Thermoforming of advanced thermoplastic composites. I: Single curvature parts

Thermoforming has been studied for a single curvature part made from various advanced thermoplastic matrix composite prepregs. For parts with acceptable shape conformity, preheating of the composite laminates to a processing temperature of 350 to 400°C is necessary prior to forming with molds maintained at 200°C. However, only PEEK/carbon fiber prepreg tapes yielded parts with acceptable microstructural integrity and a matrix crystallinity level of about 30 percent. Amorphous matrix based PXM 8505/T500 fabric prepregs also result in lamination and void free parts, but fiber matrix distribution in this case was rather poor. Parts thermoformed from other prepreg laminates contained voids and/or were delaminated, thereby indicating the need for higher mold temperature and forming pressure than that afforded by the present study, in which a standard lab-scale thermoforming machine was used.     

Thermal and air permeation properties of a carbon fiber/toughened epoxy based prepreg system

This study addresses thermal and air permeation properties of a new toughened prepreg system. Voids in the uncured prepreg structure can affect the void content in the final composite structure. A new, toughened prepreg system, commercially available for aircraft structural application, was utilized in this study. The prepreg was subjected to thermal and rheological characterization to understand the basic prepreg properties. These experiments were followed by a prepreg air permeation study to investigate prepreg processing and its influence on the prepreg structure. Crosslinking of the resin matrix was monitored with prepreg specimens without extracting resin from the prepreg. Along with thermal property measurements, the air flow rate significantly decreased in initial static experiments, followed by equilibrium permeability values. An air permeation model divided the air permeability into intralaminar and interlaminar permeabilities. Interlaminar air permeation was found to be more pronounced than intralaminar air permeation in this particular prepreg system. These permeation measurement results were explained using optical microscopy, proving that the application of vacuum could eliminate significant porosity in the laminate. Collectively, understanding prepreg thermal and air permeation properties was considered to be important; the voids in uncured prepreg may cause the voids in the final composite structure. Voids in the prepreg can be attributed to the heterogeneity and anisotropy of the toughened prepreg structure, resulting from particular prepreg processing techniques. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 5–16, 1997     

Defect reduction strategies for the manufacture of contoured laminates using vacuum BAG‐only prepregs

Complex structures manufactured using low‐pressure vacuum bag‐only (VBO) prepreg processing are more susceptible to defects than flat laminates because of complex compaction conditions present at sharp corners. Consequently, effective defect mitigation strategies are required to produce structural parts. In this study, we investigated the relationships between laminate properties, processing conditions, mold designs, and part quality in order to develop science‐based guidelines for the manufacture of complex parts. Generic laminates consisting of a central corner and two flanges were fabricated in a multipart study that considered variation in corner angle and local curvature radius, the applied pressure during layup and cure, and the prepreg material and laminate thickness. The manufactured parts were analyzed in terms of microstructural fiber bed and resin distribution, thickness variation, and void content. The results indicated that defects observed in corner laminates were influenced by both mold design and processing conditions and those optimal combinations of these factors can mitigate the defects and improve quality. POLYM. COMPOS., 38:2016–2025, 2017. © 2015 Society of Plastics Engineers     

A structure and property based process window for void free thermoset composites

A process window providing guidelines to minimize internal stress levels and to prevent void formation during cure of thermoset composite materials is presented. A model taking into account the applied pressure and the level of stress borne by the fiber assembly was introduced to calculate the hydrostatic internal stress state in the resin during cure. Based on the fundamental mechanisms of matrix shrinkage and evolution of viscoelastic properties under the given processing conditions, the internal stress in the resin was calculated as a function of fiber volume fraction, fiber stacking sequence, applied pressure and resin conversion. This level of stress is compared to a criterion for void initiation in the resin. A process window was hence constructed for preventing void formation during cure. Composite laminates with different stacking sequences and fiber volume fractions were cured with different applied pressures within and out of the process window boundaries. The composite void contents were measured and correlated perfectly with the process boundaries. This process window construction taking into account the material vis‐coelastic properties and the composite architecture is a unique tool for determining optimum process condition of composite laminates.     

