First ever attempt at casting

[piGuy]

My girlfriend told me it was time to put a ring on her finger, so I figured that was a good excuse for me to buy a Form 2 and a burnout furnace and try my hand at casting metal. I don’t know anything about jewelry, but I’ll fake it till I make it! I managed to snag 2L of Castable V1 on eBay for $75 and bought a used Jelrus Infinity dental burnout furnace. Last year I designed a simple heart shaped necklace for her that I had made in sterling silver from Shapeways. I’m using this model to get my casting process figured out. Here are pictures of my first casting attempt.

I used R&R Plasticast with Bandust (vacuum degassed after mixing and pouring) and used pure silver cast at 1050C (melted in a Thermolyne bench top lab furnace) and a flask temperature of 482C. I was expecting a total catastrophic failure for my first try, but it turned out almost decent! The bottom faces of the casting look excellent (I can make out the 50um print lines in the casting) but there is porosity on the top and side surfaces that seems to get worse near the top. The sharp edges of the part are also not well defined (shrinkage porosity?)

For my next attempt, I’m going to try sterling silver and use some borax as an antioxidant during melting. I’m also going to rig up a crude vacuum casting apparatus in the hope that these measures will reduce the porosity.

[katkramer]

Hey there, @piGuy!

I just saw this in my email feed, and wanted to check in. I know you posted awhile back, and it even warned me that I was reviving an old topic. I struggled with this in the beginning, but I think I’ve got it figured out. Seems that people who are casting successfully are pretty secretive about exactly what they’re doing. No bueno! I’m hoping that you figured it all out and are casting successfully.

You never mentioned what type of casting setup you are using…is it centrifugal or vacuum? Steam, or sling? You mentioned trying to rig up a vacuum apparatus, but then also mentioned vacuum degassing. While troubleshooting my own casting issues, I learned that a vacuum casting setup the temperature can drop 100°F per minute, so it’s sometimes recommended to set a final hold that’s slightly hotter.

Did you use Formlabs’ burnout schedule?

Yes, what you are seeing is shrinkage porosity, but also the degradation on the surface may be incomplete burnout. I’m finding that I have to hold longer…my final temperature is about 750°C (1375°F), and I’m holding for about 3 hours. Somewhere on the forum someone mentions burning out hotter at 1450°F (790°C), the R&R’s tech support says that at that temperature the burnout is too hot and oxygen is too low to eliminate the ash completely…they also said the investment breaks down at hotter temperatures, so their recommendation is to stay closer to 1350°F.

Button (opening) side down during burnout is working fine for me right now, although I’ve started to take the flask out during the top temperature and blow on the opening in an attempt to get any residual ash out, then I put it back and make sure it comes back to my final temperature of 525°C (975°F) and then put it back in the kiln for another 20 minutes before casting. To be quite honest, this might be superstition because I haven’t noticed any difference…when I burn out overnight and forget.

My metal temperature in an electric furnace is 1015°C (1860°F). I am not using any flux in the ElectroMelt because it is not necessary with the graphite crucible. The carbon in the crucible makes it unnecessary.

But here’s the main issue from my experience, and something I fought for awhile. I assumed that a bigger sprue meant that more metal was getting to the piece, so the bigger the better, right? NO. I recently took a class with a woman who’s an expert in casting and wax carving, and we talked about sprues, and the heavens opened up and it all made sense. Your sprue is holding in heat while the rest of the piece is cooling, and that’s causing the porosity near the sprue. I was having the same exact problem, and almost all of the porosity was adjacent to my very generous sprues.

Your piece seems pretty even thickness all the way around, so you need a relatively thin sprue that’s the same thickness as your piece. Also, “fillet” the connection to there are no sharp corners in the investment that can break off…it will also help the molten metal flow better. If you have a thicker area of the piece, attach the sprue at that location instead.

Most sprue waxes that are sold seem to be 8 gauge or 10 gauge, which are maybe about 2.5 to 3.5mm, so I mimic that when adding a sprue in ZBrush. Also, minimum thickness for a pendant might be 1.5-2.5mm. In a class I took awhile back, the instructor took the wax sprue and warmed it between her fingers and flattened it slightly to fit on a piece. I mimic that in my 3D program, taking a cylinder and modifying the end that goes into the piece.

Yet another consideration is the type of flask…are you doing a one-off with a 2" (50mm) diameter solid flask? That’s the easiest…I made a little diagram below. Your choice of location for the sprue is good…you have to think of the flow. I had struggled with my perforated flasks until someone mentioned that the vacuum pulls OUT instead of DOWN.
Forgive me if you already know that…I’m just putting this out there for anyone who might need the info (former tech trainer…it’s in my blood to share anything that will help others).

