> ...plaster actually labelled "Plaster of Paris" on the container is
Hmmmm, I did not realize PoP was that strong. I was intending on using dental
plaster. From what I have seen it is very strong and it is also cheaper than
resin (and a lot easier on my molds). I can get the dental
plaster pretty cheap, but it is still a bit more than PoP.
I have two masters done (both walls) and got my RTV...making progress.
:-)
Hey there,
I don't know how many responses there are to this yet - I'm online for
the
first time in 4 days and have five GZG-L digests to get through.
> Scott Said:
lazy. <g> My approach is NOT to build each and every wall.
> Instead I am just going to make one good (hopefully) set of modules.
Then create a set of molds from them using RTV silicone and cast lots and lots
of duplicates using dental cement or resin.
Something you might want to consider for your casting. While there are some
great resins out there for casting this kind of thing, I've done the same sort
of thing using Plaster of Paris, and it works really well and is a lot less
expensive.
There are several different types of casting plasters available - stuff
you get from train hobby stores, etc. But plaster actually labelled "Plaster
of Paris" on the container is what you want. It is used for outdoor garden
statues and architectural mouldings, so it is quite strong. Not as tough as
doing your walls in resin, but as long as you aren't bouncing them off the
wall, it works fine. Takes paint well, too.
You can reinforce it, if you're so inclined, with some kind of fibre in the
mold. Fabric (gauze) works well. Glass fibre. Etc. Makes it a bit stronger,
though isn't absolutely necessary.
And plaster of paris is a *lot* less expensive than casting resin. And it sets
up pretty quick too, so if you make several molds, you can have an almost
continuous production line going.
Something to think about, anyway.
The plaster I have here is labelled "LePage's - Poly - Plaster of
Paris". I've done plaster terrain bits, and it's worked fine.
My caveat: don't bang them around too much. They will take some abuse, such as
found in normal play and decent storage. If you toss them off a table onto the
floor, they *might* break or chip. If it cracks, you can
glue it and repaint - not difficult. If it shatters, well, make another
one.
These are *not* as strong as a cast resin piece, but a *lot* less expensive.
I'm satisfied trading off the strength for the cost, and then taking a bit
extra care in the "care and feeding" of my terrain. If your wall sections are
really thin, you might want to stick to resin, OR use a reinforcing method
such as glass fibre, plastic mesh (like you'd use for a window screen), gauze,
etc. That will at least stop the part from shattering all over the place if it
drops from too high up or at a funny angle. Also, as I said before, depending
on the size of your piece, the plaster sets up to a point that you can demould
pretty quickly. While I've got some odourless 2 minute resin, that stuff costs
a bundle. Some other
non-stinky resin I've used in the past is great and available in large
quantities, but is pricy and takes up to 24 hours to setup properly (unless
you boost the reaction by cooking it, but that's tricky to do properly -
you have to use a hot-air "oven" or the temp gets too hot). And there
are resins available like polyester resin for fibreglass, commonly available
all over the place, but it stinks a LOT to use and is hugely toxic.
The plaster hardens with an exothermic reaction, but won't get hot enough to
damage your moulds unless you have unusually soft RTV (or RTV with
*lot*
heat tolerance) UNLESS you make huge parts that are very thick masses of
plaster. Then the heat can build to dangerous levels. For the kind of stuff
you're talking about, you won't have a problem.
I just bought a couple of pounds of Polytek - Plastil 71-30 Silicone RTV
and it is heat-rated enough to work with melted lead, so for plaster
it'll be fine. I imagine that is going to be common to a lot of the stuff
available.
This assumes you don't have an inside line on some good cheap resin, of
course.
And if you use some kind of mould-release, it can help the mould life
quite a bit. If you have a plastic supplier anywhere nearby, you can check and
see if they carry a spray-can type mould-release suitable for RTV.
Though,
I've done a bunch of resin casting with my 2-minute setup stuff (great
for
hobby purposes - it's odourless, it mixes in a 50:50 ratio so it is
*easy* to measure accurately, and you can demould in 2 minutes or less) and my
moulds are all ok without using mould-release at all.
