- Let's put it all together.
I've opened up this assembly to demonstrate
a higher complexity real world project
that illustrates a multi-stage casting.
This simple screw driver has a total of three materials
and require two different molds.
First your master has to be split up
to follow along a seam where the split
between the hard plastic and the rubber will occur.
I did this with the split command but you may decide
to model the components separately.
As it's very difficult to make the material
meet up perfectly flush you'll be creating
something called a shut-off channel
in the design phase.
This basically amounts to creating a channel
along the seam, the meeting of the materials,
so you've got room for your mistakes.
Your parts won't line up perfectly every single time
and it really helps to add some wiggle room.
For this example I've added a small chamfer
on this edge of the initial plastic cast.
Mold A will capture the geometry of the inner layer
of hard plastic including the shank,
the main handle body, and also little mechanical keys
for the rubber to lock onto.
You'll capture this detail in a two part soft mold
in this horizontal position.
I've elected to have two funnels that serve
as both inflow and venting of the material.
If you're worried about these very small areas
getting filled in, you can elect to add small channels
between them and straight to the top.
I would place additional slim channel vents
at the vertexes of these small keys.
I've included registration blocks
so the halves line up properly.
Mold B will capture the entire volume
including the outer layer of the soft rubber grip.
You'll use a separate master form to get the mold for this
with added funnels and vents at the topmost part
of the rubber grip.
In this example I would want to create some sort of mark
so I know where the parting line
of the inner materials is so that
when I put my first inner plastic component
in the second mold that it lines up properly.
An easy way to add vents for this type of mold
are the use of chopsticks, skewers, and toothpicks
added along the seam between your mold halves
during the molding process.
Both of the molds created for the single product
start in a horizontal orientation for pouring the mold
but I'd recommend that the first mold is a soft mold
and the second mold is a harder mold.
Both molds will be filled in the vertical orientation
so the venting should be always up
following the direction of the shank.
If you do decide to do two soft molds,
make sure you're not trying to mold
silicone or urethane to itself, as it will bond
and potentially ruin your master shape.
Because you'll be molding over
this metal insert initially,
you'll have to secure the shank of your
screw driver in place before you pour
the plastic around it.
Be sure that if you decide to do a two part mold
that you don't block your materials.
Plan to make channels specifically for
the cavity you're trying to fill.
This procedure mimics overmolding in mass production
but it's a lot more labor intensive per unit.
In injection or pressure molding
the die would be metal and you could produce parts
in a matter of seconds, not hours.
If you're doing a prototyping test, think ahead
to how you plan on producing your product
en masse so you don't have to redesign it excessively.