Online monitoring and analysis of resin pressure inside composite laminate during zero‐bleeding autoclave process

Resin pressure is one of the most important parameters in manufacturing composites during autoclave process. It not only greatly influences resin flow behavior, but also has effects on void formation and elimination. Online monitoring resin pressure can provide an important guidance for the optimization of the processing parameters and the control of the quality of composites. In this study, a resin pressure online measuring system for autoclave process was established based on the principle of pressure transfer in liquid, and the size of the measuring probe of the system was optimized to increase the accuracy of measured resin pressure. The results indicate that the accuracy and the dynamic response of the system can meet the requirements of resin pressure measurement during autoclave process. Furthermore, by means of this proposed resin pressure measuring system and the measurements of compaction properties of the fabric stacks, the resin pressures inside carbon fiber fabric/epoxy resin and glass fiber fabric/epoxy resin prepreg stacks during autoclave process were investigated, especially for the zero‐bleeding process which is prevailing for aircraft composite structures. It is demonstrated that during zero‐bleeding process, the resin pressures, which conform to the spring and piston model, uniformly distribute along through‐thickness and in‐plane directions. In addition, the resin pressure profile is significantly influenced by the fiber volume fraction of the prepregs, indicating that fiber content of prepreg should be optimized for achieving free defects and uniform fiber distribution. POLYM. COMPOS., 2011. © 2010 Society of Plastics Engineers     

Micro scale flow behavior and void formation mechanism during impregnation through a unidirectional stitched fiberglass mat

Liquid composite molding (LCM) processes such as resin transfer molding (RTM) and structural reaction injection molding (SRIM) have been perceived as high potential processes for the near-net-shape manufacturing of composite parts. This paper addresses two major issues in LCM technology: fiber wetting and void formation during mold filling. Flow visualization experiments were carried out to develop a better understanding of the flow induced voids. The formation and elimination of voids were studied using several liquids and a unidirectional stitched fiberglass mat. Void formation was correlated to capillary number and liquid-fiber-air contact angle.     

Formation of voids in composite laminates: Coupled effect of moisture content and processing pressure

Void formation as a result of prepreg moisture content and processing pressure during cure was experimentally investigated in thermosetting composite laminates. This was achieved by determining the void contents of eight‐ply laminates fabricated from TenCate^® BT250/7781 E‐glass/epoxy prepreg at processing pressures of 1.7, 3.0, 4.4, and 5.8 atm. At each processing pressure, three types of laminates were fabricated using: (i) unconditioned prepregs (direct from the storage bag); (ii) prepregs conditioned at 25% relative humidity; and (iii) 99% relative humidity. Dynamics of prepreg moisture uptake during conditioning was measured using a moisture analyzer and was shown to exhibit Fickian diffusion behavior. The void contents of the cured laminates were found to vary from 1.6% to 5.0% depending on humidity environment the prepregs were exposed and the pressure applied during fabrication. The void contents of all laminates were observed to approach an asymptotic value of ∼1.6% as pressure was increased. The experimental results indicated the processing pressure applied during fabrication was increasingly carried by the fiber bed, reducing resin pressure during cure. Therefore, an enhanced void formation model was proposed through the addition of a pressure reduction factor and an asymptotic void content term. The proposed model was found to accurately predict the void content of laminates made of prepregs exposed to constant/varying humidity environments and fabricated at a wide range of processing pressures. POLYM. COMPOS., 36:376–384, 2015. © 2014 Society of Plastics Engineers     

Mechanical characterization of carbon fiber reinforced phenolic resol and epoxy condensate plastic

A matrix resin for carbon fiber reinforced composite was developed that consisted of resol type phenolic and difunctional epoxy resin (PR-EP) condensate or adduct. Carbon fiber reinforced composite with fiber volume fraction of 0.6 was prepared with PR-EP matrix containing 0, 50, 100, 150, and 175 parts of epoxy resin per hundred parts of phenolic resin (php), especially a synthesized resol type. One-shot and prepreg techniques have been adopted and the study of loss of volatiles has indicated the superiority in terms of favorable processability of prepreg technique over the other. FTIR spectroscopic analysis confirmed the PR-EP adduct formation at the prepreg preparation stage. The improvement in properties such as tensile strength and elongation at break was observed in resin matrices with epoxy and phenolic resin; however, the flexural strength and modulus remained more or less unaltered. The prepreg technique of composite preparation has resulted in substantial improvement in mechanical properties and the same was attributed to the formation of PR-EP adduct and low volatiles during cure. Composites of carbon fiber reinforced PR-EP matrix developed here are likely to meet the requirement of aerospace structures in view of the realization of a wide spectrum of properties.     

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.     

The Influence of Temperature and Pressure During the Curing of Prepreg Carbon Fiber Epoxy Resin

The physical and hence mechanical properties of carbon fiber reinforced epoxy resin are affected by the curing conditions used in their manufacture. The relationship between the cure temperature and pressure and the density, fiber volume fraction, and the void content of cured laminates, was investigated. For the unidirectional 914C prepreg material used, an optimum cure temperature was found which gave maximum fiber volume fraction and composite density, and minimum void content. This behavior is related in the paper to resin flow and cure characteristics. A linear relationship between cure pressure and fiber volume fraction is reported and explained by reference to the void content of the laminates. It is concluded that in-house trials are required to determine the optimum size of the processing window for specific systems and components.     

