Cure shrinkage characterization and modeling of a polyester resin containing low profile additives

Resin Transfer Moulding (RTM) has great potential as an efficient and economical process for fabricating large and complicated composite structural components. The low capital investment cost required and process versatility in component integration and assembly make RTM very attractive for high volume automotive applications. One of the challenges facing the automotive field is the resulting surface finish of manufactured components. The shrinkage associated with the curing of thermoset resins contributes to the poor surface quality. Low profile additives (LPA) are added to the resin to compensate for the cure shrinkage; however their effects on the thermal, rheological and morphological properties of polyester resins are not well understood. In this paper, the effect of LPA on cure kinetics, cure shrinkage and viscosity of a polyester resin is studied through differential scanning calorimetry (DSC) and special rheological techniques. Models are developed to predict cure shrinkage, LPA expansion, cure kinetics and viscosity variations of the resin as a function of processing temperature. Finally, morphological changes in the resin with and without LPA, during isothermal cure, are studied with hot stage optical microscopy. The results show that the LPA content in the range tested had no significant effect on the cure kinetics. However, higher LPA content reduced cure rate and cure shrinkage. A minimum of 10% LPA was required to compensate for cure shrinkage. Shrinkage behavior of all formulations was similar until a degree-of-cure of 0.5. However, resin formulations with higher LPA content showed expansion at later stages during curing.     

Eliminating volatile-induced surface porosity during resin transfer molding of a benzoxazine/epoxy blend

We studied the mechanism of volatile-induced surface porosity formation during the resin transfer molding (RTM) of aerospace composites using a blended benzoxazine/epoxy resin, and identified reduction strategies based on material and processing parameters. First, the influence of viscosity and pressure on resin volatilization were determined. Then, in situ data was collected during molding using a lab-scale RTM system for different cure cycles and catalyst concentrations. Finally, the surface quality of molded samples was evaluated. The results show that surface porosity occurs when cure shrinkage causes a sufficient decrease in cavity pressure prior to resin vitrification. The combination of thermal gradients and rapid gelation can generate large spatial variations in viscosity, rendering the coldest regions of a mold susceptible to porosity formation. However, material and cure cycle modifications can alter the resin cure kinetics, making it possible to delay the pressure drop until higher viscosities are attained to minimize porosity formation.     

Optimization of RTM processing parameters for Class A surface finish

Resin transfer moulding (RTM) has the potential to become an efficient and economical process for manufacturing large automotive composite parts. For body panels, the material and processing parameters must be optimized in order to achieve a Class A surface finish. In this work, the Taguchi method was used to investigate the effect of low profile additives, injection pressure, temperature gradient, filler content, styrene content and gel time on the surface finish of glass fibre polyester composite panels. The low profile additives (LPA) concentration, mixed in with the resin to compensate for its chemical shrinkage, was found to be the most influential parameter affecting surface roughness and waviness. More samples were subsequently moulded under the corresponding optimum processing conditions for validation and variability assessment.     

Numerical simulations for class A surface finish in resin transfer moulding process

Simulation tools for Liquid Composite Moulding (LCM) processes are a key to predict and solve manufacturing issues. Despite the fact that numerical process analyses are commonly used to predict mould filling, resin cure and exothermic temperatures, more comprehensive computational tools are still required. Resin additives such as low profile additives (LPA) show a significant impact on process performance and part quality. In this work, mould pre-heating experiments were compared to numerical predictions using commercial simulation software. Non-isothermal simulations were then carried out and the predicted flow and degree-of-cure evolution were compared to experiments. Finally, a volume change model, previously developed, was implemented in this work to calculate mould pressure increases in RTM of resins with four different LPA contents (0%, 5%, 10% and 40%). The predictions were compared to the results from the mould pressure transducers in the mould cavity. Simulation results matched closely with the experimental results. Pressure evolution of low profile resins was found to be very sensitive to the model parameters.     

Application of calcium carbonate in resin transfer molding process: An experimental investigation

The resin transfer molding (RTM) process is used to manufacture advanced composite materials made of continuous glass or carbon fibers embedded in a thermoset polymer matrix. In this process, a fabric preform is prepared, and is then placed into a mold cavity. After the preform is compacted between the mold parts, thermoset polymer is transferred from an injection machine to the mold cavity through injection gate(s). Resin flows through the porous fabric, and eventually flows out through the ventilation port(s). After the resin cure process (cross‐linking of the polymer), the mold is opened and the part is removed. The objective of this study is to verify the application of calcium carbonate mixed in resin in the RTM process. Several rectilinear infiltration experiments were conducted using glass fiber mat molded in a RTM system with cavity dimensions of 320 × 150 × 3.6 mm, room temperature, maximum injection pressure 0.202 bar and different content of CaCO₃ (10 and 40%) and particle size (mesh opening 38 and 75 µm). The results show that the use of filled resin with CaCO₃ influences the preform impregnation during the RTM molding, changing the filling time and flow front position, however it is possible to make composite with a good quality and low cost.     

