Reaction injection pultrusion of thermoplastic and thermoset composites

Coventional pultrusion of thermoset composites is under increasing examination for emissions of harmful volatiles from the resin wetout tank. Even though the pultrusion of thermoplastic matrix composites produces no emissions, it is difficult to wet individual fibers due to their high melt viscosities. This paper addresses both the issues of volatiles and wetting with a process called Reaction Injection Pultrusion (RIP). A prototype RIP machine was used to make both thermoplastic polyurethane and thermoset polyisocyanurate matrix composites. The RIP process produces pultruded parts with low void content, good surface finish, and acceptable mechanical properties. The low viscosity constituents used in RIP help improve fiber impregnation, while the small volume of the impregnation bath reduces emissions. Processing parameters such as line speeds, catalyst levels, and die temperaures were varied to establish processing guidelines for sustained production.     

Sensor system for monitoring impregnation and cure during resin transfer molding

In the resin transfer molding (RTM) fabrication of composites, knowledge of the position of a moving resin front during impregnation is important for process optimization. We describe here a simple, inexpensive, multi-point sensor system based on DC conductometry for determination of resin position in an RTM mold. This Resin Position Sensor (RPS) system consists of a matrix of small sensors embedded in the RTM tool, whose combined output can be used to produce a resin flow pattern at any given time after the start of impregnation. As it cures, the resin resistance increases and the sensor can then function as a cure monitor. A large, 24-sensor RTM tool was fabricated for demonstration of the RPS. Flow contour maps generated from sensor data during impregnation of both E-glass and carbon fiber preforms are shown.     

熔融浸渍工艺参数对纤维束渗透率影响的研究

在连续长纤维增强热塑性复合材料浸渍模型中,渗透率是一个十分重要的参数。准确测量熔融浸渍工艺中高黏度树脂熔体浸润纤维束的渗透率,有助于浸渍模型更好指导熔融浸渍模具设计和工艺参数优化,制备出性能优异的热塑性树脂基复合材料。本文通过自制实验装置,测定了熔融浸渍工艺中高黏度树脂浸渍单向纤维束时纤维束张力和浸渍压力变化对渗透率的影响,根据实验结果拟合出工艺参数与渗透率关系的计算公式。结果表明：纤维束张力越大渗透率越低;浸渍压力越大,纤维束渗透率越大,但增大幅度随张力增大而降低。     

In situ sensor monitoring and intelligent control of the resin transfer molding process

An intelligent closed-loop expert control system has been developed for automated control of the resin transfer molding process of a graphite fiber preform using an epoxy resin, E905L. The sensor model system has been developed to make intelligent decisions based on the achievement of landmarks in the cure process, such as full preform impregnation, the viscosity, and the degree of cure of the resin rather than time or temperature. In-situ frequency dependent electromagnetic sensor (FDEMS) and the Loos resin transfer model are used to monitor and control the processing properties of the epoxy resin during RTM impregnation and cure of an advanced fiber architecture stitched preform. Once correlated with viscosity (η) and degree of cure (α), the FDEMS sensor monitors and the RTM processing model predicts the reaction advancement of the resin, viscosity and the impregnation of the fabric. This provides a direct means for monitoring, evaluating, and controlling intelligently the progress of the RTM process in situ in the mold throughout the fabrication process and for verification of the quality of the composites.     

Experimental analysis and numerical modeling of flow channel effects in resin transfer molding

