Developing a control system for improving selective polymer laser sintering build speed and part integrity

Taylor, Joe and Rennie, Allan (2020) Developing a control system for improving selective polymer laser sintering build speed and part integrity. Masters thesis, Lancaster University.

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Abstract

Designing electronic control systems specific to additive manufacturing machines is a fast evolving practice, developments in which spur continual performance improvements, which in turn improve the quality and economic viability of parts produced (Hu & Kovacevic, 2003). Research methods used for this work comprise of; taking receipt of externally designed and built experimental rigs, recording performance data and making incremental changes in attempts to improve performance. Focus is given to the automation and speed of the processes and research is limited only by the availability of time and funding. This work has investigated several potential significant improvements to SLS cycle times and part quality, with the wider project continuing beyond the scope of this dissertation. Experimentation with serial data transmission protocols using ASCII (American Standard Code for Information Interchange) found it could provide a fast, robust link between central control system elements, which is critical and can be achieved this way without great monetary cost. Distribution of temperature across the build area surface can be optimised with a single control feedback loop to a level acceptable for the use of PA-12 (Polyamide 12) powder, though methods that are more complex may yield better results. Rapid deoxygenation of the build chamber at the beginning of each build cycle offers a slight improvement in cycle time, and proper loop feedback can assist in mitigating safety concerns. Current, commercially available stepper motor control systems are capable of greater accuracy than is necessary in such applications but are limited by the accuracy of their mechanical linkage, which can introduce significant backlash into the system. Powder can be loaded into the machine using augers fed from an external hopper in such a way as to minimise powder waste through uneven feeding. Separating power systems allows individual control of sections of the machine, improving safety, monitoring possibilities and potential for recovering failed builds. A removable build platform, comprising the build piston and associated hardware on a movable trolley frame, significantly reduces the machine cycle time by allowing part removal and cleaning to be performed concurrently with the start of the next build. Visibility of the process status via beacon stacks allows for quick human interaction where required, potentially reducing failure rates and improving cycle times.