注射条件对LCM工艺非饱和流动特性影响

双尺度多孔纤维预制体填充过程中延迟浸润的非饱和流动现象,对基于树脂流过区域为完全饱和区域的充模理论及模拟方法提出了挑战。通过控制体/有限单元（CV/FE）法结合沉浸函数实现了液体模塑成型工艺（LCM）中非饱和填充浸润的数值模拟,并对比了恒压下的实验结果,验证了其可靠性。分析讨论了注射口压力、流量和液体黏度对双尺度多孔纤维织物非饱和填充浸润特性的影响。结果表明:在允许误差内,该数值模拟结果可靠,可用于分析讨论各因素对双尺度多孔织物非饱和流动特性的影响;填充浸润过程中,纤维织物内部非饱和区域长度并非保持不变,而是随着填充浸润的进行经历了4个变化过程;不同注射条件下,压力、流量及黏度对非饱和流动特性影响不同。研究结果对合理控制注射条件及流体特性实现双尺度多孔纤维预制件的完全浸润具有指导意义。      

Finite Element Assisted Modelling of the Microscopic Impregnation Process in Thermoplastic Preforms

Fibre reinforced composite materials incorporating thermoplastic matrices are gaining increasing popularity in many industrial applications. One of the potential preforms for the manufacture of technical components is commingled yarn composed of reinforcement and matrix in fibre form. These are often employed in the pultrusion process. Another innovative preform consists of polymer powder preimpregnated sheath surrounding fibre bundles. To achieve adequate mechanical properties of the final product it is essential, when producing laminates by a process such as pultrusion with both types of preform, that sufficient matrix impregnation is achieved. The prevention of voids and dry-spots in the laminate requires a theoretical understanding of the mechanisms involved. On a microscopic scale, several finite element (FE) models can be used to simulate the progress of the matrix flow into the interstitial spaces between the single reinforcement fibres. In the present simulations, a hexagonal and a square arrangement account for two of the various fibre packings occurring in a laminate. It permits an estimation of the impregnation performance of commingled and powder impregnated yarns. For each preform the shear rate, to which the polymer matrix is subjected during the impregnation and consolidation process, can be predicted.     

Modelling microscopic flow in woven fabric reinforcements and its application in dual-scale resin infusion modelling

A physical unit cell impregnation model is proposed for the micro-scale flow in plain woven reinforcements. The modelling results show a characteristic relationship between tow impregnation speed, the surrounding local macro-scale resin pressure and the tow saturation within the unit cell. This relationship has been formulated into a mathematical algorithm which can be directly incorporated into a continuum dual-scale model to predict the ‘sink’ term. The results using the dual-scale model show a sharp resin front in inter-tow-pore spaces and a partially saturated front region in intra-tow-pore spaces. This demonstrates that the impregnation of fibre tows lags behind the resin front in the macro pore spaces. The modelling results are in agreement with two reported experimental observations. It has been shown that the unsaturated region at the flow front could increase or have a fixed length under different circumstances. These differences are due to the variation in tow impregnation speed (or the time required for the tow to become fully impregnated), the weave architecture and the nesting and packing of plies. The modelling results have also demonstrated the drooping of the inlet pressure when flow is carried out under constant injection rates. The implementation of the algorithm into a dual-scale model shows coherence with a single-scale unsaturated model, but demonstrates an advantage in flexibility, precision and convenience in application.     

The Sink Term to Different Kinds of Fibrous Mat During the Unsaturated Filling Process

The fibrous pre-form of resin transfer molding is a dual-scale porous medium with two distinct scales of pores, i.e., pores in intra- and inter-tow, which produce an unsaturated infiltration phenomenon during filling. A sink term representing the delayed flow rate from the inter-tow gap into the intra-tow one is introduced to establish governing equations. This study mainly analyzes the sink term by tow saturation during the microscopic flow. First, fiber-tow permeability is calculated by FLOTRAN of ANSYS, Second, periodic unit cells are built according to different structures, and the concrete expression of the sink term is indirectly obtained through the numerical simulation and date fitting of tow saturation under different pressure and viscosity conditions. Results indicate that: the FLOTRAN module can be used to calculate the permeability of fiber tow in two directions; Moreover, the filling time and infiltration process for diverse unit cells with the same volume fraction are different; under the same injection condition, different unit cells have different parameters for the sink term.     

