Total Curing Time Research Articles

Parametric Numerical Study and Multi-Objective Optimization of Composite Curing through Infrared Radiation

Composite curing through infrared radiation (IR) has become a popular autoclave alternative due to lower energy costs and short curing cycles. As such, understanding and measuring the effect of all parameters involved in the process can aid in selecting the proper constituents as well as curing cycles to produce parts with a high degree of cure and low curing time. In this work, a numerical model that takes inputs such as part geometry, material properties, curing-related properties and applied curing cycle is created. Its outputs include the degree of cure, maximum curing temperature and total curing time. A genetic algorithm and a design of experiments (DOE) sequence cover the range of each input variable and multiple designs are evaluated. Correlations are examined and factor analysis on each output is performed, indicating that the most important inputs are activation energy, specimen precuring, applied curing temperature and curing duration, while all the others can be considered constant. Finally, response surfaces are created in order to effectively map and provide estimations of the design space, resulting in a curing cycle optimizer given certain restrictions over the input parameters.

Automatic Pressure Gelation Analysis for Insulating Spacer of Gas Insulated Switchgear Manufactured by Bio-Based Epoxy Composite

In the case of the existing power equipment business, a variety of insulation and accessories is manufactured with petroleum-based epoxy resins. However, as petrochemical resources are gradually limited and concerns about the environment and economy grow, the power equipment industry has recently studied many insulating materials using bio-based epoxy to replace petroleum feedstock-based products in order to produce insulators using eco-friendly materials. In this paper, the simulation of the automatic pressure gelation process was performed by obtaining parameter values of curing kinetics and chemical rheology through physical properties analysis of bio-based epoxy complexes and applying them to Moldflow software. The simulation results were compared and analyzed according to the temperature control of each heater in the mold, while considering the total curing time, epoxy flow, and curing condition. A temperature condition of 140 °C/140 °C/135 °C/135 °C/130 °C/130 °C/120 °C/120 °C provided the optimal curing conditions. Based on the temperature conditions of the simulation results, the actual GIS spacer was manufactured, and x-ray inspection was performed to check the moldability.

Optimization of the curing cycle for the large-scale composite stiffened panel based on the response surface methodology and Fgoalattain algorithm

The autoclave curing process of the large-scale composite stiffened panel greatly affects the manufacturing quality of the component, especially the curing quality of stiffeners and the bonding quality between stiffeners and the panel. In this paper, a multi-objective optimization method is developed to optimize the curing cycle for the composite T-stiffened fuselage panel, which aims to minimize the temperature difference within the component to achieve better forming quality. The autoclave curing model considering thermal properties of composites is established and verified by the experiment first. The response surface models are then built up with the Finite Element Method (FEM) and the Box-Behnken Design (BBD). The fgoalattain algorithm is applied to solve the multi-objective optimization problem with nonlinear constraints. Results show that the maximum temperature difference of the two groups of monitoring points decreases by 22.91% and 21.42%, respectively. Besides, the total curing time is also reduced by 11 mins.

Multi-Objective Optimization of Curing Profile for Autoclave Processed Composites: Simultaneous Control of Curing Time and Process-Induced Defects.

The contribution of this work is introducing a multi-objective optimization method based on finite element (FE) numerical simulation to simultaneously control the curing time and cure-induced defects of C-shaped composites during a curing cycle. Thermochemical and thermomechanical coupled analysis is performed and validated experimentally to understand the evolution details of temperature, degree of cure and curing deformation. Aiming to achieve the simultaneous control of manufacturing cost and composite quality, the curing profile is optimized by employing the critical factors including the total curing time, the maximum degree of cure difference, and the curing deformation. The optimization result shows that the designed curing profile can effectively reduce the curing time and guarantee the curing quality. The total curing time of the optimization is reduced by 19%. The verification experiment is also conducted to prove the accuracy and effectiveness of the proposed optimization method.

