Additive Manufacturing allows the integration of precisely placed internal cooling channels into components. With the focus on automated post processing -- removal of residual powder and surface smoothing of these channels -- the mechanical and chemical engineering departments of the Politecnico Milano (Italy) together with Rösler Italiana S.r.l. conducted a study with the surface treatment methods mass finishing, shot blasting and chemically supported mass finishing. The results clearly demonstrated that with all three methods a significant improvement of the overall surface quality could be achieved.
With Additive Manufacturing (AM) extremely precise component geometries can be produced that are not possible with conventional manufacturing technologies. Among other things it allows the creation of highly complex components with integrated functional features such as precisely placed cooling channels. These unique AM characteristics are of great interest for tool and die making, hydraulic components and the aerospace industry. Especially for the tool and die making industry the integration of cooling channels offers significant technical advantages, because temperature fluctuations during the cool down phase increase the risk of work pieces getting warped. Moreover, with a cooling system precisely following the contours of a component, overall cool down times can be decreased considerably. Finally, because work piece faults like warping and heat checks can be prevented, a more effective cooling function for injection molded components results in a higher overall component quality.
Selective Laser Melting – Perfect shape but high surface roughness
For the production of tooling components “Selective Laser Melting” (SLM) is the primary manufacturing method. The creation of a component by selective melting of the powder in defined layers by a laser beam results in an extremely dense work piece. The downsides of this manufacturing method are that residual powder must be removed from the cooling channels and the high initial surface roughness of the components with Ra values between 10 and 20 µm. A high surface roughness as well as the powder deposits in the channels negatively impact the functionality of the work pieces resulting in reduced flow rates due to high friction, turbulences, pressure loss in the system and loose particles that can damage other equipment. Since the internal surface areas of complex components with integrated cavities cannot be treated with conventional finishing technologies, new, innovative post processing methods are required.
The choice of the most suitable surface finishing system is therefore critical for the service life of a component and the overall efficiency of a system. One option for smoothing the external and internal surface areas of AM components is mass finishing. During the finishing process the work pieces are immersed into a circular work bowl filled with special processing media. In addition, dedicated compounds are added during the process. The vibration of the work bowl causes the media and work pieces to move around the bowl in a spiral movement. The constant “rubbing” of the media against the work pieces produces a grinding/smoothing effect resulting in the desired surface quality.
Mass finishing allows the efficient smoothing of internal channel surface areas
To evaluate different treatment methods, among them mass finishing, for smoothing of the external and internal surface areas of AM components, the mechanical and chemical engineering departments of the Politecnico Milano (Italy) together with Rösler Italiana S.r.l. conducted a comprehensive study. This involved the treatment of parts with different shapes and internal passages with different diameters (3, 5, 7.5 and 10 mm) with mass finishing, shot blasting and chemically supported mass finishing. All three surface treatment systems produced surprisingly similar results. Conventional mass finishing and shot blasting consistently removed the roughness peaks and produced similar surface roughness profiles. However, the best results were achieved with chemically supported mass finishing: The work pieces had the smoothest surface, as shown in comparatively lower surface roughness readings, and displayed the typical chemically accelerated finish. With Ra values of 0.7 µm the chemically supported mass finishing method produced not only the lowest surface roughness values, but it also required the shortest cycle time. The results also showed that the final roughness values were more or less identical in the vertical and horizontal internal passages.
The study also proved that mass finishing can create the required smoothing effect on the internal surface channel areas without affecting the channel geometry. The treated surface areas were free of powder “splatters” and loose powder remnants. All three treatment methods improved the surface roughness readings on the internal channel areas. However, as already pointed out, chemically supported mass finishing produced the best results in the shortest cycle time.
Fully automatic processing in one machine
The tests were conducted on a further development of a M3 machine from AM Solutions. The latter is a brand name of the Rösler group that has specialized in the post processing of 3D printed components and offers a range of suitable equipment solutions. These include the complete spectrum of post processing functions like unpacking, removal of support structures, removal of residual powder, surface cleaning and smoothing, edge radiusing, high gloss polishing and surface preparation for subsequent coating of AM components.
The further development of the existing M3 system will not only allow the effective and targeted treatment of internal passages in the future, but it will also be a fully automated system for consistent finishing of 3D printed components without any manual work requirements. Of course, the loading and unloading of the work pieces can also be automated with a robot. The precise dosing of grinding media and compound with a special replenishment, respectively, dosing system and the equipment “Start” function are also fully automated. Depending on the surface finish requirements several grinding and polishing processes can be run in sequence. After the automatic discharge of the media from the work bowl the treated work pieces are removed from the clamping fixture. If required, a separate cleaning and drying stage can be added, of course, also fully automated. The same is true for the work piece handling including the transfer to subsequent manufacturing stages. The system controls allow the storage and call-up of multiple, work piece specific processing programs. The process parameters for the various work pieces can be selected by simply pushing a button or with a work piece recognition system.
To date internal work piece passages that are difficult to reach could not be processed at all, or only with a high degree of manual labor. The automated post processing of AM components now allows doing this work within short cycle times at a fraction of the costs and, above all, with consistent, excellent finishing results.