Mitsubishi Heavy Industries Technical Review Vol. 57 No. 1 (March 2020)
1Derivation of Low-cost and High-speed Forming ofCarbon Fiber Reinforced Plastic

YUKIO TAKEUCHI*1

KOSUKE IKEDA*2WATARU NISHIMURA*2

YUSUKE YANASE*3

KAZUTOSHI YOKOO*4

Carbon Fiber Reinforced Plastic (CFRP) is used for purposes which require high specificintensity, but the materials and molding for CFRP are relatively expensive in general and molding
takes a long time. This research applies the pultrusion method utilized in Glass Fiber Reinforced
Plastic (GFRP) to CFRP, aiming to achieve low-cost and high-speed molding. In addition, weconducted a simulation of the viscosity distribution of resin inside the mold in order to extract theoptimum molding conditions with the minimum number of prototypes. As a result, we successfully
molded a satisfactory CFRP prismatic pipe after only 2 prototypes.

|1. Introduction
In recent years, CFRP has been widely applied to products such as aircraft and automobiles.
Due to the increased volume of use, the price has decreased, but the range of application is stilllimited. On the other hand, although the pultrusion of GFRP is limited to uniform section shapes, it
is one of the least expensive molding methods for composite materials, and is currently usedmainly for molding corrugated panels. The application of this method to the molding of CFRP
beam materials would enable dramatically low-cost molding compared with the conventional
method, which would further lead to a wider range of application.
Generally speaking, however, Carbon Fiber (CF) is 5μm to 7μm in diameter, whereas Glass
Fiber (GF) is 10μm to 24μm in diameter. Therefore, CF is thinner and requires some fire retardant
to be added to the thermosetting resin to make it flame resistant, as a result, the resin viscosity
increases, it is generally difficult to fully impregnate the resin with CF. To impregnate and achieve
defect-free molding without pores, etc., the viscosity characteristics of the resin inside the moldneed to be properly controlled. In terms of GFRP, there have been numerous trial-and-error
molding tests with various different conditions conducted repeatedly to see what went right andwhat went wrong, during the process of which the conditions have been optimized. This paper
introduces a process where the optimum conditions for CFRP pultrusion are extracted byconducting a simulation of resin viscosity distribution, which has never been done before.

|2. Viscosity distribution simulation
2.1

Pultrusion utilizing thermosetting resin

Pultrusion is a method of continuously molding uniform sectional beam-shaped materials ina longitudinal direction by inserting fibers impregnated with resin in heated molds. This simulation
selected thermosetting resin, which is relatively cheap, from among the various types of resin. Theviscosity of thermosetting resin decreases before hardening when heated. Subsequently, aphenomenon called gelation occurs as the hardening progresses, where complex viscositybehaviors such as an acute increase in viscosity are observed. Figure 1 illustrates in a schematic
diagram the hardening behaviors occurring inside a mold.
*1

Research Manager, Manufacturing Technology Research Department, Research & Innovation Center*2

Manufacturing Technology Research Department, Research & Innovation Center*3

Engineering Manager, Parts Manufacturing Department, Commercial Airplanes Division*4

Engineering Manager, Engineering & Design Division, Mitsubishi Heavy Industries Compressor Corporation

Mitsubishi Heavy Industries Technical Review Vol. 57 No. 1 (March 2020)
2

Figure 1 Hardening behaviors occurring inside a mold2.2

Calculation process

Figure 2 is the calculation flowchart adopted in this research. Heating unset thermosetting
resin at a certain temperature changes the resin viscosity while inducing an exothermic
phenomenon due to chemical reactions. Since these reactions occur in succession, a simple
analytical approach was applied to the series of temperature changes accompanied by theexothermic phenomenon and the corresponding viscosity changes.

Figure 2

Calculation flowchart

Firstly, two-dimensional steady heat transfer calculations were carried out taking into
account the mixture of resin and fibers inside the mold, the mold itself and the ambient air.
Chemical reaction calculations were then conducted based on the results of the first set ofcalculations. Upon carrying out the chemical reaction calculation, the cure extent of the resin wasestimated based on the curing heat value where the general Kamal Formula was utilized as areaction formula(1). The curing heat value was measured with the Differential Scanning Calorimetry(DSC). Figure 3 presents a comparison between the calculation and experiment results in terms of
the relationship between the temperature of the resin used and its cure extent.
Secondly, the shear rate of the resin and the temperature dependence of viscosity weremeasured by the Dynamic Mechanical Analysis (DMA) where the Castro-Macosko Formula(2) wasutilized as a viscosity prediction formula combined with a curing reaction formula. Figure 4 showsMitsubishi Heavy Industries Technical Review Vol. 57 No. 1 (March 2020)
3a comparison between the calculation and experiment in terms of the relationship between thetemperature of the resin used and its viscosity.
A simulation of viscosity distribution was conducted utilizing these formulas.

