JEC COMPOSITES MAGAZINE - Issue #143 - November/December 2021 - 23

Hollow stereolithographic structures reinforced by short carbon fi bres
Fig. 4: a) Incompletely fi lled
SL structure; b) Complete
fi lling by an additional
pressure step
tensile specimens (modified dogbone
specimens
allowing
infusion) made
out of another high-performance acrylic-based
photopolymer from Henkel AG
& Co. KGaA to evaluate the achievable
mechanical properties. Both materials
were processed on the RapidShape S90
DLP machine, using the processing
parameters provided by the machine
manufacturer. The post-treatment of the
SL structures was carried out according
to the manufacturer's instructions with
ultraviolet light. A layer thickness of 50
µm was used for each geometry.
For infusion of the SL structure, a
low-viscosity epoxy resin from Hexion
Inc. and short carbon fibres from SGL
Carbon SE were used. According to the
manufacturer, the fibres have a mean
length of 150 μm and no sizing. A fibre
weight fraction of 20 wt% was used for
all tests with fibres.
Experimental setup
The experimental setup for infusing the
SL structures (see Figure 3a) consisted
of a vacuum pump (1), a desiccator (2),
the SL structure (3) and a cartridge
press (4). In the cartridge press, a 50-ml
syringe with an adapted nozzle to connect
the infusion tube was inserted. The
mixed and degassed short fibre suspension
was placed in this syringe. Before
starting the infusion, the syringe was
locked so that the suspension and the SL
structure could be degassed together at
25 mbar for five minutes.
The vacuum was then reduced to 500
mbar, the syringe locking was released
and the suspension began to flow into
the SL structure (see Figure 3b). As
soon as the SL structure was completely
filled, a tube clamp (5) on the side of the
desiccator was closed. If no additional
pressure is applied, the chambers remain
unfilled (see Figure 4a). Hence, the
pressure was increased with the cartridge
press to completely fill the SL structure
(see Figure 4b).
Then the second tube clamp (6) on the
side of the cartridge press was closed
and the infusion tubes were cut off. The
filled SL structures were then placed in
the oven for curing.
Results
Microsections
Th e suitability of the process for fi lling
complex structures was investigated
based on the chicane geometry (see
Figure 5), showing that, despite many
chambers and branches, the structure
could be fi lled completely and without
porosities or air entrapment (see Figure
5a). Th is is important as pores would
signifi cantly
reduce
the
achievable
mechanical properties [8]. Furthermore,
it can be seen that sedimentation of the
short fi bres occurred within the infusion
resin, resulting in an inhomogeneous
fi bre distribution along the gravity vector
(see Figure 5b). An optical analysis of the
fi bre orientation shows that the fi bres are
aligned in the fl ow direction, which is a
similar behaviour as with conventional
injection moulding processes [8].
Fig. 5: a) Chicane geometry completely fi lled with the short carbon fi bre suspension without pores;
b) Micrograph of an exemplary cross-section with visible sedimentation of the short fi bres
N°143 November - December 2021 / jec composites magazine
23

JEC COMPOSITES MAGAZINE - Issue #143 - November/December 2021

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