Search

Fused Deposition Modelling (FDM) is one of the Additive Manufacturing (AM) techniques that has been gaining traction due to its ability to print complex 3D shaped geometries and simplicity in manufacturing of the part. Despite its advantages, among thermoplastics polymers, amorphous polymers are predominantly used as feedstocks due to the challenges faced with semi-crystalline polymers. Due to the crystallisation kinetics in semi-crystalline polymers are highly temperature dependant and can be influenced by the FDM processing parameters, semi-crystalline polymers are challenging to print via FDM process. This study focuses on developing a simulation model to investigate the effect of process parameters on crystallisation and resulting part distortion in semi-crystalline polymer parts. Although studies have been performed in the past, there is a lack of simulation models available in the literature that can predict part distortion with respect to various FDM processing parameters in semi-crystalline polymers by taking crystallisation kinetics into account. In this thesis, processing parameters such as ambient temperature, print bed temperature, nozzle speed, raster pattern, layer thickness, overall height of the model/sample, and FEA boundary condition assumption of layer bonding are investigated. The warpage results predicted by the novel simulation model developed using the modified crystallisation physics in this study has displayed an overall accuracy of 82% on validating with the experimental warpage data.

Large hollow seamless and essentially stress-free plastic products of various size is the hallmark of the rotational moulding process. Relatively longer cooling time and warpage problems insisted researchers develop simulation models for optimisation of the polymer process to achieve better productivity. The work aimed to develop process simulation, and simulation models for direct mould heating and shrinkage-warpage. It was discovered from the literature survey that simulation works were scarce, and no simulation models existed for direct mould heating and prediction of warpage in rotomoulding. Main components of rotational moulding process simulation for modern time requires simulation models of direct mould heating and the cooling accounting shrinkage-warpage phenomenon. As a result, the simulation model work was split into three components: complete rotomoulding cycle (process) simulation, direct mould heating and shrinkage-warpage models. Development of shrinkage-warpage simulation model forms a major piece of work. It was identified that Discrete Element Modelling (DEM) has the potential for process simulation with further work. All the simulation models were developed in FE based Abaqus software. For the first time, developed simulation model of direct electric mould heating showed controlled heating could be achieved, peak internal air temperature of 223 °C matches closely with 225°C of experiment value. A novel thermal expansion coefficient model was built for warpage simulation modelling for the first time in rotational moulding. The main conclusions of simulation models are that the cooling rate has a significant effect on warpage magnitude and results are in good agreement with experimental values of literature, and in-mould pressure of 10 kPa controlled the warpage growth by 75% for a given part. The external cooling rate was found to be directly proportional to warpage growth in parts, uniform cooling of mould controls the temperature gradient in the polymer part. Temperature dependent polymer properties were readily not available and were derived from literature data. Density, expansion coefficient and Young`s modulus values are significantly important for warpage prediction as determined by sensitivity simulation investigation. Industrial application of the shrinkage-warpage model on fuel tank mould showed qualitative agreement with experimental rotomoulding data. Successfully developed simulation models for direct mould heating and warpage prediction can be exploited for optimization of the process, and feedback for mould designer and moulder. Further work on DEM, measurement of polymer properties, and cohesive parameters, different forms of pressure and more validation work helps in development of advanced rotomoulding process simulation model.

The primary reason for incorporating pigments into plastic materials is to impart the desired colour to finished articles. Some pigments however, may interact with the polymer leading to unexpected deleterious effects. Organic pigments, especially phthalocyanines, are favourable for their brilliant shade but are also well known for causing part distortion or warpage. This causes problems in parts which require good dimensional stability such as crates, containers, trays, caps and closures. Despite that, there are not many published studies on the root cause and mechanism of warpage induced by the pigment. Hence, the objective of this research is to study the influence of such pigments on the dimensional stability, crystallisation behaviour and morphology of polyethylenes in order to have a better understanding on the mechanism of warpage, which could possibly lead to a solution in overcoming this problem.