Design and manufacturing of an L-shaped thermoplastic composite beam by braid-trusion

Braid-trusion is a manufacturing process for composite materials in which a braiding machine is coupled with a pultrusion die to continuously produce beams with constant cross-section and off-axis fiber orientation. This study presents a geometrical model of the tri-axial braid which allows the design of braided preforms that achieve correct filling of the pultrusion die at the same time as it limits fiber friction on die walls. The typic design parameters are listed and used for the manufacturing of a braid-truded thermoplastic composite beam where fibers are aligned at ±69° as well as 0° with respect to the beam axis. Tensile mechanical characterization and cross-section observations are also presented.     

Multiscale modeling of unsaturated flow of dual-scale fiber preform in liquid composite molding II: Non-isothermal flows

A novel multiscale approach is developed for modeling non-isothermal flows under unsaturated conditions in the dual-scale fabrics of liquid composite molding (LCM). The flow and temperature governing equations at the global or gap or inter-tow (∼m) level and the local or intra-tow (∼mm) levels are based on a previous dual-scale volume averaging method. To solve the coupled equations at two length-scales, a coarse global mesh is used to solve the global flow over the entire domain, and a fine local mesh in form of the unit-cell of periodic fabrics is employed to solve the local tow-impregnation process. (The latter is used to compute sink terms required for solving the former.) A multiscale algorithm based on the hierarchical computational grids is then proposed to solve the dual-scale flow under non-isothermal (but non-reactive) conditions. To test the proposed multiscale model, we first carry out a validation study in which the temperature histories predicted by the multiscale method are compared with experimental data available in a publication for a simple 1-D flow. Despite the lack of information about various model parameters, a reasonably good comparison with the experimental results is achieved. Then, the non-isothermal flow through a simple 1-D flow domain is carried out and the predictions of the multiscale simulation are compared with those of a previously published two-layer model. The multiscale predictions are found to be very similar to the two-layer predictions. A significant difference between the gap and tow temperatures is observed. The ratio of pore volumes in the tow and gap regions, thermal conductivity of the tows, and fiber types are identified as the important parameters for temperature distributions in the gap and tow regions. A further comparison with the single-scale flow simulation highlights significant differences between the conventional single-scale and the proposed dual-scale modeling approaches.     

The characterization of low cost fiber reinforced thermoplastic composites produced by the DRIFT™ process

A new, low cost process for hot-melt impregnation of continuous reinforcing fibers with thermoplastic polymers is described. This technique can be used to fabricate various product forms including discontinuous, long-fiber products for compression molded parts, continuous fiber products for pultrusion, filament winding, and woven fabric applications. Mechanical data are presented for composites with various fiber and polymer combinations. Effects of fiber orientation and length on mechanical properties are discussed, and the effect of fiber–polymer bonding on impact strength and microstructure are shown. It is shown that the low cost and high performance achieved with this approach has the potential to expand applications of thermoplastic composite materials.     

双尺度纤维织物二维非饱和流动的数值模拟与实验

液体模塑成型工艺(LCM)中非饱和流动的填充模拟对于在虚拟空间中快速、高效地优化工艺参数具有重要意义。采用了一种模拟双尺度纤维织物在等温条件下非饱和流动的双尺度计算模型,通过引入沉浸函数求解宏观-微观流动控制方程组,同时考虑了在微观浸渍中毛细压力的影响,在有限元/控制体积算法中实现了对非饱和流动的数值模拟。随后对三向缝合纤维织物进行了二维径向填充实验,将实验结果与数值模拟的预测值对比。结果表明,该计算模型可以较精确地模拟双尺度纤维织物中的非饱和流动。在此计算模型的基础上,讨论了流体黏度、注射流量及纤维束孔隙率对非饱和填充浸润的影响。结果表明,不同流体黏度、注射流量及纤维束孔隙率对纤维织物填充过程中非饱和区域长度、入口压力曲线及填充时间影响不同。研究结果可以对合理预测纤维织物的浸润及树脂填充过程中入口压力提供指导。     

