Thursday, July 8, 2010

Re-designing the dry mixer

These are a few ideas I'm considering for a new dry mixer. These are all based on commercial ribbon mixers. A ribbon mixer uses a helical design to move material throughout its volume. When designed properly, its is a very effective way to mix because virtually all the material is constantly moving. Also, it is possible to design in a way that avoids corners (corners = dead space = stagnant material = no mixing).

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 1. Inner helix moves material in one direction. Outer helix moves material in opposite direction to the smaller inner helix.

Version 2. Similar to version 1 but this is a dual start design: two helixes for both inner and outer sections. This should improve mixing.

Version 3. Helixes are mirror around the middle point of the axis. Outer helixes bring material from the ends of the chamber towards the middle, while inner helixes bring it from the middle outwards. Material converges and diverges at the middle and ends of the container. One drawback to this design is that there are corner areas where material may collect and stagnate. I've highlighted one of the two starts for the outer helix to make this clearer. There are in fact 4 points where this happens (2 for the outer helixes, 2 for the inner helixes).

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... 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!

A Muller mixer

Regardless, I will build version 4 above to replace my current dry mixer for general use.

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