High-quality Composite Parts Research Articles

Optimization of Process Parameters of Fused Filament Fabrication of Polylactic Acid Composites Reinforced by Aluminum Using Taguchi Approach

The benefits of the fused filament fabrication (FFF) method, including its simplicity, affordability, and accessibility, have made it the most commonly used additive manufacturing technique. Polylactic acid (PLA) is the most widely used material in FFF, but its use has been limited by low mechanical properties and a small processing window. To address this, PLA composites are used to improve its properties. Correlating mechanical properties with process parameters is crucial for producing high-quality composite parts. This study investigated the effects of material and process parameters on mechanical properties, such as tensile strength and elongation-at-break, using a customized Delta Rostock FFF printer. Two types of filaments were used, pure PLA and PLA/Aluminum composites. Printing speed (10, 20, and 30 mm/s) and raster angle (0/90, −45/45, and −30/60) were selected as process input parameters. The Taguchi method was used for the experiment design, and signal-to-noise ratio analysis was used for statistical optimization. The optimal values for achieving maximum tensile strength of 61.85 MPa and maximum elongation-at-break of 17.7% were determined. Furthermore, the signal-to-noise ratio analysis indicated that the filament type had the greatest influence on the tensile strength, whereas printing speed had the greatest impact on the elongation-at-break.

Influence of Fibre Fill Pattern and Stacking Sequence on Open-Hole Tensile Behaviour in Additive Manufactured Fibre-Reinforced Composites

Additive manufacturing has revolutionised the field of manufacturing, allowing for the production of complex geometries with high precision and accuracy. One of the most promising applications of additive manufacturing is in the production of composites, which are materials made by combining two or more substances with different properties to achieve specific functional characteristics. In recent years, the use of Continuous Filament Fabrication (CFF) in additive manufacturing has become increasingly popular due to its ability to produce high-quality composite parts which have fibres with a complex orientation and high curvature. This paper aims to investigate the influence of fill pattern and stacking sequence on the open-hole tensile strength of composites manufactured using CFF and made of an innovative matrix composed of nylon and short carbon fibres, i.e., Onyx, and with continuous carbon fibre as reinforcement. By systematically varying the fill pattern and stacking sequence, we aim to identify the optimal combination that can achieve the highest open-hole tensile strength in these composites. The results of this study will provide valuable insights into the design and manufacture of high-strength composites using additive manufacturing. Open-hole strength and elastic properties are strongly influenced by the infill strategy and stacking sequences adopted, and show different failure modes. The results also point out a technological issue characterising the process and indicate some guidelines for designing and manufacturing 3D printing composites.

Equation for modelling energy transfers in multi-phase flows through porous media, optimised for liquid composite moulding processes

Liquid Composite Moulding LCM processes are increasingly popular in the aerospace and related industries for the production of high quality composite parts. Filling simulations are often used in the design of moulds for LCM processes for efficient and complete filling of the mould. This paper proposes an energy balance equation for modelling energy interactions in a typical LCM process for implementation in Computational Fluid Dynamics (CFD) tools such as Open Field Operation and Manipulation (OpenFOAM). The derivation of the energy balance is first described in detail, followed by a brief description of an additional equation to be solved to find the degree of cure for simulations involving a chemically reactive resin. For a preliminary validation, a simplified version of the energy equation is implemented in OpenFOAM and simulation results are compared to experiments. The temperatures predicted by the simulations are seen to closely match the experimentally observed temperatures.

