Optical Design Considerations for Additive Manufacturing
Additive manufacturing (also known as 3D printing) has been around since the early 1980s, with the first technologies using ultra-violet sources as an optics-based approach to solidify polymer resins. Over the decades, new technologies have enabled creation of functional parts using a wide variety of polymers, powders and even metals. The use of 3D printing has grown exponentially across multiple industries, none more so than medical, where multiple applications have benefitted from the ability to rapidly manufacture custom orthopedic implants for knee and hip replacement surgeries. Dentistry is one of the largest adopters of 3D printing where it is used to create highly tailored crowns, bridges, and aligners found in orthodontics.
Light as a means of directed energy has opened a myriad of opportunities of 3D printing for medical applications, where lasers, high-power LED sources and projection optics are used to reliably create high-resolution features at everincreasing production rates. Initial machine architectures leveraged available off-the-shelf (OTS) optics and system components that often required some performance trade-offs. However, more complex geometries with finer features and demands on surface finish required greater control of the energy source along with the ability to monitor and stabilize the process in-situ. Thus, current and future generation machines now employ highly customized optical designs to efficiently and repeatably direct light to the working plane.
Engineers face many advanced challenges related to designing and selecting the right optics (e.g., glass type, surface shape, optical coatings, manufacturing tolerances), when constructing industrial scale 3D printers that operate reliably over build times that can extend from hours to days. These machines often must pass strict certification processes and conform to regulatory standards of medical implants. This article covers many important technical topics for engineers to consider, including:
• Maximizing light transfer from the laser to the powder bed.
• Optical components and their function to enabling precision part manufacturing.
• The importance of optical coatings on all optics to ensure maximum light efficiency.
• The importance of optics-based realtime closed-loop metrology to verify part production.
Print resolution is a key driver in additive manufacturing adoption. The ability to print dimensionally accurate parts in a wide variety of materials including titanium and engineered polymers with prescribed surface finish and density characteristics minimizes post-processing steps such as machining and polishing. This allows printed parts customized to the patient to become not only faster to manufacture than traditional subtractive manufacturing methods, but at lower costs and delivered through a decentralized or distributed supply chain. Directed laser energy and projection systems are widely used within optical systems for additive manufacturing described in Table 1. With the right optical components, these machines tailor a beam to specific spot dimensions and intensity uniformity to impart prescribed energy profiles that yield higher quality parts at faster build rates.
Metal additive manufacturing processes based on powder bed fusion is one example of how optical components and system design play important roles in overall part quality. The basic optical layout for a laser powder bed fusion (LPBF) system is presented in Figure 1. A high-power laser source, typically a fiber laser emitting 1 kW or greater power around 1070 nm wavelength is passed through optical collimators and focusing elements prior to being directed to the working plane using multi-axis scan mirrors. The quality and stability of the focused spot are directly influenced by the selected optical and mechanical components.
Processes utilizing higher power lasers must be mindful of even small fractions of scattered energy, which can result in significant thermal effects in both the optical elements and supporting mechanics. Thermal expansion within metal mechanics can cause element positions to shift or drift, requiring special attention to both material selection and cooling using plumbed air or water. Within the optics, a phenomenon known as thermal lensing caused by heating of the glass, affects both the lens shape and the refractive index of the material. Thermal lensing is a common issue at entrance windows to the powder bed chamber as well as elements near regions of high optical intensity within the beam path. When materials and coatings are not properly considered, thermal lensing can cause the laser spot focus to drift along with spatial and temporal changes which result in lower than desired energy profiles and excess porosity in the finished part.
Design engineers should select substrate materials with minimal inclusions to avoid internal scatter and apply high performance dielectric coatings with exceptionally low absorption. High performance fused silica glass such as Corning 7980, is one example of a highly pure glass and is often one of the only options at critical locations within multi-kilowatt laser systems. More sophisticated lens designs are needed to reach high performance and may require judicious choices of glass materials to vary the refractive index. Engineers may opt for glass types with differing refractive index temperature coefficients as one way to balance out thermal effects, much in the same way doublet lenses balance dispersion profiles using crown and flint glasses.
Even perfect stability within the optical design can still leave opportunity for process variability during the melting process. Powder particle size distributions, alloy content and soot or spatter settling within the chamber and on windows can all impact the quality and porosity of a printed part. To ensure that the part being produced accurately matches the design file, in-situ, real-time monitoring of the build process is often required. This is a relatively new technology tool for 3D printing; however, it has been used for decades in optical thin-film coating deposition machines. Again, optics plays a key role by observing the melting process through emitted radiation signals at very high acquisition rates to monitor the signatures of the process as well as overall machine health.
Deviations from nominal emitted signal levels can flag issues in the process that can potentially be corrected in real time. Process errors that cannot be corrected can instead stop the build early and avoid unnecessary production time and flag potential issues within the machine. As shown in Figure 1, the use of a spectrally complex dichroic beam splitter allows blackbody radiation, emitted from the powder bed and travels back through the optical path (green line), and directed to an optical detector, typically photodiodes or an image sensor. Embedded software allows one to calculate melt pool temperatures using known multi-color pyrometry approaches that account for emissivity changes in the material during processing. Targeted spectroscopy on these optical radiation signals can also identify specific spectral signatures that can be correlated to part defects. 1
Additional optical sensors — like laser power meters — monitor the delivered power, and high-speed cameras can observe the size and shape of the melt pool. Signatures from these signals can create a detailed picture of the additive build process and provide deep understanding of printed part quality, machine health, and batch-to-batch variability across different machines installed at different locations. Beyond process monitoring, high-resolution cameras positioned above the process plane can report dimensional accuracy of the print layer by layer and flag geometry errors or material deficiencies which can ruin a print. Lastly, beam profilers and cameras can be invaluable during setup and maintenance of the machine.
Article source: Medical Design Briefs