Greetings Casting fans! Its been a while since
out last installment, so perhaps ya’ll have gained a little experience
and had a lot of fun with casting since then! We’ve learned
the basics of mold making, and how to do a little casting, now we’re going
to get a little more involved. As you my have noticed, very
few things in railroad wise have a single flat surface that makes it simple
for one to use a basic open face mold. Well that leaves only one
option, a two part mold. In reality molds can get quite complicated
depending on the part you are wanting to make, but for now we’ll keep it
simple. The first thing we need is a master, in this case I’m wanting
to update a stock Bachmann Shay truck to a more moderen version, so the
master was made up from styrene.
Now
for this part, one goes about preparing the mold in much the same way as
with out simple open face mold, roll out the clay, press in your master,
and build up a dam around the soon to be casting. just remeberthat
the dam should be AT LEAST twice as tall as the casting, even more is betteras
a thin mold can be prone to distortingthe final casting if you put weight
on top of it, or if it is bound in some way.
This is the important part to remeber in any multiple part mold.
If you are going to accurately reproduce multile items, you need to have
someway of indexing the two halves of the mold into exactly the same position
each time. This is accomplished by the use of ‘keys’. I use a small
dowel, or an unsharpened pencil to make an impression in the mold at diagonal
corners from each other, sometimes I use three or more depending on the
size of the mold. Just remeber to leave at least one corner blank
to make it easy to fit the mold together properly.
Once so prepared,
go ahead and pour the mold rubber and let it cure. This is exactly
the same as a open mold, but you only want to fill your dam halfway full.
Once the rubber has fully cured turn it over and carefully peel off the
clay, leaving your master imbedded in the rubber, then slide rummber master
and all down into the mold until it looks like this:
As you can see, this leaves plenty of room for the sencond half of the
mold. At this point you want to coat the master, the mold face, and
the sides of the dam with your release agent of choice. I prefer
an spraycan type that is avalible from Micro Mark, but whatever your preference
is is fine. Just make sure all of the mold rubber is covered so the
parts don’t bind together. Once again you pour your rubber, but in
this case leave the tops of the dowels on the back of the master just above
the top of the rubber. This will give you a way to pour the
resin and allow air to escape as you make you castings later.
We’re almost done now, once the second half has cured, remove the dam,
and master seperating the two halves of the mold. With this particular
casting it has a thin cross section so I was concerend that the resin might
not flow freely and may not let air escape from various chambers in the
mold, so an air channel was cut in the top section (the one with the holes)
to allow some extra space in there.
Its time time to give the mold a test shot! Give your mold
a shot of release agent on both halves, this both makes it easier to release
you casting later, and will help extend the life of your mold (Yep…they
wear out after a time). After you fit it togetherwith the holes up,
mix up some resin. make sure you have a relatively flat surface to
work with, you want the resin to flow, but you dont want it all running
from one part of the mold to the other.
In this case I used the right hand hole to pour in the resin, filling it
slowly til resin came out the lefthand ‘vent hole’. tappingthe mold
with my finger letthe air vent out, and pushing down slightly on the center
allowed additional air to be ‘burped’ from the mold. Allow the casting
to sit til it hardens copletely, and now the magic moment…DEMOLD TIME!!!
You
should nowbe able to turn out multiple copies of your casting with very
little problems. Sometimes you may have to go back and recut an air
channel on a particularly intricate casting, or mebbe morethan one vent
hole is needed. Draw these on the mold rubber with a permanent marker,
andcut them out with an Xacto Knife. Be patient, there is a learning
curve involved here. In our next installment, we’ll cover really
weird projects, and more than two part molds as well as another way to
make a two part casting. Good Luck!