Injection-molded Composites Research Articles

Optimizing the injection molding process for thermally and electrically conductive, carbon fiber and carbon nanotube‐reinforced poly(lactic acid) hybrid composites with enhanced mechanical properties

AbstractWe produced poly(lactic acid) (PLA) matrix carbon fiber and carbon nanotube‐reinforced hybrid composites with enhanced thermal conductivity (0.48–0.59 W/mK) and electrical conductivity (0.35–0.97 S/cm). The conductive fillers greatly decreased the toughness, which was compensated with oligomeric lactic acid (OLA). Since fillers and plasticizers greatly alter the flow and thermal properties of the material as well, it was necessary to optimize several parameters of the injection molding process that were predetermined based on theoretical considerations. Based on oscillatory shear rheometry, we explored the rheological behavior of the materials in a wide temperature and shear rate range for the optimum injection molding temperatures, injection volume rates. We showed that by adding 15 wt% OLA as a plasticizer to the composites, the optimal processing temperature decreased by 45–135°C. This remarkable change illustrates the need for rheological studies of PLA compounds. The injection molded hybrid composites containing 5% oligomeric lactic had a tensile strength, modulus of elasticity, and work of fracture higher by 41%, 10%, and 150%, respectively, compared to samples without OLA.

Fibre orientation distribution function mapping for short fibre polymer composite components from low resolution/large volume X-ray computed tomography

Short glass fibre injection moulded composites, used in interior and exterior automotive parts, are exposed to complex stress states, for example during a crash. As the fibre scale dominates the composite’s material properties, numerical models need to account for the local fibre orientation. In recent years, mould flow simulation results have been exploited to predict the fibre orientations for finite element models, albeit with limited accuracy. Alternatively, X-ray computed tomography can be used to directly image and analyse fibre orientations. Traditionally, achieving the necessary resolution to image individual fibres restricts the imaging to small regions of the component. However, this study takes advantage of recent advancements in imaging and image analysis to overcome this limitation. As a result, it introduces, for the first time, a reliable, fast, and automated fibre orientation mapping for a full component based on image analysis at the individual fibre level; even for cases where the pixel size is significantly larger than the fibre diameter. By scanning at lower resolutions, a drastically larger volume of interest can be achieved. The resulting fibre orientation analysis and mapping algorithm, based on X-ray computed tomography, is well matched to the level of information required for automotive crash modelling with a standard element-size of a few millimetres. The entire process, encompassing image acquisition, image analysis and fibre orientation mapping, can be directly integrated into an industrial full component application in a matter of hours.

Sustainable Polypropylene-Based Composites with Agro-Waste Fillers: Thermal, Morphological, Mechanical Properties and Dimensional Stability.

Natural fiber composites (NFC) are eco-friendly alternatives to synthetic polymers. However, some intrinsic natural fillers' properties hinder their widespread implementation as reinforcement in polymeric matrices and require further investigation. In the scope of this study, the thermal, rheologic, mechanical (tension and flexural modes), and morphological properties, as well as the water absorption and dimensional stability of the NF polypropylene (PP)-based injection molded composites reinforced with rice husk (rh) and olive pits (op) of 20 wt.% and 30% wt.%, respectively, were investigated. The results suggest that the higher content of the rice husk and olive pits led to a similar reduction in the melt flow index (MFI), independent of the additive type compared to virgin polypropylene (PPv). The melting and crystallization temperatures of the PPrh and PPop composites did not change with statistical significance. The composites are stiffer than the PP matrix by up to 49% and possess higher mechanical strength in the tension mode at the expense of decreased ductility. PPrh and PPop have a superior flexural modulus in the bending mode, while the flexural strength improvement was accomplished for the PP30%rh. The influence of the fibers' distribution in the bulk of the parts on their mechanical performance was confirmed based on a non-localized morphology evaluation, which constitutes a novelty of the presented research. The dimensional stability of the composites was improved as the linear shrinkage in the flow direction was decreased by 49% for PPrh and 30% for PPop, positively correlating with an increase in the filler content and stiffness. PPop was less susceptible to water sorption than PPrh due to fibers' composition and larger surface-to-area volume ratios.

