Fire, SLA 3d printer and copper alloys


About half a year ago, after an active completion by the file, the SLA 3d printer in our Kiev haklab began to produce good quality parts. Plastic is plastic, but what's really cool is having a technology at hand that translates the stl model into a metal counterpart. Therefore, the next stage, which we decided to master, is to obtain metal copies of these very prints. Who are interested in our first steps in the development of this technology, please under the cat. Attention, a lot of pictures!


Let me remind you: SLA 3d printer prints a detail of layer-by-layer polymerization of a special UV-sensitive resin using a laser. As a result, the quality and detail is much higher than that of traditional FDM printers. At the moment, our homemade beast produces such results:




The technology of making metal castings on 3d prints is not new and is actively used in jewelry, dentistry and other areas of science and technology. Nevertheless, the information about each step has to be harvested bit by bit from the network. In summary, these steps are as follows:


  1. Printing parts using a special resin that can burn out at high temperatures.
  2. The part is placed in a special metal cylinder called the flask. Then this cylinder is filled with gypsum-like form-mass.
  3. After solidification of the mass, heat the flask to the temperature of ignition of the polymer and keep it until the whole polymer burns out. When it burns out, a cavity is formed in the flask, repeating the geometry of our original model.
  4. Pouring molten metal into the cavity.
  5. Cooling, casting extraction + machining.

Stage One: Printing


Generally speaking, in addition to the option of printing a model from a special material, there is an option for making a master model of jewelry wax (the so-called “wax”). This technology is hundreds of years old and the process is well debugged. When calcined, the wax melts, and a cavity is formed to fill the metal. And in the case of 3d printing, instead of wax, we have a print, and it just burns out. This is where the main difference between the burned-out resin and the traditional one manifests itself - there is almost no ash left after burning out.


Prices for resins that can be burned (labeled as “castable”) turned out to be quite biting, about $ 150 per liter. But ebay was found resin production of the Turkish company sinertek for 60, plus $ 20 shipping cost to Kiev.





This resin turned out to be much more capricious compared to those we used to print - twice the print was unsuccessful, and, moreover, it spoiled the silicone coating of the cuvette. For the third time at a lower laser power, the print was successful.


Do not ask why we chose the Iron Man's helmet and the openwork vase as the first models!


Stage Two: Preparing Flasks


To metal flowed into the cavity requires a special channel - sprue. We just stuck a piece of thin plastic tube on the superglue and fixed the models upside down in a plasticine stand. From above, then they pressed pieces of steel pipe (flask). As a result, such a “glass” with a plasticine bottom and steel walls came out.




Formmass though similar in appearance to plaster, in fact consists of very different components. Sold in stores for jewelers. The order of mixing the dry form mass with water is very important: the mass of the mass into water, and not vice versa - then there will be no lumps. After thorough mixing, it should be evacuated to avoid frozen gas bubbles on the model.







Stage Three: Polymer Burnout


Drying the flasks took a couple of days, after that they took out plasticine, a plastic sprue from the flasks, and set it on fire in the muffle. The heating profile is quite tricky: first several hours of heating at 100 degrees, then at 200, and the longest period at 780 degrees (this profile is described in the instructions for the resin). In addition, the transition from one temperature to another should be smooth, so as not to damage the flask. For our test, we made a slightly abbreviated script, having calcined for 1 hour at 100 degrees, 2 hours at 200 and 5 hours at 800 degrees. Most likely, because of this, the final result has worsened - next time we will definitely try to approach the matter more carefully.



Stage Four: Casting


For casting, we tried 2 metals. One is silver, the other is copper alloy, bought in the same store for jewelers.


In fact, the metal will not flow into the mold, if you just melt it. This is prevented by the surface tension of the metal and the slag film on it. Slags can be handled by adding flux borax. For copper alloy, phosphorous copper is additionally added.


Further, surface tension can be fought only by physical methods, namely external pressure on the metal. There are several options. Or create a vacuum at the bottom of the form, as a result, the metal will “suck” into the cavity through the micropores in the mold mass. Or use centrifugal force. Or (our last option) try to create excess pressure on top of the molten metal. The first method requires a special vacuum unit, the second - a centrifuge, and the third requires a special ... Potatoes. Yes, half the potatoes do their job.


The principle is as simple as two and two. On a hot cast-iron with molten metal, half of a potato tuber is pressed tightly on top. The water contained in it instantly boils and creates pressure, which presses the molten metal into the cavity.



Do you see potatoes? Not? And she is ...



Here is a fungus.



After cooling in the water we get gizmos:




After processing sandblasting and sandpaper.



Results


The result is far from ideal, so we plan to try different modes of printing and annealing to find the best one. This time we abruptly moved from 200 degrees to 800 when roasting the flask, which, possibly, led to cracks. In addition, the copper alloy showed itself in a strange way and a lot of slag remained on the surface of the casting. So the field for experiments here is big.


Disclaimer


We are not wizards, we are just learning, therefore inaccuracies in terminology, technology and other incidents are possible. Constructive criticism is welcome.


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