Transient gas flow technique for inspection of fiber preforms in resin transfer molding

A transient gas flow method was developed to determine the quality of fibrous preforms in resin transfer molding (RTM) prior to resin injection. The method aims at detecting defects resulting from preform misplacement in the mold, accidental inclusions, preform density variations, race tracking, shearing, etc. Unlike the previously developed method based on steady-state gas flow, the new method allows for the acquisition of continuous time-varying pressure data from multiple ports during a single test. The validity of the method was confirmed by one-dimensional flow experiments.     

注塑成型工艺参数对制品体收缩率变化的影响及工艺参数优化

结合CAE及Taguchi DOE技术研究工艺参数对注塑制品体收缩率变化(制品中体收缩率的最大值与最小值的差值)的影响并获得优化的工艺参数以使制品的体收缩率变化最小。文中采用L9(3^4)正交矩阵进行实验,并研究了各个参数对制品体收缩率变化的影响程度,对于所选参数,保压压力和模具温度对注塑制品的体收缩率变化的影响较大。     

Surface quality and shrinkage of the composite bus housing panel manufactured by RTM

Resin transfer molding (RTM) process has been widely used in automobile industries, because products with large area can be manufactured easily with lower manufacturing cost than that of compression molding or hand lay up method. Although composite structures manufactured by RTM have light weight, good dynamic and impact characteristics, the low surface quality of composite structures made by RTM often hinders the adoption of composite automotive panels because parts made of glass fiber mat and unsaturated polyester often have shrinkage and warpage problems. To investigate the relationship between the shrinkage and the surface quality of composite part, in this work, the formation of surface contour line and the surface quality were measured experimentally with respect to stacking sequence and fiber volume fraction of glass fiber. Based on the results obtained, a real size composite bus housing panel was successfully manufactured.     

变模温与型腔气体反压辅助微孔发泡注塑技术及其产品内外泡孔结构演变

建立了一种变模温和型腔气体反压协同控制的微孔发泡注塑技术,研制了相应的变模温控制系统与型腔气体反压控制系统,构建了变模温与型腔气体反压辅助微孔发泡注塑试验线,并对变模温与型腔气体反压作用下的产品内外泡孔结构演变进行了研究。结果表明,变模温与型腔气体反压辅助工艺单独施加于微孔发泡注塑技术时,对其产品内外泡孔结构均具有双重影响:变模温可以改善产品大部分的表面形貌,但其对填充过程中的熔体发泡影响不大;型腔气体反压可以基本抑制填充过程中的熔体发泡,但却对产品内部泡孔密度有比较明显的降低影响。通过变模温与型腔气体反压的协同控制,可以实现微孔发泡注塑产品表面气泡形貌和内部泡孔结构的良好调控。     

In situ measurement and monitoring of whole-field permeability profile of fiber preform for liquid composite molding processes

For liquid composite molding (LCM) processes, such as resin transfer molding (RTM), the quality of final parts is heavily dependent on the uniformity of the fiber preform. However, the conventional permeability measurement method, which uses liquid (oil or resin) as its working fluid, only measures the average preform permeability in an off-line mode. This method cannot be used to create an in situ permeability profile because of fiber pollution. Further, the conventional method cannot be used to reveal preform's local permeability variations. This paper introduces a new permeability characterization method that uses gas flow to detect and measure preform permeability variations in a closed mold assembly before resin injection. This method is based upon two research findings: (1) resin permeability is highly correlated with air permeability for the same fiber preform with well-controlled gas flow, and (2) the whole-field air permeability profile of a preform can be obtained through measuring the pressure field of gas flow.In this study, first the validity of the gas-assisted, in situ permeability measurement technique was established. Then the technique was demonstrated as effective by qualitatively detecting non-uniformities and permeability variations in fiber performs. Finally, a two-dimensional flow model, based on the finite difference scheme, was developed to quantitatively estimate the whole-field preform permeability profile using predetermined pressure distribution. The efficacy of the new method was illustrated through experimental results.     

