I plan to continue using the 5-gallon format as its universally available, inexpensive, easily cleaned and cheap!
The following are a few iterations on a possible design. Arrows in the images represent flow of material...
Version 4. Similar to version 3 but here one side of the helixes has been rotated 90 degrees. This avoids the corners in version 3.
The following video shows a commercial ribbon blender in action... http://www.youtube.com/watch?v=LKeMvSyq_4U. The main difference between commercial units and the one I am designing is that in my design, the blades are fixed relative to the mixing chamber, which simply rotates on a ball mill unit.
I'm leaning towards version 4. I plan to use stainless or aluminum for the central shaft and perhaps Plexiglas for the helixes. I also want to make the entire helix assemble removable from the bucket for easy cleaning.
I met with an engineer today to go over this. His opinion is that while ribbon mixing is a decent approach, it won't be effective with regards to clumped materials (specifically the powdered sugar in the recipe I'm using). To really mix well, we need to consider a high-shearing mixer (for our purposes, shear = friction). We have a Muller mixer which mixes using high shear. The high shear comes from the two solid-steel wheels which rotate around the center of the machine while running over anything in their path. Any lumps of material will be quickly broken up and dispersed. Scrapers reposition material in the path of the wheels, ensuring that everything comes in contact with the wheels. We use a Muller mixer to mix plastic clay bodies on a daily basis. It is truly an awesome machine!