Composite Molding Research Articles

Reactive Processing of Acrylic-Based Thermoplastic Composites: A Mini-Review

The demand for thermoplastic composites is continuously increasing because these materials offer many advantages over their thermoset counterparts, such as high toughness, long storage time, easy repairing and recycling, and ability to be thermoformed and heat-welded. However, the manufacturing of thermoplastic composite parts using liquid composite moulding techniques (e.g. resin transfer moulding, vacuum assisted resin transfer moulding … ) is often tricky in the case of melt processing where high temperature and pressure should be chosen to impregnate the fibre reinforcement because of the high melt viscosity of thermoplastics. These issues may be overcome by means of reactive processing where a fibrous preform is first impregnated by a low viscosity mono- or oligomeric precursor and the polymerization of the thermoplastic matrix then occurs in-situ. This article draws a state of the art on the manufacturing characteristics of continuous fibre reinforced acrylic-based reactive thermoplastics (e.g. polymethymethacrylate (PMMA) such as Elium®), which are becoming more and more popular compared to other fast curing thermosets and thermoplastics for in-situ polymerization. Techniques for the in-situ polymerization of methymethacrylate monomers, characterization and modelling of the rheological properties and polymerization kinetics, and some manufacturing related issues such as polymerization shrinkage are reviewed. Particular features of the use of reactive PMMA in different manufacturing techniques of continuous fibre reinforced composites and potential industrial applications are also introduced. Finally, some perspectives for the academic research and industrial development are proposed.

Patterning High-Resolution Microstructures on Thermoplastics by Ceramic Nanoparticles Filled Epoxy Coated Molds for Duplicating Nature-Derived Functional Surfaces.

Patterning high-resolution microstructures on thermoplastic substrates is of fundamental importance for the commercialization of microfluidics, advanced functional surfaces, and optical elements. Though many methods are developed to fabricate micropatterned plastic devices with 100 μm resolution, they suffer substantially higher cost or lower productivity when the resolution of the micropatterns is to be further improved. Here, we develop low-cost molds consisting of thin ceramic-filled-epoxy composite coatings on steel substrates. By virtue of the loaded ZrO2 nanoparticle fillers, the enhanced mechanical and thermal properties of the composite molds enable the epoxy microstructures to survive harsh conditions in conventional thermoplastic processing methods including hot embossing, imprinting, and mold injection. With the ceramic-filled-epoxy coated molds, we are able to improve the fabrication resolution of microstructures on plastics to 10 μm with unprecedented low-cost and excellent durability.

Evaluation of Color and Spectral Behavior of a Novel Flowable Resin Composite after Water Aging: An In Vitro Study.

Background: This study aimed to evaluate the color matching, light transmittance, and reflectance characteristics of the novel flowable resin composite OCF-001 (OCF). Methods: Fifty-four resin composite molds were made with simulated class I cavities of A2, A3, and A4 shades by filling the rubber mold interspace with Estelite Sigma Quick (ESQ), Gracefil Putty (GP) and Filtek Supremme Ultra (FSU). After applying the adhesive, three different flowable resin composites (n = 6), OCF, Gracefil LoFlo (GLF), and Supreme Ultra Flowable (SUF), were used to fill the cavities. A colorimeter was used to measure the color parameters (CIEDE2000). The color measurements were taken immediately and after 28 days. Data were analyzed using the nonparametric Kruskal–Wallis (α = 0.05) and Wilcoxon tests. The light transmittance and reflection characteristics were measured with a black background using a spectrophotometer under D65 illumination. Results: The ΔE00, and ΔC of OCF was lower than other tested materials in A2 and A3 shades both immediately and after 28 days. OCF showed the highest transmittance characteristic, and a relatively stable reflectance curve in all the wavelengths. Conclusions: OCF showed better shade matching with the surrounding shades of A2 and A3, a relative uniform reflectance and higher light transmission properties.

Investigation of the compaction behaviour of a quasi-unidirectional non-crimp fabric during the vacuum infusion process

The compaction behaviour of technical textiles such as non-crimp fabrics (NCF) is of much interest to build high quality parts in liquid composite moulding processes (LCM). In this paper, the compaction response of a glass fibre NCF was investigated in two different ways: (1) characterisation tests via a universal testing machine and (2) height measurements via a line laser measurement unit during the infusion process. Results from both measurement systems are compared and it is found that the results of characterisation tests via testing machine can be used only to a certain extent. The pressure-thickness correlation during the infusion process cannot be described by the results of the testing machine, while the compaction results before and after the infusion process are in good agreement for both the methods. With the data of the line laser measurement, a model for the pressure-thickness correlation is derived, which can be used in future simulations. The infusion process was carried out using different scenarios at the vent, on the one hand a semi-permeable membrane and on the other hand an omega profile at the venting port. The results obtained using these two different venting scenarios were compared and it was found that using a semi-permeable membrane as an outlet can lead to thicker parts (up to 10 %).