Figure 3 Relationship between temperature of resin used and its cure degree

Figure 4 Relationship between temperature of resin used and its viscosity|3. Verification test
A molding test was conducted for verification purposes, using a mold capable of thepultrusion of a 50mm-square and 4mm-thick hollow rectangular cylinder. In the pultrusion, thetemperature of the mold is controlled by applying heat with a heater to harden the resin, but there is
no heater in the core. Therefore, the challenge is to determine the conditions to maintain thetemperature of the mold even in the through-thickness direction. Figure 5 depicts the relationshipbetween different portions of the mold and temperature, cure extent and viscosity, respectively, as
well as a comparison between the actual measurements and calculated values. In this figure, basedon the relationship between different portions of the mold and the corresponding temperature
values provided in the top chart, the cure extent and viscosity calculated according to thetemperature are shown in the middle and bottom charts,. From the figure, we can see that the actual
measurements and the calculated values are highly consistent with high accuracy. Figure 6 is acontour figure showing the optimum conditions based on the results in Figure 5. Figure 6(a) and6(b) show temperature distribution and cure extent distribution respectively. As can be seen fromFigure 6(a), the boundary between purple and blue is slanted at the entrance portion of the moldand the temperature distribution in the thorough-thickness direction is uneven, but at the exit
portion which has a significant impact on the cure extent, the boundary between red and orange
becomes more vertical and temperature distribution in the thickness direction, it becomes even .
Accordingly as the boundary of colors becomes almost vertical as shown in Figure 6(b), we canconfirm that the calculation successfully obtained desirable conditions where hardly any uneven
distribution of the cure extent was found in the through-thickness direction.

Mitsubishi Heavy Industries Technical Review Vol. 57 No. 1 (March 2020)
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Figure 5 Relationship between different portions of mold and temperature,
cure degree and viscosity

Figure 6

Contour figure showing results of optimum conditions

Next, we examined the inside of a prototype hollow rectangular cylinder molded under theoptimum conditions by cutting it at an arbitrary position and polishing the cross-section as shownin Figure 7. The image in the center is the whole cross-section and enlarged views of individual
portions are placed around it. The dark area in the center image is embedded resin for polishing andthe light-colored portion surrounding it is the molded object. On the right-hand side of this figure,two representative values of the Volume of fiber (Vf) are indicated, including where the average Vfof all the portions measured was 61.0%. Furthermore, the quality was confirmed to be excellent
with no major internal defects such as cracks or pores.
Finally, we cut out flat bar test pieces from the prototype and conducted a tensile test on it toexamine the strength. Figure 8 is a load-strain diagram of a representative test piece, which
confirms that it has a strength level equivalent to rolled steel for general structure (SS400).
Furthermore, there is high linearity in the load and strain, whereas no evidence of rupture in theresin was observed.

Mitsubishi Heavy Industries Technical Review Vol. 57 No. 1 (March 2020)
5Figure 7

Cross-section of prototype hollow rectangular cylinder

Figure 8

Load-strain diagram of cut-out flat bar test piece|4. Conclusion
This paper explained that our new viscosity distribution simulation can be used for actual
pultrusion, which resulted in successful molding of a satisfactory prototype. Viscosity distribution
simulation and pultrusion technologies are essential techniques for the manufacture of parts madeof composite materials. Since there is high demand for light-weight structural members, we will
endeavor to widen the application of these techniques to various products, while continuing towork on further sophistication so that we can contribute even more to the low-cost and high-speed
production of composite materials.

References(1)(2)

M.R. Kamal et al., The behavior of thermosetting compounds in injection molding cavities, Polymer
Engineering and Science, Vol. 20 (1980), p.859～867
Castro J. M. et al., SPE Tech. Papers, 26 (1980), p.434