Processing Of Thermoplastic Composites Research Articles

Effect of preheat treatment on carbon fiber surface properties and fiber/PEEK interfacial behavior

In this work, the effect of preheat treatment on surface properties of carbon fiber (CF) and interfacial properties of CF‐reinforced polyether ether ketone (CF/PEEK) composites was investigated in detail. The sized T700SC CFs were preheated based on practical thermoplastic composites processing conditions. The obvious mass loss was obtained due to the sizing agent degradation. Fourier transform infrared spectroscopy and X‐ray photoelectron spectroscopy demonstrated that the concentration of sizing agents and activated carbon atoms decreased apparently for preheated fibers. Preheat treatment had negligible effect on tensile strength. Transcrystallinity structures were observed at fiber/PEEK interfaces using polarizing microscope and they had similar transcrystallinity density before and after preheat treatment. The microbond test was performed to determine interfacial shear strength (IFSS) between fibers and PEEK matrix. The IFSS increased from 40.16 to 46.32 MPa by 15.34%, which was against the trend of activated carbon atoms. Furthermore, the mechanisms of interfacial adhesion enhancement were discussed. The sizing layer became weak interface region because of poor chemical bonding between sizing agents and PEEK. Removing sizing via preheat treatment could enhance the interfacial adhesion. POLYM. COMPOS., 40:E1407–E1415, 2019. © 2018 Society of Plastics Engineers

Dry Electrostatic Spray Coated Towpregs for Thermoplastic Composites

Though the processing steps for the manufacture of thermoplastic composites present little difficulty when compared to those for thermoset composites, it is not cost effective as a result of difficulty in accomplishing thorough infusion of resin into fibers and fibrous structures. Tremendously high viscosity for thermoplastic resin leads to numerous problems in the manufacturing process of thermoplastic composites. Pre-impregnation processes such as melt impregnation or solution impregnation, tend to form a stiff towpreg that cannot be used for any textile preforming operation. Powder coating can be a destined to be a cost effective as well as efficient processing technology to produce a flexible towpreg. A method for production of flexible towpregs for thorough infusion of resin into fibers using electrostatic spray gun, which involves powder coating a bundle of filaments is described. During the powder coating process using electrostatic spray gun, powder particles fetched by a conveying air stream, from a fluidizing hopper, are charged in the high voltage corona discharge by ion bombardment and transported to the fiber tow through the combination of electrostatic, aerodynamic and gravitational forces. Aerodynamic forces have the obligation to carry the powder particles to the tow where electrostatic forces dominate and cause the adherence of powder particles to the potentially grounded tow. After the powder deposition, tow is made to pass through convective oven where powder particles glue to the reinforcement as a result of partial liquefaction of the powder. The towpreg formed is then wound by the take up device. Take up device is equipped with dancer arm which ensures constant line tension and thereby consistent powder deposition. The effects of process variables of the electrostatic spray process such as corona voltage, fluidizing air pressure, conveying air pressure, distance of gun from tow and tow velocity on maximum powder deposition are investigated and reported.

Processing of thermoplastic matrix composites through automated fiber placement and tape laying methods

Fiber-reinforced composite materials are replacing metallic components due to their higher specific strength and stiffness. Automation and thermoplastics emerged to overcome the time and labor intensive manual techniques and the long curing cycles associated with processing thermoset-based composites. Thermoplastics are processed through fusion bonding which involves applying heat and pressure at the interface. Together with automated techniques (such as automated fiber placement, and automated tape laying), a fast, clean, out-of-autoclave, and automated process can be obtained. A detailed review of thermoplastic composites processing through automated methods is presented. It sheds the light on the materials used and the different heat sources incorporated with the pros and cons of each, with concentration mainly on hot gas torch, laser, and ultrasonic heating. A thorough illustration of the several mechanisms involved in a tow/tape placement process is tackled such as heat transfer, intimate contact development, molecular interdiffusion, void consolidation and growth, thermal degradation, crystallization, and so on. Few gaps and recommendations are included related to materials, laser heat source, heat transfer model, and the use of silicone rubber rollers. A review of optimization studies for tape placement processes is summarized including the main controllable variables and product quality parameters (or responses), with some of the major findings for laser and hot gas torch systems being presented. Both mechanical and physical characterizations are also reviewed including several testing techniques such as short beam shear, double cantilever beam, lap shear, wedge peel, differential scanning calorimetry, and so on. Challenges, however, still exist, such as achieving the autoclave-level mechanical properties and complying with the porosity levels required by the aerospace industry. More work is still necessary to overcome these challenges as well as increase the throughput of the process before it can be totally commercialized.

