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3D-Printing of Clay

3D printers that can extrude a clay paste are available now. And they are not cheap, there is a “gold rush” among suppliers to sell you on them. The idea is that objects themselves can be printed by extruding layers of a ceramic paste from a nozzle or by glue-bonding powder particles layer-by-layer. These are additive processes (as opposed to subtractive where material is cut away from a block). However, before you "buy in" to the hype of 3D printing clay, stop and think. A clay 3D printer is basically a tiny pugmill moving around squirting out jello-consistency goop, hoping that it will stand up. For some types of pieces (generally very vertical ones) this is OK.

Again, the practicality, quality and shapes that can be made are still big limiting factors to the technology. That being said, there are many applications that cannot be done any other way but printing with a ceramic paste. For example, refractories (like shelves, posts, supports and even entire kilns) do not need a perfectly smooth finish, but there is huge advantage in their being light-weight. By printing thin walls and a honeycomb infill within the refractory they can be made very light and well insulating. And can be made from much more high quality (and expensive) materials.

The RepRap international movement to develop open source hardware and software platforms for 3D printing has help push 3D printing in ceramics. Anyone can buy and assemble a standard filament printer to learn many details of their mechanics and operation. Once understood, a printer of any size could theoretically be constructed. In clay-printing the focus is on the print head and how to deliver a thick paste to it, extrude it an a constant rate (being able to turn the flow on and off in an instant is another matter). One solution is to pressure-feed the paste to a print head having a barrel and auger like a pugmill and control the auger using a stepper motor to feed the actual extruder nozzle.

There is a problem with scaling to a bigger size. The printhead or platform must be able to move on at least two axes. There must be a minimum of mass resisting movement in order to have precise and quick movement. But clay is heavy and if the printhead is full of it it cannot be responsive (e.g. Delta printer designs). Likewise if the item being printed is heavy the bed on which it sits cannot move too quick (because the clay, or paste, is soft). And, lower layers of very soft clay must be able to support the entire piece, this obviously limits size. And any degree of departure from vertical side walls.

A major challenge is making the clay set fast enough so that the next layer applied over it will have a firm base. When plastics are extruded in typical home 3D printers they simply cool and firm up, but clay pastes are soft and fragile. Additions of polymers can help set them after extrusion. Augmenting the water with alcohol (e.g. ethanol is preferred over isopropyl alcohol because it carries off more water as it evaporates) speeds evaporation. Pure ethanol and clay is flammable and the plasticity and dry strength are poor. But 50:50 water:ethanol is not flammable and workability is excellent. However, even though the clay feels cold evaporation does not proceed that quickly, it needs a fan to stiffen up fast enough. Under greater pressure stiffer pastes of lower water content can be delivered. When large objects are printed, fans and the extra time between layer delivery may be enough to enable structural integrity. In some types of 3D printing, support structures of a different material are printed with the item and these are later removed, this could be practical for ceramics also.

Notwithstanding all of this, printers designed for clay are appearing on the market (with lots of excitement) and more are coming. You could easily spend $10k but use caution, the laws of physics and common sense apply. Machines have differing priorities. Those that must push clay through a thin tube to a tiny pugmill in the printhead (e.g. the Lutum and WASP machines) will obviously need soft clay and to be reloaded more often and they may not work well with deflocculated bodies (the auger having trouble getting traction). The PotterBot is moving the entire piece constantly (on the x-y axes), and when that piece gets large it becomes rather like a big cube of jello on a plate being jerked around! Obviously the body will need to be stable or it will just collapse. The only solution is often to simply print really slow. This can make the novelty of 3D printing clay wear off pretty fast! Many are shocked when they realize that even normal printing time for a large piece could be ten hours!

Don’t be stuck with a fancy machine and no clay that works with it. And, you can be sure, the manufacturers are going to follow the revenue model of ink jet printers so brace yourself when you find out the price of cartridges. It might be best to make your own bodies, that is what the pros are doing. You need a propeller mixer to blend the powder and water (more powerful mixers will do this much better). Run the mixer until all air bubbles have surfaced (to de-air it) and all particle surfaces are wetted (this could take 15 minutes). Then pour it on a plaster bat to dewater to the needed consistency. A hand extruder can be used to create the diameter needed to feed the machine (the clay needs to be soft). The character and suitability of the body will be a big part of any success you have, and understanding the recipe and being able to control it will give you a big edge (especially if you want to incorporate alcohol). Click the links below for information on a mixer and plaster table.

Clay suppliers are also producing clays for this. But be skeptical. One of the bodies claims to have 40% 80 mesh grog, however our testing found no grog! Besides, grog is undesirable since it would wear out the print head quickly. Another porcelain has 25% water yet claims to have drying shrinkage of 6.5% (this is highly improbable and even if true would make it too non-plastic and fast drying). Some have shipped in, at considerable cost, non-plastic 3D printing clays and found that the plastic bodies they have used for years work better! When printing taller objects it holds up better. When printing take a long time the bottom and top of a piece are soft and the center gets stiffer, a plastic clay that dries slower is better. In addition, printing overhangs is a big issue so plastic clays that hold up better are needed.

Because of the difficulty of preparing the clay (because it needs to be so soft, bubble-free and homogeneous) suppliers are going to introduce cartridge solutions. The advantage will be more stable & repeatable performance and a ready-to-use product with good strand adhesion and plastic strength.

To make your own body follow the same pattern. Start with an existing recipe for a plastic pottery clay (your body manufacturer may give you the generic recipe of the body you already use). Many are finding that plastic bodies are best, they stick together well, hold up and do not dry too fast. To increase plasticity of a body add bentonite (or vice versa), change kaolins to ball clays or use a more plastic kaolin. Adequate silica is needed so glazes are easy to fit (usually 20% or more). And it will need enough feldspar to make the body vitreous. For example, the popular 50:25:25 recipe for cone 10 is 50 clay, 25 feldspar and 25 silica. For cone 6 it would be closer to 45:35:20. Since it is practical to make your own paste, consider trying the Zero3 porcelain for cone 03 (it contains frit to make it vitrify).

3D Printing Polar Ice Porcelain

3D Printing Polar Ice Porcelain

By Bryan Cera.

Here is what happens when an overnight 3D print goes wrong

Here is what happens when an overnight 3D print goes wrong

From Brooks Talley. At some point during the night the base could not support the layers being added and it collapsed. The printer happily just kept printing in mid air for the rest of the night!

Out Bound Links

By Tony Hansen

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