Enclosed, Internal AM Finishing & Voxel's OPECM Methodology
Updated: Sep 3
How oscillatory cathode motion can enable improved non-line-of-sight internal features in critical additive components across defense, medical device and semiconductor manufacturing.
Key Takeaways
Internal surface finishing is a primary bottleneck for scalable additive manufacturing, especially for non-line-of-sight features where conventional and flow-based methods struggle with access and consistency
OPECM enables uniform finishing of complex internal geometries by using oscillatory cathode motion, achieving order-of-magnitude surface roughness reductions (e.g., ~5 µm Ra to ~0.24–0.45 µm Ra) in inaccessible regions
Eliminating part splitting and secondary finishing steps improves both performance and manufacturability, preserving material integrity while enabling scalable, high-volume production of advanced AM components
Metal additive manufacturing (AM) has enabled engineers across critical industries to design passages and features that would have been challenging, or outright impossible just a decade ago: serpentine coolant paths, internal flow splitters, downskin-dominated curves, and other compact S-bend channels, as a few examples. These AM geometries enable a wide range of performance enhancements for critical environments, such as improved heat transfer efficiency and part consolidation.
However, there’s a critical catch: internal features on these geometries reflect many of the same problems metal-AM has encountered in other places, including, but not limited to: staircase topography, downskin sag, partially fused powder, and ±100 µm variability. The problems are further exacerbated since these issues are now trapped deep inside the part, which can be impossible to access by conventional methods without splitting the part. And other methods like abrasive flow machining can still struggle with consistency, flow shadows, and over-polishing in certain areas. It can even be argued that the primary process limitation and bottleneck to true scalability for AM is the ability to finish internal features in production environments.
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Ultimately, additive manufacturers need a consistent, uniform, and scalable internal finishing methodology for high-volume AM components, notably on unique geometries with non-line-of-sight areas. In this article, we’ll explain how Voxel’s oscillatory pulsed electrochemical machining (OPECM) methodology may enable a unique internal finishing capability for critical AM parts that can even access non-line-of-sight internal features, reducing rough as-printed additive surfaces to critical-application-ready tolerances.
OPECM is most relevant when the internal geometry, material, surface requirement, electrolyte flow path, cathode approach, and production case support a controlled finishing strategy. A part may look like a strong candidate because it has enclosed AM channels or inaccessible rough surfaces, but those signals do not confirm feasibility by themselves.
Internal Features & Scalability: AM's Biggest Bottleneck
AM is already being deployed into production environments, particularly for components where weight, complexity and part consolidation are critical performance drivers as a legitimate, proven manufacturing method. Specifically, aerospace, defense, medtech, and even semiconductor manufacturing titans are increasingly deploying AM to tackle geometrically challenging parts, such as gas manifolds for semiconductor equipment and heat exchangers for turbomachinery engines.
As AM has matured, its scalability capabilities have greatly improved, but a key problem remains: surface quality, which remains a seemingly inherent drawback of the technology itself. As-built surfaces (notably internal features) on AM parts can range from 25-40µm Ra on parts where even 5-20µm Ra would still be unacceptable, such as on features involving thermal flow paths, high temperature flux, or fatigue-critical zones. Sometimes, these issues are made even worse as AM companies continue to scale the technology and cut costs on the processing side: introducing thicker layer lines, larger powders, and faster scanning strategies, further exacerbating surface quality concerns.
The consequences of even 5µm Ra internal surfaces in critical industries can introduce serious performance issues:
In aerospace applications, rough internal channels can increase pressure drop across cooling passages (in the form of hole arrays or microchannels in heat exchangers), ultimately requiring higher pumping power or a limitation in flow rates that affect engine efficiency. Surface irregularities may also cause localized hotspots that could disrupt heat transfer and/or cause structural damage to parts.
In the world of semiconductors, direct-to-chip cooling infrastructure or gas distribution manifolds requires extremely tight internal geometries. While much of the reasoning for minimizing thermal gradients is similar to aerospace applications (heat flux efficiencies and degrading parts), superfinished internal surfaces prevent microscopic contamination (IE, UHP environments): which can be in the range of 0.2-0.8µm Ra.
A cross-industry challenge associated with internal AM parts requiring internal surface finishing is that the process of welding parts back together to reach those surfaces can introduce stress, warping, and inspection complexity, especially under cyclic loads, which will be discussed later in this article.
