SAMPE Journal - July/August 2022 - 31

Figure 2. Ultimate tensile strength of 3W 80% infill samples27
.
the number of layers the pin penetrates, typically
ranging from 8 to 20 layers. The seam offset is
defined by the number of layers deposited before
pausing to deposit pins. Typically, pins of length
L are deposited in every other pin location after
depositing L/2 layers of structure, giving a seam
offset of L/2. (Figure 1). This is to avoid a fracture
defect that occurs when the seam lines are all lined
up to one another. Lastly, the infill percentage is
defined by the amount of material deposited into
the pin cavity compared to the theoretical volume
of the cavity. Infill percentages typically used in
AM Z-pinning are 80%, 100%, and an " overflow " of
120%. An overflow pin condition is possible due to
the open structure of the rectilinear grid and can
increase mechanical interlocking of the pin with
the surrounding structure (Figure 1).
Current work in AM Z-pinning aims at improving
the ultimate tensile strength in the Z-axis, as well
as developing quasi-isotropic material properties.
The first phase of research concluded that as the
infill percentage of the pinned samples increased,
as did the ultimate tensile strength (UTS).
Meanwhile the degree of anisotropy decreased
as infill percentage increased. The 120% samples
demonstrated an increase of over 3.5 times the
0% when comparing the Z-axis ultimate tensile
strength. The 120% samples also showed the
lowest degree of anisotropy11
. The second phase
of research expanded the pin parameters used
from the first phase to collect more data at the 80%
infill level to investigate how different pin widths
and lengths effect the mechanical strength and
isotropy of printed parts. In this study, four pin
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setups were studied, a 3W 4-8 and 6-12 sample, a
4W 6-12 sample, and a 6W 4-8 sample. Analysis of
the anisotropy ratios (UTSz
/UTSx) of the structures
showed that the 6W 6-12 sample set demonstrated
the highest anisotropic ratio with a value of 0.52,
but only after decreasing the UTSx
. When compared
to an unpinned sample the 3W 4-8 sample showed
the greatest combination of increases in strength
across the X and Z axis, with a 46% and 85%
increase respectively27
. It is important to note that
it was expected that longer pins would lead to a
greater mechanical performance. This was due to
more continuous material extruded into the pin
cavities without seamlines. However, data showed
for the 3W 80 % infill, the ultimate tensile strength
decreased for X and Z axis samples (Figure 2). This
is due to defects during the printing process.
Defects
The AM Z-pinning process has promising
results, yet defects during printing stall progress
in printing more advantageous pin designs. It is
important to separate the Z-pinning process into
two phases. First a rectilinear grid is deposited.
This phase of the printing process has near zero
flaws. The next phase occurs when the actual
pins are extruded into the pin cavities. Here,
error propagation occurs. For a successful print,
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SAMPE Journal - July/August 2022

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