Optimising the energy consumption on pultrusion process

This study is based on a previous experimental work in which embedded cylindrical heaters were applied to a pultrusion machine die, and resultant energetic performance compared with that achieved with the former heating system based on planar resistances. The previous work allowed to conclude that the use of embedded resistances enhances significantly the energetic performance of pultrusion process, leading to 57% decrease of energy consumption. However, the aforementioned study was developed with basis on an existing pultrusion die, which only allowed a single relative position for the heaters.In the present work, new relative positions for the heaters were investigated in order to optimise heat distribution process and energy consumption. Finite Elements Analysis was applied as an efficient tool to identify the best relative position of the heaters into the die, taking into account the usual parameters involved in the process and the control system already tested in the previous study. The analysis was firstly developed based on eight cylindrical heaters located in four different location plans. In a second phase, in order to refine the results, a new approach was adopted using sixteen heaters with the same total power. Final results allow to conclude that the correct positioning of the heaters can contribute to about 10% of energy consumption reduction, decreasing the production costs and leading to a better eco-efficiency of pultrusion process.     

Modeling of continuous ultrasonic impregnation and consolidation of thermoplastic matrix composites

Ultrasonic propagation was used to provide heat and pressure in order to perform impregnation and consolidation during production of thermoplastic matrix composites. For this purpose, a new experimental set-up, integrating a laboratory filament winding machine with a horn and a compaction roller, was developed.The heat transfer phenomena occurring during continuous impregnation and consolidation were simulated solving by finite element (FE) analysis the energy balance equations in 2D accounting for the heat generated by ultrasonic waves, the melting characteristics of the matrix and the movement of the thermoplastic commingled roving.The temperature distribution in the composite, predicted by the numerical simulations, was validated by temperature measurements during the production of E-glass/polypropylene cylinders, with the optimized parameters obtained by the FE analysis. The ultrasonic consolidated composite cylinders were characterized by low void content and a shear modulus comparable with that obtained by the micromechanical analysis.     

Processing of Thermoplastic Composites Using a Powder Slurry Technique. I. Impregnation and Preheating

The success of thermoplastic matrix composites depends upon the development of economical methods of impregnation. The purpose of this work was to develop and understand an economical impregnation procedure using a slurry based powder technology. An impregnation and preheating line consisting of a fiber tensioner, a slurry bath, a drying heater, a coating heater, and a fiber winder was used to make resin coated fibers. The influence of the process parameters on the impregnation, preheating, coating, and consolidation were studied. Part I of this paper presents the experimental investigation of the impregnation and preheating stages of the process together with a preheating model. Luikov's coupled equations of heat and mass transfer were used to model the heating and drying of the tow in the preheater. The predictions of the preheating and drying model are compared to the experimentally obtained results.     

恒压下树脂在双尺度多孔介质中的流动特性

基于复合材料液态模塑(LCM)工艺过程中存在半饱和区域的实验现象以及对预制体双尺度效应的逐步认识, 一些学者提出用沉浸模型来研究双尺度多孔介质的不饱和流动。通过体积均匀化方法描述了双尺度多孔介质复合材料液态模塑工艺模型的特征, 得到含有沉浸项的双尺度多孔介质的质量守恒方程, 并采用有限元法对方程进行数值求解, 通过具体算例计算了考虑双尺度效应时恒压树脂注射下不同时段的压力分布状态, 得到树脂在填充过程中流动前沿半饱和区域从出现到消失的过程, 采用不同注射压力进行模拟并比较。结果表明, 与单尺度多孔介质模型不同, 双尺度多孔介质模型更能反映实际树脂填充过程中出现的半饱和区域现象。     

