2.2 3D-printing
We used the Ender-3 Pro (Creality, Shenzhen, China) Material Extrusion
3D-printer. This printer is commercially available and inexpensive
(<300 USD). The printing principle of Material Extrusion
entails advancement of thermoplastic filament into a heated extruder
barrel and exit through a nozzle whereby strands of molten plastic are
depositing, forming a layer. The object is thus built in a
layer-by-layer manner (Figure 2). The chosen material is a milled chalk
composite in a matrix of polylactic acid (PLA) sold under the tradename
Lay-brick (CC-products, Cologne, Germany). The high content of finely
milled chalk prevents the material from agglomerating and melting when
drilling. This is crucial during drilling exercises since regular PLA
based filaments will liquify from the heating caused by drilling
friction, thus agglomerating and packing in the flutes of the drill bit
making regular PLA unfit to replicate bone properties. However, the high
load of chalk filler also brittles the Lay-brick filament, which results
in the risk of filament breakage during printing, causing unsuccessful
prints. Finally, the chalk filler makes the filament slightly abrasive,
resulting in both the nozzle and the drive gear (i.e., the part of the
printer feeding the nozzle with filament—Figure 2) to wear out faster
than usual and needing replacement after approximately printing five and
twenty models, respectively.
To avoid the filament from breaking and to extend the durability of the
service parts, we made some minor modifications to the printer. First,
we installed a direct filament drive (Micro Swiss Direct Drive Extruder
for Creality Ender3, Mico Swiss, Ramsey, MN 55303 USA) costing
~100USD. Using a direct filament drive, the drive gear
is fitted directly on the hot end instead of being guided through a
Bowden tube (Figure 3) thereby avoiding tension from building up in the
Bowden tube between the drive gear and nozzle, which can otherwise cause
the filament to break. Further, we replaced the standard nozzle with a
more durable Ruby nozzle (PrimaCreator MK8, Ruby Nozzle 0.4mm,
PrimaCreator, Malmö, Sweden, ~60USD) making it possible
to print more than 50 models before nozzle replacement. The Lay-brick
filament is prone to build-up of charred residuals, which can result in
clogging of the nozzle if it is not regularly cleaned by reaming the
nozzle with a length of a Ø0.4mm piano-wire.
It takes approximately 16 hours to complete the printing of a single
model, which requires 65 grams of material corresponding to a material
cost of ~5 USD.