Composite manufacturing by Liquid Composite Molding (LCM) processes such as Resin Transfer Molding involve the impregnation of a net‐shape fiber reinforcing perform a mold cavity by a polymeric resin. The success of the process and part manufacture depends on the complete impregnation of the dry fiber preform. Race tracking refers to the common phenomenon occurring near corners, bends, airgaps and other geometrical complexities involving sharp curvatures within a mold cavity creating fiber free and highly porous regions. These regions provide paths of low flow resistance to the resin filling the mold, and may drastically affect flow front advancement, injection and mold pressures. While racetracking has traditionally been viewed as an unwanted effect, pre‐determined racetracking due to flow channels can be used to enhance the mold filling process. Advantages obtained through controlled use of racetracking include, reduction of injection and mold pressures required to fill a mold, for constant flow rate injection, or shorter mold filling times for constant pressure injection. Flow channels may also allow for the resin to be channeled to areas of the mold that need to be filled early in the process. Modeling and integration of the flow channel effects in the available LCM flow simulations based on Darcian flow equations require the determination of equivalent permeabilities to define the resistance to flow through well‐defined flow channels. These permeabilities can then be applied directly within existing LCM flow simulations. The present work experimentally investigates mold filling during resin transfer molding in the presence of flow channels within a simple mold configuration. Experimental flow frot and pressure data measurements are employed to experimentally validate and demonstrate the positive effect of flow channels. Transient flow progression and pressure data obtained during the experiments are employed to investigate and validate the analytical predictions of equivalent permeability for a rectangular flow channel. Both experimental data and numerical simulations are presented to validate and characterize the equivalent permeability model and approach, while demonstrating the role of flow channels in reducing the injection and mold pressures and redistributing the flow.     

The influence of impregnation condition on reinforced ethylene–vinyl–acetate elastomer composite

Experiments were conducted to investigate the effect of impregnation conditions on glass fiber-reinforced ethylene–vinyl–acetate elastomer. Both the matrix elastomer resin and reinforcing glass fiber were premixed and compression-molded using a specially constructed mold. The mold prevents the flowing out of the matrix resin during fabrication. The impregnation time was varied between 5 and 25 min. The level of impregnation was measured through the optical micrographs of the cross section, estimation of void contents using the ignition method, and transverse bending strength. The morphology of the fractured surface was studied using scanning electron microscopy (SEM). The results showed that the longer the impregnation time, the lower the void contents. Both the bending modulus and strength increased with increasing impregnation time. The SEM micrograph shows little adhesion between the matrix and the reinforcing glass fiber. © 1996 John Wiley & Sons, Inc.     

Study on long fiber–reinforced thermoplastic composites prepared by in situ solid‐state polycondensation

Long glass fiber–reinforced thermoplastic composites were prepared by a new process, in situ solid‐state polycondensation (INSITU SSP). In this process reinforcing continuous fibers were impregnated by the oligomer of PET melt, and then the impregnated continuous fibers were cut to a desired length (designated prepreg); finally, the prepreg was in situ polymerized in the solid state to form the high molecular weight matrix. SEM, FTIR spectra, short‐beam shear stress test, flexural strength test, impact strength test, and the intrinsic viscosity measurement were used to investigate the wetting and interfacial adhesion, the mechanical properties of the composite, and the molecular weight of matrix resin in the composite. The results showed that the molecular weight of PET in the matrix resin and mechanical properties could be adjusted by controlling the SSP time and that the high level of interfacial adhesion between reinforcing fibers and matrix resin could be achieved by this novel INSITU SSP process, which are attributed to the good wetting of reinforcing fibers with low molecular weight oligomer melt as the impregnation fluid, the in situ formation of chemical grafting of oligomer chains onto the reinforcing fiber surface, and the in situ formation of the high molecular weight PET chains in the interphase regions. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91:3959–3965, 2004     

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.     

Development of an isothermal lubricating layer model for injection pultrusion of electronic prepregs

The current process for manufacturing electronic prepregs uses solvent‐based resin systems. Solvents are environmentally unfriendly and contribute to voids in the pre‐preg, which are a source of product variability. Prepreg inconsistencies are one of the major sources of scrap on the board shops. To overcome these drawbacks, a solvent‐less process is currently being developed in our laboratory. The process is based on the concept of injection pultrusion or continuous resin transfer molding (RTM). The centerpiece of the process is an impregnation die. Glass fabric and resin are fed into the die where fiber impregnation and partial resin cure occurs depending on the material chemo‐rheology. The B‐staging is finished in an oven located immediately after the die. A key to the success of the process is being able to predict the pulling force. Fabrics used in electronic prepreg manufacturing are relatively fragile; they could be damaged if the pulling force is too high. A model to predict the pulling force, based on the formation of a continuous lubricating layer between the die surface and the moving fabric, has been developed and experimentally tested using model fluids.     