Multi-pass layup process for thermoplastic composites using robotic fiber placement

Abstract Robotic fiber placement is a promising technique to manufacture complex components for industry. Compaction pressure, hot gas torch temperature and laying velocity are the key molding factors for fiber placement process, and their optimal selection directly affects the product performance. Small compaction pressure, low gas temperature and fast laying velocity lead to incomplete interlaminar adhesion and high porosity. However, excessive compaction pressure, high gas temperature and slow laying velocity lead to excessive resin extrusion from the contact surface of the two layers, thus affecting the overall structural rigidity of the laminates. At the same time, due to the forming characteristics of fiber placement process, the compaction pressure has an impact on each underneath layer under the influence of heat transfer and laying velocity, which results in different interlaminar bonding strength between the different layers. This study aims at analyzing and optimizing the robotic fiber placement process parameters to obtain the composite laminate with the homogeneity of interlaminar bonding strength. Based on analyzing the heat transfer from current contact surface to each one underneath, the multi-pass compaction pressure from non-deformable roller affecting the underneath layers is investigated, and experiments are conducted to verify the optimizing method.

Quality Controlled UD Tape Production for the Individualised Production of Load Optimised Thermoplastic Composite Parts

A flexible and individual component manufacturing process for thermoplastic composites (TPC) has been developed at the Institut fuer Kunststoffverarbeitung in Industrie und Handwerk an der RWTH Aachen (Institute of Plastics Processing (IKV) at RWTH Aachen University). The process consists of a quality controlled tape production and a combined forming and joining process with additive manufactured functional structures. This paper describes the requirements for the unidirectional (UD) tape properties and the quality controlled tape production line in order to allow for a flexible and individual component manufacturing of load optimised thermoplastic composite parts. Besides the UD tape geometry and fibre impregnation quality an even fibre distribution over the width of the UD tape is an important characteristic. Results of investigations regarding the online measured quality data (fibre distribution) and offline measured UD tape properties (local fibre weight content) are presented and discussed.

Impact of draft plate on the inter-chamber interaction in a two-chamber spout-fluid bed

The widespread application of the spout-fluid bed in the processing of thermoplastic composites, coal gasification, municipal solid waste gasification, and biomass gasification necessitates a deeply understanding on the flow mechanism of dense two-phase flow in the system in order to improve its design optimization and operation. By tracking the gas and solid phases respectively in the Eulerian and Lagrangian framework, the dense gas-solid flow in the two-chamber spout-fluid bed with and without draft plate was carried out using the computational fluid dynamics coupled with discrete element method (CFD-DEM) to evaluate the impact of the presence of draft plates and the plate length on inter-chamber interaction. The results demonstrate that the interaction of rising bubbles with the spouting channel gives rise to the unfavorable dancing spout phenomenon, which can be alleviated by draft plates. Insertion of the plates lowers the pressure drop magnitudes of both the spouting and background inlets, and the correlation coefficient of the two signals. Furthermore, the draft plates enhance the turbulent intensity of the solid phase in the fountain region and the vertical dispersion in the spout region. Increasing the draft plate length has a significant influence on gas-solid hydrodynamics and chamber interaction.

Manufacturing procedure to make flat thermoplastic composite laminates by automated fibre placement and their mechanical properties

Automated fibre placement (AFP) is a relatively new process for the manufacturing of composite structures. Among many attractive features, it provides high-speed of material deposition, more repeatability in terms of quality of the part, less labour intensive (as compared with traditional methods of manufacturing such as Hand Lay-Up), less waste and the ability to transition more seamlessly from design to manufacturing. AFP can be used to process both thermoset composites and thermoplastic composites. Thermoplastic composites processing holds many potential benefits. This is because if the process is done right in producing parts with good quality, it is fast since it does not require a second process such as curing in an autoclave or oven. For the purpose of comparison of performance and for design, it is necessary to determine the mechanical properties of laminates made using this process. However, there are challenges in making flat coupons for the purpose of testing for mechanical properties. This article presents these challenges and the procedure developed to make flat laminates using a simple AFP machine. Mechanical properties of these laminates are also determined and compared with those obtained from laminates made using conventional autoclave moulding.

