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
eliminate)
>the chance of hurting myself.	Second, there is NO WAY my wife will let
me
near
>the stove with a ladle full of metal (leaded or not) in my hand.  So
I'm
pretty
>much stuck with a solution that keeps my (and all my nastly little
metal
bits)
>in my workshop.

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
time-consuming 
>> business, however..... and if you get it on something before
catalysing
it, 
>> then it WON'T COME OFF!
>
>Sounds rather dreadful.  I was actually thinking of using the
'putty-like'
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...

Adrian Johnson
ajohnson@idirect.com