A modeling approach to thermoplastic pultrusion. I: Formulation of models

A fundamental understanding of the effects of processing parameters and die geometry in a pultrusion process requires a mathematical model in order to minimize the number of necessary experiments. Previous investigators have suggested a variety of models for thermoset pultrusion, while comparatively little effort has been spent modeling its less well understood thermoplastic counterpart. Herein, models to describe temperature and pressure distributions within a thermoplastic composite as it travels through the pultrusion line are presented. The temperature model considers heat transfer in an infinite slab with either prescribed boundary temperature, or prescribed heat flux from the surfaces. The pressure model is based upon matrix flow relative to the fibers and incorporates a non-Newtonian matrix viscous, compaction, and friction resistance, is also presented. The models are evaluated studying and ideal pultrusion process for manufacturing of unidirectional carbon-fiber-reinforced polyether ehterketone composites.     

RIM-pultrusion of nylon-6 and rubber-toughened nylon-6 composites

Unidirectionally reinforced thermoplastic composites of Nylon-6 and polypropylene oxide-Nylon-6 block copolymers have been prepared by the reaction injection molding (RIM)-pultrusion process. This process takes advantage of both the RIM and the pultrusion techniques, while avoiding their inherent shortcomings. It also represents a novel way of incorporating toughening rubber domains into a thermoplastic composite. The composites produced exhibit excellent mechanical integrity with essentially zero void content. The chemical and physical states of the composites produced by the process were probed in terms of mechanical relaxation behavior using a dynamic mechanical spectrometer. Due to the simultaneous occurrence of both polymerization and crystallization processes in the reacting system, the resulting material is in a thermodynamically nonequilibrium state. An annealing effect is shown to correspond structurally to an increase in matrix crystallinity and the degree of phase separation, Izod impact tests were used to compare Nylon-6 and rubber-toughened Nylon-6 RIM-pultruded composites. The potential of secondary processing was, demonstrated by compression molding of the RIM-pultruded composite rods.     

Influence of heat transfer and curing on the quality of pultruded composites. II: Modeling and simulation

In this study, a computer simulation code is developed to predict the dynamics of heat transfer in the pultrusion process. The die block and heater arrangement are included in the heat transfer analysis so that the simulation can provide the temperature profile at the interface between the die and the composite. The measured interface temperature profiles are then used to validate the simulation code. Energy management, i.e. heater power control along the heating die, is also considered in the simulation code. The code is capable of carrying out transient thermal analysis for both start-up and steady-state operation of the pultrusion process. From the experimental observations on the part quality in terms of blister formation, a processing window was obtained by showing the relation of the die length and the line speed to the part quality. The processing window is then generated numerically using the computer code based on the definition of a critical die length proposed in this work, and the result shows good agreement with the experimental data.     

Development of a mathematical model for the pultrusion of unsaturated polyester resin

A mathematical model is developed for simulating the pultrusion process of unsaturated polyester resin, using a mechanistic kinetic model based on free radical polymerization. In their previous publications (Refs. 1 and 7), Han and Lee used the mechanistic model to simulate the curing behavior of unsaturated polyester resins under isothermal conditions, employing the differential scanning calorimetry data obtained for a range of single initiators and multiple initiator systems. For the sake of mathematical convenience, a pultrusion die of cylindrical geometry was considered. The mathematical model developed permits one to choose any number of initiators when predicting the distributions of the degree of cure and temperature in both the radial and axial directions of the die. The effects of material variables (e.g., the type and concentration of mixed initiators) and processing conditions (e.g., pulling speed and die temperature distribution) on the performance of the pultrusion are evaluated.     

A modeling approach to thermoplastic pultrusion. II: Verification of models

A set of mathematical models as a thermoplastic pultrusion process is evaluated. The predictions of the models are compared to experimentally obtained data in terms of composite temperature and pressure and process pulling force. The comparisons between predictions and experiments are made for two different material systems, two different die configurations, and a range of processing temperatures and speeds. The correlations between predictions and data are found to be favorable, indicating the soundness of the models.     

