Introduction

This post details our experience of producing five low poly fox chess pieces including prototype development/iteration, mold fabrication, and the casting process.

3D Printing

We began by choosing the piece we wanted to do. While we initially wanted to do a Batman bust, the limitations of the CNC machine would have made the pointy mask especially difficult. Thus, we settled on a low poly fox instead.

We initially printed a 2-inch tall piece, but following feedback from the teaching team, increased the height to 2.5 inches. =

With the proof of concept out of the way, we began CADing our positive pieces for our eventually negative molds.

Initial Model of Fox

First print!

CADing

The next step was to design the positives we would 3D print and CNC. We imported the STL into SolidWorks as an object. We chose to exclusively use SolidWorks because Justin was skilled with the software, and we needed to slice our model in half in a way that wasn’t perpendicular to the base.

We sliced the model in half using a plane defined by the tips of each ear, then rotated it so that the back half didn’t have any overhangs—allowing it to be CNC routed. Once the model was split into the front and back halves, we had to add backs to the positives. Finally, we added holes and pegs so the two negatives could be properly aligned once they were poured.

After this, we 3D printed the front positive, as it had more complex geometries and overhangs that couldn’t be handled by the CNC router. That brings us to our next step: CNC routing the back positive!

Front Positive

Back Positive

CNCing

CNCing the back positive went very smoothly because Justin had recently done 3D routing for his CNC project. We cut our model out of a 1×4 since there were leftover pieces from that project, and our part was shallow enough to fit within 1 inch. We used a 1/4-inch end mill for the roughing pass and a 1/8-inch ball nose for the finishing pass. The ball nose was highly effective because the individual passes weren’t visible in the final part.

We also had the CNC router completely cut the back positive out from the rest of the 1×4. We chose to do this so that the same method of creating a box of indeterminate size could be used for both the CNCed and 3D-printed positives. We were incredibly fortunate to have our back positive route successfully on the first try and are now ready to move on to the next step: using our positives to create the mold!

Back positive loaded into VCarve

CNC Router in the middle of cutting our back positive

Our finished back positive

Creating the mold

To start, we cut out cardboard and made two boxes of indeterminate size by bordering the CNC and 3D printed positives, using hot glue to cover any gaps. We also made sure to avoid the pegs and holes.

We did some volume calculations to figure out how much silicone we needed: 

Overall volume: 6cm*10cm*3cm = 180 cm^3 = 10.9842739 in^3

Volume of fox half: 1.62866 in^3 /2 = 0.81433 in^3

Volume of silicone for each mold: 10.9842739 – 0.81433 = 10.1699439 in^3 = 5.63529359 US ounce = 166.655522 mL

Thus, we poured about 160 mL of both parts of the silicon solution, mixed them, and poured them into the boxes. After 24 hours, we removed the cardboard and ended up with these negative molds.

We proceeded to put them together and cut a hole into the bottom of one of them to facilitate pouring the urethane for casting positives.

We used zip ties to secure the molds and make sure they didn’t split while the casting solution was drying.

Creating the foxes

The total mass of polyurethane (both parts combined) we poured for each fox was 30 grams, as we assumed a density of water and the volume of the fox itself was 26.7 mL. We put one drop of dye into the solution before mixing, and then carefully poured it into the hole. We left our first cast sitting for about 30 minutes, and it turned out well.

The base of it was not very good, as we didn’t fill up the mold beyond the top.

For our next few molds, we purposefully overfilled and used a bandsaw to cut off the excess material, then sanded down the bottom for a cleaner product.

Unfortunately, something happened when we tried to use the yellow colorant for our fourth piece. For some reason, the cast wouldn’t dry in the same time frame, and somehow leaked out into the mold. The cast was still squishy and greasy after removal. This had not happened with any of the previous colors. After conversing with the teaching team, we concluded that there may have been a problem with the colorant, which was then disposed of.

We used a very small amount of the more functional colorant to achieve a very light pink for our fifth piece. We really struggled to get this last piece out, which may have been due in part to the yellow colorant altering the properties of the silicone mold somehow. This was our best piece structurally, although the yellow colorant stuck somewhat to the piece. 

Final Results

Clean Workspace

Cost Analysis

Silicone Molds: $4.90 for 320 mL ($28.99 for 64 oz) – https://www.amazon.com/CHANGTIKEJI-Silicone-Elastic-Translucent-Casting/dp/B0C7TJ8BMT

Polyurethane Casts: $3.64 for 5*30 mL($42.99 for 60 oz) – https://www.amazon.com/LETS-RESIN-Polyurethane-Viscosity-Prototypes/dp/B0BY2PFY62/ 

Dye: $5.93 for 5 colors ($18.99 for 16 colors) – https://www.amazon.com/LETS-RESIN-Silicone-Colorant-Coloring/dp/B0DKJ16J6J/ 

Zipties: $0.42 for 30 ($27.99 for 2000) – https://www.amazon.com/Assorted-Sizes-12-Resistant-ANOSON/dp/B0C2Z4L3S6/