Hi, everyone. For this assignment, Carter and I casted the fingers and especially focused on making sure that the fine details of the fingers aren’t covered by paint.
- Creating the mold with fingers
During class, I poured 33 g of alginate and 125 g of water into a elastic silicone cup, which is used as the container to hold the mold. After that, a popsicle stick is used to stir and mix the mixture. Since this mixture only has a pot life of 5 minutes and it only takes 8 minutes to set, I finished the mixing process in about 4 minutes. Then, I inserted two of my fingers into the mixture. Thanks to the elasticity of silicone cup, I was able to reach my two fingers al most all the way down to the bottom. It took me about 4-5 minutes to wait for the mixture to actually solidify. As the mold has a very tight fit with my fingers, pulling out would almost create a vacuum. Therefore, I had to slowly and carefully pull my fingers out so that the mold itself wouldn’t be broken.
2. Creating the cast with mold
Next, I mixed plaster of Paris to water at 2:1 ratio, based on how much I think would be required to make the positive of my fingers in the mold. After the two holes in mold are filled with the mixture, I then tried to put a jig with two screws into the mixture so that I can get the fingers with screws embedded. However, the jig seems to be too long for the inner diameter of the silicone cup and the screws seems too short to reach the mixture in two holes. Therefore, I had to force the jig to fit inside the cup. The good thing is that this now becomes a very tight fit and the jig can ensure that the screws are always kept in the same position.
3. Post-processing
I waited for about three hours for the cast to solidify. However, when I took the cast out of the mold, it seems that the surface is still kind of wet. Therefore, I placed them in a piece of paper and waited for another 24 hours. After that, it seems that even the surface is dried enough for post-processing. Since primer is a preparatory coat that enhances the final finish by improving adhesion of paint to the surface and sealing the surface to prevent the underlying material from absorbing too much paint. As shown in Figure 1 is how the cast looks like after white primer is applied. One thing to note here is that in order to ensure the cast is always held upward, a slit is cut out on the paper board so that the jig can be positioned in there and the drying process would be more efficient.
Figure 1
4. Further post-processing & painting
After the primer is dried, we took them out of the jig (Figure 2). We’ve also checked the functionality of installing them by screwing them into a piece of wood log and that would also assist us in painting them. However, we noticed that all the spray paints available in OEDK seem to be quite thick and can cover the fine details made with plaster. Therefore, we decided to use a technique called zenithal highlighting with airbrush Carter brought. With this technique, we would be able to keep all the details on the fingers and make them stand out even more. Since the color we chose was deep red and it’s a darker color, we decided to apply another layer of dark primer (Figure 3). Another advantage of airbrush over the spray cans is that thick paints from spray cans were not sticking very well to the surface outdoor with the cold whether that day. As shown in Figure 4 is how the fingers look like after painting with airbrush. After the paint is dried, we proceeded to use clear coat spray can to make the final clear coating to protect the paint and create a gloss finish (Figure 5). As shown in Figure 6 is how the final result look like.
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
5. Reflection
According to the result we’ve gained, I think the first thing we can improve is to position the screws better in the cast so that they can be enclosed by the mixture better and ensure a stronger grip on the inside to sustain the torque from bearing heavier objects. Another improvement is to redesign the silicone cup so that the jig could be held in a more natural position, since one major difficulty met during demolding was to take the tight jig out of the silicone cup. One possible adjustment to the design might be to have two rectangular cutouts on two sides of the cup so that the jig can be securely positioned on top.
Cost analysis table is shown here:
| Cost Type | Cost | Price | Source | Quantity | Total |
| Materials | Alginate | $11.99 / 50 g | Amazon | 33 g | $7.9134 |
| Plaster of Paris | $9.76 / 4 lb | Amazon | ~0.16 lb | $0.3904 | |
| Screws | $0.05 / Count | Amazon | 2 | $0.1 | |
| Jig (possibly 3D printed, assuming it’s ABS) | $15.99 / kg | Amazon | 0.2 g | $0.0032 | |
| Popsicle sticks | $5.99 / 100 Count | Amazon | 1 | $0.0599 | |
| Sandpaper (grit 80, counting its depreciation) | $0.033 / min | Amazon | ~10 min | $0.33 | |
| White primer | $8.99 / 12 oz | Summit racing | ~1 oz | $0.7492 | |
| Dark primer | $12.99 / 12 oz | Summit racing | ~2 oz | $2.165 | |
| Air brush | $16.99 / 8 oz | Amazon | ~2 oz | $4.2475 | |
| Clear coat spray | $8.99 / 12 oz | Summit racing | ~1 oz | $0.7492 | |
| Labor | Prototyping Engineer (X2) | $25.91 / hour | indeed | 1.4 hr | $72.548 |
| Overhead | Utility Cost (considering both the cost of facility and electricity) | ~$5.13/ hour | TexAgs | 2 hr | $10.26 |
| Depreciation of Assets (tools versus their typical life spans) | ~$0.04/hour | ATO | 1.6 hr | $0.064 | |
| Misc. | Waste and Scrap (a collective of alginate and plasters wasted) | ~$11.67/L | Amazon | 0.003L | $0.035 |
Total=$99.6148
We’ve also cleaned the workspace after all the steps (Figure 7).
Figure 7