Preparation and Performance of High Thermal Conductivity Polypropylene-based Composites

Polypropylene-based composites with high thermal conductivity were obtained by pressure injection molding using a hybrid form of PP as the matrix and FG as the thermally conductive filler with a particle size of 37 µm. Microscopic morphologies of the material were examined by SEM to determine the effect of FG content on the thermal conductivity and mechanical properties of the composites. The study found a clear correlation between the thermal conductivity of the composites and the FG content. The research confirmed a direct link between the thermal conductivity of the composites and FG content. At 70 wt%, the material demonstrated the greatest average, axial, and radial thermal conductivity of 7.52 W·m-1·K-1, 12.6 W·m-1·K-1, and 4.50 W·m-1·K-1, respectively. However, any subsequent increase in fractional gradient (FG) content resulted in a decrease in the strength and modulus of the material. The highest tensile and flexural strength values of 34.9 and 63.7 MPa respectively, were achieved when the FG content was 60 wt%. At this particular FG content, the tensile and flexural modulus also reached 9.78 and 10.7 gigapascals (GPa), respectively. As the FG content increased, the strain on the composite material decreased. Note that the maximum tensile and flexural strains were measured at 50 wt% FG content, with values of 0.77% and 0.79%, respectively. The glass fiber sheets in the injection molded composites were uniform and predominantly vertically oriented.

Effect of Wall Thickness and Flow Distance behind an Obstacle on the Weldline Strength of Thermoplastic Composites

The strength of the weldline behind an obstacle in injection molded thermoplastic composites was investigated in this study. The effects of part thickness, glass-fiber length, and concentration on the weldline strength at different positions behind the obstacle were examined. The results showed that the mechanical properties of the specimen containing weldline were much lower as the fiber length and content increased. Considering the effect of part thickness on the weldline strength, it was observed that the weldline strength decreased as the part thickness increased. This was due to the fiber orientation and the void generation within the molded parts. The results obtained for the weldline strength at various positions along the weldline indicated that the weldline strength tended to increase with the increase of flow distance, especially for polypropylene containing 40 wt% long-glass-fibers. The explanation for this could be associated with the change in fiber orientation during the flow development.

Influence of fly ash on thermo-mechanical and mechanical behavior of injection molded polypropylene matrix composites

Polypropylene composites find widespread application in industries, including packaging, plastic parts, automotive, textiles, and specialized devices like living hinges known for their remarkable flexibility. This study focuses on the manufacturing of polypropylene composite specimens by incorporating varying weight percentages of fly ash particles with polypropylene using a twin-screw extruder and injection molding machine. The composites were comprehensively tested, evaluating tensile, compressive, and flexural strength, solid-state and polymer melt properties, modulus, damping, and thermal response. The findings reveal that the compressive strength of polypropylene increases up to 2 wt% of added fly ash particles and subsequently exhibits a slight decline. Tensile strength demonstrates an increase up to 1 wt% of fly ash, followed by a decrease with a 2 wt% addition, and then a subsequent increase. Flexural strength shows improvement up to 3 wt% fly ash addition before declining. The storage modulus curve is categorized into three regions: the glassy region (up to 0 °C), the glass transition region (0–50 °C), and the glass transition region of polypropylene (>50 °C), each corresponding to different molecular motions. Weight loss curves exhibit similar trends, indicating uniform pyrolysis behavior attributed to consistent chemical bonds. Plastic degradation commences around 440 °C and concludes near 550 °C. Additionally, elemental mapping of fly ash composition identified various elements such as O, Si, K, Mg, Ca, Cl, Na, P, Al, Fe, S, Cu, Ti, and Ni. These findings offer valuable insights into the mechanical and thermal properties of polypropylene composites reinforced with fly ash, rendering them suitable for a wide range of industrial applications necessitating strength and durability across temperature variations.

