Aerospace Resin Transfer Molding, A Multidisciplinary Approach publication

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AEROSPACE RESIN TRANSFER MOLDING, A MUL TIDISCIPLINARY APPROACH
Dimitrije Milovich, Principal Investigator
Ron H. Nelson, President
Radius Engineering and Tooling, Inc.
Salt Lake City , Utah
Presented at: "Resin Transfer Molding for the Aerospace Industry"
March 6-7, 1990
Radisson Plaza Hotel
Manhattan Beach, California
Copyrighted by SME; Technical paper number to be assigned

Aerospace Resin Transfer Molding, a Multidisciplinary ApproachRadi:JsEngineering and Tooling, Inc.
ABSTRACT
Aversatile ResinTransfer Molding (RTM) process for producing high quality aerospace composite laminates
has been developed and demonstrated in this study. High-fiber-volume. low-void-content components have
been fabricated with a standard high temperature prepregging epoxy resin. Control of process conditions,
including temperature, pressure and flow rate has been developed. In addition, geometry changes in the
mold have been incorporated to enhance laminate quality.
Analytical programs for off-line and real-time process modelling and control have been developed. This
modelling is based on the resin thermokinetics and viscosity behavior; and the time. temperature, pressure
and flow history of the resin throughout the RTM delivery system and preform. Accurate resin gelation
predictions were made and correlated experimental measurements inside the tooling. Also. flow tracking
sensors have been designed and incorporated into the tooling to monitor resin propagation throughout the
mold in real time during the RTM procedure. A process modeling and control workstation has been
developed which utilizes the above hardware and software for optimizing process parameters. Hence, this
workstation provides process control for development articles prior to costly process scale up.
Severaldemonstration and process study articles were made incorporating flat and curved geometries. Non-
destructive and subsequent mechanical testing indicates that the parts are equal to hand layup, autoclave-
cured composites utilizing the same fiber and resin systems. This, combined with the potential throughput
of the RTM process should provide lower cost processing alternatives to current manufacturing techniques
for advanced composite structures.
INmODUCTION
The Resin Transfer Molding (RTM) process has been used in the past primarily for the production of lower
performance composite structures. These structures typically have low fiber volumes and use relatively low
temperature matrices such as polyester resins. Advanced composite structures constructed with high temperature
resins require high fiber volumes with low void contents to be qualified for aircraft service.
The primary problem limiting the use of the RTM process for use on advanced composites structures is obtaining
high fiber volumes when using the high temperature thermosetting matrix materials. Most thermosetting polymer
matrix materials (i.e. epoxies, BMI's, poiyimides) exhibit a higher processing viscosity when formulated for higher
temperature service[1,2]. This increase in viscosity significantly increases the difficulty of resin transfer molding with
the higher temperature resins typically used in aerospace composite parts. The problem of the viscosity increase
is compounded by a decrease in the permeability of the fiber preform to resin flow at higher fiber volumes. The
permeability of the preform is cut in half when the fiber volume increases from 56% to 62%[3].The primary deterrent
to successful resin transfer molding of advanced composite structures is obtaining adequate flow and distribution
of the resin Into the fiber preform without forming voids or porosity.
Radius Engineering's approach to resin transfer molding utilizes variable mold cavity geometry to increase fiber bed
permeability during impregnation, followed by compaction of the laminate while maintaining high hydrostatic fluid
pressure during the high temperature cure to minimize voids. This technique uses innovative tooling and process
control, and is augmented by sophisticated process monitoring and analytical modelling.
RTM PROCESS DEVELOPMENT GOALS
The goals of this project were to fabricate laminates with the following characteristics:
1. High fiber volumes using high temperature thermosetting resins
-Greater than 60% fiber volume
-Use standard 350°F cure prepregging epoxy resin (1"9 > 410°F)
2. No voids or porosity
-Less than 1% void or porosity content
Uniform fiber volume with no fiber waviness or washout due to motion during the process3.
Mechanical properties equal to high quality autoclave cured prepreg laminates4.
All of these goals were met in this development program.
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Radius Engineering and Tooling, Inc. Aerospace Resin Transfer Molding, a Multidisciplinary Approach
BACKGROUND ON PROCESSING OF EPOXY MATRIX COMPOSITES
Severalcontracts have been awarded by the Air Force Materials Laboratory to large airframe manufacturers to study
autoclave processing of epoxy matrix composites. The "Processing Science of Epoxy Resin Composites" contract
studied the causes and solutions to void formation during cure and identified the most likely cause as water vapor
in the resin[4]. The "Exploratory Development on the Processing Science of Thick-Section Composites" developed
cure cycles for extremely thick AS4/3502 laminates[5] and the "Computer Aided Curing of Composites" contract
developed computer models of autoclave processing to optimize autoclave through-put and minimize defect
formation[6].
The autoclave process and the physical phenomena occurring during the cure of an epoxy matrix composite have
been studied in detail and analytically modeled to optimize the structural performance of the resultant laminates and
maximize autoclave through-put [3,6, 71819,1O,11,12] .This previous work allows intelligent process changes to be
made and simplifies the development of new processes because of the greater understanding of the cure process
and the ability to analytically simulate the effect of process changes on the resultant laminate quality. AJtemative
processing techniques using out-of-autoclave cures are the next step for obtaining higher production rates and lower
costs.
