6 MAY/JUNE 2018
are stronger, lighter and more durable.” Araujo explained
that the thermoplastic printing material becomes stiffer
and stronger as more carbon fiber is introduced into the
compound. But, since some applications require parts
that are more flexible than others, LHEVOSS offered the
designers a series thermoplastics with varying levels of
carbon fiber enhancement.
A decision was made early in the project to feed the
printer with pelletized material rather than the filament used
as feedstock for most FDM style machines. This decision
was driven by several factors, including the fact that
pellets retain good printing characteristics with a higher
percentage of carbon fiber than filaments can.
New Tools and Techniques
Even before work on designing the Mini 650 began, it
became apparent that printing at this scale would pose
challenges that would be challenging, or impossible for
traditional 3D printing technologies to overcome. To
address these issues, Belvisi and Cevola created OCORE
( www.ocore.it), a startup company tasked with the
development of a dedicated direct extrusion 3D printing
technology, and design tools to support it.
Instead of using one of the frame-based architectures
that most 3D printers are based on, OCORE placed its
extruder head on the end of a six-axis industrial robot arm,
manufactured by KUKA (Figure 2). Putting its business end
on the 6+ foot arm gave the printer a very large working
volume. The robot’s agility also gave the extruder more
degrees of freedom to print with than conventional machines
that can only deposit material along the vertical axis.
Besides improving the printing hardware (the robot,
extruder and nozzle) OCORE has developed a new material
deposition strategy using an algorithm inspired by fractals.
The new algorithm was created to address some of the
challenges involved with building a structure where the hull,
deck and structures are printed in a single, integrated unit.
Nearly every other FDM style printer made today is driven
by software that generates the instructions to move its print
head by slicing a mesh that defines the target object (.stl)
and deriving a polygonal geometry. In contrast, the OCORE
researchers opted for a program that translates the Bezier
surfaces that define the hull directly into the KUKA robot’s
native programming language. This approach enables the
arm to move very smoothly with higher speed and precision
than conventional control algorithms could deliver.
Advanced Structures and Strategies
Their printer’s unique abilities also made it possible to
develop the design and manufacturing strategies the design
team needed to achieve their goal of creating a boat with
an integrated structural core, covered with a thin carbon
skin that supports most of the boat’s torsional and shear
The core’s structure, shown in Figure 3, is based on an
optimized lattice structure known as an “anisogrid”, a grid-like design that exhibits different mechanical properties
along different directions (making it an-isotropic). The
primary Isogrid improves the bending rigidity of the
cylindrical wall to resist the global instability while the sub-Isogrid improves the bending rigidity of the skin enclosed
by the primary Isogrid to resist local buckling. Studies are
nearly complete that will identify any locally loaded areas
that will require additional carbon stiffening elements. This
approach enabled the development of optimized design and
manufacturing methods that are expected to yield a final
product that has an excellent performance-to-weight ratio
and a minimum of wasted material.
The boat is designed to be printed in seamless
transversal sections and then assembled lengthwise.
Conventional boats are usually built up in a longitudinal
manner, laying up or molding the entire length of the hull
and deck in separate production steps. When a composite
boat is built, the deck is glued to the hull. In the 3D printing
scenario however, it is more practical to print the boat in
transversal sections (bow up) so that hull and deck are
printed in single shot (Figure 4).
The Mini 650’s hull will be printed in four segments that
will be joined with structural adhesive and then covered
with a very thin carbon-epoxy composite shell. Once the
shell is bonded to the hull core, it adds stiffness to the
structure and overlays the rough surfaces of the core’s
Figure 3 – A CAD drawing of the
printed anosogrid lattice that serves
as the structural backbone of the
boat. (Photo courtesy of OCORE)