Casting Graphite

[bombheadster]

I'm wondering if anybody here has experience or knowledge of casting graphite. I am taking a mold making class right now, and would like to experiment with making graphite optic molds or other blow molds. I know powdered graphite is readily available, but I don't know how I would go about casting that into a mold. I'm thinking some sort of refractory liquid binding agent. Any thoughts or ideas?

[boxfan willy]

I would think that it take some serious knowledge of high heat binders, and then you would need to find some way to pressurize the "slurry" or paste for a decent length of time(2 weeks to a month).

The two most used methods I know to get a "hard" material out of graphite is through extrusion and water pressurization.

Extrusion produces the weakest materials, using larger grain sizes and more binders pushed through a die (like a badass playdo machine) to produce a lower grade amalgamate. This method requires more binders which will oxidize and burn out much quicker than iso. The advantage is that you can use dies to obtain certain shapes, just like extruding metal or wood.

Isostatic is made using smaller particles and less binders(as the particles fit closer together) and using a water pressurization chamber to produce high grade, high heat graphite amalgamate with more anti oxidizing properties. It is a time consuming process with some grades of isostatic to remain in a chamber for 90 days(kinda like kim-chi, I love kim-chi). The main drawback, besides cost, is that it comes out in slabs and must be machined to obtain desired shaping. Very costly.

I would think in the long run, machining graphite would be less costly and provide a usable mold than any type of graphite casting method, which would lend itself to very few exposures to molten glass.

If you want to cast a mold, perhaps using metal(aluminum or bronze??) and then using a slurry of aquadag(sprayable or paintable graphite) or a coat of boron nitride as a release would provide more accurate and longer lasting mold.

boxfan

[ornametalsmith]

I completely agree with B.Willy.......I'd take the machining route or cast in metal....probably brass/bronze.
dos centavos....:D

[Greymatter Glass]

Yeah, Wil summed it up better than I ever could.

As for making optic / blow molds.... there's a reason most companies use metal - it works, and is easy to cast.

[bombheadster]

Yeah, I had a feeling this would be the type of responses I got. Graphite is definitely some weird stuff. I wish we had metal casting facilitis here, but alas...Think I'll stick with plaster casts for now. What I'm experimenting with is taking a plaster cast of some of my glass work, and then casting that in slip. Anyway, thanks for the info, much appreciated. My camera broke recently, but if I get it fixed I'll post some pics of the mold-making/casting process with plaster and slip...might be interesting to some.

[NUBBLET]

here is a thought , maybe just use it as a mold flux? kinda thing , sprinkle some and pat it down on the pre-made mold so you get a real smooth finish , kinda like shelf paper or graphite liner .

Maybe make a mold then coat with the graphite to give just a few extra pushes , or just clean the surface for the push .

I just am saying waste not want not , there has got to be some use for it

[kage]

large graphite chunks and a CNC machine?

i just posted this on another thread, but i heard about a soft glass guy that makes large molds for fusing and slumping out of graphite. i have no idea what his process is, and don't think he's willing to share.

[Greymatter Glass]

... i heard about a soft glass guy that makes large molds for fusing and slumping out of graphite. ....

There's no secret or conspiracy to using graphite as a mold material - it just doesn't work very well for boro temps. it's a simple process really, works fine at the lower casting temps of soda lime glass. You take a chunk of graphite, carve your mold in it, fire it. The graphite will hold up for several firings before it starts spalling/flaking and then you replace it.

Weaver industries makes several "off the shelf" and any custom molds you want in graphite, and for a good price. They're intended as "press" molds, but could be used for kiln casting as well.

The graphite WILL off-gas some of its binders when it gets up to a high temp. that's unavoidable, and small air bubbles in the finished product are a fact of life for most kiln cast glass.

When you get to the temperatures boro slumps and fuses at (above 2200F) the graphite will break down in one firing. You might get a single impression made, but chances are you'll end up with lots of bubbles and graphite flakes in your glass. If your end piece is of enough value then sure, a one shot graphite mold may be fine... if you're thinking production or lower cost end work then the cost of the graphite will start to hurt after a while.

-Doug