While this was sadly the last project of the semester, this unit was the one I was the most excited for. The other techniques in this class are all things I’ve either done or at least come across before, but casting is something I’ve never had an opportunity to try. I’ve also done some work on a CNC before, but it was something I was eager to try out again.
This project was about creating a chess piece by downloading a file off Thingiverse, editing it in a modeling program, creating a positive solid shape using CNC, using the solid positive to create a flexible negative, then finally recreating the shape using the two negatives to produce reproducible positive copies. It was a lot. Overall we ended up attempting two different shapes, three different CNC milling blocks, three different flexible negative molding compounds, and four different solid pours until we could get consistently good results.
Model 1: Low Poly Queen
The image on the left shows the chopped version of the first model we tried for the project. We were looking for interesting versions of the Staunton set queen that would still fit the requirements of the project. The biggest challenge in selecting a model to use were that any model chosen couldn’t have a lot of overhang. Since the Carvey can only work at a 90 degree angle, anything under an overhang wouldn’t get carved. The other issue we had in choosing a file was that the base of a lot of objects was generally very large. Since the idea is to use a chess board to play chess, any bases that were larger than the square size would be problematic when playing with the pieces. Keeping these in mind, we started with an interesting file that still met all the requirements while being more interesting than the standard version.
We successfully modeled this in the modeling program then were able to use the CNC to carve out our model. The issue we ran into is that the Carvey is not very good with creating flat, angled sides. Since the entire model was this, it created serious issues with the longevity of this model. The other serious issue we ran into was that the model had several very steep angles, especially on the sides of the piece, which could not be cut well and came out as very jagged. Therefore we concluded that to get a better cut we would need a better model, which led us back to a traditional Staunton set piece. Fortunately we were able to move through the editing steps fairly quickly again as we had already succeeded in doing the first model on the Carvey.
Model 2:
The second model we decided to carve out of wax. The advantage that wax has over wood is that it is easy to sand down and get a really smooth finish. Some of the wood types with inconsistent density would chip off or create rough surfaces. With the wax the carving was smooth and the surface was easy to sand down to become very smooth and easy to cast a negative mold off of. The model was moved to the very bottom of the carving box (see image left) so that the bottom of the piece would become the pour spout that the whole model was cast off of. The only modification made to the wax model post carving was a hole was carved into the top to create leverage to help pry the negative molds out of the wax base.
The three negative molds in the image to the left are the three compounds used to create the negative molds. The reason we tried three different types is that each one was a step in the iterative process of determining which would work best.
The one on the far left is the Silicon 30 mold. This one was very good at getting the defining shape of the piece, but was improperly mixed and didn’t cure very well. It was also much stiffer than the other models and would be hard to remove pieces once they were cured. It also took 24 hours to fully set, an issue with the time we had.
The purple model in the middle was the Oomoo. This used a liquid and a partially cured element as the two compounds in the mix. This made it very had to work with and the final mix needed to be pushed into the wax mold. As a result, there were air bubbled trapped in the mix and the detail wasn’t very good. But the dried result was much easier to work with and dried in 6 hours.
The final compound we worked with was Mold Star. This was because it fit all of our requirements and we were able to easily work with it to create two negative molds. It was easy to mix, and the fully cured version was flexible and held all the detail we wanted. It also set up in half an hour, though we gave it extra time to be sure. This is afar and away the best mix to use and the one we recommended to everyone who was molding after us.
After we had our two negative molds, we needed to determine how to create consistently perfect models. We experimented with creating two halves and putting them together (turned out well, but defeated the simplicity of the mold), eyeballing lining up the mold (worked fairly well but was inconsistent), until we came to the conclusion that the only way to get consistency was to create some sort of registration key system. Since we had not introduced that in our carving step we would have to retroactively use them in our modeling.
The hardest part of retroactively doing a registration key is that the two models have to perfectly line up before you install a key. We had determined that we were going to drill a hole through the rubber and insert two wooden dowels to line up the pieces, but before we drilled we needed to line the two sides up. To do this, we used the previous technique that had created almost perfect models before: making two perfect half versions, gluing the two sides together to make a perfect full version, then using that version as a pseudo registration key to line the two negative molds up to locate them. This process actually worked very well and we were able to create really good molds from the registration keys.
From there all that was left to do was get some good casts and clean them up for the final presentation. When pouring in the liquid plastic we were sure to do a pour first, coating the inside by pouring everything out, then filling the cavity to prevent bubbles or not filing something. We had a miscast in one of our versions due to either not enough mixing or old material, but the final two versions came out really well. We sanded them to give a smoother finish as well as hide the seems.
Overall this project went pretty well. In the future some possible improvements include using the guide for Fusion 360 we wrote as a class (editing it to clean up the steps) and creating a poster for above the new molding station to explain all the different types of casting materials and what their properties are. Also adding in a section on how to create registration keys before carving would save a lot of the trial and error used on the creation of molds.
