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Development of a Resin Transfer Molding Test-bed with Process Control Capability.

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Dev Chem Eng Mineral Process., IO(IL?),pp.129-142, 2002
Development of a Resin Transfer
Molding Test-bed with Process
Control Capability
Kaushik Raghavan' and Srinivas Palanki2*
'Motorola Inc.. Chicago, Illinois, USA
2Department of Chemical Engineering, FAMU-FSU College of Engineering, Florida A&M University-FloridaState University, Tallahassee,
Florida, 3231 0-6046, USA
Fiber reinforced plastic parts have traditionally been manufactured by resin transfer
molding. This method is based on a fixed recipe and no attempt is made to control
process conditions during the manufacturing process. In this papel; an experimental
test-bed is developed which has the capability of implementing a wide variety of temperature and pressure profiles for the manufacture of plastic parts via resin tranfer
molding. The performance of the test-bed is evaluated by implementing several desired
pressure and temperature profiles in production offiber reinforced polyester parts.
Fiber reinforced plastics are widely used in automobiles and high performance aircraft
due to their high strength to weight ratio. They are also used in the chemical industry to hold corrosive liquids due to their high strength, corrosion resistance and low
cost These plastic parts have traditionally been manufactured by resin transfer molding (RTM).
RTM is a manufacturing process by which a porous media can be impregnated with
a nowNewtonian fluid. Fiber mats are layered and laid on top of each other to form a
three dimensional porous matrix of fibers This matrix is impregnated with a monomer.
The monomer polymerizes around the fiber matrix during the curing process. Impregnation or mold filling as it is called, takes place in a mold cavity which is usually sealed
from the surroundings. After mold filling, the liquid monomer flow is stopped and
polymerization or curing is carried out. Once the cure process is completed, the mold
is taken apart and the final product is removed The mechanical properties of the final
product are affected by (1) the way the fiber matrix is impregnated by the monomer
during filling and (2) the degree of polymerization and extent of cross linkages during curing. Thus, the filling pressure (which affects the filling process) as well as the
temperature (which affects the curing process) significantly affect the final mechanical
properties of the fiber reinforced plastic part being manufactured. However, current industrial practice of RTM is based on a fixed recipe and n o attempt is made to control
process conditions such as pressure and temperature during the manufacturing process.
*Authorf o r correspondence (email.
K Raghavan and S. Palanki
This method leads to a significant wastage of material and time when a large quantity of
parts are required. This provides the motivation for developing an RTM process where
one can control both the filling and the curing process by manipulating pressure and
temperature. The development of this process is further motivated by the recent interest
in developing in siru sensors for monitoring polymer properties in the mold [ 1,2,3].
In this paper, an experimental test-bed is developed which has the capability of
maintaining desired pressure and temperature profiles. The pressure and temperature is
continuously monitored and a closed-loop control scheme is implemented to keep the
system on the desired pressure and temperature trajectories. The performance of the
test-bed is evaluated by implementing several desired pressure and temperature profiles
in production of fiber reinforced polyester parts. The organization of the paper is as
follows. In section 2, a brief review of the existing RTM equipment and their shortcomings from a process control viewpoint are discussed. In section 3. the developmentof an
RTM test-bed that overcomes some of these limitations is described. The capabilities
of the test-bed in terms of controlling the filling as well as the curing are demonstrated.
Finally, in section 4, conclusions of this work are presented.
Review of Existing RTM Equipment
The industry currently uses bulky and heavy apparatus to manufacture most resin parts.
In most cases, the manufacture of composite parts is done purely on the basis of an
open-looprecipe. This equipment typically consists of no sensors for pressure or temperature and thus there is no mechanism for controlling these variables This ad hoc
approach can lead to variation in product quality from batch to batch, which results in
a substantial loss of capital, material resources and time for the manufacturer.
