MIMF

Is there something on the market I can use in place of plaster of paris to make molds from?

Spackle for sheet rock gives a longer working time than plaster and sands a bit more easily.

The problem with water-based materials is that they always shrink when drying. Trying to make a mold of an existing surface with this is very non-precise as the material will pull away from the surface when cured. There are commercial plastic mold making materials that are not water-based and do not measureably shrink. Check out the casting products on Micro-Mark and http://www.smooth-on.com/. These products can contain silicone and diisocyanate so take appropriate cautions.

UltraCal (sp) or Hydrocal is tougher than standard Plaster o'Paris. Shrinkage is minimal. Mixing plaster is an simple art, once I learned this one trick.
1. Measure out your water into a container.
2. Then sift your dry material into the water until the plaster takes up all the water. Do not stir or shake it.
3. Sift it in until the water stops "wetting".
4. Now you can mix it, with no lumps, andd just the right amount of water.
5. Keep mixing unit your stick leaves an indent than hangs, then melts back into the mix.
6. now you are ready to pour or drag-mould the mixture.
Note: multiple pots of un-stirred plaster can be staged for large pours, and can sit un-mixed for up to 45min.

Sheet rock compound sands easily but always remains soft and expands with heat and other conditions.

May be wrong here, but methinks plaster of paris is used -because- it shrinks very little. Correctly mixed, it should be nearly dry. Add a layer or two of cheese cloth to give it strength and stability, if that's a concern.

Drywall mud sands more readily, but at anything past 1/4" in thickness it will trake forever to dry and will crack as it shrinks. Useless for mold-making...

Bondo works well, and the fiberglass reinforced types offer good strength, hut it's expensive, and stinks to all hell. And it won't "flow" well, and won't reveal finer details. Oh, it can also get very hot, so it shouldn't be used on finished items...

In short, plaster of paris is used most often, for good reasons... What are you looking to make a mold of?

I also recommend Durham's Rockhard Putty. It is commonly used as a filler for nail holes. It mixes just like plaster, and cures just like plaster, but it is stronger. It is sold in cans, of different sizes, at many hardware stores.

Was always concerned that plaster of paris is exothermic - or isn't it hot enough to harm a finish? Never wanted to risk it.

I guess a point the original poster needs to clarify is whether he is making a mold to build on or making a mold of a finished piece to make multiples of it. Also what are the short comings of using plaster of paris for his application.

As Mario said, make you under-lying mother bed for the mold using cheesecloth or better, burlap. i have Plaster and Hydrocal molds that are over 10 years old. shrinkage is all dependant on using the right amount of water.

Plaster of Paris has been used for many kind of molds. I have seen people making molds using PP to make fiber glass part for cars. As well used for cast in arms legs etc. So it shouldn't shrink much IMO

Having broken more than a few bones in my time(to date.....), I can attest that Plaster Of Paris has minimal shrinkage and is rather stable.

Fact is, most of my mold-making experience comes from watching Doctors build casts around my messed up limbs...!

Frank Ford has a dandy bit about this on his site. "Restoring a K-2 Mando-Cello" Worth the read and a peek ifd you've never been to the site. Tons of good stuff. Plan on spending some time there.

Ever consider that friendly plastic?
Frank's a huge fan of it.

I could tell you about dental materials, but I'm woefully under-informed regarding the normal stuff.
How accurate? For what purpose?

If it's a small scale thing that you'll use for layups, I'd recommend PolyVinyl Siloxane.
Get the cheapest dental impression PVS and a mixing gun. Squirt it out, let it harden.
It's stable, nonstick, but somewhat expensive (as much as $20-30 per impression, I believe).

Feel free to PM me if you have any questions.

Hey guys, if you have anything further to discuss, please keep it here rather than taking it to private message, we may all be interested!

thanks for the replies. I'm am planning to make a mold of a neck to create a negative for a duplicarver until I can get a working wooden dummy neck.

Use a neck as a pattern to make the permanent pattern and do a really good job of it. This is the basic beginning of making tools to make tools, and is a good habit to get into.