Page 60 - MSAM-3-4

Materials Science in Additive Manufacturing 3D printing of anti-microbial parts

repeated layer-by-layer 26-30 . AM methods have free- digested. The reference microbe chosen to validate the anti-
forming capability, enabling the creation of customized microbial activity of the plastic disks containing Mg(OH) 2
parts . Further details on IM and PBF (SLS and HSS), as nanocrystals was Escherichia coli (E. coli K-12 MG1655;
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well as their advantages and disadvantages as production American Type Culture Collection [ATCC], USA). E. coli
methods, are discussed in Sections S1 – S5. Both SLS and is normally present in the intestines of healthy animals.
HSS technologies have evolved from rapid prototyping While E. coli is generally safe enough to work with, it is
to the manufacturing of functional parts. With further not completely harmless, as it can cause urinary tract
development in the coming years, HSS has the potential to infections.
attain comparable productivity as IM.
2.2. Part dimensions
In this work, the anti-microbial activity of PA 12 articles
manufactured through HSS and dip-coated with Mg(OH) Disks and tensile bars were manufactured using PA 12
2
NPs was compared with dip-coated IM PA 12 parts and through IM and HSS, respectively. A total of 12 test disks
with IM parts produced from PA 12 melt-compounded with a diameter of 25 mm and a thickness of 1.55 mm
with Mg(OH) NPs. were molded from melt-compounded PA 12 containing
2
5 wt.% Mg(OH) nanocrystal powder. PA 12 disks with
2
2. Materials and methods the same dimensions were also produced by HSS. The
2.1. Materials disk dimensions were tailored to fit into pods designed to
introduce bacterial broth, for testing the anti-microbial
Polymer powders are typically used as raw materials in activity of the disks. The tensile bars were produced to
HSS; PA 12 (type B; voxeljet AG, Germany) was used in assess whether the Mg(OH) coating led to significant
2
this study. The virgin powder may be used for printing deterioration of mechanical properties. A total of 15 tensile
articles, but “used powder” mixed with virgin powder can bars were printed by HSS, in accordance with ISO 572-2,
be employed; this is a common practice due to the high but with a shortened overall length of 150 mm instead of
cost of polymer powders for PBF. The term “used powder” 170 mm (the grip area was reduced). Both types of HSS-
refers to leftover heat-exposed materials from a previous printed specimens (disks and bars) were dip-coated with
printing episode. Notably, a 100% “used PA 12 powder” a suspension of Mg(OH) . Pure PA 12 IM disks were also
cannot be employed for printing because its molecular dip-coated. 2
weight increases greatly during the long exposure of the
powder to heat during HSS , resulting in printing defects . 2.3. Manufacturing of test specimens (disks and
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The manufacturing of HSS specimens utilized a mixture of tensile bars) with HSS
used: virgin powder in a ratio of 70:30, and the samples
were sieved using a Russell Finex Minisifter vibration sieve The HSS manufacturing technology is described in Sections
(the UK) with a mesh size of 180 µm. The 70:30 ratio was S1 – S4; the inkjet module of the HSS machine is presented
in Section S5. For manufacturing the specimens, a HSS
known to print without gross defects in the articles. The machine (Voxeljet AG VX200 HSS beta version), located
used and virgin PA 12 powders were mixed within a mixer
machine (PM 10; Dr. Fritsch, Germany) for 30 min with a at the Chair of Manufacturing and Remanufacturing
rotation speed of 48 revolutions per minute (RPM). Technologies of the University of Bayreuth, was used. The
arrangement of both types of test specimens in the build
The anti-microbial biocide selected to fabricate the chamber is featured in Figure 1. All test specimens were
anti-microbial plastic articles was Mg(OH) nanocrystals. arranged at a distance of 10 mm to reduce the influence of
2
A concentrated aqueous suspension of Mg(OH) NPs the mutual heat output arising from the crystallization of
2
(7 wt.% Mg(OH) and 93 wt.% of water) was obtained the parts after sintering.
2
from Aqua Inc. (United States of America [USA]). This
suspension was diluted and applied to the plastic parts. After arranging the test specimens in the build chamber,
the CAD files for the print head were created layer-by-
The Mg(OH) nanocrystals were also used as a dry layer in the form of bitmaps using digital slicing. The
2
powder (Aqua Inc., USA) by melt-compounding and HSS process parameters used for manufacturing the test
IM with the same PA 12 used for HSS. The dimensions specimens are listed in Table 1. The greyscale determines
of the nanocrystals in the powder were the same as the the amount of ink applied to the powder bed. As stated
nanocrystals from the suspension. below, the areas within the powder bed where the parts are
As this work is about plastic articles that can deactivate located are printed with 18 pL of ink per voxel. A voxel
infectious microbes, a microbe was selected that is safe is the unit cell determined by the smallest printable area
enough to handle in the laboratory but is infectious if (x-y-direction) and the layer thickness (z-direction).

Volume 3 Issue 4 (2024) 3 doi: 10.36922/msam.4970