Hi guys, welcome back to another one of Ravi’s projects in BIOE 555. This time, we’re making a finger cast. This was one of the simpler projects but figuring out the mixing ratios and waiting times proved a little difficult.

To start, we organized a group of people (at 6 PM on Wednesday!) to do molding and casting together so we could make a large batch of material for each step. This looked like adding material in a bucket, slowly pouring water, and using a hand drill to mix with the appropriate attachment. Here’s our molding/casting party:

For the molding step, we tried mixing at a 1:1 ratio but realized failed. There was a discrepancy with another manual sheet (included in the mixing kit) that stated to do a 1:2 ratio of alginate to water. When we tried this, we added 1 lb. alginate and 2 quarts of water and mixed. Unfortunately, the battery of the hand drill died midway and as I was trying to get a new one, I missed my opportunity to cast new mold since the mixture had agglomerated. This blog documents the process I went through, but for my final submission, I used my previous casts that were done in class.

Next, for the casting step, this was also a little ambiguous. We tried a 1:2 ratio but realized it was too “liquidy”. Two of us felt courageous enough to try this mixture, and the rest of us waited to remake it. This time, we did a 2:1 ratio (the opposite ratio lol) but the others felt that it was too thick and “bouncy” in the mold which signifies trapped air bubbles that can’t escape. I’m curious to see how their parts turned out – I will follow up with them later.

Finally, everyone set their screws up using tape. In my previous process, I remember doing this, but when I went to recover my casts from their mold, one screw was missing – presumably sunken into the abyss of my finger. Ah well – at least now my finger is reinforced from the inside and will not break as easily. Unfortunately, I managed to recover my index, middle, and pinky fingers – the ring finger broke. Here’s an image of my casts from the previous class:

Lastly, after my casts had sufficiently dried, I spray painted my finger and used a ceramic clear coat – here are the final results. This was a fun process, very artsy.

Lastly, here are key realizations and things I would’ve changed. The longer the plaster sits in the mold, the more brittle it becomes (I think) because the plaster releases water which causes it to dry. I would try another larger object to see how it would work. Another key problem is how to suspend objects such as the screw inside the plaster. The tape was definitely a low-fidelity solution but using a popsicle stick would’ve been sturdier and could potentially prevent screws slipping into the casts. Also, the spray paint I used was running out, so I sprayed continuously so there was definitely pooling of paint.

Anyway, here’s the cost breakdown:

COST BREAKDOWN (for a group batch):

Cost	Price	Source	Quantity	Total
Alja-Safe Mold Mixture	$55/3 lb.	Amazon	~ 1 lb.	~$19
DAP Plaster of Paris	$9/4 lb.	Amazon	~ 1 lb.	~$2.5
operator cost (me)	$15	me, the operator	2 hrs.	$30
			TOTAL:	~$61.5