AJ and I took a slightly different approach to our final project to cast a chess piece. As I am extremely allergic to components of the uncured final casting resin, we decided to try our hands at slip casting porcelain.

As we are both associates at Wiess (and Grant a Wiess alum as well), we decided to make a warpig. We found this very happy critter on Printables. After a bit of consideration about how delicate the wings would be, we chose to modify the model to be wingless. A bit of time off-screen in Rhino, and we have a terrestrial critter again. The pig may fly again someday, but that may involve a rocket pack or maybe a hang glider. While in Rhino, I re-meshed it to a much more reasonable 20k triangles from the 80k+ it had been, which made working with it in Solidworks much less time consuming.

Rather than simply splitting the model straight down the center, its geometry required a non-planar mold line. Solidworks has a nice draft analysis tool… that utterly crawls when dealing with imported STLs. Great for things made natively, not so much for high-poly models. It took hours to do the analysis, and the parting line had to be edited extensively, with 30 seconds to rebuild between clicks. After all that was done, it failed to generate usable geometry, so I used splines to hand-make a much nicer parting surface.

As we were doing slip casting, our molds needed to be somewhat different. The final part would be cast from Laguna’s Very White Casting Slip. That goes into a plaster mold that absorbs the moisture until the clay is hard enough to de-mold. As the plaster is very rigid, that means the positive from which it is molded needs to be somewhat flexible. Thankfully I have access to some nice elastomeric resins. For that reason, and because the large CNC at the OEDK was down so machining time on the smaller mill would have been almost 12 hours, I printed both halves of the mold. The negative molds were made from basic Plaster of Paris. If this were for production casting, we probably would have used potter’s plaster or some other more durable gypsum-based materials, some of which can even be made air-permeable so you can embed a perforated air hose into the mold and eject the pieces with compressed air! After it set and cured a bit, an overnight bake in the oven drove off the moisture in the plaster.Casting the pigs was simple enough with a syringe, which conveniently let us know that it ultimately took 45cc to fill one casting.

As the slip shrinks as it dries, but dries where it contacts the plaster, it naturally turns into a hollow mold. We found that pouring out extra after sitting for about 30 minutes gave us the best results. We played around with making them entirely solid as well, but the backs tended to suck in a bit. We dried the green casts overnight again in the oven, then bisque fired them in a small kiln at the Ion.

We will be glazing the pigs black and goldenrod, but the kiln didn’t have shelves in it yet, so we held off on that so we wouldn’t accidentally put glaze all over the firebrick. For now we painted half of them black with acrylic. They turned out quite well for our first time slip casting!

Costs are as follows:

Cost Type	Cost	Price	Source	Quantity	Total
Materials	EPU 41 resin	$117/liter	Carbon 3D	136 mL	$15.91
	Plaster of Paris	$10.98 / 4lb	The Home Depot	~1 lb	$3
	Laguna Very White Casting Slip	$23.96 / gal	Ceramic Store of Houston	45 mL x 10	$2.88
	Paint (Liquitex Acrlyic Medium)	$9.30	https://www.texasart.com/category/150/acrylics.html	1/10 bottle	$0.93
Labor	Artist	$20/hr	Indeed	5 hr	$100
	Prototyping Engineer	$60/hr	OoI rate	20 hr	$1200
Overhead	Facility Cost (Makerspace Subscription)	$50/month	TXRX	1 month	$50

 

Development cost: $1318.91

Per item cost assuming 20 minutes of artist time per item at scale: $7.05