Improvement of mechanical properties of structural thermoplastic composites using a reactive (low molecular weight) matrix component

An innovative manufacturing process for continuous fiber composites with the polymeric matrix made up of polypropylene and epoxy resin, as a model reactive low molecular weight component, was developed; variable process parameters give rise to different morphologies of matrix components surrounding the woven fabric reinforcement. Furthermore, the combination of both thermoplastic and thermosetting polymers permitted intimate fibers impregnation, typical of thermosetting matrix composites, with short process cycle time, which usually occurs in manufacturing process of thermoplastic matrix composites. Polypropylene (PP) films, glass fibers fabric, and epoxy resin film were used to produce flat composite through film‐stacking technique. The preparation process focused on control of both epoxy resin cure process and polypropylene melting. The process was able to induce the two matrix components to form either a planar (sandwich‐like) structure or a three‐dimensional (3D) network by means of controlling the process parameters such as pressure and heating rate. The strong enhancement of the mechanical properties (Young's modulus and tensile strength of the composites with the 3D structure were almost twice as high of those of the composites with sandwich‐like matrix structure) was due to the different microstructures produced by the interplanar flow of the thermoplastic polymer. POLYM. COMPOS., 31:1762–1769, 2010. © 2010 Society of Plastics Engineers.     

Impregnation of thermoplastic resin in jute fiber mat

Impregnation rate of thermoplastic resin (polypropylene) in jute fiber mat and influence of relative factors on impregnation were studied, aiming to develop the continuous melt impregnation technique and to investigate the effect of impregnation rate and temperature on processing conditions and mechanical properties of natural fiber mat-reinforced thermoplastics. Influence of pressure on porosity of fiber mat and effect of melt viscosity on impregnation rate were also investigated. The modified capillary rheometer was used as apparatus and experimental data were analyzed based on the one-dimension Darcy’s law. Results showed that at a given pressure, the impregnation rate is inversely proportional to melt viscosity and jute fiber mat has higher porosity than glass fiber mat. The architecture, compressibility, permeability and fiber diameter of jute fiber mat were compared with those of glass fiber mat and their effects on impregnation were discussed further. It could be seen that the average diameter of jute fiber is much bigger; the porosity of jute fiber mat is significantly higher and inner bundle impregnation does not exist in jute fiber mat. Therefore, it is not difficult to understand why the impregnation rate in jute fiber mat is 3.5 times higher and permeability is 14 times greater. Kozeny constants of jute and glass fiber mats calculated based on the capillary model are 2950 and 442, respectively.     

Flow enhancement in liquid composite molding processes by online microwave resin preheating

Liquid composite molding techniques are increasingly applied for the manufacturing of fiber-reinforced plastic components for civil, aerospace, and automotive applications. Being the preform impregnation a key step during the process, resin viscosity should meet the precise requirements. Opportune resin preheating increases its fluency, thus enhancing the impregnation and saturation flow through the fabric and reducing the mold filling time. This paper explores the application of microwave technology for resin preheating. The integration of an online microwave preheating system within a demonstrative resin infusion facility is described and the effects of preheating on the infusion time are discussed. Parallel-plate dielectric sensors were embedded into the mold to track the unheated and preheated resin flow through the fiber preform. The obtained results highlighted the effectiveness of online microwave heating to reduce the time required for the impregnation of the dry fiber reinforcement.     

