The ring opening polymerization of cyclic monomers that yield thermoplastic polymers of interest in composite processing is reviewed. In addition, the chemistry, kinetics, and rheology of the ring opening polymerization of caprolactam to nylon 6 are presented. Finally, the rheo-kinetics models for polycaprolactam are applied to the composite process of reaction injection pultrusion.
This chapter presents the curing reactions of thermosetting resins and reviews and discusses the widely used three approaches to characterize and model the reactions: kinetic models (phenomenological and mechanistic), gelation theory, and rheological models. Tables containing different phenomenological kinetic models for neat resins (epoxies and vinyl esters) and for polymeric composites are listed, along with the kinetic parameters. The equations used to describe the coupled phenomena, such as resin flow, heat transfer, and cure reactions, inherent in any polymer and polymer composites manufacturing process, are also presented and discussed. Some optimization and control strategies for obtaining optimum cure cycles are introduced.
The phase separation behavior during curing of polyetherimide (PEI) modified diglycidyl ether of bisphenol A (DGEBA) epoxy and PEI modified bisphenol A dicyanate (BPACY) were studied using SEM, light scattering, and dynamic mechanical analyzer.
In situ frequency dependent electromagnetic–impedence measurements provide a sensitive, convenient, automated technique to monitor the changes in macroscopic cure processing properties and the advancement of the reaction in situ in the fabrication tool. This chapter discusses the instrumentation, theory, and several applications of the techniques, including isothermal cure, complex time–temperature cure, resin film infusion, thick laminates, and smart, automated control of the cure process.
Based on a local volume averaging concept general balance equations for transport of mass, momentum, and energy in stationary and moving porous media have been developed. In turn, these equations have been used to obtain specific governing equations in a number of important polymer matrix composite manufacturing processes, such as resin transfer molding, injected pultrusion, and autoclave processing. Moreover, appropriate numerical techniques for solution of these coupled partial differential equations have been briefly outlined and a few example simulations have been performed.
The stability, growth, and transport of voids during composite processing is reviewed. As a framework for this model, the autoclave process was selected, but the concepts and equations may be applied equally effectively in a variety of processes, including resin transfer molding, compression molding, and filament winding. In addition, the problem of resin transport and its intimate connection with void suppression are analyzed.
To overcome the shortcomings in thermosetting resins there is considerable interest in the use of thermoplastic resins as matrix materials for fiber-reinforced composites. Thermoplastic resins are high toughness materials; subsequently, they can improve the damage tolerance of composites. Fabrication costs of thermoplastic composites are potentially lower than are those for thermosetting composites due to reduced processing times and high-speed manufacturing methods. It has been established that thermoplastic composites consolidate by autohesive bonding of interply interfaces. The resulting autohesive bond strength is a function of the processing cycle to which the interply interfaces are subjected. This chapter reviews developments in modeling and analysis of thermoplastic composite consolidation. The effects of the processing cycle on mechanisms of intimate contact formation and polymer healing that occur during autohesive bonding are discussed.
Residual stresses are inherent to composite materials. They arise because the two (or more) materials that constitute the composite behave differently when subjected to a nonmechanical load (e.g., temperature). Processinginduced residual stresses occur as a result of nonmechanical loading during cure. Residual stresses induced during processing can be traced to one of two causes: chemical shrinkage strains or thermal expansion/contraction strains. A thermosetting matrix typically undergoes 5 percent volumetric shrinkage during crosslinking. Thermal expansion coefficients of polymer matrices are usually an order of magnitude higher than the reinforcement. The effect of processing-induced residual stresses can be dramatic. In some cases they are high enough to cause matrix cracking even before mechanical loading. In all cases a preloading of the fibers occurs. This chapter begins with a review of process models used to predict residual stresses in polymer matrix composites. A process model is then developed using phenomenological cure kinetics and curedependent mechanical properties. Both elastic and viscoelastic predictions are examined. Finally, the effect of processing conditions on residual stresses is discussed and ways of reducing them are explored.
Quality control is an important issue in composite manufacturing because of the high cost associated with the processing steps. The lack of accurate processing models, inadequate sensors to monitor quality on-line, and the batch or sequential nature of processing all make quality control difficult to achieve. Advances in computer hardware, artificial intelligence, model predictive control, and sensor technology have enabled the implementation of so-called intelligent or smart control strategies in composites processing. Such strategies attempt to combine prior experiential knowledge and available processing models to predict product quality so that any deviations from the quality specifications can be detected and corrected before the processing is completed. This chapter examines the evolution of these intelligent processing strategies and the current research in this field.