There are two key steps during a typical autoclave cure cycle which produce high laminate quality. The first step is
an intermediate temperature hold used with a vacuum to remove volatiles such as water vapor, and pressure applied
to compact the laminate and bleed resin from the laminate. The second step is the high temperature hold when the
vacuum is vented to the atmosphere to increase the hydrostatic pressure in the resin and the autoclave pressure
is raised slightly. Hydrostatic pressure in the resin must be maintained above a critical level during this step to
prevent void formation due to the expansion of volatiles. Eliminating voids and porosity in the laminate depends
primarily on removing volatiles from the laminate before the end of the intermediate hold and/or suppressing their
growth at later stages of the cure by maintaining a high hydrostatic pressure in the resin.
A similar hydrostatic pressure which can be created during the RTM process was developed by this project to
suppress void formation. The presence of volatiles in the resin was minimized by vacuum degassing of the resin prior
to injection which served the same purpose as the vacuum in the intermediate hold of an autoclave cure. This step
can be accomplished much more thoroughly prior to the molding cycle in a much shorter time than the autoclave
hold. This results in shorter overall cycles and lower void contents. The vacuum applied during the resin
impregnation into the mold is primarily for avoiding entrapment of air in the laminate, and to dry the preform.
The advanced RTM process contains many features which are designed to duplicate key parts of processes
developed in previously AFWAL funded processing science contracts. The thorough degassing of the fiber preform
and resin prior to consolidation is similar to the emphasis which was placed on this aspect of the process during
the "Manufacturing Technology for Nonautoclave Fabrication of Composite Structures" program sponsored by the
Air Force Materials L.aboratory[13].This project used special tooling (termed tooling "hardbacks") to apply a vacuum
to a layed-up prepreg laminate without applying any mechanical load to the laminate. This allowed thorough
degassing of the laminate, and allowed the subsequent cure to be performed under only vacuum pressure
(atmospheric pressure).
The emphasis on maintaining a high fluid pressure in the mold cavity during the advanced resin transfer molding
process is derived in part from work performed during the "Processing Science of Epoxy Resin Composites"
contract[4]. One processing variation developed during the this contract was the "bagless" cure. This cure consisted
of eliminating the vacuum bag during the second half of the cure in order to produce a uniform and high hydrostatic
fluid pressure in the resin. The initial compaction and debulking of the laminate during the intermediate hold was then
performed in the traditional fashion. This required essentially two autoclave cures, but it did reduce void content.
Another processing variation (pressure bag technique) involved applying pressure to the vacuum port outside of the
autoclave which is connected to the vacuum bag on the part in the autoclave. The pressure was applied during the
second half of the cure, and had the same effect of increasing the hydrostatic fluid pressure in the resin.
ANALYTICAL BASIS OF THE RESIN IMPREGNATION PROCESS
The flow of thermosetting resins through beds of reinforcing fibers have been studied in detail and several analytical
models of the process are available[3,6,8, 14,15,16] .Resin flow through a laminate can be accurately predicted using
analytical models for flow through a porous media or D'arcy's law. Fiber preform permeability data has been
obtained for varying fiber volumes and agrees with predictions on the effect of fiber volume changes on
permeability[15] .The permeability of the fiber is basically dependent on the wetted surface area and free volume for
resin flow. Thermokinetic and viscosity models have been developed and are available for several prepregging
resins[6,7]. These models allow the prediction of degree of cure and viscosity as an arbitrary function of time and
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Aerospace Resin Transfer Molding, a Multidisciplinary ApproachRadius Engineering and Tooling, Inc.
temperature history imposed. These models allow an accurate prediction of the degree of cure, viscosity, and
gelation point[11 ] of all the resin in an RTM process. This information can be used to predict resin flow for different
processing parameters.
Using D'arcy's law for one-dimensional flow through a porous media and conservation of mass, the time (t) required
for the resin front to move a distance (x) in the mold can be derived and is shown in equation [1] below. The
viscosity was assumed to be constant for simplification of the governing equations. This allows the importance of
the various processing variables to be evaluated.
[1]t = ~ / (2PkJ where: u = resin viscosity
kx = permeability of fiber bed
p = pressure differential
As seen in equation[1] above. the time required increases as the square of the distance traveled. The permeability
of the fiber bed can be calculated using a model developed by Gutowski at M.I.T.[3].
[2]k,. = (r /4kJ(1-VJ3 /Vf2 where: r = fiber diameter
ko = permeability constant
vf = fiber volume fraction
Equation [2] illustrates the relatively large changes in permeability of the fiber bed with small changes in fiber
volume. Table I below lists the change in permeability of the fiber bed relative to a 62% fiber volume laminate and
illustrates the drastic reduction in permeability which occurs at high fiber volumes. The high permeabilities in low
fiber volume laminates allow successful RTM using conventional. low performance processes.
Table I. Change in Permeability of Fiber Bed with Fiber Volume[3].
The ability to successfully obtain high fiber volumes depends on creating pathways for resin flow and reducing the
resistance to resin flow in the laminate during the impregnation. This was accomplished in this project by slightly
overfilling the cavity with resin during the initial impregnation. Then the subsequent compaction debulked the
laminate prior to gelation. Uniform resin content in the laminate was obtained during the compaction step by resin
flow within the laminate and excess resin flowed into specially designed resin flash cavities.