Composite lamination—analysis and modeling

Multilayer printed circuits are a prime example of complex polymer composites. One of the critical steps in manufacturing of the circuit boards is the lamination process, which is frequently simulated by the aborted flows test (AFT). In order to obtain a composite free of voids and good interlaminar adhesion, it is imperative for the resin to flow into metal interfaces as well as intermix in the resin interfaces. On an empirical basis, the AFT is used as a process monitor for making prepreg, the glass fiber reinforced dielectric which supports the circuitry. Under a given set of conditions (temperature profile, pressure, layup), AFT measures advancement and reactivity of prepreg. By using a given batch of prepreg, the optimum laminating conditions can be identified, which are mainly a function of the temperature profile. Here, a calorimetric and rheological study of prepreg cure kinetics during lamination is presented. The flow process is being modeled by calculating the viscosity from combining a temperature dependent viscosity term with a kinetically controlled molecular weight term.     

Processing parameters for filament winding thick-section PEEK/carbon fiber composites

The consolidation pressure and winding speed for thermoplastic filament winding were studied. Thermoplastic composite parts were manufactured from tape prepreg (APC-2); powder-coated, semi-consolidated towpreg; and commingled fiber towpreg. The material used was carbon fiber (AS-4) (60 vol%) in a PEEK matrix. The parts made were open-ended cylinders of the three materials, 177.8-mm ID, 228.6 mm long, 17 plies thick with a 0° lay-up angle; and rings, 50 plies of APC-2 thick, 6.35 mm wide (one strip wide), 177.8-mm ID, and a lay-up of 0°. Their quality was determined by surface finish and void percentage. The tubes made from APC-2 appeared to have the best quality of the three prepregs. For the rings, the speed of lay-down had a significant effect (at a 99% confidence level) on both the final width of the parts and on the percentage of voids. The pressure of the roller had a significant effect on the final widths at a 99% confidence level, but a significant effect on the percentage of voids at only a 95% confidence level.     

Stress‐initiated void formation during cure of a three‐dimensionally constrained thermoset resin

During cure of epoxy resins, polymerization induces an increase in mechanical properties, which is accompanied by a volumetric shrinkage. When the resin is cured in a constrained mold to which it adheres, tensile stresses will hence develop, which may exceed the stremgth of the resin at a given curing stage. Voids will then form. The origin and governing parameters of void formation are studied using an epoxy resin cured in a three‐dimensionally constrained glass mold following isothermal cure cycles. Two types of voids are shown to appear during cure, one early in the process and a second around the gelation point. A viscoelastic analysis of the material stress state over the whole range of cure is performed. Both the viscoelastic modulus obtained from a time‐cure‐temperature superposition and the volumetric shrinkage, which was continuously measured by density change, are taken into account. A value for the critical internal stress at void initiation is thus proposed. This criterion can be used to provide guidelines for tailoring the material properties toward an increase of the critical stress for void initiation. Also, since during theprocessing of composite materials, cases may arise where the resin cures within the interstices left between consolidated fibres that do not move, this critical stress failure criterion can be of use in the eastablishment of a process window providing guidelines for the production of void free composites.     

Development and hot-melt impregnation of a model controlled flow prepreg system

A controlled-flow epoxy-based model prepreg resin system was developed. The formulation of the model controlled-flow resin was designed from performance information obtained from a commercially available controlled-flow resin, presently used in the aircraft industry. Thermoanalytical techniques including rheometry were used to provide the necessary information to develop the model system along with a formulation methodology developed by Seferis and co-workers. The model resin formulation, which was a combination of tetraglycidyl ether of methylenedianiline (TGMDA), diglycidyl ether of bisphenol-A (DGEBA), carboxyl-modified butadiene/acrylonitrile rubber (CMBN), carboxyl-terminated butadiene/acrylonitrile rubber (CTBN), bisphenol-A (BPA), diaminodiphenyl sulfone (DDS), and dicyandiamide (DICY), was hot-melt impregnated into unidirectional carbon fibers on a laboratory scale hot-melt prepreg machine. A two-parameter, three-level design of experiments was performed on the prepreg processing parameters in which impregnation temperature and pressure were varied. Thus, a total of nine different experimental prepregs were produced and characterized by resin content, extent of impregnation, and tack. The results from the characterization of the nine experimental prepregs are compared with the effects of the prepreg processing conditions. These results are also compared with the results generated for the commercial controlled-flow resin. Collectively, this work provides a fundamental basis by which the analysis and rational utilization of controlled-flow matrix prepregs can be effected.     