Another thing I noticed over time…Castable V2 gives a significantly smoother print surface, so if you’re doing this a lot, you might give it a shot. Be sure that your pieces are also cured completely (they will turn darker). I would sometimes cure for a day. Also, my LED nail lamp never cured my pieces, but one with a UV fluorescent tube worked beautifully. Weird, because the LED lamp works faster and better on my nails.

I’ve also become less afraid of seeing lines in my pieces…I don’t sand the model beforehand, and use a sprue clipper ($$) or jeweler’s saw ($) to remove the sprue CLOSE to the piece, then use a 4-cut hand file to grind off the excess, blending it into the piece. I then use emery sticks (sandpaper glued to a stick), or sanding boards from a nail supply and work from 400 to 600 to 800 to buffing. There are some from Tropical Shine that were recommended by another jewelry instructor in a class I took recently. You can literally go from rough casting to high polish with this inexpensive $4 four-way buffer.

Anyway, curious if you’ve made progress…the heart is adorable and I’m hoping your girlfriend loved it!

[katkramer]

An interesting update…I just cast a few small pieces in a solid flask and ended up with a little porosity. The pieces I cast from the same resin in a perforated flask were just fine. Metal temperatures were the same, everything else the same. Hmmmm…come to think of it, every casting I’ve made in a perforated flask has worked perfectly.

Update after a recent class with Kate Wolf…her preference on casting technique is 1) perforated flask, 2) centrifugal, 3) solid flask.

[billb]

Presumably the perforated flask is allowing more air to get to the resin during burnout?

[katkramer]

That’s a good question…I think it’s more about air flow from the main trunk, but I wonder if the perforated flask allows more oxygen in? Food for thought!

By the way, I’ve just had 5 flasks of near-perfect casts this week. My new process is to print at least three pendants on the Form 2, and at least one cast should be good, I finish the piece, then make a rubber mold with Castaldo ICE and shoot waxes. For production, it means a better starting point and less cleanup, and the waxes are much faster to burnout. I have not been a production jeweler in the past, so I’m in uncharted territory. I sold eighteen this week in my Etsy store and my website! I’m on a roll…

Gotta save up some money for whatever Formlabs comes up with next!

[piGuy]

A brief update:
Shortly after my original post, I did more casting experiments and got some significantly improved results. Here are some more details:
I’m using a 2.5" non-perforated flask
I switched from .999 silver to .925 sterling casting grain
Added borax to my graphite crucible (maybe not necessary according to @katkramer)
Reduced melt temperature to 980C

Incorporated a homemade vacuum casting apparatus. This was a simple metal plate with a vacuum port and a high temp silicone ring to seal the flask against the plate while under vacuum. When pouring the plasticast into the flask I wouldn’t fill to the absolute top of the flask so I would have a few mm air gap. This would allow the entire bottom surface of the flask to be exposed to vacuum when placed onto the metal plate. The setup is similar to this: https://youtu.be/H4vSYcYwdNU?t=4m56s

Without changing anything to the 3D printed part shown in the first post, I got a really good looking result. I had just a few bubbles here and there, but otherwise the surface looked great. Unfortunately I wound up remelting that piece before I took any pictures!

[katkramer]

I have not cast .999 fine silver yet, but I have heard that it’s more difficult, and I haven’t found a casting house yet that will do it. I wanted fine silver for making enameling frames, because sterling interferes with the colors of transparent enamels.

I have been using the anti-firescale casting grain from Rio Grande, and it works nicely. I’ve been putting the melting temperature at about 1860°F with a 975°F flask temperature. The casting comes out VERY clean, and only requires pickling and tumbling, then normal finishing. The only drawback I’ve found is that it seems not to take a patina/oxidation well…it’s just not as dark.

Yup…if it’s an enclosed dental melting furnace with a graphite crucible, no borax necessary. I used it a couple of times when I got my ElectroMelt, but was told it wasn’t necessary. It left residue in the crucible. I had assumed that the sticky molten borax would “grab” any crud that was floating on the top of the molten metal when I put a graphite rod in there, but learned that my thinking was incorrect…someone told me that the metal flows out of the crucible from under whatever is floating on top, so that’s not an issue. The goal of the flux is to eliminate oxygen from the environment, however, that’s what the graphite crucible is for.

Did you decrease the size of the sprue at all? Another test I did recently was to decrease the size of my sprue. Not being a traditionally trained jeweler, I don’t know what the “normal” sizes of sprues or even limits are. I recently decreased the diameter to 2mm on one of my smaller pieces, and it worked perfectly fine. I think that’s as small as I need to go. I was making the “clapper” on a small bell pendant.

I can’t wait to see one of these if you cast again.