Solid-state electrolytes play a pivotal role in advancing lithium-ion battery technologies by providing a stable and secure medium for ion transport within batteries. Unlike conventional liquid electrolytes, solid-state alternatives enhance the mechanical integrity of battery devices, preventing issues like leakage and ensuring longevity. Additionally, these electrolytes facilitate the design of thinner and more lightweight batteries, addressing the demand for portable technology solutions. There are many solid electrolyte designs and processes, however, with inconsistent results, along with many different setups that lead to varying conclusions. In this research, experiments were conducted to study the furnace sintering process of solid electrolytes. The samples were sandwiched between two pieces of high temperature plates to provide a compression force. This process is sought to solve the issue of warpages when manufacturing solid-state electrolytes. The results indicate that while fused quartz glass cannot effectively decrease the warpage, other materials were successful in this development. The study explores the effects of compression, sintering temperature, and heating and cooling rate on warpage of furnace sintered solid electrolyte discs.

Abstract: Polyolefin thermoplastics form a significant fraction of the world scale production of plastics and are of high interest due to their toughness, low cost, good chemical resistance and excellent IR properties. Two grades of PE: Linear Low-Density Polyethylene (LLDPE) and High-Density Polyethylene (HDPE). LLDPE and HDPE have a huge number of applications in a diverse range of areas such as radiative passive heating and cooling, infrared (IR) and visible camouflage, additive manufacturing, defense sector and many more. However, LLDPE faces the challenge of being visibly semi-transparent, while HDPE exhibits severe warpage during 3D printing. This dissertation aims to address these challenges and enhance the performance of PE for expanded applications in IR camouflage and radiative passive heating and cooling applications. Chapter one focuses on resolving the semi-transparency issue of LLDPE by pigmenting the material using a simple and easier compounding method while making sure to maintain its highly desirable IR transparency. A comprehensive FTIR, UV-VIS, mechanical testing, and IR analysis is implemented to confirm the successful achievement of a visibly opaque LLDPE material without compromising its exceptional IR transparency. This breakthrough enables the utilization of LLDPE in various applications, including radiative passive heating and cooling, IR and visible camouflage, and IR shielding applications. Chapter two addresses the warpage challenges experienced during the 3D printing of HDPE. Incorporation of 10 wt% LLDPE into HDPE helps reduce crystallinity and mitigate stresses during 3D printing, by optimizing printing parameters and exploring different print bed options. Through this approach, this research provides a successful solution to provide significant reduction in warpage for 3D printing of HDPE leading to its expansion of potential applications in additive manufacturing. Overall, this thesis dissertation highlights the excellence of PE as a material, the challenges it faces and addresses those challenges. Through custom compounding of PE, we have successfully produced PE based metamaterials which have high visible opacity, are available in diverse range of colors and still maintain their high IR transparency properties. HDPE based grades of PE can be successfully 3D printed which pave the way for 3D printing functionality of these metamaterials which can further be drawn into fibers to make fabrics for civilian and military camouflage and radiative heating/cooling applications.

