Tilting separation analysis of bottom-up mask projection stereolithography based on cohesive zone model

Compared with top-down stereolithography, bottom-up mask projection stereolithography can reduce the start filling volume of vat and is able to build components with high-viscosity materials. For general photosensitive materials, a separation process is required to detach the cured layer from the resin vat surface in order to accomplish the fabrication of current layer. The separation process can be achieved without damaging the part by utilizing appropriate platform motions including pulling-up, tilting and shearing, and covering inert film on the vat surface. The tilting separation is used in both industrial and academic area. However, there is a limited corresponding study compared with pulling-up separation. The mechanism of tilting separation and its effects on separation force and fabrication process are not clear. In this paper, an analytical model based on cohesive zone model was formed and a specialized experimental system was built. Experimental studies on the tilting effects on cohesive stiffness and fracture energy were conducted by collecting and analyzing separation force data. The results showed that changing exposure area function or the part fabrication orientation changed the cohesive stiffness, and increasing tilting separation velocity caused different increase in fracture energy when using different inert films. The results of this investigation can be used to choose the reasonable platform motion and process parameters by considering the part geometry and the characteristics of both inert film and materials

Design considerations for eco-friendly palm-strand reinforced concrete for low-cost housing

This research was sponsored by the overseas PhD scholarship scheme of the Nigerian Petroleum Technology Development Fund (PTDF). The authors also wish to thank the University of Exeter for providing the open access fund to cover the publication cost of this research.Peer reviewe

Design considerations for eco-friendly palm-strand reinforced concrete for low-cost housing

This is the final version. Available from Elsevier via the DOI in this record. Recent campaigns towards reducing the housing deficits and enhancing environmental sustainability in developing countries have led to increasing research efforts towards incorporating abundant natural materials into housing construction. One of such materials is oil palm broom fibres (OPBF) which began to attract research attention only recently for having the potential of being used as longitudinal reinforcement for concrete beams when combined as strands. This study provides some practical considerations and guidance for the design of OPBF-strand reinforced concrete using the flexural behaviour curves of 100 × 100 x 500 mm palm strand – reinforced prisms obtained from experimental investigation and parametric studies using finite element modelling. The study recommends the use of allowable stress design methodology for OPBF-strand reinforced concrete. A comparison of the carbon footprint between the OPBF- strand reinforced concrete beam and an equivalent steel reinforced concrete beam shows that the former could provide cheaper and eco-friendlier building material.Nigerian Petroleum Technology Development Fund (PTDF

Investigation of an Optically Created Dead Zone by Low-One Photon Polymerization for Silicone Stereolithography

Silicones have a variety of applications in many fields because of their unique properties such as bio-applicable, corrosion resistive, mechanically elastic and tough, and stable under a high temperature condition. 3D printing techniques have been actively studied freedom to fabricate complex geometries. Vat photopolymerization (VP) provides a high printing resolution, better mechanical isotropy, minimal structure defects, and fine surface finish. This study evaluated the printability of the silicone photopolymer with digital light processing. The results showed a good dimensional accuracy and better mechanical isotropy. However, it was found that the strong adhesion between the cured polymer and the bottom surface of a vat. The separation force to overcome the adhesion caused a slow printing speed and printing failures. To eliminate the separation force, an optical method to create a gap between the interface was investigated. Using a low one-photon polymerization (LOPP), a limited curing at the focal spot was successfully demonstrated. Three different wavelengths with different absorbance rates were selected and tested under the stationary and moving exposure conditions. the ultra-low absorbance wavelength showed a higher printing resolution and lower geometrical variation. With a small error, the data from the stationary exposure condition converted to the parameter for the moving exposure condition. However, it also found that the ultra-low absorbance wavelength required extremely high irradiance to compensate the time loss from its ultra-low absorbance. The time could not be linearly scaled by power due to the non-steady state polymerization kinetics

3D Printing of Functional Materials: Surface Technology and Structural Optimization