Modeling of heat transfer and unsaturated flow in woven fiber reinforcements during direct injection-pultrusion process of thermoplastic composites

This paper provides a methodology for the modeling of heat transfer and polymer flow during direct thermoplastic injection pultrusion process. Pultrusion was initially developed with thermosets which have low viscosity. But the impregnation becomes a critical point with thermoplastics which exhibit higher viscosity. There are very few reported works on direct thermoplastic impregnation with injection within the die. In addition, the rare studies have not adequately addressed the issue of unsaturated flow in woven fiber reinforcements. The solution proposed here, models the polymer flow through dual-scale porous media. A heat transfer model is coupled to a flow model enriched with a sink term. Specific changes of variables are made so as to model the steady state solution of unsaturation along a continuous process. The sink term, added to the continuity equation, represents the absorption rate of polymer by the bundles. Data were measured on a pultrusion line and micrographs confirmed the modeling strategy with an unsaturated flow approach. The flow modeling coupled to heat transfer of the thermoplastic pultrusion process aims at determining the saturation evolution through the die so as to manufacture pultruded profiles with the lowest residual porosity.

Optimization of Stamp Forming Process for Thermoplastic Composites

The present study is focused on the development of a two-dimensional stamping method for the manufacturing of fiber reinforced composites with thermoplastic matrix resins. Materials investigated are carbon fiber reinforced polyamide-6. Taguchi L16 orthogonal array is used in split-plot designs. The processing conditions include thermoforming temperature, mold temperature, pressure and time, required to establish high-quality parts. From the experimental results, we derive a set of best combination, A1 (90°), B2 (263°C), C1 (105°C), D1 (33 kg/cm2) and E2 (48 sec) and carry out an estimated equation for the short-beam shear strength. The results have described the correlations between processing parameters and shear stress. Finally, for verifying the prediction ability of the estimated equation, the confirmation experiments are conducted. The confirmation test result is 48.67 kg/mm², fall in the confidence interval. It shows that the prediction ability of estimated equation and the repetition of the experimental results has confirmed and accepted by the tests.

Dynamic finite element simulation and transient temperature field analysis in thermoplastic composite tape lay-up process

In the lay-up process of thermoplastic composite tape, the thermal conditions have great influence on the end quality and performance of composites. In the lay-up process of thermoplastic matrix composites, a semi-infinite solid model is used to analyse the heat transfer by simulating the heating of hot gas torch. Temperature of the control volume is calculated based on the model. A dynamic finite element model is developed to obtain the transient temperature field. The birth–death element strategy in ANSYS was used to simulate the movement of lay-up head during dynamic loading and laying-up prepreg onto the composite substrate. The simulation results of using the transient heat transfer model and the finite element model were compared well. The bonding temperature increases linearly with the heater temperature and is closely related to the moving speed of the roller. The bonding temperature changed slowly when the roller moved faster.

A method for investigating blending quality of commingled yarns

A prospective technology for manufacturing thermoplastic composites is based on hybrid commingled yarns. The hybrid yarns are further processed into semi-impregnated thermoplastic preforms in the form of woven or non-crimped fabrics, which are consolidated and molded in a single processing step. The quality of the component distribution in yarn is known to affect the mechanical properties of the manufactured composites. As part of a quality management system for the manufacturing process of thermoplastic composites based on commingling yarns, a new method for analyzing the blending quality along the length of commingled structures is presented in this work. By using this method, a new blending index ( kb,yarn) is introduced as a measure of the quality of commingled yarns. The blending index combines the existing coefficients of the lateral and radial distribution of fibers in the cross-section of hybrid yarns. Within this paper, the new analysis method is explained in detail using the example of glass fiber/polyamide 6 (GF/PA) commingled yarns. Due to the combination of the yarn analysis along the yarn axis and in its cross section, the new method allows for the first time a reliable comparison of the blending quality in commingled yarns used for the manufacturing of thermoplastic composites.