TECHNICAL APPROACH
Resin transfer molding is more complex than the prepreg autoclave cure system because an entire sequence of
steps in the manufacturing process is bypassed and incorporated into the final assembly sequence. The step of
forming the prepreg material from the fiber and resin is eliminated. The additional complexity can be compensated
for with additional process monitoring and control equipment. In addition to higher production rates, lower material
costs, and lower unit costs, resin transfer molding allows greater control of the process by allowing separate
degassing of the resin and drying of the fiber to eliminate water and air, independent control of the hydrostatic
pressure in the resin, and mechanical pressure applied by the faces of the tooling.
Our successful approach to resin transfer molding is based on three primary technologies: innovative tooling and
hardware to control impregnation and debulking, sophisticated process monitoring and data acquisition to allow
more intelligent changes to be made to the process during the development, and multi-level analytical process
modeling to aid the process design.
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Radius Engineering and Tooling, Inc. Aerospace Resin Transfer Molding, a Multidisciplinary Approach
temperature history imposed. These models allow an accurate prediction of the degree of cure, viscosity, and
gelation point[11] of all the resin in an RTM process. This information can be used to predict resin flow for different
processing parameters.
Using D'arcy's law for one-dimensional flow through a porous media and conservation of mass, the time (t) required
for the resin front to move a distance (x) in the mold can be derived and is shown in equation [1] below. The
viscosity was assumed to be constant for simplification of the governing equations. This allows the importance of
the various processing variables to be evaluated.
[1]t = ~ / (2PkJ where: u = resin viscosity
k,. = permeability of fiber bed
p = pressure differential
As seen in equation[1] above. the time required increases as the square of the distance traveled. The permeability
of the fiber bed can be calculated using a model developed by Gutowski at M.I.T.[3].
[2]kx = (f2/4kJ(1-VJ3/V,2 where: r = fiber diameter
ko = permeability constant
v, = fiber volume fraction
Equation [2] illustrates the relatively large changes in permeability of the fiber bed with small changes in fiber
volume. Table I below lists the change in permeability of the fiber bed relative to a 62% fiber volume laminate and
illustrates the drastic reduction in permeability which occurs at high fiber volumes. The high permeabilities in low
fiber volume laminates allow successful RTM using conventional. low performance processes.
Table I. Change in Permeability of Fiber Bed with Fiber Volume[3].
The ability to successfully obtain high fiber volumes depends on creating pathways for resin flow and reducing the
resistance to resin flow in the laminate during the impregnation. This was accomplished in this project by slightly
overfilling the cavity with resin during the initial impregnation. Then the subsequent compaction debulked the
laminate prior to gelation. Uniform resin content in the laminate was obtained during the compaction step by resin
flow within the laminate and excess resin flowed into specially designed resin flash cavities.
TECHNICAL APPROACH
Resin transfer molding is more complex than the prepreg autoclave cure system because an entire sequence of
steps in the manufacturing process is bypassed and incorporated into the final assembly sequence. The step of
forming the prepreg material from the fiber and resin is eliminated. The additional complexity can be compensated
for with additional process monitoring and control equipment. In addition to higher production rates, lower material
costs, and lower unit costs, resin transfer molding allows greater control of the process by allowing separate
degassing of the resin and drying of the fiber to eliminate water and air, independent control of the hydrostatic
pressure in the resin, and mechanical pressure applied by the faces of the tooling.
Our successful approach to resin transfer molding is based on three primary technologies: innovative tooling and
hardware to control impregnation and debulking, sophisticated process monitoring and data acquisition to allow
more intelligent changes to be made to the process during the development, and multi-level analytical process
modeling to aid the process design.
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TOOUNG AND HARDWARE DEVELOPMENT
Integrally heated tooling was used for the flat plate tools, which were machined from aluminum, as well as the tooling
for the subscale radome, which was fabricated using Radius Engineering ArctoolTM composite tooling technology.
This provided rapid uniform heatup and close control of the process temperatures. Arctools TMare made by thermally
spraying metal on a master pattern and then reinforcing the resulting metal shell with a conductive isotropic
composite backing material. Figure 1 shows the thermal spraying step and Figure 2 shows a typical set of matched
tooling which can be made economically and is well-suited for RTM.
Resin Delivery System
High performance prepregging epoxy resins, such as the Fiberite 976 used in this work, are generally supplied as
pre-formulated, single-component materials which are solid at room temperature and require heating to
approximately 200°F to reduce the viscosity to a processable level. Therefore, commercial equipment designed to
mix and dispense resins and curatives was not required. The delivery system used a dual hydraulic cylinder
arrangement. One cylinder, the injection ram cylinder, held the resin and the second cylinder supplied the force to
inject the resin. The injection ram cylinder was modified to have teflon components or teflon coated components
in contact with the resin. The power cylinder was modified to accurately control resin delivery pressure and flow rate.
The ram injection cylinder has a low thermal mass and high conductivity for precise control of the resin temperature.
A relatively long cylinder was chosen so that the resin would be in contact with a large surface area which reduced
the chance of resin exotherm. The resin delivery line connecting the resin injection cylinder head and the mold
injection port was lead through a heater and was disposable.
Tooling Proximity Sensors
One critical aspect of successful resin transfer molding is control of the cavity geometry (mold spacing) in order to
allow efficient resin movement through the fiber bed during injection. and to compact the laminate during cure to
the desired fiber volume. The proximity sensing system used on the tooling was invaluable for monitoring and
controlling the mold surfaces relative to each other. The sensors provided a reliable. non-contact method for real-
time mold distance measurement that was displayed on the computer screen. Special integrated circuit sensors
were assembled to perform at the 350°F operating temperature.
