- [Instructor] With all of the considerations
we've talked about in mind so far,
it's time for a look at the practical applications.
In prototype molding, there are also different
kinds of casting.
I'll discuss solid casting, combination casting,
also known as overmolding, hollow rotational casting,
and expanding casting.
The first type of casting we'll talk about is
the most common type, a solid casting.
In solid, or standard, casting, the entire cavity
of your mold is filled with one material,
resulting in a solid cast component.
I recommend solid casting for small to medium parts.
If you decide to cast a larger solid,
the weight could become an issue and the cost increases.
The more material you use, the more expensive it is,
so many designs minimize unnecessary bulk.
Large, volumetric plastic parts made with factory methods
found around you are generally hollow with structural
design features and consistent wall thickness.
These parts would still be considered a solid casting
because the mold itself creates a solid vessel,
even if it has a complex topography.
Combination casting, or overmolding, is a process
of casting a part in more than one cycle,
usually combining materials into a single product.
The most common example you might know
with overmolding is a toothbrush.
Handles are often made of three or more types
of material to create the desired textural combination
of soft grip and structural handle.
This makes use of the unique characteristics
of multiple materials.
A specific portion of the part is molded,
usually with the harder material, and cooled.
It is then placed in a second mold with bristles
and to add the rubberized grip material,
which will bond with final part and becomes one toothbrush.
Sometimes plastic parts are molded over metal inserts,
which can be very helpful for electronics.
And lastly, overmolding often requires a mechanical method
of bonding the two materials together so that
it can grip together and bond.
Adhesive bonds are useful, but the strongest bond
is a combination of chemical bonding and mechanical bonding.
Rotational casting, or rotocasting, is a method
whereby the mold cavity is filled partially
while being rotated so that the curing, or cooling,
materials fills a near equal layer of material
on every surface of the mold.
This results in hollow components that main their
exterior detail and texture,
but don't have an exact wall thickness.
The material melts, flows, and contacts all parts
of the mold at its lowest gravitational point
because it's not filled by force.
There are limited inherent stresses and can be very strong.
Rotational casting is well suited for projects
that don't have interior mechanisms or specific project
needs for exact wall thicknesses.
I didn't name these methods,
and they can get a bit confusing,
but rotational casting is generally for thermoplastics
and involves a heating element where it heats up the plastic
and then cools it down.
Rotational molding using thermoset resin materials
that combine and cure, doesn't heat up at all,
and can be done by hand for short run projects.
Another kind of casting is with the use of expanding foams.
Some of them can expand up to 18 times
their original volume.
In this example, I have a cushion.
I'm gonna expand the subassembly and then just make
this back part transparent.
Close that back up.
This type of casting has some special considerations
for where your inflow will go and how many outflow
ports you'll need to design into your project,
which are these.
Contributing factors, like heat and moisture,
will affect the process so pay close attention
to those factors on your specific material.
This method is often used for project that will have
an unseen back face.
This is an opportunity to use back pressure
and have an open-faced pour.
This board is what would be considered back pressure
and has these outflow ports, here.
You need those so that after pouring material,
the mold will fill all the way up to the corners
before evacuating via the outlets.
Foams are also sometimes used to fill hollow rotational
molds for things like insulated coolers
or flotation devices.
I hope this has given you a few ideas for how you can
combine casting methods to make the most of your projects.
Each method has it's own pros and cons, so choose wisely.