We made fingers!
This was a great project and I really enjoyed learning how to use hydrogels like sodium alginate, as well as dry mix Plaster of Paris.
We started by mixing sodium alginate with water at a ratio of 0.98 oz (28 g) alginate to 4 oz (~118 g) water, in accordance with the recommended ratio on the bag of material shown below.
Bag of alginate molding material
I mixed the alginate and water and the placed two fingers into the cup to form a negative as the hydrogel dried around it.
Forming a negative with two fingers so that the alginate mold solidifies around it
After about 5 minutes, the hydrogel cross-links are strong enough to the point that we can remove our fingers. I then mixed Plaster of Paris in a 2:1 ratio with water (60 mL plaster to 30 mL water) and mixed it before pouring it into the mold.
Plaster of Paris, which we mix in a 2:1 volume ratio with water.
Plaster of Paris inside the mold. I was unable to locate the plastic bracket to stabilize the screws, so I rested them diagonally on the mold.
After about 30 minutes, the plaster had dried enough to the point that I could remove the alginate mold and allow the fingers to dry.
Removing the alginate casing from the fingers
Finger replicas, removed from alginate mold
Unfortunately, the screw was not well-attached to the second finger, so I decided to wait for it to dry before hot-glueing it.
Hot-glued screw to finger
Finally, I spray-painted the fingers deep blue and applied a clear gloss finish.
Spray painting the fingers deep blue
Clear gloss finish for the fingers
What the final fingers look like
Clean outdoor workspace after finishing
Overall, the geometry of the fingers was preserved, but probably could have been better if I had bent my fingers slightly more while making the mold.
The cost analysis of the project is below:
Cost Analysis
| Cost Type | Cost | Price | Source | Quantity | Total |
| Materials
|
Alginate | $11.99/1.5 lbs | Hobby Lobby | 50 g | $2 |
| Water | $6/1,000 gal | City of Houston | .25 gallons | $0.0015 | |
| Plaster of Paris | $7.99/4 lb | Michael’s | 30 g | $0.13 | |
| Screws | $46.98/25 lbs | Home Depot | 6 g | $0.02 | |
| Rust-Oleum Spray Paint (Deep Blue) | $5.98 | Home Depot | 6.60 g (Based on a rate of 0.33 g/sec) | $0.12 | |
| Rust-Oleum Spray Paint (Clear Gloss) | $5.98 | Home Depot | 6.60 g (Based on a rate of 0.33 g/sec) | $0.12 | |
| Fix-all adhesive Super glue | $11.49 | Amazon | 0.001 oz | $0.03 | |
| Design | Engineering and Development | $8/hour | https://www.careerexplorer.com/careers/molding-and-casting-worker/salary/ | 1 hr | $8 |
| Iterations/Quality Control | Same as production of successful iteration | See above | 1 | 10.58 | |
| Misc. | Waste and Scrap | $0 (in city of Houston) | https://houstonlanding.org/city-council-oks-study-of-houstons-solid-waste-department-that-could-recommend-garbage-fee/?utm_source=chatgpt.com | NA | $0 |