Modi,fications to Industrial RTM Equipment
Several modifications have been proposed to this standard industrial equipment, particularly in the laboratory scale. Yalvac et al. [4] used an experimental set-up which
consisted of a rectangular mold fitted with six pressure transducers located along the
linear axis of the mold. Thermocoupleswere used to measure the temperature at various points in the mold. The preform consisted of six to Seven plies of fiber matrix This
resulted in a fiber volume of around 50 percent. The mold was designed with nine possible vent port locations, one in each comer, one equally spaced between each successive
comer and one in the center of the mold. An inexpensive,disposableball valve was connected to each vent port to allow opening and closure of port as desired during injection
or cure. The press used for the experiment was a 150 ton, four-post upacting Wabash
hydraulic press. Spacer plates were placed between the mold amid plates. The spacer
plates transfer the closing force from the press to the mold but contain various grooves
and cut out sections so as not to crush the vents and pressure transducers. The necessity
of including spacer plates removed the possibility of efficient temperature control of
the mold surface through conduction only. For this reason heating of the mold was accomplished by hot oil lines machined into both the top and bottom halves of the mold.
The injection machine used was an impingement head mixing machine capable of injecting a liquid with a melting temperatureof less than 150OC.Initial preparation of the
machine included charging the machine with resin and hardener, adjusting the system
Resin Transfer Molding Test-bed with Process Control Capability
motion controls to the desired flow rate and adjusting the system heating controls to the
desired temperature set points and degassing the resin components with vacuum.
Two different types of sensors, thermocouples and pressure transducers were used
in the molding experiments. Seven thermocouples were used to measure temperature
in the composite during fill and cure. The diameters of the thermocouples were small
enough to not interfere with the flow pattern; yet the thermocouple could be positioned
anywhere in the preform without much difficulty in terms of handling and breakage
An additional thermocouple was positioned to measure the bottom mold surface temperature. This thermocouple was shielded. All the thermocouples were ungrounded
and required a special configuration on terminal panels of the data acquisition hardware for grounded and ungrounded use since the graphite-epoxy environment provided
conductive and non-conductive environments during early and late stages of the injection process, respectively. The pressure distribution during the process was monitored
by six pressure transducers equispaced along this centerline of the mold cavity in the
bottom mold surface. Each pressure transducer was connected to external signal conditioningheadout instrumentation with a six-wire shielded cable assembly.
Before the experiment was executed, full active vacuum was applied to the mold.
The resin components in the material holding pots were heated and de-gassed. After
the machine and resin components had reached the set point temperature, vacuum was
removed from the system and replaced by a nitrogen blanket. Upon reaching the set
point temperature the injection unit was set to continually recirculate the components
through the system to maintain thermal steady state. During the recirculation period
prior to injection the orifice openings in the impingement head were adjusted to create
enough back pressure in the system to ensure proper mixing of the resin components
during injection.
Kendall and Rudd [5] examined and compared the pressure and temperature histories observed in mold cavities during impregnation, heating and impregnation for both
RTM and SRIM using polyester, vinyl ester and polyurethane resins in combination
with continuous strand mats. In their work, they used a single pressure pot and a pneumatically actuated single acting reciprocating pump during the RTM studies while the
RIM metering system consisted of two lance metering system and an impingement mix
head The mold cavity was defined by a frame located between two water heated steel
plates which had chromium plated surfaces to improve release and minimize buildup of
surface deposits The mold was mounted in a converted injection molding press. The
injection of the resin was through the center of the top platen while vents were provided
at each comer No provision was made for sealing the vents although this did occur
in practice if the resin gelled very shortly after injection. Eight thermocouples, eleven
flush mounting pressure transducers and one dielectric sensor could be permanently
mounted in the mold. Although the mold had facility for cavity temperature monitoring
they did not provide sufficiently short response times for SRIM cycles. TOsupplement
these, sacrificial thermocouples made using thin wire, were adhesively bonded to the
mid plane of the preform. The sensor information was collected, processed and displayed in real time on a computer. A Micromet Instruments ICAM 1000 dielectric
system was employed to monitor dielectric response using a ceramic, flush mounted
sensor to provide a non-intrusive method of obtaining readings.
Shields and Colton [6] studied the effect of incomplete fiber wetting on final product
K Raghavan and S.Palanki
properties. For this they studied the use of tows that were pre-coated with a powdered
version of the liquid molding resin (towpregs) The resin was heated to 124°C and the
empty mold was pre-heated to 177°C. The preform was placed in the mold just before
resin injection so that the powdered resin in the preform would not cure before the
liquid resin was injected. The mold was maintained at the same temperature and kept
under vacuum.The resin was injected into the mold at a pressure of 68.9 kF’a and was
allowed to cure in the mold at 177°C for 2 hours. They did not, however, measure the
extent of cure. They used the manufacturer’s processing instructions and assumed that
it was adequate to ensure full cure. This is again a typical example where one follows a
“predefined recipe”.