Variable Cavity Mold Seals
A proprietary methOOof mold cavity sealing was developed to provide a controllable seal which can be turned on
or off at different points during the injection cycle. and to provide sealing over a wide range of cavity separations.
This allows another means of controlling hydrostatic resin fluid pressure in the laminate.
Mold Separation Device
The pneumatic mold separation device performs two functions during the advanced resin transfer molding process.
First it holds the mold open during injection step, and then it allows the molds to close during compaction steps.
The load which must be carried by the mold separation device is dependent solely on the area of unsupported cavity
(approximately 15 psi. times the cavity area). The maximum force required to hold the mold open during injection
(15 psi. times the cavity area) is approximately one eighth the load exerted during the final compaction
(approximately 125 psi. times the cavity area).
PROCESS DATA ACQUISITION SYSTEM DEVELOPMENT
One advantage of resin transfer molding over autoclave processing is the more accurate monitoring and control that
is possible. Since the autoclave is eliminated, the process instrumentation, monitoring, and control becomes easier
because the monitoring and control equipment can be located physically close to the tooling and not be exposed
to the hostile environment inside an autoclave. The basic nature of the RTM process allows much greater control
over the process since the resin impregnation and cure occur all in one step. For example, hydrostatic pressure in
the resin during cure is only indirectly controlled in an autoclave. The temperature and pressure cycles, and auxiliary
bleeder materials used in an autoclave cure typically apply a mechanical pressure to the laminate and fluid pressure
is dependent on the amount of flow which occurs. By contrast, this work demonstrated that resin pressure can be
directly controlled during resin transfer molding with injection ram pressure, mold seal actuation, and cavity spacing.
Temperature control in the heated tooling used with resin transfer molding is more accurate and controllable than
that achieved in an autoclave during heatup and cooldown. The gas flow inside an autoclave makes the part heat-
up temperature dependant on gas flow profiles inside the autoclave which change as different tool loads are used.
Such intimate process control provides an opportunity for improved quality and production rate.
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A computerized data acquisition system for real time processing monitoring was designed and assembled. The
system allows the following data to be monitored and stored simultaneously in a personal computer:
1.
2.
3.
4.
5.
6.
Mold and resin delivery system temperatures using standard thermocouple input
Mold cavity pressures using standard pressure transducers
Control system pressures for various actuators. clamping press. and vacuum system
Mold closure spacing to within ..::t..002 inches using Hall effect proximity sensors
Resin injector ram displacement using a linear potentiometer
In-mold resin position sensing
A sequential display of data acquisition screen display over several minutes can be seen in Figure 3. Figure 4 shows
a X-V plot of the pressure data which was stored during an injection for later analysis.
Resin Position Sensors
The presence or absence of the resin in the laminate is a major resin transfer molding process variable which differs
from autoclave processing. Aerospace quality parts require high temperature, high pressure tooling that cannot be
made transparent. Optimum design of the tooling and process sequencing for the development of new composite
parts using RTM requires information on resin flow inside the tool during the process. Determination of resin flow
paths without some form of real time monitoring inside the mold is based largely on speculation. There are several
requirements for afunctional and cost effective in-situ resin sensing system. The cost of installing and using it must
be offset by the reduced cost of fewer trial parts produced during development of new structures.
A unique low cost method for continuously detecting the level of resin impregnation at 96 different locations in the
mold was developed. based on local sensing of the dielectric properties of the materials in the mold. The sensors
can be embedded in the tooling during or after tool fabrication. The sensors are capable of sensing three distinct
levels of resin impregnation of the fiber bed at up to 96 locations in the mold. This provided 96 sensors on the 9"
x 13" plate. and 37 sensors on the 12Mspherical radome section. The computerized data acquisition system and
resin position sensing subsystem can scan and display all 96 locations in approximately 1 to 2 seconds while
simultaneously performing all the other data acquisition and real time analytical process modelling. A sequential
display of the resin flow can be seen in Figure 3.
ANALYTICAL PROCESS MODEL DEVELOPMENT
Proprietary analytical process models have been developed to predict resinflow and pressure distribution throughout
the injection system and laminate during injection. The models incorporate the thermokinetic and viscosity behavior
of the resin, the injection system and tool geometry, the layup, the system temperatures, and the prescribed injection
pressure/cavity separation during the injection.
Analytical process modelling was conducted to aid in the selection of the optimum processing parameters. These
parameters include: selection of temperatures of the various tooling elements, initial cavity spacing, injection pressure
sequencing, sequencing of degassing and injection, and compaction sequencing. The analytical modeling was
performed at three distinct levels.
Thermal Profile Evaluation
The First Level analytical modelling involved relatively simple analytical modeling and prediction of the resin
thermokinetics and viscosity during an arbitrary time-temperature profile defined by the user. The user can estimate
the thermal history that the resin will be exposed to, and predict its viscosity profile during the "cycle". An example
of a thermal history is: time and temperature for low temperature degassing in vacuum bell jar, time and temperature
for high temperature degassing in the injection cylinder, injection time, time and temperature in the mold cavity after
injection. Different thermal profiles can be quickly evaluated to allow selection of the various process temperatures
and times.
