- [Narrator] With the advances of
material science and manufacturing,
we've been able to create large volumes
out of a minimum amount of material.
This is primarily for costs,
but includes other considerations like weight
and temperature resistance.
A typical injection-molded design
is called a clamshell,
where there are two parts that meet along a shared plane
enclosing the mechanics within an easily handled volume.
They are so called clamshell
because they are volumetric and thin walled.
In order to make parts
that are the same thickness throughout,
assembled together, and hold the motor within,
certain structures are necessary to the B side of the part.
Structures like:
ribs and bosses.
Let's look at this stool.
This is a step stool that needs to hold up
the weight of a person,
defeating their height challenge.
It needs to hold up under that weight
and so if we look underneath
we can see some of those necessary structures.
These ribs aren't here for looks.
They're there to create structural strength.
At the intersection of the ribs,
I've added some generous bosses.
A boss is a circular support
that can house screws for assembly,
or in this case, to create strength
and maintain material thickness throughout.
If we zoom in a little more,
you can see I've added these circular marks.
Not for style,
but to represent the mark left behind
from an ejector pin.
Ejector pins are a handy part
of both injection and compression molding
to help eject the parts.
The material won't come out as easily
or in one piece without them,
especially thin ribs like this.
So, it's a necessary design feature.
The amount and spacing will be determined
by the manufacturer,
but the designer will have some say
over which face the ejector pins will be.
Almost always on the inside,
or the B side of the part.
These pins are usually circular in standard bar stock sizes
so that they're easy to replace.
You can use a non-standard ejector pin,
but they're more expensive and rare.
Ejector pins are held in place during molding
on something called an ejector plate.
Which is another sliding,
moving part of the mold,
so that all of the pins become part of the mold
during the injection process,
and then pop out at the same time with one mechanism.
So, if I move the entire block,
you can see the pins move in tandem.
They can be different sizes and will be cut
to roughly match the curvature of your surface.
For plastic formed with a liquid introduction,
there will be an injection site
depending upon the size of your part.
It could be anywhere,
but is usually opposite of the ejector plate.
Products like round plastic bottle caps
tend to have it in the direct center,
on the top of the cap for aesthetics,
and also for function.
The plastic will fill from that site outwards.
The mold can also have something called
a flash and tear trim gate,
where any excess of material,
along this edge for example,
can overflow along the parting line
and can be trimmed automatically
by the mold separation each cycle.
This is usually planned when the design is finalized
for preparation of the complete tooling.
In my stool example,
this gate would trim along the outer bottom edge
following the parting line of the mold.
The injection site is where the material flows in.
Vents release the air plastic displaces,
and allowances for flash,
help ensure the mold fills completely.
You want that bit of overflow
because it's a lot easier to trim off excess,
than hide an incomplete ragged edge.
With some care,
you can allow for material to flow
in, throughout, and out of the mold smoothly.