Metalworking Thread (aka Haus indulging his inner mountain dwarf)

Well, the first test was an absolute fail. The oven I have isn't hot enough to really liquify the PLA to extract it, so I had to go with the "burn it out using the molten metal you pour in" method, which isn't optimal by far (see the red eyed skull above for what that process yielded). Don't know if I'll keep this around for my "Gallery of failures" , I mean it's still an all copper skull weighing in at around 3lb 7oz.

I think the reason for the fail was in part due to the melting plastic popping and "exploding" minimally which broke up the inner plaster part of the mold. Modifying to try again, minimizing plastic in the print (print with a single shell wall , i.e. "vase mode") and print the feeder tube so there's a block to keep the metal from free falling into the mold...

The bigger tube will be the downpour feeder for metal, the slim tube will be the vent for gasses escaping the mold cavity.

I might also use a method not doing the "complete encase in plaster" stuff. And instead use "artificial sandstone". That's where I do the base layers of plaster around the mold, but then instead of putting it in a flask and filling it with plaster like I did this time, I just put the shell in a container and pack around it with sand mixed with Sodium SIlicate (aka waterglass). That works a lot like greensand for casting, except that as the sodium silicate catalyzes with air it becomes silicate and turns the sand into essentially sandstone.

For reference : this is what the filled plaster flasks looked like :

The taller flask I still haven't poured. Will probably pour that one tonight, and use a lower temp metal like aluminum rather than copper/brass.

OK, so I didn't forget about all this, but I had a medical situation wholly divert me from any of my metalworking for a bit (see health issues thread if you're curious).

OK, I was attempting "lost PLA" but in the end I was not able to "lose" the PLA before casting, so it was just melting and incinerating the PLA with the molten metal which was causing all the fails you saw. So I decided to try true lost wax casting. I made a silicone mold of the skull I was trying, cast it in wax, then did that in plaster. Still experiencing fails but not as bad.... (That's what you saw in the previous post, here's an example of the results :

Better, but molds still cracking, still fails. So I decided to take a step back and analyze.

What is causing this? Two things apparently, air trapped in the plaster expanding (causing mold cracks), and moisture trapped in the plaster also expanding causing much the same set of problems. So I looked at how they address this with traditional lost wax processing :

A kiln and a vacuum chamber.

• You use the Vac chamber before pouring the plaster to suck the air out of it for the most part.
• You use the kiln to fire the molds. By taking the molds slowly all the way up to around 1000 degrees or so force out all the moisture.

I also identified that I was trying to do my first runs at this with a REALLY complicated model with tons of detail filigree and whatnot. I was also doing this in copper and brass (the highest temp metals I work with), and that will come back up later in this wall of text.

• OK, Vac chamber is pretty easy, under $100 from Amazon.
• Easier forms to try? Also easy, ordered some silicone molds from amazon that people use for resin figure casting, but I cast wax instead.

A kiln? Damn... Not too expensive to pick one up (Hi2u Craigslist and FB Marketplace) , but the problem is space. A traditional kiln large enough to do the molds I want to do would be around 3'x3'x3' and need a 220/240 volt plug. I could easily run the power in my garage/workshop, but I just don't have the space for a kiln at the moment. When we rebuilt the garage we took an oath to keep it clear enough that we could park our cars in there at night, which means a big bulky kiln has little space to live with the amount of stuff we currently have in there. I even have a shed, but that shed is also full enough storing a kiln would be a real trick. So I am going to try to work around the kiln need.

I cast the simple wax figures. Then when I mixed the plaster I put it in the vac chamber first and ran it for a couple minutes and sure enough huge bubbling and a "foam" on top of the plaster from all the air being pushed out. So feeling good about that.

Poured the molds, they set well. Now the question is what to do about the moisture. With the kiln you go north of 1000 degrees for hours to "fire" the plaster. I thought I would try going to a temp I could go to, and just go longer. So I set up a cheap toster oven I picked up at a garage sale to heat them in. Along with engineering it so the taster ovens normally time regulated on/off would just stay on. And I got to this :

That will hold two molds at a time of those cans (and taller cans). So I put them in there, set it to "max" which is around 475 degrees, and literally let it run for around 36 hours. An electronic moisture meter (used in construction) I have showed it as having a lower than 3% moisture content at the end of this.

Then I thought about metal temp and how thermal shock of the water/air and higher temps could be causing a lot of the volatility in the mold. Of the metals I use I have three classes in terms of temp. Copper and brass , the highest temps (1900-2000 degrees Freedom), Aluminum in the middle at around 1100 degrees, then Zinc/Die Cast/Pewter/tin at the low end (around 750, which i can literally do with a blowtorch). So I opted for a low temp metal (Die Cast, which is a tin, zinc, etc alloy ), vac chamber for the plaster, simple mold, and a day and a half in a toaster oven for moisture. And I give you... the results :

Still a little rough, you can see where there was some moisture/steam pockets. But WORLDS better.

This of course got me cocky... So I decided to go with a high temp metal (copper), simple form, vac chamber, toaster over for a day and a half... and got this :


Still better, but as you can see large gaps from steam/air pockets.

I am seriously thinking about how to apply some next tier Tetris skills to the shed/garage to make more space so I can store a kiln now. This hobby is obviously getting out of hand on a near autistic scale for me, heh.

Overindulgent Hobby Update!

I tweaked a couple things.

Added a retired handheld massager (previously used for calf cramping, but the massage head kinda broke) to the side of the Vac chamber. Now that vibrates things as they're being vacuumed which seemed to cause a LOT of of frothy bubbles on top of me tapping the chamber to agitate it some. This caused a LOT fewer bubbles (still a couple, but FAR fewer).

With the toaster oven outside and rigged all these molds got to bask in 450 degree heat for 48 hours straight to force out as much moisture as possible. This caused FAR less volatility when pouring the metal (less steam) and I got through a run with no molds cracking (as was evidenced above around Necronomikittys ears).

With those changes I was able to do low temp (die cast/zinc), medium temp (aluminum), and high temp (brass and "scrap dust metal") without major failures :

Left to right :

• Raven in Brass
• Lab puppy in Aluminum
• Necronomikitty II in Zinc
• Sleeping puppy and kitten in "dust metal" which is literally just all the metal dust I accumulate in sanding, cutting, and doing other post pouring work that I accumulate and when I have enough I melt it into something, so it's a Heinz 67 alloy of everything I work in)

Still some tiny pitting in the Aluminum and Zinc. And there's a bubble on the far side of the raven's head which makes it's other eye look all Marty Feldman-ish. But worlds better.