Simple modelling of impregnation in pultrusion process of thermoplastic composites

Pultrusion is a continuous process with application in the manufacture of fibre–reinforced composites. This work reports the impregnation process which is one of the major concerns in pultrusion. We set out to develop a model that describes the flow advancement for a thermoplastic matrix when the reinforcements pass through the die. Therefore, the influence of several processing conditions on the impregnation state has been assessed. An analytical model based upon the Darcy's law and Stokes equation was applied to describe the progression of the radial flow front through porous medium. In this investigation, a unidirectional fibrous medium is adopted and the injected fluid is considered to be Newtonian and incompressible. Results show that the geometry of the die, pulling velocity and obviously the fibres permeability are the major factors influencing the degree of wetting. This analytical methodology is a useful tool to control the matrix impregnation.

Design Simulation of Glass-Fiber-Loaded Flow in an Internally Spout-Fluidized Bed for Processing of Thermoplastic Composites. I. Flow Characterization

A novel fiber dispersion process suitable for the manufacture of composites consisting of long glass fibers and thermoplastic is discussed. An internally spout-fluidized bed with a draft tube and disk baffle has been designed to disperse fiber bundles and perform preimpregnation, which can improve the hydrodynamic behavior inside the spout-fluidized bed for more effective dispersion of fiber bundles and impregnation with resin powder when applying such beds in the manufacture of fiber-reinforced thermoplastics. Features of the flow field in the spout-fluidized bed are important for fiber dispersion. Therefore, this article discusses the design results in terms of numerical simulations [large-eddy simulations (LESs)], focusing on the likely hydrodynamic impact on fiber dispersion in this particular spout-fluidized bed. Because of a strong interaction between the flow in the spout-fluidized bed and the fibers in the actual process and the limitations on direct measurements of fiber floc breakup in the spout-fluidized bed, the strength of the fiber floc was obtained by strength measurements with a rheometer. In the LES runs, the motion of the fibers was neglected because of their low volume fraction (lower than 0.001%). The turbulence kinetic energy, Reynolds stress, and strain rate were determined by statistical analysis. The LES results clearly indicate that the addition of internals significantly alters the flow patterns in the spout-fluidized bed, in particular changing the characteristics of the round turbulent jet and confined impinging jet. The hydrodynamics analysis showed that the introduction of internals into a spout-fluidized bed improves its capacity and efficiency for dispersing fiber flocs. These results provide important information on the flow fields within spout-fluidized beds for process design and scaleup.

Reactive processing of anionic polyamide-6 for application in fiber composites: A comparitive study with melt processed polyamides and nanocomposites

The present study describes reactively processed anionic polyamide-6 (APA-6) as improved matrix for continuous fibre composites and compares it with melt processed PA-6 and PA-6 nanocomposites. Whereas the reinforcing effect of nanoparticles is already extensively documented, this study shows that reactive processing results in a particular crystal morphology, which itself already causes a similar reinforcing effect. The effect of processing parameters like temperature, pressure and demoulding time on the reactively processed material is discussed. Additionally, it will be shown that both the reactively processed polyamide-6 and the nanocomposite show improved property retention at elevated temperature and are less affected by moisture absorption. It is expected that APA-6 will improve the performance of fibre composites in the same way as nanocomposites. Additionally, reactive processing of polyamide-6 composites using, for instance, vacuum infusion will make it possible to manufacture larger and thicker parts with a higher level of integration and complexity than is possible with traditional melt processing of thermoplastic composites.

Fracture and impact behaviour of solid phase formed polypropylene composites with long glass fibres

Solid phase forming is prominent in the processing of thermoplastic composites, but not much work has been done in this area. In the present study, the effect of fibre content and prestrain on the fracture and impact behaviour of solid phase formed glass–polypropylene composites was investigated. Fracture tests were conducted using specimens with three fibre contents (20, 30 and 40%). Impact tests were carried out with the application of three different fibre contents (20, 30 and 40%) and three levels of prestrain (10, 20 and 30%). After fracture and impact testing, SEM examination was carried out of the fracture surfaces to investigate the failure mechanism of the solid phase formed glass–polypropylene composites. The results showed that fracture toughness and impact strength increased with increasing fibre content. Fracture toughness and impact strength increased by ∼15% and ∼190%, respectively, when the fibre content was increased from 20 to 40%. The results also showed that, for fibre contents of 20 and 30%, the impact strength increased with increasing prestrain. For a fibre content of 40%, however, the impact strength increased until the prestrain reached , 10%, but decreased thereafter. Examination by SEM indicated that plastic deformation and matrix fracture played an important role in the case of low fibre content and prestrain. Fibre breakage and debonding functioned as dominant failure mechanisms in the case of high fibre content and prestrain.

