Prototype Experiments: Strategies and Trade-offs

Throughout the whole product development process, there is always a question on whether the proposed product is “up to its task”, and often it is up to the engineer or designer to answer these questions. In many cases, this calls for experiments in form of prototype testing, to explore, verify, and validate the product performance. This paper connects the overall approach of the development process, in form of point-based, set-based, and agile strategies, and connects them to what seems to be the fundamental tradeoff in prototype experiments, exemplified by real cases from an industrial-academic development project

The effects of voids on structural properties of fused deposition modelled parts: a probabilistic approach

In the search to understand the functional capabilities and limitations of fused deposition modelling (FDM) manufactured components, control over their structural behaviour is crucial. For example, voids introduced during the production phase are a large contributor to anisotropy, yet the magnitude of this contribution remains unquantified. As a baseline model for quantifying strength reduction due to process-induced voids, a statistical method for evaluation of the minimum residual (net) cross section is proposed and tested. Our new method serves to predict the reduction in ultimate tensile strength of transversely printed specimens relative to solid or longitudinally printed specimens, based on void sizes identified from microscopy images of the centre plane of a tensile specimen. ImageJ is used to identify void sizes from the microscopy images, and residual cross sections are determined using a bit counting MATLAB script. From the distribution of residual cross sections, the weakest link for a given sample size is estimated. The accuracy of the proposed method is determined through comparison with experimental test data for samples of polylactic acid (PLA). The results reveal a close yet slightly under-predicted strength estimate, which for the case considered predicted approximately 5 MPa (12%) lower strength than observed in the experiments. Based on our findings, we have established evidence that the anisotropic behaviour of FDM specimens in PLA can to a large extent be explained by the reduction in residual cross section. This implies that other effects such as fracture mechanics and atomic diffusion of polymer chains play a secondary role for the phenomena observed.publishedVersion© The Author(s) 2018 Open Access. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/

Dimensional accuracy of threads manufactured by fused deposition modeling

For parts manufactured by fused deposition modeling (FDM), helical threads drawn in accordance with normal standards (e.g. ISO 68-1), tend to end up deformed when manufactured. These dimensional imperfections typically include violation of radial tolerances and often distorted thread profile. Therefore, making successful threaded connections using FDM would often require looser tolerances than standard ones, unless the nominal threaded sections are in the very loose end of their tolerance band. As a proof-of-concept, this paper aims to characterize such dimensional inaccuracies through image analysis. It also investigates the implications of the defects and partially compensate for the defects by using a strategy of narrowing the thread profiles. In addition, an analytical model for defects on printed inclined planes is presented and verified experimentally by manufacturing three M10x1.5 screws using layer heights corresponding to approximately 10, 8 and 5 layers per pitch. The results show that a lower layer height significantly reduces the defects. However, there is a significant nominal targeting error independent of layer height. On most performance measures, the results show that narrowing the thread profiles with 1/16th of the pitch has only an effect on specimens modelled with 10 layers per pitch. The coarser samples did show improvement in some measures, yet leaving others unchanged. In addition to the quantitative results, key points for investigating threads by microscopy is highlighted in this study

Eliciting unknown unknowns with prototypes: Introducing prototrials and prototrial-driven cultures

This paper maps and describes how prototypes are used to elicit requirements of unknown unknowns in industry. Eight engineering design companies serve as a dataset for a multi-case investigation. By semi-quantitatively analysing 19 prototypes in terms of functionality, timing, stakeholder involvement and requirement elicitation, we present a wide spectrum of prototype utilizations. However, this broad span leads to misunderstandings of what the term ‘prototype’ encompasses, hindering exploitation of its full potential. Hence, we introduce the term ‘prototrial’ that covers functional prototypes utilized in the early stages of the design process, prototypes that effectively elicit unknown unknowns. With this contribution, we encourage introducing mind-sets and behaviours that aim at exploration and learning rather than lean implementation – a prototrial-driven culture

Structural and Biomedical Properties of Common Additively Manufactured Biomaterials: A Concise Review

Biomaterials are in high demand due to the increasing geriatric population and a high prevalence of cardiovascular and orthopedic disorders. The combination of additive manufacturing (AM) and biomaterials is promising, especially towards patient-specific applications. With AM, unique and complex structures can be manufactured. Furthermore, the direct link to computer-aided design and digital scans allows for a direct replicable product. However, the appropriate selection of biomaterials and corresponding AM methods can be challenging but is a key factor for success. This article provides a concise material selection guide for the AM biomedical field. After providing a general description of biomaterial classes—biotolerant, bioinert, bioactive, and biodegradable—we give an overview of common ceramic, polymeric, and metallic biomaterials that can be produced by AM and review their biomedical and mechanical properties. As the field of load-bearing metallic implants experiences rapid growth, we dedicate a large portion of this review to this field and portray interesting future research directions. This article provides a general overview of the field, but it also provides possibilities for deepening the knowledge in specific aspects as it comprises comprehensive tables including materials, applications, AM techniques, and references

Investigating pressure advance algorithms for filament-based melt extrusion additive manufacturing: theory, practice and simulations

