Autoclave manufacturing of thick composites

Autoclave manufacturing of thermoset composites is determined mainly by heat transfer phenomena. As a matter of fact, the consolidation of composite laminates takes place by the progress of the polymerization, which is activated thermally. The design and control of the autoclave process relies on the capability to manage the relationship between the temperature-pressure cycle of the heat carrier fluid and the temperature distribution through the manufacturing part. In particular, in industrial cases, the main limitations reside in the correct evaluation of the local convective heat transfer conditions through the autoclave and in the evaluation of the local thermal inertia arising from the bagging-tooling system. In this study, the autoclave manufacturing of thick laminates has been addressed by modeling the heat transport phenomena occurring through the composite, the bagging and the tooling system. A new methodology for the evaluation of the energy transfer regimes has been proposed accounting for the heat fluxes from the bag and the tool side, the temperature through-the-thickness gradients and the heat generated by the resin polymerization reaction. The proposed approach enables the prediction of the temperature history of the autoclave assembly without knowledge of the effective thermal inertia of the two external layers, which could be difficult to evaluate owing to possible deformations of the bag during the manufacturing cycle and nonuniform shape of the metallic tool along the part. Experimental data from industrial autoclave runs have been collected and analyzed to validate the method.     

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.     

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.     

VARI成型V型构件固化变形和影响因素分析

复合材料在成型过程中会伴随着复杂的热化学和物理变化,此现象会导致复合材料内部的残余应力积累并进一步使复合材料在脱模后产生固化变形。采用顺序耦合热应力的方法建立了预测复合材料固化变形的三维有限元模型,对真空辅助成型(VARI)的复合材料V型构件的固化变形进行了数值模拟和试验验证,并进一步分析了固化温度、对流换热系数、构件厚度等因素对复合材料V型构件的影响。模拟结果表明,复合材料V型构件的回弹角会随着固化温度和对流换热系数的升高而增大,随着构件的厚度增加而减小。固化温度从150℃升高至190℃,回弹角增加了72%;对流换热系数从1 W/(m2 K)升高至100 W/(m2 K),回弹角增加了15%;构件厚度从2.4 mm增加至4.8 mm,回弹角减小了43%。     

Effects of tool‐embedded dielectric sensors on heat transfer phenomena during composite cure

The present study examines the effects that placement of tool‐mounted dielectric sensors in the composite cure assembly has on the local thermal and degree‐of‐cure fields. A nonlinear heat transfer model, incorporating appropriate thermoset cure and thermal property submodels, is used to simulate the cure of carbon and glass composites of various thicknesses. The model is validated against experimental data obtained during the cure of composite samples in a resin‐transfer molding tool. The results of the simulations show that embedding a sensor can have a significant effect on the thermal field, because of the thermal conductivity mismatch between the metal tool and the sensor substrate material. In the heating‐up stage of the cure profile, sensors embedded in the heated tool side intensify thermal gradients, which causes a corresponding lag in the progress of the reaction in the area adjacent to the sensor. Exothermic effects are also intensified by the sensor presence, especially in the case of thick composite curing. These results suggest that control strategies based on the utilization of embedded sensors should take these effects into account. POLYM. COMPOS., 28:139–152, 2007. © 2007 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     

橡胶导热助剂的优选

探讨导热助剂和填料对胶料导热性能的影响。结果表明：导热增强剂TB-1对橡胶的补强作用和导热效应优于氧化锌、三氧化二铁、三氧化二铝,可以用作导热型补强助剂;用于轮胎胎侧胶和胎体胶,可以提高胶料的导热性能,对胶料物理性能影响不大;用于水胎胎体胶,可以提高水胎传热速度,缩短硫化时间,提高硫化效率,降低硫化能耗。不同种类的炭黑对胶料导热性能影响不同;炭黑用量增大,胶料的导热系数增大。     

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     

Numerical analysis of parametric effects on consolidation of angle‐bended composite laminates