Some issues on impregnation in manufacturing of thermoplastic composites by using a double belt press

The manufacturing of thermoplastic composite intermediates by a continuously running double belt press (System of Held Comp., Germany) has become one of the most effective techniques for high quantity production. The process of combining thermoplastic materials and reinforcing fabrics during the manufacturing results in impregnation phenomena of the reinforcing layers distinct from one in resin transfer molding (RTM). Here, the work is focused on the clarification of the impregnation process that occurred in such a continuous manufacturing process. A composite intermediate of 50 wt% fibers consisting of E-glass fabrics and nylon 66 films (Zytel, DuPont) was produced at different processing conditions to exhibit the influence of the degree of impregnation on mechanical properties and damage patterns of thermoplastic composites. It can be proved that because of strong inhomogeneity in the fabric concerning the permeability of the yarns and the weaving structure, respectively, the time required to impregnate the fabrics is governed by transversal micro-flow into the fiber bundles rather than macro-infiltration of the polymer into the fabric structure. Imperfect impregnation resulted in specific damage pattern in the center of the compressed yarns after flexural loading. The results are to be applied to guide the optimization of the manufacturing process with respect to material selection and preselection of processing conditions.     

Effect of compaction treatment on laminated CFRP composites fabricated by vacuum‐assisted resin‐transfer molding

Carbon fiber‐reinforced polymer (CFRP) composites were fabricated using ordinary and compaction setups (OS and CS, respectively) in the vacuum‐assisted resin‐transfer molding (VARTM) process. The mechanical properties and acoustic emission (AE) spectra of the CFRP composites were compared among fabricated samples. The CFRP plates with sequences of [+30/−30]₆ were sectioned to make specimens for Mode I interlaminar fracture tests and three‐point bending tests. The difference between the material properties and AE characteristics of the OS and CS specimens were statistically compared using one‐way analysis of variance. The OS specimens had a thicker resin layer, a higher resin fraction, larger average fracture toughness, and AE energy corresponding to the Mode I fracture, whereas the CS specimens had more macro‐scale voids and higher bending strength. AE analysis showed that frequency bands in the interlaminar fracture tests corresponding to matrix‐related fracture were dominant for the OS specimens, whereas those corresponding to the mixed fracture mode of the fiber and matrix fracture were dominant for the CS specimens. In the bending tests, mixed fiber‐matrix fractures were dominant for the OS specimens, and fiber‐related fractures were dominant for the CS specimens. In conclusion, the compaction treatment diminished interlaminar fracture toughness, due to the enhanced formation of macro‐scale voids around the fiber bundles during the resin impregnation stage. However, the bending strength improved with an increased fiber volume fraction. POLYM. COMPOS., 38:217–226, 2017. © 2015 Society of Plastics Engineers     

Processing effects in production of composite prepreg by hot melt impregnation

The hot melt impregnation process for producing composite prepreg has been studied. The role of the exit die is highlighted by operating without impregnation bars. Experimental results show that when a fiber two is pulled through a resin bath and then through a wedge shaped die, the total resin mass fraction and the extent of resin impregnation in the two increase with the processing viscosity. The penetration of resin into a fiber bundle is greater when the resin viscosity is higher. This trend is unchanged over a range of two speeds up to the breaking point. A theoretical model is developed to describe the effect of processing conditions and die geometry on the degree of impregnation. Calculations with this model indicate that for a given die geometry, the degree of impregnation increases from 58% to 90% as the ratio of the clearance between the two and the die wall, to the total die gap is decreased from 0.15 to 0.05. Physical arguments elated to the effective viscosity of the prepreg show that the clearance ratio is independent of the two speed, but decreases as the ratio of the effective shear viscosity of the prepareg to the resin viscosity increases. This provides a connection between the experimental results obtained with varying resin viscosity and the computational results obtained with varying clearance values at the die inlet.     

