The nature of a surface is defined by three characteristics: lay, surface roughness, and waviness. It comprises the small, local deviations of a surface from the perfectly flat ideal, or a true plane.
Recently, additive manufacturers have been addressing surface finishes and have seen remarkable results. Some metal 3D printing companies, like 3DEO, can achieve a surface roughness of sub-10 μin (0.3 μm) Ra with the likes of polishing, black nitriding, bead blasting, and a few other options. Let’s dive more into surface finishes with additive manufacturing.
SURFACE ROUGHNESS
Surface roughness is a measure of the variance in a part’s surface topology. Roughness can affect part aesthetics (e.g. shiny or maNe) and mechanical behavior like crack initiation, wear resistance, fatigue life, marine, sealing, bearing, and fluid dynamics.
The rougher the surfaces in an integrated dynamic machine, the less quietly, efficiently, and safely it will operate.
Reducing roughness, or friction, is critical in mechanical parts like pistons, bearings, and
seal surfaces where too much contact between moving surfaces can lead to rapid wear and tear. Specifications may differ by case, but the smoothness required of an end-use part can be an important factor in costing it.
Surface roughness analysis includes the use of parameters to inspect and determine whether the part manufactured meets quality control standards. This helps manufacturers and designers quantify the roughness of the surface finish they select.
MEASURING SURFACE ROUGHNESS
A surface’s profile is measured using a profilometer which detects and records a surface’s step, curvature, and flatness. This data is then used to calculate the roughness of the surface, typically using one of the parameters below.
Ra – Roughness Average
Ra is the arithmetic average of surface heights measured across an entire surface. Simply put, it’s the average height of the microscopic peaks and valleys on any given surface.
*This is most commonly used in North America.
Rz – Mean Roughness Depth
Rz is calculated by measuring and averaging the vertical distances from the highest peak to the lowest valley within five sampling lengths.
*This is most commonly used in Europe.
RMS – Root Mean Square
RMS is calculated as the Root Mean Square of a surface’s measured microscopic peaks and valleys.
SURFACE FINISH OPTIONS IN METAL 3D PRINTING
Metal 3D printing is quickly making a name for itself as an up-and-coming manufacturing technology. Rather than manufacturing parts conventionally through MIM or CNC, metal 3D printers build parts layer-by-layer to produce complete objects. Shifting from traditional manufacturing to metal 3D printing brings a variety of advantages, including product development flexibility, design freedom, and low-supply chain risk. While these are significant advantages, surface finish has historically been a difficult challenge for end use applications, particularly by the finishes yielded in the as-printed state. As such, it makes post-processing a big opportunity for high volume 3D printers in the long run.
While metal 3D printing brings with it a host of surface finishing options, the “right” finish depends on the part’s application.
Since each case is different, understanding surface finishes helps design and engineering teams grasp the costs and benefits associated with bringing the most suitable finish to the printed parts. 3DEO offers options for almost every use case and has production parts in a wide variety of industries today.
DIFFERENT METAL SURFACE FINISH PROCESSES
There are five main categories of surface finishing processes, each of which deliver different aesthetic, feel, and part properties.
- Standard finish – Parts are “as sintered” out of the standard 3D printing process
- Undefined cutting edge finish – Abrasive blasting, vibratory finishing
- Chemical additive finish – Isotropic superfinishing, vibratory finishing
- Electric power finish – Electropolishing, metal DryLyte
- Solidification by plastic deformation – Shot peening
Here are 7 finishing processes expanded on with more detail.
- Bead Blasting Finish – Finishing process to smooth surface without affecting tolerances. MIM-like surface finish, great for non-reflective applications, 32-64 μin (0.8-1.6 μm) Ra surface roughness.
- Vibratory Finish – Machining process with an undefined cutting edge. The goal is to improve surface quality for small parts. This is done by rounding edges, smoothing processes, and grinding.
- Polished Finish – Mirror finish with a unique production polishing process. Ideal for aesthetic and functional requirements, 10 μin (0.3 μm) Ra surface roughness.
- Centrifugal Finish – Fully automated surface finishing which deburrs and polishes large metal parts.
- Black Nitride Finish – Chemical hardening of steel that produces a black finish. Harder, more durable surface quality. Ideal for applications requiring a black finish.
- Shot Peening Finish – The goal of shot peening is to strengthen the surface and solidify it through plastic deformation, thus modifying the mechanical properties of the surface.
- As-Printed Finish – Surface finish in the “as printed” state with no secondary ops. Includes a matte finish comparable to investment casting with 100 μin (2.5 μm) Ra surface roughness. The surface can be polished to a glossier finish with some simple carbide tooling.
3DEO AND SURFACE FINISH
As a metal 3D printing company focused on supplying small, complex stainless steel components to customers in the medical, defense, aerospace, and industrial equipment markets, 3DEO realizes the importance of surface finish. They offer polished finish, bead blasting, black nitride, electroless nickel, cerakote, and matte finish. Utilizing the company’s breakthrough Intelligent Layering® technology, part properties exceed the high industry benchmark MPIF Standard 35 while achieving tight tolerances and a superior surface finish.
3DEO’s 17-4 PH Stainless Steel Material Specifications in the As-Printed State
- Relative Density: 99.5%
- As Printed Surface Roughness: 100-120 μin (2.5-3.1 μm) Ra
- Hardness: 34-42 HRC
CONCLUSION
The reasons for using a surface finish apply both to 3D-printed parts and parts manufactured with traditional methods. Regardless, all aspects of your part, such as the component’s material, shape, thickness, and weight, its planned use, and the environment in which it will be used, must be considered before deciding on the best surface finishing process. Of course, different finishing options come with different costs, but ultimately, the decision on which option to use depends on the part’s application and performance requirements.
This article has been written by Matt Sand, Founder & CEO at 3DEO.