Effect of saw-tooth ply drops on the mechanical performance of tapered composite laminates

Automated Fibre Placement (AFP) is a manufacturing technique to produce large, high quality composite parts, where preimpregnated carbon fibre tapes are laid side-by-side to generate the composite preform. Thickness changes within a component are realised through internal ply terminations, with the tapes being cut perpendicular to the fibre direction. In plies laid up at an angle to the taper direction, the AFP tape cuts create saw-tooth shaped ply drop tips. This increases the size of resin rich zones and enlarges the region where stress concentrations and cracks can develop, which amplifies the risk of failure. This study investigates the effect of simulated AFP saw-tooth ply drop tips, created by hand layup, on the mechanical properties and failure behaviour of carbon fibre/epoxy composites and compares its results to reference broad goods layups. The stiffness was found to be unaffected and the strength was reduced by ~ 10%. The failure mode was governed by delamination for the saw-tooth ply drop tips, whereas the reference specimens failed by rupture of the unidirectional fibres. A finite element modelling technique was used to select the layup to be tested. The models were then refined in the light of experimental results, to accurately predict the failure and explain the failure mode transition.

ИССЛЕДОВАНИЕ ПОРИСТОСТИ КОМПОЗИТНЫХ КОНСТРУКЦИЙ НА ОСНОВЕ 3D-ПЕЧАТНЫХ КАРКАСОВ, ПРОПИТАННЫХ ЭПОКСИДНОЙ СМОЛОЙ

In the designs of modern machines, more and more polymer parts are used, at the same time, there is a problem of their quick replacement in case of failure. Reducing the cost and repair time can be achieved by using 3D printing by FDM method, but such parts do not always demonstrate the necessary strength. To improve their mechanical properties, a method of their impregnation after printing in epoxy resins was previously proposed. (Research purpose) The research purpose is in studying the dependence of the porosity of composite structures based on 3D-printed frames impregnated with resin on the parameters of their manufacture. (Materials and methods) Authors used samples for the first stage of the work, which are 3D-printed cylinders with different wall thicknesses and internal geometries, impregnated with ED-20 epoxy resin. The samples were cut in several sections and the number of pores in these sections was calculated. The second stage of the experiment was to evaluate the porosity of a part of complex geometry. (Results and discussion) With an increase in the percentage of filling and thickening of the wall in 3D printing, there is a tendency to reduce the number of pores. With a less dense filling of the frame and a thinner wall, the resin is worse retained in the product and partially flows out after impregnation. The best filling of a part of a complex shape was observed when it was cured in the position of the massive part up. (Conclusions) For the production of high-quality composite parts based on 3D-printed frames impregnated with epoxy resin, it is recommended to choose the largest possible percentage of filling during 3D printing and strive to position the part during the curing process after impregnation with the massive part up.

Thermomechanical Characterization of a Fast Curing Epoxy Resin for Simulation of Surface Waviness

The interaction of process parameters and material properties represents a major challenge for reproducible manufacture of high quality composite parts with polymer matrix. Major phenomena that have to be taken into account are thermal expansion, chemical shrinkage as well as process-dependent curing and mechanical properties that lead to residual stresses and geometrical deviations on different length scales. The analysis of these effects is even more challenging when considering fast curing resins. This contribution presents the development of a simulation approach dedicated to the prediction of surface waviness originating from liquid injection molding (LCM). A fast curing epoxy resin used in automotive industry was characterized in terms of curing kinetics and thermomechanical properties. A generic cure cycle was derived. A simulation approach is presented which aims at the numerical analysis of process-related parameters and their influence on surface waviness. In a first step, the influence of the resin thermomechanical behavior and the tool boundary condition on surface deformation is evaluated during cooling. The presented work provides a basis for a comprehensive understanding of the underlying effects and prospectively allows a linkage to internal residual stresses.