Now, having rambled on about all that, if you *like* your dental plaster and
if it's good for the purpose... tell us about it! Let us know how it goes.
> From: "Scott Clinton" <grumbling_grognard@hotmail.com>
> dental plaster. From what I have seen it is very strong and it is also
> cheaper than resin (and a lot easier on my molds). I can get the
:-)
********************************************
> adrian.johnson@sympatico.ca wrote:
Apparently (ie I've read about it but never tried it) cotton thread works well
as reinforcement in plaster castings. Cut lengths of around
1" - 2" and mix them into the plaster before pouring.
> I just bought a couple of pounds of Polytek - Plastil 71-30 Silicone
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.
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?
> This assumes you don't have an inside line on some good cheap resin,
The prices I can get locally are about two or three times (or more) then
what I will pay for my dental plaster. From what I hear the right kind of
dental plaster can be thrown against a concrete floor and not crack. It's
drawback is that it is heavy and more expensive than PoP.
> And if you use some kind of mould-release, it can help the mould life
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.
> Now, having rambled on about all that, if you *like* your dental
I have yet to pick up the dental plaster, but again from what I hear it is
what I need. I do I will let you know.
Scott Clinton The Grumbling Grognard
> From: adrian.johnson@sympatico.ca
Though,
> I've done a bunch of resin casting with my 2-minute setup stuff (great
From: "Scott Clinton" <grumbling_grognard@hotmail.com>
> >And if you use some kind of mould-release, it can help the mould life
Camie Campbell sells several mold release sprays which work as well as, and
are rather less expensive then, the 3M stuff (something like $4/can
IIRC).
Try www.camie.com and see if you can locate a distributor nearby--and
call that distributor to be sure they sell single cans over the counter (some
only sell cases). Or just call a plastics distributor (eg Ain, Laird,
Cadillac, Commercial Plastics, Piedmont, maybe Trident). I don't know how well
Vaseline works, none of our customers molders ever
mentioned it to me--of course, they were also mostly high volume, high
speed
> At 6:41 PM -0400 10/15/01, adrian.johnson@sympatico.ca wrote:
Hows this for an idea?
Pour in a small amount of the plaster in the mold then spread it around inside
the mold. Then take some medical gauze or some other kind of open weave cloth
(Burlap perhaps...?) of about the right size and place it inside the mold near
the outer perimeter of the mold, then fill the balance of the mold. Thump the
mold to relieve any bubbles and add the balance of the PoP that you need to
add or remove. Let it set. That should act as a nice reinforcement to the
casting near the major ares of stress (corners).
> And plaster of paris is a *lot* less expensive than casting resin. And
Very true. There are a few 1/300 scale buildings out there cast in
Plaster that work quite well. You just have to pad them slightly better in the
carry all for your terrain.
Hi again,
> From: "Scott Clinton" <grumbling_grognard@hotmail.com>
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
> (about a dozen times), but I am still a little worried that I have no
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
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,
In a message dated 10/28/01 11:01:41 PM Pacific Standard Time,
> owner-gzg-digest@lists.CSUA.Berkeley.EDU writes:
> . I had no big problems on my
Scott, Before springing for a vibrating table, try finding an old ultrasonic
contact lens cleaner at a garage sale (or in your own garage!). I've used one
myself, and it worked fine. although it was a bit fiddly keeping the mold on
top of the thing once it was turned on and started vibrating. I mostly don't
bother with it any more as I can get rid of most of the bubbles by coating
each surface of the mold with a thin layer of resin using a disposable brush
before doing the pour. I've even heard that you can use your washing machine
as a vibrating table, but I haven't tried that one out yet... Jed
> ...try finding an old ultrasonic contact lens
Thanks for the tip!
Scott,
Use a rubber band to hold the mold to the ultrasonic cleaner. We've used one
of those engravers people buy to mark their property against burglary.
KR, Geo-Hex
[quoted original message omitted]