In-process control of epoxy composite by microdielectrometric analysis. Part II: On-line real-time dielectric measurements during a compression molding process

The curing of a fiberglass epoxy composite based on diglycidyl ether of bisphenol A (DGEBA) with dicyanodiamide (DDA) as the hardener and imidazole as the catalyst agent was analyzed using microdielectrometry. The curing behavior of thick epoxy composite parts was examined in a production environment for compression molding process. The particular focus of this paper is to present the method used to collect on-line real-time conversion measurements during an epoxy/fiberglass composite cure. For this purpose, temperature and ionic conductivity profiles during industrial moldings of a thick epoxy part were recorded. Corresponding conversion profiles were deducted from a previous empirically established correlation and discussed in terms of cure gradients as a function of the through-the-thickness location and of the cure cycle time.     

Comprehensive thermal optimization of liquid composite molding to reduce cycle time and processing stresses

Liquid composite molding (LCM) is a well‐established and flexible composite manufacturing technology capable of producing large parts at a relatively low cost. In this family of related injection processes, a large number of design variables have strong impact on manufacturing performance. The determination of adequate process parameters is key to yield successful molding conditions and reduce cycle time. In addition, properties and durability of composite parts are strongly affected by internal stresses. Excessive stress levels may lead to important defects in the part at the curing stage and after processing, when the part is cooled to room temperature. In this investigation, a comprehensive curing optimization algorithm is proposed to reduce internal stresses during composite processing. This study focuses on the minimization of the macroscopic residual stresses that appear during cure and cooling in thermoset composite laminates as a result of temperature and degree of cure gradients. The proposed fitness function to be minimized is based on the physics of the matrix material transformation and on the mechanical behavior of the composite material. An evolutionary strategy based on genetic algorithms (GA) is implemented for the minimization of the fitness function. Optimization is carried out for thin and thick glass/polyester laminated composites. Different optimization schemes with thermo‐elastic and viscoelastic models of the composite mechanical properties are studied. The advantages and drawbacks of each model are stated and discussed. POLYM. COMPOS., 26:209–230, 2005. © 2005 Society of Plastics Engineers     

Optimal curing for thermoset matrix composites: Thermochemical and consolidation considerations

A design sensitivity method is used to find optimal autoclave temperature and pressure histories for curing of thermoset-matrix composite laminates. The method uses a finite element simulation of the heat transfer, curing reaction, and consolidation in the laminate. Analytical sensitivities, based on the direct differentiation method, are used within the finite element simulation to find the design sensitivities, i.e., the derivatives of the objectives function and the constraints with respect to the design variables. Standard gradient-based optimization techniques are then used to systematically improve the design, until an optimal process design is reached. In this study the objective is to minimize the total time of the cure cycle, while the constraints include a maximum temperature in the laminate (to avoid thermal degradation) and a maximum deviation of the final fiber volume fraction from its target value (to achieve proper consolidation). The simulations of curing process are performed for EPON 862/W epoxy under a conventional cure cycle, for both thin and thick parts. Time-optimal cure cycles are found using the optimization program. Simulations of fast-curing cycles are also examined. The optimal cycles are similar in form to conventional cure cycles, but give substantially shorter cure times. The entire scheme works automatically and efficiently, simultaneously adjusting multiple design variables at each iteration.     

Optimum heating system design for compression molds

Compression molding is a widely used method of forming composite materials where long fibers are necessary for strength requirements. Compression molding involves putting the charge through a specific, material dependent temperature and pressure path to induce thermochemical cure. During cure, certain temperatures are required for a time. Spatial variation of the cavity temperature can lengthen time needed for curing and cause voids and residual stresses in the part. Towards the goal of uniform cavity surface temperature, an interactive graphics based computer aided system for compression mold heating design has been developed. The system employs a boundary element method treating long, thin cylindrical electric heating elements as singular line sources. It is coupled with a CONMIN algorithm, a nonlinear constrained minimization procedure to, optimize the heating system for uniform temperature over the cavity surface. Realistic constraints are featured to insure design feasibility. The problem is also decomposed in such a way to allow easy redesign and a sensitivity study. Through the optimization process, it was found that uniformities can be obtained which are far better than anything that could be achieved through common sense.     

