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Aihemaiti, et al.
specimen center in the 3DFSM image. The red area in 3.3. Analysis of cross-sectional geometry of the
the front image is on the right side of the sample, which single deposited line
corresponds to the distorted and collapsed part shown in
the photograph on the right side of Figure 4E, indicating Figure 5 shows the geometry, width, and height of the cross-
that the stress concentration here was significant. In the section of the single deposited line which was observed and
bottom image, there were several scattered red areas measured by the digital microscope. The cross-sectional shapes
instead of continuous red areas in the middle area. As and sizes of the deposited lines were significantly different.
shown in the photograph on the right in Figure 4F, the The cross-sectional shapes of the deposited lines were mostly
distortion deformation led to the dislocation of parallel flat. For schemes 1 and 10, the widths of the deposited lines
deposited lines, tearing in some areas, and uneven stress were slightly larger than the diameters of the bottom surface of
distribution on the bottom. When the indenter was raised, the nozzle. The width of other deposited lines did not exceed
the distortion of the specimen largely recovered, but the diameter of the bottom surface (0.65 mm), and its height
curved and cracked deposited lines were still visible. The was equivalent to the set layer thickness. This showed that
failure modes of schemes 5 and 9 were similar to that of the nozzle bottom surface had the function of flattening and
scheme 3. spreading the molten filament from the nozzle hole. When the
width of the deposited line was <0.4 mm, there was no contact
3.2. Internal defect analysis of specimen between adjacent deposited lines. The widths of the deposited
lines in schemes 3, 5, and 9 were less than the print spacing;
Figure 2 shows the scanned and reconstructed
images of the specimens. The colored areas in the hence, the deposited lines did not overlap in the horizontal
figure represent pores, and the volumes of the pores direction. As a result, several unconnected thin walls formed.
are represented by different color ranges. The porosity Figure 6 shows the relationships between overlap
and corresponding bending strength of each specimen rate, porosity and bending strength for all the schemes.
are shown in Table 4. The porosity of the specimen As shown, the schemes with high bending strength have
affects its bending strength. The specimens with higher smaller porosity and larger overlap rate.
bending strength have small porosity. The porosity of 3.4. Analysis of printing temperature
scheme 3 was the highest, reaching 49.71%, and the
corresponding bending strength was 11.7 ± 0.35 MPa. Figure 7 shows the top view appearance and temperature
The porosity of the specimen with optimized parameters contours of a single line, which was printed by ten schemes.
(scheme 10) was 0.18%, and the bending strength was Based on the data shown in Table 2, some orthogonal
103.1 ± 5.24 MPa. experiment schemes with relatively good bending
The porosities of the specimens built in schemes strengths had larger high-temperature areas during the
3, 9, and 5 ranked in the top three of all the schemes. printing process, and the shapes of the high-temperature
The 3D reconstruction model of the internal defects of areas were uniform and showed comet-like appearances.
the specimens showed that the interiors of the specimens The top view of the corresponding deposited line shows
were composed of several unconnected thin-walled that the width of the deposited line was larger and had
walls, and gaps between the thin walls were evident. The a uniform shape. The large width of the deposited line
stability of the thin-walled wall structure was poor, and it indicated that the extrusion of material per unit time was
was easily destabilized under the bending loads. Thus, the greater. The cooling speed was relatively low, which was
specimen distorted and collapsed as shown in Figure 4E. more conducive to full integration with the surrounding
material. In contrast, schemes with poor bending strengths
Table 4. Porosity analysis results and flexural strengths of had small areas of high-temperature field during the
specimens printing process, and the shapes of the high-temperature
Scheme Porosity (%) Flexural strength (MPa) areas were irregular. For example, the temperature field
1 0.62 97.2±7.85 image of scheme 5 showed that the overall temperature
2 10.37 82.1±4.81 was low and varied with a wavy shape, and the shape of
3 49.71 11.7±0.35 the corresponding deposited line also showed a wide and
narrow uneven shape. The irregular width of the deposited
4 3.08 96.2±4.58 line could lead to insufficient binding and gap formation.
5 35.35 30.9±12.86 Low temperatures affected the bonding quality between
6 11.37 80.4±8.08 adjacent lines when overlapping.
7 12.44 63.5±3.65 The temperature of the extruded material at the nozzle
8 11.76 71.7±13.70 outlet and in contact with the substrate was measured from
9 45.9 10.6±2.16 the side using an infrared thermal camera and the results
10 0.18 103.1±5.24 are shown in Table 5. In general, the outlet temperatures of

International Journal of Bioprinting (2022)–Volume 8, Issue 1 159