The viscosity and thermokinetic models were obtained from McDonnell Aircraft Company, and were developed as
a part of the "Smart* cure program [6] under AFWAL funding. The model was developed for Hercules 3502 resin
which is very similar to Fiberite 976 the resin used in the project. The analytical viscosity predictions were compared
to test data for Hercules 3502 for a step-hold cure, and to several tests conducted by Fiberite at our request on
straight ramps and step-hold cures. This comparison allowed the 3502 model to be used with reasonable confidence
for the 976 resin. The Fiberite viscosity testing data and the analytical model predictions provided valuable
information in selection of the processing temperatures and times at the various steps in the process. An independent
prediction for the gelation point was calculated with the Hoffrnan-Boll gelation model [11 ]. The use of a second
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model for predicting gelation provided greater confidence. Example Output from the Thermal Profile Evaluation
program is shown in Figure 5.
Real Time Analytical Process Modelling
The Second Level analytical modelling was a real time adaption of the First Level model. The thermokinetic, viscosity,
and gelation models were added to the real time data acquisition and display system discussed previously. The
temperatures at the thermocouples were used as input to the model which then tracked the degree of cure, viscosity,
and percentage gelation of resin at each thermocouple location. These parameters were stored and displayed on
the computer screen real time (Figure 3). The system was improved to actually track the resin temperature, degree
of cure, viscosity, and percentage gelation as the resin moves through the system. The program takes the degree
of cure and percentage gelation (the only true path dependent variables which must move with the resin) from the
injection cylinder and transfers them to the mold cavity when the ram displacement is moved a predetermined
amount. The net result from the Second Level Analytical Model is that the control operator is provided with an
estimation of the resin viscosity throughout the injection system. This information is valuable for making processing
decisions "on the fly" as the process is occurring. Real time process sequencing decisions were made during the
process development on the flat plates and the subscale radomes, and will probably be made during the process
development phase of any new structure. Figure 6 is an X-Y plot of the Real Time Analytical Process Modeling output
which was stored during an injection for later analysis. The viscosity and gelation model output shown in Figure 6
indicate that resin gelation was predicted to occur at approximately 52 to 56 minutes. The resin fluid pressure
measured in the mold cavity for the same injection is shown in Figure 4. The cavity fluid pressure dropped rapidly
at approximately 48 to 53 minutes with very little change in any of the other system process parameters such as
temperatures and pressures. The most likely cause of resin fluid pressure drop is chemical shrinkage of the epoxy
resin occurring near resin gelation. The resin must have had a sufficiently high viscosity to prohibit resin flow into
the recessed pressure transducers as the resin underwent a volumetric chemical shrinkage.
Integrated Thermokinetic, Viscosity, and Resin Flow Model
The Third Level analytical process model consisted of an integrated thermokinetic, viscosity, and resin flow model
for the entire system. A graphical representation of the model is shown in Figure 7. The system was modelled as
three separate sections, 1.) the injection cylinder, 2.) the injection line, and 3.) the mold. The user enters the
geometry and temperature for each section. The number of plies of fabric, the areal fabric weight, the fiber density,
and fraction of fibers oriented in the lengthwise direction of the cavity are also entered. The applied pressure is then
input and the computer program uses a finite difference solution to predict the one dimensional resin flow throughout
the system as the resin is injected. The resin is subdivided into a large number of volume elements whose degree
of cure and viscosity are tracked through the system. The position and temperature of each volume element are
tracked as they move from one section to another. Each section is also divided into spatial or distance elements.
One volume element is equal to one distance element throughout the system (the distance elements vary in length
through the system). The resistance to flow and pressure drop throughout the system from the injection cylinder to
the free edge of the resin in the cavity is calculated at each time step.
The resistance to flow in the cavity is calculated by one of two means determined by the user. The flow resistance
in the actual fiber bed is calculated with a solution for the permeability of the unidirectional fiber bed in the axial
direction using a model developed by Gutowski at M.I.T.[3]. This solution was compared to several other solutions
[17,18,19].The permeability of the fiber bed transverse to the fibers is approximately two orders of magnitude higher
than in the fiber direction and therefore the flow in these layers was neglected. The total flow in the mold cavity is
calculated by the program, depending on whether the user selects the "no gap" or "gap" option. The "gap" option
was added because work at McDonnell Aircraft on the "Smart" Cure program [19] has indicated that a significant
amount of resin flow occurs between layers rather than through the fiber bed. These conclusions [ibid.] were
reached after extensive analytical modeling and testing of the prepreg autoclave cure process. The final version of
the "Smart" cure program developed by McDonnell Aircraft assumes that 10% of the resin in the prepreg is initially
contained in the area between the plies, and flows horizontally through the laminate. The analytical modeling work
at Radius Engineering, when coupled with observations of flow actually observed in cavity with resin position sensing
system, suggests that resin must be flowing in thin layers or gaps between the fiber bed and the mold. This
conclusion seems to agree with the McDonnell Douglas work. Graphical Outputfrom the model is shown in Figures
8, 9 and 10. Several different model runs are shown in each Figure to illustrate the effect of varying one input
parameter. The Figures plot the advancement of the leading edge flow front as function of time. The flow rate
decreases rapidly with time (constant injection pressure) due to the increasing channel length and increasing
viscosity as the resin polymerizes. Figure 8 illustrates the relatively short lengths the resin is predicted to travel if the
"no gap" assumption is made. Figure 9 in contrast, shows the resin can flow much further if some channel flow is
assumed to occur between the fiber bed and the mold, ie. "gapMoption. Figure 10 illustrates the effect of different
mold temperatures. In general, increasing the temperature produces greater flow until approximately 330°F.~
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Aerospace Resin Transfer Molding, a Multidisciplinary ApproachRadius Enghleering and Tooling, Inc.
