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Produced in one piece by
selective laser sintering:
the Faltstuhl One_Shot
.MGX of Materialise N.V.
The chair is on display,
among other places, at
the Museum of Modern
Art in New york
Laser sintering enables layers
only millimeters thick
When it comes to polymers, additive manufacturing
competes with extrusion and injection molding.
One process that is particularly well suited to
plastics is selective laser sintering (SLS), which
can produce layers 0.15 mm thick. Far thinner layers
(down to 0.08 mm) are also possible, although
at this level the powder becomes hard to handle
because interior forces of attraction prevent the
tiny particles from trickling. While chemical flow
aids can prevent adhesion, there is the risk that
the thermal conductivity of the sintered layer will
change at the edges, to the detriment of the process
capability. “Laser sintering” is an historical
term and somewhat misleading: it refers to a
pressure-free process in which only a short processing
time is required for each layer—just
enough time for the areas that form the part to
melt and form a closed melted film.
With polymers, a CO 2 laser is used to directly
stimulate the polymer chain without the need for
an absorber. The speed of the laser is normally
between 5 and 10 m/sec. Under these conditions,
a component will “grow” two to three centimeters
per hour. But a variety of components can be
produced in the same layer almost without losing
speed—the production area can be packed full of
parts. Compared to other additive manufacturing
technologies, powder-based laser sintering has the
advantage that the powder bed around the component
assumes the function of an all-round support—special
supporting structures that hold projecting
forms in position are unnecessary.
Because not only the material itself but the
production process influences the technical properties
of a part, the parameters of AM products
differ from those of injection-molded products.
Comparative measurements show that the density
and elongation of a part produced in layers are
lower, and that the elastic modulus and tensile
strength show higher values (Fig. 4).
New demands through low-volume
production
In addition to new material properties, the use of
laser sintering in low-volume production places
even more demands on the laser sintering process.
Reproducibility and reliability take on a
whole new meaning: the entire quality assurance
system must be ensured—something that played
essentially no role in the creation of prototypes.
Since last year, the Direct Manufacturing
Research Center (DMRC) at the University of Pa derborn
has worked on the challenge of making
22 elements32 evonik science newsletter
Photo: .MGX by Materialise