Prototyping Engineer: Xinyin (Sing)
In this assignment, I created finger hooks in the same way dentists create tooth molds.
Materials
The material I used for the mold is alginate1 (Fig. 1a). It is a polysaccharide derived from brown algae (Fig. 1b) and bacteria and looks white and yellowish (Fig. 3b left).
Figure 1. (a) Alginate chemical structure2. (b) Brown algae3.
The Plaster of Paris was used to create the cast. It is made from a soft mineral called gypsum, commonly found in sedimentary rocks4.
Figure 2. Gypsum4.
Additional materials include water, screws, a plastic screw holder, superglue, colorful and clear spray paint, and Vallejo paints shown in Figure 3 and also in the Cost Table (Table 1).
Figure 3. Materials and tools. (a) Cup and popsicle stick, (b) Alginate (left) and Plaster of Paris (right), (c) Skews on a plastic holder, (d) Superglue, (e) Colorful spray paints, (f) Clear coat, (g) Vallejo paints.
Methods
I weighed 33g of alginate in a cup and added 125ml of room-temperature water (Fig. 4a). Then, I stirred the mixture with a popsicle stick until no powder was left (Fig. 4b). Next, I inserted my index finger and middle finger on my left hand into the mixture (Fig. 4c). I was careful not to move my fingers until the mixture solidified, which took about 5 minutes. Then, I slowly wiggled my fingers out of the alginate, and the mold was down (Fig. 4d).
Figure 4. Steps for molding and casting.
For the casting, I mixed approximately 60 mL Plaster of Paris with 30 mL water (Fig. 4e). The texture was between gluey and liquidy. Next, I inserted the screws into a plastic piece with holes and aligned the screws to the holes left by the fingers (Fig, 4f). The plastic piece is longer than the diameter of the cup, so I had to squeeze the cup to let the screws enter the holes. As a result, I expected flatter finger products. Afterward, I poured the plaster solution into the finger mold (Fig. 4g) and set it to dry for 24 hours before I demolded it (Fig. 4h).
Before demolding, I observed water flowing out as I tilted the cup. To take out the fingers, I broke the alginate mold. However, when I lifted the fingers, both broke into two pieces, possibly due to the moisture inside (Fig. 4i).
I stuck the pieces of fingers together using superglue (Fig. 3d). The superglue stuck them together quickly, but there were cracks that the superglue could not fix. As a result, I tried to spray layers of paint to not only make the fingers colorful but also to fill in the crack. After the paint dries, I added a clear coat to the fingers to make them shiny. Finally, I decorated the finger hooks with the Vallejo paints and a shiny ring.
Result
Video 1. Final product.
The finger hooks have a theme of spring, where purple and red flowers open on a green field under the sun (Video 1). The inspiration comes from the Chinese saying 妙手回春 (magical hands that bring back the spring), which is used as praise for doctors who cured patients, (which makes my product somewhat medical-related).
The superglue stuck the fingers together strongly and left the clack behind. The cracks got shallower after I applied paint but they were still apparent. Therefore, I brought a ring to hide the crack and to increase the livelihood of the piece of art. The fingers came out with natural folds. The larger folds at joints are still detectable, but the small folds were hidden under the paint.
The fingers appeared flat as expected, but I can still recognize them as mine 🙂
Discussion
From this assignment, I learned about alginate and plaster from molecular structures to their texture and feelings. The post-processing allows me to exercise my creativity.
To improve the quality of the finger hooks, specifically about the crack and the deformity, I have two thoughts. First, to avoid breaking the fingers, I could try to demold the fingers earlier. According to the instruction, the plaster releases water as it dries, which matches my observation. The water could make the plaster soft and prone to be broken. As a result, if I were to create the fingers again, I would try to demold it after half an hour as suggested by the instruction.
Second, the mold underwent deformity due to the mismatch in length between the plastic screw holder and the cup, causing the fingers to be flattened. One way to address the problem is to use a larger cup or use an alternative tool to position the screws. Alternatively, I can flatten the cup of alginate mixture in such a way that the plastic holder can fit nicely into the cup, and then put my fingers in, let the alginate solidify, and put the screws.
Nevertheless, I love my current product, for they remind me of Venus with arms broken. The imperfection is beautiful as well.
The cost of the fingers is around 32 dollars (Table 1), where the labor cost counts for around 80% of the total cost.
| Cost Type | Cost | Price | Source | Quantity | Total |
| Materials | Perfect Mold Alginate | $0.76 / ounce | Amazon.com | 33 g = 1.16404 ounce | $0.88 |
| Plaster of Paris | $1.8725 / pound | Lowes.com | 60 mL = 0.1123 pound* | $0.21 | |
| Spray paint | $0.5 / ounce | Amazon.com | 0.11 ounce | $0.055** | |
| Vallejo Paints | $4.13 / Fl Oz | Amazon.com | 0.012 Fl Oz | $0.05 | |
| Screws | $0.0394 / piece | Amazon.com | 2 pieces | $0.079 | |
| Superglue | $0.96 / g | Walmart.com | 0.5 g | $0.47 | |
| Plastic piece for holding screws | NA | NA | 1 piece | $0.03*** | |
| LED light-up ring | $0.5 / count | Amazon.com | 1 count | $0.5 | |
| Labor | Prototyping Engineer (You!) | $17 / hour | ZipRecruiter.com | 1.5 hours | $25.5 |
| Overhead | Cups | NA | NA | 24 hours | NA |
| Scale | NA | NA | 5 minutes | NA | |
| Popsicle sticks | $0.0194 / piece | Walmart.com | 2 pieces | $0.0388 | |
| Quality Control | $27.5 / hour | LinkedIn.com | 0.1 hours | $2.75 | |
| Design | Engineering and Development | $17 / hour | ZipRecruiter.com | 0.05 hr | $0.85 |
| Misc. | Waste and Scrap**** | $0.66/Gal= $0.17/L | Houstontx.gov | 1 L | $0.17 |
Sum: $31.58
Table 1. Cost Table for the set of finger hooks.
* 53.00134 pounds [lbs] of Plaster fit into 1 cubic foot. 60 mL x 3.53147e-5 cubit foot/mL x 53.00134 pounds/cubic foot = 0.1123 pounds
**I estimate one finger to be 13 cm^2. Two fingers are 26 cm^2. The fingers received about 3 layers of colorful paint and one layer of clear paint, so the area sprayed is 26 x (3+1) = 104 cm^2 = 0.11 feet^2. 12 ounces of spray paint provides coverage for approximately 10 to 15 square feet, so 0.11 feet^2 takes around 12 x 0.11 / [(10+15)/2] = 0.1056 ounces. Given that I sprayed some paint on the cardboard, I estimate that I used 0.11 ounces of paint.
***The string is made of plastic and has a simple outlook, so I assume it is cheap.
****Include used alginate, leftover plaster, and cardboard. The cardboard was from the dumpster, so I did not count its price.
Clean up
Wen-Yi and I used cardboard to prevent spraying paint onto the ground (Fig. 5). However, there was an old bottle of red paints that fired in two directions, and I sprayed some to the ground. I cleaned it right away with paper, and there was a trace of red left (Fig. 6). I trashed the broken alginate mode and returned the cup to the shelf (Fig. 7).
Figure 5. Cardboard for spraying paint.
Figure 6. The stain of red paint on the ground.
Figure 7. Cups returned to the shelf.