Eliminating porosity via reformulation of a benzoxazine–epoxy resin transfer molding resin

Use of benzoxazine resins in composites is limited by volatile-induced porosity, which degrades the thermomechanical properties of the product. In the present study, we demonstrate how to eliminate cure-induced volatilization and volatile-induced defects in benzoxazine composite laminates, using a chemistry-based approach. Like most resins formulated for high-temperature service, benzoxazine and benzoxazine–epoxy blends generally include solvent additives. Consequently, composite parts produced with such resins exhibit higher levels of cure-induced volatile release, often leading to porosity in the final manufactured part. Here, we develop a method to eliminate porosity by analyzing volatile release and the effects of residual solvent in a pre-commercial benzoxazine–epoxy system designed for liquid molding by resin transfer molding. Utilizing thermogravimetric analysis, nuclear magnetic resonance spectroscopy, and dynamic mechanical analysis, we correlate the concentration of residual solvent remaining within the final manufactured part with the Tg, degradation temperature, and dynamic modulus. Lastly, a resin synthesis method is demonstrated that eliminates residual solvent in order to produce composite parts with optimal surface finish and thermomechanical properties. The report outlines a methodology for optimizing blended resin chemistry for production of high-quality composite parts.

Thermomechanical Effects in PVTα Measurements

Knowledge of the different properties of thermoset composite materials is of great importance for the manufacturing of high quality composite parts. The resin bulk modulus is one of them and is essential to define the composite parts compressive behaviour under uniform compression. The evolution of this property with temperature and conversion degree of reaction is a challenging task and has been tentatively measured with a home-made apparatus, named PVTα, on which temperature, volume change and degree of cure are simultaneously recorded. But as the sample is contained in a non-reactive and deformable capsule, which mechanical behaviour may interfere with the measurement, a validation is required. The aim of this work is to develop a finite element model of the problem in order to simulate the thermal mechanical behaviour of the sample and the capsule, and so to validate the measurement process. The multiphysical numerical model accounts for phase change kinetics and non-linear thermal properties as well as thermo-dependent elastic properties, all problems being solved through a strong iterative coupling scheme. Mechanical contact problems between the capsule and the resin sample are handled through a penalization method contact algorithm which enables to capture the effects of chemical and thermal shrinkage in the sample and the capsule. The heterogeneous stress state generated by the material transformation is assumed to induce heterogeneous strain states which may lead to misinterpretations of macroscopic measurements. This model is a first approach and will be improved using a more sophisticated rheological model. Nevertheless, results show that the usual experimental analysis method can be used as long as the gel point is not reached. After a certain conversion degree, the measured bulk modulus is different from the effective one so corrections have to be brought.

Chemical shrinkage characterization techniques for thermoset resins and associated composites

Control and optimization of curing process is very important for the production of high quality composite parts. Crosslinking of molecules of thermoset resin occurs in this phase, which involves exothermy of reaction, chemical shrinkage (Sh) and development of thermo-physical and thermo-mechanical properties. Exact knowledge of the evolution of all these parameters is required for the better understanding and improvement of the fabrication process. Sh is one such property of thermoset matrix, which is difficult to characterize due to its coupling with thermal expansion/contraction. A number of techniques have been used to determine volume Sh of thermoset matrix, which later on has been used to find tensor of Sh for the simulation of residual stresses and shape distortion of composite part, etc. Direct characterization of volume Sh of composites has also been made by some authors. Though not much, but some work has also been reported to determine the Sh of composite part in a specific direction. In this article, all the techniques used in the literature for the characterization of Sh of resin and composite are reported briefly with their respective advantages, disadvantage and important results.

Elevated-temperature vacuum-assisted resin transfer molding process for high performance aerospace composites

Vacuum-assisted resin transfer molding (VARTM) is commonly used for general temperature applications (<150 °C) such as boat hulls and secondary aircraft structures. With growing demands for applications of composites in elevated temperature environments, significant cost savings can be achieved by employing the VARTM process. However, implementation of the VARTM process for fabricating elevated temperature composites presents unique challenges such as high porosity and low fiber volume contents. In the present work, a low cost and reliable VARTM process is developed to manufacture elevated temperature composites for aerospace applications. Modified single vacuum bagging infusion and double vacuum bagging infusion processes were evaluated. Details of the method to obtain high quality composite parts and the challenging issues related to the manufacturing process are presented. Density and fiber volume fraction testing of manufactured panels showed that high quality composite parts with void content less than 1% have been consistently manufactured. A property database of the resin system and the composites was developed. A three-dimensional mathematical model has also been developed for flow simulation and implemented in the ABAQUS finite element package code to predict the resin flow front during the infusion process and to optimize the flow parameters. The results of the present study indicate that aircraft grade composite parts with high fiber volume fractions can be manufactured using the developed elevated temperature VARTM process. © 2013 Society of Chemical Industry

