Over the past few weeks, Brandon Peoples and I have been working on the ENGI 210 final project, which was to create a reusable mold for a chess piece and use it to cast 8 identical pieces. This blog post will cover our process, areas for improvement, and a cost analysis for our project.
Figure 1: Our eight completed chess pieces.
Our Process
We began this project by searching for a chess piece on Thingiverse. After some looking, we found a woodlands-themed chess set with a full set of owl pieces and a full set of squirrel pieces. We really liked the look of the squirrel “knight” piece because it did not have any overhangs that would make it difficult to 3D print or cast from a mold, and it also was symmetrical about the middle of its body, which would make it easier to create a mold.
Figure 2: Squirrel knight piece on Thingiverse.
Once we had the model of our piece, we began to create the positive molds. We went through the file processing steps in the file for this assignment – slicing the file in half through its plane of symmetry, approximating the shape with 10,000 triangles, importing it into SolidWorks, and making a flat bed with pegs. To make the other end of the positive mold, we simply mirrored the file and turned the pegs into holes. Once we had the file for our positive molds, we scaled it to our preferred size, and made that file into both an Easel file and sliced it to be 3D printed with the OEDK’s FDM printers.
Figure 3: Our FDM-printed positive mold.
After getting our positive mold, we prepared to create the negative mold that we would use to cast our chess pieces. We began by hot gluing cardboard strips around the edges of our positive mold in order to create a box that the mold material could harden in. We then mixed Part A to Part B in a 1/1 ratio by mass, mixed thoroughly, and poured it into our positive mold. After 24 hours, we came back and our negative mold had solidified into silicone rubber.
Figure 4: Part A of the silicone rubber.
Figure 5: Part B of the silicone rubber.
Figure 6: Our negative mold.
Now we could begin to cast our chess pieces out of rubber plastic. This process consisted of lining up our negative mold pieces, wrapping rubber bands around them to hold them against each other, mixing Part A and Part B in a 1/1 ratio by volume, mixing them together, and pouring them into the cavity formed by our negative mold pieces. Initially, the solution looked clear with bubbles slowly forming. After a few minutes, it would suddenly transform into a white solid, solidifying outwards from the middle. After around 15 minutes, we would remove the newly-formed chess piece from the mold. We completed this process 8 times to create a total of 8 squirrel chess pieces. We then worked on post-processing our pieces – scraping off extra material and lines that were formed from where the two negative mold pieces were touching. At this point, we could call this project a success.
Figure 7: Part A of the liquid plastic.
Figure 8: Part B of the liquid plastic.
Figure 9: One of our chess pieces.
Areas for Improvement
Although we are very happy with how our final products turned out, there certainly some aspects of our project that could have been done better.
The main issue with our project has to do with the way that we designed our positive mold, and the effects were felt all the way down to our final product. When we were designing our positive mold on SolidWorks, we made our pegs a little bit smaller than we expected – their diameter was fine, they just did not stick out very far. We figured this would not be an issue, because from the positive mold it still was clear that the pegs were there and that it should have formed a peg large enough to fit into the hole of the other piece so that it would not slide around freely.
Figure 10: Sideways view of some of the pegs on our positive mold.
Unfortunately, after we took our negative mold off of our positive mold, only one of the pegs had actually formed. The other pegs had a circle marking where they were supposed to be, but did not stick out nearly as far as we had intended them to. Since the mold material was able to capture lots of details across the rest of the positive mold, we are pretty sure that air pockets got trapped within the cavities of the peg holes, preventing the material from fully forming the pegs. However, this likely would have not been an issue at all had we designed the pegs to be larger. As a result, we had to align the parts of the negative molds on our own, trying to reduce asymmetries in the mold cavity. We did our best, but it was not perfect – on some of our pieces, there is a clear line going across various parts across their planes of symmetry where it is clear that the negative mold pieces were not exactly aligned. Part of our post processing was shaving off some material near those lines to reduce the appearance of asymmetry, but there was only so much that we could do.
Figure 11: Sideways view of some of the pegs on our negative mold.
There was another issue we had, which arose during the casting of our negative mold. Once we took it off, we realized some irregular globular pockets in our mold, connected to edges of the shapes of our chess piece’s cavity. When we casted from the mold, these globules would show up like little tumors on the squirrels’ bodies. We were able to scrape these off for the most part during post-processing, but there are usually still marks from where they originally had been.
Figure 12: Image of the line from casting and globules from irregularities in the mold.
Cost Analysis
The elements of this project that could have accrued cost are the chemicals used for the negative mold, the chemicals used for the casting material, the FDM filament used to make the positive molds, time spent using the 3D printer, and the opportunity cost of our time. I will assume the cardboard to be of negligible price since it is a very common, low-fidelity material, and the hand tools we used in post-processing to be of no price since we used them very briefly and they were simple, cheap tools.
Silicone Rubber Chemicals
I found a 2lb set of the silicon rubber (1lb of Part A, 1lb of Part B) for $$63. While we did not use a full pound of each component for our project, we did use enough for it not to be negligible. We will make a generous estimate of $10 for the amount of silicon rubber chemicals we used.
Liquid Plastic Chemicals
We found the exact EasyFlo 60 Liquid Plastic Part A and Part B that we used for this project online for $141.45 together in 7.6 lb jugs. While we did not use anywhere near the entire jugs for our casting, we did use a noticeable portion of a jug, maybe 1/8th as a generous estimate. Therefore I will estimate a cost of $141.45/8 = $17.69 for the cost of the liquid plastic we used.
FDM Filament
The cost of the filament used to produce our FDM prints (as calculated by 3DPrinterOS) was $1.29 each. Since we made 2 of these, they would cost a total of $1.29 x 2 = $2.58.
Time Spent Using the 3D Printer
Our FDM prints (printed together) took about 8.5 hours for the 3D printer to print. We can’t find a figure for a standard hourly rate of using a 3D printer, but we have seen a figure a few times online where a $2000 printer would have 2000 hours of expected printing life, which equates to $1/hr. Someone who bought a 3D printer would want to profit, however, so we will assume $5 for the cost of using a 3D printer for an hour. So, the cost of this element is $8.5 x 5 hours which is $42.50.
Opportunity Cost of Time
If you add up the number of hours that Brandon and I each spent working on this project, it probably adds up to around 11 hours total. Since both of us have worked jobs that pay around $12/hr, we will estimate this as the value of our time. Therefore, the opportunity cost of this project was $12 x 11 = $132.
Altogether, the cost of this project was $10 + $17.69 + $2.58 + $42.50 + $132 = $204.77. This was definitely one of the more expensive projects done for this class, but one of the cool things about it is that now that this process is over, we have a mold that we can use over and over again to make more pieces, roughly 1 piece every 10 minutes. If we sold each piece for even $2, this would come even with our opportunity cost. If we raise the price to even $5, we could theoretically generate $30 of revenue per hour, and could quickly offset the cost of creating the mold and the cost of the chemicals used for casting. This is an interesting component of casting and molding – it takes an initial investment to create the mold, but then manufacturing using the mold is relatively quick and easy from then on.
