Printing complex organic shapes with a Makerbot

UPDATE: Anna Galovich has been generous enough to translate this fantastic blog post into Estonian. You can find the translation right here.

For a recent project, I had to print this awesome crocodile skull from the University of Texas Digimorph project. At first it looked daunting, but it turned out to be surprisingly easy to print. I really like this print because it is a complex organic shape, and it is really impressive that it came from a Makerbot.

Crocodile skull, printed on a Makerbot

I used Netfabb Studio and ReplicatorG to prepare for print. I’m not posting the print-ready files because a) not sure if UT will let me and b) the following process is easy and you will learn a lot. I basically used Netfabb Studio to re-orient, repair, scale and split the model, in order to get it ready for print. The procedure I followed is generally applicable to all kinds of complex prints.

Keep reading for instructions on how to do it.

The crocodile skull data is derived from a CT scan of a real skull. It is very detailed, preserving information about the internal volumes of the skull. That makes it fragile. I quickly realized that I was going to have to slice it into two parts in order to be able to print it without support. Cutting support off a fragile piece can be very difficult!

The first step is optional: the model is very high resolution, more than will really be noticeable in the print. If you want, open the model in Meshlab, and use Quadratic Decimation to reduce the number of polygons to 50% or 25% of the original number. This will make things faster.  However, I had no issues slicing the model in Skeinforge at full resolution, though it took longer.

The next step is to get it into the right orientation. You can do this in ReplicatorG, which works fine, but since I was going to use Netfabb Studio for the rest of the process, I just used that.

Rotated skull after transformation is applied

Open the model in Netfabb Studio. Select Part/Rotate, and select 90 degrees on the X-Axis. The crocodile is now standing on its head.

At this point, might as well apply Netfabb’s automatic repair procedure. Do this by choosing Extras/Repair Part and then pressing the Automatic Repair button. As far as I can tell it doesn’t do much, but it does not take long. Netfabb creates a new part in the tree. You have to click the Apply Repair button for the repairs to “stick”. Choose to keep the old part. Select the newly repaired part (with suffix “fixed”) in the tree.

Next comes scaling. Netfabb provides a handy display of the total size of your part in the Information panel in the lower right hand corner. Be aware that you are going to cut it into two pieces vertically, so you will probably have enough room on the Z axis. Use the Part/Scale command to scale the piece. I used 40% but 50% will likely fit too.

Slicing a model in Netfabb Studio

Now slicing. Models in Netfabb are centered on the origin ie. Z = zero is the middle of the object. That makes slicing easy. Move the Z-Slice slider to 0mm. Netfabb conveniently highlights the section to be cut in blue. Select the “triangulate cuts” checkbox, that makes sure that the cut edges are filled in with a solid surface. Click Apply Cuts. The object is now shown as two separate parts. Double check in the Information panel that the parts will actually fit the build platform on your Makerbot.  Select one part and then the other and export them as STL, in the Part/Export menu. If it gives you warning about non-manifold parts, go ahead and apply the repair.

You are almost ready to print.

Open the parts in ReplicatorG. One of them will be upside down (below the build platform) so use RepG’s Rotate button to get it into the right orientation. Use the Move/Center and Move/Put On Platform buttons to get it into the right place.

Now is time to print. If you haven’t already done so, make yourself a fine profile with a layer height of 0.25mm or less, and a feed rate of 30mm/sec or less.  If you use Makerblock’s Profilemaker, try a thread width of 0.5mm. Having a high ratio of thread width to height will lead to much better handling of overhangs.

You might get some warnings from Skeinforge about unsupported overhangs, but you can ignore them.

Back of crocodile skull on build platform

Forget using a raft – you want a nice smooth surface between the two pieces so you can glue them. I personally find thin Kapton over aluminium to give the best results. Wipe the platform with acetone before starting, and make sure your Z-height is properly dialled in. You don’t really need to squash down that hard if your platform is level and you’ve wiped with acetone – ABS will stick really well.

Two crocodile skulls

The picture shows two versions of the skull, the one in back printed at 0.36mm layer height and the one in the front at 0.25mm. You can see that the teeth in particular are more finely rendered at the higher resolution. On a Thing-O-Matic with a stepper extruder, the piece required almost no cleanup.

When the parts are done printing, let them cool so you can take them off the platform without damage, and carefully glue them together. Presto, beautiful crocodile skull. The Digimorph project doesn’t have a lot of STL files, but they are adding more all the time. Might be fun to try printing some of the others, I have my eye on the Leopard Seal skull.