Casting using RTV Was: Re: Metal Molder (was Re: Casting your own...)

> The reason I wanted to start with this silly little kit is two fold.
First,
> it's nice to work with a lower temperature metal to reduce (actually

Heh.  You can get a single hot-plate for not-very-much money, and that
is plenty hot enough for casting. Keep in mind if you use lead that you'll
have to work in a VERY well ventilated area. Lead fumes are BAD for you. Work
outdoors or in a garage, or somewhere like a basement under a
vent-hood if you can set one up.

> Be aware that working with RTV-31 is a nasty, messy and
mold
> making stuff. It sounds a bit more forgiving to beginners.

It could be more forgiving, but is it going to be as heat resistant? RTV is
kind of gooey to work with, and as someone else pointed out you have to be
really careful not to get any on your cat 'cause it's impossible to get
off when not catalized :)   But I've done lots of casting using RTV
moulds, and have not had many problems. Simple precautions are all that is
necessary - newspaper a couple of layers thick on your work bench, and a
pair of latex surgical-type gloves (you can often buy these very
inexpensively in packs of 50 or a hundred from places that supply home
furniture refinishing products) are enough to keep the goo from your hands.

The only real trick to working with RTV is getting the mix of catalyst to RTV
right. The catalyst usually is a very thin clear liquid that comes in a small
bottle with the RTV kit. The RTV itself is really thick and goopey. You'll
find that you need catalyst at about 3 to 5% of the weight of RTV, which means
that you'll need a scale to weigh the RTV you are going to use.

RULE OF THUMB: I know this is getting further off track, but it might help
someone out. I used to work at a rapid prototyping company making big RTV
moulds, and when you are using a lot of the material, you want to mix an
amount as close as possible to what you will need to eliminate wastage
(which is costly 'cause RTV ain't cheap :)   We used the following rule
of thumb, which though it mixes measuring systems, really works well.

When you create your mould box (around the object to be moulded - for a
greater description of setting up a mould box and other details like this,
see one of the sites with a casting/mouldmaking FAQ that people posted a
day or two ago) the box should be open at the top. Measure the rough internal
volume in CUBIC INCHES. Multiply that total by 21.2 That gives you a weight of
RTV needed in GRAMS to fill the mould. You should overestimate a bit to
account for the material that will stick to the sides of the mixing container.

If the total is, say 100g, you need to figure out how much RTV plus Catalyst
you neet to make 100g total material if you want to be accurate. In casting
for hobby stuff where the moulds are small, I don't bother doing that. I just
add the catalyst on top of the total needed, and that gives me my
overestimate. So, for a 100g requirement, I would use 100g of RTV and 4g of
catalyst (assuming you need 4% catalyst). Best thing to do is to have several
projects ready to go at once so that you can mix up a larger volume. That
makes it a lot easier to figure out the catalyst volume and to measure that
amount... I use the small plastic measuring cups with graduated markings on
the side. Mine are have markings for cc, mL, fl oz, drams, and table spoons...
Anyway, it isn't rocket science, and as long as you get the proportion of
catalyst as close as possible, you're laughing. Depending on the make of RTV
and the catalyst used, it will take up to about 24 hours to harden (ideally),
and then you've got your mold (or first half if you're doing it in 2 parts,
which is best for gravity feeding cast metal).

As a plug here, you can get the surgical gloves and measuring cups from Lee
Valley tools by mailorder.

http://www.leevalley.com/

They are an excellent outfit, primarily focusing on tools and supplies for
wood working, but have lots of other cool stuff. Their products are generally
excellent, and they have the best catalogue I've ever seen, often with a
written evaluation of the products being sold.

Before I ramble on forever here, if anyone has other questions about this,
they can see the FAQ's we saw mentioned earlier in this thread, or speak to me
off list...