SAMPE Journal - July/August 2016 - 32

Feature Article
control strips are simple unidirectional laminates
used to measure the bulk factor of placed material.
Full consolidation correlates to a 0% bulk factor.
There are three independent variables changed
to determine their effect on consolidation/bulk
factor: 1) machine infrared (IR) heater settings
2) lay-up speed, and 3) compaction force. In
addition, compaction force can also be impacted
by changing the roller durometer (hardness) on the
AFP machine. Since the temperature and relative
humidity affects the tack and consolidation of the
material, they need to remain fixed at nominal
values for these trials.
Evaluation of AFP process parameters is
accomplished by laying multiple process control
strips, each changing one of the independent
variables listed above, to correlate a bulk factor
impact with each parameter. The strips are onecourse wide and eight plies thick. The thickness of
each strip is measured at six locations throughout
the strip, avoiding the beginning and ending.
Thickness is measured using a micrometer while
gently gripping the process control strip to avoid
deforming the prepreg stack. The total thickness
of the strip is divided by the number of plies to
obtain the average measured ply thickness (MPT).
The average thickness of each strip is then used to
calculate the bulk factor of each strip.
Using these measurements, a material specific
processing parameter envelope can be established
for panels at the lowest possible bulk factor. A matrix
of test panels is built at these process conditions to
validate the process envelope.
Different processing envelopes apply to
monolithic versus sandwich structure. Because

the core material acts as a thermal insulator, heat
does not dissipate through the material when laying
up the plies over the core as it would on most
metal or composite tool surfaces. Therefore, fiber
placement over core will require development of
different process parameters (e.g. heat and speed
settings) to ensure successful fabrication and avoid
imperfections such as material blistering.
AFP developments, as described in this paper,
emphasize the critical impact precise application of
high nip temperatures have on the manufacture of
quality OoA composites. For this reason Lockheed
Martin has continuously worked to improve the
heater system on the Mongoose machine as well
as investigate advances in heating technology
permitting revolutionary improvements to AFP
productivity and panel quality. To that end, we also
conducted internally funded assessments of laser
based heating technology.
Bag Technique and Cure Methodology
Having established the AFP process parameters
for flat monolithic and sandwich core parts, the next
step is optimizing the bagging and cure procedures.
While the objective in processing OoA materials
with fiber placement is to use the AFP machine
to consolidate-on-the-fly, any potential bagging
and cure strategies leading to further reductions in
porosity in the cured parts should also be explored.
Bag Technique Optimization. Evaluation of bagging
techniques is conducted on flat panels where
the focus is on promoting evacuation of air and
volatiles in the z-direction to produce the least
amount of porosity. Procedures designed to aid

Figure 3. Bridging is a common
defect at AFP joggles resulting
in extensive labor hours
to repair.

SAMPE Journal, Volume 52, No. 4, July/August 2016

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