STEP 11: Making Rubber Molds and Casting the Parts
Continued ...
The Erie Railroad Boxcars Part 3 - 11
48: The silicone rubber has been poured and is setting
up in the molds. This was one of those moments of
“I hope this all works and I can get the mold out of
the mold box.” Needless to say, it all worked, or this
article wouldn’t have been written.
I found a product called Smooth Cast 300, which is a
two-part casting plastic from an art supply store. This
is a white liquid plastic that you simply mix
The material I used to make the molds is a silicone
RTV manufactured by Tap. It is a two-part mix that
is a 9:1 ratio. You should be able to find this at
most plastic or art supply houses.
One of the keys in having a good mold is no air
bubbles, especially on a finished surface. Since I
do not have a vacuum system to remove any air, I
use the next-best thing. I turned on my jig saw and
let it vibrate for three to five minutes until I didn’t
see any more air bubbles rising to the surface.
I let a mold cure in the form for at least 24 hours
before I remove it. I then let the mold cure another
day or so before I try and use it. I don’t know that
all of the cure time is necessary, but it doesn't hurt
to have a little patience.
This was the first time that I had made rubber
molds. Those who have done it before know it’s
not difficult. To those who haven’t, don’t be afraid
to try it. It looks and sounds a lot more difficult
than it is.
STEP 11: Making Rubber Molds and Casting the Parts
Continued ...
equal parts of the A and B component in a small
plastic cup, stir the mix together and then pour into
the mold.
There are different versions available. I used the 300
which has a three-minute pot life and a de-mold time
of 10 minutes. Hindsight being 20/20 I wished I had
picked up the 305 which has a seven-minute pot life
and a 30 min de-mold time.
Once you hit the pot life limit, it begins to set up. I
also discovered that the pot life is very temperature-
sensitive. The higher the ambient temperature, the
faster it sets up, so you need to have everything
ready to go. You will also need a release agent to
spray on the molds. I use a mold release agent from
Casting Craft. If in doubt, talk with the sales people
at the art store. They were a tremendous help to me.
48
MRH-Jun 2013
