For our midterm project, we have to cast a chess piece. There are 4 steps to casting a chess piece from an .stl file of the chess piece. The first step is modifying the .stl file to 3D print and CNC the positive mold. Next is to actually create a positive mold using 3D printing and CNC. Next, we cast the negative mold using silicone. Finally, we cast the chess piece by pouring a quick setting plastic mixture into the silicone mold.
For this project, we decided to create a Barack Obama chess piece using an .stl file we found on Thingiverse.
For the first step, modifying the .stl file to 3D print and CNC the positive mold, we used MeshMixer to split the .stl file in half vertically.
We then used the fill holes feature to remove any hollow aspects of the mold and make it more defined
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More defined meant we had more triangles so that the 3D printed and Carved molds could be as detailed as possible.
Afterwards, we added a platform and pins and sockets to each side to help align the silicone molds when we cast the final piece, all of which was done in Fusion360.
Initially, our Fusion360 model was had a smaller backboard with the model shifts down some with an extrusion for the fill hole. This was problematic because having a smaller backboard means it is possible for the mold to warp when pressure is applied when creating the mold. The funnel idea was replaced with just using the bottom part of the empty space pressed against the end of the backboard to serve as the funnel instead.
For the 3D printing file, we saved the file as an .stl and used the slicer on 3DPritnerOS.com to slice the file into a gcode file that the 3D printer can read. In order to make the 3D print as detailed as possible, the layer height was reduced to .1 mm and the infill density was increased to 20% for the Prusa i3 MK3.
Next, we then sent the file gcode from 3DPrinterOS to the 3D printer to print the face side of the positive mold. The finished print is pictured below before the mold is poured.
For the CNC file, we used the manufacturing mode in Fusion360 to create the gcode files for Carvey, our CNC machine. We created 2 CNC files. One is for the first pass to cut the material to roughly the correct shape using a ⅛” bit, and the second with a 1/16” bit is to get sharper details.
For the CNC, we uploaded the gcode files to Easel and used Easel to print the file using Carvey. We followed the instructions on Easel of measuring and securing the wood inside Carvey before carving it. For the material, we used a small piece of 2”x4” wood, approximately 8 inches long. The first pass took approximately an hour, and the second pass took 10 minutes.
Unfortunately, the Carvey was beginning to break down during our carving process. Thus, it was unable to line up with the 2nd pass with the final detail with the rough cut. As a result, it just ended up cutting through the entire rough cut which forced us to stop the cut. Therefore, we had to make do with just the rough cut for the mold pictured above.
After we have the two positive molds, one from 3D printing and one from CNC, the next step is to cast the negative mold using silicone. First, we use cardboard to make walls to contain the silicone (pictured above). Next, we mix the two silicone compounds in a 1:1 ratio by mass, and pour it into each of the positive molds. The silicone takes approximately 6 hours to set.
After the silicone molds have set, we remove them from the 3D printed and CNC molds and put them together.
We then put rubberbands around them to keep them sealed.
Next, we mix the plastic mixture in a 1:1 ratio by volume and pour it into the silicone molds.
It takes approximately 15 minutes to set. After it was set, we removed the rubber bands and took the mold apart to remove the casted chess piece.
The final two pieces were achieved after several attempts since we had issues with lining up the two halves and avoiding air pockets in the nose and chin of the model (example pictured below).
Cost
For the 3D printed positive mold, the cost of the PLA plastic is $0.50, and it took 5.17 hours to print. For the CNC, the cost of the 2”x4” plywood is 0.018$ per in^3 of wood (used 64in^3), and it took 70 minutes total to carve. To cast the silicone mold, the cost is $165 for 2 X half-gallon bottles and we used approximately 200g of the material. Creating the cardboard boundaries for the CNC and 3D printed part along with pouring the mold and waiting for it to set took about 7.5 hours. The plastic costs $70 for 2 X half-gallon bottles and we used approximately 50mL of the materials. Creating the plastic solution and pouring the plastic into the mold took approximately .5 hours.
Wood: 64 * 0.018$= $1.15
Silicone: 0.02gal(200g) *165$/gal= $3.3
Plastic. 0.013gal(50ml)*70$/gal= $ 0.91
Total Monetary Cost: $5.46
Total Time Cost: 14.34 hours
Failures
Although it wasn’t a failure, we made modifications to our initial design for the positive mold because it would be easier to pour and cast the piece without the pour spout. There was one failed CNC carving due to the placement of the positioning of the cutting route being in contact with the clamps, so we have to move the positioning slightly over. The last failure we had was when Carvey decided to shift each level slightly for our 2nd try at carving the positive mold. It resulted in the piece being completely skewed and unusable.
