SAMPE Journal - January/February 2023 - 11

(a) Invar tool cleaned with
Loctite® Frekote® PMIC mold
cleaner.
(b) Invar tool treated with
Loctite® Frekote® B-15™ mold
sealer.
(c) Invar tool treated with
Loctite® Frekote® B-15™ mold
sealerand
710-NC™
agent.
release
Figure 1. Standard steps of tool preparation in aerospace composites manufacturing: a) remove
residue and create a clean tool surface with mold cleaner, b) cover surface pores and scratches with
mold sealer, and c) minimize surface free energy with release agent.
INTRODUCTION
Aerospace composites manufacturing is a series
of complex and interdependent processes that
may lead to the formation of residual stresses
in parts and, consequently, process-induced
deformations (PIDs)1-4
. Residual stresses develop
during processing primarily due to a mismatch of
free strains at the micro-, macro-, coupon-, and
component-levels1
. Other factors, however, may
also significantly contribute to the formation of
residual stresses and PIDs, including interfacial
bonding and/or friction between a production tool
and composite part (i.e., tool-part interaction)5-13
.
In the aerospace industry, production tools are
typically prepared with release coatings prior to
part lay-up to mitigate residual stress formation
and PIDs induced by tool-part interaction. Release
coatings also facilitate the removal of cured parts
from tools after processing14
.
Aerospace manufacturers typically use a threestep
process to prepare production tools such as
Invar with release coatings. First, the tool is treated
with solvent-based mold cleaner to remove any
residue from previous processing/handling and
create a clean mold surface (Figure 1a). Depending
on the severity and amount of resin residue on
the tool surface, abrasive cleaning methods (e.g.,
coarse pads) or nonabrasive industrial methods
(e.g., dry ice blasting) may be used in addition
to solvent cleaning. However, abrasive methods
may increase the tool surface roughness and
consequently promote tool-part interaction,
residual stresses, and PIDs. Next, a mold sealer is
applied directly on the tool surface to fill microcavities,
cover scratches, and prevent mechanical
interlocking (i.e., physical bonding) between the
www. sampe.org
tool and part (Figure 1b). Then, several layers
of release agent (RA) are applied on top of the
sealed surface to form a low-energy barrier that
prevents the formation of strong adhesive bonds
and minimizes tool-part friction (Figure 1c)14
.
Mold sealer and RA combine to form an abhesive
and impenetrable release coating between the tool
and part during lay-up and processing. Figure 1
was generated using an Olympus LEXT OLS4100
laser scanning digital microscope on an Invar tool
surface prepared with aerospace-grade Frekote
release coating products.
The aerospace industry typically uses siliconebased
release coatings due to their low adhesion
(i.e., high abhesion) and high thermal/chemical
stability properties. Silicone-based coatings are
usually applied as liquids and cured through
an evaporation reaction to form thin polymer
barriers on the tool surface. During the crosslinking
reaction of thermoset composites, some
release coating molecules transfer from the tool
to the part's surface and become physically and
chemically integrated into its resin network. The
transfer of molecules decreases the thickness of
the remaining coating layer on the tool surface15
.
Elevated temperatures and pressures during
autoclave processing may also cause resin to
bleed from the prepreg onto the tool. If the release
coating has undergone many processing cycles,
some of this resin may infiltrate the tool, stick into
its cavities, and contaminate its surface during the
demolding process. Consequently, each successive
processing cycle results in a further reduction
of
coating
thickness,
increased
to contamination, and decreased overall tool
abhesion properties. The resultant change in
JANUARY FEBRUARY 2023
|
SAMPE JOURNAL |
11
vulnerability

http://www.sampe.org

SAMPE Journal - January/February 2023

Table of Contents for the Digital Edition of SAMPE Journal - January/February 2023