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Purpose Directly printing molten metal droplets on a build platform to create full dense metal parts is a promising additive manufacturing process. This study aims of to analyse the effects of the thermal conditions on the resulting tensile properties of parts made from aluminium 4047A built in droplet-based metal printing. Design/methodology/approach A drop-on-demand print head with pneumatic actuation is used to eject droplets on a nickel sheet mounted on the heated build platform. Tensile specimens are machined from cuboid blocks built by successive droplet deposition and tested in a universal testing machine. The ultimate tensile strength, uniform elongation and yield strength are evaluated and presented. Micro-sections are taken from the printed blocks to examine the internal pores and the metal’s microstructure. Findings With an increase in the interface temperature the uniform elongation increases from 0.5 to 12%, while the yield strength decreases from 130 to 90 MPa. The ultimate tensile strength increases from 130 MPa to a maximum of 190 MPa at an interface temperature of 530º C and slightly falls for higher interface temperatures. Those values are in the same range as conventionally casted parts of the same alloy. The authors’ hypothesis is that the main effect responsible for the mechanical properties is the wetting of solid material by the liquid droplet and not remelting, as has been reported in literature. Originality/value To the best of the authors’ knowledge, this is the first time that mechanical properties of aluminium 4047A built by a droplet-based additive manufacturing process are published for different interface temperatures. It is also the first time that the main effect on mechanical properties is attributed to wetting instead of remelting.

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This paper introduces a chemical resistant piezoelectrically driven microdrop generator which can be fabricated in a cost and time saving manner by using rapid prototyping techniques. Thus it is especially suitable as an experimentation platform. For the adaption of microdrop generators to various fluids, an experimentation platform is needed which allows the rapid change of geometry, dimensions, and material parameters of the microdrop generator. The size of the nozzle, the geometry of the pumping chamber, and the thickness of the used piezo-transducer have to be adaptable to various fluids to achieve drops of the size, speed, and uniformity that are needed. This microdrop generator uses a sandwich structure which consists of a silicon wafer, a Pyrex diaphragm, and a PZT transducer. A pumping chamber is milled into the silicon by laser micromachining; and the Pyrex is anodically bonded on top of the silicon plate to seal off the pumping chamber. The piezo-transducer is then glued to the diaphragm with an epoxy adhesive to obtain a bimorph actuator. When electrically driven, the actuator bends inwards into the pumping chamber which in turn creates a pressure wave inside the chamber that finally leads to the ejection of a drop out of the lateral nozzle. Since only the Pyrex and the silicon are in contact with the fluid the assembly is very resistant to aggressive media like solvents, adhesives, or acids. The thickness of the piezo-actuator can be varied according to the intended application. Depending on the piezoceramic used, the operating temperature is up to 250 °C. Single- and multi-nozzle arrays as well as the integration of a heated fluid reservoir can be realized. The drop volume is set by proper dimensioning of the microdrop generator. Manufacturing, assembly, and interconnection technology of the droplet generator will be described later in this paper. The electro-mechanical behaviour of the droplet generator is analyzed by determining the step response function and by measuring the frequency-dependant impedance. For the first fluidic validation of the experimentation platform, isopropanol is used because of its well known properties. The relationship between drop velocity and drive voltage on the PZT transducer is established. Special attention is paid to the calculation of the microdrop generator material cost which only amounts to $25 for a multi-nozzle array. By using rapid prototyping techniques the microdrop generator is manufactured within 180 min. This shows the potential for a low-cost and rapidly producible experimentation platform.

Dispensing minute amounts of fluid is used in many industries, such as in life science, bioengineering, 3D printing, or in electronics manufacturing. Each application for drop-on-demand (DoD) printheads requires different drop volumes and drop velocities. Furthermore, it is necessary to eject droplets made of fluids with different fluid properties, like viscosity, surface tension, or density. Due to this wide range of different applications and demands on printheads it is important to investigate the influence of relevant factors on the droplet formation process. Therefore, the influence of the fluid properties, the printhead geometry, and the electrical excitation form on the droplet formation process are described in this project. In detail, the influence of the surface tension as well as the viscosity of the fluid, the nozzle length and its width, and the amplitude of the applied voltage at different pulse widths on the droplet characteristics are investigated. The used printhead consists of a silicon chip, which includes the fluidic components, and of a bimorph piezoelectric actuator. The printhead is manufactured with rapid manufacturing techniques, such as laser micromachining. The advantage of this method is that the printhead is adaptable to new boundary conditions in a time- and cost-saving manner. In this project, the nozzles have a square shape with a sidelength between 50 and 100 μm and the nozzle length varies between 50 and 200 μm. A fluid mixture is provided which can be varied in its fluid properties. Therefore, the possibility for the independent adjustment of its viscosity and its surface tension is given. The mixture consists of glycerin, distilled water, and isopropanol. An analytical description for each amount of its substances enables to provide a fluid with defined properties. Three kinds of experiments are carried out in order to determine the influence of the fluid properties, the printhead geometry, and the electrical excitation on the droplet formation process. The determination of the minimum excitation voltage needed for droplet ejection and the determination of the droplet volume and its velocity. The main results are: The higher the surface tension, viscosity, and nozzle length, the higher is the minimum excitation voltage. Furthermore, the droplet velocity decreases for an increased surface tension, viscosity, and nozzle length. On the other hand, the droplet velocity increases with an enlarged amplitude of the voltage and pulse width. The droplet volume increases for an increased surface tension, nozzle width, pulse width, and amplitude of the voltage. In general, the reasons for these correlations are the interaction between the strength of the pressure pulse, friction forces, fluidic resistances, and fluid properties. Overall, the possibility to achieve microdroplets made of different fluids and with a specific velocity and volume is described. Furthermore, a fluid mixture, which can be varied in its fluid properties, is presented.