Influence of alumina powder (Al2O3) on mechanical and tribological properties of injection molded polyoxymethylene composites

Composite materials combine two or more materials with mechanical and chemical properties that differ from one another. The matrix is one element, while the reinforcement is another. In this study, alumina powder (Al2O3) is used as filler while polyoxymethylene (POM) is used as the matrix. The injection moulding method is used to produce the composite specimens in this investigation. Prior to injection moulding, composite pellets are made using a twin-screw extruder. Compounding is done to produce a homogeneous mixture. The injection moulding method is the most efficient solution for mass-producing material with the slightest faults in samples. The advantage of injection moulding is that it only takes a few minutes to complete a cycle.The composite was put through mechanical, tribological, and morphological testing as part of this investigation to determine its strength. Tensile, flexural, and compression tests are examples of mechanical tests. A wear test is conducted with the help of a pin-on-disc setup. The specific wear rate of all the samples is calculated. The SEM is done to study the surface of the sample after the tensile test. From the tensile test, it is found that with the increase in the amount of tensile strength of composite decreases. The maximum flexural strength (180.92 N/mm2) is obtained for a sample with 1% of alumina powder. The maximum compression strength (91.88 N/mm2) is achieved for a sample with 4% of alumina powder. The SEM image shows the brittle failure of a composite.

Injection moulded composites from high biomass filled biodegradable plastic: Properties and performance evaluation for single-use applications

Biodegradable plastic-based items play an essential role in ensuring the sustainability of the food packaging industry due to their high biodegradability and minimized use of fossil fuels. The incorporation of low-cost waste biomass into bio-based polymers to produce biodegradable composites supports the circular economy model and reduces landfilling and carbon footprint challenges associated with petroleum-based plastics. This work discusses the utilization of waste almond shell powder (ASP) up to 50 wt.% with poly(butylene succinate-co-butylene adipate) (PBSA) to develop sustainable biocomposites through injection moulding for rigid packaging applications. At a lower angular frequency (0.1 s − 1), the complex viscosity of the PBSA/50%ASP biocomposite was reduced by ∼65% after adding 5 wt.% compatibilizer, as confirmed by a rheological analysis. The heat deflection temperature, flexural strength, and tensile and flexural moduli of the PBSA/50% ASP biocomposite with 5 wt.% compatibilizer were improved by ∼24, 125, 368, and 385%, respectively, compared to pristine PBSA. These improvements are attributed to the high stiffness and load-bearing capacity of ASP and the enhanced interfacial adhesion and particle dispersion caused by the compatibilizer, as corroborated by SEM analyses. Hence, the formulated biocomposites show a suitable structure-property-processing co-relationship for injection moulding of single-use products.

Influence of flow–fiber coupling during mold-filling on the stress field in short-fiber reinforced composites

The anisotropic elastic properties of injection molded composites are fundamentally coupled to the flow of the fiber suspension during mold-filling. Regarding the modeling of mold-filling processes, both a decoupled and a flow–fiber coupled approach are possible. In the latter, the fiber-induced viscous anisotropy is considered in the computation of the flow field. This in turn influences the evolution of the fiber orientation compared to the decoupled case. This study investigates how flow–fiber coupling in mold-filling simulation affects the stress field in the solid composite under load based on the final elastic properties after fluid–solid transition. Furthermore, the effects of Newtonian and non-Newtonian polymer matrix behavior are investigated and compared. The entire process is modeled micromechanically unified based on mean-field homogenization, both for the fiber suspension and for the solid composite. Different numerical stabilization methods of the mold-filling simulation are discussed in detail. Short glass fibers with a typical aspect ratio of 20 and a volume fraction of 20% are considered, embedded in polypropylene matrix material. The results show that the flow–fiber coupling has a large effect on the fiber orientation tensor in the range of over ± 30% with respect to the decoupled simulation. As a consequence, the flow–fiber coupling affects the stress field in the solid composite under load in the range of over ± 10%. In addition, the predictions based on a non-Newtonian modeling of the matrix fluid differ significantly from the Newtonian setup and thus the necessity to consider the shear-thinning behavior is justified in a quantifiable manner.