Cure simulations of thick adhesive bondlines for wind energy applications

Curing of adhesive bondlines is a critical and time-consuming operation in wind turbine blade manufacturing. Significant variation in adhesive thickness can lead to important differences in thermal histories trough the adhesive bonds due to the exothermic nature of the cure process. Reducing bondline cure cycle time and avoiding adhesive overheating are two competing factors in the design of cure temperature cycles. Predictive models on the impact of adhesive thickness variability in bondline cure temperature cycle is currently limited. Adhesive curing and temperature evolution can be simulated by finite element (FE) models coupling the heat transfer problem with the cure kinetics of the adhesive. The cure kinetics of the adhesive system was characterized by isothermal differential scanning calorimetry experiments and implemented in the FE software Abaqus/CAE by user subroutines. Predictions from the FE model were validated experimentally against temperature readings from the curing of 10, 20, and 30 mm thick adhesive bondlines. To highlight the role that predictive models potentially have in the optimization of bondline cure cycles a 2D cross section model representing the trailing edge of a wind turbine blade was used as case study. It was demonstrated that computational models enable customizing cure profiles for nonuniform adhesive thicknesses, ensuring fully cured bondlines with acceptable mechanical properties.     

Sequential heat release: an innovative approach for the control of curing profiles during composite processing based on dual‐curing systems

The sequential heat release (SHR) taking place in dual‐curing systems can facilitate thermal management and control of conversion and temperature gradients during processing of thick composite parts, hence reducing the appearance of internal stresses that compromise the quality of processed parts. This concept is demonstrated in this work by means of numerical simulation of conversion and temperature profiles during processing of an off‐stoichiometric thiol–epoxy dual‐curable system. The simulated processing scenario is the curing stage during resin transfer moulding processing (i.e. after injection or infusion), assuming one‐dimensional heat transfer across the thickness of the composite part. The kinetics of both polymerization stages of the dual‐curing system and thermophysical properties needed for the simulations have been determined using thermal analysis techniques and suitable phenomenological models. The simulations show that SHR makes it possible to reach a stable and uniform intermediate material after completion of the first polymerization process, and enables a better control of the subsequent crosslinking taking place during the second polymerization process due to the lower remaining exothermicity. A simple optimization of curing cycles for composite parts of different thickness has been performed on the basis of quality–time criteria, producing results that are very close to the Pareto‐optimal front obtained by genetic algorithm optimization procedures. © 2018 Society of Chemical Industry     

Processing‐interphase‐property relationship in fiber‐reinforced thermosetting‐matrix composites

Fabrication of thermosetting‐matrix composites is based on a critical step of cure, which involves applying a predefined temperature cycle to a fiber‐resin mixture. Several temperature‐dependent mass transport processes occur in the vicinity of the reinforcement fiber, leading to the formation of an interphase region with different chemical and physical properties from the bulk resin. The cure cycles applied on the macroscopic boundaries of the composite govern the microscopic cure kinetics near the fiber surface, which in turn determines the interphase and composite properties. A predictive approach to directly linking the cure cycles and final composite properties is not presently available and is established for the first time in this paper. A multiscale thermochemical model is developed to predict the concentration profile evolution with time near fiber surfaces at various locations across the composite thickness. The concentration profiles at the gelation time are mapped to modulus profiles within the interphase region, and a finite element analysis is used to determine the overall composite modulus in terms of the constituent interphase, fiber, and matrix properties. Relevant numerical results are presented for the first time where the composite modulus is directly linked to the cure cycle and interphase formation parameters without assumed structures or properties of the interphase. The results provide useful information for selecting material components and cure cycles parameters to achieve desired interphase and composite properties. POLYM. COMPOS., 26:193–208, 2005. © 2005 Society of Plastics Engineers     

橡胶厚制品硫化工艺优化的数值模拟研究

利用ANSYS热分析模块,得到圆柱形橡胶厚制品在传统工艺条件下的温度变化过程,并绘出制品不同位置胶料的硫化效应曲线。根据橡胶材料的硫化条件,计算得到其最小硫化效应及最大硫化效应,结果发现传统工艺条件下的制品在内层达到最小硫化效应时外层严重过硫。改变工艺条件,通过APDL参数化语言设计,实现由5级硫化温度与9级硫化加热时间组成的45组不同工艺条件的循环分析、结果存储及自动筛选,得到硫化质量满足要求且效率相对较高的工艺组别。结果表明,采用APDL参数化语言,进行数值模拟分析,能够实现橡胶厚制品硫化工艺条件的优化设计。     

Curing processes simulation of complex shape carbon fiber reinforced composite components produced by vacuum infusion