Adhesive strength analysis and real-scale simulation for smart curing in a large turbine blade with carbon fiber-reinforced plastic spar cap

Various approaches for the smart curing processes in the fabrication of large hybrid composite blades have been employed to determine whether smart curing, which has multiple heating and cooling processes during the total curing time, is applicable to the adhesive process between carbon fiber-reinforced plastic (CFRP) and glass fiber-reinforced plastic (GFRP). Therefore, investigating the adhesive strength and residual thermal stress between CFRP and GFRP is very important. In an experimental approach, the adhesive strengths of CFRP/GFRP single-lap joint specimens were compared according to surface treatment techniques and curing conditions. The proper roughness of the CFRP surface and the application of smart curing are very effective for the manufacture of CFRP spar cap wind turbines. The simulation results from the analytical approach demonstrated that when the final curing temperature was reduced by the smart curing process, the residual thermal stress and thermal shear stress in the blade were also reduced, resulting in an increased adhesive strength in the adhesive layers between the CFRP spar caps and GFRP skin. It was determined that large hybrid wind blades with CFRP spar caps could be efficiently manufactured using a smart curing process.

Optimization of a high-pressure microwave curing process for T800/X850 carbon fiber-reinforced plastic

Compared with the conventional composite curing processes, high-pressure microwave curing is a promising technology. In this study, a set of devices for high-pressure microwave curing was built and equipped with real-time temperature measurement capability and a microwave input control system. The orthogonal experimental method was applied to optimize three process parameters, including the heating rate, curing temperature, and holding time, for the high-pressure microwave curing of T800/X850 composites. The effects of the three parameters on the curing quality were studied by measuring the interlaminar shear strength (ILSS) and conducting differential scanning calorimeter tests. The fracture surface of the samples was also examined by scanning electron microscopy. The results showed that the heating rate had a significant effect on the ILSS of the laminates, and the degree of cure of all samples was more than 95% in the tests. Furthermore, the optimal process parameters were determined as follows: heat up to 170°C with a heating rate of 6°C min−1 and a holding time of 90 min. The total curing time of the sample was 42.4%, and the ILSS of the sample was slightly enhanced by 0.31% compared with standard thermal curing. These results could serve to make trade-offs between reducing manufacturing time and preserving the mechanical properties of microwave-cured composites.

Steam-cured recycled aggregate concrete incorporating moderately high early strength cement: effect of binder content and curing conditions

Steam curing employed for faster strength gain due to higher hydration rate may lead to shrinkage issues, and hence the use of moderately high early strength cement (MESC) is proposed instead of typically used high early strength cement (HESC) which has a high C3S content. This paper evaluates the suitability of MESC for use in precast concrete. The objective was to (i) determine the feasibility of MESC in recycled aggregate concrete (RAC), (ii) determine the optimum steam curing conditions (curing duration, maximum temperature, and duration of holding maximum temperature), and (iii) evaluating the beneficial use of reduced binder content to compensate for steam curing. Four series of RAC were developed which were subjected to nine different steam curing condition, and the resulting strength properties were determined. Though all the developed concrete specimens achieved the design strength of 30 MPa, those cured at 50 °C with 1 h duration of exposure to maximum temperature exhibited the best performance. Further, it was found that even though reducing the binder content reduces the ultimate strength at 28-day age, curing at 70 °C with 2 h exposure to maximum temperature duration (for 7% reduction in binder volume) leads to merely 11.8% strength decline. It is concluded that MESC can be effectively used in steam cured RAC instead of OPC and HESC with meagre strength loss when cured at maximum temperature of 50 °C maintained for 1 h while the total curing time being 4.5 h. Thus, utilizing MESC in RAC shall not only help combat the persistently declining natural resources but also reduced the associated carbon dioxide emissions, thereby promoting sustainable development.