A Dynamic Transient Model to Simulate the Time Dependent Pultrusion Process of Glass/Polyester Composites

The objective of this paper is to introduce a novel dynamic transient model to simulate the time dependent pultrusion process of glass/polyester composites. The model is able to simulate the resin curing process systematically. The resin curing process is divided in two liquid and gel-solid phases. Physical properties of the resin including resin specific heat, viscosity and thermal conductivity change by altering the resin temperature and the degree of cure. It is shown that in liquid and gel-solid phases, some of the resin physical properties have significant role in heat transfer phenomenon and affect simulation results. The physical and mechanical properties of fibers do not change during the curing process of composites; therefore, an equivalent material is introduced instead of the resin-fiber compound. The model simulates the heat generation during the resin curing process. The degree of cure of the resin, used for the resin viscosity calculation, is an important parameter indicating the final stage of simulation of resin curing process. The components of the model are integrated in a finite element method. As case studies, the process of pultrusion of circular, rectangular and I cross-sections are simulated by the model. The results show that the model is able to simulate the pultrusion process very well.     

Squeeze flow characterisation of thermoplastic polymer

New manufacturing concepts developed by the CRC-ACS involve co-curing a thermoplastic polymer layer onto the surface of a composite component. The layer can be reshaped to alter the dimensions of the laminate locally. A major physical phenomenon involved in these uses is squeeze flow of the thermoplastic polymer.

In the present work, the squeeze flow of a selected thermoplastic at high temperature and under given pressure was studied. A power-law model for viscosity variation with shear rate was assumed to describe the non-Newtonian behaviour of the thermoplastic at high temperatures. Based on the viscosity model, a fluid mechanics model was derived for the squeeze flow between approaching parallel plates of infinite length. The model relates the plate approaching speed to the applied pressure, thermoplastic geometry (both thickness and width) and power-law viscosity model parameters.

An experimental method and data analysis procedure were developed to determine the parameters of the power-law model that describes the viscosity of the thermoplastic material. Tests were conducted under isothermal conditions by squeezing composite laminates with integrated thermoplastic films. The transient thickness of the thermoplastic film was continuously measured and used to calculate the power-law model parameters for each test. A temperature range of 180–200 °C was investigated to establish the dependence of the power-law model parameters upon temperature.

Using the squeeze flow model, and experimentally determined power-law viscosity model parameters, the effect of various process conditions on the thermoplastic thickness was investigated. Predictions were found to be valid for situations where the shear rate range matched that achieved during the power-law viscosity model parameters tests.     

Cross-sectional monitoring of resin impregnation using an area-sensor array in an RTM process

It is difficult to visualize the flow in the cross-section direction, and most conventional methods for monitoring resin flow are limited to the in-plane direction. This study investigates the monitoring of the cross-section of resin impregnation using an area-sensor array during a resin transfer molding (RTM) process. The area-sensor array is mounted on a thin polyimide film that is integrated with the inter-digital electrode array and associated wiring, and forms the bottom layer of the stacked composite laminates. Each area-sensor is square-shaped and measures the capacitance and electrical resistance of the sensor region. First, we constructed the equivalent electrical circuit model of in-plane and out-of-plane impregnation. Using this model, we proposed a method to identify the flow direction and the ratio of the impregnation thickness by measuring the electrical capacitance and resistance. The validity of the model was confirmed by comparison with the experimental results. To demonstrate the applicability of the proposed method, the area-sensor array was applied to monitoring the resin injection through-thickness to the glass fabric laminates. As a result, the cross-section of the impregnated area could be estimated and the estimated area provided a good match to the actual impregnated area.     

