Hello and welcome to this week’s Insight.
This time around we’re going to be talking
about a really useful piece of design and
how to tackle it with 3D printing – living
hinges.
Now, even if you haven’t heard the name
before, you probably know what a living hinge
is. They’re thin, flexible hinges that are
made of the same material as the pieces it
connects – think of the lid of a box of
mints or even the middle of those fold-up,
disposable spoons you get when you buy a yogurt.
They’re great little pieces of design – cheap,
easy to make and highly effective – but
while injection moulding can easily produce
living hinges that have a long, reliable lifespan
on finished products, it can be a little trickier
to get them to work when you’re 3D printing
prototypes.
While it is possible to create robust and
functional living hinges with technologies
like selective laser sintering and Multi Jet
Fusion, you need to take a little more care
during the design stage. This may even require
that the hinge be designed one way during
the prototyping stage, and then redesigned
before moving to injection moulding.
This all raises the question of why bother
with 3D printing living hinges in the first
place? Well, there are a few reasons.
First, 3D printing technology is just a great
way to rapidly iterate your designs, and doing
that can strip out many of the technical niggles.
Secondly, you can use it to prototype parts
in nylon before shifting to injection moulding.
And finally, you can use it for creating functional,
end-use parts if your actual quantity needs
are low.
So, how do we go about making living hinges
work as well as they can with 3D printing?
The first principle to keep in mind is that
the hinge always needs to be the weakest area
of the part. If the living hinge is roughly
the same thickness as the rest of the geometry,
then the part will distort and flex when you’re
attempting to use the hinge – not ideal.
Next up, you need to bear in mind that the
force needed to bend a hinge is proportional
not only to the thickness of the hinge, but
also its width. To make things a little easier,
you can think about modifying one big hinge
into several smaller ones. Rather than have
a single 20mm hinge, you could have, say,
three 5mm ones with gaps between them. That
reduced width means you need less force to
operate the hinge and therefore less stress
is placed on the clasps, snaps, and the geometry
as a whole.
Cutting this down is important because living
hinges are under constant stress when in use.
One side of the hinge is going to be under
compression, while the other side will be
under tension. As well as making them as narrow
as possible, we also want to make them as
thin as possible. If you’re 3D printing
a living hinge, you generally want it to be
the minimum feature size for the technology
you’re using.
On top of this, the layout of the hinge design
can be really important. To explain why, we
need to get a little technical, and though
I appreciate that not everyone wants a excessive
maths in their videos, we need to go over
a couple of formulas if we want to understand
what’s going on.
The important one for living hinges is L = πR,
where L is the length of the hinge and R is
the distance between the hinge and the attachment
point.
This little formula is the ideal relationship
between those two values, as it ensures that
the hinge will form a nice, neat semicircle
when it’s closed. This should evenly distribute
the stress along the hinge and minimise it
at the attachment point.
If the hinge is too short or the distance
to the attachment point is too large, the
hinge will be under tension in its closed
position. Essentially, the curve the hinge
makes is too shallow and it’ll try to pull
itself apart.
On the other hand, if the hinge is too long
or the attachment points are too close, stress
will be concentrated at the attachment points
as well as the middle of the hinge. The excess
hinge material will act as a spring and try
to pry the pieces apart – which is rather
the opposite of what you want it to do.
So, if you keep all this in mind there’s
no reason why you can’t make use of living
hinges in your 3D printed prototypes. Just
make sure to speak with your manufacture if
you have any queries.
Right that’s it from me this week. I look
forward to seeing you again next Friday.
