Slides from the Molding & Casting class from Saverio Silli, instructor in the Fab Academy 2016 at WOMA/Volumes in Paris.

1. MOLDING & CASTING
SAVERIO SILLI

2. RUBBERMOLD&CASTINGRESIN
TIPS ON HOW TO MAKE A
PLUS “COLD CASTING”
TECHNIQUE

3. MAKING THE ORIGINAL PIECE (MASTER)
The piece we want to cast can be anything, in this case it is a model
made with plastic sheets and tubing, but it can be a 3D printed part or
a CNC milled part. It could be something that was already existing like
a toy or a part of a device, but in this case be sure not to infringe any
copyright (in the prop-making industry it is called “recasting” and if you
are caught doing it your reputation is dead).

4. PREPARING THE TWO VALVES MOLD
The master is surrounded by clay around its parting line.We use
clay to make the channel used to pour the resin.We build a simple
box with wood and hot glue and we make “registration keys” into
the clay, so the two valves will match automatically later.

5. POURING RUBBER IN THE MOLD
The first layer of rubber is brushed on the mold to avoid bubbles
being trapped on fine details.Then the rest is poured in the box
which stays on a leveled table.When the first valve is cured, you turn
the box, remove the clay and pour the second valve. Remember to
add demolding compound on the first valve or…

6. COMPLETED TWO VALVES MOLD - ISSUES
…you’ll end up with a solid block of rubber which you’ll have to
cut with a knife like in the picture. By the way, if you manage to
align the two valves well when you assemble the mold for casting,
you can get nice casts with no flashing on the parting line.

7. “COLD CASTING” AN OBJECT IN A ONE VALVE MOLD
"Cold-Casting" is a term used to describe the process of mixing metal
powder with a translucent casting resin and applying the mixture into a
mold.The resulting cast give the appearance and weight of solid
metal. I started downloading a Star Wars' Jedi model and I modified it
to be used as a cufflink, then I used the DWS stereolitography 3D
printer to achieve a great level of detail.

8. MAKING THE MOLD BOX
This time I used poor material I had laying in the lab: a block of
MDF as the base, four rectangles of cardboard and hot glue
gun. Once the box is made, I taped the master to the box using
double sided tape.

9. MIXING THE SILICONE RUBBER
The first thing to do is to mix the silicone rubber alone, then you
weight it following the mixing ratio described by the
manufacturer, and mix the rubber with the hardener. Use a sturdy
stick of wood to mix rubber, cause it has high viscosity and you’ll
spend five minutes stirring, so better use something comfortable.
Remember to scrap many times the sides of your cup, you want
ALL the rubber in your cup to be stirred and mixed.

10. POURING THE SILICONE RUBBER
As usual, brush the first layer of rubber on
the master, then pour the rest of the resin.
Remember to pour it holding the cup high
over the mold box, so the rubber will go
down in a thin string. Start from the bottom
of the box and let it grow up until it
submerge the highest point of the box.
Wait a day, then break the box and
demold: cross your finger for bubbles!

11. COLD CASTING
For the process you need
urethane resin, aluminum
powder and a black tint
that works with urethane
(similar to Smooth-on So
Strong tint).
Put a small amount of
aluminum powder in the
silicone mold and "flour"
the mold to lay a thin layer
of pure aluminum.Mix the
resin with the black tint a
little amount of aluminum
powder and pour it in the
mold.
Optionally you might sink
something into the resin
when it is still liquid. I put
the cufflink support in the
mold and used a piece of
wood as a support.

12. BRINGING OUT THE SHINE AND FINISHING
After an hour, you can take out the piece and you see the finish
is dull because of the first layer of pure aluminum.
To polish the piece you can use steel wool or work it over a
fabric disk mounted on a drill press. You can use polishing
compound for a very shiny effect.

13. STEP BY STEP COLD CASTING PROCESS
‣Put a small amount of aluminum powder in the silicone mold and "flour"
the mold like you would do with flour when preparing a cake pan. This will
lay a thin layer of pure aluminum that will dramatically increase the finish.
Aluminum powder sticks to the silicone very well.
‣Pour the amount you need of part A in a cup and add one drop of black
tint, and stir well.
‣Put in the same cup the same amount in volume of aluminum powder and
stirr well.
‣In another cup pour part B, it has to be the same amount of part A in
volume (mixing ratio is 1:1, the aluminum powder is an extra).
‣Pour part B in the cup with part A that you already mixed with black tint
and aluminum, and stirr very well: the reaction and pot life starts now
‣Pour the mix into the mold.
‣Optionally you might sink something into the resin when it is still liquid. I
put the cufflink support (bought at a cheap jewelery supply store) in the
mold and used a piece of wood as a support to keep it in place.
‣After an hour, you can take out the piece and you see the finish is dull
because of the first layer of pure aluminum
‣To polish the piece you can use steel wool or work it over a fabric disk
mounted on a drill press. You can use polishing compound for a very shiny
effect.
http://fabacademy.org/archives/2015/eu/students/silli.saverio/exercise09.html