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.