For our final project, we (Bonnie and Natalie) chose a Moai pawn chess piece we found on Thingiverse by tetralite (Figure 1). The goal of this project is to (1) create two positive halves of the shape (2) create two negative molds from the positive halves and (3) cast the shape using the negative molds. To create the two positive halves of the shape, one side will be 3D printed and the other will be CNC milled wood. Before we started the project we 3D printed the part by itself to get an idea of the scale of the part (Figure 2). After printing the black part, we increased the scale and printed the blue half.

Figure 1. Moai Chess Pawn on Thingiverse

Figure 2. 3D printed shapes compared to final shape

The first step is to download the file and prepare it to make two positive halves of the shape. To do this, we used Meshmixer to cut the shape in half. Then, we opened the files in Fusion 360. In Fusion 360, we added a base and a pour hole to each of the two halves. Additionally, we added air channels which will help ensure the complex section of the shape will be filled when we cast the negative mold. Additionally, we added pegs to align the two halves when we will cast the negative mold. (Figure 3).

Figure 3. Mesh before (left) and after (right) air channels, pour hole and pegs were added

For the 3D printed half, the file was ready to be sent to the printer (Figure 4). It successfully printed on our first try.

Figure 4. 3D Printer in action

For the CNC printed half, we followed additional instructions to set up the CAM and g-code for the Carvey machine. Figure 5 shows a screenshot of the CAM simulation.

Figure 5. Screenshot of CAM simulation

Unfortunately, due to a missing screw in the Carvey which caused several errors in the machine homing and securing the source material, we were unable to use the Carvey. To work around this, we 3D printed both halves which can be seen in Figure 6.

Figure 6. Finished 3D printed parts

From these 3D printed parts, we created negative molds by pouring 2-part liquid silicone rubber and letting it cure over 5 hours (Figure 7).

Figure 7. (left) Setup to create molds (center) Poured liquid silicone rubber (right) Complete negative molds

Next, we secured the two negative molds together and poured Easy Flow (with different dyes) to cast the Moai shape. This process can be seen in Figure 8.

Figure 8. (top left) Assembled negative mold (top right) Shape right after it is removed from the mold (bottom) Examples of cast pieces.

During this step, we had some challenges aligning the two halves of the mold to minimize the seam between the two parts.  After we cast several pieces, we sanded the seam to make the boundary between the two sides less noticeable. Then, we used the belt sander to remove the pour hole (Figure 9).

Figure 9. Using the belt sander to remove the pour hole

  Finally, we had our finished parts!

Cost Analysis:

Labor time:

Labor time includes modifying the .stl file, preparing the CAM and g-code, creating the negative molds, casting the molds, and post processing the parts. This totals to about 15 hours between two people. Working at $12 an hour, the total labor costs would be $180.

Raw Materials:

2 3D Printed Parts at $2 each: $4

Silicone Liquid Rubber: 1/2 * $45 = $22.50

Easy Flow: 1/2 * $24 = $12

Total Raw Materials Cost: $38.50

Machine Time: 

3 hours 3D Printer (one day at local makerspace): 1/31 * $50 = $1.61

Total Cost: = $180 + $38.50 + $1.61 = $220.11