In the previous study, the finite element formulation has been developed by our group based on two‐dimensional resin flow and fiber compaction model. Good agreement between simulations and experimental results was found under the one‐dimensional flow condition. In this article, the two‐dimensional model was used to simulate the consolidation of angle‐bended laminates with the convex tool in autoclave process. The effects of material properties on the consolidation were studied. It was found that the fiber bed shear modulus significantly affects the compaction behavior in the corner section of angle‐bended laminate, the fiber bed compaction property decide the laminate deformation, and the resin viscosity and fiber bed permeability affect the rate of laminate compaction and consolidation time. The angle‐bended T700/BMI QY8911‐Ilaminates were manufactured in autoclave process. The experimental data validate the numerical simulation method for the consolidation of the angle‐bended laminates. These results are greatly helpful for the optimization of processing parameters, improvement of composite parts quality, and reduction of the fabrication cost. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Alumina–Aluminide Alloys (3A) Technology: I, Model Development

The general reaction behavior of the 3A process under the thermal explosion mode of synthesis has been investigated via a continuum model. The continuum model uses mass and energy balances to predict temperature difference ( T _s,avg− T _f) curves, as well as profile curves of the reactant conversions and sample temperature. In particular, the effect of the dimensionless parameters associated with the rate of local heat generation (β, the thermicity factor), the activation energy (γ, the Arrhenius number), the rate of heat redistribution (α, the modified thermal diffusivity), the rate of heat transfer by convection (Bi, the Biot number or convective heat transfer parameter), and the rate of heat transfer by radiation (Ω, the radiative heat transfer parameter) were investigated. Conditions to control the reaction process, which should produce high-density final products, were determined. It was found that the overall maximum temperature may be reduced for high γ, low β, high α, and high Bi and Ω. In terms of processing conditions, this may be obtained by reducing the initial reactant concentrations, optimizing the particle size, using small sample sizes and high compaction pressure, and increasing the heat loss by using a high thermal conductivity inert gas.     

Comparison of the guarded‐heat‐flow and transient‐plane‐source methods for carbon‐filled nylon 6,6 composites: Experiments and modeling

In this study, two different carbons (synthetic graphite particles and carbon fiber) were added to nylon 6,6, and the resulting composites were tested for thermal conductivity. The first goal of this work was to compare through‐plane thermal conductivity results from the guarded‐heat‐flow method and the transient‐plane‐source method. The results showed that both test methods gave similar through‐plane thermal conductivity results for composites containing 10–40 wt % synthetic graphite and for composites containing 5–40 wt % carbon fiber. The advantages of using the transient‐plane‐source method were that the in‐plane thermal conductivity was also measured and the experimental time was shorter than that of the guarded‐heat‐flow method. The second goal of this work was to develop and use a detailed finite‐element analysis to model heat transfer within a carbon‐filled nylon 6,6 composite sample for the transient‐plane‐source method and compare these results to actual experimental results. The results showed that the finite‐element model compared well with the actual experimental data. The finite‐element model could be used in the future as a design tool to predict the dynamic thermal response of different composite materials for many applications. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006     

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     

A Theoretical Model for the Nongray Radiation Drying of Polyvinylalcohol/Water Solutions

A theoretical analysis of heat transfer and moisture variation was performed while a PVA solution was exposed to high-intensity nongray irradiation and/or air flow convection. Effective absorption coefficients were incorporated in the radiative transfer analysis. The influence of various radiation and convection parameters on the transfer of heat and moisture variation in the coated layers on an optically thick substrate was investigated. The effects of radiation and convection parameters on the transfer process were presented in terms of the rate of water content removal, heat transfer, and moisture distribution. Results were compared to those of drying when using convective heat. It is evident that the use of thermal radiation combined with convective heat will help in improving the drying rate. Numerical results show that both the radiative energy absorbed by the solution and the substrate and the distribution of water mass fraction in the solution are closely related to the rate of water removal from the solution during the process.     

