SAMPE Journal - November/December 2021 - 40

FEATURE / LARGE COMPOSITE AEROSTRUCTURES
alignment during extrusion, make determining
a correct compensation value very difficult and
prone to error.
Much work is being done in the industry to
address the CTE concern. Feedstock development,
hardware changes to the AM equipment, and
modeling efforts to better predict complex tool CTE
are ongoing, but none of these have fully addressed
the issue. The study presented in this paper takes a
different, more brute force approach. A hybrid tool
construction is used that begins with an AM shell
structure printed net shape to the final part. The
AM shell is laminated with a conventional tooling
prepreg, which is then cured and machined to final
shape. The AM shell allows for a " tool-less " layup
of the tooling prepreg. The prepreg provides a
more durable and nonporous surface as compared
to the AM shell, and it also physically restrains the
AM core from expanding at temperature. The end
result is a tool shown to exhibit invar-like CTE values
isotropically across the tool face at a significant
cost reduction as compared to traditional tooling.
EXPERIMENTATION
Geometry
Tool Geometry
For this study, a C-spar from an Airbus tooling
development program was selected as an ideal
candidate to investigate this tooling approach. The
C-spar is over 10 meters in length. A representative
3 meter long demonstrator of the C-spar was used
to better match facility capabilities.
A female tool was selected so that the critical
part surfaces were hard-tooled surfaces. The spar
caps had a slight inward angle that would create a
negative draft and tool lock condition in a single
piece tool. This necessitated a split tool design.
Materials
AM Feedstock
SABIC provided the AM feedstock used in this
project. A 20% carbon fiber-filled polyetherimide
(PEI) (Thermocomp™ EX004EXAR1 Ultem®
) was
selected due to the researchers' familiarity with the
material and its proven ability to withstand typical
autoclave cure cycles when used in composite
tooling.
Tooling Prepreg
Hexcel's Hextool™ M81 epoxy tooling prepreg
was selected for lamination of the AM shell. The
material is a 2000 grams per square meter mat
made of chopped carbon fiber impregnated
with Hexcel's 8552 toughened epoxy system. The
material boasts nearly isotropic properties that
Figure 1. Cincinnati Incorporated's Big Area Additive Manufacturing printer.
allows for post-machining without tool movement
due to residual stress relief. It is a proven material
for composite tooling. A bismaleimide matrix
version of Hextool is also available, but with the
limited expected lifespan of this tooling, the extra
cost for this material was deemed unwarranted.
Equipment
Additive Manufacturing Equipment
Cincinnati Incorporated's (CI) Big Area Additive
Manufacturing (BAAM) 1000 equipment was used
in this study to print the thermoplastic composite
tooling (Figure 1). BAAM was the first among the
large
format
additive
manufacturing
(LFAM)
platforms now on the market, and it has been used
for countless aerospace tooling demonstrations.
BAAM printers at CI (Harrison, Ohio) and Additive
Engineering Solutions (AES) (Akron, Ohio) were
used during this program.
Figure 2. Test panel with Hextool lamination and
metal dowels installed. Y-direction is the printed
in-flow direction, while the x-direction
printed cross-flow direction.
is the
40 | SAMPE JOURNAL | NOVEMBER/DECEMBER 2021
www. sampe.org
http://www.sampe.org
SAMPE Journal - November/December 2021

Table of Contents for the Digital Edition of SAMPE Journal - November/December 2021