Tow impregnation during resin transfer molding of bi-directional nonwoven fabrics

A model has been developed for analyzing resin impregnation of fiber tows during resin transfer molding of bi-directional nonwoven fiber performs. The model is based on the existence of two main regions of resin flow: the macropore space formed among fiber tows and the micropore space formed among individual fiber filaments within a tow. The large difference in permeability between these two regions of flow leads to the potential for void formation during resin transfer molding. The model was formulated for both constant flow rate and constant pressure mold filling. For ambient pressure mold filling, the model predicts a difference in the size of the voids and distribution between axial tows (oriented along the flow direction) and transverse tows (oriented in the transverse direction). When vacuum is imposed on the mold, the model predicts the same resin impregnation behavior for both axial and transverse tows. Furthermore, given sufficient time, voids generated under vacuum mold filling will eventually collapse because of the absence of an opposing internal void pressure. In addition to insights on void formation, the model also provides a basis for the study of the relationship between resin transfer molding parameters and the resin impregnation process.     

Prepreg processing science and engineering

Resin flow and fiber spreading during the prepregging process were investigated experimentally using a scale-down version of a commercial hot-melt prepregger with bismaleimides and carbon fibers as model systems. Specifically, several dimensionless parameters such as the Prepreg Flow Number, fractional resin uptake, resin distribution function, fractional width change, and prepregging efficiency were defined to characterize the prepregging process as well as the quality of the prepreg. Fiber spreading during the prepregging process was observed to be a viscoelastic phenomenon depending on the impregnation temperature. For resin impregnation into collimated fiber tows, all the experimental data points obtained at different operating conditions were superimposed onto a single line showing a temperature-pressure-velocity superposition for the prepregging process as predicted by the definition of the Prepreg Flow Number (PFN). Finally, three fundamental steps of the prepregging process were identified and confirmed with cross-sectional micrographs of unaged prepregs produced at different temperatures.     

The consolidation of commingled thermoplastic fabrics

Commingled fabrics composed of yarns containing both the reinforcement and the matrix in fiber form are an innovative preform for thermoplastic composite materials. The material is consolidated into a rigid structure by the application of heat and pressure. A mathematical model of the consolidation process for commingled fabrics has been developed. The model predicts the variation of laminate thickness, fiber volume fraction, and void content during the consolidation process as well as the time required to reach full consolidation. Materials composed of initially separate, commingled or cowoven fiber bundles are considered. The influence of fiber velocity induced by compaction on the flow of matrix is accounted for. An equivalence factor has been derived so that a one-dimensional flow analysis may be used to model the impregnation of elliptical bundles of varying aspect ratio. This permits an analytic solution to the governing equation for fiber bundle impregnation. The model was utilized to examine the influence of various material and processing parameters on consolidation behavior.     

Reactive melt infiltrated Cf/SiC composites with robust matrix derived from novel engineered pyrolytic carbon structure

Needle-punched C_f/SiC composites were fabricated by a novel pore tuned reactive melt infiltration (RMI) process. The novel hierarchically porous carbon structure in the fiber preform with the porosity well open to liquid silicon was engineered by impregnation of phenolic resin with addition of a pore former. Neither residual bulk carbon nor residual bulk silicon is detected in the matrix of the C_f/SiC composites prepared by the pore tuned RMI, indicating that a robust matrix with homogenous SiC can be formed. The composite prepared by the pore tuned RMI exhibits a tensile strength of 159±5 MPa, which is 46% higher than that without addition of pore former.     

In-situ monitoring of the resin transfer molding impregnation and cure process

Resin transfer molding (RTM) of advanced fiber architecture materials promises to be a cost effective process for obtaining composite parts with exceptional strength. However there are a larger number of material processing parameters that must be observed, known, and/or controlled during the resin transfer molding process. These include the viscosity both during impregnation and cure. In-situ sensors which can observe these processing properties within the RTM tool during the fabrication process are essential. This paper will discuss recent work on the use of frequency dependent electromagnetic sensing (FDMS) techniques to monitor these properties in the RTM tool. Our objective is to use these sensing techniques to address problems of RTM scaleup for large complex parts and to develop a closed loop, intelligent, sensor controlled RTM fabrication process.