The effect of process parameters on volatile release for a benzoxazine–epoxy RTM resin

Volatile release during cure is a potential cause of void formation during the resin transfer molding of complex thermosetting resins. In this study, a blended benzoxazine–epoxy resin system is analyzed to determine the rate at which volatiles are evolved, as well as the dependence of that rate on process parameters. The evolution of thermophysical and thermochemical resin properties is characterized using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The identity and rate of evolution of the gaseous byproducts released during cure are determined at ambient pressure using a Fourier transform infrared spectrometer (FTIR) linked to a reaction cell. The results show that gas release during cure can be reduced but not eliminated by degassing at elevated temperature. Furthermore, the results indicate that the nature and rate of volatile release can be modified by judicious selection of cure cycle, as shown by a preliminary analysis of manufactured neat resin panels.     

Numerical prediction of saturation in dual scale fibrous reinforcements during Liquid Composite Molding

This paper presents a fractional flow model based on two-phase flow, resin and air, through a porous medium to simulate numerically Liquid Composites Molding (LCM) processes. It allows predicting the formation, transport and compression of voids in the modeling of LCM. The equations are derived by combining Darcy’s law and mass conservation for each phase (resin/air). In the model, the relative permeability and capillary pressure depend on saturation. The resin is incompressible and the air slightly compressible. Introducing some simplifications, the fractional flow model consists of a saturation equation coupled with a pressure/velocity equation including the effects of air solubility and compressibility. The introduction of air compressibility in the pressure equation allows for the numerical prediction of the experimental behavior at low constant resin injection flow rate. A good agreement was obtained between the numerical prediction of saturation in a glass fiber reinforcement and the experimental observations during the filling of a test mold by Resin Transfer Molding (RTM).     

Non‐isothermal ‘2‐D flow/3‐D thermal’ developments encompassing process modelling of composites: flow/thermal/cure formulations and validations

In the manufacturing process of large geometrically complex components comprising of fibre‐reinforced composite materials by resin transfer molding (RTM), the process involves injection of resin into a mold cavity filled with porous fibre preforms. The overall success of the RTM manufacturing process depends on the complete impregnation of the fibre mat by the polymer resin, prevention of polymer gelation during filling, and subsequent avoidance of dry spots. Since a cold resin is injected into a hot mold, the associated physics encompasses a moving boundary value problem in conjunction with the multi‐disciplinary study of flow/thermal and cure kinetics inside the mold cavity. Although experimental validations are indispensable, routine manufacture of large complex structural geometries can only be enhanced via computational simulations, thus eliminating costly trial runs and helping the designer in the set‐up of the manufacturing process. This study describes the computational developments towards formulating an effective simulation‐based design methodology using the finite element method. The specific application is for thin shell‐like geometries with the thickness being much smaller than the other dimensions of the part. Due to the highly advective nature of the non‐isothermal conditions involving thermal and polymerization reactions, special computational considerations and stabilization techniques are also proposed. Validations and comparisons with experimental results are presented whenever available. Copyright © 2001 John Wiley & Sons, Ltd.     

Temperature and pressure evolution in fast heat cycle injection molding

An accurate mold temperature control during injection molding processes allows obtaining objects with better surface finishing, more accurate surface replication, and reduced frozen-in orientation. The control of these properties is particularly important when the thickness of the molded part is very small as in the case of microinjection. In this work, thin multilayer heaters are adopted to obtain a very fast mold surface temperature evolution during the process of an isotactic polypropylene (iPP). The effect of mold temperature history on the pressure developed inside the cavity is analyzed and correlated to both gate solidification and mold deformation. Results confirmed that, with a fast control of the cavity surface temperature, a reduction of the injection pressure is achieved, without affecting the cycle time. The understanding of the phenomena that occur during the fast temperature evolution on the cavity surface can allow the microstructural calibration of the molded parts.     

Development of adaptive injection flow rate and pressure control algorithms for resin transfer molding

To prevent dry spot formation during fabrication of composite parts by Resin Transfer Molding (RTM), a control interface and four different adaptive control algorithms have been developed and tested with numerical simulations. The interface is capable of controlling the flow pattern of resin as it fills a mold containing a preform of fiber reinforcement, provided that the mold is equipped with multiple inlet gates, a single vent and a spinal sensor system that continuously feeds the interface with the resin flow front locations along the spine lines connecting the inlet gates to the vent. Four different adaptive control algorithms targeting on injection flow rate control, injection pressure control, linearly-corrected pressure control, and the combined flow rate and linearly-corrected pressure control have been proposed and incorporated with the control interface. To provide desirable controllability of the filling process and effective utilization of the resin dispensing equipment, the final formulations were optimized by means of numerical simulations of a rectangular RTM part containing different permeability distributions. The results were compared to investigate the strengths and weaknesses of the spinal adaptive control algorithms in terms of dry spot size, filling speed, and the minimum responding speed of injection pump. Finally, a complex geometry case study was conducted to validate and highlight the spinal adaptive control algorithms’ capability in handling flow disturbance for a complex RTM mold filling process which involves irregular mold geometry, multiple inserts, significant permeability and racetracking variations, and non-straight spinal sensors.     

