Rapid Prototyping Journal

Hochan Kim; Jae‐Won Choi; Ryan Wicker

2010 Rapid Prototyping Journal

doi: 10.1108/13552541011049243

Purpose – To operate a multiple material stereolithography (MMSL) system, a material build schedule is required. The purpose of this paper is to describe a scheduling and process‐planning software system developed for MMSL and designed to minimize the number of material changeovers by using low‐viscosity resins that do not require sweeping. Design/methodology/approach – This paper employs the concept of using low‐viscosity resins that do not require sweeping to minimize the number of material changeovers required in MMSL fabrication. A scheduling and process‐planning software system specific to MMSL is introduced that implements four simple rules. Two rules are used to select the material to be built in the current layer, and two rules are used to determine at which layer a material changeover is required. The schedule for the material to be built depends on the material properties stored in a user‐defined materials library. The developed algorithm produces sliced loop data for each material using the predetermined layer thickness from an input CAD model, and the four rules are applied at each layer. The algorithm then determines the build order for each material, the material‐specific number of layers to be built, and whether or not sweeping is required. Output data from the program are the scheduling and process‐planning report and the partitioned computer‐aided design models to be built before changing a material according to the process planning. Two examples of the algorithm applied to multiple material parts are provided. Findings – The MMSL scheduling and process‐planning software system is developed using Microsoft Visual C++7.0. For verification, a simple demonstration is conducted on a two material part where the process plan could be easily determined through intuition. A more complex multiple material part is also tested that consisted of four subparts. Several cases of resin assignment are tested showing that the ultimate scheduling and process planning vary significantly depending on the material combinations and specifications. These examples demonstrate that the strategy, method, and software developed in this paper can be successfully applied to prepare for MMSL fabrication. Research limitations/implications – Although the software system is demonstrated on two multiple material parts, more extensive work will be performed in the future on fabricating multiple material parts using the MMSL machine. It is expected that additional rules will be developed as additional limitations of MMSL are identified. It is also anticipated that particular emphasis will be placed on building without sweeping as well as development of advanced non‐contact recoating processes. Originality/value – As designs incorporating multiple materials increase in the future and additive manufacturing (AM) technologies advance in both building out of multiple materials and fabricating production parts, the scheduling and process‐planning concepts presented here can be applied to virtually any AM technology.

Jonathan Hiller; Hod Lipson

2010 Rapid Prototyping Journal

doi: 10.1108/13552541011049252

Purpose – Digital materials are composed of many discrete voxels placed in a massively parallel layer deposition process, as opposed to continuous (analog) deposition techniques. The purpose of this paper is to explore the wide range of material properties attainable using a voxel‐based freeform fabrication process, and demonstrate in simulation the versatility of fabricating with multiple materials in this manner. Design/methodology/approach – A representative interlocking voxel geometry was selected, and a nonlinear physics simulator was implemented to perform virtual tensile tests on blocks of assembled voxels of varying materials. Surface contact between tiles, plastic deformation of the individual voxels, and varying manufacturing precision were all modeled. Findings – By varying the precision, geometry, and material of the individual voxels, continuous control over the density, elastic modulus, coefficient of thermal expansion, ductility, and failure mode of the material is obtained. Also, the effects of several hierarchical voxel “microstructures” are demonstrated, resulting in interesting properties such as negative Poisson's ratio. Research limitations/implications – This analysis is a case study of a specific voxel geometry, which is representative of 2.5D interlocking shapes but not necessarily all types of interlocking voxels. Practical implications – The results imply that digital materials can exhibit widely varying and tunable properties in a single desktop fabrication process. Originality/value – The paper explores the vast potential of tunable materials, especially using the concept of voxel microstructure, applicable primarily to 3D voxel printers but also to other multi‐material freeform fabrication processes.