A Potential Solution: OPECM
Most, if not all, internal finishing methods inherently require some sort of tool access. Mechanical tools and brushes are limited by line-of-sight, and non-contact methods like EDM or laser methods still require direct access (also introducing thermal issues). Abrasive flow machining, a notable exception that involves a viscous medium put through channels, may over-polish high-velocity regions or under-polished ‘dead zones’ and often struggles in uniform machining around elbows and tight ends.
Voxel’s OPECM methodology extends the key principles of PECM (no mechanical contact, thermal loading, and minimal tool wear) into new territory by using a multi-axis oscillatory cathode motion rather than a single-axis feed. The oscillatory motion allows all surfaces in a given region to spend comparable time in the inter-electrode gap (IEG) zone and access unique internal areas not otherwise accessible with a single-axis cathode/tool motion. This is critical as electrochemical material removal is extremely sensitive to both cathode proximity (IEG) and the exposure time, and OPECM enables even and uniform material removal across a wide range of areas within a given part.
A common mistake is treating OPECM as simply “PECM with more motion.” The oscillation is important, but the method still depends on field localization, electrolyte exchange, cathode design, fixturing, and inspection strategy. Those variables determine whether the process can improve a difficult internal surface without creating new dimensional or uniformity problems.
In one series of Inconel 718 “serpentine” test coupons, as-printed, non-line-of-sight, internal channel walls that showed typical AM artifacts (staircase features, partially fused powders) were reduced from ~5µm into roughly the 0.24-0.45µm range, representing about an order-of-magnitude reduction in roughness in internal areas impossible to reach by conventional methods without splitting the part.
Internal AM finishing is often oversimplified as an access problem. Access matters, but the harder question is whether a finishing method can remove material where needed while protecting the surfaces, flow paths, and dimensions that already work. That judgment depends on more than surface roughness alone.
Other OPECM testing on more conventional geometries, such as testing on printed shrouded turbine blade sections, showed other promising results, mainly on downskin surfaces. Regions that were 300-400µm off-nominal were brought down to about 20-40µm using an oscillatory cathode, and across the blades, ~7um Ra was reduced to 1.3-1.6um Ra while faithfully preserving overall profile curvature.
This is where Voxel’s vertically integrated process model matters. OPECM requires coordination between cathode/tooling design, electrolyte methodology, machine motion, fixturing, inspection, and production planning. Owning more of that process stack helps Voxel evaluate whether an internal AM feature is a realistic finishing candidate, what must be proven first, and how the method could scale beyond a lab result.
OPECM still uses the fundamentals of ECM and PECM, including non-contact machining, non-thermal material removal, minimal tool wear, and potential parallelization through multi-feature cathodes. The scalability case still depends on geometry, fixturing, electrolyte management, inspection requirements, and whether the application justifies dedicated process development.
Use Cases
Beyond improving internal surface quality itself, OPECM also addresses several manufacturing bottlenecks that engineers routinely encounter when working with complex AM components.
Eliminating Split-Part Designs & Re-Welding/Re-Brazing
A common workaround in AM is to split a part in two, machine/polish the internal surfaces, and then re-weld or braze the halves back together. While this methodology solves one key problem, it tends to introduce several more in its stead.
The key issue is that these thermal processes like welding and brazing can inherently change the microstructure of a given material, as the localized heat can produce heat-affected zones (HAZ) with potentially differing grain size, hardness, and residual stress resistance. These molecular changes are known to introduce a variety of issues, including reduced fatigue performance and microcracks. Even without these issues, a perfect weld can still come with measurable angular distortions or peaking, more notable in thin-walled or curved components. These issues may even be interpreted as undermining one of AM’s key advantages in the first place: to avoid multi-part assemblies.
Method | Line-of-Sight Required | Thermal Effects | Uniformity in Bends | Internal Feature Access |
Mechanical tools | Yes | Yes | Poor | Limited |
EDM / Laser | Yes | Yes | Moderate | Limited |
Abrasive Flow Machining | No | No | Inconsistent | Moderate |
OPECM | No | No | High | High |
This table summarizes general tendencies, not a universal process-selection rule. OPECM may be attractive when enclosed geometry, surface quality, or re-welding risk creates a serious constraint. Feasibility still depends on whether the cathode approach, electrolyte flow, removal target, dimensional requirements, and production plan can work together.