Purpose – This paper aims to present the mathematical foundation of so-called advance algorithms, developed to compensate for defects during acceleration and deacceleration of the print head in filament-based melt extrusion additive processes. It then investigates the validity of the mathematical foundation, its performance on a low-cost system and the effect of changing layer height on the algorithm’s associated process parameter. Design/methodology/approach – This study starts with a compilation and review of literature associated with advance algorithms, then elaborates on its mathematical foundation and methods of implementation. Then an experiment displaying the performance of the algorithm implemented in Marlin machine firmware, Linear Advance 1.0, is performed using three different layer heights. The results are then compared with simulations of the system using Simulink. Findings – Findings suggests that advance algorithms following the presented approach is capable of eliminating defects because of acceleration and deacceleration of the print head. The results indicate a layer height dependency on the associated process parameter, requiring higher compensation values for lower layer heights. It also shows higher compensation values for acceleration than deacceleration. Results from the simulated mathematical model correspond well with the experimental results but predict some rapid variations in flow rate that is not reflected in the experimental results. Research limitations/implications – As there are large variations in printer design and materials, deviation between different setups must be expected. Originality/value – To the best of authors’ knowledge, this study is the first to describe and investigate advance algorithms in academic literature

Novel in-situ residual strain measurements in additive manufacturing specimens by using the Optical Backscatter Reflectometry

Material extrusion (MEX) is a well established production method in additive manufacturing. However, internal residual strains are accumulated during the layer-by-layer fabrication process. They bring about shape distortions and a degradation of mechanical properties. In this paper, an in-situ distributed measurement of residual strains in MEX fabricated thermoplastic specimens is achieved for the first time. This innovative measuring system consists of an Optical Backscatter Reflectometry (OBR) interrogation unit connected to a distributed fiber optic strain sensor which is embedded during the MEX process. The characteristic residual strain distribution inside 3D printed components is revealed and numerically validated. The main mechanisms of residual strain creation and the sensing principles of in-situ OBR are described. A minimum measuring range of 4 mm and a spatial resolution of 0.15 mm were experimentally demonstrated. The potential of in-situ OBR technology for detecting invisible manufacturing defects was shown by a trial experiment

A new method for assessing anisotropy in fused deposition modeled parts using computed tomography data

Voids in fused deposition modeled (FDM) parts are assumed to be a key driver for their anisotropic behavior. However, these assumptions are based on investigations of voids using only 2D data (microscopy images). This paper presents a new method to measure such voids by analyzing 3D-data of from X-ray computed tomography (CT), and application of this data for assessment of mechanical parameters. The article is divided into three parts, where the first part elaborates on a proposed method to assess and characterize the void geometry throughout uniaxial printed FDM parts using CT-data. The second part presents an investigation of the void configurations in samples manufactured using different process parameters, aiming to understand how variation in extrusion rate and compensation for non-linear dynamic extrusion behavior affects the void sizes. The third part displays how the information regarding void sizes could be further related to global mechanical properties, using a multiscale finite element approach. The present method of CT-data analysis gives a clear overview of the spatial variation of the void geometry, and findings from the investigated samples suggest that the size of voids have a large non-random spatial variation, highly dependent on the turning points of the toolpaths, and also significantly affected by accumulation of excess material. Printing at a low extrusion rate increases the void sizes considerably, while implementation of an extrusion dynamics compensation algorithm was found to have low impact on the void sizes. The multiscale finite element approach predicts anisotropic elastic behavior, significantly more compliant in the vertical and transversal direction, relative to the printing direction of the infill. It also predicts a non-isotropic strain energy density throughout the specimen, where the location and magnitude of the most energy dense locations vary significantly for different directions of loading, which implicates an anisotropic behavior in terms of failure, in accordance with literature

Revealing the influence of electron beam melted Ti-6Al-4V scaffolds on osteogenesis of human bone marrow-derived mesenchymal stromal cells

Porous Titanium-6Aluminum-4Vanadium scaffolds made by electron beam-based additive manufacturing (AM) have emerged as state-of-the-art implant devices. However, there is still limited knowledge on how they influence the osteogenic differentiation of bone marrow-derived mesenchymal stromal cells (BMSCs). In this study, BMSCs are cultured on such porous scaffolds to determine how the scaffolds influence the osteogenic differentiation of the cells. The scaffolds are biocompatible, as revealed by the increasing cell viability. Cells are evenly distributed on the scaffolds after 3 days of culturing followed by an increase in bone matrix development after 21 days of culturing. qPCR analysis provides insight into the cells' osteogenic differentiation, where RUNX2 expression indicate the onset of differentiation towards osteoblasts. The COL1A1 expression suggests that the differentiated osteoblasts can produce the osteoid. Alkaline phosphatase staining indicates an onset of mineralization at day 7 in OM. The even deposits of calcium at day 21 further supports a successful bone mineralization. This work shines light on the interplay between AM Ti64 scaffolds and bone growth, which may ultimately lead to a new way of creating long lasting bone implants with fast recovery times