Casting Beginner Questions

[Zhatan]

Hello everyone

I am complete newbie at molding and casting so I thought it would be a good idéa to get some questions answered before I start casting anything.
Now I have read alot of info and watched tutorials on blogs and on this forum but some things are unclear to me still.

Before I come tto that however I would like to get your opinion on the materials I bought for my casting. I read alot of stff before ordering Mold Star 15 and Smooth Cast 300 along with some ease release 200 and Chavant Clay Soft to place the master in. I hope this stuff will do even though the Mold Star is a pretty soft rubber.

Now to the questions:
After watching this tutorial: http://www.youtube.com/watch?v=FQ1A7ZjTsx8 I though I could just make a simple mold like that and pour the resin right in from the top however I see that people on here pour the resin in a seperate channel and have the resin fill up from the bottom. Is this really required and if so how come the tutorial made by smooth on doesn't use this approach?

How thick should I make the "walls" of the silicone mold? Will 1 cm be enough? Also can the "walls" separating the different parts in the mold be thinner then the outer walls?

I have loked at some fillers to add to my resin the make it less expensive to cast but I am unsure how this will affect the resin. The two fillers I can get my hands on here is Sweden at a resonable cost is microballoons and something called fillite. Would any of these be appropriate when casting small models or do you think it will thicken the resin too much? What kind of fillers, if any do you use?

*Edit*
I just remembered another thing. Do you have any tips on how to measure the amount of rubber and resin required for any given taks so you don't mix too much. Given the high cost it would be nice to minimize leftover rubber and resin.

Thanks for any help!

[mangozac]

Hi Zhatan,

The answer to many of your questions is "it depends on what you're casting". Any chance you could give us an idea (even if it's a vague description, i.e. "a large 28mm gun" or "a 32mm figure")?

Someone else who uses them will be able to comment on the Smooth-On products, but in general they get a pretty good rap.

Actually, barely any of us are using a separate pour channel. You will sometimes need a second channel to help get the air out of the cavity as the resin is poured in, but otherwise you can generally get away with a single (although larger) pour hole.

Are you planning to pressure and/or vacuum cast? If not then the technique you want to use is a bit different. Have you read up on the ? Most of us are using that.

The wall thickness depends a lot on what is being moulded/cast. For smaller parts 10mm in the direction parallel to the mould seam is about right and you only need about 5mm in the direction perpendicular to the mould seam.

I wouldn't play with fillers yet. They aren't really suitable for small parts and just complicate things.

To work out the amount of silicon I just calculate the volume of the mould in centimetres and that generally corresponds to the amount of silicone required in grams. For resin you just get good at guessing

[Zhatan]

Thanks for your answers.

I will mainly be casting bits & pieces I sculpt to use with 28 mm figures but later on when I get better at the sculpting I plan to try sculpting entire figures and larger monsters.

As I am a beginner I have none of the fancy stuff like a vacuum chamber which is why I went with products that should work well without it. My plan was just to make two piece molds and pour the silicone and resin.

I have read about the split mould but I am unclear as to why it is better then a two piece mold. It seems more complicated to me as you have to cut the piece out of the rubber.

[mangozac]

Trust me: slicing apart the rubber is far less work than claying up the parts and having to pour two lots of silicone

[Deadmeat30]

As zac says, by the time you've clayed up one side, you could of cut open about 3 moulds. Its really easy to do. Just make a cut, pull either side apart, and keep cutting down. Remember not to cut all the way through the mould, leave a CM or 2 at the bottom so you have 2 flaps of silicone.

[blind pig]

[Zhatan]

[mangozac]

For ease of use the split mould is still on top. Since you don't cut the split all the way through, it is (to a large degree) self aligning and thus keys aren't needed.

By all means though, feel free to try out both methods and see which gives you better results. In my experience no two people cast using the exact same technique and you just have to work out the best process for the subject and materials you have. You should budget for screwing up at least one or two moulds as part of the learning process

[Anvils Hammer]

SPlit molds are quicker, but the main reason I prefer them is the quality is much much higher!! you get no mold line at all around most of the part casting.

If you dont have vacuum/pressure though, split molds will not work. You will need a two part mold with a pour channel which fills the component from the bottom, and lets air out the top.

If you have pressure, you can use split molds but its time consuming as you have to squeeze each mold a lot to get air out and resin in.

I use vaseline to seal my mold boxes, so for me pouring rice etc into the mold box does not work. I caculate the volume of the mold box up to the level I want to rubber to come, then make a rough guess at volume of the parts/channels, its normally about 25%, then remember to multiply by the density factor ie 100 square centimeters will be 120-130g depending on the silicone.

For caculating resin, guess the first tiem, use to much, then weight the entire sprue with parts, then you know exactly how much you need each time!

Good luck!

Joel

[Magnetic Duckling]

[Zhatan]

Thanks for all the replies.
I am doing some test molds to get a feel for how it works and used rice to measure the amount of rubber needed. It worked pretty good, I ended up with some left over but that was not a problem due to the 50 min pot life of the silicone. I just made another moldbox and poured it in.

I now have another question though. Do you guys use disposable cups when measuring parts a and b or do you clean the cups after each use? And if you do how do you do it?

[mangozac]

Just throw the cups out. Once you're more comfortable with pouring the silicones and resins you can just pour the two parts into the one cup while they're on the scales. Then once the leftovers are cured you can just pull it out and reuse the cup

[Anvils Hammer]

rubber in plastic pint cup, measure the catalyst in the lid of the catalyst pot. When the rubber has cured you can peel it out, been using the same two plastic cups for 6 months now..

Resin I use smaller plastic cups, reuse them as many time as I can till there is too much resin in the bottom..

AH

[Magnetic Duckling]

[Zhatan]