There has been a surge in interest of 3D printing technology in the recent 5 years with respect to the equipment and materials, because this technology allows one to create sophisticated and customized parts in a manner that is more efficient regarding both material and time consumption. However, 3D printing has not yet become a mainstream technology within the established manufacturing routes. One primary factor accounting for this slow progress is the lack of a broad variety of 3D printable materials, resulting in limited functions of 3D printed parts. To bridge this gap, I present an integrated strategy to fabricate a variety of functional materials/devices through the post-printing surface modification and target-motivated structural topology. A reusable 3D printed filter was first demonstrated to remove metal ions from water. This filter was functionalized with a layer of bio-adsorbent grown on its surface using post-printing modification, and the capacity was improved through structural optimization. To further improve the working efficiency, a customized 3D all-in-one printable material system was employed, which uses only one 3D printing material, but can realize various functionalities through a post-printing process. This material system is applicable for all types of photo-polymerization based 3D printing routes, including DLP, SLA, polyjet and other emerging technologies. It has significantly extended the capacity of current 3D printing technology. The 3D printed structures were converted into useful devices with new functions or new structural metamaterials with novel properties, that are attributed to both their materials composition and structural design. For example, we have showcased the magnetically manipulated robot, strength-enhanced lattice materials with high effective strength, ultralight metal materials and mechanical-metamaterials. In this thesis, a new generation of initiator-integrated material system was also developed. Beyond being able to successfully 3D print functional devices/materials with desirable properties, I also demonstrated that this initiator-laden material can be utilized to locally repair the surface damage, allowing a self-healing ability. In general, the developed 3D printing process that incorporates surface modification and structural topology enables a new class of functional devices/materials to be produced, and opens a door for further research and development of an increasing variety of 3D printing applications. Through the work presented in this dissertation, I substantially build upon and further establish the strategy and material system for 3D printing functional devices/materials, keeping in mind components, design, engineering and application

Continuous Liquid Interface Production (CLIP) for the Fabrication of Porous Architected Structures

Porous structures have long been investigated for advanced material properties however conventional fabrication methods do not have the necessary specificity to dictate void volume size, shape and distribution. Additive manufacturing (AM), or more commonly 3D printing, is a rapidly growing field in which material is selectively deposited in a layer wise manner as instructed by a computer-aided design (CAD). Therefore, AM has been seen as an attractive route for fabrication of porous structures. While many AM platforms have been investigated, the overall disadvantage with these processes has been the layer wise assembly method, which yields mechanically weak parts. Additionally, many methods impart an unintentional porosity on the resulting structure that deviates from CAD and aids in the mechanical failure mechanisms. This work sought to investigate and apply a novel AM platform, continuous liquid interface production (CLIP), to the fabrication of porous architected structures. The platform utilizes photopolymerization to reconstruct the CAD in a continuous manner. The platform was investigated for porous structure compatibility through assessment of the fabrication mechanism. It was found that CLIP structures fabricated continuously were layerless, addressing one of the key disadvantages with other platforms. The resolution of CLIP was preliminaryily explored and several contributing factors were identified. The resolution of the CLIP platform was investigated for the fabrication of porous architected structures. Void volumes in the hundreds of microns regime was explored through the fabrication of microlattices. Structures were systematically varied by unit cell type, size, orientation, resin formulation, and CLIP fabrication parameters. The resulting mechanical and physical property space was investigated. The lessons of the importance of low viscosity resin and exposure were carried forward. The tens of micron size range was explored through the fabrication of chromatography columns containg ordered internal architectures with CLIP. Functional resins to enable different separation mechanisms were developed and optimized for CLIP. The pore size of the internal architecture of the column was systematically reduced. The computational limit for CAD generation of complex structures was found and an angular hexagonally packed unit cell designed to facilitate computation as well as mimic monolithic column flow profiles was developed. Columns assembled with CLIP fabricated external housings were assessed for stability. Methods to circumvent computational constraints were developed to allow direct exposure of the light source which enabled the fabrication of smaller pore sizes approaching the theoretical limit of resolution.Doctor of Philosoph

Optimising laser stereolithography for constructing electronic packages onto novel substrates

