SAMPE Journal - July/August 2022 - 30

FEATURE / Z-PINNING ADDITIVE MANUFACTURING
An Arizona State University research team added a
laser to increase the temperature of the deposition
surface during the pre-deposition printing process.
This work demonstrated an increase of 50% in
the flexural strength of a 0.03% Carbon Black
ABS structure16,17
. Alternatively, work done at
Montana State University focused on increasing
the interface temperature by introducing a stream
of hot air. This method was not effective due to
the additional air flow effecting the geometry of
the build and not improving the strength of the
material or structure18
. Additionally, Oak Ridge
National Laboratory used infrared lamps on the
BAAM system to preheat the underlying substrate
just prior to deposition. This resulted in a 100 %
increase in the fracture strength of 20% Carbon
Fiber ABS19,20
. Most of the prior work to eliminate
the Z-axis drop in ultimate tensile strength for FFF
printed structures has been focused on adding
pre-heating steps during the printing process
or continuing to modify existing layer printing
styles. This leaves a gap for a novel process that
uses 3D printed structures built into the existing
part. Additional work done at the University of
Tennessee and Oak Ridge National Lab focused
on developing a novel printing approach, called
Z-pinning which was first presented at SFF in 2017 .
Z-PINNING
Work done at the University of Tennessee and Oak
Ridge National Lab focused on developing a novel
printing approach, call Z-pinning, which was first
presented at SFF in 201720,21
. The Z-pinning printing
method was developed to reduce mechanical
anisotropy and improve the Z-axis strength of
printed structures. This Z-pinning method involves
creating a 3D printed structure with pre-determined
cavities in a layer-by-layer fashion. Then after a
set number of layers, introducing another printed
structure into those cavities to reinforce the
Figure 1. The Z-pinning process20
.
originally structure. The Z-pinning AM process
is similar to the Z-pinning process that has been
used in
traditional composites manufacturing.
Traditional composites manufacturing has utilized
a Z-pinning process to eliminate delamination,
improve upon the through-thickness properties,
and increase joint and bearing strengths. There
are a variety of different Z-pinning process used
in
traditional composites manufacturing, with
the most common being the UAZ® process. This
process uses an ultrasonic tool to insert Z-pins into
an uncured pre-peg stack.22-26
The Z-pinning AM process can be divided into
two parts, first a multi-layer structure is created with
intentionally created cavities spanning several,
defined, layers. After a pre-determined number of
layers have been deposited, the extruder positions
itself over the cavities and deposits material into
the pin holes. This extra deposition step deposits
the same material (although a different material
could be used in a multi-material structure) to fill
the cavities in the Z-axis, hence the name " Z-pins " .
The pins are deposited such that they are staggered
at half of the length of the prior row of pins. This
removes a continuous " seamline " that would have
spanned the width of the structure that occurs
when two pins are laid on top of each other. Then
using the same material, pins are extruded into the
voids (Figure 1).
When setting up a print incorporating
Z-pinning, certain parameters are used to define
the pins and how they are printed. These are
known as the " Pin parameters " and define the pin
width, pin length, infill percentage, and the seam
offset. The traditional pins have a square cross
section defined by the spacing between adjacent
beads in the rectilinear grid. Typical pin widths
are integer multiples of the nozzle exit diameter
(W = 0.5 mm), such as 1.5 mm (3W), 2 mm (4W),
and 3 mm (6W). The pin length (L) is defined by
30 | SAMPE JOURNAL | JULY AUGUST 2022
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