Mold making question

[Factoid]

Does anyone have any experience using this material?

http://www.smooth-on.com/Rebound%3D-25-Self/c1132/index.html

I've never made a mold or casting before, but I'm going to try it on this sculpture (picture attached). I'm guessing that without the wings and the little plate it has a surface area of around 2.5-3 square feet.

The wings and plate are removeable and I think they'll both be easy to cast in a simple 1 or 2 piece silicone mold, but I'm trying to do this cheaply, so I thought a brush-on method would be most efficient for the large piece.

My main question is how much I need to buy. They sell it in a "trial size" or by the gallon, but I have no idea how much area either will cover. I'd rather buy 2 or 3 trial size kits if need be than spend 175 on a gallon of silicone rubber that I'll never use again.

Hoping to spend 50-75 dollars to make this mold. My goal is to make replicas experimenting in different materials like resin, plaster, even foam-rubber. Original is carved wood with a painted/laquered finish.

[racine]

a bit difficult to advise from this photo,although in theory mouldmaking is fairly easy as a concept... there are a hundred things to know through practise, i sincerely suggest a trial with a smaller object [large enough to make a small 2 part mould]. this approach will likely save money and produce a better result.
to help people to help u..
what medium do you expect the resulting casts to be and what is the height of this object...?

[ahirschman]

Hi Factoid, you are going to have to do some basic math to figure out how much material you will need.

First, you have to figure out the surface area of your sculpture. You can approximate it by using similar shapes that are simplified a bit. Spheres, rectangles, etc. The area equations for all of these are easy to find or figure out. Once you have the surface area of your sculpture you will have to multiply the surface area by the thickness of your desired final mold. You will have to add the extra amounts you will need for seams, etc. Rebound 25 weighs 1 lb per 23.6 cubic inches so you can figure out the total weight you will need. I would always order extra (How much extra depends on the level of confidence over your measurements and skill).

For example, if you were making a mold of a sphere with a 5" radius you would use the formula 4 pi r squared (basic formulas for area (http://www.math.com/tables/geometry/surfareas.htm)) and you would come up with 4 * 3.1415 * 5" * 5" = 314 square inches.

If you want to make the mold 1/3" thick, then you will have a volume (In cubic inches) of 314 * 1/3 = 105 cubic inches.

If you will make a seam that is 1" x 1" and will cover the entire molds circumference (a two part mold) then you will need an additional amount to go around the sphere. The length is going to be equal to the circumference or 2 * pi * r = 2 * 3.14 * 5" = 31.4" and the volume of something 1" x 1" x 31.4" in length is 1 * 1 * 31.4 or just 31.4 cubic inches.

The total volume will be the sum of all volumes, or 31.4 + 105 cubic inches which equals 136.4 (make it 137 to simplify).

Rebound 25 (from their website) weighs 1 lb per 23.6 cubic inches, so you will need 136.4 / 23.6 or 5.8 lbs.

If I am making a mold and compute that I would use 5.8 lbs, I would probably have at least 8 lbs around.

But basically, you have to figure out the volume of rubber or silicone that you will need, and the easiest way to do that is to estimate the surface area and just multiply by the thickness (Keeping all measurements in the same units).

Hope this helps.

Ari.

[Factoid]

Sorry, I forgot to include the dimensions. It is around 18 inches tall and about 5 inches across once you take off the removeable pieces.

My first step to making a mold was going to be pouring a simple one piece version of the wings, since that has a flat side on it. I figure that should give me a little primer on how to do things without too much difficulty.

How does this idea sound for gauging the surface area? I have a few extra rolls of blue painters tape laying around. I could just wrap the thing from top to bottom, measuring my tape out as I go. Then I just multiply the width of my tape by the length I pulled off the roll and voila...surface area.

Then I just add maybe 10 or 20% extra and that's how much coverage I need.

I could similarly calculate volume by wrapping it in a couple of layers of plastic wrap and dunking it in a 5 gallon bucket of water, measuring the displacement.


As for the material for casting, I'm planning on experimenting with different stuff. I will probably try resin, maybe a clear acrylic. I also really want to try foam-rubber.

I would like to cast the materials hollow if possible, but that might be tricky because of the legs. Still, I think if I work slowly and build up in layers I can do one leg at a time.

If I want to be really cheap I'll just fill it solid with plaster.

[The Forge]

It would be cheaper and much less frustrating to just buy another one.

[ahirschman]

One piece molds are much simpler. You can make a little box by using wood, cardboard, aluminum, etc. Porous materials should be sealed with at least a couple of coats of wax.

Then the amount of mold material you will need is just the volume of the box (Up to the height you are going to pour) minus the volume of the sculpture. As simple as it is, you have to be careful not to trap air bubbles when pouring.

Volumes of sculptures are only useful for block molds, and poured molds. The volume I was talking about was the volume you would need for a brush on mold. Any way, it is always the same. You are trying to figure out how much material you will use.

Block molds are great, in that they are quick to make, but they use much more material (Generally). You trade time for $.

Casting hollow is simple. We can go there when you have the molds. I use a polyurethane rigid foam to fill the voids (After casting the outside walls of the sculpture), making the casts solid, but light weight.

Ari.

[Factoid]

I think based on my math if I was going to make a block mold it would cost me at least 200-300 dollars worth of silicone. or maybe 175-250 in urethane.

I don't think I can use urethane because this is a finished, painted sculpture and I don't want to damage it. I'm pretty sure urethane would stick to everything, even if I sealed and released it well. Silicone sounds like a much more user-friendly material for new mold-makers.

How would one make a one-piece block mold? wouldn't it have to be at least two pieces? Otherwise I'd never be able to get the original out. I was thinking that a one-piece block might work work well for the wings, though, since those have a flat side and should release very easily...no undercuts, just a simple wing shape with feather etching.

I went ahead and made an order for some smooth-on Rebound 25 brush-on silicone. I ordered two of the trial size containers which I'm almost certain will be enough to give me 3/8ths inch all the way around the main body. If I have left-over I will use it to make a brush-mold of the plate, and if there's STILL any left over I might use the excess to pour a block-mold for the wings, even though that's not really what it's designed for. It's worth a shot, though, because I won't have any other use for it after the other two pieces.

Since I'm planning on casting with lightweight materials or doing a hollow pieces I'm going to try to get away with a really cheap mother mold to hold the rubber in place. I ordered a few rolls of plaster gauss, just get it wet and apply like papier mache. Should be hard enough to hold it in place, but probably not strong enough to hold a really heavy casting material. For that I might need to use trowel-on plastic or a heavy plaster. It's worth a few bucks as an experiment, though.


My latest idea for casting material is actually to try and make an ice sculpture! I am worried that the weight of the material will cause it to leak, through the cut seams in the silicone, though. My wife will be very mad if I leave a frozen mess of plaster and ice in our basement freezer.