低轮廓不饱和聚酯树脂的中低温固化形态

研究了加有低轮廓添加剂的不饱和聚酯树脂在中低温固化时的形态,结果表明:温度对加入聚醋酸乙烯酯类LPA试样的固化形态影响不大,而加入聚苯乙烯类LPA对试样的固化形态则有较大的影响.在固化过程中极性较大的LPA更有利于从UPR相中分离出来,形成有利于补偿收缩的两相交互连续的相态结构,而玻璃转化温度与UPR的差别大并且低于固化温度的LPA,使得固化试样形成微孔有更多的时间和更高的效率对于加入聚醋酸乙烯酯类LPA的试样,试样的固化形态随着LPA含量的增加发生两次明显的转变.对于具有较高分子量的LPA,只需要加入较低的含量就能使试样形成相互连续的两相结构,而加入聚苯乙烯类LPA的试样,固化的形态随着LPA含量的增加没有明显的改变.     

RTM工艺树脂流动过程数值模拟

RTM工艺过程数值模拟对模具设计、工艺过程控制及参数优化非常重要。本文作者介绍了RTM工艺过程及特点,给出了树脂渗流控制方程,阐述了RTM工艺过程数值模拟存在的主要问题,采用贴体坐标/有限差分法和网格分区划分法模拟了模具内有插入物情形下的RTM工艺树脂渗流过程,给出了不同时刻树脂流动前沿曲线、计算网格及终止时刻压力场分布,确定了排气孔位置,计算结果与其它研究结果吻合良好。结果表明:贴体坐标/有限差分法和网格分区划分法适合解决复杂边界及可移动边界问题。     

Cure simulation with resistively in situ heated CFRP molds: Implementation and validation

In composite processing of parts with varying cross-sections, homogeneous cure is sought but poses a significant challenge. Electrically heated molds for resin transfer molding (RTM) processes offer the potential to locally introduce heat and, thus, achieve more homogeneous cure and enhanced part quality. However, low conductivity of CFRP poses a risk of uncontrolled exothermic reactions. To target this potential, an appropriate and efficient numerical method is presented in this study to simulate part cure governed by resistive heated CFRP molds. A numerical control algorithm for 3D finite element cure simulations is developed, which uses the reaction flux of a temperature boundary condition to calculate the arising tool temperature field. The capability of this method to predict non-uniform tool temperatures of self-heated CFRP molds with close to thermocouple accuracy during the cure process is shown by means of numerical verification and experimental validation on a self-heated CFRP plate.     

Three-dimensional process cycle simulation of composite parts manufactured by resin transfer molding

A process cycle of resin transfer molding (RTM) consists of two sequential stages, i.e. filling and curing stages. These two stages are interrelated in non-isothermal processes so that the curing stage is dominated by the resin flow as well as temperature and conversion distributions during the filling stage. Therefore, it is necessary to take into account both filling and curing stages to analyze the process cycle accurately. In this paper, a full three-dimensional process cycle simulation of RTM is performed. Full three-dimensional analysis is necessary for thick parts or parts having complex shape. A computer code is developed based on the control volume/finite element method (CV/FEM). The resulting computer code can provide information regarding flow progression and pressure field during mold filling; and temperature distribution and degree of cure distribution for a process cycle. The computer code can also be used for process cycle simulation of composite structures with complex geometry and with various molding strategies including switching injection strategy, multiple gate injection strategy and variable mold wall temperature. Numerical examples provided in the present work show the capabilities of the computer code in analyzing the process cycle.     

RTM工艺注模过程边缘效应模拟分析

RTM工艺需将纤维预制体预置到模具中,由于纤维预制体结构不均匀性和模具形状、尺寸等影响,极易产生边缘效应。边缘效应会严重影响树脂流场发展和压力场分布。本文作者采用等效渗透系数方法模拟边缘效应,得到了其影响下的树脂流动前峰曲线和压力场。研究表明:一方面边缘效应可能导致不期望的树脂流场发展而形成工艺缺陷——干斑;另一方面可以利用边缘效应提高工艺效率:常流率注射时减小合模压力和注射压力,常压力注射时可以减少注模时间。