Effect of Manufacture-Induced Interfaces on the Tensile Properties of 3D Printed Polyamide and Short Carbon Fibre-Reinforced Polyamide Composites.

This study aims to elucidate the structure-property-process relationship of 3D printed polyamide and short carbon fibre-reinforced polyamide composites. The macroscopic properties (tensile modulus) of the 3D printed samples are quantitatively correlated to the printing process-induced intrinsic microstructure with multiple interfaces. The samples were printed with different layer thicknesses (0.1, 0.125 and 0.2 mm) to obtain the varied number of interface densities (number of interfaces per unit sample thickness). The result shows that the printed short carbon fibre-reinforced polyamide composites had inferior partially bonded interfaces compared to the printed polyamide, and consequently exhibited interface-dependent elastic performance. The tensile modulus of 3 mm thick composites decreased up to 18% as a function of interface density, whilst the other influencing aspects including porosity, crystallinity and fibre volume fraction (9%) were the same. Injection moulding was also employed to fabricate samples without induced interfaces, and their tensile properties were used as a benchmark. Predictions based on the shear-lag model were in close agreement (<5%) with the experimental data for the injection-moulded composites, whereas the tensile modulus of the printed composites was up to 38% lower than the predicted modulus due to the partial bonded interfaces.

A critical review on mechanical and morphological characteristics of injection molded biodegradable composites

AbstractFactors like environmental and societal awareness often lead to a significant change of action in the development of eco‐friendly materials like the family of green composites. Green composites are becoming increasingly apparent as a sensible and potentially viable alternative to the synthetic composites. A healthy synergism of factors to include weight reduction, cost, performance improvement, biodegradable nature, nontoxic properties, and flexibility are often the key motives that provide the much needed impetus for the development of green composites for the purpose of selection and use in a spectrum of applications. The biodegradable or green composites became a viable substitute for the long‐established materials and often a challenge to the family of synthetic polymer‐based composites. The present study draws attention to the issues and challenges that come in the way of development and characterization of “green” composites primarily on the use of advanced manufacturing processes like injection molding. In this article, the chemical composition, structure, mechanical properties, and classification of the natural fibers are presented and briefly discussed. The mechanical properties, to include tensile response, flexural behavior, and impact strength, of the injection molded composites are presented and adequately discussed. The effect and role of surface treatments and coupling agents on mechanical properties of the family of polymer composites, to include the nonbiodegradable composites, partially green composites, and fully green composites, are examined and adequately discussed.

Life Cycle Assessment of Calcium-Rich Industrial Waste Reinforced Polypropylene and Low-Density Polyethylene Composites Using the Cradle-to-Gate Approach

The first life cycle assessment (LCA) of injection-molded composites made of low-density polyethylene (LDPE) and polypropylene (PP) reinforced with calcium-rich flue gas desulfurization (FGD) gypsum and marble waste is shown in this work. The life cycle phases of the manufacturing of polymer composites were assessed using the methodological framework of IS/ISO 14044:2006. Midpoint indicators were evaluated by the ReCiPe method. To determine the composition, the physicochemical properties of the used FGD gypsum and marble waste were analyzed to ensure utilization in subsequent composite materials. American Society for Testing and Materials (ASTM) standards were followed for mechanical testing to check the quality of the developed composites. The flexural strength of the developed polymer composites was higher than that of pristine specimens with a 20–80 wt % filler concentration. Using the openLCA software and ecoinvent database, a cradle-to-gate LCA was performed for the developed composites. The global warming potential of the MW-PP composites is 1.01–1.06 times less than those of the other developed reinforced composites. The results of the present research are useful for creating environmentally friendly technologies, particularly the injection-molding practice.