Kamran Mumtaz; Neil Hopkinson

2010 Rapid Prototyping Journal

doi: 10.1108/13552541011049261

Purpose – The purpose of this paper is to investigate the selective laser melting (SLM) of Inconel 625 using pulse shape control to vary the energy distribution within a single laser pulse. It aims to discuss the effectiveness of pulse shaping, including potential benefits for use within SLM. Design/methodology/approach – Laser parameters were varied in order to identify optimal parameters that produced thin wall parts with a low surface roughness without the use of pulse shape control. Pulse shape control was then employed to provide gradual heating or a prolonged cooling effect with a variety of peak power/pulse energy combinations. Properties of pulse shaped and nonpulse shaped parts were compared, with particular attention focused on part surface roughness and width. Findings – High peak powers tended to reduce top surface roughness and reduce side roughness as recoil pressures flatten out the melt pool and inhibit melt pool instabilities from developing. Ramp up energy distribution can reduce the maximum peak power required to melt material and reduce material spatter generation during processing due to a localized preheating effect. Ramp down energy distribution prolonged melt pool solidification allowing more time for molten material to redistribute, subsequently reducing the top surface roughness of parts. However, larger melt pools and longer solidification times increased the side roughness of parts due to a possible lateral expulsion of material from the melt pool. Originality/value – This paper is the first of its kind to employ laser pulse shape control during SLM to process material from powder bed. It is a useful aid in unveiling relationships between laser energy distribution and the formation of parts.

Cheekur Krishnamurthy Srinivasa; Chinnakurli Suryanarayana Ramesh; S.K. Prabhakar

2010 Rapid Prototyping Journal

doi: 10.1108/13552541011049270

Purpose – The purpose of this paper is to study the effect of blending time, SiC content and fill ratio on the homogeneity of iron‐silicon carbide powder mixture, blended in double‐cone blender; to evaluate density, microstructure and micro hardness of laser sintered iron and iron‐SiC specimens; and study the feasibility of building a complex iron‐SiC metal matrix composite (MMC) part by direct metal laser sintering (DMLS) process. Design/methodology/approach – The morphology and particle size of iron and silicon carbide powders were evaluated. Nickel coating was carried out on silicon carbide particles. Blending of iron‐SiC powders were carried out in two phases in a double‐cone blending equipment. In the first phase, three tests were conducted with fill ratios (ratio of volume of conical blender to volume of powder mixture) of 1.68, 3.39, and 6.8 percent while iron‐SiC weight ratio was kept constant at 97:3. In the second phase, four tests were conducted with iron‐SiC weight ratios of 99:1, 98:2, 97:3, and 95:5 while keeping a constant fill ratio of 1.68 percent. In both the phases, blending was carried out for duration of 43 minutes. Homogeneity of the powder mixture was evaluated at different intervals of time by adopting sampling process. Sintering was carried out on iron and iron‐SiC powder mixture using DMLS machine at laser speed of 50, 75, 100, and 125 mm/s. Microstructure, density and micro hardness studies were carried out on the sintered specimens. A 3D model of a part with complex geometry was modeled using Unigraphics CAD/CAM software and prototype part was built by DMLS technology using the blended iron‐2 weight percent SiC powder. Findings – A reduction in blending time was observed with increase in SiC content and decrease in fill ratio. Microstructure and micro hardness tests conducted on laser sintered iron‐silicon carbide specimens, reveal the homogeneity of blended powder. The density of the iron‐SiC composites sintered at a laser speed of 50 and 75 mm/s, decreased with increase in SiC content. Further, an increase in the micro hardness of iron‐SiC composites was observed with increase in SiC content and decrease in laser speed. Complex functional part was built by DMLS technology with out any supports. Research limitations/implications – The experiments were conducted with standard blending equipment in which the speed is limited to 48 revolutions per minute only. Originality/value – Meager information is available on blending of powders for producing MMCs by laser sintering process. The work presented in this paper will be a guideline for researchers to carry out further work in blending of powders for producing MMCs by rapid prototyping process.

Kaushik Alayavalli; David L. Bourell

2010 Rapid Prototyping Journal

doi: 10.1108/13552541011049289

Purpose – The purpose of this paper is to produce electrically conductive, fluid impermeable graphite bipolar plates for a direct methanol fuel cell, using indirect selective laser sintering (SLS) and suitable post processing techniques. Design/methodology/approach – Bipolar plates are made by the indirect SLS of graphite powder and phenolic resin mixture. The phenolic resin binder is then burnt off at a high temperature in a vacuum furnace to produce a 100 per cent carbon part. This brown part is then infiltrated using a low‐viscosity (∼5‐10 cps) cyanoacrylate to seal up the open pores, rendering the plates fluid impermeable. Findings – It has been found that the electrical conductivity increases significantly (> 220 S/cm) with a corresponding increase in pyrolyzing temperature which correlates well with literature on the carbonization of phenol formaldehyde resins. The cyanoacrylate infiltrated parts tested under fluid pressure demonstrated no leakage through the plate, indicating full closure of open porosity. Originality/value – This work demonstrates the capability of the SLS process to produce working bipolar plates with complex flow field designs that can be tested to verify its efficacy in a working fuel cell, thereby saving time and cost in machining natural graphite.

