by Erik Es - Amsterdam, July 5, 2018  
  Apart from my fascination for ships in bottles (see the prequel to this
blog: INSERTION PRINTS #1) I had another reason to create the
3D printed ship-in-a-bottle. I wanted to finally bring to life an idea
which I had AGES ago - to make a bottle-in-a-ship.
Does humour belong in 3D printing ? Yes, it does.

This turned out to be a lot harder than the regular ship-in-a-bottle. The
reason for this: you can not print any desired shape as a hollow object.

This is because the print is built up in layers from the bottom upwards.
So printing a cup is no problem: first the bottom is printed, then the
wall of the cup grows upwards, no problem.

The problem is the top. Or sometimes it is. With some objects, hollow
printing can be done without too many problems.
(If you know all about "bridging" you probably want to skip the next bit.)

For example, it is possible to print a hollow cube. The bottom is
printed first, then the walls (from bottom top top), then the printer
starts printing the top. There is nothing to support the first layer of the
top as the printer is crossing over the hollow inside - the printer is
extruding molten plastic into thin air.

However, this produces thin strands of plastic that get spun over the
top (a bit like a spider does), providing the distance to be crossed is
not too high. This is called "bridging".

This first top layer is quite irregular - sometimes a few strands are
broken, and they tend to sag... However, it provides some support for
the next layer, which is printed at 90 degrees across the first top layer.

Because of this support, this second layer will be less irregular. And
the next one will be much better again. After a few layers there is
enough support for the rest of the top to be printed with a clean finish
on the outside. The "saggy" first layers of the top are now hidden
inside - unless you are using transparent plastic for the print.

Another example: you can also print a hollow cylinder with a dome
shaped roof. In that case there is no need for bridging. The printer will
print bottom and wall, and will then start to "shrink" the deposited
circles as it finishes the domed top.

So that's two examples of objects that can be printed hollow.
The same can be done with many other shapes: a cone, or a pyramid,
or a quartz crystal, or a church - all possible.

But there are plenty of objects that are impossible to print as hollow
prints. Example: the same domed cylinder as before, but with the
dome upside down, like an upside-down beer can.

Here's what happens:

At some point the printer starts to print the first layer of the hollow
dome: just a small circle, right into empty space. There is nothing to
support it, and it is not bridging from wall to wall. So all that happens is
that some blob of extruded plastic will end up in the wall of the cylinder
as the printer continues with the next layer.

The same thing will happen in the next layer, and all the ones after that.
So you get no top, just a messy cylinder.
In this case a solution could be to add a central pillar inside the cylinder - but then it is no longer a fully hollow object...

A similar problem occurs if a flat top of an object has holes in it.
Bridging is a nice trick, but you can't expect these spider strands to
suddenly end in mid-air and then pick up again as the print head
passes across the place where there is a hole in the top...

So I had to take all this into account while designing the ship, because
it would need to be printed hollow.

Here's the ship I made:

As you can see, the deck of the ship forms a curved top that closes
the hull. The hull must be hollow to hold the bottle. The bridge of the
ship needs to be hollow as well - looks much better and saves a lot of
print time compared to a solid block. The bridge has a flat top, so
bridging is no problem there. The chimney is also hollow and has an
open top - no problem either.

However, there was one problem to be solved: the top of the hull is
curved. It had to be, otherwise the boat would look stupid.
But that slightly curved top would cause some nasty problems when
the bridging occurs. The "ceiling" of the inside of the hull would not look
pretty, and that would definitely be visible with the ship being transparent.

I managed to solve this problem. How I solved it is a lot of tech-babble
which I will save for another blog.
The short version: I made the model in such a way that the bridging will
always happen "port-to-starboard and back".

To make a nice set, I decided to use the same shapes for the ship and
the bottle in both the "regular" and the "goofy" version. With one
exception: the bottle that goes into the ship does not have a flat side,
so it can roll around freely inside the ship.

Here you see some bottles being printed:

Next the ship is printed. Inserting the bottle into the ship (just before
the top of the hull starts to close up) is MUCH easier than inserting the
ship into the bottle (the "regular" version), because in this "goofy"
version the bottle is small and the hull of the ship is much larger,
as you can see here:
After the insertion, the bottle gets blown around by the cooling fan in a
dangerously frantic dance, as if it is trying to escape the imminent
enclosure... But as the deck closes up, it settles down, enclosed
forever inside the ship.
The ship needs just a little bit of clean-up work after printing due to
some traveling (print head jumping from one part of the print to
another) and then it is done:
A bottle-in-a-ship. Yippee - a very old idea, finally did it !

Just one of many great 3D prints for sale at Printed In Space.