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potters clay as your mold?

 
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djtrickdog
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Joined: 17 Jun 2007
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PostPosted: Thu Jun 21, 2007 11:16 pm    Post subject: potters clay as your mold? Reply with quote

i am good with clay, i took clay arts classes and sculpting, so when i made my stormtrooper faceplate it took about an hour if that to complete it. my question is, since im going to be only making one stormtrooper costume, can i use dried clay on the mold surface? its fully dry, and pretty sollid and the walls are pretty thick. oh and the back to keep it from falling is compacted with compact newspaper, its so compact, i cant even get the damn thing out if i wanted it to

but im askin has anyone tried dried clay as their mold?
how many pulls so you think it would last?
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Plasticman
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PostPosted: Thu Jun 21, 2007 11:57 pm    Post subject: Reply with quote

1 with a vacuum pump and maybe 10-12 with a shop vac, i dunno...
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Solo
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PostPosted: Fri Jun 22, 2007 9:52 am    Post subject: Reply with quote

If you do try it, please let us know how it worked for you.

I really don't know, but maybe you could use your clay mold and form a negative wax casting, and then make a plaster mold out of the casting.

Just an idea...
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jegner
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PostPosted: Fri Jun 22, 2007 10:15 am    Post subject: Reply with quote

Plain old clay will shrink about 10% when dry. For a one off pull, dry clay will work. Very brittle. For a one-off, not a bad option. But, if you could bisque fire that mold, to vitrify the clay, you would have a much stronger mold to work with. You just run the risk of a kiln explosion, if there is any air or water trapped.
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cod
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PostPosted: Fri Jun 22, 2007 12:51 pm    Post subject: Reply with quote

Another option that I'm considering(using a clay mold) is in the TJ book:

make your clay mold, vacuform a very thin sheet of plastic directly on top and keep the plastic and the clay together and vacuform on top of these two. The thin plastic becomes a protective sheet over the clay.


I was thinking of taking this technique a step further by removing the clay from the formed thin sheet, and replacing with some harder, more permanent material(plaster?). At this point you could keep the thin plastic skin on , or take it off for more original detail.

Basically the thin sheet becomes (in theory) a quick detailed negative or positive mold.
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djtrickdog
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PostPosted: Fri Jun 22, 2007 11:01 pm    Post subject: Reply with quote

yeah its brittle, but i made it THICK, like inch and half. and its been dry for like 2 weeks now. im using a shop vac. im not gonna make negatives because i simply wont need it again.. about the thin layer of plastic idea, cant afford to buy all that plastic lol
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drcrash
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PostPosted: Sat Jun 23, 2007 6:08 am    Post subject: Reply with quote

djtrickdog wrote:
yeah its brittle, but i made it THICK, like inch and half. and its been dry for like 2 weeks now. im using a shop vac. im not gonna make negatives because i simply wont need it again.. about the thin layer of plastic idea, cant afford to buy all that plastic lol


Keep in mind that a good shop vac can pull maybe 2.5 pounds per square inch. That's about 350 pounds per square foot.

(With high vacuum, the pressure is several times higher---about a ton per square foot.)

I wouldn't espect a large sculpt made of 2 inch thick clay to withstand that kind of pressure. Maybe with just a shop vac, but maybe not. With high vacuum, I'd expect it to break.

Your molds should generally be strong enough to stand on without breaking in half.

Filling it with paper is likely not to keep it from breaking, unless the paper is so well compacted that it doesn't give significantly when you apply hundreds of pounds of force to it. If it gives even a sixteenth of an inch, the clay may break, because clay is so weak and brittle.

I wouldn't risk it. I'd make an alginate mold (with a plaster mother mold) and cast a solid buck to vacuum form over. (Like we did the modeling clay mask sculpt in this thread: http://www.tk560.com/phpBB2/viewtopic.php?t=635 )

You can cast it out of plaster, if you're not going to be making a lot of pulls over it. You can get #1 molding plaster (i.e., plaster of Paris) at a local ceramics supply place (use the yellow pages again) for about $25 for a 100-lb bag. That's about a quarter the pice at Home Depot or Lowe's. You can get Hydrocal White, which is stronger, for about $10 more. (They also sell 50-lb bags, for a bit more per pound but not a lot.)

Plaster's another thing it's worth shopping for locally. You don't want to pay shipping on 100 lbs of rock.
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djtrickdog
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PostPosted: Sat Jun 23, 2007 10:28 am    Post subject: Reply with quote

uhhh i dont think you can make molds from negatives of face plate because of the complex structure. because things get bigger and smaller, it will get stuck. get it?

for instance:

Code:

-0---I---0-
|    I    |
|    I    |
-0---I---0-

I= plaster negative mold seam (where the two would pull apart)


it wouldnt work because of the bumps ("0"). it would just get caught when trying to pull the plaster mold out. let alone this, i dont even think you could make it to making the negative mold!
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djtrickdog
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PostPosted: Sat Jun 23, 2007 10:28 am    Post subject: Reply with quote

double post
Laughing
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drcrash
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PostPosted: Sat Jun 23, 2007 4:56 pm    Post subject: Reply with quote

djtrickdog wrote:
uhhh i dont think you can make molds from negatives of face plate because of the complex structure. because things get bigger and smaller, it will get stuck. get it?

for instance:

Code:

-0---I---0-
|    I    |
|    I    |
-0---I---0-

I= plaster negative mold seam (where the two would pull apart)


it wouldnt work because of the bumps ("0"). it would just get caught when trying to pull the plaster mold out. let alone this, i dont even think you could make it to making the negative mold!


It sounds like you're talking about "undercuts", which are places where the shape wraps around more than 180 degrees.

If you can demold the vacuum-formed part pretty easily, you should be able to demold the cast buck as well---in either case, you need just a little draft. (That is, for things to wrap around less than 180 degrees.)

If there are actual undercuts, but they're minor, you can use a flexible alginate mold with a plaster mother mold, with thick alginate in the right places, and the alginate will squish a little as you pull the buck out, without tearing.

Alternatively, you can break the mold away, if you're only making one copy. (That's called a "waste mold".) Just break the mother mold and tear the alginate away.

(Or you can use a two-piece plaster mother mold around an alginate mold, but that's a little tricky because alginate dries fast and you don't want it to shrink or warp too much while you've got one piece made and are working on the other. Still, I think it's doable; that would let you make a couple of copies.)

If you do have undercuts, you need to think about how you're going to get the vacuum formed part off the mold without breaking one or the other.
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Tired of buying cheap plastic crap? Now you can make your own! www.VacuumFormerPlans.com
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