For the final project for ENGI 210, our task was to create a mold of a chess piece and cast 8 identical plastic versions of the chess piece. To do this, I found a 3d model of a chess piece (bishop) on thingiverse and used Blender to create a positive for the mold. In Blender, I merged the bishop with a rectangular prism and then added holes and pins by adding and subtracting cylinders to the prism. Then, I 3d printed the positives.

At this point, I knew the holes were not going to line up because a simple mirror I used to get the other half of the mold would not properly mirror the pins, but I decided to proceed anyways and create my own alignment system later.

To create the mold negatives, I cut and hot glued cardboard to the sides of the positives and then mixed polymer solutions 1:1 by mass. I then poured the mixture into the cardboard box and waited a day for it to harden. For my particular mold, 90g of each solution made the perfect amount of silicon.

Finally, it was time to cast the pieces. This time, the polymer mixture was different and would create a white hard plastic instead of green silicon and require a 1:1 mixture by volume. I found that a little under 1/2 oz of each liquid made a good amount for each cast. My first molds did not come out well because my bishop model has sharp edges (and therefore tight crevices in the mold) that the polymer solution did not enter easily, causing bubble voids in the cast pieces.

To combat this, I started shaking and tapping the mold against the table to try and get the liquid to enter the crevices. This worked, but introduced new issues as the tapping misaligned the mold pieces slightly. To fix this, I inserted 2 and later 4 nails to ensure the mold stayed in place and stopped tapping the mold since shaking seemed to be enough. To get to this final process with trial and error, I casted about 4-5 times.

To reach the goal of 8 pieces, I casted 4 more pieces but made sure that they were as good as I could get them to prove that I could reproduce the process. The 4 good pieces are shown in the left image and in the right image, the back 4 pieces show the trial and error I used to get my final product.

I learned a great deal about molding and casting over the course of this project, including how to better prepare the mold so that the pins align properly and how to better select a model that is conducive to easy casting. I learned how to pour the liquid so that even with tight crevices, the liquid will fill the entire mold. In the future if I need to cast again, I am confident that I will be able to get perfect casts.

Cost analysis:

Labor: $120 – 6 hours at $20 per hour

3D print: $1.50 – cost from 3dprinteros

Silicon mold: $5 – approximated from ~$50 bottles

Liquid plastic: $15 – approximated from $150 bottles

Total cost: $141.50