Additive manufacturing (AM) has matured from its initial concept as a prototyping technique to an established industrial manufacturing process for which compliance with relevant standards is required. Here, we investigate inserting components into geometries constructed directly onto non-standard substrates using stereolithography (SLA), for the purpose of electronics packaging. Silicon nitride is of particular interest as the interfacial material when packaging semiconductors and technologies such as silicon interconnect fabric. Compared to conventional encapsulation processes, SLA avoids elevated temperatures and stresses while permitting much greater flexibility to arrange components in three dimensions. This can enable an increased feature density, optimised packages for confined spaces, functional packaging to complement the operation of the device, and enables rapid production for bespoke applications. One of the key challenges is the ability to bond the product to the substrate sufficiently to adhere to the industry standard. Additionally, interactions between the SLA process, the photopolymer and the substrate can result in distortion and compromise the ability to deliver products to the required tolerances. To support this initiative, relevant literature has been reviewed to determine current knowledge and the gaps to be filled through further investigation. From doing so, active adhesion mechanisms were identified and methods to enhance them explored. Moreover, novel experimental processes had to be developed to produce suitable test samples. Characterising the substrate and photopolymer materials allowed potential changes in properties during the curing process to be determined, and comparison with conventional adhesion models. Furthermore, the shear stress generated from shrinkage during post build curing (PBC) has been measured to make a significant contribution to the stress at separation on untreated silicon nitride. The investigation concluded that the application of a TMSPMA monolayer to the plasma treated substrate, combined with PBC, substantially increases the strength of adhesion to an extent compliant with the industry standard and above the cohesive strength of the polymer. In addition to adhesion, the influences of process parameters are analysed, and their potential to distort the beam and the resulting product. Distorting effects investigated include divergence, ellipticity, refraction, reflectance, over-exposure, and low intensity noise present in the beam. These have been modelled with consideration given to the influence of superposition and the machine architecture. A non-linear relationship between distorting effects and the dimensions of the build area is identified which has implications for the scalability of production. This allows the extent to which the build area can be increased, until the combined influence of these distorting effects compromises the ability to meet manufacturing tolerances, to be determined. Further modelling supported by purpose designed experimentation, revealed the potential for significant distortions from refraction and reflectance. If sufficiently energetic, reflections from the substrate can produce spurious curing, and distort the product. Modelling the limits of exposure at which the onset of spurious curing and distortion occurs, allows an operating window to be obtained, within which the construction process can be optimised. This allows for significant savings in construction time with a reduction of 26% demonstrated. Methods to mitigate distortion by optimising the beam’s focal point, the design of parabolic mirror profile, anti-reflective coatings, modulating the laser power, and the potential to modify the photopolymer, are reported. Lastly, by building on prior work, a process to insert multiple large and complex geometries into the SLA build process, and to connect the installed components electrically for the construction of 3D electronic packages, is demonstrated. It is concluded, by using the processes described, electronic packages can be constructed directly onto silicon nitride using SLA and meet the required standard of adhesion. Moreover, by applying the tools developed, the process can be optimised for scale and time, while complying with the necessary manufacturing tolerances

Optimising laser stereolithography for constructing electronic packages onto novel substrates

Additive manufacturing (AM) has matured from its initial concept as a prototyping technique to an established industrial manufacturing process for which compliance with relevant standards is required. Here, we investigate inserting components into geometries constructed directly onto non-standard substrates using stereolithography (SLA), for the purpose of electronics packaging. Silicon nitride is of particular interest as the interfacial material when packaging semiconductors and technologies such as silicon interconnect fabric. Compared to conventional encapsulation processes, SLA avoids elevated temperatures and stresses while permitting much greater flexibility to arrange components in three dimensions. This can enable an increased feature density, optimised packages for confined spaces, functional packaging to complement the operation of the device, and enables rapid production for bespoke applications. One of the key challenges is the ability to bond the product to the substrate sufficiently to adhere to the industry standard. Additionally, interactions between the SLA process, the photopolymer and the substrate can result in distortion and compromise the ability to deliver products to the required tolerances. To support this initiative, relevant literature has been reviewed to determine current knowledge and the gaps to be filled through further investigation. From doing so, active adhesion mechanisms were identified and methods to enhance them explored. Moreover, novel experimental processes had to be developed to produce suitable test samples. Characterising the substrate and photopolymer materials allowed potential changes in properties during the curing process to be determined, and comparison with conventional adhesion models. Furthermore, the shear stress generated from shrinkage during post build curing (PBC) has been measured to make a significant contribution to the stress at separation on untreated silicon nitride. The investigation concluded that the application of a TMSPMA monolayer to the plasma treated substrate, combined with PBC, substantially increases the strength of adhesion to an extent compliant with the industry standard and above the cohesive strength of the polymer. In addition to adhesion, the influences of process parameters are analysed, and their potential to distort the beam and the resulting product. Distorting effects investigated include divergence, ellipticity, refraction, reflectance, over-exposure, and low intensity noise present in the beam. These have been modelled with consideration given to the influence of superposition and the machine architecture. A non-linear relationship between distorting effects and the dimensions of the build area is identified which has implications for the scalability of production. This allows the extent to which the build area can be increased, until the combined influence of these distorting effects compromises the ability to meet manufacturing tolerances, to be determined. Further modelling supported by purpose designed experimentation, revealed the potential for significant distortions from refraction and reflectance. If sufficiently energetic, reflections from the substrate can produce spurious curing, and distort the product. Modelling the limits of exposure at which the onset of spurious curing and distortion occurs, allows an operating window to be obtained, within which the construction process can be optimised. This allows for significant savings in construction time with a reduction of 26% demonstrated. Methods to mitigate distortion by optimising the beam’s focal point, the design of parabolic mirror profile, anti-reflective coatings, modulating the laser power, and the potential to modify the photopolymer, are reported. Lastly, by building on prior work, a process to insert multiple large and complex geometries into the SLA build process, and to connect the installed components electrically for the construction of 3D electronic packages, is demonstrated. It is concluded, by using the processes described, electronic packages can be constructed directly onto silicon nitride using SLA and meet the required standard of adhesion. Moreover, by applying the tools developed, the process can be optimised for scale and time, while complying with the necessary manufacturing tolerances