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Products used in this How To:
Tail Light Molding w/ Equipment
Use this mold to Cast a piece > |
This
How To process will walk you through step by step the procedure
for making a two piece mold of a tail light lense with the assistance
of vacuum.
Choose
a lense that is with no scratches, blemishes, dull spots, or cracks
to reproduce.
You
will need a plastic base and Alumilite's Modeling Clay to get started.
Set
the clay on the plastic base and trim it into the basic shape of
the lense. Leave approximately 3/4" around the outside of
the lense. 
Use
a section of PVC pipe that is approximately 1" in diameter
more than the lense. Press the PVC down around the lense. This
will serve as your mold box. This will leave approximately 1/2" around
the outside of your lense.
Once
you have the clay line as good as you can get it, press the PVC
tube back over your lense and into the clay. Then trim the excess
clay away from the PVC which will serve as your mold box. 
The
lense is now clayed up and the mold box is sealed. You are almost
ready to pour the first half of your rubber mold.
The
last thing you want to do is to clean the lense before pouring
the rubber. Use some rubbing alcohol and some paper towel to gentley
clean all fingerprints and clay off of the lense.
You
are now ready to pour the rubber. Before we mix, we first must
figure out how much rubber we will need. The equation is as followed:
radius squared x 3.14 x height The radius is half the diameter
(distance across the entire pipe). You also get to subtract the
volume of the lense itself. Take a look at the Calculations page
for more indepth instruction for calculating product requirements. 
We
calculated for this half of the mold we need approximately 3/4" lb
of silicone rubber. Therefore we will mix .7 lbs of base and .07
lbs of catalyst to be on the proper 10:1 mix ratio. Measure and
mix the rubber in a container much larger than the liquid volume
required. When we put the rubber in the vacuum chamber it will
grow up to 5 times its liquid volume as it is being degassed. 
Do
to the fact that we will be vacuuming the rubber to help us make
sure we will not have any air entrappment in our rubber mold, we
have chosen to use Dow Corning's Silastic M-2 silicone mold making
rubber. The M-2 is a thick, high durometer, very strong, silicone
rubber that has great dimensional stability and is able to reproduce
and keep very fine detail for many cast parts. It is one of the
absolute best silicone rubbers on the market for two piece rubber
molds when you have the ability to vacuum. 
Once
you've added the catalyst, mix it thorougly until every swirl on
the container wall and throughout the rubber is gone. The M-2 has
a 90 minute work time so you have plenty of time to mix it thoroughly
and vacuum it. 
Place
the container in a vacuum chamber and turn the pump on. Allow the
M-2 to rise up and fall on its own before removing it from the
vacuum. The magic number is 29" of Hg (mercury) of vacuum
to effectively degas the rubber. It is necessary for the rubber
to rise up on its own and collapse without allowing atomospheric
pressure back in the chamber. 
Here
you can see the material sagging back down the walls of the container
after the material has risen, crested, and fallen. Keep your hand
on the release valve to allow atmospheric pressure back in the
chamber if the rubber is going to rise up over the top of the container. 
Once
the silicone rubber has cured (typically takes 8 hours at room
temp for the M-2), remove the mold from the plastic base.
Using
Alumilite's clay tools, dig out all of the clay away from the second
half of the mold.
Remove
every little piece of the clay and be sure not to remove the lense
from the mold or the mold from the PVC tube.
Once
you have removed all of the clay, use an Exacto knife and cut some "V" shaped
wedges in the outside flange area (rubber between the lense and
the mold box).
Then
peel or pull away the rubber so you have a small groove in the
cured rubber. These will act as locators to line up the two halves
of the rubber.
Next
mold release the rubber using Alumilite's UMR (Universal Mold Release
- coming soon!). Silicone rubber will stick to cured silicone rubber
and therefore needs to be released. Simply spray on a thin layer
where ever the next pour of liquid silicone will be in contact
with the first half of the mold that is already cured. 
Before
pouring the second half of the silicone rubber, make sure to clean
the lense one last time to make sure there is no clay or other
contaminates on the surface of the lense. 
Calculate
the proper amount of silicone needed as we did for the first half
of the mold and make sure to mix it in a container much bigger
to allow the rubber room to rise and degas. 
Measure
the proper amount of catalyst and begin mixing.
Mix
the rubber thoroughly. Make sure the sides have been scraped and
that there are absolutely not swirls in the liquid rubber.
Place
the rubber in the vacuum chamber and allow the liquid rubber to
grow until it collapses on its own.
Once
you have effectively degassed the rubber, slowly pour the second
half of the mold allowing the rubber to flow from one side of the
mold on its own. Do not dump the material in but rather pour from
one spot and allow the material to flow naturally. 
Allow
the material to cure. The M-2 demold time is 8 hours. Temperature
will affect the cure rate. The warmer the temperature, the faster
it will cure. The warm temperature will also reduce your work/open
time.
Once
the rubber has cured, break the tension between the outside edge
of the rubber mold and the mold box and push the rubber mold out
of the PVC.
Using
your hands, work the line where the two pours of rubber meet until
they begin to separate. |
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