Adding fillers and reducing crystallinity are the commonly adopted strategies to overcome warpage and poor first layer bed adhesion which make 3D-printing (3DP) of polypropylene (PP) challenging. The primary goal of this study was to investigate 3DP of homopolymer isotactic PP (iPP) without sacrificing mechanical properties and processability. This work included three polymer systems for 3DP, and three bed materials to address the challenges of warpage and first layer adhesion. The first polymer system considered the effect of 1,2,3-trideoxy-4,6:5,7-bis-O-[(4-propylphenyl) methylene]-nonitol (TBPMN), an α-nucleating agent, dispersed as a masterbatch in UltraClear NX8000, produced by Milliken on 3DP of iPP. The objective was to study the effects of faster and uniform crystal growth on reduction of warpage. The results indicate that faster crystallization rates enabled extrusion of filaments with lowest ovality while the increase in storage modulus caused by fibrillar network formation resulted in poor interlayer adhesion during 3DP. In the second part, the effect of complex formation between NX8000 and a nanofiller, tri-silanol phenyl POSS (Tri-POSS) on 3DP of iPP was investigated. It was hypothesized that lower melt viscosity afforded by the complex could aid in 3D-printing, thus retaining the mechanical properties of iPP. Although a significant reduction in viscosity was found at lower shear rates in the case of 0.5 MFI iPP, the viscosity advantage disappeared at higher shear rates, especially in the regime of interesting processing flows. Also, high normal stresses resulted in shape instabilities thus narrowing the 3DP processing window. Some improvements in surface qualities were observed with the complex, but all the 3DP parts showed warpage. A similar trend of melt viscosity reduction was found for 2.7 MFI iPP. However, no shape instabilities were observed. Rheological measurements using rotational rheometer revealed that the complex between Tri-POSS and the nucleating agent became unstable in this grade of iPP, and the fibrillar networks of the nucleating agent formed easily. Interlayer delamination was observed during 3DP of 2.7 MFI iPP compound containing the complex.The third part of the project investigated the effects of only Tri-POSS on 3DP of iPP. The viscosity values evaluated using capillary rheometer revealed a reduction compared to iPP. Poor dispersion of Tri-POSS led to a lower magnitude of viscosity reduction compared to the complex. The 3D-printability of 2.7 MFI iPP with and without Tri-POSS was investigated on bed materials having similar surface energy as iPP. The printability window was narrow when styrene-ethylene-butylene-styrene (SEBS), and 0.5 MFI iPP were used as the bed materials. Interlayer delamination attributed to fibrillar network formation was exploited by adding NX8000 to the 0.5 MFI bed. At commercially feasible 3DP speeds, such as 30 mm/s, 2.7 MFI iPP material showed 18% lower tensile modulus and 10% lower tensile strength compared to virgin compression molded iPP specimens. The addition of Tri-POSS helped retain the tensile modulus values close to iPP, but a similar reduction in tensile strength was observed.

Recent demonstrations with fused filament fabrication (FFF) 3D printing have shown to produce prototypes as well as production components. Additionally, due to the FFF process platforms being low-cost and readily available there has been a high-demand to produce on-demand parts for various applications in automotive, in-space manufacturing and electronic industries. However, current limitations such as limited availability of advanced composites materials, and guidelines for design-for-manufacturing make the process prone to trial-and-error experiments both at the materials development, product design and manufacturing stage. In this work, new thermomechanical process simulations platform, Digimat-AM has been evaluated to address and demonstrate digital design and manufacturing of FFF process by performing simulation and experiments. With the use of Acrylonitrile butadiene (ABS) material and soyhull fibers reinforced ABS composite (ABS-SFRC) as a basis, an L9 Taguchi design-of-experiment (DOE) was setup by varying key process input parameters for FFF 3D printing such as layer thickness, melt temperature and extrusion multiplier were varied for three levels. A total of 9 DOE simulations and experiments were performed to compare part properties such as dimensions, warpage, and print time were analysed. Additionally, ANOVA analysis was performed to identify the optimum and the worst conditions for printing and correlate them with their effect on the mechanical properties of the printed samples. Furthermore, from the simulation results, a reverse warpage geometry, 3D model was generated that factors for part warpage, shrinkage, or other defects to enable 3D printing parts to design dimensions. Subsequently, using the generated reversed warpage geometry was used to perform 3D printed experiments and analyzed for part dimensions and defects. As a case study, a functional prototype [Two different geometries] was designed and simulated on Digimat-AM and using the above guide, 3D printing was performed to obtain part to specific dimensions. In addition to that, the thermomechanical properties of ABS-SFRC were needed to perform the Digimat simulation of geometries printed with ABS-SFRC. However, the materials property database of ABS-SFRC is very limited and experimental measurements can be expensive and time consuming. This work investigates models that can predict soyhull fibers reinforced polymer material composite properties that are required as input parameters for simulation using the Digimat process design platform for fused filament fabrication. ABS-SFRC filaments were made from 90%ABS 10% soyhull fibers feedstock using pilot scale filament extrusion system. Density, specific heat, thermal conductivity, and Young's modulus were calculated using models. The modeled material properties were used to conduct simulations to understand material-processing-geometry interactions.