Thermal processes of carbon fiber‐reinforced composite parts curing cycle were studied experimentally and by mathematical simulation. Flat panel, T‐stringer, and five‐stringer detail based on tetrafunctional MY721 epoxy resin were investigated. Application of standard curing cycle (heating followed by isothermal exposure at 180°C) leads to the overheating of 28°C in flat panel and of 30°C in T‐stringer of 24 mm thickness. The model considers the vacuum bag with auxiliary materials as separate layers. This approach allows to simulate the shifting of thermal field to the vacuum bag side. Simulation results show good agreement with the experimentally observed temperature fields. The model helps to optimize the curing cycle to reduce the local overheating. The same temperature regime could be used for all three geometries. Therefore, the optimization can be carried out only for the simple‐shape parts to save the calculation time and simplify experiments at the stage of model testing. POLYM. COMPOS., 37:2252–2259, 2016. © 2015 Society of Plastics Engineers     

Material systems for rapid manufacture of composite structures

A manufacturing process is described that builds complex composite parts using a layered building process in which each layer of pre‐preg composite is laid and cured as the build progresses. In order to employ on‐line curing without molds, resin technologies that provide fast curing at room temperature—ultraviolet curable and epoxy/polyamide—were investigated. UV‐curable resins were tested for their ability to “shadow” cure by exposing carbon fiber composites to ultraviolet light to determine if the cure propagated from areas directly exposed to areas under fibers. Though ultraviolet curing showed advantages in cure time and low volatile production, very minimal “shadow” curing was achieved. A low temperature curing epoxy/polyamide mixture was tested for the effects of cure temperature, cure time, and mix ratio on the final degree of cure (%DOC) and glass transition temperature (T_g). Layers were made using different resin mixtures, partially cured, and used to build layered parts to determine curing characteristics during the lay‐up process. In the epoxy/polyamide mixtures, mix ratio had little effect on the reaction rate but did affect the T_g. A kinetic model was established for the resin epoxy/polyamide system for optimizing processing conditions during fabrication. However, the model failed to correctly predict the fabrication. The reaction of the material was different during the fabrication process than during the isothermal cure due to the presence of oxygen. During the build process, the degree of cure in each layer increased significantly over the prestaged degree of cure in less time than theoretically predicted. However, the final resin properties, such as T_g, were still below the specifications for high performance parts.     

A three‐function numerical model for the prediction of vulcanization‐reversion of rubber during sulfur curing

The cross‐linking mechanisms of sulfur vulcanization are not analytically known and, therefore, reticulation kinetics has to be deduced macroscopically from standardized tests. One of the most popular laboratory test to characterize curing and reversion is the oscillating disk rheometer ODR, which gives a quantitative assessment of scorch, cure rate, and state of cure. In this article, a numerical two‐step approach, which is based on the utilization of experimental ODR data and aimed at predicting the degree of vulcanization of thick rubber items cured with accelerated sulfur, is presented. In step one, a composite numerical three‐function curve is used to fit experimental rheometer data, able to describe the increases of the viscosity at successive curing times and at different controlled temperatures, requiring only few points of the experimental cure curve to predict the global behavior. Both the case of indefinite increase of the torque and reversion can be reproduced with the model. In step two, considering the same rubber compound of step one, numerical cure curves at different temperatures are collected in a database and successively implemented in a Finite Element software, which is specifically developed to perform thermal analyzes on complex 2D/3D geometries. As an example, an extruded thick EPDM section is considered and meshed through eight‐noded isoparametric plane elements. Several FEM simulations are repeated by changing exposition time t_c and external curing temperature T_n, to evaluate for each (t_c,T_n) couple the corresponding mechanical properties of the item at the end of the thermal treatment. A recently presented bisectional approach, alternating tangent (AT), is used to drastically reduce the computational efforts required to converge to the optimal solution associated with the maximum value of an output property, tensile strength. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

A machine system for the rapid production of composite structures

A rapid manufacturing process for thermosetting materials was developed to produce composite structures in a manner similar to rapid prototyping. The machine system builds the part layer by layer with continuous curing and consolidation, making traditional curing methods unnecessary. Each part layer, which has been prestaged to provide a desired degree of cure, moves under a heat gun and a roller for curing and consolidation. The heat gun initiates the cure in the part layer, partially curing it and making it tacky enough for the next layer to stick to it when it goes through the consolidation process. The cure continues to advance as each layer is placed. Testing was performed by fabricating three‐layer parts and performing tests on single strips of towpreg to determine how temperature and time affect the degree of cure. The material was IM7 carbon fiber/977‐3 epoxy towpreg, prestaged in a tunnel oven to an approximate 30% degree of cure. Hot air temperatures of 150 – 380°C (302 – 716°F), roller forces of 236 – 472 N (53 – 106 Ibf), table speeds of 13 – 38 mm/sec (0.5 – 1.5 in/sec), and times of exposure to hot air of 5, 10, and 20 seconds were used. The results showed that the machine system can continuously cure and consolidate parts. The cure was advanced to any level up to 100% degree of cure with an air temperature of 380°C. Parts were produced with layers well stuck together. Within the limitations of the experiments, temperature was determined to be the most significant factor affecting degree of cure. With the correct processing conditions, the machine system has the capability to produce well‐consolidated, high quality parts without using traditional curing methods. The design of the machine system allows for the addition of components for the fabrication of arbitrarily shaped parts and for different curing methods.     

Spatially homogeneous gelation in liquid composite molding

On‐line mixing of the resin with its curing agents prior to injection into a mold is a common industrial technique for fabricating composite parts. For vinyl‐ester resins that cure via free radical polymerization, the concentrations of retarder, accelerator, and initiator are pre‐selected and cannot be changed during the injection. Hence, the resin that enters the mold the earliest has cured longer than the resin that enters the mold later, since the gel time for the resin is the same, owing to the fixed ratio of the curing agents. This approach leads to inhomogeneous cure of the resin and consequently to longer residence time of the resin in the mold. It requires an additional 50 to 75 percent of the filling time before a part can be de‐molded. In this study, it is shown that by adjusting the concentration of curing agents during the injection, a more homogeneous gel time throughout the mold can be achieved. The time to de‐mold is reduced to 18‐24 percent of the filling time. Sensors that measure the conductivity of the resin were used to detect the location and monitor the cure of vinyl‐ester. This approach could be extended to other resin systems to control the spatial curing of the resin in the mold.     

On‐line mixing during injection and simultaneous curing in liquid composite molding processes

Liquid composite molding (LCM) processes such as resin transfer molding (RTM) and vacuum assisted RTM (VARTM) are used to manufacture high quality and net‐shape fiber reinforced composite parts. All LCM processes impregnate fiber preforms packed in a mold cavity with a thermoset resin. After the preform is fully saturated, the injection is discontinued but the resin continues to cure. Once the curing step is complete, the part is de‐molded. The resin has to be mixed with a curing agent to cure. Typically, the resin and the curing agent are mixed together in a pressure pot before the injection. This has several disadvantages, such as storage of large amounts of hazardous polymerizing resin, wastage, and cleaning of cured resin from the injection line. This paper proposes the implementation and calibration of an alternative to this technique. The approach is to mix the curing agent with the resin as the resin enters the mold through a separate system featuring two feed‐lines. Such a system will enable one to maintain a uniform gel time throughout the part by varying the mixing ratio of resin and the catalyst during the injection. An experimental study of such on‐line mixing to obtain simultaneous curing and to reduce the overall curing time is conducted and presented in this paper. Implementation of a control scheme that varies the curing agent during injection and its effect on cure time is benchmarked with the process in which the percentage of curing agent is held constant. The gel time for the fabricated parts was reduced by 20–25% by continuously varying the percentage of curing agent during injection. POLYM. COMPOS., 26:74–83, 2005. © 2004 Society of Plastics Engineers     

Effects of storage aging on the cure kinetics of T700/BMI prepregs for advanced composites

The effects of room temperature aging on the cure kinetics of a bismaleimide (BMI) matrix prepreg have been characterized by different time and storage conditions. The study has focused on the stability of BMI matrix carbon fiber prepregs, when exposed to controlled environmental conditions before being used in composite manufacturing. The effects of aging on reactivity, glass transition temperature, and process window have been investigated by differential scanning calorimetrer through dynamic and isothermal tests. A theoretical kinetic model for epoxy matrix prepregs, developed in previous studies, has been applied to the cure of both aged and virgin BMI matrix. The model is able to satisfactorily describe the effect of processing variables such as temperature and degree of cure during the curing of the composite under different conditions (curing temperature and heating rate). The effects of diffusion‐controlled phenomena on the cure kinetics, associated with changes in glass transition temperature as a function of the degree of cure, have been taken into account in the formulation of an nth‐order kinetic model. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Equivalent processing time analysis of glass transition development in epoxy/carbon fiber composite systems

The cure kinetics and glass transition development of a commercially available epoxy/carbon fiber prepreg system, DMS 2224 (Hexel F584), was investigated by isothermal and dynamic‐heating experiments. The curing kinetics of the model prepreg system exhibited a limited degree of cure as a function of isothermal curing temperatures seemingly due to the rate‐determining diffusion of growing polymer chains. Incorporating the obtained maximum degree of cure to the kinetic model development, the developed kinetic equation accurately described both isothermal and dynamic‐heating behavior of the model prepreg system. The glass transition temperature was also described by a modified DiBeneditto equation as a function of degree of cure. Finally, the equivalent processing time (EPT) was used to investigate the development of glass transition temperature for various curing conditions envisioning the internal stress buildup during curing and cooling stages of epoxy‐based composite processing. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 144–154, 2002; DOI 10.1002/app.10282     

Cure reaction of multi‐walled carbon nanotubes/diglycidyl ether of bisphenol A/2‐ethyl‐4‐methylimidazole (MWCNTs/DGEBA/EMI‐2,4) nanocomposites: effect of carboxylic functionalization of MWCNTs

BACKGROUND: The aim of this work was to study, using differential scanning calorimetry, the effect of carboxylic functionalization of multi‐walled carbon nanotubes (MWCNTs) on the cure reaction of MWCNTs/diglycidyl ether of bisphenol A/2‐ethyl‐4‐methylimidazole (MWCNTs/DGEBA/EMI‐2,4) nanocomposites. This is important for the practical design, analysis and optimization of novel materials processing. RESULTS: Comparing the influence of non‐functionalized MWCNTs and carboxyl‐functionalized MWCNTs, it was found that, at the initial curing stage, both MWCNTs act as catalyst and COOH functionalization of MWCNTs has a catalytic effect on the curing process. Then, at the later curing stage, non‐functionalized MWCNTs prevent the occurrence of vitrification, whereas COOH functionalization of MWCNTs promotes vitrification. Non‐functionalized MWCNTs decrease the degree of curing, as evidenced by lower total heat of reaction and lower glass transition temperatures of nanocomposites compared to neat epoxy; however, COOH functionalization of MWCNTs increases the degree of curing. CONCLUSION: For the development of composites, COOH functionalization of MWCNTs could bring a positive influence to the composite process. Its acceleration of cure could help shorten pre‐cure time or lower pre‐treatment temperature, and its effect of promoting vitrification could help shorten post‐cure time or lower post‐treatment temperature. Copyright © 2009 Society of Chemical Industry     

Simultaneous optimization of die‐heating and pull‐speed in pultrusion of thermosetting composites

Die‐temperature and pull‐speed are the key parameters in pultrusion that affect the degree and uniformity of the curing of thermosetting composites, which in turn influence their mechanical properties and in‐service performance. This paper presents the development, implementation, and validation of a numerical procedure for arriving at an optimum combination of die‐heater temperatures and pull‐speed for producing a uniformly cured composite pultrudate. An objective function representing the effects of both parameters on the distribution of degree‐of‐cure across the cross section of a pultruded part at the die exit was established. The function was used to develop an algorithm for calculating the required changes in die‐heater temperatures and pull‐speed to achieve a desirable degree‐of‐cure with maximum possible uniformity. The algorithm was implemented using the three‐dimensional. Finite Element/Nodal Control Volume (FE/NCV) approach for process simlation, in which a general‐purpose FE package was used to perform heat transfer analysis, together with other routines developed to perform cure modeling and the optimization. The application of the developed procedure was demonstrated by simulating pultrusion of a graphite/epoxy C‐section. The results of the studies show that the procedure is numerically stable and works well for a die with multiple heaters.     

Effect of cure kinetic simulation model on optimized thermal cure cycle for thin‐sectioned composite parts

In this article, the optimal design of the thermal cure cycle of a prepreg material has been presented. Three different kinetic methods have been considered to determine the kinetic parameters of the curing reaction of an epoxy/glass prepreg. The capability of these kinetic models in simultaneously predicting kinetic parameters in isothermal and non‐isothermal conditions differs with each other. For the simulation of the cure cycle, a zero‐dimensional model has been used and coupled with the genetic algorithm to optimize the cure cycle of thin composite parts. The effect of the kinetic models on the optimized cure cycle has been investigated. Of these models, only one model could predict the kinetic parameters outside the experimental temperature regions and resulted in a reliable and acceptable optimized cure cycle. The validity of the optimized cure cycle has been also verified experimentally. POLYM. COMPOS., 34:1172–1179, 2013. © 2013 Society of Plastics Engineers