Material response of a semicrystalline thermoplastic polymer and composite in relation to process cooling history

A model capable of predicting the process-induced macroscopic in-plane material response of semicrystalline thermoplastic matrices and their composites was developed. Thi sinvestigation focused on the material response of a single layer or ply of neat PEEK matrix and its carbon fiber composite (APC-2) when subjected to various processing histories. Specifically, the response of the material moduli and processing strains as a function of temperature and the degree of crystallinity were studied. The kinetic-viscoelastic response of the matrix was determined from a modified form of the Standard Linear Solid model. A constitutive relation was proposed to quantify resign shrinkage as a function of thermal history, which incorporated crystallization. For a specific process history, the effective composite mechanical properties were determined from micromechanics models. Both neat and composite processing strains were evaluated to show the effect of fibers on the matrix dominated response (90° direction). In addition, comparisons of model moduli predictions with experimental measurements were performed. This study demonstrated that an increase in the degree of crystallinity results in an asymmetric shift of the modulus in the glass transition region to higher temperatures. Also, strains due to crystallization were predicted to be much smaller in comparison to the strains resulting from thermal contraction of the PEEK matrix.     

Simulation of the effects of thermal history on the morphology of thermoplastic composites

This paper concerns the simulation of the development of the degree of crystallinity and the size of spherulites that arise during the cooling of a slab of a semicrystalline polymer reinforced with long, continuous carbon fibers. This situation is commonly found during the processing of semicrystalline thermoplastic composites. Whereas published attempts at simulating this process have treated the composite as a continuum, and thereby used mass averaged physical properties (such as thermal conductivity, density, and specific heat), we use a quasicontinuum approach in which we consider the properties of the matrix and fiber separately. Once a temperature distribution is calculated using the continuum approach, the finite element method is applied locally at various points in the slab to calculate the degree of crystallinity and the size of the developing spherulites. This is done by using the Avrami equation and the Hoffman and Lauritzen radial growth equation. The degree of crystallinity and the spherulite size are predicted as a function of fiber spacing and packing geometry, and the predictions are in good agreement with experimental results obtained on poly(phenylene sulfide)/carbon fiber composites. The advantages of our approach over the continuum approach is that a relatively accurate prediction of the spherulite size is possible owing to constraints imposed by the fiber on the spherulitic growth.     

The development of an epoxy resin system for the injection molding of long-fiber epoxy composties

The Combination of reaction injection molding and pultrusion has resulted in a new processing technique, RIM-Pultrusion, Which has been used to produce a thermoplastic epoxy prepreg. This prepreg has been used to produce a long-fiber injection molded phenoxy/carbon fiber composite with near-Zero void content. A heat-activated curing system has been developed, which allows injecton molding of the prepreg to form a thermostet long-finer epoxy/carbon finber composite. The RIM- pultrusion conditions for producing an injection moldable prepreg are described. Capillary rheomety is used to study the epoxy resin to determine the proper molar ratio for RIM-Pultrusion. The long-fiber epoxy compostie is analyzed with dynamic mechanical analysis (DMA) and Fourier transform infrared spectroscopy (FTIR). Also., the impact strength and solvent resistance of the long-fiber composite are examined. The properties of the thermoset long-fiber epoxy xomposite are compared to those of a thermoplastic injection molded long-fiber phenoxy composite.     

Solidification in anisotropic thermoplastic composites

Heat transfer analysis in thermoplastic composites manufacturing offers several challenges. The melting/solidification process results in the appearance of a molving solid-fluid interface or two-phase zone. The presence of reinforcing fibers with thermal conductivities substantially different than those of matrix meterials causes the composite material to behave anisotropically. Furthermore, an analysis should be adaptable to complex geometries if it is to be useful for practical applications. In this paper, a numerical approach is presented to develop an analysis tool for the processing of thermoplastic-matrix composites. A numerical grid generation technique is employed to determine the temperature distribution within the part while accounting for the heat absorption and liberation during the solidification stage. The influence of anisotropy in the domain is accounted for by considering thermal conductivity as a second-order tensor which varies continuously throughout the domain. Sample part configurations are used to demonstrate the applicability of this technique to thermoplastic composites processing.     

