ABSTRACT
CONTENTS 14.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 573
14.1.1 The Liquid Composite Molding (LCM) Processes . . . . . . . . . . . . . 573 14.1.2 The Physics in LCM .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 576 14.1.3 LCM Simulations for Optimization and Control. . . . . . . . . . . . . . . 577
14.2 Modeling and Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 579 14.2.1 Flow in LCM.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 579 14.2.2 Heat Transfer in LCM .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 580 14.2.3 Chemical Reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 585
14.3 Process Control and Optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 588 14.3.1 Injection Gates and Vents and Flow Distribution
Network Optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 589 14.3.2 Online Permeability Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . 593 14.3.3 Flow Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 595 14.3.4 Temperature and Resin Cure Cycle Optimization . . . . . . . . . . . . . 597
14.4 Conclusions and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 599 Acknowledgment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 601 Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 601 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 602
14.1.1 The Liquid Composite Molding (LCM) Processes
Polymer composite structures are fabricated using fibers as reinforcements held in position with a polymer matrix. There are a variety of processes to
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manufacture composites, dependingupon the typeof applications, numberof parts to be made, the geometry of the parts and the performance desired. For an introduction to this, readers may refer to the following texts on composite manufacturing [1-4]. Liquid composite molding (LCM) represents a class of composite manu-
facturing processes in which the fiber preforms are placed in a closed mold and the liquid polymer is impregnated to saturate the empty spaces between the fibers to create the composite structure. The reinforcing fiber preforms are usually fabrics formed from continuous strands or tows of a few hundred to 48,000 glass fibers, carbon fibers, or aramid fibers (such as Kevlar) by stitching, knitting, or weaving them as shown in Figure 14.1. The ability to tailor fiber directions allows the designer to build the structure for desired mechanical properties. The polymer matrix used to bind the framework of fabrics can be either thermoplastic or thermoset resin. Thermoplastic resins are usually in solid phase at room temperature but at elevated temperatures they melt into viscous liquids with viscosities of the order of about a million times higher than that of water. It is very difficult to impregnate the tiny empty spaces between and within the fiber preforms with the thermoplastic resin. Hence, thermoplastic resins are rarely used for LCM processes. On the other hand, most thermoset resins are in liquid phase at room temperature. The viscosities of the thermoset resins are about 50 to 300 times higher than water and relatively easier to saturate the fiber preform. However, thermoset resins undergo an exothermal chemical reaction and cross-link, and hence are difficult to recycle. Thermoset resins are used for LCM processes mostly due to their low vis-
cosities, which enable them to infiltrate into the small spaces between the fibers. The thermosets used are usually epoxies, vinylesters, or polyesters with desired chemical or environmental resistance. In this chapter, we will focus on a class of manufacturing processes for long fibers/thermoset resin composites called liquid composite molding (LCM). LCM includes resin transfer molding (RTM), vacuum assisted resin transfer molding (VARTM), and structure reaction injectionmolding (SRIM). These processing techniques arewidelyusedbecause they lend themselves toautomation, readily reducing
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cost and time, and allows one to produce the nearly net-shaped composite parts. Different industries may have different expectations on LCM processes. For example, the automotive industry emphasizes the potential of high volume manufacturing and good surface finish. On the other hand, the requirement in the defense and aerospace industry [5-7] is to produce light, high quality, complex composite structures. Civil or transportation applications, such as composite bridge decks, ship hulls, and wind turbine blades, are usually large structures; hence the key constraints pertaining to them are to reduce the mold tooling cost and enable resin infusion into the fabric structure within reasonable time. The LCM process is versatile and flexible enough to accommodate these needs and constraints. Thus, over the last decade researchers have focused on gaining a scientific understanding of this process. Many mathematical models and simulations of the process have been developed to create a virtual manufacturing environment as this would help reduce the prototype development cost and time. One of the representative LCM processes is the RTM, which can loosely
be divided into five steps, as illustrated in Figure 14.2. The first step is to manufacture the fiber preform from glass, carbon, or Kevlar in a form as shown in Figure 14.1. The second step is to stack the preforms in the mold cavity. The mold is then closed, which compresses the fiber preforms into the designed thickness andfiber volume fraction. This stationary compactedfiber preform is a fibrous porous medium. The third step is to inject a thermoset resin into the mold cavity and impregnate the fibrous porous mediumwith a low viscosity thermoset resin. The fourth step is to initiate and accelerate the cure process of the thermoset resin either by adding a catalyst or by heating the resin that has saturated the empty pores between the fibers of the preform
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and then cooling the solidified composite to room temperature. The last step is to demold the net shaped composite part from the mold. One of the limitations of RTM was that the cost of tooling and injection
machinery went up exponentially as the part size increased. VARTM was invented to overcome this limitation. In the VARTM process, the preform is placed on a flat tool surface and enveloped with a plastic bag. A vacuum is applied to compact the preform and draw the resin from a reservoir at atmospheric pressure into the mold cavity to saturate the preform as shown in Figure 14.3. Thus, VARTM uses low pressures and one-sided tools to make large composite structures. To reduce the infusion time of the resin into the preform, flow channels and the distribution media are used to accelerate the flow infusion process. The flow in the channels and/or the distribution media makes the flow of resin in the anisotropic fibrous porous media truly three-dimensional.
