracer particles)...

For conical hoppers the slope angle required to ensure mass flow depends on the powder/powder friction and the powder/wall friction. Later we will see how these are quantified and how it is possible to determine the conditions which give rise to mass flow. Note that there is no such thing as a mass flow hopper a hopper which gives mass flow with one powder may give core flow with another. 

Explain with the aid of sketches what is meant by the terms mass flow and core flow with respect to solids flow in storage hoppers. 

Explain how core flow of free-flowing particulate mixture from a hopper gives rise to a size-segregated discharge. 

Once the cavity and core insert sets had been machined, the student then assembled these components into a standardized master mold base. The mold was then placed in a injection-molding machine. The student then selected a process based on the results of his commuter-aided flow filling simulation. The process was set up on the injection-molding machine. The resin was loaded into the hopper after drying, when it was required. The student then molded approximately 50 plastic parts for evaluation. A first article inspection procedure was done, and small capability studies were completed. 

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