ИССЛЕДОВАНИЕ ЭФФЕКТИВНОСТИ РАБОТЫ КОНФОРМНЫХ КАНАЛОВ ОХЛАЖДЕНИЯ КОМПОЗИТНЫХ ПРЕСС-ФОРМ

The article deals with the issue of evaluating the performance of a composite metal-metal-polymer mold. The use of a composite mold, including a forming plate consisting of a metal, and a metal polymer introduced with a low cooling efficiency of such a mold. For this reason, it is important to develop technological solutions that ensure the efficient operation of the cooling system. The article provides mathematical calculations of the required amount of heat flow when molding thermoplastics into a mold. Further, based on the calculated data, a study is made using finite elements of the thermal conductivity of the metal-polymer part. The design of the composite forming plate of the mold described in the article involves the manufacture of a cooling channel profile as close as possible to the forming part of the mold and located in the metal-polymer part. Based on the analysis of the temperature viscosity of the metal-polymer part of the mold, conclusions and recommendations are given for the design of a curved investment mold. It allows designers to calculate the optimal geometric parameters of the cooling system. The design and manufacture of conformal cooling stages in the metal-polymer part is likely to be preferable for cooling the forming part of the mold by bringing the cooling reversal surface closer to the surface of the forming mold, resulting in increased durability and productivity of the composite mold.

Saturated out-of-plane permeability and deformation metrology of textiles at high levels of injection pressure

Out-of-plane impregnation and high levels of injection pressure are key strategies for cycle time reduction in Liquid Composite Molding processes. The combination of these two strategies provides a promising approach for large volume production of automotive components. In this context, a novel test system is presented, which allows the textile reaction characterization to saturated out-of-plane fluid flow at injection pressure levels of up to 200 bar. For any given engineering textile, the resulting out-of-plane permeability and total hydrodynamic compaction can be measured for different combinations of initial fiber volume content, number of layers and injection pressure. Initial tests on a conventional non-crimp fabric show a compaction-induced out-of-plane permeability decrease for pressure levels up to 95 bar, while for pressure levels between 95 and 170 bar the permeability remains constant. In other words above 95 bar, a further increase in pressure directly pays off in terms of increased flow rate. The identification of such processing windows can be very valuable for process design.

Ultrasonic Anisotropy in Composites: Effects and Applications

Stiffness anisotropy is a natural consequence of a fibrous structure of composite materials. The effect of anisotropy can be two-fold: it is highly desirable in some cases to assure a proper material response, while it might be even harmful for the applications based on “isotropic” composite materials. To provide a controllable flexibility in material architecture by corresponding fibre alignment, the methodologies for the precise non-destructive evaluation of elastic anisotropy and the fibre orientation are required. The tasks of monitoring the anisotropy and assessing the fibre fields in composites are analyzed by using the two types of ultrasonic waves suitable for regular plate-shaped composite profiles. In the plate wave approach, the effect of “dispersion of anisotropy” has been shown to make the wave velocity anisotropy to be a function of frequency. As a result, the in-plane velocity pattern measured at a certain frequency is affected by the difference in the wave structure, which activates different elasticity against the background of intrinsic material anisotropy. Phase velocity anisotropy and its frequency dependence provide a frequency variation of the beam steering angle for plate waves (dispersion of beam steering). In strongly anisotropic composite materials, the beam steering effect is shown to provide a strong focusing of ultrasonic energy (phonon focusing). For bulk shear waves, the orthotropic composite anisotropy causes the effect of acoustic birefringence. The birefringent acoustic field provides information on stiffness anisotropy which can be caused by internal stresses, texture, molecular or/and fibre orientation. On this basis, a simple experimental technique is developed and applied for mapping of fibre orientation in composite materials. Various modes of acoustic birefringence are analyzed and applied to assessing the fibre fields in injection moulding composites and to identify the fibre lay-ups in multiply materials. The birefringence pattern is also shown to be sensitive and applicable to characterizing impact- and mechanical stress-induced damage in composites.