PROCESS DEMONSTRATION ARTICLE FABRICATION
Two tools and the sophisticated data acquisition system were constructed to develop the process and fabricate
components. A flat cavity mold was first used to fabricate plates. These plates underwent extensive physical and
structural testing to verify that the structural and physical properties obtained in parts manufactured with the
advanced RTM process were equal to those manufactured in the prepreg autoclave cure manufacturing process.
A three dimensional mold was used for fabrication of subscale radomes. The subscale radome was fabricated to
verify that more complex three dimensional shapes which are more representative of actual aircraft structural
components could be fabricated with the process.
Materials Selection
A standard uncatalyzed 350°F cure prepregging epoxy resin based on the Ciba Geigy MY-720 epoxy was desired.
Discussions were held with chemists form the various resin suppliers to assess the differences between Hercules
3502, Narmco 5208, and Fiberite 976. The 976 resin was chosen and samples were obtained.
Hercules AS4 fiber in the form of the A370-5H fabric was chosen as the baseline fiber perform material for the flat
plates which would undergo the extensive testing. This material was chosen because it would allow direct
comparison of the flexural strength test data from plates fabricated with our advanced resin transfer molding process
to data available from Hercules on hand layed-up autoclave cured prepreg laminates constructed of AS4 A370-5H
fabric and 3502 resin. The Hercules data was obtained from internal Hercules testing and reported in their marketing
literature with a description of the test method, number of samples, and standard deviation [20].
Flat Plate Testing
Twelve plates have been fabricated with the high temperature Fiberite 976 resin. One of the plates was unidirectional
graphite and one was a balanced [0°/90j plate constructed from unidirectional fiberglass layers. The other ten plates
used the AS4 A370-5H graphite preforms. Plate #3289 used the unidirectional E-glass preform to allow visual
inspection of the plate. Visual inspection of the plate for voids is possible by back lighting it with a bright light, even
though the 976 resin is relatively dark. The last eight of these 976 plates were subjected to various quantitative
physical inspection, physical testing, and structural testing as shown in Tables II, 111,and IV. All the flat plates were
9.125 inches by 13.125 inches, and ranged from .062 inches to .079 inches thick. A typical plate is shown in
Figure 11.
Table II. Test Matrix for Physical and Mechanical Testing of Flat Plates.
Fiber Volume and Void Content Testing
The fiber volumes and void contents were measured on four of the plates using the acid matrix digestion technique.
Two samples were taken from each of the four plates and tested. The testing was performed at Brigham Young
University. A summary of the acid matrix digestion fiber volume and void content testing is shown in Table III along
with the fiber volume determined with two alternative methods. The two alternative methods agree fairly well with
the acid matrix digestion testing.
The 67% fiber volume plate had a void of .28%, which makes it an exceptionally high fiber volume/low void content
plate. All the average void content measurements are under .33%, which is very low. These low void content
measurements are collaborated by the ultrasonic inspection. Each acid digest test covers an area of approximately
1 inch by 2 inches. Two tests were performed on each plate to test different areas of the plate. The ultrasonic
inspection covers the entire plate, and when combined with the two local void content tests, provides a high degree
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Aerospace Resin Transfer Molding, a Multidisciplinary ApproachRadills Engi,qeeringand Tooling, Inc.
of certainty that the plates have no areas of high void content.
Three plates were tested using a pulse-echo ultrasonic inspection technique by ICI Fiberite Composite Materials. The
ultrasonic inspection was performed to establish that the entire area of these three plates were free of voids or
porosity, and that the acid digest test results which were performed on relatively small samples. were indicative of
all the areas of the plates. The majority of the area in all the plates had an attenuation of less than -2 decibels with
only a few very small spots showing attenuation greater than -4 decibels. These results indicate that the plates are
of very high quality, with essentially no voids or porosity.
A Photomicrograph study was performed on 4 plates. Two 1 inch by 2 inch specimens from each plate were
examined, one near the center, one near the comer. The specimens were mounted and polished, and then examined
at SOx,1OOx,2OOx,and 25Ox magnification. The primary feature of interest were the presence of voids, porosity, and
the quality of fiber wetting. No voids or porosity were found in any of the specimens and the laminate quality did not
appear to vary from the center to the comer specimens. The fiber appears thoroughly "wet out" with resin by
examination at 25Ox magnification, even in the 67% fiber volume plate as shown in Figure 12.
