# 3D-Printing the Almost Impossible

Hovering Fish and Rotating Turtles: An optimization method for 3D-printed objects gives them remarkable geometric properties.

A plastic fish is thrown into the water and hovers close to the surface. Inside it has a hollow space with exactly the right shape and size to make the fish hover and stay upright. Using standard methods, countless test runs would be necessary in order to produce such a perfectly balanced object. Now there is a rather simple way to implement such complex requirements. Scientists from TU Wien (Vienna) and RWTH Aachen have developed a method to adjust the hollow space inside a 3D-printed object in such a way that its balance or other physical properties can be tuned with great accuracy.

Toys Containing Science
At first glance, the objects produced in the computer graphics lab at TU Wien look like toys but from the scientific point of view they are indeed quite interesting. The rotational axis of a turtle was adjusted in such a way that it can spin like a gyroscope. An oddly shaped magic bottle has particularly amazing properties: when it is filled with water, it topples and spills its contents. When it is filled with alcohol it stands still. The reason for this is that the density of alcohol is slightly lower. The bottle was optimized in such a way that this discrepancy in density makes all the difference between toppling and standing upright.

To achieve that, the thickness of the bottle’s walls must be customized. On one side it is much thicker than on the other, so that the centre of gravity is kept in exactly the right place. The required shape is calculated on the computer using a mathematical optimization method developed by Przemyslaw Musialski (TU Wien) and Leif Kobbelt (RWTH Aachen) with their colleagues. “We can input the outer shape of the object and certain requirements we want it to meet – such as the rotational axis or its orientation when hovering in water”, says Musialski. “Then the software calculates the shape of the hollow space inside the object, which we need to meet these criteria.”

A few more requirements have to be fulfilled to make sure that the object can be produced by a 3D printer. Complicated, jagged shapes are hard to build, the computer code therefore favours smooth shapes. The user can decide whether the code is also allowed to slightly alter the outer shape of the object.

“Our method has several big advantages”, says Przemyslaw Musialski. “It is fast, the calculations can be done within a few seconds, it is almost fail-safe, and, as we could show it, it can be used for many different optimization tasks.” In future many objects – from decorative sculptures to mechanical spare parts – will not be bought in stores but designed on the computer and then printed. Optimization methods will then make sure that these objects have the right physical properties.

Further information:
Dr. Przemyslaw Musialski
TU Wien
Favoritenstr. 9-11,
T: +43-1-58801-18623
przemyslaw.musialski@tuwien.ac.at