The Direct Sandwich Composite Molding (D-SCM) Process: Sandwich Manufacturing and Characterization

Sandwich structures benefit from the geometrical stiffening effect due to their high cross-sectional area moment of inertia. Transferred to carbon fiber-reinforced plastic (CFRP) components, the needed amount of carbon fiber (CF) material can be reduced and with it the CO2 footprint. The combination of a light foam core with continuous fiber-reinforced face sheets is a suitable material combination for lightweight design. Traditionally, CFRP sandwich structures with a foam core are manufactured in a two-step process by combining a prefabricated foam core with fiber-reinforced face sheets. However, in addition to the reduction in the used CFRP material, manufacturing processes with a high efficiency are needed. The objective of this paper is the sandwich manufacturing and characterization by using the Direct Sandwich Composite Molding (D-SCM) process for the one-step production of CFRP sandwich structures. The D-SCM process utilizes the resulting foaming pressure during the reactive polyurethane (PUR) foam system expansion for the impregnation of the CF-reinforced face sheets. The results of this work show that the production of sandwich structures with the novel D-SCM process strategy is feasible in one single manufacturing step and achieves good impregnation qualities. The foam density and morphology significantly influence the core shear properties and thus the component behavior under a bending load.

ИСПЫТАНИЕ КОНФОРМНОЙ СИСТЕМЫ ОХЛАЖДЕНИЯ КОМПОЗИТНОЙ ПРЕСС-ФОРМЫ

The article deals with the issue of manufacturing conformal cooling channels to ensure efficient cooling of composite metal-metalpolymer molds. A brief description of both the design of the composite mold and the technology for its production is given. Conformal cooling channels in the composite mold are made in the metal-polymer part, which reduces the distance from the surface of the cooling channel to the cooled forming surface. Experimental studies are given on the example of a composite mold for the "collar" part. Fixing the temperature data of heating and cooling of the metal-polymer molding is carried out using a thermal imager. Evaluation of the results of the study show the efficiency of manufacturing conformal cooling channels directly in the metal-polymer part of the mold. An analysis of the experimental results allows numerically evaluating the cooling efficiency and comparing it with the cooling of a composite mold, in which the cooling channels are made in a metal cage. An assessment of the efficiency of using a composite mold with cooling channels made in the metal-polymer part in the context of saving the main technological time for the production of a batch of parts is given. In conclusion, the difficulties that arose in the process of manufacturing conformal cooling channels in the metal-polymer part and the tasks that require further research in the technology of manufacturing composite molds are described.

Capillary Effects in Fiber Reinforced Polymer Composite Processing: A Review

Capillarity plays a crucial role in many natural and engineered systems, ranging from nutrient delivery in plants to functional textiles for wear comfort or thermal heat pipes for heat dissipation. Unlike nano- or microfluidic systems with well-defined pore network geometries and well-understood capillary flow, fiber textiles or preforms used in composite structures exhibit highly anisotropic pore networks that span from micron scale pores between fibers to millimeter scale pores between fiber yarns that are woven or stitched into a textile preform. Owing to the nature of the composite manufacturing processes, capillary action taking place in the complex network is usually coupled with hydrodynamics as well as the (chemo) rheology of the polymer matrices; these phenomena are known to play a crucial role in producing high quality composites. Despite its importance, the role of capillary effects in composite processing largely remained overlooked. Their magnitude is indeed rather low as compared to hydrodynamic effects, and it is difficult to characterize them due to a lack of adequate monitoring techniques to capture the time and spatial scale on which the capillary effects take place. There is a renewed interest in this topic, due to a combination of increasing demand for high performance composites and recent advances in experimental techniques as well as numerical modeling methods. The present review covers the developments in the identification, measurement and exploitation of capillary effects in composite manufacturing. A special focus is placed on Liquid Composite Molding processes, where a dry stack is impregnated with a low viscosity thermoset resin mainly via in-plane flow, thus exacerbating the capillary effects within the anisotropic pore network of the reinforcements. Experimental techniques to investigate the capillary effects and their evolution from post-mortem analyses to in-situ/rapid techniques compatible with both translucent and non-translucent reinforcements are reviewed. Approaches to control and enhance the capillary effects for improving composite quality are then introduced. This is complemented by a survey of numerical techniques to incorporate capillary effects in process simulation, material characterization and by the remaining challenges in the study of capillary effects in composite manufacturing.

Failure mechanism of the Y2O3 doped BaZrO3/Al2O3 composite ceramic mould during directional solidification of TiAl-Based alloys

Directional solidification of Ti–46Al–8Nb (at.%) intermetallic in the Y2O3 doped BaZrO3/Al2O3 composite ceramic mould was carried out using Bridgman apparatus. To increase the success rate of Ti–46Al–8Nb single crystals preparation and improve the quality of the ingots, the failure mechanism of the mould in directional solidification experiments was evaluated. Nucleation and propagation of cracks in the moulds were investigated by tracing each key process of the experiment, the effect of pores in the mould on the target alloy was revealed by studying the mould/metal interface. The results show that the macrocracks in the facecoat of the mould would lead to the leakage of the alloy melt. Furthermore, the alloy melt would infiltrate into the mould through large-size pores, which would increase the oxygen content of the target alloy, and also form inclusions containing O, Zr, Si and Y elements in the alloy ingot.