Flatness of wood composite panels, such as Laminated Veneer Lumber, is often difficult to control during the manufacturing process. Out-of-plane deformation, or warpage, of wood veneers caused by changes in moisture content affects the ability to press flat panels. To understand wood panel warpage, experimental methods are developed to create and measure moisture-induced deformation of wood veneers on five species of various thicknesses. Three moisture induction methods are investigated and evaluated to determine the increase in moisture content. Experiments are developed to produce moisture gradients of two concentrations in the veneers to examine the effect on warpage behavior. Additionally, the surface area of applied moisture and veneer thickness is also investigated. Three-dimensional scanning technology is used to measure warpage of veneers. A procedure using a structured-light scanner is developed to analyze the surface curvatures to observe the effect of moisture-induced warpage. After moisture-induction treatment of the veneer, surface deformation data is measured using the scanner and the data is converted into a 3D solid body model that is used for curvature comb analysis. The results show that curvature comb analysis can be used to analyze the geometry of moisture-induce warpage. The method can be used to analyze the effect of moisture gradient variables on warpage behavior including concentration, veneer thickness, and surface area. The experimental methods developed can be used by future researchers to validate theoretical warpage prediction models.

This work has been concerned with the dimensional stability and the structure-property relationships of thermoplastics reinforced with particulate and fiber fillers. The first part of this study was concerned with ascertaining the main causes of warpage observed for injection-molded thermoplastics reinforced with high aspect ratio fibers. Typically, warpage in injection-molded fiber reinforced thermoplastics is primarily attributed to residual thermal stresses associated with shrinkage and thermal contraction of the parts. Therefore, it is assumed that flow-induced stresses generated during mold filling do not play a significant role in the warpage. The warpage of PP composites reinforced with TLCP fibers was found to increase with an increase in fiber loading. The shrinkage and the thermal expansion of the TLCP/PP composites and, hence, the thermally induced stresses decreased with an increase in fiber loading while the flow-induced stresses increased. The increase in the flow-induced stresses was attributed to an inhibition of stress relaxation and greater generation of orientation of the polymer chains with an increase in fiber loading. Therefore, it was found that in order to accurately predict the warpage of fiber reinforced thermoplastics, the flow-induced residual stresses must be accounted for. The second part of this work was concerned with minimizing the particle loading of reinforced PC/PBT composites while maintaining the stiffness, i.e. modulus, and the dimensional stability of injection molded flat panels. This was accomplished by using high aspect ratio (≈100-150) nano-clays as opposed to micron-size talc (≈5-10). It was found that by using nano-clays surface modified with a quaternary ammonium salt that contained two hydroxyl groups as opposed to fine talc particles, the level of particle reinforcement could be reduced from 6 to 1 wt% without sacrificing the modulus of the reinforced PC/PBT composites. Further benefits included a 26% increase in flexural strength, 77% increase in the tensile toughness and 3% reduction in the density of the reinforced PC/PBT composites. An increase in the modulus and tensile toughness was observed even though there was evidence of loss in molecular weight of the PC/PBT matrix, which was supported by the rheological behavior of the composites.

