F. Wolff-Fabris
V. Altstädt
U. Arnold
M. Döring
Electron Beam Curing
of Composites
Sample Chapter 3:
Electron Beam Curing Applied to Composite Molding Technologies
ISBNs
978-1-56990-473-2
1-56990-473-1
HANSER
Hanser Publishers, Munich • Hanser Publications, Cincinnati
3
Electron Beam Curing Applied to Composite
Molding Technologies
Electron beam curing of composites must be combined with a suitable molding technology.
This can be an already established method for thermal curing, such as tape or tow placement,
hand-layup with hot debulk, vacuum assisted resin transfer molding, pultrusion, and others,
or a molding technology specifically used for electron beam curing, such as lost wax molding
techniques.
Numerous demonstration parts were manufactured in order to illustrate the flexibility of
electron beam curing of composites. Some of them are presented in this section together with
a critical discussion of the suitability of the molding technology to fabricate parts by electron
beam curing.
One of the advantages of electron beam curing compared to thermal curing is that the tools
do not have to tolerate the typically high curing temperatures. Lightweight inexpensive
tooling materials such as injection-molded polyethylene, wood, and cardboard are used for
many electron beam cured composite parts. On the other hand, electron beam curing under
pressure, although being possible, is considered to be impractical for most products because
two of the benefits of electron beam curing, the overall production speed and the simplicity
of the process, are lost [68].
Finally, before going into the details of each composite molding technology, one should be
aware that some materials, such as releasing agents, may inhibit curing on the surface of the
part [72]. This surface inhibition evaluation, primarily due to “proprietary” mold releases, and
a judicious analysis of the effect of such materials should be carried out prior to the use.
3.1
Layer-by-Layer Assembly
This technique uses a single-sided female mold, as shown schematically in Fig. 3.1. The method
consists of the lay-up of the reinforcement fibers and subsequently the resin is worked into the
reinforcement with a brush or a roller. This process is repeated for each layer of reinforcement
until the required thickness is built up.
After preparing the laminate, the part is irradiated with the electron beam focused on the top
surface. However, it is known that interface adhesion has a major influence on mechanical
properties of composites, and this technique does not assure a complete coating of the fibers
by the matrix. There might be a high fraction of voids. Therefore, only low quality electron
beam cured composites are expected to be manufactured by this molding method.
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3 Electron Beam Curing Applied to Composite Molding Technologies
Prepreg
Mould
Figure 3.1: Schematic composite preparation by layer-by-layer method
Figure 3.2: A portable electron beam gun for in situ curing was fabricated at Science Research
Laboratory, Inc. before installation on the ATP head at NASA Marshall Space Flight Center
[12] (Reprinted with permission of the publisher)
Main advantage of this technique are the low costs involved with the preparation of the
composites, which renders the technique to be effective for low-performance composite
applications.
An automated processing method derived from the manual layer-by-layer technique, which is
already used for electron beam curing applications, is called automated tape placement (ATP)
[260]. In this case, the e-beam gun combines the two steps into one process, leading to the
added benefit of potentially faster throughput, since ATP lay-up and the curing process are
completed at the same time (Fig. 3.2).
As curing takes place at each thin layer placement, low electron beam energies are required.
These equipments are easier to install and cost less than high-energy systems.
To obtain good consolidation, researchers often use a warm debulk and vacuum bag pressure
prior to high energy electron beam curing. On the other hand, the auto tape placement (ATP)
head applies heat and pressure for only a very short time, leaving little room for possible
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3.2 Prepregging
87
errors. Good tape quality with full fiber wetting is critical to produce in situ cured composites
without voids. This is because unlike conventional autoclave curing, there is no time for resin
flow to fill tape voids, and thus flaws must be avoided during tape fabrication. Voids can also
occur if the top layer springs back due to insufficient “memory” before it is cured. The degree
of spring-back as a function of EB cure dose, as well as the tack, drape, and memory of the
uncured tape are determined by the viscoelastic properties of the EB-curable resin.
3.2
Prepregging
A “prepreg” is a preimpregnated fiber reinforced material where the resin is partially cured
or thickened. The fibers are arranged in a unidirectional tape, a woven fabric, or random
chopped fiber sheets. Prepregs are made using a hot melt impregnation method. A strict
control of viscosity of the resin is a major issue to impregnate the fibers with the desired
amount of resin.