Sun et al. [7] performed permeability measurements on the fiber reinforced preforms typically used in resin transfer molding. They tried to enforce a constant flow
rate of impregnating resin during the resin injection process. The standard rnethodology for a constant flow rate impregnation is to use a piston arrangement which would
force the resin through the preform at constant feed rate. However, in 171 a different
methodology was used for implementing constant flow rate for resin flow. They varied
pressure of impregnation accordingly to produce a constant flow rate. For this, the flow
rate was sensed using a flow metering device and the pressure control was accordingly
implemented. The test fluid was placed in a pot which could hold up to seven liters of
fluid. The test fluid used for the experiments was corn syrup diluted with water. The
fluid exhibited Newtonian behavior. The study was thus very good for low viscosity
unreacted thermosets like epoxy. A tube ran from this pot through an in-line flow meter
and a pressure transducer and then to the inlet of the mold. The fluid traveled through
the tube from the pot to the mold The flow meter and the pressure transducer were connected to a computer through a data acquisition system which recorded their responses
and also controlled the process. A closed loop control system was then established to
provide constant flow rate by varying the pot pressure. A digital pressure regulator was
used to control the pressure in the pot.
Higuerey et al. [8] investigated the need for sensing subsystems for the monitoring of resin flow dynamics during molding process. Their embedded electronic sensor
concept was based on the positioning of electrically conductive wires within mold cavities to form orthogonal grid patterns with non intersecting grid junctions. The region
between adjacent grid layers is a sensing gap, which in its initial unfilled state leads to
open electrical circuits for all interlayer wire pairs. Their setup included a l-D transparent mold with embedded electronic sensors, a power supply and a fixed resistor, a
CF-920 Ono-Sokki dynamic analyzer, a resin supply tank and a pump.The mold was
composed of two transparent Plexiglas plates and one aluminum spacer plate with the
inlet gate located at one end of the mold. The spacer plate had a U-shaped internal
slot to create a flow cavity. Electrically conductive wires were embedded in a nonintersecting manner. The resin flow rate was maintained at a low value in order to study
both the molding process and the monitoring technique. Two inlet gates were located
at one end of the mold and four vent gates were placed at the other end of the manifold
Limitations of Existing Experimental Setups
The RTM equipment in use currently has several limitations. Some of the major limitations are described below:
Resin Transfer Molding Test-bed with Process Control Capabiliw
1. Flexibility: Most experimental setups are built to operate at specific conditions
(like pressure, temperature, mold shape). Any change in these parameters requires a lot
of modification to the equipment.
2. Portability: Another limitation among existing setups is the size of the setup and
their ability to be relocated without disturbing the system settings and calibration. In
the event of relocation most apparati have to be completely dismantled and reassembled
at the point of relocation. This makes it very easy for fresh errors and irregularities to
creep into the working of the apparatus. Also, most apparati are sensitive to physical
shock and are required to be mounted on special surfaces.
3. Measuring Devices: A drawback found in most machines is the use of old and
error-prone measuring devices. These are particularly susceptible to losing accuracy
and have to be recalibrated frequently Some components like thermocouple wire are
quite delicate and cannot be subjected to tensile load or such physically destabilizing
actions. Also, these wires contribute to electrical resistance by every unit length of
wire. Thus, they are not suitable for long distances. Some test-beds are equipped with
an insufficient number of sensors or with incorrectly placed sensors.
Development of RTM Test-bed with Process Control
In order to study and experiment with different manufacturing conditions to produce an
effective resin part, one needs to try out various processing “paths” to find the one that
is optimal. For this reason it is necessary to develop an RTM test-bed which provides
reproducible results, and in which process conditions such as temperature and pressure
can be changed easily This provides the motivation to develop an RTM test-bed with
process control capability.