Enabling Design Freedom & Miniaturization
By eliminating some of the constraints of internal additive finishing, manufacturers can unlock further potential of additive methods without sacrificing surface quality or dimensional stability. For instance, designers can make tighter channel bends, smaller hydraulic diameters, and more compact flow networks for heat exchangers.
As previously mentioned, internal surface quality is essential for many critical industries: thermal pathways within direct-to-chip cooling infrastructure relies on smooth surfaces to distribute heat efficiently, and the sterility and safety of a number of medical devices rely on smooth surfaces, such as to mitigate microscopic bacterial growth in surface irregularities and to prevent the chipping of potentially cytotoxic materials in implantable medical devices.
The ability to machine these small, non-line-of-sight internal features can also allow further miniaturization efforts already prevalent in critical industries, already enabled by AM:
Aerospace components are already reliant on tightly-integrated features for the purpose of maximizing space: to allow designers to place materials precisely where they are needed, while removing it elsewhere. Additionally, miniaturized features result in less weight, improving fuel efficiency at the system level.
Medical device components benefit from miniaturization for other reasons: smaller tools can reduce surgical invasiveness (shortening recovery times and potentially enabling new surgical methods altogether). For implantable devices, miniaturization can improve patient comfort, mobility, and long-term outcomes of implantation.
We have extensively written on the topic of part miniaturization: why it’s happening and what it can enable for critical industries. Consider reading more.
Enabling Scalability
Deploying additive methods across critical industries also means manufacturers must optimize for build rate and cost-per-part. Unfortunately, this sometimes results in cost-cutting methods including thicker layer heights, larger powder sizes and faster scanning strategies. While these methods do help enable better throughput, they also result in inferior surface qualities and more effort for postprocessing/finishing methods. As a few examples, thicker layer lines can produce staircase effects and incomplete melt overlaps, larger powder sizes can result in non-uniform particle fusing, and faster scanning strategies can lead to sloppier overlapping and layer-to-layer precision. If the postprocessing methodology is inefficient, AM companies are trading off improved process throughput for more post-processing efforts that can undermine or even eliminate the gains from these throughput efforts.
OPECM may help some manufacturers separate print-throughput decisions from final internal surface requirements. In the right application, this could allow teams to prioritize build efficiency first, then recover critical internal surfaces through a controlled finishing step. That strategy still requires review of dimensional margins, removal targets, inspection criteria, and production economics.
For applications where split-finishing and rejoining create meaningful lead-time, distortion, inspection, or qualification burdens, OPECM may offer a cleaner route. The value case depends on the part geometry, the surfaces that need finishing, and the cost of the current workaround.
In Summary
Limitations continue to mount for additive processes as the industry moves towards scalability; the industry will increasingly care less about what can be accomplished geometrically, but what can be both finished and scaled. Internal surface finishing has proven to be a critical bottleneck, most notably for critical-environment components that rely on thermal performance, flow behaviour and fatigue life to succeed.
Why early review matters: OPECM review can help engineering teams identify which internal surfaces drive performance, which roughness targets matter, which dimensions must be protected, and whether the AM design should change before production. In some cases, OPECM may support enclosed features that would otherwise require splitting, joining, or accepting rough internal surfaces. In other cases, the review may show that abrasive flow machining, design changes, or a different finishing sequence is the better path.
Geometry Type | As-Printed Condition | Post-OPECM Result | Key Outcome |
Internal serpentine channels (Inconel 718) | ~5 µm Ra | 0.24–0.45 µm Ra | ~10× Ra reduction |
Turbine blade downskins | 300–400 µm form error | 20–40 µm deviation | Form correction |
Blade surface roughness | ~7 µm Ra | 1.3–1.6 µm Ra | Smooth + shape preserved |
Voxel’s OPECM methodology may offer a path forward for selected AM components with enclosed or non-line-of-sight surfaces. By using oscillatory cathode motion to finish internal regions, OPECM can reduce roughness, support design options that avoid splitting and rejoining, and help manufacturers evaluate whether more efficient print strategies can still meet final surface and dimensional requirements. The right fit depends on the internal geometry, material, removal target, tolerance stack, inspection method, and production case.
If you work with AM components that rely on finished internal features, send Voxel the material, internal geometry, surface target, tolerance requirements, current finishing method, and annual volume. We can help assess whether OPECM is a realistic fit or whether another finishing path is more appropriate.
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