To do sand casting a muller is almost a necessity. It is difficult to mix up foundry sand either green sand or oil bonded sand without a muller. I have been thinking about how to build a muller for some time and recently I started to build one. Basically a muller mixes and squeezes the sand a binder to get a uniform mixture with the binder coating the sand.
The basic design I chose uses a rotating cylinder with a plow and heavy cast iron wheels for mixing and squeezing fixed to a bar above the cylinder. A transaxle from a garden tractor is used to reduce the speed of an electric motor and turn the cylinder. Besides the transaxle the cylinder is supported by four casters mounted to a frame under the cylinder.
I made a sketch using Fusion 360 to determine the size of the frame based on a drum diameter of 18 inches. I wanted the frame to be as small as possible but be able to mount the drum support rollers on top of the frame on each side.
The first thing to build was the cylinder. It consists of a 18 inch diameter 1/4 inch thick steel plate and a 6 inch wide strip of 12 gauge steel. I had the disk plasma cut by the steel supplier and I should have had the strip rolled into a ring. Bending and wrapping the strip around the disk was a bit fiddly. I clamped the disk to my welding table and used a ratchet strap to tighten the strip around the disk.
The ratchet strap and the tack welding on the inside tended to make the rim lean inward. To fix this I used the jack to push it open before I welded the side seam.
The frame is build out of 1.5 inch square tubing. It took almost all of a 24 foot stick of tubing to build the frame. 
Here is the frame with the garden tractor transaxle installed for a test fit. The bottom of the legs have plates welded on that are tapped for casters so that I can easily roll the muller around the shop. The frame did not come out exactly square so it would rock when sitting on a flat floor. I fixed that by putting a washer on top of two of the casters. The two longer legs will support a crossbar on top of the muller that holds the roller and plows.
The pully that came with the transaxle was going to give a drum RPM that was higher than I wanted. This larger pully has been laying around and would give a suitable RPM. Problem is the original pully was attached with a special spline so I cut the hub with the spline out of the original pully trued it up in the lathe and welded it into the bigger pully.
The final part of the construction was building the plows and roller assembly. It took a bit of experimenting to get it right. One plow pushes the sand away from the center of the drum and the other plow pushes the sand away from the outside. This piles the sand right in the path of the roller. The roller is made of 4 five pound barbell weights glued together. After experimenting with this I may add two more weights to cover more of the sand with each revolution.
Above is the completed muller. It was necessary to clamp the bottom axle so that the drum would turn.
The first thing I tried with the muller was reconditioning some oil bonded sand. This worked well until the sand got well mixed. Then the oil bonded sand got sticky and built up on the roller until the roller stalled. At this point the sand was mulled well enough so it was not too big of a problem.
One of the reasons for building the muller was to be able to make and use water bonded sand (green sand). So I made a few batches of green sand. The bentonite clay that I used is oil adsorbent which is fairly larger granules. I probably could have just dumped it in with the sand and mulled away but I thought it would work better if I ground up the bentonite first. The muller did a good job of grinding up the clay and after about 30 minutes in the muller it was most a fine powder. To make the green sand I added the bentonite and fine silica sand to the muller in the correct proportions and let it run dry for a bit to mix it well. Then water was added in small amounts until the correct consistency was obtained.
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