Plasma electrolytic polishing (PeP)
The perfect surface finish for complex and rough 3D printed metal parts.
1. Introduction: The challenge of rough surfaces
One of the most persistent problems with additive manufacturing of metals (especially with powder bed processes such as SLM or EBM) is the relatively high surface roughness (Ra values often between 10 and 20 µm). Adhering, incompletely melted powder particles and the step-like layer structure (staircase effect) not only reduce the optical quality, but also drastically reduce the fatigue strength of components, as rough valleys act like notches (predetermined breaking points).
While classic mechanical polishing often fails with the complex, organic geometries of 3D printing, plasma electrolytic polishing (PeP) offers a highly efficient, shape-independent solution.
How does PeP work compared to electropolishing?
In classic electropolishing, the component is immersed as an anode in a usually highly toxic acid bath. Instead, PeP uses environmentally friendly, aqueous salt solutions and very high electrical voltages (often > 200 V). This creates a thin plasma layer around the component, which evaporates the metal at the microscopic roughness peaks.
2. The PeP process in detail
The component is immersed in the electrolyte bath and contacted anodically. The high voltage creates a gas jacket around the entire component, in which a plasma ignites.
- Tip removal: The electric field strength is highest at the geometric elevations (the micro-roughness and adhering powder particles). This is exactly where the plasma removes the material, while the valleys remain largely protected.
- Speed: PeP is extremely fast. A polishing cycle often only takes a few minutes, compared to hours of mass finishing (trowalizing).
- Environmentally friendly: In contrast to mixed acids in electropolishing, which have to be disposed of at great expense, the salt solutions used in PeP (often ammonium salts) are far less harmful to health and ecology.
3. PeP for complex 3D printing structures
The greatest strength of PeP lies in the processing of complex geometries, for which 3D printing is currently used.
- Cavities and cooling channels: Mechanical processes such as milling or grinding do not reach the interior of conformal cooling channels. The electrolyte solution in PeP, on the other hand, flows through even the finest cavities. With special auxiliary cathodes, the internal roughness of nozzles can even be drastically reduced.
- Lattice structures: Fine bionic lattice structures, such as those often used in lightweight titanium components, would be immediately destroyed during machine grinding. PeP smooths these delicate bars gently and without any force.
4. Effects on the component properties
Plasma electrolytic polishing not only improves the aesthetics (high gloss), but also has essential effects on function:
- Fatigue Strength: By leveling the microscopic notches on the surface, the formation of microcracks is drastically delayed. This is crucial for highly stressed aircraft or engine components.
- Corrosion resistance: The process cleans the surface and at the same time promotes the formation of a passive oxide layer (e.g. in stainless steel and titanium), which massively increases chemical resistance.
- No mechanical stress: Since PeP works purely electrochemically and physically, no internal stresses are induced in the surface, as would be the case with shot peening.
5. Conclusion: The key to the perfect surface
With increasing quality requirements in additive manufacturing, post-processing is becoming increasingly important. Plasma electrolytic polishing (PeP) has established itself as one of the most effective processes for refining 3D printed metal components. It combines speed, shape independence and environmentally friendly chemistry into a process that enables Ra values of less than 0.1 µm even with highly complex titanium and stainless steel geometries.