Optical coherence tomography imaging of SLS parts

Background

Selective Laser Sintering (SLS) creates 3-D parts through the application of laser energy to powder beds via the 3-D CAD description of the part geometry from which it derives a 2-D stack of layers which represent the part. Each layer is then created by scanning a laser spot over the required cross-sectional area, and using the laser to melt, sinter, and bond particles together in a thin lamina. By spreading a further layer of powder on top of the previously processed layer and repeating the scanning process, subsequent layers are created and simultaneously bonded to already existing layers until such time as the entire stack of 2-D layers has been created and bonded together to form the geometry described by the original 3-D CAD solid model.

Technology description

Researchers at The University of Texas at Austin have developed a portfolio of technologies related to additive manufacturing. One technology is a technique that allows for in-situ monitoring of any form of additive manufacturing by using optical coherence tomography (OCT). OCT enables not only a view of the visible surface of the build process, but a penetrating 3D view through the build layer to enable evaluation of properties where layer to layer bonding is critical. The monitoring process may be completed without disturbing the fusion process. A variety of signals (layer thickness, phase changes, surface roughness, etc.) may be derived from the OCT signal, and the OCT may be used in a feedback mode to better control the process. Defects can be detected and corrected before new layers are added to increase part yield and maintain design specifications.

Results

The technology has been used to investigate nylon 12 components fabricated via SLS, and the resulting images displayed artifacts that had not been previously reported. Nylon powder and sintered parts were imaged in air and in an index matching liquid. The results of the investigation have practical implications for the use of OCT for ex-situ optical nondestructive evaluation of parts, and in-situ process monitoring of SLS. Additional details may be found in the peer reviewed article here.

 

Figure 1. (a) OCT image of empty dish with top and bottom surfaces labeled. (b) OCT image of nylon in a similar dish with a signal-tail imaging artifact label

 

Figure 2. (a) OCT scan of nylon powder in ethyl cinnamate. (b) The average intensity of the OCT scan is shown versus depth. The imaging depth (6-dB above background) of nylon 12 in ethyl cinnamate is 499 μm.