Development of quality system for additive manufacturing

Abstract

Selective laser sintering (SLS) and fused filament fabrication (FFF) are significant methods in additive manufacturing (AM). As AM is increasingly being used to manufacture functional parts, there is a need to have quality systems for AM process, to ensure repeatability of properties or quality of part made by the process. The primary aim of this research is to develop quality systems for SLS and FFF processes of AM. In order to develop a quality system for SLS process based on defining a minimum set of tests to qualify a build, two SLS materials of Nylon 11 and Nylon 12 were investigated. Melt flow index (MFI), impact, tensile and flexural tests were assessed, along with density, surface roughness, dimensional measurements and scanning electron microscopic (SEM). Two benchmark parts were designed for manufacture to track changes in key parameters from one build to another, and tests on this validated against ISO standards. Similarly, to develop a quality system for FFF process, the various mechanical properties of tensile, flexural properties, notched and un-notched impact strengths and sample mass of specimens made from biodegradable polylatic acid (PLA) FFF material were investigated. In order to identify the tests that can be most sensitive to changes in processing conditions and differences in interlayer bond strength which affect the structural integrity of part made by FFF. Analysis of variance (ANOVA) was used to compare the significance of the effect of processing parameters on the mechanical properties, while optical microscopy was also used to investigate failure pattern. A novel low cost method for evaluating fracture strength of FFF made parts was also developed for low cost FFF machines. Benchmark specimens and a low cost test jig were designed and fabricated to track changes in key quality characteristics of FFF made parts from one build to another. Tests conducted on the test jig were validated against those conducted on standard machine. Very good correlation was observed between them. On the basis of the data from experiments, impact strength was adopted as a key test of interlayer bond strengths which determines overall structural strengths of the materials for both SLS and FFF AM processes. A positive correlation exist between density and modulus of SLS parts, and also between sample mass and modulus of FFF made parts. ii This led to the impact strength and density/mass of parts being adopted as key indicators of mechanical integrity, with MFI a good indicator of input material quality, and dimensional accuracy of machine calibration. These tests were thus adopted as a quality assurance system in the respective developed quality system for AM processes of SLS and FFF. If the quality system is implemented, repeatability of properties can be achieved and the quality of product assured.