For our final project, we worked on casting 4 identical chess pieces using a combination of prototyping and fabrication techniques we have learned, including using the CNC miss, 3D printing, and molding and casting. We selected a creative piece that portrays our group’s creativity and humor. So Gabby creatively suggested we create a piece using Dwayne Johnson’s head. And so “The Rook” was born.
Step I: 3D Printing
Once we decided on our chess piece design, we downloaded a .STL file imported into Mesh Mixer and edited it to create the whole piece, well adjusted to the head. After we were happy with the piece’s shape, we moved on to 3D printing it on a Bamboo printer available at the OEDK.
Next, we cut our piece vertically in half, exported it as an STL file, and modified it on SolidWorks (SW). Once the file was successfully loaded onto SW, we created additional geometry of our object and extruded it to a ~10 mm thickness. Next, we made four equal-sized circles to be our holes/pegs, each with a diameter of ~5mm and an extrusion thickness of ~2.25 mm. Lastly, we made an air vent from the tip of the object to the bottom to prevent air bubbles in our cast and 3D printed this file using the printer mentioned above.
Step II: CNC Machining
After we finalized the SW file (ensuring that “The Rook’s” neck wrinkles were intact), we set up the file in VCarve to generate the corresponding g-code. Using the methods and precautions learned in class and the previous CNC homework, we set up the Shapeoko CNC machine to carve our piece out. The total cutting time was 1hr 11min (46 min ¼” drill bit for roughing and 25 min ⅛” drill bit). The piece was sanded to ensure smoothness and maximum neck-wrinkle retention.
Step III: Creating Molds
Once we were satisfied with our halves, we calculated the amount of silicone we needed to create our mold using a 1:1 ratio of parts A and B. Before mixing and pouring, we added spray mold-release on the CNC half.
We created a cardboard frame casing around the 3D printed and CNC cut halves to pour the 60 mL silicon solution into. To do this, we used cardboard, hot glue, and duct tape for reinforcement.
Later, we poured the silicone, and it was left out overnight. Then, we checked that the molds were solid the next day.
Step IV: Casting
To finish, we freed our dried silicone molds, ensured tight-fitting, and put several rubber bands tightly to avoid leaking from the sides or bottom. We prepared the polyurethane using a 1:1 ratio of parts A and B, stirred the mixture for <2, poured a bit of the mixture into the sealed mold, and moved it around to prevent bubbles from forming on the sides; then poured the rest of the mix into the mold and waited for 10 minutes for it to cure. We repeated this process three more times to get all our pieces. We finished the pieces using an X-Acto knife to remove excess.
Cost Analysis
| Type | Cost | Price | Source | Quantity | Total |
| Silicone mold release spray | $ 16.99 | Home Depot | 14.4 oz | $ 1.00 | |
| Materials | Silicone | $250/2 Gallons | BJB materials | 460 mL | $ 30.18 |
| Wood | $8.08/8ft | Lowes | 3.5 in | $ 0.30 | |
| PLA | $19.99/kg | Bambu | 100g | $ 2.00 | |
| Sandpaper | $7.99 / 36 sheets | Amazon | 1/8 of a sheet | $ 0.03 | |
| Xacto | $ 5.49 | Michaels | 1 | $ 5.49 | |
| Rubber bands | 12.39/bag | Office Depot | 6 | $ 1.00 | |
| Polyurethane | $40.99/60oz | Amazon | 300g | $ 20.50 | |
| Hot Glue | $17.99/50 sticks | Amazon | 4 sticks | $ 1.44 | |
| Labor | Wood Working Operator | $21/hr | Ziprecruiter | 2 hrs | $ 42.00 |
| 3D Printer Operator | $26/hr | Ziprecruiter | 0.15 hr | $ 3.90 | |
| Mold Maker and Caster | $8.26/hr | CareerExplorer | 2 hrs | $ 16.52 | |
| Prototyping Engineer (You!) | $36/hr | Ziprecruiter | 7 hrs | $ 252.00 | |
| Overhead | Facility Cost (Machine Time) | $40/month | Makers Barn | 4 hrs | $ 10.00 |
| Prototyping Total | $ 385.36 | ||||
| Production Total | $ 140.94 |
Once all the materials are obtained and the mold is made, the cost of subsequent pieces will be much lower in production. The initial costs involve one-time or bulk purchase items: silicone for the mold, PLA for 3D printing, and other materials such as wood, sandpaper, and hot glue. The main recurring costs for future production are the polyurethane used for casting and minor consumables such as hot glue and sandpaper. The silicone mold can be used many times, so the initial cost of silicone is divided by the number of pieces cast, making it much less significant in the cost per piece. Furthermore, the labor cost for mold-making would not be there, leaving only the labor of casting and finishing. The machine time overheads might also reduce when the process is more efficient. Overall, the per-piece cost will go down greatly, and the production will get more and more economical with more units.
