Re: [SGII] Modular Starship Terrain AND CASTING

Hi again,

>From: "Scott Clinton" <grumbling_grognard@hotmail.com>
>
>Yep, this is the same stuff I picked up this last weekend.  I have yet
to 
>bust the seal on it though.  I am REAL, REAL cautious about new
techiques.  
>Especially when I get only one chance to do it right, and it will cost
me 
>$5-10 a shot.
>
>It's not a fortune, but then again I am pretty 'anal' about some things
(I 
>am told <g>).	Heck, I just re-built one of my masters last night
because I 
>did not like the way the seam was going to lay in the first one.

Well, that's not such a bad thing.  If you're doing a 2 part mould, then
getting the seam right is pretty important.  Makes your job a LOT easier
later.

>That is another thing.  I am new to this casting stuff, and one of my
first 
>molds is going to have to be a two part mold.	I have read how to do it

>(about a dozen times), but I am still a little worried that I have no
way to 
>de-air my RTV or my plaster when I cast.
>
>Do you de-air your RTV, plaster or resins before your use them?

I used to work at a rapid prototyping company (way back as a summer job
when I was in school) and we had a vacuum degassing chamber for this
kind
of thing.  Works great, but unless you are prepared for a serious $$
outlay, not the sort of thing you can mockup at home.  Also, messing
with
pressure vessels is kinda dangerous.  Ours was "homebuilt", but
certified
by an engineer and had a government approval.  I don't know about the
regs
where you're living, but here in Ontario, you have to have pressure
vessels
certified safe otherwise you can get in trouble.  And it's stupid not
to,
besides...

At home, given that I *can't* degas, then I don't worry about it.

The trick then is in how you set up your mould, how you pour the RTV
when
you're making the mould, and how you pour your resin/plaster.

If you're making a 2-part mould, always make sure that the master is on
the
bottom when you're pouring the RTV.  This sounds obvious, but it's
important.  As long as you don't have RTV seeping UNDER the master
model,
air bubbles will largely rise up OFF the surface of the master.  Mostly.
You might get trapped air bubbles if you have lots of little fiddly
detail,
so once the mould has been poured and the RTV has had a chance to settle
for five minutes or so, bang the mould on the table a few times.  This
is
to try to get any trapped air bubble to rise.  Obviously you don't bang
it
hard enough to slop your RTV around...

If you're doing wall sections, you could to a 2 part open mould.  What I
mean by that is that you'll have a "front" and "back" of the wall, but
the
"bottom" (the bit that would be sitting on the table) is left open - no
RTV
there.	Then, when you want to cast, you put the mould in a box that is
exactly the size of the rubber mould, with the now open edge (the bottom
of
the part) facing upwards.  Effectively, your mould is sitting "upside
down".	You pour into the open space.  Once it is full, you can smooth
off
the plaster to give it a relatively flat bottom, but this can be sanded
later.	Again, bang the mould once or twice to loosen any air bubbles.

The box has two functions.  It puts pressure on the mould so that the
casting material (plaster, resin) doesn't seep out the bottom, and it
allows you to use a THINNER mould.  If you rely on the mould for it's
own
strength - to hold it's own form while being filled, then you need
relatively thick walls.  If you're using an outside support of some kind
(ie the box) then you can make the walls thinner.  Not so thin that
they'll
tear, but think enough that you save $$ on the RTV and it's easier to
demould later.	The box can be made from any material you like, as long
as
it is stiff.  I've seen these done so that the mould box is in 2 parts,
with clamps to hold them together.  Woodworking bench clamps, or
overcenter
latches like on a toolbox.

If you have a "standard" size box, you can have slightly different size
moulds in it, and then pad out any empty space with thin bits of wood or
something.

When you're pouring the RTV and later doing the resin/plaster, try to do
it
in a very steady flow, keeping "turbulence" to a minimum.  With the RTV,
if
you have fine detail in your masters, dribble RTV into the detail area
with
a medical applicator stick or toothpick, little bits at a time, to fill
in
the small detail without trapping air.	Then top up the rest of the
mould
with a steady pour.

>>And if you use some kind of mould-release, it can help the mould life
>>quite a bit.
>
>I will be using Vasiline.  I have been told that if you heat it a bit
and 
>rub it on it works well.  And it is dirt cheap.

Vaseline does work, and it shouldn't inhibit the setting of the plaster,
I
think.	The problem comes with getting the vaseline off your walls
afterwards...

I've used vaseline (thinned out with a solvent - methylene chloride,
which
also works as a great glue for plastic) to keep MOULD HALVES separate
when
I'm making the mould.  Not as a mould release.

The other drawback with useing vaseline as a mould release is that if
you
brush it into your mould too thickly, it can obscure detail, or you
might
see brush marks/smears on your cast pieces.

If you use the mould-box method and relatively thin walled RTV moulds,
and
you are really careful pulling the part out of the mould (which is a lot
easier using a 2 part mould), then you can probably go without release
at
all, and save yourself the hassle of cleaning off the vaseline.

This whole moulding business sounds kinda scary to start with, but it's
really not that difficult.  And you can get some *really* great hobby
stuff
going if you put the time into it.

Have fun,