Table III. Fiber Volume and Void Content Tests Using Acid Matrix Digestion
Brigham Young University-College of Engineering-Technology Department
Fiber Volume and Void Content Testa Using Acid Matrix Digestion
Plate' 3289 3389 3689 3789 3989 31089 31489 31789
Specimen 1 Volume Fraction ---48.24' ---65.17' 64.57' 64.43'Void Content ---0.37' ---0.31' 0.19' -0.31'Specimen 2 Volume Fraction ---49.84' ---68.81' 65.31' 64.09'Void Content ---0.29' ---0.24' 0.19' -0.09'Average Volume Fraction ---49.04' ---66.99' 64.94' 64.26'Void Content ---0.33' ---0.28' 0.19' -0.20'* ASTM Test Methods D792, D3171, D2734
Fiber Volume Fraction Calculated Usinq The Fabric Areal Weiqht Method
Average Fiber Volume 53.80' 51.01' 64.12' 66.55% 63.32% 62.06% 64.77% 59.39%
Fiber Volume Fraction Calculated Usinq The Component Weiqht Fraction Method
Average Fiber Volume 54.71% 50.10% 64.58% 66.70% 64.23% 61.13% 66.30% 61.60%
Flexural Strength Testing
Aexural testing was conducted In-house at Radius Engineering, and at the University of Utah -Engineering
Experiment Station. The test results obtained from in-house testing agreed closely with those obtained from the
University of Utah. The flexural strengths from all the plates were normalized to 0.014 in/piy, or 58.5% fiber volume,
for comparison to prepreg autoclave cure data. The highest recorded unnormalized strengths was 187 ksi. Two
piates had average normalized strengths of over 150 ksi. For comparison, the average flexural strength quoted from
the Hercules literature for AS4 A370-5H/3502laminates is 147. ksi., and the average strength quoted by Fiberite for
T300/976 (T300 in a 370 gr/m2 areal weight fabric and 5 harness weave) is 121. ksi. [21]. The flexural strengths for
the resin transfer molded plates and comparative data for autoclave cured prepreg laminates is listed in Table IV
and shown in Figure 13.The variation in plate to plate strength is probably due to layup sequence of the warp sides
of each layer. The 5 harness weave cloth is inherently unsymetric because the warp fibers are located predominately
on one side of the fabric. The flexural laminate strength appeared to be dependent on the sequencing of the warp
sides in the layup.
Subscale Radome Fabrication
Subscale radome tooling was designed and fabricated after the fabrication and testing of the flat plates. The inner
mold half which also contained 37 flow sensors is shown in Figure 14. The subscale radome is 12 inches in diameter
at the base, 3.5 inches high in the center, and has a constant radius of curvature of 7 inches as shown in Figure 15.
The subscale radomes varied in thickness from .080 inches to .097 inches and had an average fiber volumes ranging
from 47.3% to 57.4% determined with fabric areal weight, fiber density, and finished plate thickness. The first three
subscale radomes are constructed of 10 plies of 7781 weave E glass supplied by Hexcel with the F16 finish. The
fourth subscale radome was constructed of 11 plies of the same fabric. The average thicknesses and fiber volumes
are listed in Table V.
8

Aerospace Resin Transfer Molding, a Multidisciplinary ApproachRadius Englqeering and Tooling, Inc.
Table IV. Comparison of Aexural Strengths to Autoclave Cure_~ta.
COMPARISON OF FLEXUBAL TEST DATA TO PREPREG AUTOCLAVE CURE DATA
Plate Number
3389 3789 3989 31089 31789
Averaae Plate Fiber Volume 51.01' 66.55' 63.32' 62.06' 59.39'
Average RTM Plate Strengths[l]
Radius Engineering Average ---118,027 142,976 123,361 153,273 psi
Standard Deviation ---11,121 21,046 7,071 12,314 psi
University of Utah Average 163,411 120,818 138,115 119,164 ---psi
Standard Deviation 5.939 10.194 25.275 6.347 ---psi
ComDarative PreDreq Autoclave Cured Strenqths[1.2j
Average 1'7000 psi
Standard Deviation 13300 psi
Number Tests 25
[1] Normalized to .01' in/ply which corresponds to 58.5' fiber volume.
[2] Hercules Inc. Testing ot Preimpregnated Autoclave Cured AS' A370-5H/3502 Laminates
Table V. Subscale Radome Fiber Volumes.
Patents are pending on many aspects of this work, including tooling and sensing technologies. All elements
described herein are available to the composites industry on a contractual or licensing basis.
ACKNOWLEDGEMENT
A large portion of this effort was performed under contract to the Air Force Materials Laboratory, Air Force Wright
Aeronautical Laboratories, Materials Engineering Branch, Mr. Theodore J. Reinhart, Branch Chief.
9

FIGURES
10

Figure 3. Resin flow inside the mold during injection is shown in this sequence of data
acquisition system display screens.
System and Mold Cav ty Pressures
160
150
140
130
120
110
100
90
80
70
60
So
40
30
20
10
0
-10
-20
-30
-40
r
In
0.
u
..
'-
~
In
In
..
d:
MAIN VACUUM
I I I I I I I'
a 20 -40 60 ea
Elaspea Ti~ (min)
Figure 4. System and Mold Cavity Pressures.
11

DEGREE OF CURE..
Figure 5. Analytical process modelling of resin behavior using the Thermal Profile Evaluation
Model for a candidate injection cycle.
Calculated Viscosity; Degree of
Percentage Ge tat ion at Thermocoup tes
Cure..
0.9
0.8
&
"
.,
0:
..
.,
:lJ
"
...
"0
"
O
...
u
.,
at
0.7
0.6
o.~
0...
0.3
0.2
0.1
0
60 800 20 40
EJaspea Time (min)
-Cure 0-1 -Gel 0-100-Log Viscosity 0-4
Figure 6. Calculated viscosity, degree of cure, and percentage gelation at thermocouple
location from the Real Time Analytical Process Model.
12

..sJ;;OUENCE MODELLEIJ
~
0%
20';
~
80"
Figure 7. Integrated thermokinetic, viscosity, and resin flow model.
13

Aerospace Resin Transfer Molding, a Multidisciplinary ApproachRadf'JsEngineering and Tooling, Inc.
~
"'
"
+'
"
c
E
I.J
~
~
C
O
+'
~
'C
Figure 8. Resin injection time as a function of cavity depth without "gap" option.
I"'
UI
.,
..
