Material jetting (MJT) is a category of additive manufacturing processes where the build material is deposited in the form of individual droplets. MJT has recently been expanded into the field of metal processing due to a potentially high printing speed at low equipment and raw material cost. For full 3D capability, support structures are needed that have to be removed after the print job. We examine water soluble salts and suitable nozzle materials to realise the printing of molten salt in a MJT process. Here, the wetting characteristics of the melt and nozzle are crucial because pronounced wetting is problematic for the ejection of droplets. A sessile-drop contact angle test stand was set up to evaluate the wetting characteristics of three salts or salt mixtures (NaCl, KCl–NaCl and NaCl–$${\hbox {Na}_{2}\hbox {CO}_{3}}$$ Na 2 CO 3 ) on six different nozzle materials (various ceramics and graphite), i.e. potential nozzle materials. The results indicate a high wetting tendency of most of the examined samples with the exception of KCl-NaCl on graphite. Application of these materials on a MJT test stand confirm the feasibility of our findings.

Like most additive manufacturing processes for metals, material jetting processes require support structures in order to attain full 3D capability. The support structures have to be removed in subsequent operations, which increases costs and slows down the manufacturing process. One approach to this issue is the use of water-soluble support structures made from salts that allow a fast and economic support removal. In this paper, we analyze the influence of salt support structures on material jetted aluminum parts. The salt is applied in its molten state, and because molten salts are typically corrosive substances, it is important to investigate the interaction between support and build material. Other characteristic properties of salts are high melting temperatures and low thermal conductivity, which could potentially lead to remelting of already printed structures and might influence the microstructure of aluminum that is printed on top of the salt due to low cooling rates. Three different sample geometries have been examined using optical microscopy, confocal laser scanning microscopy, energy-dispersive X-ray spectroscopy and micro-hardness testing. The results indicate that there is no distinct influence on the process with respect to remelting, micro-hardness and chemical reactions. However, a larger dendrite arm spacing is observed in aluminum that is printed on salt.

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The clogging of piezoelectric nozzles is a typical problem in various additive binder jetting processes, such as the manufacturing of casting molds. This work aims at print head monitoring in these binder jetting processes. The structure-born noise of piezoelectric print modules is analyzed with an Artificial Neural Network to classify whether the nozzles are functional or clogged. The acoustic data are studied in the frequency domain and utilized as input for an Artificial Neural Network. We found that it is possible to successfully classify individual nozzles well enough to implement a print head monitoring, which automatically determines whether the print head needs maintenance.

In Material Jetting, build material is deposited as single droplets onto a platform. This offers potential advantages such as faster processing and cheaper raw material compared to powder based processes. For metals, this technology is subject of several research projects. Due to variations in droplet size, the process inevitably results in deviations between the desired and the actual height of a printed layer. Such deviations can add up over several layers and thus lead to an unacceptably high overall geometrical deviation of the component. One possible solution to this problem is the compensation of local height deviations by adjusting the build strategy (droplet size, droplet spacing) in the next layer. For this, it is necessary to measure the geometric deviations of the local layer heights. However, the temperatures of up to 300 °C inside the build chamber pose a challenge for the integration of a measuring system. In this work, a process monitoring system was integrated into a previously developed printer for Material Jetting of aluminum. The system consists of an optical confocal sensor that enables contactless distance measurement. To avoid overheating of the sensor, it is located outside the build chamber. An infrared filter glass allows measurement from the outside, while heat radiation from the build platform is absorbed by the glass. The sensor is water cooled to ensure a safe operating temperature. A calibration object and printed aluminum components were measured to validate the system. The measurement results show the potential of the system for inline process monitoring for Material Jetting. Based on this, the development of a closed-loop layer height control is now possible.