Development and Validation of RTM Process Control Test-bed
Figure 1 shows the schematic of the RTM Process Control Test-bed that was built as a
part of this research. The test-bed was built using “off-the-shelf” parts. The test-bed
consists of the following units:
1 A personal computer with two microprocessors
2. A transportable four-wheel, three-sided cart
3. A data acquisition system
4. Mold plates and frame
5. Vacuum pump
6 . Solenoid valve system
7. Resin reservoir with sliding mechanism
8. Injection gun
9. Resin overflow pot
10. Pressure and temperature sensors
11 Digital and analog termination boards
12. Condensation trap system
13. An auto-tuning PID temperature controller
14. A 16 channel relay mounting panel with low power switching relays
K Raghavan and S. Palanki
FB1:Pressure feedback loop for mold inlet
FB2: Pressure feedback loop for mold outlet
P1: Pressure sensor at mold inlet
P2: Pressure sensor at mold outlet
R1: Resin reservoir
R2: Resin overflow pot
T1: Temperature sensor
TC: Temperature controller (PID)
TSP: Computer algorithm to compute temperature set-point
V1, V2: Solenoid valve
Figure 1. Schematic of RTM test-bed.
15. Visual BASIC graphical user interface front end
Details of various components and their assembly can be found in [9].
The desired set-points of the pressure are entered in the computer prior to the experiment by the user. The computer calculates suitable ramps in the pressure set point
profile and accordingly activates the solenoid valves. This “on-off’ control strategy is
implemented at a rate of a thousand times a second.
The desired set-point profile of the temperature is entered in the computer prior to
the experiment The temperature of the mold is controlled using an auto-tuning PID
temperature controller.
Several experiments were performed to test the capabilities of the RTM machine.
The objective was to test the efficacy of the machine to track a desired pressure trajectory while filling the mold, and a desired temperature trajectory while curing. A series
of runs were made with initiated resin to test the performance of the test-bed during
actual polymerization. The resin used here was polyester resin which is quick-setting.
Figure 2 shows the pressure profiles implemented during mold filling. Since the
curing process is believed to exhibit non-linear behavior, it is expected that the optimal
temperature profile that results in parts of desired quality could vary with time. The
Resin Transfer Molding Test-bed with Process Control Capability
desired time varying temperature trajectories are shown in Figure 3
Two concave profiles, two convex profiles and a linear temperature profile were
studied as they represented a wide range of temperature trajectories The temperature
is varied only for the first thirty minutes as the resin cures during this time. After
thirty minutes, the controller attempts to keep the temperature constant. The results are
shown in Figures 4-13. It is observed that the test-bed is able to track both the desired
pressure as well as temperature profiles quite well. A small lag is observed between the
temperature of the mold and the desired set-point. More aggressive settings in controller
parameters can improve performance. In Figure 6 there is a sharp drop in temperature
set point because of user error during input of the desired temperature trajectory. The
resulting temperature trajectory helped in studying the variation in mold temperature
when there is a sudden drop in temperature set point. The respective pressure profiles
have been plotted for each temperature trajectory. The pressure profiles tell us how
the process inside the mold has progressed Pressure control is imposed on the mold
only for the duration of mold filling. After the mold has been filled with resin, the inlet
and outlet are sealed with a pinch cock in order to leave the polymerization process
undisturbed. In all the graphs one can note a point where the pressure suddenly drops
in the mold. This indicates that the polyester in the mold has started to gel. Gelling
point is the first time when the viscosity of the initiated monomer increases by the order
of a couple of magnitudes. One can thus estimate the gelling time of the process in each
of the runs The gelling times for each of the temperature runs are shown in Table 1.
Table 1. Efect of different temperature pmjiles on gelling time.
Temperature Profile
Linear Profile
Concave Profile 1
Concave Profile 2
Convex Profile 1
Convex Profile 2
Gelling Time
1250 seconds
1550 seconds
1400 seconds
2000 seconds
In Figure 13, one notes that the pressure profile has flattened out at zero immediately after the pressure control has been disabled. This tells us that there is a leak in the
mold sealing. This normally would not have been obvious until the final cured product
had been removed from the mold if the pressure was not being tracked. If a leak is
detected in the initial stage, the curing process can be stopped and necessary action can
be taken to fix the leak. Thus the test-bed can be used not only as an apparatus to test
manufacturing methods, but also to monitor the manufacturing process for inconsistencies In this particular run, the leak did not maintain the pressure in the mold thereby
masking the drop in pressure at the start of gelling. Thus the gelling time could not be
determined in this run.
