Microhole Arrays, "EHAR", and PECM
PECM is finding new applications across critical industries in machining challenging features such as microhole arrays and high-aspect ratio features. To learn more about PECM's capabilities, or to simply learn how it works, see our other articles.
Modern market trends are creating new challenges for manufacturers-- among the most prevalent of these being miniaturization, or the process of critical components and features gradually reducing in size to meet market demands. Orthopedic and surgical device components, for instance, may be miniaturizing to reduce the invasiveness of surgical procedures or to improve the mobility of certain implants. Within aerospace applications, certain components such as heat exchangers, when miniaturized, can lightweight the aircraft and improve fuel efficiency of the engines.
One challenging feature to miniaturize is microholes. Microholes are generally considered to be holes with diameters below 2-3mm (.07-.11in), and conventional manufacturing methods are increasingly struggling when tasked with machining these features -- notably when holes are in arrays, and/or when the holes have extreme high-aspect ratio (EHAR) features. These features may become even more challenging on certain materials. Despite these challenges, it is crucial manufacturers find efficient ways to accurately machine microholes for a variety of critical purposes.
Microholes can be advantageous for a few reasons depending on the application. For instance, small microhole arrays may allow increased control of microfluidics (IE the study of the travel of fluids through microscopic channels) which can be utilized to enhance the precision capabilities of drug delivery devices for the medical industry, among other critical applications.
On the other hand, microholes with EHAR features can be used for cooling channels in industrial or aerospace turbine blades, among other critical applications.
In this article, we will discuss the use cases for both microhole arrays and extreme high-aspect ratio features, why they are challenging to machine conventionally, and how pulsed electrochemical machining (PECM) can be applied to machine these unique features.
As a brief disclaimer, for the purposes of this article, EHAR will refer to holes above a 30:1 aspect ratio, but keep in mind that depending on the application this may be as high as 50:1 or as low as 20:1.
Conventional Struggles
There are a few scenarios in which typical material removal processes may struggle to accurately and repeatedly machine microhole features, notably when in-parallel.
For instance, consider processes that utilize heat as a means of material removal such as laser-drilling. Microholes can be especially vulnerable to recast layers, which are deposits of molten material that are removed from the workpiece material but “re-cast” on the workpiece surface due to the heat of the machining process. Recast layers are generally formed where workpiece materials are ejected after machining, such as the entrance of the hole.
These surface irregularities, even when microscopic, can cause significant effects on the hole’s microfluidic capabilities and structural integrity; these effects are intensified further as these hole sizes decrease for more precise applications (such as drug delivery for medical devices). Furthermore, recast layers can impact other material properties such as corrosion resistance and overall material hardness, and can also lead to stress concentrations.
While conventional processes like laser hole drilling are capable of producing these features without recast layers, there is usually a significant caveat in the form of speed and efficiency. Furthermore, even more advanced laser drilling methods often create tapered holes than can limit aspect ratio.
Conventional machining challenges are further exacerbated when creating dense microhole arrays, as even the smallest tooling misalignments or machining inaccuracies can lead to complete part failure or breaking small end mills. With laser methods, processing microhole arrays can be extremely time intensive. Ultimately, difficulties arise when attempting to make the process repeatable and scalable for dense, tight, arrays.
EHAR features are also challenging for conventional processes, including laser-direct drilling methods. microholes may become tapered and have poorly rounded outlets, notably in high-aspect ratio holes when machined via CNC or with laser drilling. The removal of debris and swarf becomes more challenging for these processes as the holes become deeper.
Voxel: Dense Arrays
Voxel’s pulsed electrochemical machining (PECM) technology is well-suited to machining small, dense arrays of microscopic holes for critical manufacturing applications.
Most conventional heat and/or contact-based material removal processes remove larger chunks of the workpiece material at a time, which may lead to a variety of surface or structural irregularities, especially when machining micro-hole features. PECM, on the other hand, utilizes an electrochemical reaction that converts individual metal atoms on the workpiece surface into metal hydroxides that are quickly removed from the machined area via the electrolytic fluid. This atom-by-atom material removal process offers an additional layer of precision, allowing PECM to machine microhole features with precision-- including small diameters, the correct aspect ratios, superfinished internal surface qualities and even the correct angle of the holes.
Most importantly, PECM is particularly effective for applications involving microhole arrays, where dozens or even thousands of holes are required on a single component.
For applications with repeated features, PECM has two distinct advantages over conventional processes. The process’s primary advantage is its significantly reduced rate of tool wear; as there is no heat or contact involved in PECM, a cathode is less likely to be chipped, worn, or distorted by friction or vibration when machining hundreds, or even thousands of features such as microhole arrays. Voxel’s PECM process thereby has high repeatability-- down to <10μm (0.0004 in.).
Another critical advantage of PECM is its ability to machine multiple part features in-parallel in a single cathode operation. Multiple tools can be affixed to a single cathode without the need for additional machining time. This repeatability can occur feature-to-feature or even part-to-part, allowing PECM to machine identical features on multiple parts in a single operation, vastly increasing throughput and production efficiency for high-volume parts.
For instance, Voxel has previously utilized PECM to machine 0.5mm microholes in a large array with only 0.1mm in between adjacent holes. Furthermore, Voxel developed a cathode capable of machining up to hundreds of these microholes simultaneously, machining each array in seconds.