This work discusses the deformations of various unsymmetric composite panels due to thermal and mechanical loads. Chapter 2 focuses on the warpage of large unsymmetric curved composite panels due manufacturing anomalies. These panels are subjected to a temperature change of -280°F to simulate the cooling from the autoclave cure temperature. Sixteen layer quasi-isotropic, axial-stiff, and circumferentially-stiff laminates are considered. These panels are intended to be symmetric laminates, but are slightly unsymmetric due to the manufacturing anomalies. Rayleigh-Ritz and finite-element models are developed to predict the deformations. Initially, to serve as a basis for comparison, warpage effects due to orthotropic thermal expansion properties in perfect panels are investigated and are found to produce deformations not captured in two-dimensional theories. This is followed by the investigation of the effects of ply misalignments. Ply misalignments of 5° are incorporated into the laminate, one layer at a time, to produce unsymmetric laminates. It is found that ply misalignments produce warpages much larger than those induced by orthotropic thermal expansion properties. Next, unsymmetric laminates resulting from ply thickness variations are investigated. Layers 10% thicker than nominal are incorporated into the laminate, one layer at a time, while the remaining layers are of uniform thickness. Due to the change in fiber volume fraction of the thicker layers, corresponding material properties are modified to reflect this change. The results show that ply thickness variations cause warpages of about 25-50% of those induced by ply misalignments. Finally, warpage of panels due to nonuniform cooling due to inplane thermal gradients during cure is investigated. A thermal gradient of 0.1°F/in. is used to construct six inplane distributions. It is found that the warpages induced by thermal gradients are very small. The warpages are negligible with respect to those induced by ply thickness variations or ply misalignments. Deformations induced by thermal gradients depend primarily on the magnitude of the thermal gradient, but not on the pattern of distribution. Overall, ply misalignments cause the most warpage, followed by ply thickness variations. Important variables for these imperfections are, the through-thickness location of the imperfections, the orientation of the layer containing the imperfections, and the lamination sequence. All cases show that geometric nonlinearities are important to accurately predict the deformations induced by these imperfections. Chapter 3 discusses the deformations of composite plates that are intentionally fabricated to be unsymmetric. Such plates, if flat, might be considered in applications where bending-stretching coupling effects can be used to advantage. It is assumed the laminates are cured at an elevated temperature and then cooled 280°F. Significant deformations result because of the high level of asymmetry in the laminate construction. Accordingly, geometric nonlinearities are included in the models. Four cross-ply laminates and three angle-ply laminates are considered. Four-term and 14-term Rayleigh-Ritz models are developed, together with finite-element models to model the deformations. Actual specimens were constructed and the deformations measured to compare with predictions. The results show that agreement between predictions and the experimental results are good. The 14-term Rayleigh-Ritz model is found to be the most useful due to its ability to find multiple solutions, its physical basis, and computational efficiency. Chapter 4 discusses the deformations of initially flat aluminum, symmetric, and unsymmetric composite plates due to axial endshortening under various boundary conditions, the aluminum and symmetric plates serving as a baseline. Seven plates are considered, each with three boundary condition combinations, namely, clamped ends and sides (CL-CL), clamped ends with simply-supported sides (CL-SS), and simply-supported ends and sides (SS-SS). Generally, the boundary conditions play a key role in the deformation characteristics of the plates. The aluminum and symmetric cross-ply plates have no out-of-plane deformations until classic buckling, or primary instability, then each exhibits two stable solutions. Each also exhibits secondary instability that results in two stable solutions. The symmetric laminates show less of a dependence on the boundary conditions compared to the unsymmetric laminates. Unsymmetric laminates show a mixture of characteristics. Some cases exhibit primary instability, other cases do not. Some cases exhibit secondary instability, while some case do not. The unsymmetric cross-ply laminates have only one stable solution after secondary buckling, while most other laminates and boundary condition combinations have two stable solutions. It is interesting to note that for the unbalanced unsymmetric [302/90/0]2T laminate, the boundary conditions controlled the sign of the out-of-plane deflection from the onset of axial endshortening. Generally speaking, the CL-CL cases carry the most load, followed by the CL-SS, and then the SS-SS cases. Like all the problems discussed in Chapter 2 and 3, geometric nonlinearities are found to be important for this case as well.