Effects of vacuum pressure, inlet pressure, and mold temperature on the void content, volume fraction of polyester/e-glass fiber composites manufactured with VARTM process

Vacuum-assisted resin transfer molding (VARTM) process is one of the liquid composite molding (LCM) processes aimed at producing high-quality composite parts. The void content and fiber volume fraction of a VARTM part can be affected by many parameters and is critical to the mechanical properties and the quality of the part. In this paper, a series of experiments were conducted with a heated dual pressure control VARTM setup for investigating the effects of vacuum pressure, inlet pressure, and mold temperature on the void content and fiber volume fraction of polyester/E-glass fiber composite. It was found that stronger vacuum and higher mold temperature can better control and increase the fiber volume fraction; however, such a combination of strong vacuum and high mold temperature may also require a reduced inlet pressure for minimizing the void content. The need of pressure reduction can be explained with the compatibility between Darcy's flow and capillary flow in the fiber preform and can be calculated based on the room temperature VARTM results. The experimental results suggest that high mold temperature, high vacuum, and appropriately reduced inlet pressure can produce a VARTM part with high fiber volume fraction and low void content.

Investigation of process-induced strains development by fibre Bragg grating sensors in resin transfer moulded composites

A comprehensive understanding of the development of residual strains in composite processing is essential to manufacture high quality composite parts. In this paper, the use of fibre Bragg grating (FBG) optical sensors was investigated to measure in situ the build-up of the process-induced strains in composite panels manufactured by resin transfer moulding. The FBG sensors, embedded in the composite laminate, successfully measured the evolution of the composite in-plane strains due to the temperature history. The sensors also captured a strain discontinuity during the cool-down related to the debonding of the composite from the mould. Finite element models were then proposed to simulate the strain development measured by the FBG sensors. Three different contact interactions between the tool and the laminate were investigated: no bonding, perfect bonding and frictional contact. The model using the frictional contact interaction described well the evolution of the measured strains.

Inspection of Drop-weight Impact Damage in Woven CFRP Laminates Fabricated by Different Processes

The influence of manufacturing process on the drop-weight impact damage in woven carbon/epoxy laminates was inspected by visual observation, dye-penetrant X-ray technique, and optical microscopy observation. The MTM56/ CF0300 woven quasi-isotropic laminates were fabricated by two processes: the autoclave and the Quickstep processes. QuickstepTM is a novel composite manufacturing process, which was designed for the out-of-autoclave production of high-quality composite parts at lower cost. It utilizes higher heat conduction of fluid other than gas to transfer heat to components, which results in much shorter cure cycles. The laminates cured by this fast heating process showed different impact failure modes from those cured by the conventional autoclave process. The residual indentation in the top side of the Quickstep-cured laminates had a bigger diameter, but a smaller depth at the same impact energy level. Dye-penetrant X-ray revealed more intense and connected impact damage regions in the autoclave-cured laminates. Optical micrography as a supplementary method showed less severe matrix damage in the quickstep-cured laminates indicating a more ductile property of the resin matrix cured at a faster heating rate.

