Comprehensive investigation of mechanical properties of fused deposition modelling : a dissertation presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Engineering at Massey University, Albany, New Zealand
Fused depositing modelling (FDM) is a layer wise manufacturing method whereby parts are printed from the bottom up through the extrusion and deposition of a filament onto a print base. Various test methods exist for the determination of part mechanical properties. These include tensile, flexural, and impact testing and are conducted using a variety of standards including those of ASTM and ISO. Many researchers have investigated the effects of factors such as road width, raster orientation, layer height, and air gap on the mechanical properties of FDM parts. However, there are many unexplored factors that also impact on the properties of printed parts. For example, the printers used in characterisation studies are mostly commercially available or consumer market printers which allow only limited control over the print parameters and print with a limited set of materials. Similarly, the life of the printer can also affect the print quality but this has not been studied before.
Control over machines could be achieved by purchasing additional print profiles from the manufacturers or by open-sourcing legacy hardware through retrofitment with new electronics and software. The latter option is more economically viable as there are a large number of decommissioned legacy machines that have superior hardware cheaply/freely available. A retrofitted commercial 3D printer would allow control over print parameters and printing with materials outside the ones sold by the manufacturers. This can open new avenues to study the properties of the printed parts. In this work, a Stratasys Vantage X 3D printer has been retrofitted and made open-source through a combination of hardware, software, and firmware modifications. These modifications result in complete control by the user over all print variables along with the ability to use any feedstock including custom made feed stocks and ones that are locked by the manufacturer. The printing accuracy of the machine is evaluated by optical imaging of the printed samples and destructive testing in accordance with the ASTM D638 standard. .
To study the effect of the machine’s life on the properties, a longitudinal study is designed in which two groups of parts (with 0° and 90° orientations) are printed at two different times during the course of this research. The temporal spacing between the parts is eighteen months. The parts are designed according to ASTM D638 standard and printed on identical printers using the same parameters on both occasions. The parts are subjected to tensile testing for the mechanical characterization while scanning electron microscopy (SEM) is used for the examination of the sample’s fracture and topographical surfaces.
A difference is discovered between the Young’s moduli of old and new groups. The orthotropic nature of FDM parts becomes prevalent in the strain responses of samples with 0° samples experiencing the largest strain. Distinct differences exist between the diffusion levels of the chronological sample groups, with the original batch exhibiting greater diffusion resulting in almost indistinguishable layers and higher tensile strengths. Individual layers are easily observed in the newer sample groups. Topographical analysis of samples shows up to 0.1mm difference between the road widths with the older samples roads being the narrowest. Results from this research show that the age of the printer affects the mechanical properties of the parts with the older parts exhibiting greater strength compared to their new counterparts even though both were printer under identical conditions. Therefore, a significant difference exists between temporally spaced FDM parts.
To conclude, this research has successfully retrofitted an old FDM system which is capable of printing various materials through a choice of user parameters. The longitudinal study conducted to study the effect of the machine age on the printed parts purports that as the printing machines get older their print quality deteriorates and this factor should be considered by designers when designing parts for functional purposes.