In all the runs conducted for making polyester parts, the mold cavity was de-gassed
with vacuum. This proved beneficial in a number of ways. Firstly, this reduced the void
content of the final product. Secondly, the impregnation of the resin tow was better.
Oxygen present in air is a potential initiator and its presence in the fiber matrix would
cause localized variations in initiator concentration and cause deformations in the final
K Raghavan and S.Palanki
Figure 2. Pressure projile setpoints.
50 0
40 0
20 0
10 0
Figure 3. Temperatureprofile setpoints
Resin TransferMolding Test-bed with Process Control Capability
Time in seconds
Figure 4. Linear temperature profile
20 0
10 0
Inlet pressure
Outlet pressure
-10 0
C 1
1000 0
2000 0
3000 0
4c 00
Time in seconds
Figure 5. Pressures in linear temperature profile run.
K Raghavan and S. Palanki
Time in seconds
Figure 6. Concave temperature prvjiie I .
."-".." -
'.-. -...
-.-- _. ,.
Figure 7. Pressures in concave temperature profile I run.
Resin Transfer Molding Test-bed with Process Conrrol Capability
40 -
35 -
10 0
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ :----.:
-_ _ __ _ _ _ _ _ _ _ _ _
-100 1
Outlet pressure
Figure 9. Pressures in concave temperature projile 2 run.
K Raghavan and S. Palanki
Temperature Set Poinl
Actual Temperature
c 25 ,-./&
Outlet Pressure1
1000 0
2000 0
Time in seconds
3000 0
4000 0
Figure 11. Pressures in convex temperature profile 1 run.
Resin Transfer Molding Test-bed with Process Control Capability
40 -
0 35
Figure 12. Convex temperature profile 2.
Figure 13. Pressures in convex temperature projle 2 run
K Raghavan and S. Palanki
In this research, a test-bed was constructed in order to study the RTM process. This testbed has the capability to produce fiber reinforced plastic parts using various polymers
under reproducible conditions, and to allow the study of the effect of various pressure
and temperature profiles on the quality of the final product. A range of temperature
profiles were implemented and their effect on gelling time was observed
The authors gratefully acknowledge funding from the National Science Foundation
@MI 95 12251) and the help of Dr. Richard Pamas at NIST for assistance in building the RTM test-bed.
Hunst0n.D. McDonough. W , Fanconi. B , Mopsik, F , Wang , F , Phelan, F , and Chiang, M 1991
Assessment of the state-of-the-art for process monitoring sensors for polymer composites Report NISTIT
4514, US Department of Commerce
Fildes. J , Altkom. R , Haidle, R , Milkovich, S , and Neatour, M 1993 lntelligent 1R spectroscopy in
composites materials fabrication Great Lakes Composite Consortium, Kenosha, WI
Dunkers, J Flynn. K , and Pamas, R 1997 A mid-infraread attenuated total internal reflection cure
sensor for control of resin transfer moulding for a pre-ceramic polymer Composites Parr A. 28, 163-170
Yalvac, S , Calhoun, D and Wetters, D 1996 Mold filling analysis in resin transfer molding Polymer
Composites, 17(2). 251-264
Kendall. K and Rudd, C 1994 Flow and cure phenomena in liquid composite molding Polymer
Composites, 15(5), 334-348
Shields, K and Colton. J 1993 Resin transfer molding with powder coated preforms Polymer Composites, 14(4), 341-348
Sun, J Mogavero, J , and Advani, S G 1997 A nonlinear control method for resin transfer molding
Polymer Composires, 18(3), 412-417
Higuerey. E , Kikuchi, A , and Coulter, J 1995 An experimental investigation of resin flow sensing
during molding process J Engng Materials Techno1 , 117(1), 86-93
Raghavan, K 1998 Development of Resin Transfer Molding Testbed for Manufacture of Composite
Polymer Parts M S Thesis, Florida State University, Tallahassee, FL, USA
Received 7 November 2000;Accepted after revision: 19 March 2001.
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development, test, process, capability, transfer, resins, bed, molding, control
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