OK, I know that gluing stuff together to make a casting master is
kinda tacky. And yet, that’s what I need to do here. I have a carved
acrylic piece (which will survive the vulcanizing process), and I
need to attach a bail, and it’ll be easier to make the bail in metal
and glue it on, then make the mold, then do the castings.
So: what glue suggestions do people have for glues that will survive
the vulcanizing process? A certain amount of viscosity would not be
amiss, though I can probably live without it.
Right now I’m wondering about gel cyanoacetate, or maybe epoxy. Any
thoughts or experiences?
I have found that JB Weld is good for vulcanizing temps. Miserable
stuff to work with, just make a nice joint and it should not fall
apart. Do a good job packing the frame to avoid stress on the join.
In our studio, mold making is part of our program. We use two
different methods.
For molding models which can survive 307 degrees and lots of
pressure, as in vulcanizing, we prefer to do it that way. Most of
our metal “masters” are molded this way. “No-shrink” mold rubber
sheets work well, and molds are quick to make (1/2 hr heating time in
the vulcanizer for small molds).
For models which won’t take heat or pressure, we use a room-temp mold
compound which takes much longer to set up and cure (overnight), but
provides great detail and minimal shrinkage. Models can be of
absolutely any material, and parts can be glued with anything or
stuck together with wax, if you want. The liquid mold compound is
quite gentle with models, and will not disturb delicate patinas or
finishes. Antique carved ivory, as an example, is unaffected by this
type of molding, and molds well.
Companies which sell the room-temp curing mold rubber have varying
degrees of “durometer” or stiffness, so that aspect of the mold’s
character can be adjusted from batch to batch, if desired, by just
ordering a higher or lower durometer of the liquid mold compound and
catalyst. One nice part of making room temperature molds is that you
can make your own “U” shaped mold forms out of wood, and clear
plexiglass “windows” for each side. These mold forms have an open
top, and the thickness, width, and height are easy to make in wood.
We use wide masking tape to hold the frame and plexiglass together,
and then vacuum the liquid mold compound under a bell jar to get the
bubbles out. A pretty simple process, especially if you have a
vacuum casting machine.
How about avoiding the heat question and glue compatibility issue
altogether by using an rtv compound like castaldo liquicast?
Liquicast doesn’t like water so avoid water based glues and it also
is incompatible with styrene plastic. I save used it several on
models
made of acrylic plastic (plex) with good results. One piece did
crack probably due to exposure to vacuuming ( the piece was carved
from a cast autoclave cured block). I still got a great mold.
Have you considered using RTV for your rubber mould then you have a
multitude of choices because heat is not a problem. I usually use wax
to attach sprues or fabricate bails. With hot vulcanising you might
get away with epoxy (but not 5 minute epoxy which has a lower melting
point). The problem might be having the glue stick to the acrylic.
Acrylic is usually glued using a solvent based glue so perhaps if you
are confident the Acrylic will survive vulcanising temperatures you
should consider using acrylic for the bail and sprue and use acrylic
glue.
I save used it several on models made of acrylic plastic (plex)
with good results.
Reading throught the replies has made me remember something else, I
don’t know if it’s useful in this application or not. Tap plastics,
for one, carries an acrylic glue that’s an actual welding agent. It’s
used for things like fish tanks and things because it actually
dissolves the plastic and welds the edges when it evaporates,
leaving solid plastic. But it needs to be plastic-to-plastic, I would
think. Look under “plastic adhesives”, towards the bottom left, on http://www.tapplastics.com/
On the other hand, the general rule of thumb is ~don’t mold
findings~. If you want or need to, then disregard that - it can be
done. But the reasons not to are many: They are generally at odd
angles to the main piece, sometimes they are long, sometimes they are
skinny and/or thin, sometimes the wax and metal has to jump at
unnatural angles to fill them, and they definately are in the way
when it comes to finishing and polishing. Very, very often it is
penny wise and pound foolish to mold pieces with findings attached.
There, I said it - I know people will continue to do it, and there
ARE times when it’s perfectly fine to do. Just remember that little
voice in the back of your head when you (whoever) have a problem
sometime…
I’ve definitely made RTV molds out of various things. With this one,
though, I’m hoping to go straight from my acrylic/metal “master” to
a rubber mold that will be usable for production. Previously I’d:
carve the acrylic, do the RTV mold, use that to make a series of
waxes until I got one I was happy with, send that to the casters, get
back the metal master, tweak it, and then send it back for the final
mold.
It would be lovely to skip some of those intervening steps! So,
since the acrylic seems to mold OK- it’s destroyed in the process,
but the mold looks decent- the only thing (I hope!) that stands
between that and a final rubber mold is an adhesive so I can add the
metal bits to the carved piece. If such a shortcut were possible,
it’d save me a lot of work!
Thanks for the RTV recommendations, but I am hoping to bypass that
step if at all possible (and it may not be…!).
I’ve used 5 minute epoxy in vulcanized molds for years. It comes
unstuck during the molding process, but it holds together long enough
for everything to stay in the right spots. Dunno how it’d work with
acrylic, but there’s one way to find out…