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.

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