- Often, when forming a part, you'll be dealing
with liquid material at one stage or another.
Different plastics have different characteristics
while molten.
Depending on how thick it is,
the harder it will be to completely pack into the mold.
Think about trying to squeeze honey into your mold
and what that honey would stick to.
When I have the point of material introduction
at one end of the model and only allow for
one avenue of venting, this is an exaggeration
of the problems you may find.
A properly designed part has enough gaps, vents, and avenues
for the molten plastic to flow into
and fill the part completely.
It's also helpful in a larger surface area part
to fill from the center of volume
and vent at the outer most extents of the model.
Most importantly, it reduces built in tension and stress
as much as possible.
Non uniform filling and cooling can introduce stress.
This can lead to your part warping out of shape
or even cracking.
I can't give advice for every project but on basic level,
more friction in the mold equals more interior stresses.
Friction can be introduced
whenever the flow has to change direction,
especially acute angles or when your wall transitions are
aggressive instead of incremental.
Running simulations and working to eliminate material stress
in the design phase is better than
on the spot problem solving later.
On some plastic parts, you'll be able to see flow lines
where there are visible waves, following the path
of the molten plastic as it fills and cools in the mold.
When plastic is flung from different directions,
and these masses meet, you'll get something called
a weld line or a knit line.
Some level of texture and the right amount of materials
can emphasize or camouflage this mark of manufacture.
Whether you're working with thermoplastics or thermosets,
you'll have to beware of your worst enemy, bubbles.
The bubbles that I'm talking about aren't fun soap bubbles
with low surface tension.
They're tolerance blasting problematic voids.
In prototype patches, bubbles can be introduced to both
mold and casting material during mixing
from the ambient air.
Any bubbles in your mold will be present on
every single cast.
Bubbles in your casting material can disrupt clarity
and leave ragged corners.
Depending on your material choice,
there are ways to mitigate these little dudes.
In general the longer curing time you have,
the more time you have to work the problem before it sets.
You can use that time to suction the bubbles out
with vacuum chambers
and shrink down the remainder in pressure pots.
In any mold, bubbles can also form due to the surface
tension of your corners and it'll have trouble getting
these acute sharps to fill in.
Bubbles in voids intend to stick to acute edges,
corners of things, and also with gravity, float to the top.
You should be doing material tests and if you, for example,
find that the half inch top of your mold always fills with
bubbles, work that into your design with funnels and vents.
You can also add design elements like screws
above your part in the mold to be cut off later.
Gas traps are usually due to inadequate venting.
The air being displaced by the plastic has to go somewhere.
Ideally out of the mold.
For example, two masses of material move toward each other
but there's no where for the preexisting gas to escape to
so it gets trapped in the plastic.
Watch out for voids along knit lines,
in tight areas, or features that are too thin
to fill in properly.
This would be an example that has an inflow at one point
of material introduction and has out flow in the middle of
this figure eight.
So you've got the draft required,
you've made sure your project is assigned for manufacture
and you get your first run of parts in.
Wait, did these get smaller?
They did!
Less exciting than it sounds, but when polymer parts
cure or cool they get smaller.
And that rate of shrinkage depends on a coupe of factors.
Since polymers are long chains, think cooked spaghetti,
they can have a grain direction, like wood.
Even 3D prints have shrinkage.
Just for kicks, some polymers will shrink at different rates
along the access of the grain
than they do perpendicular to the grain.
Different materials shrink at different rates,
so play close attention to the shrink rate
when working with material suppliers.
Other factors that can affect shrinkage have to do with
how long the material is kept under pressure in the mold,
the thickness of the part, temperature, flow rate,
et cetera.
If you aren't filling the part uniformly or features
of your parts are too thick,
they can shrink at different rates.
A part with the feature exceeding recommended thickness
for the material may still be functional,
but due to shrinkage will look oddly deflated.
There's a whole science to predicting shrinkage,
so do your homework and check with your manufacturer.
In addition to dreaming about your void-less,
non stressed part, you should also consider
the gravity of the situation.
Gravity comes into play when your casting material is
being introduced without external pressure, i.e. molding
by hand.
It's less important with pressurized
mass production methods.
If you're pouring material into the mold from the top,
your mold with be subject to the force of gravity.
Even the strongest of wills will still be subject
to this force, so part of your mold design will require
picking the best orientation of your project to the ground.
Positioning helps aid my elephantine example.
If it's integral to my design to have the parting line
along the midplane of the elephant,
this curling trunk is going to be a problem.
If you can put the elephant on it's side, these problems
are solved.
Because you're pouring down the material,
it's going to have a very hard time traveling
up and around corners like this.
My solution to this would be
to add more than one point of material introduction
with plenty of vent toothpicks to trim off later.
The principle works with pressurized molding too.
It's hard to pack curly features like this
when your injection site is too far away from it.
Keep these gooey dynamics in mind
by leaving your material enough outs
and you'll be good to go.