The Process

As the semester comes to a close, Amanda and I (Alex) partnered up for our final project with the hopes to creating a magnificent, multicolored cat chess pieces set. These pieces would be created using various methods, including 3D printing, CNC machining, and molding/casting.

After selecting our Thingiverse cat bishop design by Rollerpaw, we downloaded and imported the file into Meshmixer to reduce and cut the piece in half. After both halves were created, we opened the front half as an .stl in Solidworks and added a mold base as well as peg negative and positives. Once this file was ready, we used the Bambu slicer to 3D print the front half of the chess piece (scaled at 155% for better sizing and maintenance of facial details).

Next, using the “back” half .stl of the chess piece, we used VCarve to prepare the CNC half. Referencing the provided VCarve CAM Tutorial Video, a .crv file was generated and G-codes were imported into Carbide Motion on the Shapeoko CNC computer in the OEDK. The job time for the roughing pass alone exceeded 4 hours, so files were adjusted and the 1/8″ end mill bit was swapped out for a 1/4″ end mill bit. For the finishing passes, 1/8″ and 1/16″ ball nose bits were used.

 

After both halves were either printed or cut using the CNC machine, we were ready to create the silicone molds. Since we did not print or cut a barrier on the 3D printed or CNC pieces, we had to create boxes of indeterminant size, using cardboard and hot glue, to contain the silicone once poured. While the glue dried, we calculated the volume of silicone needed for each mold half by subtracting half of the volume of the chess piece from the volume of the box where the height is set 1 cm above the chess piece, which brought us to 160 total mL of silicone (80 mL would be of Part A and 80 mL of Part B). After measuring out the proper amounts of solutions A and B, we mixed them together in a third, larger cup before pouring into the boxes of indeterminant size.

We repeated the same process for the CNC-cut half, and left them to sit overnight to cure.

Returning the next day, we found the silicone molds fully cured. After carefully removing them from the mold halves and the boxes of indeterminant sizes, they were ready for polyurethane casting.

We fit the two halves together and secured the mold using rubber bands. Next, we poured 20 mL of Part A polyurethane solution, adding in our desired color dye to this part, and 20 mL of Part B in a separate container. Acknowledging our pot time of ~2 min, we combined the two parts in a third container and quickly mixed the solution until it approached homogeneity. This solution was then poured into the silicone mold and left to sit for 10 minutes while it hardened. We repeated this casting process 3 more times, with different colors for each piece, until we had our litter of 4 cat chess pieces. After the kitties were de-molded, a small set of wire cutters were used to trim the excess “fur” off the seams where extra polyurethane minimally seeped out.

Meet Beth, Forrest, Isaac, and Hunter (from left to right)! They are just purrrfect.

Clean Up

Before leaving after each stage throughout this process, we made sure our workspaces were as clean as we had found them.

Reflection

As a whole, we found this project to be very fun and successful. However, we had a few difficulties along the way. The first issue we ran into was sizing the piece. We originally printed it the same size it was originally designed to be straight from Thingiverse. After this print, we decided to size up the model by 140% and 150% to maintain more details in the cat’s face. Although the larger print seemed to be sufficient, before printing the base for the silicon mold we sized up once more to 155% and the features came out great! 

The next issue we ran into was with the CNC machining. Despite using a roughing and smoothing pass our CNC piece came out a little rough to the touch. We tried rerouting with a smaller bit but the ridges remained. We later learned that these ridges were caused by too small of an overlap. If we were to repeat the project and optimize for a smooth finish, we would increase the path overlap for the smoothing passes. In the end we decided to keep the natural CNC look for the mold half as we thought the difference in textures could be fun. After casting the chess pieces, the rough CNC portion looks like hairy fur on the back of the cat, which is a finish we absolutely love!

Finally, the last thing we learned was the importance of adjusting values when working with precise and expensive materials. During the polyurethane casting portion, we started by adding ~20mL of Solution A and ~20mL of Solution B. After pouring in the mixture, we had a bit of polyurethane left in the cup, which could be an expensive mistake, especially if repeated over time. We therefore reduced the mixture by 2mL for each part and the solution filled the mold perfectly without any waste. After optimizing the mold and solution amount, this process was easy to replicate and very successful!

Cost Analysis

Cost Type	Cost	Price	Source	Quantity	Total
Materials	PLA filament	$19.99/1kg	BambuLab.com	.03866 kg	$0.77
	Wooden Block	$3.76 (2x4x8)	HomeDepot.com	1	$3.76
	Silicon Cups	$1.99	Temu.com	3	$5.97
	Silicone Mold Kit	$19.99	Amazon.com	1	$19.99
	Polyurethane Resin Casting Kit	$36.07	Amazon.com	1	$36.07
	Dye Kit	$9.99	Amazon.com	1	9.99
Labor	Prototyping Engineer	$25/hr	Personal Rate	20 hrs	$500.00
Overhead	Quality Control	$19/hr	ZipRecruiter.com	N/A	$0
Machine Cost	CNC Machine (~1kwh CNC)	$.14/kwh	ChooseEnergy.com	2 hrs	$0.28
	Facility Cost (Bamboo-.07kwh)	$.14/kwh	ChooseEnergy.com	5 hrs	$0.05
Total					$576.88

Above is the price breakdown for this project. By far the most expensive aspect was the time spent creating the mold. This is the most difficult and time consuming part of the process and after the mold is created, it is much quicker, easier, and cheaper to mass create pieces. In the future we would save money by making one mold and many castings. We would also invest in bulk materials which would lower the unit price of the silicon and polyurethane.