Optimizing curing process of graphene oxide/waterborne epoxy blends by curing kinetics simulation considering the coupling of heat conduction and curing reaction

Curing kinetics of the waterborne epoxy emulsion (EPWE)/Graphene oxide (GO) system was investigated by non-isothermal differential scanning calorimetry (DSC), which shows an autocatalytic behavior and can be well predicted by model fitting kinetic method. By combining with the kinetics simulation and the conventional extrapolation method, an novel three-stage curing process with a high initial cure temperature and a cooling stage was proposed to inhibit the thermal shock inside materials and the uneven curing reaction during the curing process of the materials. The experimental results indicate that compared to the conventional two-stage curing process, the new curing procedure containing a cooling section significantly increased the bending strength by 14.9%, elastic modulus by 7.3% and Tg by 8 K of the cured GO/EPWE material, and the total curing time clearly decreased.

A new process control method for microwave curing of carbon fibre reinforced composites in aerospace applications

Abstract For the fabrication of carbon fibre reinforced composites used in aerospace industry, microwave curing technologies are more effective than traditional thermal curing technologies. However, the manufacturer's recommended cure cycles used in traditional autoclave curing are directly adopted into current microwave curing technologies without thorough validation. Here, a new cyclic heating and cooling methodology for microwave curing process control of composite is proposed by analyzing mechanisms of heat conduction, stress generation and curing kinetics. The results of the experiment carried out show significant reductions in residual strain, warpage, total curing time and energy consumption, compared with both traditional thermal curing and current microwave curing technologies. The mechanical properties of samples cured by the new process are compared with the autoclave cured ones.

MECHANICAL STRENGTH OF SEALED-CURED NEAT CEMENT-PASTE

The strength of Portland cement concrete is dependent on the strength of the cement paste binding the aggregates together. An understanding of the cement paste is the first step to assess the concrete strength. To this end, a program for strength testing of cement paste was implemented for water to cement ratios of 0.35 and 0.40. The compressive strength was evaluated using two inch cubes and four inch diameter cylinders; while the tensile strength evaluation was based on flexure and direct tension tests. All the specimens were cured in the mold the first day, followed by curing in sealed vacuum bags for the remaining of the time. The total curing time ranged from three, seven, fourteen and twenty-eight days. Three replicate specimens were prepared, cured, and tested for each combination of water to cement ratio, type of specimen, and time of curing. The results of the testing program show that while the compressive strength increases for increasing curing time, the tensile strength shows a consistent decline. These results indicate that the sealed curing forced that further hydration caused “self-desiccation” of the CSH, inducing new cracks or extension of existing cracks that would be responsible for the loss of tensile strength.

Optimization of the reactive injection moulding process

The aim of the paper is to discuss the possibility of applying a genetic algorithm (GA) in conjunction with the finite-volume (FV) FLUENT package to the optimisation of the reactive injection moulding (RIM) technological process. Firstly, the fundamental flow, heat, mass, and rheokinetic relations are examined, and the possibility of numerically simulating this class of problems is examined, with a view to optimising the parameters governing the progress of the process. Then, the optimisation objective is formulated, taking into account both the total curing time of the RIM process and the set of control parameters, called the degree of cure, that is used as a measure of the effectiveness of the technological process. From among the various parameters characterising the process, the temperatures of the heat sources (electric heaters placed around the mould) are chosen as the design variables (in discretised form). A 2-D planar problem is solved to illustrate the effectiveness of the proposed method.

Modeling Thermal Residual Stresses in Composite Patch Repairs During Multitemperature Bonding Cycles

Generally, thermal residual stresses in composite patch repairs can be reduced by lowering the cure temperature of the adhesive used. However, lowering the cure temperature requires a longer curing time to achieve the fully developed performance of the repair. The total curing time can be shortened by using a multistep cure-temperature cycle. In modeling of the cure cycle, a cure kinetic model was established first based on the result of differential scanning calorimetry tests that were performed to estimate the development of the degree of cure of the adhesive. Because stress relaxation depends on the degree of cure of the adhesive, this kinetic model was used in conjunction with the Maxwell viscous model to estimate stress relaxation in the adhesive during the curing cycle. Thus, the model has the capability of estimating the thermal residual stresses in a composite patch repair undergoing a multistep curing schedule. Thermal residual stresses induced by a cure cycle were quantified by ΔT eff defined as the temperature difference between the stress-free temperature and room temperature. This model was used to establish a guide for selecting an efficient two-step cure cycle, which yields a ΔT eff lower than that of the cure cycle recommended by the adhesive manufacturer.

