Hi everyone, welcome back to Ravi and Sid’s last blog post. It’s so sad that this class is ending, but at least we’ll take a lot of prototyping knowledge away. In this project, we’re tasked with creating molds and eight casts of “chess-sized” figures. Our initial idea was a pawn but after receiving feedback that this would’ve been too small given the project constraints, we switched to a king piece. Usage of Meshmixer and SolidWorks was used to scale and design the appropriate .STL files for the positive mold – images of both the pawn and king are displayed below.

After creating half of the positive shape using 3D printing, the other half was created using CNC techniques. Ravi had experience using VCarve and Shapeoko Pro (large-format CNC) as he made Mt. Everest for the previous project. Full details can be found in his previous blog – “Conquering Mt. Everest with Shapeoko Pro”. To summarize, two passes using the ¼” end mill (45% stepover) and 1/8” ball nose mill (8%  stepover) were made – the process took about ~45 min. Here are images of both positive shapes – one 3d printed and the other CNC’ed from wood.

Next, it was time for the molding/casting process. Cardboard was used to create boundaries around each positive shape. Hot glue was used to seal the crack/crevices around each piece to prevent the mold mixture from leaking. After making appropriate volume calculations, we estimated that ~500 mL of mold mixture was needed to achieve a height of 1cm above positive shape. Equals parts of A and B were mixed thoroughly and added into each volume to create the negative mold – images of before and after pouring the mixture are depicted here.

After waiting ~5 hours, we de-molded from the positive shapes and began casting. Using equal parts of A and B to create a total solution volume (EasyFlo 60 Plastic) of ~14 mL (~7 mL each), we mixed for 2 minutes and poured into the cast for setting. A drop of different dyes was used in each mixture to create different colored king chess pieces. Minimal post-processing was done like (1) sanding the base and (2) using a box cutter to remove the excess material on the seam. Here’s the final result! Overall, it was a pretty fun process.

We could’ve fixed a couple of things in this process even though the final result came out really well. We had some leakage, especially with the wood’s positive shape, due to the way the cardboard was aligned and hot glued. Luckily, we poured some excess so the cracks were filled and it didn’t cause too much of a problem. Our air bleed lines worked well except for one spot – on one of the cross edges on the king made from the 3D-printed positive mold. We hypothesize air getting trapped there and mold wasn’t able to enter so all our kings lack part of the top cross – this is imaged below. Some of the post-processing could’ve been better, we wanted to use a Dremel but we were afraid to remove too much material. Overall, we’re happy with our final products regarding the work we’ve put in – thanks!

It would’ve been better if the bleed line was positioned with the hole as depicted above so the mold mixture was able to penetrate better.

COST BREAKDOWN:

Material	Cost	Source	Quantity	Total
FDM Filament	$13.99/spool	FDM Filament	0.2 spool	$2.80
Silicone	$79.99/gallon	Silicone	24 inches^3	$8.65
Plastic	$145/gallon	Plastic	40 inches^3 (5 per king)	$25
Prototyping Engineer	$56/hour	Prototyping Engineer	6 hours	$336
CNC Operator	$24/hour	CNC Operator	1 hour	$24
Overhead	14 cents/kWh	Overhead	0.5 kWh	0.07
Design	Free	Noun Project		$0
Total				$396.52