Group Members: Eva Montenegro & Favour Williams

For the final project, we decided to do the intermediate level, wanting to 3D print one half of the mold and CNC the other half. We knew we wanted to make an animal chess piece, and after exploring Thingiverse, we chose a very cute bear.

After downloading the model from Thingiverse, we scaled it to be the size of the chess piece and 3D printed it. Following some feedback from the Engi teaching team, we decided to make the piece a bit bigger and re-printed it.

Next, we modified the shapefile from Thingiverse to create the mold halves. To do this, we first opened the .stl file in Meshmixer to scale and cut the piece in half. Then we opened one of the halves in SolidWorks and added the mold base from Canvas by creating an assembly. To fix the bear to the base, we added 3 mates: one to connect the bottom of the bear half to the top of the base, one to distance the bottom of the bear 0.25” from the edge of the base, and one to center the bear on the base. We also added a pour hole and an air tunnel to the mold. We did the same for both halves of the chess piece and saved our modified mold halves as .stl files.

We decided to 3D print the front half of our piece because it had more details, like the bear’s face. We imported the mold .stl file into Bambu slicer to prepare the print and then printed it using PLA. 

To make the front half of the silicone mold, we first used carboard and hot glue to make a box of indeterminate size around the 3D-printed positive. We also duct taped the edges to ensure a tight seal. After determining the amount of silicone needed by subtracting the volume of half of the bear from the volume of the box, we mixed together the two silicone parts in a 1:1 ratio by weight. We then poured the silicone in and waited overnight to demold.

Originally, we wanted to CNC the back half of chess piece mold. In VCarve we set the dimensions based off of our piece of wood. Then we uploaded our mold .stl file and adjusted the position on the workpiece. Next, we created our toolpaths. For our roughing pass, we used a 1/8” square endmill, and for the finishing pass, we used a 1/8” ballnose endmill. We saved the toolpaths and then opened up the fill in Carbide Motion.

Before running our cut, we taped our wood piece onto the platform, initialized the Nomad, and zeroed and probed the corner. The roughing cut took about 2.5 hours and turned out really well. The finishing pass took a little over 30 minutes.

Unfortunately, when we ran the finishing pass, it only smoothed out part of the bear and hovered around the other areas. We tried re-measuring the tool, zeroing and probing the piece and running the finishing pass a second time, but the same thing happened. We also talked to an ENGI TA and multiple lab assistants and had them look over our VCarve file, but no one was sure how to fix our issue.

To move forward with the project, we decided to also 3D print the back half of the mold positive. We then used the same process as we used for the front half to pour the back half of the silicone mold.

Finally, it was time to cast our chess pieces. We used rubber bands to hold the two sides of our mold together. After determining the amount of resin needed, we mixed together the two parts in a 1:1 ratio by mass. We only mixed together enough for one chess piece at a time to avoid wasting material. We then poured the mixture into the mold and waited for about 10 minutes before demolding the piece. We repeated the seven more times, adding yellow dye to 4 of our bears.

To finish the pieces, we sanded off the extra material attached to bears’ feet from the pour hole. We also sanded the edges where the two halves of each bear met, since there was a bit of misalignment.

This was a very fun last assignment. Even though we were a little disappointed that we couldn’t figure out the smoothing for our cnc half, we were very happy with the final product: a little army of bears!!

Cost Analysis:

Item	Price	Quantity	Total	Source
3D printing Machining cost	$50 monthly membership	1	$50	https://apps.txrxlabs.org/join-us/
CNC machining				https://apps.txrxlabs.org/join-us/
3d printing filament (PLA)	$1.48	1	$1.48	3dPrinterOS
Cardboard	$2	1	$2	Packaging Hero
2 in. x 4 in. x 12 ft Pine wood	$5.43	2 in. x 4 in. x 6 “ (0.042)	$0.22	https://www.homedepot.com/
TC-802 Castable Resin Part A and Part B	$129.00/gallon kit	0.114 gallon	14.71	Brick in the Yard Mold Supply
Magikmold P-525 Silicone Catalyst and Silicone Rubber Base	$265.00/16 lb. kit	320 gram	$11.68	Brick in the Yard Mold Supply
Dye pack	$1.88 for assorted pack	1	$1.88	https://tinyurl.com/ufdn9tv
Multicolor Rubber Bands	$5.99	1/10 of bag	$0.60	Amazon.com
Labor	$12/hour	16 hours * 2 people	$384	Our fair rate
Total			$466.57