Optimization for CFRP pultrusion process based on genetic algorithm-neural network

The temperature and the degree of cure of carbon fiber reinforced polymer (CFRP) are coupled during pultrusion. In this paper, the governing equations for heat transfer and resin curing are solved by the combination of finite element method, finite different method and indirect decoupling method. The kinetic parameters needed for simulation are obtained from differential scanning calorimetry (DSC) measurement. The temperature of composites on real time during pultrusion is monitored by the fiber Bragg grating (FBG) sensor. And the final degree of cure of composites is also measured through Sorbitic extraction. It shows that the simulation procedure is effective and reliable and predicts temperature and degree of cure which are in good agreement with the experimental results. On the basis of the simulated results, the relationship between processing parameters and degree of cure is formed by artificial neural network. And genetic algorithm combines with the artificial neural network to solve the bi-objective optimization for CFRP pultrusion. It shows that there are considerable improvements in pull speeds and die temperatures after optimization by ANN and GA.     

Simulating tape resin infiltration during thermoset pultrusion process

The thermoset tape pultrusion is a widely adopted manufacturing process to produce long, constant cross-section composite structural parts. For high volume production, low cost can be achieved by maximizing the production rate which is a function of the material and process parameters, more specifically the rate of resin infiltration and resin cure. During resin infiltration, the resin saturates the dry reinforcement either under positive pressure in the pressure chamber, or, by the action of capillary and surface forces, within the resin bath. In either case, the saturation must be completed as the tape is squeezed into the final cross-sectional form at the entrance of the heated mold where the resin will be cured to form the composite part.This paper models the resin infiltration process during pultrusion, by modifying the pre-existing simulation tool for liquid molding processes. The formulated capability can be used not only to optimize the impregnation dynamics within the pressure chamber, but can also be used to predict the required forces for the selected pulling rate. The proposed model does allow one to handle a variety of tape cross-sections, not just rectangular prisms.     

玻璃纤维/聚丙烯复合纱拉挤型材制备及其性能研究

以玻璃纤维/聚丙烯复合纱为原料,采用拉挤成型方式制备连续纤维增强热塑性复合材料,通过组建的拉挤试验线获得了拉挤型材试样,探究了复合纱穿纱方式、模具型腔结构、模具温度和拉挤速率对制品性能的影响,并观察其截面形态。结果表明:采用收敛式型腔结构、提高模具温度、降低拉挤速率,可有效改善玻纤/树脂间结合能力,提高纤维在制品中的分布均匀性,降低制品的孔隙率,提高其力学性能。      

Investigation of the pressure behavior in a pultrusion die for graphite/epoxy composites

There are a variety of ways to process composite materials. One such way is pultrusion which is a continuous process for manufacturing composite materials of constant cross-sections. The pultrusion process involves a number of variables and processing parameters which can affect the quality of a pultruded product. One variable of particular interest is the fluid resin pressure rise in the tapered inlet region of the die. The liquid resin pressure rise in the die inlet can have a significant impact on the quality of a pultruded product. An appreciable pressure rise can suppress void formations and enhance fiber “wet out”. Darcy’s law for flow in a porous media is used to mathematically model the fiber/resin system of the pultrusion process, while employing the finite volume solution method to predict the pressure and velocity fields as a function of various process control parameters. The results obtained by the numerical model can establish a foundation by which process control parameters are selected to achieve an appreciable pressure rise which will enhance the quality of the pultruded composite. The results can also be applied to die inlet design.     

The heating of polymer composites by electromagnetic induction – A review

Since the late 1980s a small number of research groups have been attracted with the idea of using induction heating technology for the processing of fibre reinforced polymer composites. Induction technology is suitable for the processing of thermoplastic and thermoset polymer materials but requires special susceptor additives (conductive materials) either in the form of structured fibres and fabric or particulate that can transform the electromagnetic energy into heat. This paper aims to summarize the principles of induction heating with respect to polymer composites processing taking a look first at material and equipment based process influences. State of the art applications and research activities are then reviewed, from thermoplastic composite welding, thermoset curing, selective material heating and fast mould heating technologies. Current simulation possibilities and available software tools have also been covered. Finally, some new ideas and possibilities for future developments in the field of polymer composites processing have been discussed.