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I'm hoping that someone, one of you experts out there has experience making stainless steel molds/moulds?

I recently spoke with my local metal fabricator and he thinks that SS will soften and bend with heat? I assured him it does not soften at slumping (1200F) temperatures, but, I suppose it depends what is mixed with the steel? Another problem I am concerned with is that some SS does spall (sp?) when heated?

This mold will be quite large, about 40" by 20" and I hope I can reuse it many times. I would like to make it out of as thin a material as possible because I want to be able to bend it.

Is there a specific grade/thickness of SS we should use? Might I have a problem if I use really (really) thin stainless steel? Is there a solder I can use that will hold up to slumping temperatures? Any advice will be greatly appreciated. If I pull this off I will post photos.

Shawn- I can't talk about SS grades, I don't know what mine are, but I've used SS as thin as 18g and as thick as ⅛" from slump temps to 1500F (briefly) and gotten no spalling ever.
Over time and repeated firings the stuff does begin to oxidize but all that means is more maintenance- light sandblasting and recoating with kw between uses.
I did have an issue with the thick stuff- the glass was cracking- but that was cured with extra insulation between the steel and the glass (Doug Randall nailed that one for me). The steel will heat up and cool much faster than the glass.
I've also had issues with very heavy pieces scraping through the kw and sticking to the metal (and breaking) but a bevel on the glass took care of that.
With the thinner metal, for example a free-form bent sheet, I've supported the stuff with kiln furniture or pieces of brick underneath because I didn't trust the metal to hold its shape at heat.
Welds will hold up, solder won't. Maybe your metal guy (it's always a guy, isn't it?) can weld a frame around or under after you've bent it?

Can't add anything that Marty didn't cover, except to say that when I've experienced a bit of spalling, it hasn't occurred at a temp where the bits become embedded in my glass. I usually open the kiln, panic at the bits scattered about, then sigh with relief as I realize they aren't attached.

Shawn,

Ask for 316 Stainless. It has higher temperature resistance than other stainless steels.

I've never seen spalling with any of my SS molds, only discoloration.

Tony

I have been using stainless molds for quite a few years and they have held up well. I have only used them at slumping temps though and not at fusing temps. I have mainly used 304 SS because it seems to be slightly more available than 316 SS. Tony, have you noticed a significant difference in corrosion resistance at high temps between the 304 and the 316?

Hi Shawn,

I'm no expert, but I've never experienced spalling at slumping temperatures. As Valerie described, I often get a lot of spalling at high temps (wire melts/combing), but the spall rarely stocks to the glass.

If I recall correctly T304 SS is considered "high temp" grade and is more resistent to deforming with heat than other grades, but all grades can/will distort. If the mold is made of thin/flexible sheet, it will of course have a greater tendency to bend under the weight of the glass and so I believe supporting the mold would be more important than the grade of SS you decide to use.

That's one heck of a big mold you've set your sights on! Why am I not surprised

Sharol

Spalling/rusting is an issue if you use cheap stainless steel also known as thrift store pre-made shapes of dubious stainless grading imported to North America from far away.
In my experience, they will, once heated reveal their true inner rusting selves if allowed to be exposed to normal atmospheric conditions.
If you continually fire them as in never allow them to be removed from the kiln or let the kiln become truly 'cold', they may work for some time before needing rust removal.
Makes you wonder what additives you were adding to your food preparation and serving on this sort of stainless......

I use regular 304 stainless, which is the most common grade. I have used very thin and up to ⅛" thick. It just works. Remember that glass shrinks more than stainless. I have spherical shaped molds that will grab the glass if cooled all the way to room temp. When warm the glass will come out. If it does stick. I just invert the mold and heat to 300º, and at some point it falls out. I've never gotten spalling or distortion.

Thank you everyone for the advice, I'm ready to do forward and make the mold. This is the "go to" place for the latest and the best fusing information!

Shawn

Bert, don't you mean that ss shrinks more than glass, thus the potential of a ss mold trapping the glass? I've not has a problem unless the ss mold is a full hemisphere.

Stainless has a higher coefficient of thermal expansion than glass, yes, so it'll get bigger with heat, and shrink more rapidly when cooled. If glass gets in the way in either direction (so that the metal encloses the glass and it can't escape), there can be enough force to crack the glass.

I've used 304 and 316 stainless, mostly, in the kiln, and definitely seen it deform. I've deformed quarter-inch square stock in the kiln, for example in this blogpost: http://www.morganica.com/glass/fusing/d ... -in-glass/ These bars were 24 inches long, heated in a casting cycle with billet on top of the bars in the arrangement you can see in the picture:

The bars held, but sagged down about 1.25 inches from the perfectly straight shown in the pic. I've also had 304 kitchenware deform after extended use (after a couple years of repeated slumping/firing), unless it was thicker gauge stuff.

At casting temps, especially if you're also steaming out molds while 304 is in the kiln, it can corrode after a few sessions. So yep, it can rust. Once there's a hint of corrosion, spalling isn't so awfully surprising.

And I've also seen what looks like spalling but turns out to be thin layers of glass melted onto the stainless, which pop off as the steel cools. Either way, they don't stick to the glass. I do, however, make sure that I clean the stainless well before I use it--I suspect that if someone has problems with stainless spalling into the glass it's something leftover from a previous firing that pops off on the way up. The spalling I've seen seems to be a post-processing product, so it's ejected from the cooling metal after the glass is already too hard for anything to stick. (haven't used mild steel or other non-stainless in the kiln, so I can't say this would be true for other metals)

316 tends to hold up where 304 doesn't, I've found, but it's more expensive and harder to find.

Beam deflection increases as a cube of the beam length, so it's important to place the supports (fire bricks) as close to the glass as possible. In the image above, there's probably 25% of the length that does nothing to support the glass, but it increases deflection by about 2.4 times.

Tony

Nikki ONeill wrote:Bert, don't you mean that ss shrinks more than glass, thus the potential of a ss mold trapping the glass? I've not has a problem unless the ss mold is a full hemisphere.

Yes Nikki. I have many drafted ss molds that work fine. The glass is able to ride up, and does;t get stuck. I also have hemispheric shaped molds which will grab hold if let cool all the way to room temp. The mitigation is to turn the mold upside down, and reheat. Somewhere between 300 and 400ºF, the glass will fall out.

Shawn, you can get away with very thin 304 ss sheet, if you manage to support it well. When I make bowl molds, I like to use a rolled top to create stability, I have ring molds made by bending 1x1 angle irons. These too are very stable .

I have a buddy who made some bending molds using very thin gauge ss sheet. This was for the horizontal part. Then he put slits in the vertical pieces, to define the shape. He used a water jet machine to cut the steel and the slits in it.

Thank Bert!

Thank you Cynthia and Tony!

Tony Smith wrote:Beam deflection increases as a cube of the beam length, so it's important to place the supports (fire bricks) as close to the glass as possible. In the image above, there's probably 25% of the length that does nothing to support the glass, but it increases deflection by about 2.4 times.

Tony

True. It's a trade-off here because the firebrick is also an excellent insulator. I've found that when I put the firebrick right up against the mold in these applications, I get too much insulation baffling those sides of the mold. Backing off a couple of inches on each side alleviates the issue, but it does increase the deflection, yes. I was responding, however, to the "never gotten spalling or distortion" comment. I've certainly gotten both with 304, just depends on the setup.