Success in metal casting is achieved through ever-tighter process control. This week I spent many hours on chasing the wax models to remove any small concave geometry left from removal of sprues & scaffolding required to cast in silicone. When wax is steamed out in the pressure canner, any small concave negatives become small investment positives. Even though UltraVest is high quality, a few bits of the material hanging on a thin neck will always break off due to gravity, shaking, or by force of metal entering the mold.
To take care of bubbles getting stuck in concave corners (the ones I have to leave in because they are part of the design) VacuFilm can be mixed with Methyl Alcohol and brushed on, almost like mold release when silicon casting:
Here's also a botched test cast from this week - still encouraging because almost the entire model came out this time.
To fight bubbles embedded in the casting I got a spray bottle to apply VacuFilm on the wax models more evenly - this worked and I got a thin mist layer instead of large drops as when using the brush.
I also got a new vacuum chamber for degassing before and after investing the wax model. I tried to do that with the vacuum oven for a long time (it was large enough), but after using it for wax, it no longer pulls full vacuum unless it's hot. With the new chamber, time to full vacuum is short enough that I can vacuum twice, all while agitating aggressively with the vibratory tumbler.
The resulting casts no longer have bubbles stuck inside concave geometry. There are actually a few bubbles on convex geometry (perhaps just a spot I missed with VacuFilm) but those tend to be simple to just grind away.
The last problem are the inclusions - and I noticed that making sprues that are bent several times before reaching their target may be the answer. The model that had two of these bends next to each other had the least inclusions of the entire batch (almost perfect).
I automated the investment/water weight calculation to save myself some time & trouble. I wanted to experiment with Preact and Recompose for a while for building small but potent web apps, and also to see how simple it would be to build a site that's pre-rendered and hosted in a static S3 bucket on the Amazon cloud even though it was using all the features of an advanced view engine. Publishing it on AWS was super easy - now I am taking a UI design class and learning Sketch to design the graphics to go with the functionality.
Of course I will mostly be using it on my phone, so I have the mobile comp as well.
I found out that the last problem I have with inclusions occurs due to thermal shock weakening the investment (inside the mold) and vacuum then breaking weak pieces off so they end up being embedded in cosmetic surfaces. To shorten the working time I decided to try making ceramic fixtures that have riser & filter pre-inserted. That way I can take 5-6 minutes it takes me to attach riser & filter down to 20 seconds (which is the max time before the thermal shock event).
The idea is to have the crucible and the vacuum rig already in the kiln room, so all I have to do is take the flask out of the kiln, close the kiln door, place the flask in the vacuum chamber, and attach the fixture. Can this be done in 20 seconds? We'll find out, but first I have to fire the fixtures themselves and glaze them. So, I have some involuntary clay art to do
Tire polish pictured here is used as mold release for clay.
The experiment with ceramic fixtures made out of pottery clay failed because the clay shrinks when drying (and especially during firing) - I mounted the clay onto steel flasks to try to imprint the flask surface on the inside which meant the fixtures had no available space to shrink and blew up in the kiln.
Plan B - I am going to try a quality ceramic casting compound for dense and high-detail casts with 1% shrinkage: Ceramacast. This time I will use wax cores cast from silicone molds to create the inside detail - after hardening, no firing is necessary and the wax core can be steamed out.
This weekend I finally completed fixtures to increase the casting quality. After refractory which cracked and fell apart, clay which exploded in the kiln, CeramaCast which fell apart despite the high price, and finally a cost-inefficient CNC quote, I iterated onto something that works.
The inner part sized to accept the riser is held inside the outer part that connects to vacuum flask lip with 1/2" screws. The holes took two weekends to drill and tap, but the result is holding well. The cavity between the pipes is filled with refractory. which no longer needs to hold together by itself.
I estimated the CAD model used for the CNC quote with steel pipes of diameters stocked by the nearest steel yard - features like bevels, tapers and 1-piece construction had to be given up but the resulting cost was around $90, down from $900 from the CNC process. The estimated model:
The last two fixtures got fused together because the metal filled the cavity where pipe diameter didn't quite match the flask. I managed to beat the chunk of metal out of one of the fixtures, but both got destroyed in the process.
Just when I was in talks with a plant that manufactures custom riser/filter combinations from CAD drawings, one of the engineers here at the hackerspace got interested in the problem and helped create a fixture that worked to increase casting quality:
The new fixture uses a ceramic blanket and more precisely machined & welded pieces (now I know how to weld!)
Thanks! I was waiting until this last issue has been resolved to write a series of articles on DIY investment casting - I think this might be time to start. I will ping you when it's done, if you can find time to review.
I did a detailed inventory this weekend - I am going to cast between 16 and 26 parts (out of 245) and fast-track the rest through CNC. I am glad I built out my foundry infrastructure, but I still want to finish this project in no more than a year. So far I have 3-4 casts ready to cut off the sprues and start polishing.
Last week I re-cast two parts with better detail and completed four other parts I already had molds for.
I also started working on casting some of the largest parts: hip and chest plates
...and ordered a new batch of risers with inline ceramic filters.
Finally, I started working on an investment casting article. I want it to be the best reference for DIY investment casting on the Internet, so I'm taking time with reviews, edits, and graphics. Seeking feedback from anyone here: