Analysis of heat transfer in autoclave technology

In autoclave technology, polymer based composites are manufactured under the application of pressure and heat. The heat transferred between the energy carrying fluid and the bag‐composite‐tool element activates exothermic curing reactions, leading to composite consolidation. The convective heat transfer mechanism is the most relevant aspect controlling the rate of chemical and physical transformations associated with composite curing. Moreover, the fluidodynamic regime that results from the interactions between the autoclave and the tool geometry, even if totally predictable in theory, is unattainable in practice. In this study, the heat transfer phenomena occurring during the autoclave manufacturing cycle have been analyzed. The assumption of a negligible through‐the‐thickness thermal gradient led to simplified energy balance equations. In this case, the thermal evolution of the manufacturing elements has been completely determined by two parameters: the global convective heat exchange coefficient, setting the rate of the heat transfer between the autoclave environment and the bag‐composite‐tool element, and the adiabatic temperature rise, establishing the relevance of the polymerization exotherm. A scaling analysis has been performed in order to identify the dimensionless parameters controlling the autoclave process. The developed semitheoretical methodology has been extensively tested by comparison with experimental data from an industrial autoclave.     

High temperature vacuum assisted resin transfer molding of phenylethynyl terminated imide composites

Vacuum Assisted Resin Transfer Molding (VARTM) has proven to be a cost effective process for manufacturing composite structures compared with prepreg/autoclave and traditional resin transfer molding (RTM) processes. However, VARTM has not been accomplished with high temperature resins (such as polyimides) until recently, primarily because no resins had low melt viscosity and long melt stability that are required by VARTM. With the recent invention of phenylethynyl terminated imides (PETIs), high temperature VARTM has been achieved. Two processing methods, in‐plane and through‐thickness resin flow, were proposed and tested. Both methods are capable of fabricating polyimide matrix composites; and the carbon fiber laminates yield good fiber‐resin interfacial bonding and comparable mechanical properties to those laminates fabricated using RTM. POLYM. COMPOS., 2011. © 2010 Society of Plastics Engineers     

Use of eco‐friendly epoxy resins from renewable resources as potential substitutes of petrochemical epoxy resins for ambient cured composites with flax reinforcements

In the last years, some high renewable content epoxy resins, derived from vegetable oils, have been developed at industrial level and are now commercially available; these can compete with petroleum‐based resins as thermoset matrices for composite materials. Nevertheless, due to the relatively high cost in comparison to petroleum‐based resins, their use is still restricted to applications with relatively low volume consumption such as model making, tuning components, nautical parts, special effects, outdoor sculptures, etc. in which, the use of composite laminates with carbon, aramid and, mainly, glass fibers is generalized by using hand layup and vacuum assisted resin transfer molding (VARTM) techniques due to low manufacturing costs and easy implementation. In this work, we study the behavior of two high renewable content epoxy resins derived from vegetable oils as potential substitutes of petroleum‐based epoxies in composite laminates with flax reinforcements by using the VARTM technique. The curing behavior of the different epoxy resins is compared in terms of the gel point and exothermicity profile by differential scanning calorimetry (DSC). In addition, overall performance of flax‐epoxy composites is compared with standardized mechanical (tensile, flexural and impact) and thermal (Vicat softening temperature, heat deflection temperature, thermo‐mechanical analysis) tests. The curing DSC profiles of the two eco‐friendly epoxy resins are similar to a conventional epoxy resin. They can be easily handled and processed by conventional VARTM process thus leading to composite laminates with flax with balanced mechanical and thermal properties, similar or even higher to a multipurpose epoxy resin. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

非热压罐预浸料制备及成型工艺研究

采用单面胶膜浸渍的方法制备非热压罐(Out of Autoclave,以下简称"OoA")预浸料。采用三种方法测定预浸料的浸渍度,通过预浸料的细观形貌、层压板孔隙率及力学性能,系统地分析了浸渍度对碳纤维增强复合材料(CFRP)质量的影响,22%的浸渍度时性能最优。与OoA预浸料相匹配的固化工艺至关重要,通过无损检测、孔隙率、微观结构及力学性能对比分析,120℃/2 h作为第一阶段的固化工艺最适合,同时层压板热性能、力学性能与热压罐相媲美。     

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.     

Optimal curing for thermoset matrix composites: Thermochemical considerations

A design sensitivity analysis is used to optimize the applied wall temperature vs. time in autoclave curing for thermoset matrix composites. The calculation minimizes the cure time and obeys a maximum temperature constraint in the composite. The transient, coupled thermal and cure problem is solved by a finite element method. Design sensitivity information is extracted efficiently from this primal analysis, based on an analytical, direct differentiation approach. The sensitivities are then used with gradient‐based optimization techniques to systematically improve the curing process. The optimal cure cycles for different numbers of temperature dwells may be similar (for a 2 mm thick part) or very different (for a 4 cm thick part), depending on the nature of the problem. In the latter case a large reduction of cure time is obtained when a three‐dwell cure cycle is used, and the optimizer has more flexibility to adjust the cure cycle. This systematic optimization approach provides a powerful and practical means of optimizing composite manufacturing processes.     

Microwave and conventional curing of thick‐section thermoset composite laminates: Experiment and simulation

In conventional processing, thermal gradients cause differential curing of thick laminates and undesirable outside‐in solidification. To reduce thermal gradients, thick laminates are processed at lower cure temperatures and heated with slow heating rates, resulting in excessive cure times. Microwaves can transmit energy volumetrically and instantaneously through direct interaction of materials with applied electromagnetic fields. The more efficient energy transfer of microwaves can alleviate the problems associated with differential curing, and the preferred inside‐out solidification can be obtained. In this work, both microwave curing and thermal curing of 24.5 mm (1 inch) thick‐section glass/epoxy laminates are investigated through the development of a numerical process simulation and conducting experiments in processing thick laminates in a conventional autoclave and a microwave furnace. Outside‐in curing of the autoclave‐processed laminate resulted in visible matrix cracks, while cracks were not visible in the microwave‐processed laminate. Both numerical and experimental results show that volumetric heating due to microwaves promotes an inside‐out cure and can dramatically reduce the overall processing time.     

Hybrid fiber fabric composites from poly ether ether ketone and glass fiber

Poly ether ether ketone (PEEK) polymer was extruded into filaments and cowoven into unidirectional hybrid fabric with glass as reinforcement fiber. The hybrid fabrics were then converted into laminates and their properties with special reference to crystallization behavior has been studied. The composite laminates have been evaluated for mechanical properties, such as tensile strength, interlaminar shear strength (ILSS), and flexural strength. The thermal behavior of the composite laminates were analyzed using differential scanning calorimeter, thermogravimetric analyzer, dynamic mechanical analyzer (DMA), and thermomechanical analyzer (TMA). The exposure of the fabricated composite laminates to high temperature (400 and 500°C) using radiant heat source resulted in an improvement in the crystallanity. The morphological behavior and PEEK resin distribution in the composite laminates were confirmed using scanning electron microscope (SEM) and nondestructive testing (NDT). Although DMA results showed a loss in modulus above glass transition temperature (T_g), a fair retention in properties was noticed up to 300°C. The ability of the composite laminates to undergo positive thermal expansion as confirmed through TMA suggests the potential application of glass–PEEK composites in aerospace sector. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117:1446–1459, 2010     

A model for the thermal and chemorheological behavior of thermosets. I: Processing of epoxy-based composites

The rheological and thermokinetic aspects of the cure of epoxy based composite laminates are analyzed by means of a computer program developed using the heat transfer and heat generating characteristics of a polymerizable system. In particular, the temperature and degree of cure influence on the resin viscosity have been first considered, then the temperature profiles, calculated according to an appropriate kinetic and heat transfer modeling, have been used to predict the corresponding viscosity profiles. Molecular and thermocalorimetric parameters are used for the prediction of the theoretical chemorheological behavior. Commercial epoxy systems commonly used in the preparation of carbon fiber laminates have been characterized by Differential Scanning Calorimetry (DSC) and dynamic viscosity measurements and the results are compared with the theoretically predicted values.     

Mechanical characterization in laminated composite for cryogenic application

Damage in cryogenic composite fuel tanks induced during manufacturing and advanced by thermomechanical cycling can accelerate leakage of the propellant. Whether the leakage exceeds tolerable levels depends on many factors, including pressure gradients, microcrack density, other damage such as delamination, connectivity of the cracks, residual stresses from manufacture, service‐induced stresses from thermal and mechanical loads, and composite lay‐up. This article is concerned with the creation and the detection of damage in cryogenic composites due to thermomechanical loading. The first part deals with cryocycling (cycling between two temperatures) test procedures, that were developed to understand the damage produced in cryogenic composite laminates under thermomechanical loading. An apparatus was developed to thermomechanically cycle coupon test specimens under different thermomechanical loadings. IM7/977‐2 and IM7/5250‐4 graphite/epoxy cross‐ply [90₂/0₂]_S and angle‐ply [0/‐45/90/45/0/45090/‐45/0]_S laminates were tested using these systems. Ply‐by‐ply microcrack density was measured as a function of thermomechanical cycles. The second part of this article deals with effect of thermal gradients due to sudden exposure to a cryogenic temperature. The sudden exposure to cryogenic temperatures may have caused the microcracks, due to large temperature variations through the thickness and the resulting thermal stresses. In this work, an investigation of the sudden exposure to cryogenic temperatures is conducted experimentally with and without an insulating layer. POLYM. COMPOS., 2013. © 2013 Society of Plastics Engineers     

Cure characterization of Cycom 977‐2A carbon/epoxy composites for quickstep processing

In this effort, Quickstep, a relatively a new technique, have been employed for manufacturing of composite materials. The cure schedule provided by a prepreg manufacturer is usually designed for autoclave or other traditional processing techniques and thermosetting resin systems are formulated for ramp rate curing 2–3 K min^?1. While in case of Quickstep processing, ramp rates of 15 K min^?1 can be achieved, thus changing the chemorheology of resin. The cure process of 977‐2A carbon/epoxy composites was evaluated for Quickstep processing using differential scanning calorimetry (DSC), dynamic mechanical and thermal analysis, and Fourier transformed infrared and results were compared with cure cycle employed for autoclave curing. Optimum hold time for Quickstep processing at upper curing temperature (180°C) was determined using DSC. The hold time of 120 min at 180°C was found to be suitable for Quickstep cure cycle, producing a panel of similar degree of cure to that achieved through autoclave processing schedule. Final degree of cure was dependent on time spent at upper cure temperature and slightly on initial steps of the cure cycle which was used to control the resin flow, fiber wetting, and void removal. Quickstep processed samples exhibited higher T_g and crosslink density and similar molecular network structure to the autoclave cured samples. POLYM. ENG. SCI., 54:887–898, 2014. © 2013 Society of Plastics Engineers     

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     

Effect of processing conditions on physical properties of 3‐aminophenoxyphthalonitrile/epoxy laminates

To develop high performances of polymer composite laminates, differential scanning calorimetry and dynamic rheological analysis studies were conducted to show curing behaviors of 3‐aminophenoxyphthalonitrile/epoxy resin (3‐APN/EP) matrix and define cure parameters of manufacturing processes. Glass fiber reinforced 3‐APN/EP (GF/3‐APN/EP) composite laminates were successfully prepared through different processing conditions with three parameters such as pressures, temperatures, and time. Based on flexure tests, dynamic mechanical analysis, thermal gravimetric analysis, and scanning electron microscope, the complementary catalytic effect of the three processing parameters is investigated by studying mechanical behavior, thermomechanical behavior, thermal behavior, and fracture morphology of GF/3‐APN/EP laminates. The 50/50 GF/3‐APN/EP laminates showed a significant improvement in flexural strength, glass transition temperature (T_g), and thermal stability with favorable processing parameters. It was also found that the T_g and thermal stability were significantly improved by the postheated treatment method. The effect of manufacturing process provides a new and simple route for the polymer–matrix composites application, which indicates that the composites can be manufactured at low temperatures. But, they can be used in a high temperature environment. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39746.     

Rapid thermal cycling with low thermal inertia tools

Solid freeform fabrication processes offer the manufacturing flexibility for tools with low thermal inertia and internal conformal channels for rapid thermal cycling in injection molding. This article analyzed each step of a rapid thermal cycling process and provided quantitative guidance for tooling design. The proposed design methodology was tested on a three‐dimensional (3D) printed benchmark tool with truss support. Rapid cooling test on the benchmark tool resulted in the mold time constant shorter than 2.3 s and the cavity temperature uniformity better than 3°C. Preliminary tests demonstrated the technical feasibility of using a solid freeform fabrication process to fabricate low thermal inertia tools for improved heat management in injection molding. POLYM. ENG. SCI., 2009. © 2008 Society of Plastics Engineers     

Modeling of heat transfer and crystallization kinetics in thermoplastic composites manufacturing: Pultrusion

Whereas pultrusion with thermoset resins has been widely analyzed, there is a scarcity of knowledge about pultrusion with thermoplastic resins. The aims of this work were to strive towards deeper knowledge of temperature distribution and degree of crystallinity in the composite during processing, to develop a tool for process simulations with a variety of processing parameters, and to provide input data to models for pressure and matrix flow. The heat transfer model presented herein describes the temperature distribution within the composite throughout the thermoplastic pultrusion process. The transient heat transfer situation is modeled one-dimensionally through a transverse cross section far from the edges of a high aspect ratio composite. Temperature-dependent thermal properties, partly non-infinite contact conductance, and heat contribution from crystallization were taken into consideration. The degree of crystallinity within the composite was determined as well. The analysis also includes an experimental verification of the heat transfer model. Results show good agreement with experimental data.     