Dimensionless Formulation of Convective Heat Transfer in Fry‐Drying of Sewage Sludge

Fry‐drying is an alternative for heat and mass transfer intensification. The process reuses waste oil as a heating medium for drying by contact with the wet sludge. At the end of the process, a stable derived fuel is obtained, a granular solid composed of the dried indigenous sewage solid and the impregnated oil. The fry‐dried sludge is storable and transportable without any pathogen elements. Knowledge about heat and mass transfer rates during the frying process is essential in order to assess the quality of the final product such as calorific value, oil uptake, porosity changes, etc. The heat transfer properties including transfer by free convection between the solid and the frying oil are fundamental for the process design and manufacturing of the fry‐dried product. The convective heat coefficient by temperature measurement and overall energy balance calculation is determined. The heat flux is calculated from the fry‐drying kinetics including moisture loss and oil intake kinetics. Various hydrodynamic regimes for convective heat transfer during the frying process are discussed (non‐boiling, boiling, and low‐boiling regime). A dimensionless formulation for estimating the convective transfer is proposed.     

工程胎胶囊定型机及硫化罐胶囊硫化工艺装备的采用

将胶囊硫化与水胎硫化作了比较,强调了工程胎生产胶囊化的重要性和采用硫化罐胶囊多模硫化的原因。介绍了自行研制开发的工程胎胶囊定型机的结构、功能、主要特点、操作步骤及硫化罐胶囊多模硫化工艺,并对其社会及经济效益作了分析。     

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.     

A Theoretical Model for the Nongray Radiation Drying of Polyvinylalcohol/Water Solutions

Abstract

A theoretical analysis of heat transfer and moisture variation was performed while a PVA solution was exposed to high-intensity nongray irradiation and/or air flow convection. Effective absorption coefficients were incorporated in the radiative transfer analysis. The influence of various radiation and convection parameters on the transfer of heat and moisture variation in the coated layers on an optically thick substrate was investigated. The effects of radiation and convection parameters on the transfer process were presented in terms of the rate of water content removal, heat transfer, and moisture distribution. Results were compared to those of drying when using convective heat. It is evident that the use of thermal radiation combined with convective heat will help in improving the drying rate. Numerical results show that both the radiative energy absorbed by the solution and the substrate and the distribution of water mass fraction in the solution are closely related to the rate of water removal from the solution during the process.     

石膏基相变储能构件的数值模拟分析

以石蜡为相变储能材料,石膏为基础材料,制备不同配比的石膏基-石蜡相变储能构件并利用现有热工测试方法进行了热工性能测试。将测试数据代入经验证过的相变模型模拟了北京地区被动式建筑中相变构件的应用效果,并对结果进行了优化处理。结果表明：随着石蜡含量的增大,相变构件的热导率随之减小;制备的四种配比中PCM质量分数为33%的相变构件在北京地区被动式建筑中应用效果最好,在过渡季中较传统建筑可节能10%;增大构件的热导率与提高墙体表面对流传热系数均可提高室内舒适度与节能效果,且对流传热系数的影响程度更大。     

辅助工装在解决复合材料制件表面质量问题中的应用

主要研究了橡胶、热收缩塑料用作辅助工装在真空袋/热压罐成型复合材料制件中的应用。结合实验研究总结了在复杂的层压件、管状结构件及蜂窝夹层结构件成型时辅助工装选用材料要点、制作工艺控制要点等,有效地解决了复杂复合材料制件在成型中易出现的表面缺胶、发白、架桥等缺陷,提高了产品的质量。     

相变微胶囊悬浮液储能系统放冷特性实验研究

利用相变材料定温储能特性,搭建了以水为换热流体、相变微胶囊（MPCM）悬浮液为储能介质的潜热储能系统。采用放冷速率、相变完成率、单位体积放冷量和对流传热系数表征实验系统的放冷特性,通过该潜热储能系统与以纯水为工作介质的显热储能系统的对比,分析了循环水体积流量以及搅拌速率对系统放冷性能的影响。结果表明：MPCM主要在17~19℃范围内发生相变,当悬浮液温度到达20℃时,其相变完成率接近90%;增大循环水流量可以提高放冷速率,循环水体积流量为6 L·min^-1时,MPCM悬浮液的放冷速率在相变区间最高可达1.52 kW,相较于显热储能系统提升了70%;在0~200 r·min^-1的范围内,增大搅拌速率可增大MPCM悬浮液的单位体积放冷量和对流传热系数,搅拌速率为200 r·min^-1时,MPCM悬浮液的单位体积放冷量和对流传热系数分别为73.86 MJ·m^-3和2176 W·m^-2·K^-1,比显热储能系统分别高1.66倍和1.87倍。