Jochen Kern presents the design engineer’s guide to photo-chemical etching
As a precision sheet metal machining technology, photo-chemical etching (PCE) achieves exacting tolerances, is highly repeatable and in many instances is the only technology that can cost-effectively manufacture precision metal components with the accuracy necessary in demanding and often safety-critical applications.
Once design engineers select PCE as their preferred metalworking process, it is important that they fully appreciate not only its versatility, but also the specific aspects of the technology that can affect – and in many instances enhance – product design. Here, we analyse the key considerations that design engineers must appreciate to get the most from PCE, and benchmark the process against other metal machining technologies.
PCE has many attributes that can truly stimulate innovation and ’stretch the boundaries’ with the inclusion of challenging product features, enhancements, complexity and efficiency. It is vital that design engineers fully exploit its potential, and micrometal (incorporating HP Etch and Etchform) advocates that its customers view it as a product development partner – not just a sub-contract manufacturer – early in the design stage, allowing OEMs to optimise the potentials that this versatile metal machining process can offer.
General Technology Attributes
Photo etching can be applied to a vast spectrum of metals in a variety of thicknesses, grades, tempers and sheet sizes. Each supplier is able to process different thicknesses of metal with different tolerances, and when selecting a PCE partner to work with, it is important to interrogate precisely what capabilities they have. For example, when working with micrometal’s etching group, the process can be applied to thin metal sheets ranging from 10 microns to 2,000 microns (0.010-2mm) with a maximum sheet/component size of up to 600 x 800mm. Metals that can be processed include: steel and stainless steels, nickel and nickel alloys, copper and copper alloys, tin, silver, gold, molybdenum, aluminium and hard to machine metals including highly corrosive materials such as titanium and its alloys.
Tolerances are a key consideration in any design and with PCE vary dependent on the material thickness, the material, and the skill and experience of the PCE supplier.
The micro metal etching group’s process is able to produce intricate parts with tolerances as low as ±7 microns depending upon material and its thickness, this tolerance attainment being unique among all alternative metal fabrication technologies. The company uses a special liquid resist system to obtain ultra-thin (2-8 micron) photoresist layers, enabling a higher degree of precision in the chemical etching process. It allows the group to achieve extremely small feature sizes of 25 microns, a minimum hole diameter 80% of the material thickness, and single digit micron tolerances repeatably.
As a guide, the etching group can process stainless steel, nickel and copper alloys up to 400 micron thick with feature sizes down to 80% of material thickness and tolerances of ±10% of thickness. Stainless, nickel and copper above 400 micron thickness, and other materials such as tin, aluminium, silver, gold, molybdenum and titanium can have feature sizes down to 120% of material thickness with tolerances of ±10% of thickness.
Traditional PCE uses relatively thick dry film resist, which compromises ultimate part precision and the tolerances that are available, and it is only able to achieve 100 micron feature sizes and a minimum hole diameter of 100-200% material thickness.
In some instances, traditional metal machining technologies can achieve tighter tolerances, but there are limitations. For example, laser cutting can achieve accuracy to 5% of the metal thickness, but it is limited to a minimum feature size of 0.2mm. PCE can achieve a minimum standard feature size of 0.1mm and openings below 0.050mm are possible. In addition, it must be appreciated that laser cutting is a “single point” metal processing technology, which means that it is usually more expensive for complex parts such a meshes, and it cannot achieve the depth/engraved features necessary for fluidic devices such as fuel cells and heat exchangers, which are easily attained using depth etching.
Burr- and Stress-Free Machining
When it comes to the ability to replicate the exacting accuracy and minimum feature size capabilities of PCE, stamping probably gets the closest, but the trade-off is the stress that is imparted in the metal when it is processed and the residual burrs that are a characteristic of press-working. Stamped parts need costly post-processing, and as expensive steel tooling is used to produce the parts, it is not viable for short runs. In addition, when processing hard metals, tool wear is an issue, with expensive and time-consuming refurbishment often necessary. Many flexure spring designers and designers of intricate metal components specify PCE due to its burr- and stress-free nature, zero tool wear and speed of supply.
Unique Features At No Additional Cost
Unique characteristics can be designed into products manufactured using photo etching due to the inherent edge “cusp” during the process. By controlling etch cusp a range of profiles can be introduced allowing the manufacture of sharp cutting edges, such as those used in medical blades, or conical openings, such as those used to direct fluid flow in filtration meshes.
For OEMs across all industry sectors looking for feature-rich, complex and precise metal parts and components, PCE is now the technology of choice as it not only copes well with difficult geometries, it also allows design engineers enormous flexibility, facilitating the adjustment of designs right up to the point of manufacture.
One major factor that allows for this is the use of digital or glass tooling, which is inexpensive to produce, and therefore inexpensive to change even up to a few minutes before manufacturing commences. Unlike stamping, the cost of digital tooling does not increase with part complexity, which stimulates innovation, as designers focus on optimised part functionality rather than cost.
For traditional metal machining technologies, it is fair to say that increased part complexity equals increased cost, much of this being a product of the necessary costly and intricate tooling. Costs also escalate when traditional technologies have to cope with non-standard materials, thicknesses, and grades, none of which have any effect on the cost of PCE.
As PCE does not use hard tools, distortion and stress are eliminated. Also, components produced are absolutely flat, with a clean surface and no burrs as the metal is dissolved away uniformly and evenly until the desired geometries are achieved.
With PCE, you pay by the sheet, not by the part, which means components with different geometries can be processed at the same time from a single tool. This ability to produce many part types in one production run is the key to the enormous cost savings inherent in the process.
PCE can be applied to virtually any metal type whether soft, hard or brittle. Aluminium is renowned for being difficult to stamp due to its softness, and difficult to laser cut due to its reflectivity. Equally, titanium’s hardness can prove challenging. Micrometal has developed proprietary processes and etching chemistries for both of these specialist materials and is one of the few etching companies in the world with a titanium etching facility.
Add into the mix the fact that PCE is inherently speedy, and the rationale behind the exponential growth in uptake of this technology in recent years is obvious.
Design engineers are increasingly turning to PCE as they are under pressure to create smaller and increasingly complex precision metal components. As with any process selection, designers need to be aware of the specific attributes of the chosen manufacturing technology when looking at design attributes and parameters. The versatility of photo etching and its unique advantages as a precision sheet metal machining technology makes it an engine for design innovation and can truly be used to manufacture parts that would be deemed impossible if using alternative metalworking technologies.
Jochen Kern is with Micrometal