SAMPE Journal - January/February 2024 - 33

and then consolidated between the nozzle face and
the previous surface. The outside diameter of the
face of the nozzle is manufactured to be greater
than the width of the spread commingled tow. For
this nozzle-based process, the tow is shaped as it
moves through the nozzle, to approximately the
nozzle dimensions, and is then spread under the
nozzle face. This results in a processed tow width
narrower than that of the original 6 mm tow. A
cooling manifold is attached to the end effector
with two adjustable ducts for computer-controlled
compressed air flow to locally cool and rapidly
rigidize the newly placed composite. Warlick
et al. developed this cooling system to increase
positional fidelity when tow shearing of continuous
fiber reinforced thermoplastic composite (CFRTC),
using EG/PP, was implemented to enable enhanced
tow steering13
. By switching the cooling air on
and off for localized cooling, this feature provides
advantages of enhanced tow spreading and
consolidation during programmed manufacture.
In order to improve consolidation quality and
uniformity over surface variations and to eliminate
voids, Rodriguez et al.10
included a spring-loaded
system which supports the nozzle. This springloaded
end effector has a spring constant of
approximately 7.5 N/mm which results in the
application of a repeatable consolidation force on
the placed tows. The amount of force applied is
controlled by positioning the nozzle in the build
direction (Z axis) at a height above the previous
position such that a known amount of spring
compression is introduced. To reduce the thickness
of the composite tow and increase the tow width,
it is possible to program a negative height value in
relation to the build plate which results in a greater
spring displacement and a correspondingly greater
force. This enables the nozzle to exert pressure on
the build plate, or on previously positioned tows,
and spread the tows being currently placed. Load
cells in the dual gantry system, supporting the
build plate, are available to monitor the resultant
consolidation force while continuous fiber
placement is underway.
2.2.1 Sample Preparation
System
To manufacture grid
for Nozzle-Based
stiffeners and grid
intersections, a similar approach to that described
by Hogan et al. was used to generate a continuous
print path incorporating loops outside the basic
stiffener profile11
. The benefit of this approach is
that it avoids the need to manually cut and restart
the fibers after each layer is printed as was the
case for the demonstration intersection shown
in Figure 1. This is convenient as multilayer grid
stiffener specimens can take many hours to
produce and the use of a continuous path removes
the need for operator intervention. The deposition
path program and toolpaths were defined using
CAD software. Once the coordinates of the first
layer are generated within the software, the X
and Y coordinates are transferred to FullControl
GCode Designer17
to generate the points for the
subsequent layers of the part without having to
manually change the Z position for every layer. The
layer height, or Z step, was set to a predetermined
0.3 mm. The consolidation of the layers is controlled
by controlling the Z position of the gantry system.
The initial Z position of the system is set by reading
a force output from load cells installed under the
build plate. The value for these tests was set to
11N, enabling the initial layer to effectively tack
to the build plate. With a goal of a uniform grid
height at the intersections, even with the local
doubling of fiber count, the consolidation force, at
the crossover region, the Z step was programmed
to a value consistent with a 22N force. The higher
consolidation force at the grid intersections allows
for the tows to spread out, thus reducing the
individual layer thickness, as depicted in Figure 3.
The specific continuous tool path used to
create the reinforcing grids for the samples of this
work is shown in Figure 4. Only the straight regions
adjacent to the 3 intersections and the intersections
themselves are used in this effort.
Figure 3. Image of spread of commingled yarn: Front view (left) and top view (right).14
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