Study of the Possibilities of Obtaining Powder Materials by Rotational Turning with Multi-Faceted Cutters

Composite materials obtained through powder metallurgy methods are increasingly applied in various industries, particularly in aviation and rocket and space equipment which use their high specific strength, resistance to high temperatures and other properties. Producers of composite materials use various metallic and non-metallic materials (fibres and powders) as fillers [1-2]. For example, the high plasticity (moldability) of aluminium powders allows utilizing them as a matrix material in moulding of composites using various methods of rolling, extrusion, and intense plastic deformation [3-5]. However, the widespread use of chip as a raw material for the production of composites is hampered by the complexities in obtaining powders with granules of the necessary shape and size.

Mechanical and Rheological Properties of Bamboo Pulp Fiber Reinforced High Density Polyethylene Composites: Influence of Nano CaCO3 Treatment and Manufacturing Process with Different Pressure Ratings

In order to investigate the effect of the relative motion of nano CaCO3 reinforced bamboo pulp fiber (BPF)/HDPE composite components on the mechanical performance, a comparative study was performed. BPF was treated by nano CaCO3 blending (BM) and impregnation modification (IM) technology. The composites were produced using hot press (HPMP), extrusion (EMP) and injection molding process (IMP). The physical morphology of BPF was similar at different manufacturing processes. Compared to the samples manufactured by HPMP, a decrease in the (specific) flexural strength of BPF/HDPE composites and an increase in those of composites treated by nano CaCO3 manufactured by EMP and IMP were observed. The injection molded composites exhibited the best values in the (specific) impact strength, (specific) tensile properties. IM had a greater effect on the rheological behavior of the composites than BM, and nano CaCO3 treatment most effectively affected the performance of the extrusion molded composites.

Application of Graphite Composite Slow Cooling Mold to Optimize the Composition and Microstructure of Ti-Si Alloy During Esr for Recycle Ti-Bearing Slag

Application of Graphite Composite Slow Cooling Mold to Optimize the Composition and Microstructure of Ti-Si Alloy During Esr for Recycle Ti-Bearing Slag

Theoretical models for stiffness prediction of short fibre composites

The short natural fibre reinforced polymer matrix composites are a good cost-effective option in a multitude of non-structural applications. The estimation of elastic moduli of such composites is important during the initial design phase. In this work, a brief introduction to the theoretical models to determine elastic moduli of short fibre composites is presented. Various theoretical models like Halpin-Tsai model, Mori-Tanaka model with fibre orientation averaging, alongwith the rule-of-mixtures and the Hashin-Shtrikman bounds are employed to obtain the elastic properties of composites. Although some of these models have been used for short glass fibre composites, in this study, the applicability of these models to natural fibre composites has been explored. The elastic properties obtained from the theoretical model are compared with experimentally obtained values of injection moulded short jute fibre-polypropylene composites from our other work. The model which satisfactorily results in better elastic property predictions of short fibre composites amongst the presented theoretical models has been highlighted. It has been observed that the fibre orientation averaging is imperative to account for random fibre orientation and yield better elastic property predictions.

Sisal fiber reinforced polyethylene terephthalate composites; Fabrication, characterization and possible application

The use of thermoplastics (TPs) for natural fiber composites is restricted to commodity ones like polypropylene and polyethylene However, using engineered TPs such as polyethylene terephthalate (PET) will benefit from its technical and economic advantages. The research aims to characterize injection molded PET composites reinforced with sisal fibers treated differently. Polyethylene terephthalate composites containing 40 wt.% of untreated, alkaline-treated, and alkali/acetylation treated sisal fibers were prepared using compounding and injection molding processes and then characterized. It has been found that production of sisal-PET composites by compounding and injection molding has been shown to be possible. Thermal damage to sisal fiber was noticed during composite production. Based on the thermogravimetric analysis analysis, a net weight loss (excluding water loss) of 11.1%–14.0% was observed at the operating temperatures of the two processes. The addition of 40 wt.% of sisal to the PET matrix improved the tensile modulus by 137%. Further improvement by 179% was observed when alkali-treated sisal fiber was used. The combined alkali/acetylation treatment of sisal yields more enhancement by 233%. This is a significant advancement because modulus is the most influential parameter during the design and service of an engineering product. Generally, compared to the raw sisal composite (RSC) the interfacial, mechanical, thermal, and water absorption properties of the alkali treated sisal composite (Al-SC) and alkali/acetylated sisal composite (Al-ASC) specimens recorded an improvement. Relative to the natural fiber reinforced thermoplastic composites that were commercialized in the automotive industry, the produced sisal–PET composites resulted in a considerable improvement of 66.6%–190% in flexural strength and by 110.5%–410.0% in flexural modulus, depending on sisal fiber treatment and the composite to be compared. Thus, the studied composites can be recommended for various parts of automobiles.