Recycled HDPE/Natural Fiber Composites Modified with Waste Tire Rubber: A Comparison between Injection and Compression Molding.

With the objective of turning wastes into added-value materials, sustainable and fully recycled wood-plastic composites were reinforced by waste tire rubber particles to show balanced properties and potentially low-cost materials. Recycled high density polyethylene (rHDPE) was compounded (melt extrusion) with flax fiber (FF) and waste regenerated tire rubber (RR) to investigate the effect of mixing ratio, coupling agent (maleated polyethylene, MAPE) and molding process (injection and compression molding) on the properties of hybrid composites. In particular, a complete set of characterization was performed including thermal stability, phase morphology and mechanical properties in terms of tension, flexion and impact, as well as hardness and density. Adding 40 wt.% of flax fibers (FF) increased the tensile (17%) and flexural (15%) modulus of rHDPE, while the impact strength decreased by 58%. Substitution of FF by waste rubber particles improved by 75% the impact strength due to the elasticity and energy absorption of the rubber phase. The effects of impact modification were more pronounced for rHDPE/(FF/RR) compatibilized with MAPE (10 wt.%) due to highly improved interfacial adhesion and compatibility. The results also suggest that, for a fixed hybrid composition (FF/RR, 25/55 wt.%), the injection molded composites have a more homogenous morphology with a uniform distribution of well embedded reinforcements in the matrix. This better morphology produced higher tensile strain at break (12%) and impact strength (9%) compared to compression molded samples.

Oleylamine functionalization of boron nitride nano-platelets for Polyamide-6 thermally conductive injection moulded composites

Boron nitride nano-platelets have been functionalized with oleylamine using a simple method without ultrasonication. The functionalization is effective with the formation of a coordination bonds between the boron nitride nano-platelets and the oleylamine molecules. The oleylamine functionalization allows for the production of 25% by weight loaded polyamide 6 – boron nitride composites having higher mechanical properties (15% tensile strength increase) and better through-plane thermal conductivity (0.85 to 0.99 W/m·K) with respect to the untreated samples. A better particle-matrix interaction is confirmed by SEM imaging and rheological analysis. The injection moulding process determines a filler orientation which influences the thermal conductivity.

Comparison of the accuracy of analytical models for basalt fiber–reinforced poly(lactic acid) composites prepared by injection molding and fused filament fabrication

We compared the accuracy of analytical models for short fiber–reinforced composites prepared by injection molding and fused filament fabrication (FFF). The microstructural features define the strength of the composites, and they are greatly dependent on the processing conditions. We collected data on fiber length, orientation, and porosity via X-ray micro-computed tomography (µ-CT) and determined the critical fiber length experimentally. We used this data as input for the modified rule of mixtures and the modeling framework based on the Halpin–Tsai method, and found that the cumulative error for FFF is more than twice that for injection-molded composites. We also showed that experimentally determined matrix strength for FFF gives a lower strength limit which is applicable for engineering parts. We presented a new approach for the modeling of the tensile strength of neat FFF products, in which the printed structure is divided into contact zones and bulk material zones. The matrix strength calculated this way was found to approximate the experimental results with an error of 5%.

Study on the Properties of Chemical Foaming Injection Molded Polyolefin/Chitosan Composites

Chitosan is a biodegradable material with good biocompatibility. It can be used in medicine, foodstuff, the chemical industry and heavy metal adsorption. In this study, an exothermic foaming agent (Azodicarbonamide) injection molded was added to polypropylene (PP), maleic anhydride (MA) grafted PP (PPgMA) and Chitosan composites. MA served as a compatibilizer due to the poor bonding between PP and chitosan. This study investigated the effects of the modifier and chitosan loading on the tensile strength, thermal properties and morphology in chemical foam injection-molded PP and PPgMA composites. The results showed that the tensile strength decreased with the addition of chitosan, but Young’s modulus increased with the added chitosan loading. The enhancement was significant for foam injection molding. The cell size decreased and the cell density increased with the addition of chitosan for the PP/PPgMA composites. The thermogravimetric analysis (TGA) results showed that the thermal degradation could be decreased with the addition of chitosan in both the PP and PPgMA composites. The use of foamed chitosan composites will be further investigated in the removal of heavy metal in waste water.

