Dimensional limitations for Additive Manufacturing design
Part dimensions in Additive Manufacturing are primarily restricted by the available machine technology. An upper limit is given by the build volume that is defined by a machine’s dimension and its gantry or mechanical axis design. The largest part dimensions are possible to build with DED technologies. Such welding designs can stretch for several meters, for example at frames for airplanes.
Parts fabricated with PBF technology are typically significantly smaller. Multiple parts fit into the build volume to be manufactured simultaneously during one build job. However, there are applications which take up the whole building envelope, such as rocket propulsion engines that have large diameters and a height filling the complete machine chamber. Typical PBF chamber heights are around 400 mm, while certain machines are available with up to 800 mm in height. Some machine manufactures offer customization of their machines to allow for even larger build volumes. While such customizations may be cost-intensive, meeting the optimal part specifications can still result in a positive business case.
However, not only the build volume restricts the upper limit of part dimensions in PBF. Residual stresses and other process related factors may restrict the part size, the wall thickness or the feasible building orientation.
Limitations in sinter-based Additive Manufacturing technologies arise from the subsequent process steps after green body printing. In Metal Injection molding (MIM), where similar debindering and sintering process steps take place, part dimensions in credit card size are favored. With increasing part size, the debindering process becomes more complex. Furthermore, the shrinkage of up to 20% during the sintering process may not be controllable. In Binder Jetting, green body handling is critical due to low volume percentage of binder in the range of 1 %. Therefore, there are low binding forces and low stability in the green body. It is possible to break or damage the part in this state. To use these sinter-based AM technologies, deep knowledge about debindering and sintering has to be acquired. Additionally, a reliable way to predict, control and compensate shrinkage has to be developed.
In general, the lower limit of part dimensions and resolution is restricted by the process itself. Fine features in the range of a tenth of a millimeter can be manufactured by Laser Beam Powder Bed Fusion, where the resolution is in the dimension of the diameter of the laser spot. The optical setup in smaller LB-PBF machines allow for higher resolution and thus finer features, however, with the downside of lower productivity. In BJT high resolution printing heads and small powder particle distribution facilitates building fine features. In general, powder-based technologies have higher resolution than rod or wire-based technologies.