SAMPE Journal - January/February 2016 - 47

PHD STUDENT DIVISION AWARD ARTICLE
in the time and temperature of an intermediate
dwell, summarized in Table 1. Cycle A is considered
the baseline case, with no intermediate dwell, and is
included to demonstrate the need for a 2-stage cycle
and to show how surface porosity forms when rapid
gelation is induced. Cycle B corresponds to previous
work5, and represents the cure cycle that has been
shown to minimize surface porosity. Cycles C through
G, with "extended" dwells, denote intermediate dwells
that were held until the pressure drop occurred, and
were used to determine the maximum possible hold
time at each intermediate temperature as described
in section on Process Map. Finally, cycle H was
designed using the methods described in this paper,
and represents an attempt at reducing the cycle time
compared to cycle B while achieving similarly low
surface porosity levels.
The lab-scale RTM used for fabricating molded
samples is shown in Figure 1. The main tool body is an
aluminum block containing heating elements, inlet/
outlet ports, thermocouples, and a pressure sensor.
A "picture frame" style spacer plate determines the
thickness (3.2 mm) and in-plane dimensions (76
x 127 mm) of the molded part, and a 20 mm thick
tempered glass plate acts as the second tool-face.
All molded samples were reinforced with 8 layers
of five-harness satin (5HS) carbon fiber fabric (364 g/
m2 areal weight, 3000 fiber/tow count, Sigmatex Ltd.)
stacked in a quasi-isotropic layup. Resin was vacuumdegassed for 45 minutes at 80°C before being loaded
into a pneumatic injector (Radius 2100cc). Injections
were performed at 110°C and 300 kPa, with vacuum
being applied to the mold cavity both before and
during injection. Excess resin was flushed through
the outlet port until no bubbles remained in the
mold cavity, as observed through the glass tool plate.
Injection was completed by closing the outlet valve,
leaving the inlet valve open, and applying a post-fill
hydrostatic pressure of 450 kPa using the regulator
on the injector's compressed air supply. Finally, a USB
microscope was aimed at the window and used to
record time-lapse videos, allowing the exact timing of
porosity formation to be determined.
After de-molding, surface porosity was quantified by
a measure of percent defective area. High-resolution
photographs were converted to binary maps of void
distribution using an open-source image processing
software (ImageJ v1.48). Due to the configuration
of the molding tool (with heating elements on only
one side of the sample), the window-side was always
cooler than the tool-side, and consequently, surface
porosity appeared exclusively on the window-side
SAMPE Journal, Volume 52, No. 1, January/February 2016

of molded samples. Thus, only the window-side was
considered when evaluating the surface quality.
Additionally, to directly measure the magnitude
of the through-thickness temperature gradient,
thermocouple wires were embedded into each side of
the samples cured under cycles A, E, and H, by running
the wires in through the resin outlet port and placing
the tips between the preform and the tool faces prior
to sealing the mold.

Cure Kinetics Model
The heat of the exothermic cure reaction was
measured by DSC. The degree of cure (α), which
ranges from 0 for resin in the uncured state to 1 for
fully cured resin, was computed using a normalized
running integral of the heat release. H(t) represents
the heat flow over time, and the average measured
total heat HT = 473±4 J/g.
(1)
(2)

Cure kinetics of thermoset resins are generally
modeled using a rate equation that contains an
Arrhenius temperature dependence times a function
¦(α), which describes the behavior of the particular
type of reaction under consideration7.
(3)

(4a)

(4b)
Common choices for ¦(α) include nth-order
(Equation (4a)) and autocatalytic (Equation (4b))
expressions, but many variations have been used
to capture more complex phenomena, for example
the inclusion of a diffusion factor to account for the
reduced reaction rate when in the diffusion-limited
regime. However, for resins that undergo multiple
reactions, a single-term expression as shown in
Equation (3) may not suffice to fully describe the cure
kinetics behavior.
The blended resin in this study exhibits a complex
set of overlapping and interdependent reactions,
whose exothermic profile cannot be adequately
described using Equation (3). Modeling the cure
kinetics of a variant of this resin has been previously

Table of Contents for the Digital Edition of SAMPE Journal - January/February 2016