Selective laser sintering (SLS) printers have been used for rapid prototyping, and the prototypes of part assemblies have been reported to expand or shrink over time. This paper examines the hygroscopic swelling behavior of 3D printed parts from SLS printers. A total of 10 hexahedron samples were produced using nylon-12, which is a common material used for prototyping. Half of the samples were exposed to a high temperature to reduce the moisture content, and the rest were left at a room temperature. In the meantime, 13 dimensions of each sample were measured periodically along with the local weather records including relative humidity in order to track the hygroscopic swelling behavior of the samples. The results showed that the deformation was mostly occurred to the dimensions parallel to the sintering layers. Also, changes in these dimensions were found to have a high correlation with the relative humidity regardless of temperature conditions. These results imply that changes in environmental conditions such as relative humidity result in the deformation of 3D printed parts after production. The high correlation between dimension change and relative humidity also indicates the layup orientation is a decisive factor to predict the deformation of 3D printed parts. Thus, unexpected deformation of 3D printed parts can be avoided by optimizing the parts design considering the layup orientation and by controlling the environmental conditions.
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