we were using a couple batter mixers for water based inks because it's easy with the pressure washer to clean the mixers off. SPA uses bench mounted mini drill presses to mix ink. They buy 2 or 3" wide flat holesaw bits, then grind off the center tip and sand the final flat bottom to be a little rounded. Then when mixing plastisols, it's an easy wipe of the two flat sides. They do a ton of custom mixed inks for customers that they supply and the flat bottom bits seem to really mix all the way to the bottom of the ink bucket or quart cups very well.