Image Processing (IP) Assisted Tools for Pre- and Post-Processing Operation in Additive Manufacturing (AM) A thesis submitted to the Graduate School of the University of Cincinnati In partial fulfillment of the requirements for the Degree of MASTER OF SCIENCE In the Department of Mechanical and Materials Engineering of the College of Engineering and Applied Science (CEAS) By Rohit Vaidya Bachelor of Engineering (B.E.) – Mechanical Engineering University of Pune, India. – 2010. Committee Chair: Dr. Sam Anand Abstract Additive Manufacturing (AM) or 3D printing (3DP) process is a layer by layer manufacturing approach for manufacturing parts with complex geometries. The STL (Stereolithography) file format is the de-facto industry standard for manufacturing parts by AM/3DP. The STL file is a triangular mesh representation of CAD geometry in non-uniform rational B-spline (NURBS) surface format and thus is an approximation of actual part geometry. During the conversion of a complex CAD geometry to an STL file, geometric errors are introduced in the model. These drawbacks associated with the STL file may translate into a faulty or inaccurate final manufactured part. This thesis presents a novel Image Processing (IP) based Direct CAD Slicer, IPSlicer, which can be used to manufacture components directly from CAD geometry (without converting to STL file). Using sectional image snapshots of a part, captured normal to the build direction, and sectional 2D bounding box data, contour points for each section are identified by performing boundary tracing operation followed by application of Contour Mapping Algorithm (CMA). The method slices the actual NURBS geometry and thus parts manufactured by this method have reduced GD&T errors such as flatness, cylindricity, and profile error. In addition, a support removability analysis tool is developed by performing color-based segmentation on sectional image snapshots in conjunction with a pixel traversal approach. Using the segmented images and sectional bounding boxes, a sintering area and time calculation tool for each layer is also developed. Lastly, a CAD tool has been developed to identify and highlight sharp features and corners which violate Design for Additive Manufacturing (DFAM) guidelines. The effectiveness of IPSlicer is verified by virtually manufacturing test components and calculating GD&T errors by the application of computational metrology algorithms on virtually manufactured i data. The support removability analysis tool, sintering area and calculator, and sharp feature detection tool are validated using test cases. ii iii Acknowledgement I would like to take this opportunity to express my gratitude to the people who contributed, in different ways, to the completion of this work. First and foremost, I would like to thank my academic advisor Dr. Sam Anand and express my deep appreciation for his guidance and support for successful completion of this research. I also take this opportunity to thank Dr. David Thompson and Dr. Jing Shi for serving as members on my master’s thesis defense committee. I would like to dedicate this thesis to my parents and my sister. Thank you for bearing with my idiosyncrasies and crazy ideas. I would like to thank my past and present lab mates who were always there to help me when I was stuck during the research work. I would like to thank Rohan Vaidya, Rajit Ranjan, Sushmit Chowdhury, Nikhil Kabbur, Kunal Mhapsekar, Anay Joshi and Ranjan Melpal for making this MS journey a ride to remember. iv Table of Contents Abstract ............................................................................................................................................ i Acknowledgement ......................................................................................................................... iv Table of Contents ............................................................................................................................ v List of Figures ............................................................................................................................... vii List of Tables ................................................................................................................................. ix 1 Introduction ............................................................................................................................. 1 1.1 Motivation of Research .................................................................................................... 2 1.2 Objective and Impact of Research ................................................................................... 3 1.3 Thesis Outline .................................................................................................................. 5 2 Literature Review .................................................................................................................... 6 2.1 STL File Defects and Input File Modifications ............................................................... 6 2.2 Direct CAD Slicing .......................................................................................................... 7 2.3 Support Structures Removability Analysis ...................................................................... 7 2.4 Sintering Area and Time Calculation ............................................................................... 8 2.5 Design for Additive Manufacturing (DFAM) Guidelines................................................ 8 3 Methodology ............................................................................................................................ 9 3.1 Direct CAD Slicing using Image Processing Algorithms: IPSlicer ................................. 9 3.1.1 Sectional Snapshots and Extraction of Bounding Box Data (Step 1) ..................... 10 3.1.2 Boundary Tracing Operation (Step 2)..................................................................... 11 3.1.3 Contour Mapping Algorithm (Step 3)..................................................................... 13 3.2 Support Structure Removability Calculator using Image Processing ............................ 17 3.2.1 Sectional snapshot generation and color-based segmentation ................................ 18 v 3.2.2 Segregation of Support pixels into Removable and Non-removable...................... 21 3.3 Sintering Area and Time Calculator............................................................................... 24 3.4 Sharp Corner Detector .................................................................................................... 27 4 Examples and Results ............................................................................................................ 31 4.1 Direct CAD Slicing using IPSlicer................................................................................. 31 4.1.1 Test Case 1: Component with Cylindrical and Spherical features ......................... 31 4.1.2 Test Case 2: Component with Freeform Surface .................................................... 33 4.2 Support Structures Removability Calculator.................................................................. 35 4.2.1 Test Case 1: Component with 100% Removability Score ...................................... 35 4.2.2 Test Case 2: Component with Non-removable Supports ........................................ 36 4.3 Sintering Area and Time Calculator............................................................................... 37 4.4 Sharp Corner Detector .................................................................................................... 39 5 Conclusion and Future Scope ................................................................................................ 41 References ..................................................................................................................................... 42 vi List of Figures Figure 1: AM/3D printing process flow.......................................................................................... 2 Figure 2: Structure of image processing assisted tools ................................................................... 4 Figure 3: CAD to STL conversion error ......................................................................................... 6 Figure 4: Working principle of IPSlicer ......................................................................................... 9 Figure 5: 2D sectional bounding box of actual contour ................................................................ 10 Figure 6: a) Moore-neighborhood of pixel P. b) Moore-neighbor tracing algorithm (adapted from [29])............................................................................................................................................... 11 Figure 7: Boundary tracing applied on multiple contours ............................................................ 12 Figure 8: Boundary tracing operation workflow .......................................................................... 12 Figure 9: Flowchart for CMA ....................................................................................................... 13 Figure 10: Extreme pixels from boundary pixels ......................................................................... 14 Figure 11: Translation operation ................................................................................................... 15 Figure 12: Scaling operation ........................................................................................................