A major drawback of thermal curable prepregs is the limited life time and the need for storage
at sub-freezing temperatures until required for usage. Prepregs made from electron beam
curable resins would therefore be an advantage, as these prepregs can be stored for a much
longer time at ambient temperature.
This method can be employed also for electron beam curable materials. One of the main issues
that should be taken into account for cationically cured epoxy resins is the stability of the
initiator to visible light. In cases where these initiators react under light, particular care should
be taken during the process to avoid exposure and consequently curing of the material.
In respect to viscosity, the resins should be prepared to have a viscosity suitable for the
processing window, and the maximum temperature should not exceed the temperature where
cationic initiators undergo thermal scission (this is usually between 150 and 200 °C).
A low energy electron beam accelerator might have to be incorporated to the prepreg line, in
order to partially cure the resin to assure the handability of the material for further processing.
The preparation of pregregs with electron beam curable resins still requires further investigations. A research team [59] highlighted several problems with the prepregging of electron
beam curable resins, many of which related to the use of new fiber surface treatments and
new resin systems.
In 1996, Oak Ridge National Laboratory successfully produced electron beam curable glass
fiber prepregs that were used for the manufacturing of composite fuel tanks and engine inlets
for the US Army LONGFOG anti-tank missile. Although complicated, the usage of prepregs
is, together with filament winding, one of the mostly used composite molding technologies
for the manufacturing of electron beam cured composite parts.
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3.3
3 Electron Beam Curing Applied to Composite Molding Technologies
Vacuum Bagging
Reinforcement and resin are laid into the one-sided tool. After placing the stack of fiber layers,
this is then covered with perforated PTFE release film followed by a porous woven nylon peel
ply fabric. This layer provides a barrier between the laminate stack and subsequent stack and
offers an escape route through the laminate thickness for air and excess resin. A bleeder cloth
is then added which provides a uniform compaction pressure over the laminate surface due to
its high bulk factor. Finally, the vacuum bag film, usually nylon, is used to enclose the entire
mold and sealed using a butyl mastic tape. The vacuum connection can be made either via a
connection fitted to the mold or a fastening in the bag itself.
Curing of composites manufactured by vacuum bagging is already used in electron beam
curing. An example is shown in Fig. 3.3, where an aircraft part for military applications is
passing under the electron beam and is being cured [262].
The accelerated electrons traverse the bag film to reach the composite and to cure the resin.
This means the material from which the bag film is made should not undergo degradation
or deformation during irradiation. The same statement is valid for the sealing component of
the vacuum bag.
Figure 3.3: Composite part manufactured by vacuum bagging under electron beam irradiation [262]
(Reprinted with permission of the publisher)
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3.5 Filament Winding
3.4
89
Pultrusion
A collection of fibers in the form of rovings, tows, mats, or fabrics is pulled through a resin
bath and then through a heated die. Products range from a simple round bar to complex
prismatic sections of up to about 1.5 m maximum dimensions (the shape of the section is
similar to the pultrusion die). A flying cut-off saw is programmed to cut the product to the
desired length.
This is an important process to manufacture continuous composite sections, mainly from glass/
vinyl ester or polyester resins. The continuous character of this process permits a high production rate, and can therefore be adequately adapted to electron beam technology [261].
Up to now, there are still no operational electron beam pultrusion lines. In this case, some
modifications would have to be taken into account. The replacement of the hot die by a lowdensity die is the first point, in order to assure penetration of the accelerated beams throughout
the material. Moreover, the resin bath would have to be completely isolated from the curing
area; otherwise, radiation would also cure the material in the bath. Problems associated with
the impregnation of the fibers, as described in the prepreg process, would also have to be
faced in pultrusion.
From an economical point of view, the pultrusion of glass/vinyl ester composites curable by
electron beam irradiation would not be suitable, as curing of such resins is relatively fast.
Therefore, one of the main advantages of electron beam curing would not be used: the drastic
reduction in the curing time.
However, for most high performance composites, such as those reinforced by carbon fibers,
electron beam curing associated with pultrusion would have a major significance, as this
molding technology can assure a high material production rate.
