- Concepts like parting line, direction of pull,
sprues, runners, and vents are important to understand
in the complexity of molding.
It will help with your design
to consider some of the mold physics.
We've talked about the parting line
a bit in previous chapters.
To review, it's this surface that meets that flesh
between the mold parts.
On a cast component, the parting line
is the sometimes visible line
where the two mold halves meet.
In many cases, the parting line is plainer and flat,
though sometimes it could fall along a curve
due to design constraints.
The direction of pull is perpendicular to the parting line,
so let's look closer.
If your parting line splits your part in half like this,
along the plane, then your direction of pull
is straight up and down.
That parting line is important to one of the most important
aspects of molding, which is draft.
Draft is the angle of taper away from perpendicular,
measured in degrees.
When your material is cooled,
it needs to be ejected from the machine.
You don't want your part to get stuck.
If your part has texture, you may need more
depending on how deep the texture is.
For most parts,
the starting minimum degree of draft is 2 degrees.
Without draft, your part could get stuck, or,
have drag marks and scuffing.
Your parts can even get misshapen.
Save yourself a world of heartache by consulting
with your molder as soon as you can in the design process
so you know exactly how much draft you have
to built into your part.
As you can see here,
I've added just a bit of draft to these bolts
to make them come out easily.
Another necessary component to mold design
is the inclusion of rounded edges called fillets.
The viscous material that supporter ejected into your mold
has surface tension.
If your design has very sharp edges or acute angles,
the material will have a lot of trouble flowing smoothly
throughout the mold, introducing stress into the part
and can catch bubbles.
There are fillets right along these edges here,
to aid with demolding.
Fillets and smooth wall transitions between volumes
make it that much easier for your material to flow
and fill the entire cavity, as per your design.
On the note of physics, you have to build in some vents.
The air is being displaced by the liquid plastic;
it has to go somewhere.
I've added some vents along the edges of the parting line.
Plastic flows into the cavity
with the help of sprues and runners.
Sprues are often the first river of the material
to then splits into runner streams
to different parts of your mold.
Larger surface area parts benefit
from having more than one point of material introduction,
making it easier and faster to fill the entire mold.
I've also added vents here,
and here.
In prototype molding, you wanna add them
to the highest part of your mold
to release any air that might get trapped in your mold.
As you can see, even some of the simplest parts
end up requiring engineering to keep it all moving
like a well-oiled machine.
Knowing some of what goes into making it work
helps save time on the design of your projects.