An Investigation on Enhancing Material Extrusion Additive Manufacturing by Introducing Rheo-Printing Technology

Abstract

In this dissertation, all the work done is concerning the evolution and enhancement of material extrusion additive manufacturing. In particular, the focus of this research study is on AM machines that receive polymers in the form of pellets and extrude them to form a three-dimensional object. A novel printing technology is prepared and developed to investigate its benefits to the additive manufacturing industry. The Rheo-Printer provides the ability to apply controlled circumferential and axial shear rate to the polymer melt prior to the extrusion process with the purpose of enhancing the final product’s properties. A rotational nozzle is retrofitted into an AM machine to apply circumferential shearing. A modified mount with a conical cavity is retrofitted into the machine to apply controlled axial shearing. The controlled application of different shear rates provided control over the rheology of the polymer melt. This effect is studied to investigate the possible beneficial aspects of the Rheo-Printing technology. Theoretical and numerical analysis was used to investigate the degree to which the axial shearing mechanism can alter the rheology of the polymer melt. Experimental investigations were then performed to study the effect of axial shearing on enhancing the thermal and mechanical properties of the produced samples. Different characterization techniques were used to investigate the effect of the technology on PLA samples. It was concluded that the axial shearing mechanism was able to increase the crystallinity percentage by 59%. The cold crystallization temperature and the melting temperature were also increased by 2.7% and 1.8% respectively. The glass transition temperature was reduced by 4.2%. The axial shearing mechanism was also able to shift the crystallization structure from the alpha phase to the alpha prime phase. These alterations in the properties of the printed polymer are solely the result of the application of axial shearing. Changes in these thermal properties along with the crystalline structure evolution can have a tremendous effect on the overall quality of the printed objects. The circumferential shearing was investigated theoretically and numerically. We found that the higher the rotational velocity of the nozzle the higher the shear rate applied to the polymer melt. Also, the effect of shearing was seen to be higher around the shell of the printed rod compared to the core. PLA, a pseudoplastic, was used in the investigation. As a non-Newtonian, its viscosity decreases when subjected to shear rate. Hence, the possible impact on enhancing the interlayer adhesion and reduction of porosity was investigated. Flat-wise tensile testing was used to measure interlayer adhesions strength between layers. It was concluded that the rotational shearing was able to increase the strength of the interlayer adhesion by up to 25%. It was also concluded that the low viscosity of the polymer melt led to a reduction in porosity by 50% and up to complete elimination of porosity. The Rheo-Printing concept was theoretically studied and numerically validated to have an impact on the rheological properties of extruded polymer melts. The concept was also validated experimentally to investigate the aspects from which the user can benefit. Material extrusion AM with locally induced crystallization and control over mechanical and thermal properties as well as products with a reduced degree of anisotropy and percentage of porosity is achieved utilizing the Rheo-Printing concept. Theoretical and numerical simulations provided results suggesting that the RheoPrinting technology is a plausible tool in enhancing the interlayer adhesion quality as well as reducing the porosity percentage of products made of High-Performance Engineering materials and in products made utilizing Big area Additive