At first we tried watering down modeling clay and this works but its hard to get consistent results. Now we always start from clay powder that you can find in the better ceramics supply store. I found out lately by buying a different type of clay that I had been lucky with the first clay that I bought because the last one had a really tough time NOT to collapse during print. The difference? Fat or lean clay, the one I always use is semi-fat (or semi lean, depends on your ideologies) and the one that collapses is lean. I can't find a good explanation in english online but fat clay particles are more plates and have a better cohesion. (if anyone has the explanation somewhere in english?)
So you'll want semi-fat or fat clay for 3d printing with extrusion. Next you need to balance the viscosity so that it extrudes around 4-5 bar (60-70 psi) but has enough strength to support itself as a build object. A water : powder ratio of 1 : 2.2 gives the best results for the clay powder we use here, this may vary for the powder clay you buy in the store but this might be a good starting point.
You could also reinforce your clay with cellulose fibers (paperclay). I have only done a quick test to see if it extrudes but did not build an object with it. I can imagine that it clogs faster.
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