Introduction

For the Cast a Piece Final Project assignment in ENGI 210, Boyuan and I decided to create Peeps chess pieces, modeled after the popular bunny shaped marshmallow candies. Throughout this project we took a 3D model of the peeps from Thingiverse, modified the model to fit our needs and dimensions, CAD’ed the peeps model with a backstop for negative molding, 3D printed it using a Prusa, used V-Carve to convert the CAD model into g-code, CNC machined the other negative mold using the Shapeoko, used these negatives to fabricate the positive two halves of the two piece silicone mold, and finally casted the model using a polyurethane mixture.  This process reinforced our knowledge of CNC machining and 3D printing with mold making and resin casting techniques. This project was completed on the intermediate level because of the peeps design, which while pretty cool, is simple and does not meet the proficient mark. We also wanted to be considerate of time because all of this was also occurring during a busy final schedule for the both of us.

The Process

Design

We began this project by selecting the simplified Peeps model from Thingiverse and scaling it to fit within a 3.5″ × 2.0″ × 1.5″ mold box size. We initially 3D printed a couple to find a preferred size, and after some iteration we found a mold size that worked for us. Once we did, we used Meshmixer to split the Peeps model into two pieces and saved the bottom and top halves to be used for CNCing and 3D printing later. We used the top half of the peeps model in Solidworks to create the 3D printed model that would be used for casting the top half of the negative mold. Once this was completed we, printed this model on one of the Prusas in the OEDK.

First 3D printed peeps model, 3D model for negative mold, 3D printed model for negative mold

To create the other half of the negative mold for our Peeps chess piece, we were required to use the CNC. To set this up we followed the same process as we did for the top half, creating an identical model with the bottom half replacing the top half. We then imported this model into V-Carve and generated the g-code for the CNC in order to create a negative mold inside a 7.5″ x 3.5″ x 1.5″ block of wood. To actually CNC the negative mold on the Shapeoko Pro 5, the g-code was split into two sections, a roughing cut (using a 1/4 inch bit), and a finishing cut (using a 1/8 inch bit). The roughing cut was fast, taking only 4 minutes, but the finishing cut was significantly slower, taking around 2 and a half hours! Later we found out that we were actually able to speed the Shapeoko up, but this was as we were almost done with our finishing cut. We struggled through this section of the fabrication process because of the time it took to CNC (requiring us to be there, watching the entire cut), access issues due to Fabman (Permissions switched as we were 90% through a finishing cut one time), issues with the spindle not turning, and later on with issues clamping the wood block to the Shapeoko bed. However, we fought through these issues and we were finally able to finish a successful CNC cut of the negative mold.

Steps for CNC cut on Shapeoko:

1. Loaded the g-code from carbide motion
2. Secured the wood block onto the Shapeoko bed using clamps
3. Initialized the Shapeoko
4. Jogged Shapeoko head to wooden block location
5. Taped the zeroing sensor to the wood block and connected to Shapeoko head for zeroing
6. Zeroed the machine at the bottom left corner of the wooden
7. Removed the sensor once zeroed
8. Moved protective doors in front of the Shapeoko and started the cut.

Top half models in V-Carve (We changed this to bottom half model)

Photo #1 of CNC Cutting Process

Photo #2 of CNC Cutting Process

Failed CNC Cut (Stopped 90% through)

Successful CNC Cut

Fabrication 

Casting Positive Molds out of the Negative Cuts

Once the CNC cut and 3D printed negatives were finished, we mixed a silicone mold using the class instructions and specifying the volume based on the size of the peeps negative mold. While one of us was mixing, the other set up a box of indeterminate size for the 3D printed and CNC’ed negative molds. Once these were set up with hot glue and the silicone was thoroughly mixed, we poured the silicone into the boxes at a specified height and let them set for 24 hours. When we returned the next day, we removed the cardboard and removed the silicone positive molds from the 3D and CNCed pieces.

Mixing Silicone for Positive molds

Box of indeterminate size for 3D printed negative mold

Box of indeterminate size for CNC’ed negative mold and silicone poured

Positive mold from CNCing and Positive mold from 3D printing

Casting the Peeps Chess Pieces

When the positive molds were completed and the negative molds were removed, we secured them together using their insert poles/holes and also used rubber bands around the two pieces of wood to more tightly secure them together before casting. The mixing process for the polyurethane was also unfortunately a struggle for us throughout this final step in the process. We followed the instructions for the mixture and pouring from class and the first few castings seemed to be successful. However, once we switched to our third casting using the green pigment for coloring, the mixture performed badly, sticking to the positive molds and not forming well. Additionally these castings were taking longer to form then the projected 10 minutes from the project description (Taking around 30 minutes to form). After a few more attempts trying different methods of fixing this problem, we switched to a new mixture of solutions for the polyurethane casting which I’ll show below here (Also shown are some of the mixtures we attempted):

Setup for securing the two positive molds together and pouring polyurethane

First casted chess piece

All casted chess pieces

Finished Product 

After a lot of failed casts and iteration we were able to complete some clean looking Peeps chess pieces. While they do have a little bit of post processing that they still need to go through, they look solid enough for photos, and we will post process them more a little bit later.

Successful casted chess pieces

Clean workspace photo

Reflection 

This assignment helped us to better understand computer aided design, transforming STLs into g-code, using the Shapeoko CNC, creating molds out of silicone, and polyurethane casting. If we were to repeat this process again we would have made sure that Fabman access was secured during the whole of our first CNC cut and we would have possibly tried various mixtures before initially using the mixtures from the class project description so that we would have started using the mixture we did in the end. Overall the entire process did take a while, mostly due to some of the struggles we had with the CNC and during casting, but troubleshooting through these steps and ending with the finished Peeps pieces taught me a lot about the process and was very rewarding.

Materials Used

Wood Blocks (3.5″ × 2.0″ × 1.5″)
4 blocks ($8.00 at $2.00 each)

Cardboard for mold boxes
$2.00 (total box stock)

Hot-melt glue
$3.00 (two sticks)

Silicone kit (Part A + 300Q + 305B)
1 kit = $35.00

Polyurethane resin + pigment
For 10 Peep casts = $40.00

Labor (design, setup, molding, casting, post-processing)
5 hrs ($37.50 at $7.50/hr)

CNC machining time (Nomad 883 Pro)
5 hrs ($75.00 at $15.00/hr)

3D-printer time (including PLA)(Prusa MK3S+)
3 hrs ($15.00 at $7.50/hr)

SolidWorks (Free)(university license)

V-Carve Pro (One time license = $699)(We’ll assume free from university license)

Meshmixer (Free)

Carbide Create & Carbide Motion (Free with Shapeoko

Total Cost
$215.50