Mass Casting Complex Parts (with a parting line)
Mass Casting a
part that does not have a flat side requires a more complex
process involving the creation of a parting line.
This is the model that we’re going to duplicate. As you can
see, there are no flat surfaces, so no matter how we orient
the model, if we just built a frame and poured on top of
it, the material would flow underneath and the model would
get locked in the mold.
Therefore we will create a two-part mold, which will have a
parting line at the bottom of the car, about halfway
through the wheels.
First, we machined four cavities in a piece of wood using a
Bridgeport mill. This will allow our model to sit flush
against the flat board. This board is now the location of
our parting line and by sitting flush, it allows a tight
seal and therefore a clean parting line.
We used Plast-econ modeling clay
to secure the
model to the plywood and also to fill in the slight gaps
along the parting line.
On the bottom surface, we are applying the clay to seal off
the bottom of the car. This will prevent material from
flowing underneath the car.
Next we press the car onto the plywood. And you can the see
the excess clay has squeezed out all around the model.
We trim the excess clay and then we press in some
additional clay at the wheels and any other area that
didn’t get completely sealed. To do this, we’re using one
of the Freeman fillet shaper
tools.
Next we apply some Freeman Wax release with a white bristle
throwaway brush, then lightly buff the wax using a
cotton cloth. We are also applying a layer to the model
itself to aid in the releasing – technically, a release
agent isn’t needed when working with silicone rubber,
but the layer of Freeman Wax Release lubricates the
surface and makes the demold process even easier.
Finally, using an exacto-kinfe, we trim the remaining
excess clay around the detail of the wheels.
Here is our completed master model with the wheels set
below the plywood and clayed in to create a clean parting
line.
To contain the poured material, we have constructed a two
part wooden frame. The first half is deep enough to cover
the majority of the model and the second half is deep
enough to cover the wheels.
Now we need a mechanism for the two halves of the mold to
interlock. Additionally we need to create a path to pour
the material into the mold, called a gate.
To accomplish this, we’ve chosen our Freeman half-round pattern wax,
which is easy to
handle and non-reactive with the silicone.
We choose a length and cut the ends at an angle so the
surface will draw easily from the mold. And then we make an
identical piece for the other side of the model.
We adhere the wax to the wood using clay and then trim the
excess.
This will create an indentation in the rubber which will
later be filled during the second half of the pour,
producing a perfect alignment & locking mechanism.
Using a larger size of half-round pattern wax, we create a
gate. We mark the end of the frame with a pencil and then
cut a piece of the wax to fit exactly between the model and
the frame. Again, we use clay to adhere it to the wood.
Our model is now ready for pouring.
We clamp the frame to secure it in place, making sure the
gate is touching both the model and the frame.
We are using Rhodia’s V-340 silicone
rubber, which has already been
properly weighed, mixed and degassed. Please view our
other mass casting video which details this process.
This material requires 16-18 hours to solidfy, so we need
to wait until the next day to create the second half of our
mold.
Day 2
It has been 16 hours since we’ve poured the first half of
our mold and the Rhodia V-340 has now solidified.
Here we are taking off our clamps and then wedging apart
the frame to reveal the underside of our pour.
The first thing we must do is remove the residual clay with
our fillet shaper tool. Notice how the clay has prevented
the silicone rubber from pouring underneath the model,
resulting in a very clean parting line for our second half.
Next, we remove the locking mechanism. You can see, a
little bit of material has flowed beneath the half-round
pattern wax, which we can cut off with a knife.
To complete the cleanup, we use a small piece of folded
sandpaper in the groove. The harder silicone rubbers like
V-340 can be sanded fairly easily to remove small amounts
of excess material. The softer rubbers are more difficult
to work with sandpaper.
After
cleaning the excess flash around the locking mechanism, we
need to create the other half of our gate to form a wider,
round hole through which the material will flow. On one
end, we are angling the tip and on the other side, which
will butt up against the frame, we cut at 90 degrees.
Before we pour the second half to the mold, it is very
important to apply a release agent to the first half.
Silicone rubber doesn’t stick very well to most materials,
but it does stick very well to other silicone rubber, so we
are applying Pattern Release 202, which is an aerosol form
of Vaseline. A light, uniform coat will be enough to
give us a clean release.
Next, we place the other half of our mold frame on top and
clamp it down.
Now we’re ready to pour the second half of our mold. Once
again, our V-340 has been weighed, mixed and degassed, and
we flow the material into the mold as before.
Day 3
It has been 16 hours since we poured the second part of our
mold. The silicone is now hard and ready to be demolded.
First we take the clamps off, and using the same wedges as
before, we separate the frame at our parting line. Since we
waxed our frame, it actually comes off first. And then we
separate the mold.
Notice our locking device has transferred perfectly from
one half to the other.
We push the first half out of its frame and then remove the
lower part of the gate.
Notice that our gate needs to be cleaned up just a little
bit, which we take care of with sandpaper, much as we did
yesterday when cleaning the flash around the locking
mechanism.
In order to pour a closed mold, we need more than just an
entrance for the material. We also need a vent for the air
to escape. We could cast these vents, but since V-340 is a
harder rubber and cuts very easily, we’ve decided to cut
the vents on one side of the cured rubber.
All we need are small vents, just enough for the air to
escape but not too big to require a large repair on our
finished part.
Since silicone rubber is self-releasing, it doesn’t need a
release agent. However, in order to make the demolding
process easier and to increase the life of our mold, we’re
going to apply a light coat of silicone spray parting
agent.
Notice how it gives the mold a higher shine as well.
Now we’re ready to put the two halves together, and since
we’ve built in the locking mechanism, it aligns perfectly.
We tape the mold together with masking tape. We could use
clamps, but then we’d also have to use boards on each side
to prevent any localized pressure that might distort the
mold.
This is our Repro 10 Tan which has been agitated,
weighed and mixed.
Because of its low viscosity, Repro 10 pours very well.
To make sure that we have no air bubbles on the surface of
the mold, which might naturally occur around the headlights
or other detailed parts of the model, we pour in a little
material, then rotate the mold to make sure the entire
surface is covered first. And then we finished filling the
mold.
About a 30 minutes later, our Repro has partially cured,
but it is still a little rubbery. We want to demold the
part now so it is easier to remove the gating system.
After removing the tape, we separate the two halves of the
mold to reveal our new model.
A little material did flow up into the vents, but notice
that there is no air entrapment in the casting itself.
Now is the best time to cut our gate -- while the Repro is
hard enough to handle and yet soft enough to cut with a
knife. We also remove the part of the gate underneath our
model. If we had let this fully cure, we would have had to
machine or sand this off, which would take a lot more time.
Finally, here is our finished model car, along with our two
part mold. Since we used a high quality silicone rubber
from Rhodia, this mold will produce numerous high quality
parts.