Analysis of uncertainty using stochastic flow simulation of LCM at constant thickness

Liquid Composite Moulding (LCM) process design requires that the resin impregnation minimizes both mould filling time, as well as the probability of occurrence of dry, unsaturated zones. This can be challenging, as the degree of variability present in material properties and the manufacturing process itself, can be substantial. The main purpose of this study is to understand and quantify how the different sources of variability, present in LCM, may affect the performance of the mould filling process, creating defects and to quantify how lower scale interactions may affect macro-scale flow. In order to transfer the meso-scale stochastic behaviour towards a complete macro-scale model, an uncertainty propagation model is proposed, whereby local statistical properties are used to predict the upscaled flow behaviour. This is done by means of flow simulation, using stochastic models for the input process variables. Local fabric distortions, combined with fabric permeability, pressure and race-tracking serving as inputs to estimate the joint probability distribution that characterizes the global uncertainty in flow performance. Preliminary uncertainty analyses investigating individual sources of variability are available in the literature, however, a complete framework combining the different variables into a mould filling scenario, was not found. Therefore, this study tries to develop a new multi-variable aggregating concept that is applied to mould filling simulation. The individual contribution of each source of variability used in this work is quantified, providing a better understanding of the reliability of alternative process designs.

Analysis of uncertainty using stochastic flow simulation of LCM at constant thickness

Liquid Composite Moulding (LCM) process design requires that the resin impregnation minimizes both mould filling time, as well as the probability of occurrence of dry, unsaturated zones. This can be challenging, as the degree of variability present in material properties and the manufacturing process itself, can be substantial. The main purpose of this study is to understand and quantify how the different sources of variability, present in LCM, may affect the performance of the mould filling process, creating defects and to quantify how lower scale interactions may affect macro-scale flow. In order to transfer the meso-scale stochastic behaviour towards a complete macro-scale model, an uncertainty propagation model is proposed, whereby local statistical properties are used to predict the upscaled flow behaviour. This is done by means of flow simulation, using stochastic models for the input process variables. Local fabric distortions, combined with fabric permeability, pressure and race-tracking serving as inputs to estimate the joint probability distribution that characterizes the global uncertainty in flow performance. Preliminary uncertainty analyses investigating individual sources of variability are available in the literature, however, a complete framework combining the different variables into a mould filling scenario, was not found. Therefore, this study tries to develop a new multi-variable aggregating concept that is applied to mould filling simulation. The individual contribution of each source of variability used in this work is quantified, providing a better understanding of the reliability of alternative process designs.

Development of preforming and RTM processes for highly integrated structures

Landing gear doors / traps are usually composed of sandwich structures manufactured by hand lay up and cured in autoclave. In the present development, this type of construction and its associated manufacturing process have been replaced by a monolithic design manufactured by liquid composite moulding. Specifically, the selected process has been RTM (Resin Transfer Moulding) in a single process of impregnation / curing (one shot) with the objective of eliminating subsequent operations. In order to obtain a 3D integrated preform close to the final shape, pre-forming processes have been developed for the sub-elements of the part (skin, double T stiffeners, C stiffeners, etc.) as well as the processes of integration of the various preforms until obtaining an integrated preform. The preforming and manufacturing processes have been fully validated by obtaining real-size prototypes of the target part. This work is included within the ATEAERO project, with the support of the ETORGAI program (SPRI - Basque Government).

Effect of interleaved fiber mats on the manufacturability and mechanical properties of carbon/epoxy composites

Fracture toughness is a critical property of composites, especially in the case of materials manufactured by liquid composite moulding processes because of the brittleness of the thermoset polymeric matrix. The use of thermoplastic interleaving veils can improve the fracture behaviour affecting only slightly other mechanical properties. Also, the veils can affect the manufacturability of the composites. This work presents a study of the influence of commercial and homemade veils on the manufacturability process and mechanical properties of carbon fibre reinforced composites. Results obtained show a significant improvement of fracture toughness without affecting manufactubarility.