bioplasticsMAGAZINE_1406

3D printing
Fig. 4: Filament from a PLA blend (Source: IKT)
The melt strand is not produced by extrusion, as is usual in
plastics series processes, but out of a mono-filament (Fig. 4),
which is melted completely in the FDM nozzle by contact
heat. The nozzle-infeed (depending on the different machine
producers) usually has a diameter of exactly 3.0 or exactly
1.75 mm, whereas the nozzle outlet is 0.2 to 1,0 mm, depending
on the machine. The production pressure is raised by pushing
the filament into the heated nozzle. For this purpose, the
machine has pressure rolls or wheels (see Fig. 5).
Fig. 5: Detail of the printer head of the
FDM process (Source: IKT)
There are three process steps to produce 3D printed,
biobased, plastics parts (Fig. 6). The first step is the
compounding step that upgrades biopolymers to processable
bioplastics. The second step is the production of printable
monofilaments and the third step is the 3D printing process
itself.
Compounding: To achieve 3D printable bioplastic filaments
IKT Engineer Linda Goebel (Fig. 7) has to develop Bio-Blends
on one of the twin screw extruders in the compounding
technical centre of IKT.
Requirements of the material:
The chosen material has to be thermoplastic and needs to
consolidate quickly. In the solid state, the filament has to be
strong enough, to avoid breakage during its transport and
the filament´s surface needs a certain roughness, to prevent
slipping effects. In the molten state, the viscosity must be
high enough to avoid filament rupture, dripping off of melt
from the nozzle as well as keeping the upper new layer on
top of the layer laid down shortly beforehand. But, viscosity
should not be too high, to allow entanglements across the
layers´ surfaces and thus a fusion. The re-solidified state of
the material must meet the requirements of the later part.
Requirements of the filaments:
The filament diameter must be perfectly round to allow
pushing by means of the rolls and wheels as well as to make
sure that the there is enough contact to the inner nozzle wall.
If a filament were slightly oval it would probably neither be
pushed into the nozzle, nor would it have enough contact for
an efficient and fast heat transfer. In addition the filament’s
diameter should not pulsate along its length, i.e. the diameter
must be precisely the same over the whole length. This is not
easy, since the thermoplastic melt produced through a die
contains molecular orientations, which will relax after leaving
the nozzle. A so-called die swell occurs and will influence the
filament´s diameter even after production.
Compounding
Production of
the filaments
3D-printing
Fig. 6: Three process steps from the biopolymer
to the 3D part (Source: IKT)
Fig. 7: Linda Goebel during 3D printing
experiments (Source: IKT)
www.ikt.uni-stuttgart.de
bioplastics MAGAZINE [06/14] Vol. 9 17