Tinghan and I are on a mission to build a city that has never been seen by the likes of women and men before…We are on our way to building a mini NEW YORK CITY!!!! Tinghan and I wanted a challenge when we were tasked with simply printing our chess pieces during the first gate of this project. Several options possessed the same geometry and multiple planes of symmetry no matter where you cut. We had the option to build the Empire State Building, but we saw that someone had already done that. We still looked for a geometrically complex piece, which was a little challenging. From looking at different buildings, we eventually found the Chrysler building. We should have realized that the base of the Chrysler building is not a perfect square but an irregular trapezoid. Understanding that we could not use the same file for creating a CNC and 3-D printing mold. Understanding the regularities in the dimensions, we knew that we needed to be very careful with our CAD files.
The first piece we 3-D printed was relatively small and around an inch and a half, so we sized it up by a factor of 3 to be around 4 inches. We first split the CAD file into two parts using the mesh mixer software and ensured they were the same size.
3-D Print
After splitting the STL files in two, we worked on solid works to create the base for the CNC and 3-D printed hold. Since we were working with two different files, the position of the pin and hole could be easily messed up by the slightest millimeter, and then we would have naughty models. To get around this issue, I based each piece’s center or origin on the center of the lateral line from the middle of the base to the tip of the building. After developing the CAD file for the 3-D printing part of the mold, we printed our first mold, as seen below.
However, the pins were considered more significant, and we needed to reposition the air-hole tunnel. My reason for putting the air tunnel on the base of the building was that putting it on the tip would have little to no air as the thickness of the tip was less than half an mm. To get around this, we restructured our CAD model to have a slightly thicker air hole that was 0.1 thicker, as seen below.
After getting our mold checked and validated, we then poured the liquid silicon to create our mold. Using the positive 3-D printed piece, we used cardboard, hot glue, and duct tape to make an unleashed case to cast the liquid silicone. We had parts A and B of the liquid silicon and assumed that they had a density similar to water. We measured the width and length of our piece(3″ by 4.5″) and a height of around an inch, which gave us a required volume of 13.5 cubic inches, roughly 8 ounces. We then determined we should use 8.8 ounces of the part A and part B mixture just in case we needed more. We knew we needed a ratio of 1:1 by weight to create the silicon mold material. We measured 4.4 ounces of B and 4.4 ounces of A, then mixed them in a cup for 10 minutes, since we had 15 minutes before it hardened, and then poured it over our mold.
CNC Piece
I know quite a few people had early issues with using the Shapeoko. I definitely found it to be a learning curve as well. Overall, regarding how the CNCing worked, we used one roughing pass with the 1/16″ and followed with the smooth pass with the 1/8″. I needed to use as many passes as possible because some parts of the Chrysler building required extraordinary attention to detail.
For asking about the CNC piece, many people found using the Shapeoko complex and getting the wood piece to stick on the platform adduct with our green clamps. I took inspiration from the NOMAD and used double-sided tape and clamps. The double-sided tape helped secure its Z position, while the clamps helped secure its x and y positions.
Like how we cast the 3-D print, we used duct tape, hot glue, and cardboard to create an efficient seal to hold the silicon layers. The dimensions of the CNC piece were the same as the 3-D print, so we used identical amounts of Silicon parts A and B to create the mold(~8.8 ounces). We stirred for ten minutes and then poured since we had 15 minutes before the mixture would solidify.
Casting
We put ourselves in an excellent position for casting. We had two molds that looked great, precise, and amazing, and we were ready to start casting immediately. However, when we looked at our molds, we realized we needed to rent and position the holes and pins correctly. If we were to close it, the two holders would face each other, and so would the pins.
However, this was a minor setback as we cut off all the pins and replaced two open holes with wooden sticks that I cut out to lock in place with the holes they faced.
We got the exact volume from Solid Works regarding how much liquid plastic we needed. From solid works, it was determined that half a piece was around 17 cm^3 or 0.55 ounces. We would roughly measure 5/8th of the part A solution and another 5/8th of the part B solution. We had everything laid out nicely and neatly when casting the liquid silicon. We had a reservoir cup for the liquid parts A and B, each kept on top of napkins for their respective areas. We the
n had another napkin to be used as the mixing station. Next to the mixing station, we kept our two silicon mold pieces clamped together. We could yield the best pieces with two wooden blocks on the back and cardboard sheets on the side. We then made sure to put rubber bands uniformly.
When it came to mixing, we quickly tuned on a timer for 2.5 minutes and stirred it as best as possible to reduce the air bubbles. At the 30-minute mark, we slowly poured the mixture in since we did not want more bubbles to appear.
While we did not have the luxury of aligning it as best as possible with the four missing pins, we still tried to align it with my eye by flashing a light down the mold and trying our best to connect it as best as possible.
Pieces
Overall, Our pieces were lovely, as many were nicely aligned. We even had quite a lot of fun playing with the different colors we could use. My personal favorite is what I call “dark red.” I mixed the red, yellow, and black dye to see if I could get a dark, not bright, form of red. The product can be seen below. It is a very pastel color, and I’m happy it turned out quite nicely.
Tinghan’s favorite, on the other hand, was his attempt to replicate the actual color of the Chrysler building. He was trying to go for the bronze color it was supposed to be. He used a very minute amount of brown and got an elegant haki brown color.
Post Processing
For post-processing steps, we used a box cutter to cut the extra edges of the buildings after being cast for around 8 minutes. It was essential to cut these Roush uneven edges immediately after release since they were still warm and easy to take off. We also used sandpaper and the sand belt to give a nice, uniform, flat surface at the bottom to rest.
Personal Critiques:
It would have been more advantageous to consider how the mold would fit together in the future so we did not have to improve making our own make-shift dolly pins. This error could have cost us some accuracy.
Furthermore, we could have cleaned up after each mold better. If we were using a dye, we should have waited a little longer or cleaned the air more thoroughly before casting the next one. Some of our samples have dye stains that are the color of the previous ones we just made. Better job cleaning should have been done.
Overall, here is our little city:
| Cost Type | Cost | Price | Source | Quantity | Total |
| Materials | 3″x3.5″x8ft | $14/8 feet | Lowes | 4 inches | $1 |
| PLA filament | $18.99/kg | Amazon | 300 g | $5.70 | |
| Liquid Silicon mold(Parts A & B) | $130.99/gallon | Speciality Resin | 0.0865 gal | $11.20 | |
| Liquid Plastic Resin(Part A and B) | $57/gal | Speciality Resin | 0.039 gal | $2.23 | |
| Labor | Injection Molding Engineer | $18.12/hr | ZipRecuiter – https://www.ziprecruiter.com/Salaries/Woodworker-Salary–in-Texas#:~:text=As%20of%20Nov%2013%2C%202023,(75th%20percentile)%20in%20Texas. | 3 hours | $54.36 |
| Prototyping Engineer | $15.00 | Zippia | 4 hour | $15.00 | |
| CNC Engineer – Wood Working Engineer | $17.41/hour | ZipRecruiter | 3 hour | $52.23 | |
| Overhead | CNC Machine | $40/hr | CAD Crowd | 3 hours | $120.00 |
| 3-D printer | $27 | Sculpteo | 2 hours | $54.00 | |
| Misc. | Excess wood | $14/8ft | Lowes | 1.5 | $9.93 |
| Excess Liquid Plastic Resin(Part A and B) | $57/gal | Speciality Resin | 0.001 gal | $0.06 | |
| Total | $325 |