Numerical and experimental analysis of resin flow and cure in resin injection pultrusion (RIP)

This work presents the results of modeling, numerical simulation, and experimental study of resin flow and heat transfer in the resin injection pultrusion (RIP) process. A control volume/finite element method (CV/FEM) was used to solve the flow governing equations, together with heat transfer and chemical reaction models. Resin viscosity, degree of cure, and fiber stack compressibility and permeability were measured in order to understand their influences on the process. An analytical flow model has also been develped based on the one‐dimensional flow approximation for the resin flow in the injection die. A high‐pressure small‐taper injection die was tested with different line speeds. Experimental data were used to verify the simulation results and the analytical solutions.     

Properties of poly(butylene terephthatlate) polymerized from cyclic oligomers and its composites

The high viscosity of thermoplastic matrices hampers fiber impregnation. This problem can be overcome by using low viscous polymeric precursors such as cyclic butylene terephthalate (CBT^® resins), which polymerize to form a thermoplastic matrix. This allows thermoset production techniques, like resin transfer molding (RTM), to be used for the production of textile reinforced thermoplastics. Due to the processing route and more specifically the time-temperature profile, inherent to the RTM process, the crystallites of the matrix consist out of well-defined, thick and well-oriented crystal lamellae. Together with a high overall degree of crystallinity and a low density of tie molecules, these large and perfect crystals cause polymer brittleness. Matrix brittleness lowers the transverse strength of unidirectional composites to below the matrix strength, but leaves the mechanical properties in the fiber direction unaffected. Although not a valid option for the RTM production route, crystallization from a truly random melt and at a sufficiently high cooling rate would substantially improve the ductility.     

复合材料拉挤成型过程中温度和固化度的数值模拟研究

本文对拉挤成型过程中热传导方程和固化度动力学方程进行了数值分析，确定了拉挤模具内温度和固化度属于强耦合关系。使用有限元软件IFEPG和FORTRAN语言为平台开发出一计算机程序，并使用该程序模拟出在一定工艺参数下拉挤模具内温度和固化度的分布，着重探讨了拉挤速度对模具内温度和固化度分布的影响。     

纳米SiO2包覆微晶纤维素对PBS/PLA复合材料性能的影响

以纳米二氧化硅(nano-SiO2)表面包覆的微晶纤维素(MCC)为填料,采用熔融共混的方法制备了聚乳酸/聚丁二酸丁二醇酯（PLA/PBS）复合材料。运用扫描电子显微镜、热重分析仪、差示扫描量热仪、动态热力学分析仪等方法研究了nano-SiO2对PLA/PBS/MCC复合材料的力学性能、热稳定性以及结晶行为的影响。结果表明,nano-SiO2包覆在MCC表面后与PLA/PBS熔融共混提高了nano-SiO2在聚合物材料中的分散性,改善了MCC与树脂基体的相容性;添加5 %（质量分数,下同）MCC的PLA/PBS/MCC复合材料,与同样添加量的PLA/PBS/nano-SiO2-MCC复合材料相比,其储能模量、冲击强度以及结晶度分别提高了13.04 %、11.70 %、71.92 %。     

Fiber reinforced nylon-6 composites produced by the reaction injection pultrusion process

Reaction injection pultrusion (RIP) combines the injection pultrusion process with reaction injection molding (RIM) techniques to yield one of the more novel methods of thermoplastic matrix pultrusion. An experimental set-up was designed and built to pultrude nylon-6 RIM material and continuous E-glassfiber. Well-impregnated nylon-6 composites with 66.5, 68.8, 71.1, and 73.3 vol% fiber were produced. Internal temperature profile within the die was recorded during the process, and physical properties of resulting composites were measured. This paper presents results of the effect of fiber content, die temperature profile and pulling speed variations on internal temperature profile, monomer conversion, and physical properties. The study showed that increasing pulling speed lowered both peak temperature and monomer conversion. Higher die temperatures accelerated the reaction, resulting in a higher exotherm, a higher peak temperature, and a higher monomer conversion within the range investigated. Shear strength, flexual strength, flexual modulus, and transverse tensile strength were proportional to monomer conversion. Flexual modulus increased with higher fiber content within the range observed. Data allow the proper combination of die temperature profile and pulling speed to be selected to achieve a desired level of monomer conversion and physical properties. Results of this study provide basic information required for product design with nylon-6 composites as well as tool design, selection of processing conditions, and quality control for the process.     