Effect of thermal treatment on the tensile and in‐plane shear behavior of carbon fiber‐reinforced poly(phenylene sulfide) composite specimens

The effect of PPS matrix evolution occurring during thermal treatment of carbon fiber‐reinforced PPS plies prior to their consolidation to laminates on the mechanical behavior of the composite material has been investigated. The thermal treatments were performed at temperatures and times, which are relevant for processing PPS composites. All thermal treatments were carried out in an oven in air to facilitate the presence of oxygen, while the subsequent consolidation was performed in an autoclave. The tensile and in‐plane shear behavior of both, thermal‐treated and untreated materials, was investigated. Differential scanning calorimetry and microscopy analyses were made to evaluate the effect of the performed thermal treatments on degree of crystallinity and porosity of the laminates. The mechanical tests carried out have shown an appreciable degradation of the mechanical properties investigated. The observed degradation increases with increasing thermal treatment temperature and time when thermal treatments were carried out on each single composite ply prior to the consolidation. On the other hand, when, prior to the consolidation, the whole set of plies was subjected to thermal treatment, improved mechanical properties were observed. The results were discussed under the viewpoint of PPS matrix evolution during processing of the composite plies in the presence of oxygen. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

3D finite element method for the simulation of the filling stage in injection molding

During the molding of industrial parts using injection molding, the molten polymer flow through converging and diverging sections as well as in areas presenting thickness and flow direction changes. A good understanding of the flow behavior and thermal history is important in order to optimize the part design and molding conditions. This is particularly true in the case of automotive and electronic applications where the coupled phenomena of fluid flow and heat transfer determine to a large extent the final properties of the part. This paper presents a 3D finite element model capable of predicting the velocity, pressure, and temperature fields, as well as the position of the flow fronts. The velocity and pressure fields are governed by the generalized Stokes equations. The fluid behavior is predicted through the Carreau Law and Arrhenius constitutive models. These equations are solved using a Galerkin formulation. A mixed formulation is used to satisfy the continuity equation. The tracking of the flow front is modeled by using a pseudo-concentration method and the model equations are solved using a Petrov-Galerkin formulation. The validity of the method has been tested through the analysis of the flow in simple geometries. Its practical relevance has been proven through the analysis of an industrial part.     

Effects of tool coating materials and coating thickness on cutting temperature distribution with coated tools

Coated tools are currently widely used tool technology in machining. The influence of tool coating on heat transfer has become an active field of research enjoying constantly increasing attention in the field of machining. This paper is devoted to the cutting temperature in machining H13 hardened steel with monolayer coated tools (TiN, TiAlN, and Al₂O₃) and multilayer coated tools (TiN/TiC/TiN and TiAlN/TiN). Equivalent composite thermal conductivity and thermal diffusivity of multilayer coated tools were calculated using the equivalent approach. The established heat transfer analytical models estimated coating temperature in turning. The effect of tool coating in steady and transient heat transfer was studied, as well as the cutting temperature distribution. It reveals that the tool coating material and coating thickness can influence the cutting temperature distribution of coated tool. Thermal conductivity of coating material affects the steady cutting temperature distribution, and thermal diffusivity of coating material affects the transient cutting temperature distribution of coating tools.     

Use of a simple,inexpensive pressure sensor to measure hydrostatic resin pressure during processing of composite laminates

A simple and cheap method of measuring the resin pressure within a composite laminate during processing is presented. The method consists of using a small diameter, long needle filled with inert fluid and connected to an external pressure sensor, to measure the resin pressure at a point inside a composite laminate. This method can be used to investigate resin flow, laminate compaction, the control of voids, and in several composite material processing methods such as autoclave processing, hot press curing and resin transfer molding. The sensors are suitable for research and development or troubleshooting, but not for production. Sensor assemblies were developed and tested to show that their response is reproducible, linear and stable with temperature and time. Resin pressure profiles for two AS4/3501-6 laminates were generated and compared. The experimental results were also compared to the resin flow simulation of a general processing model for composites, COMPRO. It is shown that the resin profile in the laminate is influenced by the presence of the bleeder cloth and the vacuum bag pressure. A significant pressure drop corresponded to the point of minimum viscosity of the resin. Finally, the resin pressure was stabilized when the resin reached gelation.     

Influence of cure conditions on out‐of‐autoclave bismaleimide composite laminates

Bismaleimides (BMI) are thermosetting polymers that are widely used in the aerospace industry due to their good physical properties at elevated temperatures and humid environments. BMI‐based composites are used as a replacement for conventional epoxy resins at higher service temperatures. Out‐of‐Autoclave (OOA) processing of BMI composites is similar to that of epoxies but requires higher cure temperatures. Polymer properties such as degree of cure and crosslink density are dependent on the cure cycle used. These properties affect mechanical strength as well as glass transition temperature of the composite. In the current research, carbon fiber/BMI composite laminates were manufactured by OOA processing. The void content was measured using acid digestion techniques. The influence of cure cycle variations on glass transition temperature and mechanical strength was investigated. Properties of manufactured specimens were compared with that of conventional autoclave cured BMI composites. Laminates fabricated via OOA processing exhibited properties comparable to that of autoclave cured composites. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43984.