Enclosed, Internal AM Finishing & Voxel's OPECM Methodology
- Kirk Abolafia
- 39 minutes ago
- 7 min read
How oscillatory cathode motion can enable improved non-line-of-sight internal features in critical additive components across defense, medical device and semiconductor manufacturing.
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.
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.
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.
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.
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.
Importantly, OPECM still utilizes the key fundamentals of ECM and PECM, offering similar benefits such as minimized tool wear, apathy for material hardness, and the potential ability to process these features with multi-featured cathodes across features and/or parts for high scalability.
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 |
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.
However, the surfaces and tolerances produced by these lean methodologies are not an insurmountable challenge for OPECM. In fact, OPECM may help manufacturers continue to prioritize print efficiency first, then recover internal surface qualities afterwards, allowing them to maximize throughput without significant worry of out-of-tolerance components on the production line.
A recurring point that can further enable scalability is the ability of OPECM to internally finish monolithic parts without the need to split and re-weld or re-braze components. Between fixture setup, part splitting, internal finishing methods, welding/brazing time, cooling, and inspection, this process can result in significant lead time increases, notably for volume-production environments that OPECM may be capable of eliminating altogether.
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.
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 |
A potential path forward may be found in Voxel’s OPECM methodology, utilizing an oscillatory cathode to machine and finishing internal, non-line-of-sight surfaces and features within critical AM components. OPECM can eliminate key bottlenecks for additive manufacturers by eliminating the need to re-weld or re-braze, enable new design opportunities, and allow manufacturers to continue lean additive methods such as larger powders or thicker layer lines as they continue to scale.
Let’s talk! If you work with AM components that rely on finished internal features, consider contacting us.
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