Properties of carbon fiber composite laminates fabricated by coresin film infusion process for different prepreg materials

A new cocured process called coresin film infusion (co‐RFI) process, which combines RFI process and prepreg/autoclave process, was introduced and four kinds of commercial carbon fiber prepreg material systems and a kind of resin film were applied to fabricate co‐RFI laminates. The compatibility between the resin film and the prepreg matrix and the application of co‐RFI process were investigated based on the resin flowability, glass transition temperature of cured resin, processing quality of laminate, and variation in resin modulus on cocured interphase region measured by nanoindentation. Furthermore, mode I (G_IC), mode II (G_IIC) delamination fracture toughness, and flexural strength and modulus were measured to evaluate the mechanical properties of cocured laminates with different prepreg materials. The experimental results show that thickness and fiber volume fraction of co‐RFI laminates with the four kinds of prepreg materials are similar to those of prepreg laminates and RFI laminate with acceptable differences. In addition, there are no obvious defects in co‐RFI laminates. Moreover, the reduced modulus of resin at cocured interface and glass transition temperature values of the mixed resin reflect good compatibility between prepreg matrix resin and RFI resin. The G_IC, G_IIC values, and flexural performances of cocured laminates lie between and even exceed those of prepreg laminates and RFI laminates, indicating no weakening effect in the cocured interface. Therefore, the co‐RFI process is believed to effectively fabricate composite with low cost and it can be applied using various prepreg systems. POLYM. COMPOS., 34:2008–2018, 2013. © 2013 Society of Plastics Engineers     

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.     

Formation and elimination of voids during the processing of thermoplastic of matrix composites

The mechanisms of void formation and elimination during the stamping process of a polypropylene/glass fiber composite have been infestigated as a function of temperature, pressure, and time. Experiments were performed in a temperature and pressure controlled chamber, equipped with a rapid cooling facility. Samples of the composite as well as model polypropylene specimens containing calibrated voids were held at 200°C at different levels of hydrostatic pressure (1,10,100, and 300 bars), for a period of either 1 or 10 min. The speciments were subsequently characterized by denstiy measurements and morphologival observation. It was shown that: i) the expansion of the composite observed during the heating at 200°C under atmospheric pressure is largely induced by the gases previously dissolved in the polymeric matrix, ii) the rapid increase of the pressure during the stamping process leads to the closing of voids and, iii) the final holding under high calculation of void growth and shrinkage is in agreement with the morphological observations.     

Determination of prepreg tack

Prepreg materials — fibre-reinforcement preimpregnated with uncured resin — are widely used for the manufacture of large composite material components. Current commercially available prepreg materials often show significant variations in tack strength, from point to point within a sheet and from sheet to sheet. Such inconsistencies can lead to void formation in the final composites laminate. This paper describes the techniques and apparatus developed for the investigation of the surface tack strength of adhesives in general and of prepreg materials in particular, as a function of contact time and pressure and of rate of separation. It is hoped that the more detailed knowledge of prepreg tack will enable the production of more consistent material and hence the manufacture of improved quality composite laminates.     

直升机柔性杆用环氧树脂预浸料的研制

研制了用于制备直升机主传力系统中复合材料柔性杆件的改性环氧树脂基体及其RC10．800连续玻璃纤维预浸料。探讨了树脂基体组成、连续玻璃纤维窄带预浸工艺参数等变量对预浸料和复合材料性能的影响。选用的促进剂满足直升机柔性杆的技术要求和热熔工艺要求，满足此生产工艺要求。     

Effects of fiber mat architecture on void formation and removal in liquid composite molding

Liquid composite molding (LCM) processes such as resin transfer molding and structural reaction injection molding are considered to be high potential processes for the mass production of composite parts. The resin injection step in LCM consists of two simultaneous flows: bulk mold filling and tow wetting. This complexity often results in the entrapment of air in the composite part, which is known to result in degradation of part performance, In this work, systematic investigation of the resin flow behavior through various types of glass fiber reinforcements is carried out by flow visualization. The objective is to relate the fiber mat architecture to the micro scale flow pattern and void formation, movement, and removal. An optical image analysis and processing technique is developed to help quantify void formation. Void formation is related to liquid properties and fiber-liquid contact angle. Although the focus of the study is LCM, the results can be directly applied to other composite manufacturing processes that involve advancement of resin in a dry fiber reinforcement.