Strongest / Toughest material moldable in a Silicone RTV Mold?

Hello, I want to cast some fine pitch gears in a Silicone RTV mold. I want to know what the toughest and/or strongest material is moldable
in silicone RTV that would also serve well to make well detailed,resiliant and strong gears.
Thanks Steve
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Probably some exotic graphite and A/B-cure epoxy.
If you want metal, spincasters have cast aluminum diecasting alloys in silicone rubber. The rubber is a very hard, high-temperature version. It doesn't last for many shots but it does work. It doesn't much resemble the common RTV type that's used for low-temperature casting.
Look up spincasting on the web. One company that I knew of years ago, in Mount Vernon, NY, used to sell the rubber and the casting machines. The lower-temp, RTV mold rubber is used for casting zinc-based alloys, some of which mave mechanical properties that are very similar to those of medium-strength aluminum. The Zamak family of alloys (look into ZA-8) have tensile and compression strengths on the order of 40 kpsi and good bearing properties. Their only disadvantage against aluminum is their tendency to creep. Under high loads, running continuously, the teeth eventually will deform. But the gears probably will wear out long before that happens.
Good luck.
Ed Huntress
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I would suggest ZA-27 (27% aluminum, balance zinc, you can whip it up with a dollar or two of 1983-or-newer pennies and some scrap aluminum). Strong as mild steel, hard as heck, melting point less than aluminum (circa 1000 degrees).

Oh...there you have it ;)
I don't like the looks of ZA-8, at least the stuff I've made; makes big huge crystals, visible when you break a face.
Tim
-- "I have misplaced my pants." - Homer Simpson | Electronics, - - - - - - - - - - - - - - - - - - - - - - --+ Metalcasting and Games: http://webpages.charter.net/dawill/tmoranwms
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What kind of mold are you casting it in? If you're using plaster, it chills too slowly for ZA-8.
Ed Huntress
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chills
Was just an ingot in a sheetmetal mold. Probably was cooling too slowly, but still. ZA-27 makes a nice and fine structure though, almost like a hardened and broken steel surface! Heh, strength testing a button of the stuff is what broke the cast iron drill press vice... Which reminds me, my cast aluminum replacement is starting to crack up..(fatigue limit reasons I imagine)..should cast a new one. To get back on topic, maybe I should cast it with ZA-27.. :)
Tim
-- "I have misplaced my pants." - Homer Simpson | Electronics, - - - - - - - - - - - - - - - - - - - - - - --+ Metalcasting and Games: http://webpages.charter.net/dawill/tmoranwms
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Hi Ed,
I know nothing about casting (and probably won't be doing any in the forseeable future), but I've followed this thread and been intrigued by your last comment "If you're using plaster, it chills too slowly for ZA-8."
Why did you suppose he might be using plaster? And what does "chills too slowly for ZA-8" mean/imply?
Just curious.
Roger
Ed Huntress wrote:

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ZA-8."
Tim is doing a lot of experimenting with melting metal and casting, and one of the mold materials experimenters often try is plaster of Paris. Since he was experiencing grain growth, I thought he might be using it.
Plaster insulates the melted metal and chills it relatively slowly. This promotes crystal growth and weakness in the finished part, as well as a coarse crystal structure where it fractures. To get fine grain and the resulting ductility and, to varying degrees, higher strength, you have to chill the molten metal more quickly than plaster allows.
Grain growth varies by metal type. Some metals are immune to the problem. Zinc alloys vary widely in this regard, as do aluminum alloys. Generally speaking, aluminum casting alloys, such as A356, have similar properties whether sand-cast or plaster-cast in sections up to around 1/4". Anything thicker than that will benefit noticeably from being cast in green sand, because the water in the sand helps to chill the casting more quickly.
There is a hierarchy of chill rates for different mold types. Metal molds generally chill quickest. Graphite molds are next. Green sand is next. Petrobond and baked sand are next. Plaster and plaster-based investments are the slowest (most insulating) of common mold materials.
Ed Huntress
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Hi Ed,
Thanks, very interesting. I always learn something from your posts. So many questions can be answered in a couple of words, but you generally go the extra yard with a bit of background or expansion on the subject to back up your statements. Keep up the good work.
Regards,
Roger
Ed Huntress wrote:

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Snip >

Hi Ed, (and everyone else) thanks for the replies. I have talked to some spincasting Gurus and it seems that the because of the huge amount of pressure and heat that goes into making the rubber mold, the spincasting process is a little hit and miss when it comes to casting accurate gears. The rubber changes shape concentric to the middle of the mold as it cools and pressure is released.
I was wondering whether it would be possible to cast Nylon or Delrin in a Silicone Room Temp Vulcanizing mold?
Steve
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the
That's true about distortion. I didn't think you were considering spincasting itself, but that you had some method for casting and you were going to use silicone for the molds. I should have asked.

I've never heard of it. They're both thermoplastics, and would have to be pressure-molded in some way, such as by injection molding. I believe you'd have the same distortion problem, in spades. Compression molding, transfer molding, and injection molding of thermoplastics are done in metal molds.
Why silicone rubber? Refractory gypsum cements are handy. US Gypsum makes one that's designed for casting aluminum or lower-melting-point metals. Then you could cast zinc alloys, on a camp stove out of doors, if needs be.
If you want to use silicone rubber for a mold and you want a plastic gear, I think you're stuck with the thermosets: epoxy, polyester, vinylester, and polyurethane are the primary ones. I don't know if any of them would work for you but polyurethane sounds like the most likely one to me.
Ed Huntress
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If you use the silicone RTV mold for making a wax casting, you can use the lost wax process to cast any metal. If I were doing it, I would probably just go for silicon bronze. I don't have any way to melt steel or higher temp metals.
Dan
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