Synthesis and characterization of melt-processable polyimides derived from diaminodiphenylsulfone and benzophenone tetracarboxylic dianhydride with excellent heat resistance and thermoplasticity

Polyimide has excellent heat resistance, dielectric properties, and mechanical properties, and has a wide range of applications in aerospace, electronic packaging, and insulating materials. However, traditional polyimide is difficult to melt and dissolve, and its processing is difficult, which has become an important reason limiting its practical application. Therefore, the development of high temperature-resistant thermoplastic polyimide has become a research hotspot. To prepare high temperature-resistant thermoplastic polyimide materials, a series of thermoplastic polyimides was successfully prepared using 3,3′,4,4′-benzophenone tetracarboxylic dianhydride, 3,3′-diaminodiphenylsulfone, 2,3′,3,4′-benzophenone tetracarboxylic dianhydride, 9,9-bis(4-aminophenyl)fluorene, and 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane via a two-step method. The effects of non-coplanar structure and bulky groups on the solubility, processability, and thermal properties of polyimide were studied. The structure, heat resistance and thermoplasticity of polyimide were characterized via various methods. The results show that the glass transition temperature of the prepared thermoplastic polyimide is between 292 and 302°C, and has excellent thermal resistance. The processing viscosity of polyimides is as low as 9210 Pa.s, and it has a certain degree of processing properties. It may be designed to be used in high temperature-resistant hot melt adhesives for structural components, high temperature-resistant melt processing resins, or thermoplastic composite materials used in the field of aerospace in the future.     

Heat transfer in injection molding of crystallizable polymers

A model is proposed for the treatment of heat transfer with crystallization during plastics processing in general, and injection molding in particular. The model incorporates experimentally determined crystallization kinetics parameters. It permits the calculation of the distribution of both temperature and crystallinity in the molding. Theoretical predictions are in good agreement with experimental measurements in both injection molding and a prototype apparatus.     

Modeling compression molding of all‐thermoplastic honeycomb core sandwich components. Part A: Model development

The compression molding process is studied with the aim of modeling the instantaneous degree of face‐core bonding in all‐thermoplastic sandwich components during molding. The theory of bonding is briefly discussed and it is concluded that for most thermoplastic materials, processing is performed at temperatures where full bond strength is seemingly immmediately established at positions in full face core contact. A two‐dimensional model is developed to predict increase in contact area due to flow of melted core material during molding. Further, heat transfer during processing is modeled in order to determine the extent of melted core material. The two models are coupled into one process model and a numerical example is presented illustrating the predicted behavior of polypropylene‐based sandwich components in compresion molding. The process model suggests that the face‐core bond strength may be significantly increased through flow of the melted core wall.     

Simultaneous optimization of die‐heating and pull‐speed in pultrusion of thermosetting composites

Die‐temperature and pull‐speed are the key parameters in pultrusion that affect the degree and uniformity of the curing of thermosetting composites, which in turn influence their mechanical properties and in‐service performance. This paper presents the development, implementation, and validation of a numerical procedure for arriving at an optimum combination of die‐heater temperatures and pull‐speed for producing a uniformly cured composite pultrudate. An objective function representing the effects of both parameters on the distribution of degree‐of‐cure across the cross section of a pultruded part at the die exit was established. The function was used to develop an algorithm for calculating the required changes in die‐heater temperatures and pull‐speed to achieve a desirable degree‐of‐cure with maximum possible uniformity. The algorithm was implemented using the three‐dimensional. Finite Element/Nodal Control Volume (FE/NCV) approach for process simlation, in which a general‐purpose FE package was used to perform heat transfer analysis, together with other routines developed to perform cure modeling and the optimization. The application of the developed procedure was demonstrated by simulating pultrusion of a graphite/epoxy C‐section. The results of the studies show that the procedure is numerically stable and works well for a die with multiple heaters.