Innovative Injection Molding Process for the Fabrication of Woven Fabric Reinforced Thermoplastic Composites.

Woven fabric reinforced thermoplastic composites have been gaining significant attention as a lightweight alternative to metal in various industrial fields owing to their high stiffness and strength. Conventional manufacturing processes of woven fabric reinforced thermoplastic composites can be divided into two steps: first, the manufacturing of intermediate material, known as prepreg; then, the formation of the final products from the prepregs. This two-step process increases the manufacturing cost and time of the final composite products. This study demonstrated that woven fabric reinforced thermoplastic composites could be fabricated by an innovative injection molding process instead of the two-step process. A structure placing an extra mesh, which is a new and key component, on the mold-side of woven fabric was devised so that the thermoplastic matrix could be impregnated up to the surface of the woven fabric during injection molding. Tensile tests were performed in the direction parallel to the yarns of the fabric on the injection-molded composites to confirm their mechanical properties. As a result, it was possible to fabricate woven fabric reinforced thermoplastic composites with increased mechanical properties using injection molding without prepreg, and the composites could be molded with a much shorter cycle time than the conventional process, such as thermoforming or over-molding process.

The Role of Fibre Length on the Fatigue Failure of Injection-Moulded Composites at Elevated Temperatures under a Range of Axial Loading Conditions

The effect of fibre length distribution on the fatigue behaviour of an injection-moulded PA66 carbon fibre composite is investigated. Two materials, short carbon fibre with a mean length of 100 microns, and long carbon fibre with a mean length of 580 microns, are subjected to fully reversed fatigue loading at room temperature and three stress ratios at 120 °C. The fatigue results are compared, and fracture surfaces are analyzed to determine the differing failure modes between the materials and loading conditions. At 120 °C, the fibre length has a significant effect on the fatigue behaviour with order of magnitudes of different fatigue life for a given stress amplitude during tensile fatigue loading. Under tensile loading, fatigue failure initates as fibre matrix debonding with pits present due to end effects in the short carbon fibre material. Under compression–compression loading, the fatigue life is matrix-dominated and should be treated as a maximum stress failure. Under this loading, a smooth crack propagates across the sample with buckling as the final failure mode.

Tensile and water absorption properties of notched glass fiber/polypropylene and glass fiber/polyoxymethylene injection-molded composites

In this work, the tensile properties and hydrothermal aging behavior of notched glass fiber/polypropylene (GF/PP) and glass fiber/polyoxymethylene (GF/POM) prepared by injection molding were investigated. It is shown that the notch strength ratio (NSR) of GF/PP was higher than that of GF/POM with the same fiber mass fraction. With the increase of the fiber mass fraction, the NSR decreased. After 30 times cyclic loading–unloading tensile process, the decrease rate of tensile strength was not evident at 55% level cyclic loading, whereas the decrease rate was over 15% when cyclic loading reached 85% level. The specimens were exposed to 80 °C hot water to investigate the hydrothermal aging behavior. For the water absorption properties, the weight gain and weight loss of GF/PP barely changed after 1600 h. The weight of GF/POM increased within 25 h, and then reached equilibrium. After 1600 h, the weight loss of GF/POM was below 0.4%. The effect of immersion time on the tensile strength of notched GF/POM was noticeable. The decrease rate of tensile strength of GF/POM reached 21% after 1600 h, owing to the hydrolysis of matrix and the weakened interface between matrix and glass fibers.

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.