"
"
E
U
.,
E
f-
"
0
..
U
.,
C'
Figure 9. Resin injection time as a function of cavity depth assuming a "gap" forms.
14

15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
r
..
Q)
+'
:J
c
E
u
~
~
C
O
+'
U
Q)
c
Figure 10. Resin injection time as a function of mold temperature assuming a "gap" forms.
Figure 11. Resin transfer molded graphite/epoxy plate using high temperature
prepregging resin.
15

Aerospace Resin Transfer Molding, a Multidisciplinary ApproachRadjl.JsEngineering and Tooling, Inc.
Figure 12. Photomicrograph of 67% fiber volume plate at 250X magnification.
Summaly oT Flexulal Testing
Radius Engineering & University of Utah
170
160
150
1"'0
130
120
110
100
90
80
70
60
50
"'0
30
20
10
0
"
-"
In In
a.~
~o
+'In
01"
C O
..r:.
'-1-
+'-.1
"'
3989 31089 31789
Plate Nu11:)er
~ University/Strength
~cu I es
PREPREGI AUTOClAVE
3389 3789
~ ~dius/5trength
Figure 13. Comparison of RTM flexural strengths to prepreg autoclave cure strengths.
16

CFigure14: Inner mold line tool for subscale radome has 37 resin position sensors
imbedded beneath the surface of the tool face.
Figure 15. The fiberglass subscaJe radome used the high temperature 976
prepregging resin.
17

Aerospace Resin Transfer Molding, a Multidisciplinary ApproachRadi",s Engineering and Tooling, Inc.
BIBLIOGRAPHY
1. Stark, Eric, "New Non-MDA Epoxy Resin Systems For Resin Transfer Molding (RTM) and Filament Winding", 32nd
International SAMPE Symposium, Apri11987, Shell Development Company.
2. Potter, Kevin, "Bismaleimide Formulations for Resin Transfer Moulding", 32nd International SAMPE Symposium,
April 1987, British Petroleum Advanced Composites Ltd.
3. Gutowski, T.G., "Applications of the Resin Flow /Fiber Deformation Model", 31st International SAMPE Symposium,
April 1986, Massachusetts Institute of Technology.
4. Brand, R.A., McKague, E.L, "Processing Science of Epoxy resin Composites", Final Report Jan. 1984, General
Dynamics Convair Division, Technical Director Dr. C. Browning, AFWAL/MLBC.
5. A. Kayes, Lockheed-Georgia,"Exploratory Development on Processing Science of Thick-Section Composites",
Contract No. F33615-82-C-5059, Technical Director Dr. C. Browning, AFWAL/MLBC.
6. Campbell, F., Mallow, A., "Computer-Aided Curing of Composites", Contract No. F33615-83-C-5088, Interim
Technical Reports act. 1985 -Dec. 1987, Technical Director Dr. C. Browning, AFWAL/MLBC.
7. Lee, Loos, Springer, "Heat of Reaction, Degree of Cure, and Viscosity of Hercules 3501-6 Resin," Journal of
Composite Materials, Vol. 16 pp. 510- 520.
8. Springer,"Resin Flow During the Cure of Fiber Reinforced Composites," Journal of Composite Materials, Vol. 16,
pp. 400-410.
9. Loos, Springer, "Curing of Epoxy Matrix Composites," Journal of Composite Materials, Vol. 17, pp. 135-169.
10. Kardos, Dudukovic, Mckague, Lehman, "Void Formation and Transport during Composite Laminate Processing,"
Composite Materials, Quality Assurance and Processing, ASTM STP 797, pp.96-109.
11. Hoffman, Boll, "The Use of Dynamic Gel Temperature to Develop Cure Cycles" , 29th National SAMPE
Symposium, Apr. 1984, pp. 1411- 1421.
12. Dave, R., Kardos, J. L, "Autoclave vs. Nonautoclave Composite Processing", 32nd International SAMPE
Symposium, April 1987.
13. Burroughs, B., "Manufacturing Technology for Nonautoclave Fabrication of Composite Structures", Contract No.
F33615-80-C-5080, Final Report, June 1985, Air Force Program Monitor Mr. D. Beeler, AFWAL, MLTN.
14. Eckler, J.,"Development of Flow Models For Srim Process Design", Proceedings of the Third Annual Conference
on Advanced Composites, Sponsored by ASM, Sept. 1987.
15. Coulter, John, "Experimental and Numerical Analysis of Resin Impregnation During the Manufacturing of
Composite Materials", Proceedings of the American Society for Composites 2nd Technical Conference, Sept.
1987, Center for Composite Materials, University of Delaware.
16. Gaurin, R.,"The Modeling of Pressure Distribution in Resin Transfer Molding", 4151Annual Conference, Jan. 1982.
17. S.J. Claus, AC. Loos, W.T.Freeman, "RTM Process Modelling for Textile Composites", Fiber- Tex 88 Proceedings.
18. R.C. Lam, J.L Kardos, "The Permeability and Compressibility of Aligned and Cross-Plied Carbon Fiber Beds
During Processing of Composites" (Washington University).
19. A.R. Mallow, F.R. Muncaster, "Computer-Aided Curing of Composites -Software Documentation", Contract No.
F33615-83-D-5088, AFWAL/MLBC.
20. "Graphite Prepreg Tape and Fabric Module, AS4/3502", pp. 14, Hercules Inc.
21. "Composite Materials Handboo~ , ICI Fiberite.
18

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