Potential use cases: Drug Delivery
One of the most common ways PECM can utilize its microhole-array machining capabilities is in the medical device manufacturing industry.
Manufacturers’ ability to machine dense microhole arrays can shape the future of both medicine and technology. One critical usage of microhole arrays and their improved control of microfluidics are for certain “drug delivery” devices--topically or implanted devices that provide highly precise microdoses of drugs for patients.
A variety of factors may warrant drugs to be administered via microhole arrays in drug delivery devices as opposed to conventional hypodermic needles or oral delivery, such as for needle-adverse patients, the need to minimize side-effects via microdosing, or the drugs themselves metabolizing too quickly to be used in the digestive system via oral delivery.
Consider, for instance, specialized drug pumps that use osmotic pressure to administer vital drugs to patients. For these systems, the ideal size of the delivery orifice can range from 600μm to 1mm (0.02-0.039in), and may have an array of up to several hundred orifices on a single device.
There are a few methods in which PECM’s high repeatability and superfinishing capabilities could be helpful for producing and improving these types of microhole array features.
For instance, PECM’s superfinishing capabilities may improve the sterilization of medical devices, as low-Ra surfaces may be more susceptible to bacterial growth, or may corrode at quicker rates.
Moreover, improved part features and surface quality (especially internal features of the microholes) may increase the microfluidic capabilities of drug delivery devices. Manufacturers may be able to achieve smaller hole sizes to increase microfluidic precision and sharper features may penetrate the skin easier.
PECM’s ability to efficiently produce microhole arrays from feature-to-feature and part-to-part could increase throughput and reduce long-term manufacturing costs for drug delivery devices, as industry leaders are looking to broaden the usage of drug delivery devices that can be self-administered to be used in developing countries.
Voxel: EHAR
A distinct advantage of Voxel’s approach to PECM is its unique, patented electrolytic flow methods which allow PECM to machine complex, hard-to-reach and non-line-of-sight areas, including extreme high-aspect ratio (EHAR) holes found in many critical applications. The electrolyte’s composition and flow rate are often optimized for different materials and applications in PECM.
Not only is the electrolyte the catalyst for the electrochemical reaction itself, but also acts as a flushing agent that stores the dissolved metal hydroxide particles after the reaction and swiftly removes debris and swarf away from the workpiece. When machining and/or internally finishing EHAR holes, PECM’s ability to quickly dissolve the workpiece material and rapidly remove it from the machined area allows PECM to machine up to 300:1 aspect-ratio holes without any burrs or the formation of recast layers.
Furthermore, PECM can machine variable-diameter holes in high-aspect ratios using patented tooling and electrolyte flow technology.
Potential Use: STEM
Among the most critical applications for PECM’s high-aspect ratio features are found in critical aerospace and energy parts such as turbine blades.
As the role of gas turbines becomes increasingly important for critical aerospace and industrial applications, the issue of heat transfer and cooling capabilities becomes increasingly important, as the amount of power an engine produces is ultimately limited by how much heat that the turbine is capable of withstanding. As temperatures in turbine engines can approach thousand-degree temperatures (capable of melting the superalloys that compose them), the use of cooling features not only significantly extend the lifetime of critical engine parts, but are vital to the engine’s performance and safety.
PECM is adept at machining other critical components in industrial turbine engines, such as microchannel heat exchangers. Learn more here.
One commonly utilized cooling solution lies in deep, EHAR holes in the superalloy turbine blades (or vanes). These cooling holes slowly “bleed” colder air as the turbines are rotated, and this steady stream of air also creates a thin layer of cooler air over the blade, protecting it from exposure to the hotter gases, known as film-cooling. Ideal dimensions for these holes are often between 0.2 and 0.8mm wide, with extreme aspect ratios (30:1 to 300:1 in some cases) to maximize cooling capabilities.
Some common machining challenges occur with the materials used (often superalloys) as well as the unique geometries of the holes themselves; the shape of the blades may sometimes warrant curved holes or complex angles in the blades, and these non-line-of-sight geometries may not be ideal for conventional processes like CNC. Additionally, internal finishing may be needed in these deep holes to ensure proper airflow.
Electrochemical machining technology has been previously utilized to machine these features in a process known as STEM (Shape-Tube Electrochemical Machining) which is capable of machining EHAR features in turbine blades and vanes, and can even be advantageous for curved or complex-angled holes in these blades.
Furthermore, as these EHAR features are often identical across the part, PECM can be advantageous at machining multiple of these features in-parallel (as well as machining multiple features on multiple parts at once) with a single cathode.
These deep channels can contribute to a reduction in internal temperatures-- research suggests cooling channels in turbine blades can contribute to temperature reductions from 1500°C cooled down to 1000°C (2732°F to 1832°F).
Conclusion
Microhole features-- whether in a dense array or with high-aspect ratios-- have important utility for a variety of applications across the medical, energy and aerospace industries. However, machining these features with high tolerances in tough-to-machine materials such as superalloys can be challenging for many conventional material removal processes. PECM, however, can utilize its ability to parallel-process small features and its lack of thermal stresses or recast layers to machine microhole arrays or EHAR features for applications such as drug delivery devices or STEM drilling in turbine blades.
PECM’s ability to machine microhole arrays and EHAR features with high precision, repeatability, and surface quality makes it an indispensable technology for industries demanding miniaturization and performance.
If you have an application that would require tight arrays of micro-machined holes and/or high-aspect ratio features, contact us.
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