Autoclave processing remains the mainstay for fabricating continuous fiber-reinforced thermoset composite parts. During autoclave processing, several complex and interrelated phenomena occur, including heat transfer, resin viscosity changes, resin flow, chemical crosslinking, and void formation, growth, and transport. Numerous factors influence the cure cycle, but the ultimate goal remains the same: to produce a high-quality composite part free of voids and porosity. To achieve this goal, researchers have developed a fundamental understanding of many aspects of autoclave processing. In some cases this understanding is a mathematical representation of one aspect of the cure process. These models allow process engineers to conduct off-line interrogations of the autoclave process. Numerous models are available for simulating heat transfer and resin flow, viscosity and kinetic changes, and void formation, growth, and transport. For other cases, this fundamental understanding is a general principle. These general principles address the difficult to model, but more influential factors that affect autoclave processing, including bagging method, incoming material quality, prepreg moisture content, and dehulking procedures. This fundamental understanding has been further explored for making off-line process decisions, for real-time process control, for processing of high temperature condensation polymers, and for extensions to nonautoclave composite processes. This chapter provides an overview and reference guide for this fundamental understanding of autoclave processing.
The pultrusion process is briefly introduced in terms of machinery, materials, markets, process characteristics, and technology issues, where-upon modeling of matrix-flow–related issues is discussed in some detail. Despite the fact that pultrusion in its thermoset incarnation is a very economical and widely used technique to manufacture polymer-matrix composites, and that its thermoplastic counterpart has received considerable interest from both industry and academia, they are still—from a scientific point of view—relatively poorly understood processes. Although most of the early modeling work on the thermoset process focused on heat transfer and crosslinking kinetics, it has gradually been recognized that the issue of matrix flow is of great importance in terms of impregnation, consolidation, void migration, and the like. To shed light on the relevance of flow phenomena in both thermoset and thermoplastic pultrusion, models to predict pressure distributions within the composite during molding and models to predict the pulling resistance of the die are presented. Experimental verifications of the models are discussed and the usefulness of the models in their dimensionless forms is illustrated with numerical examples.
Liquid Composite Molding (LCM) is the name of a class of manufacturing processes that have the common feature that a mold cavity filled with dry reinforcement is injected with liquid resin. Well-known examples of this class of processes are Resin Transfer Molding (RTM) and Structural Reaction Injection Molding (S-RIM). The science and technology of LCM will be reviewed in this chapter. The process steps of preforming, mold filling, and in-mold cure will be discussed in detail, followed by a more cursory discussion of mold design for LCM. In the discussion of preforming the characteristic features of reinforcements relevant to preforming will be described. The most common preforming methods will then be discussed. In mold filling the basic theory and relevant general results will be summarized, followed by a discussion of mold filling problems. Useful rules-of-thumb for different injection strategies will be described and illustrated with an example. For in-mold cure the relevant theory will be summarized followed by a discussion of optimization of cure and possible cure-related problems.
Filament winding offers a high-speed, automated, precise technique for manufacture of closed-surface composite structures. Although traditionally applied to cylindrical and spherical parts, the winding technology has also been extended to nonsymmetric parts with complex curvature. In the winding process, a computer-controlled head positions fiber tows on a rotating mandrel. This technique is cost effective, particularly when compared with traditional methods such as autoclave curing. Several aspects of filament winding will be discussed including: winding methodology, winding equipment, cylinder design considerations, process simulation models, and characterization of filament wound parts. An emphasis is placed on the relationship between processing conditions and final part quality for both thermosetting and thermoplastic matrix filament wound cylinders. As such, descriptions of process simulation models and part characterization will be presented in detail.
Dieless forming is directed at production of large singly curved continuous-fiber composite components, generating the shape by using an array of small rollers. An initial version focused on tapered components with a variable cross-section along their length. Successes included the use of induction heating for rapid through-thickness temperature change, introduction of bends at a free edge and propagation of them into the laminate, and control of wrinkling on the inside of the bends. Complete success, however, was not attained due to the development of “overcurvature” in the formed components. An energy minimization model showed that the over-curvature was due to an “incompatibility strain” that arises between the formed and not-yet-formed regions, and was inherent to the process. A subsequent version of dieless forming imparted curvature simultaneously across the entire width of the workpiece. This version (“continuous dieless forming”) successfully formed laminates and preserved high quality, as measured by ultrasonic examination (6 dB loss) and shear strength tests. Predrying and forming within a vacuum bag were important ingredients, as was manipulation of the temperature gradient across the forming roller. Continuous die-less forming was used to form components with arbitrary and variable curvature, with a good shape accuracy (maximum deviation ±0.4 mm or 0.016 in.) being attained.
A series of computer aids for the control of polymer processing will be discussed, emphasizing the practical aspects of choosing the proper tool for the application. Where possible, examples of production, or at least developmental experience, will be included. Because there are a variety of processes used in polymer processing, the advantages and disadvantages of each method will be discussed in the context of the types of processes. The chapter is divided into a section on development of process cycles or plans and a section on in-process control. The tools to be discussed include design of experiments, expert systems, models, neural networks, and a variety of combinations of these techniques. The processes to be discussed include injection molding, resin transfer molding, autoclave curing, and prepreg manufacturing. The relative cost and difficulty of developing tools for these applications will be discussed where data is available.