Jane Chu; Sarah Engelbrecht; Gregory Graf; David W. Rosen

2010 Rapid Prototyping Journal

doi: 10.1108/13552541011049298

Purpose – The purpose of this paper is to investigate design synthesis methods for designing lattice cellular structures to achieve desired stiffnesses. More generally, to find appropriate design problem formulations and solution algorithms for searching the large, complex design spaces associated with cellular structures. Design/methodology/approach – Two optimization algorithms were tested: particle swarm optimization (PSO) and Levenburg‐Marquardt (LM), based on a least‐squares minimization formulation. Two example problems of limited complexity, specifically a two‐dimensional cantilever beam and a two‐dimensional simply‐supported plate, were investigated. Computational characteristics of the algorithms were reported for design problems with hundreds of variables. Constraints from additive manufacturing processes were incorporated to ensure that resulting designs are realizable. Findings – Both PSO and LM succeeded in searching the design spaces and finding good designs. LM is one to two orders of magnitude more efficient for this class of problems. Research limitations/implications – Three‐dimensional problems are not investigated in this paper. Practical implications – LM appears to be a viable algorithm for optimizing structures of complex geometry for minimum weight and desired stiffness. Originality/value – The testing of design synthesis methods (problem formulations and algorithms) for lattice cellular structures, and the testing of PSO and LM algorithms, are of particular value.

James M. Gibert; Eric M. Austin; Georges Fadel

2010 Rapid Prototyping Journal

doi: 10.1108/13552541011049306

Purpose – The purpose of this paper is to focus on the changing dynamics of the ultrasonic consolidation (UC) process due to changes in substrate geometry. Past research points to a limiting height to width ranging from 0.7 to 1.2 on build features. Design/methodology/approach – Resonances of a build feature due to a change in geometry are examined and then a simple non‐linear dynamic model of the UC process is constructed that examines how the geometry change may influence the overall dynamics of the process. This simple model is used to provide estimates of how substrate geometry affects the differential motion at the bonding interface and the amount of energy emitted by friction change due to build height. The trends of changes in natural frequency, differential motion, and frictional energy are compared to experimental limits on build height. Findings – The paper shows that, at the nominal build, dimensions of the feature the excitation caused by the UC approach two resonances in the feature. In addition trends in regions of changes of differential motion, force of friction, and frictional energy follow the experimental limit on build height. Originality/value – This paper explores several aspects of the UC process not currently found in the current literature: examining the modal properties of build features, and a lumped parameter dynamic model to account for the changes in of the substrate geometry.

Lino Costa; Deepak Rajput; Kathleen Lansford; Wenqiang Yue; Alexander Terekhov; William Hofmeister

2010 Rapid Prototyping Journal

doi: 10.1108/13552541011049315

Purpose – The purpose of this paper is to develop a simple, easy to implement powder delivery strategy for solid freeform fabrication (SFF) processing. Design/methodology/approach – A specially designed “tower nozzle” located at the center of the processing area dispenses the feedstock powders continuously and uniformly onto the processing area, where powders accumulate progressively as a flat powder bed. During the dispensing, powders are selectively consolidated by melting and solidification using a laser beam which was scanned in a predefined pattern using a galvo‐mirror scan head. Findings – Experiments performed with AISI H13 steel show that the tower nozzle powder delivery strategy is suitable for SFF processing. Practical implications – Both powder delivery and laser consolidation are performed simultaneously and without interruption with simple apparatus. No powder delivery scrapers or rollers are used. Originality/value – The main characteristics of a prototype tower nozzle and the typical processing conditions used to form thin wall AISI H13 steel shapes are presented.