PMR Type Polyimides by Aqueous Cyclization Methods — Chemistry

PMR-15 has long been the leading resin system for high performance composite applications requiring long-term use temperatures up to 300°C. A mixture of carboxylic esters and acids, aromatic diamine and norbornyl end caps capable of crosslinking by a reverse Diels—Alder mechanism in methanol is pre-pregged into cloth and under careful processing conditions produces high quality composite parts. Some problems with handling and processing remain in this commercial system and in an attempt to overcome these problems we have synthesized a series of fully cyclized PMR type prepolymers under aqueous imidization conditions. Their solubility and cure behavior have been determined with a view to producing resins that will form good quality, high performance composites. While resins with these characteristics have been obtained, the best of them utilize significant proportions of relatively expensive co-monomers. Almost all of the materials presented in this study have very high temperature stability and high glass transition temperatures, although no one system out performs PMR-15 on a monomer cost basis, the advantages of water as the synthesis solvent includes both its low cost and low toxicity and its ability to fully react the monomers to cyclic imide structures.

Flow sensing and control strategies to address race-tracking disturbances in resin transfer molding—part II: automation and validation

Abstract Resin Transfer Molding (RTM) is an advanced processing technology for composites manufacturing. In this process, the fiber preform is placed in a closed mold and the resin is injected into the mold to saturate the preform. After the resin cures, the mold is opened and the net shape composite part is removed. RTM is capable of making complex and high quality composite parts in fairly short cycle times. However, by introducing more complexity into the part, one also introduces higher probability of disturbances, such as race tracking along preform edges, into the molding system and increases the risk of failure. Part-I [Hsiao KT, Advani SG. Flow sensing and control strategies to address the race-tracking disturbances in resin transfer molding—Part I: design and algorithm development. Composites Part A: Appl Sci Manuf 2004;35(10):1149–59] outlined the methodology using process models and simulations along with sensing and control strategies to address flow disturbances. It was demonstrated in a virtual environment that one can increase the success rate significantly by selectively placing the sensors to identify the location of the disturbance and strategically opening and closing auxiliary gates to steer the resin flow to impregnate all the fibers. In part-II, an automated system designed to implement the on-line control strategy of part-I is demonstrated. Once the design of on-line control strategy is completed as delineated in part-I, the results can be seamlessly interfaced with the manufacturing platform to communicate with the sensors in the mold and control the injection and vent valves on an experimental platform. Thus, this streamlines the feedback control in RTM from the design stage to the automation stage. An experimental case study is presented to validate the use of model based on-line control of the RTM process and to demonstrate how it lends itself to automation in the manufacturing process.

On-Line Characterization of Bulk Permeability and Race-Tracking During the Filling Stage in Resin Transfer Molding Process

Resin Transfer Molding (RTM) is widely used to manufacture polymer composite materials. In this process, the fiber preform is placed in a closed mold and thermoset resin is injected to saturate the preform. After the resin cures the mold is opened and the net shape composite part is obtained. With RTM, one is capable of making complex and high quality composite parts with short cycle times. However, by introducing more complexity into the part, one also introduces higher probability of disturbances, such as race-tracking of resin during impregnation along preform edges. This can lead to incomplete saturation of fiber preform forming flaws such as dry spots in the composite part. The strength and existence of race-tracking is a function of the fabric type, perform manufacturing method, and their placement in the mold. It can vary from one part to the next in the same production run and usually it is not repeatable. The characterization of race-tracking and preform permeability is a key input for simulation tools that optimize the gates and vents locations and for designing advanced sensing and flow control strategies. This paper describes a method to estimate the strength and location of race-tracking and characterize the preform bulk permeability during the resin impregnation stage of RTM. The approach can be described in two steps. The first step generates simulations off line that map the overall range of potential disturbances likely to occur during the injection. The second step uses sensors placed in the mold, which allow one to identify the simulation within the database that resembles the experiment in terms of the filling behavior during the filling stage on the manufacturing platform. In the final step, this information is used to estimate race-tracking strength and location as well as the bulk permeability. Both experimental and numerical case studies show the reliability and accuracy of this method. It could be utilized for a broad range of applications such as active flow control, vent location optimization, and process monitoring of mold filling processes.

Space age materials come back to Earth

Abstract An Australian company has patented a processing technology which it believes will allow the mass production of high quality composite parts. Deryck Gra