Comparative efficiency of plasma and halogen light sources on composite micro-hardness in different curing conditions

Objectives. Recent developments have led to the introduction of high power curing lights, which are claimed to greatly reduce the total curing time. This study evaluated the effectiveness of a plasma-curing device (Apollo 95 E) and a halogen device (Heliolux DLX), in different curing conditions. Method. Vicker's micro-hardness values were performed on 1 and 2 mm thick composite discs cured in a natural tooth mold by direct irradiation or indirect irradiation through composite material (2 or 4 mm) and dental tissues (1 mm enamel or 2 mm enamel-dentin). Measures were, respectively, performed after a 1, 3, 6 s (SC, step curing mode) or 18 s (3×SC) exposure to the plasma light, and a 5, 10, 20 or 40 s exposure to the halogen light. Results. With the PAC light used, a 3 s irradiation in the direct curing condition was necessary to reach hardness values similar to those obtained after a 40 s exposure to the halogen light. Using the indirect curing condition, hardness values reached after an 18 s exposure (3×SC mode) with the plasma light were either equivalent or inferior to those obtained with 40 s halogen irradiation. Significance. Direct polymerization with the plasma light used requires longer exposure times than those initially proposed by the manufacturer. The effectiveness of plasma generated light was lowered by composite or natural tissues, and therefore requires an important increase in the irradiation time when applied to indirect polymerization. The practical advantage of this polymerization method is less than expected, when compared to traditional halogen curing.

Reduction of polymerization shrinkage stress and marginal microleakage using soft-start polymerization.

This study evaluated the influence of a soft-start light-curing exposure on polymerization shrinkage stress and marginal integrity of adhesive restorations. Six resin-based composites (Pertac II, Tetric Ceram, Definite, Surefil, Solitaire, and Visio-Molar) were adhesively bonded to a cylindrical cavity (n = 9 per material/light) in a photoelastic material. Visible light-curing was applied using either the standard polymerization mode (800 mW/cm2 exposure duration 40 s) of the curing light (Elipar TriLight, 3M ESPE) or the exponential mode from the same device (ramp-curing: 150 mW/cm2 to 800 mW/cm2 within the first 15 s of a total curing time of 40 s). Polymerization stress was calculated at 5 minutes, 1 hour, and 24 hours postexposure from the second-order isochromatic curves obtained from photoelastic images (Matrox-Inspector). Two standardized Class V preparations were made each on the facial and lingual surfaces of 80 extracted human molars and premolars. Resin restorative systems (Pertac II/EBS Multi, Tetric Ceram/Syntac, Definite/Etch&Prime 3.0, and Surefil/Prime & Bond 2.1) were exposed using both light exposure modes (n = 20). Marginal dye penetration (2% methylene blue) was investigated separately for enamel and cementum margins after thermocycling. To obtain information on equivalent depth of cure, relative surface hardness measurements were performed on resin samples of the same material at the top surface and at 1.5 mm and 3.0 mm thickness (Zwick 3212, 10 N). A significant (p < .01) reduction in polymerization stress of 7.1% for Pertac II, 4.1% for Tetric Ceram, 3.6% for Definite, 3.7% for Surefil, and 6.2% for Solitaire was observed when using the exponential mode as opposed to the standard. A significant (p = .04) reduction of marginal dye penetration was found only for Pertac II/EBS Multi at the cementum margins when the soft-start polymerization was used. For the sample thickness of 3 mm, a significant higher relative bottom to top surface ratio in favor of the standard exposure mode was found (p = .001). Depending on the restorative material, soft-start polymerization may lead to a significant reduction in marginal microleakage of adhesive Class V restorations. This effect might be attributable to a significantly lower polymerization stress, as seen from photoelastic analysis, and/or a decrease in the degree of conversion, as deducted from surface hardness ratios. However, the effect of soft-start curing mode depends on the material itself, with the most effective response from hybrid resin-based composites.