3.5
Filament Winding
Filament wound composites are produced by resin-preimpregnated fiber tows over a rotating
male mandrel. Typically, thermal cure takes place after the part is wound. Process variables
such as winding pattern, fiber tension, winding speed, and impregnation techniques are directly
related to the properties and quality of filament wound composites.
This composite mold method is possibly the most frequently used technique for electron beam
curing composites. The French company Aerospatiale has manufactured solid propellant
rocket motors using this technique for more than 25 years. The casings were traditionally made
from filament wound carbon fibers with heat-cured epoxy resins. Researchers found that a
combination of electron beam and X-ray curing could reduce the curing time from four days
to less than eight hours, while keeping the structures nearly at room temperature.
After filament winding, the casing is taken to the irradiation facility. The product rotates, at
a specified speed, with its axis perpendicular to the beam scan direction. The casing is slowly
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3 Electron Beam Curing Applied to Composite Molding Technologies
moved under the scan horn, in the direction of its axis. It is moved back and forth under the
electron beam until a specified dose is applied. To ensure uniform energy deposition, the
product must rotate at least once before the conveyor moves the product a distance equal to
the radius of the electron beam spot. The speed of rotation must also be different from the
speed that the electron beam traverses across the scan horn. The scan width is also set to be
less than the minimum diameter of the product, ensuring that the entire beam is directed
onto the product surface.
3.6
Resin Transfer Molding (RTM) and
Vacuum Assisted RTM (VARTM)
In resin transfer molding (RTM), a dry fiber preform is placed between the molds and
impregnated with the resin, which is injected, either by gravitational or pump pressure. The
resin is then cured and the mold opened to release the cured component, which may require
subsequent finishing operations.
The vacuum assisted resin transfer molding (VARTM) technology is almost identical to RTM.
The mold for VARTM is linked to a vacuum pump, and therefore a high sealing level of the
mold is required. The vacuum system allows the evacuation of air from the fibers, leading to
a composite presenting lower void volume than those manufactured by RTM.
Both RTM and VARTM are considered to be the principal viable alternatives to prepregs for
the manufacture of high-quality composite moldings. The key processing parameters are the
viscosity of the resin, which should be below 500 cps, and the length of the required resin
flow path [261].
These methods were already adapted to electron beam curing. A major modification is that
the mold should be made from a low-density material that allows the passage of the electron
beam without undergoing degradation or deformation. Moreover, this mold should stand
some mechanical pressure due to the injection pump and should provide adequate sealing
for VARTM.
A maritime part [66] was produced by this technique. The composite part was layed up on
an inexpensive wood/formica tool. A vacuum-assisted resin transfer molding technique was
used for the infusion of vinyl ester resin. Immediately after resin infusion, the part was irradiated to achieve cure. Because electron beam curing can be achieved at room temperature,
no special consideration for high temperature operation was included in the tool design and
construction. Unfortunately, the elevated temperature, resulting from the exothermic reaction during cure, exceeded the desired operating temperature of the mold. This resulted in
a loss of vacuum during the EB cure, exacerbating the tendency of the part to delaminate.
Nevertheless, the VARTM method showed to be efficient for electron beam curing and the
use of a more appropriate mold material will have an impact on the improvement of the
composite properties.
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3.7 Lost Core Molding
3.7
91
Lost Core Molding
A difficult problem is to fabricate essentially closed parts that are supported internally during
cure and before tooling removal. To overcome this problem, the metal industry developed
a method called the “lost core process”, which has been adapted for electron beam curing of
composites.
Wax is a common, inexpensive material that can be easily cast. Because it melts at relatively
low temperatures, it is also an excellent candidate for single use applications where destruction of the mandrel during part removal is not a problem or where an encapsulated tool must
be removed. The wax is relatively easy to melt out and can be recast through at least several
cycles. Similarly, styrofoam is a common, inexpensive material that can be easily cast or cut
to shape. A relatively rapid dissolution by common solvents such as acetone is possible. These
characteristics make styrofoam a good candidate for single use applications where destruction
of the mandrel during part removal is not a problem.
However, one should be aware about the sensitivity of the styrofoam and wax to elevated
temperatures in which the curing exotherm from the curing of the part actually may melt the
core. With electron beam curing, the cure rate and associated exotherms can be controlled,
and these parameters allow the part to be successfully cured to the desired shape.
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