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Strongest / Toughest material moldable in a Silicone RTV Mold?



 
 
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  #1  
Old March 5th 04, 07:37 AM
Scorsi
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Default 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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  #2  
Old March 5th 04, 11:45 AM
Ed Huntress
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Default Strongest / Toughest material moldable in a Silicone RTV Mold?

"Scorsi" wrote in message
om...
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.


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


  #3  
Old March 5th 04, 05:02 PM
Tim Williams
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Default Strongest / Toughest material moldable in a Silicone RTV Mold?

"Ed Huntress" wrote in message
...
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.


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).

The Zamak family of alloys (look into ZA-8) have
tensile and compression strengths on the order of 40 kpsi and good bearing
properties.


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


  #4  
Old March 5th 04, 05:37 PM
Ed Huntress
external usenet poster
 
Posts: n/a
Default Strongest / Toughest material moldable in a Silicone RTV Mold?

"Tim Williams" wrote in message
...
"Ed Huntress" wrote in message
...
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.


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).

The Zamak family of alloys (look into ZA-8) have
tensile and compression strengths on the order of 40 kpsi and good

bearing
properties.


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


What kind of mold are you casting it in? If you're using plaster, it chills
too slowly for ZA-8.

Ed Huntress


  #5  
Old March 5th 04, 05:50 PM
Dan Caster
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Posts: n/a
Default Strongest / Toughest material moldable in a Silicone RTV Mold?

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



(Scorsi) wrote in message . com...
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

  #6  
Old March 5th 04, 09:51 PM
Tim Williams
external usenet poster
 
Posts: n/a
Default Strongest / Toughest material moldable in a Silicone RTV Mold?

"Ed Huntress" wrote in message
et...
What kind of mold are you casting it in? If you're using plaster, it

chills
too slowly for ZA-8.


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


  #7  
Old March 6th 04, 05:32 AM
Scorsi
external usenet poster
 
Posts: n/a
Default Strongest / Toughest material moldable in a Silicone RTV Mold?

"Ed Huntress" wrote in message t...
"Scorsi" wrote in message
om...
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.


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.


Snip
Ed Huntress


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
  #8  
Old March 6th 04, 06:41 AM
Ed Huntress
external usenet poster
 
Posts: n/a
Default Strongest / Toughest material moldable in a Silicone RTV Mold?

"Scorsi" wrote in message
om...
"Ed Huntress" wrote in message

t...
"Scorsi" wrote in message
om...
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.


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.


Snip
Ed Huntress


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.


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 was wondering whether it would be possible to cast Nylon or Delrin
in a Silicone Room Temp Vulcanizing mold?


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


  #9  
Old March 6th 04, 09:04 AM
Roger Head
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Posts: n/a
Default Strongest / Toughest material moldable in a Silicone RTV Mold?

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:

"Tim Williams" wrote in message
...

"Ed Huntress" wrote in message
et...

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.

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).


The Zamak family of alloys (look into ZA-8) have
tensile and compression strengths on the order of 40 kpsi and good

bearing

properties.

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


What kind of mold are you casting it in? If you're using plaster, it chills
too slowly for ZA-8.

Ed Huntress




  #10  
Old March 6th 04, 02:51 PM
Ed Huntress
external usenet poster
 
Posts: n/a
Default Strongest / Toughest material moldable in a Silicone RTV Mold?

"Roger Head" wrote in message
...
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?


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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