A compilation of design principles and guidelines for selective laser sintering
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The term Additive Manufacturing (AM) is used to describe several manufacturing technologies that share the same basic principle of producing parts directly from their CAD models without the need for special tooling, by adding material selectively one layer at a time. Current research focuses on one such technology called Selective Laser Sintering (SLS) where thin layers of powdered thermoplastic material are fused using a laser beam. With no part-specific tooling required, the product development cycle is drastically shortened. This lack of tooling, coupled with freedom of placement of material, opens the door to several design opportunities unique to AM such as increased geometrical design freedom and the ability to manufacture low production volumes economically. Gradual improvements in process accuracy and selection of materials over time have resulted in a shift in application of AM from rapid prototyping to direct manufacturing and even ‘democratization’ of the product development process in which even non-professional users can rapidly manufacture products as long as there is a CAD model for the part. However, the move to direct manufacturing of end-use parts also means that part quality in terms of conformance to product specification becomes important for the product to successfully perform its function. The research in this thesis is focused on documenting these manufacturability capabilities and limitations for Selective Laser Sintering. It focuses specifically on thermoplastics, especially Nylon 12 polyamide materials known by the trade names PA 2200 and Duraform PA. While several design resources have been created based on industry best practices developed through experience, they are scattered throughout the literature and are not readily available to designers. It is also difficult to compare and draw quantitative inferences from existing guidelines as they are developed independently under dissimilar process conditions. Therefore, a prime focus of this research is to synthesize and compile existing guidelines into a comprehensive document. The first objective of this research is to compile a user-friendly resource, in the form of design principles and guidelines, to help designers make early process selection decisions, optimize part quality and minimize manufacturing cost. A systematic literature review of available guidelines, exploratory studies and case studies is conducted to develop actionable design recommendations that are within the scope of the designer. The second objective of this research is to address the lack of adequate process tolerance information that can reliably predict the quality of parts produced by the selective laser sintering process. This information is important to accurately evaluate the process during early process selection. A test part is proposed to measure dimensional deviations for various features (such as holes, gaps, cylinders, walls, clearances, etc.) across a range of dimensions and along different orientations. Finally, a sampling plan that represents sources of variability in the process is put forward to collect statistical data in an economical manner.