Metal additive manufacturing is on the rise. The most recent numbers show that over 50% of service providers are producing some level of metals parts, and almost 25% are producing metal exclusively. However, metals have more challenges when it comes to meeting the final surface finish specifications. Almost one-third (32.8%) of the total part cost is attributed to post-processing (2018 Wohlers Report). For most, this includes significant time and labor to meet various surface finish requirements. To mitigate these costs, there are some key factors to think through along the way. We have identified 5 considerations below in question format to create an actionable list.
Print Technology – What print technology best supports my criteria?
Whether starting a design from scratch or migrating a traditionally manufactured component to additive, there is more to consider than just build tray size and speed of print. It is critical to understand which metals are available and tested for different print technologies. Material properties such as density, hardness, and grain structure and size are important in determining the optimal print technology and subsequent surface finishing solution. For example, a powder bed technology like Direct Metal Laser Sintering (DMLS) can provide initial Ra values between 200 µin to 400 µin, while Electron Beam Melting (EBM) produces much rougher parts closer to 1000 µin, yet typically results in lower residual stresses. However, EBM only offers titanium, cobalt chrome, and limited nickel alloys, while DMLS offers additional aluminum alloys, Inconel, and stainless steel options. Some designs call for strength and only require light smoothing, some prioritize appearance requirements like polished finishes, while others require precise specifications related to dimensions and surface characteristics in industries like medical and aerospace.
– What to do? Outline print technologies and capabilities, including material availability, with consideration for final surface finish requirements.
Design – Is the part designed for metal printing?
Whether the material is being sintered, melted, or deposited, it is safe to assume that there will be a ‘gross’ surface finishing operation for most metal printed parts. This means most applications will involve a process to smooth and/or deburr all external surfaces, even if machining or other secondary processes are taking place. During this initial surface smoothing process, depending on material there can be a loss of material up to .005”. This can be more extreme at sharp corners. Consequently, there should be a consideration during the design process to factor in some loss of material on all external surfaces. Equally important in the design phase is to think about complexity. Although it is desirable to push the boundaries and create a massive assembly in one build, consider how it may limit the access to surfaces and potentially trap material that ultimately needs to be removed, including powder.
– What to do? Start with individual components to understand how the material particle size and print resolution impact external surfaces. Then determine what geometries may need to be built up to compensate for the initial surface finish process. Nail this then increase complexity.
Geometry – What are the critical surfaces and where are they?
This may seem obvious, but this is often overlooked. More importantly, it should be a starting place that drives your process, not something that is thought about only after the part is printed. These criteria should be objective and measurable. For instance, what is your target roughness average, or Ra? Knowing this will help you work backward and identify a reasonable target Ra from the printer, and start setting expectations for the material Rate of Removal (RoR) during the finishing stage. That’s the ‘what’, but in additive manufacturing of metals, the ‘where’ often presents more of a challenge. Vibratory systems can work well on external surfaces but struggle to address small or complex internal surfaces. Therefore the most common challenges are internal channels, especially in metals because they typically need to meet testing specifications such as flow rate. Similar to this example, the internal surface needed to be ‘reached’ through other means. In some cases, it may be important that the treatment of the internal surface does not over-process or damage surrounding surfaces. This would require capabilities of targeting specific geometries or features.
– What to do? Identify your surface finish criteria before printing: what (Ra) and where (critical geometries and locations).
Support Structures and Build Orientation – How do support structures affect the build?
Most metal printing technologies, especially the powder bed variety, require support structures even on flat surfaces to act as a heat sink. During print, vertical print surfaces can have a roughness average at least 50% higher compared to horizontal surfaces. Depending on geometry, some parts can be printed at an angle to balance the layer stepping and provide a more uniform surface finish across the build. Because support structures are the same material as the build in metal printing, it is important to think through the placement of these supports because those areas will require additional attention during surface finishing.
– What to do? Simulate support structure designs for 3 different builds of the same part: default, rotated 90 degrees and another oriented at a 30° angle. This will provide guidance for optimizing support structure placement and layering of critical geometries.
Scalability – Is the surface finishing process as repeatable as the print?
When it comes to the actual process of surface finishing metals, many begin by relying on hand tools and equipment already present at their facility. Not only can this be unsafe, i.e. grinding near titanium powders can create an explosion hazard, but this can also quickly become a workflow issue.
Inconsistency – even if the perfect finish is produced once, can it be produced again? Scrapping parts can be quite costly, especially in metal printing.
Throughput – at what pace do parts need to be finished to justify the business case for printing? This is more critical with metals since there is more likely to be a comparison to an existing process with traditional manufacturing.
Productivity – if volume increases, is the surface finishing process still justifiable with existing resources (i.e. labor and tools)? This often results in printing less because surface finishing becomes a bottleneck.
The goal for most is to achieve consistent results while minimizing the time a technician needs to spend handling a single part. There are different technologies to monitor key process parameters and manage energy to match the sophistication of metal 3D printing.
– What to do? Begin with the end in mind. Research surface finishing technologies that can leverage the data and automation of 3D printing metal to balance and scale the entire workflow.
Printed metals bring additional challenges to the surface finishing of additive parts: tighter tolerances, additional support structure considerations, and even new safety hazards. Thus it is more important to think about surface finishing at each step of the workflow. New technology might mean re-thinking the same problem from a new perspective. From design to preparing the final surface for the customer, there is no one technology that fits all applications. This can take some testing and experimentation, but if the above considerations are taken into account it should make for a much smoother adoption of additively manufactured metals.
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