Influence of Consolidation Process on the Drilling Performance and Machinability of PIXA-M and PEEK Thermoplastic Composites

In this paper, the drilling performance of two composite systems, namely graphite-PIXA-M (PIXA-M) and graphite-PEEK (PEEK) induction heat-pressed thermoplastic composites, are compared with their counterpart from autoclave processing. Machinability is evaluated in terms of drilling forces and hole quality. Hole quality characteristics are evaluated in terms of geometric accuracy and surface finish. The experimental observations discuss unique chip characteristics during drilling both autoclaved and induction heat-pressed thermoplastic composites. The mechanism of material removal in drilling of thermoplastic composites is investigated through the further calculation of the specific cutting energy. The induction-processed composite material produced equivalent or better holes than the autoclave-processed composites although it required larger cutting force magnitudes and protruded fibers on the bottom ply are produced. The induction heat press may be a viable alternative for the processing of thermoplastic composites in terms of drilling performance.

Fibre Technology for the Production and Processing of Thermoplastic Composites

Fibre Technology for the Production and Processing of Thermoplastic Composites

Characteristics of CF/PEI tape winding process with on-line consolidation

The effects of processing conditions on consolidation quality for the tape winding process of thermoplastic composites with on-line consolidation have been investigated. Composite rings were manufactured using carbon fibre (CF)/polyetherimide (PEI) tape for both cases with and without an insulated ring. Based on heat transfer and intimate contact/autohesion mechanisms, a steady-state finite element method (FEM) model was developed to analyze consolidation quality and overheating in composite tapes. The processing windows with the upper bound for good consolidation and the lower bound for overheating were established for winding speed as a function of processing temperature, when the compaction pressure was kept constant. Good correlation was obtained between the steady-state FEM model and experimental data.

Material phenomena controlling rapid processing of thermoplastic composites

Driven by the need for more cost-effective applications, researchers and design engineers of polymer composites are facing increasing demands for the development of faster and more environment-friendly processing methods. Rapid processing techniques require a better understanding and a more efficient control of phenomena such as heating, impregnation, bonding and solidification. Most of the new techniques promote non-isothermal and transient phenomena in order to achieve faster transformation of the materials. Material physical properties, material microstructure and processing conditions set the theoretical limits for rapid processing. The time scales of the material phenomena controlling the processing of thermoplastic composites are presented. It is shown for example, how novel composite microstructures influence the consolidation time, or how the fusion bonding time can be reduced when non-isothermal conditions are promoted. Furthermore, the application of the results to a novel integrated processing technology illustrates the new potentials of thermoplastic composites. The description of the phenomena controlling the processing rate is useful, knowing that better tailoring and control of the material transformation, adaptation of the material constituents, and combination of process sequences are intensively required for the development of more efficient manufacturing routes for composites.

Fractal Description of Interlaminar Contact Development during Thermoplastic Composites Processing

Fabrication of layered thermoplastics and thermoplastic-matrix composites using processes such as tow placement, tape laying, and resistance welding, is fundamentally based on the principle of fusion bonding, which involves applying heat and pressure to contacting thermoplastic surfaces. One of the important processing steps— intimate contact—is considered in this paper. Interlaminar intimate contact development is a strong function of thermoplastic surface geometry. Profilometric measurements of thermoplastic prepreg tows show that surface roughness features can be found at several length scales, which implies that the surfaces have a fractal structure. In this paper, principles of fractal geometry are used to describe prepreg surfaces. Based on this description, an axisymmetric squeeze flow model is developed to relate degree of intimate contact to the process parameters—pressure, temperature, and time—and the fractal parameters of the surface. The model development and comparisons with available experimental data are presented and discussed in this paper.