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Albert Tsang, Technical Manager, Precision Micro
Despite the fact that photochemical etching has been a machining technology for over 50 years, it is still a relatively low profile process, and its practical use in a variety of manufacturing scenarios with an ever broadening number of materials is one of industry’s best kept secrets.
Commonly misrepresented as a prototyping technology, photochemical etching is in fact a versatile and increasingly sophisticated metal machining technology, with an ability to mass manufacture complex and feature-rich metal parts and components.
Using photo-resist and etchants to chemically machine selected areas accurately, the process is characterised by retention of material properties, burr free and stress free parts with clean profiles, and no heat-affected zones.
Coupled with the fact that photochemical etching uses easily re- iterated and low cost digital tooling, it provides a cost-effective, highly accurate, and speedy manufacturing alternative to traditional machining technologies such as stamping, pressing, punching, and laser and waterjet cutting.
However, for any machining technology to remain viable and relevant in modern and innovative manufacturing projects and environments, it must adapt to the needs of industry. Specifically, machining technologies these days must be able to work to extremely high tolerances, and on more ‘modern’ metals which are sought after in various industrial applications.
As such, companies such as Precision Micro that have invested in research and development in the field of photochemical etching have focused huge amounts of time and money refining the process for use on aluminium, a metal that is used more and more due to its inherent characteristics, but which is notoriously difficult to work.
Many companies that have used traditional manufacturing processes on aluminium, for example, will be familiar with some issues such as the reflective nature of aluminium (which makes laser cutting problematic), and the fact that aluminium coats dies which makes punching unreliable.
This article analyses developments in the area of aluminium machining via photochemical etching, and looks at the opportunities that are therefore opened up for product designers in various industry sectors.
Aluminium — A Versatile Material
Aluminium alloy is the material of choice in a number of industrial applications due to the fact that it is corrosion resistant, light and strong. In this way, it exhibits many of the attributes of titanium, the strength-to-weight ratios of each metal being a key attribute. However, aluminium is significantly cheaper than titanium, this lower cost not so much being a product of its abundance and ease of production (it is in fact quite expensive to extract from the ore bauxite) so much as its recyclability. It is in fact suggested that within 20 years there will be no need to mine for more aluminium as enough will have been mined and will be continually recycled to meet demand.
Aluminium is also considerably less dense than titanium. Whereas titanium is stronger and more corrosion resistant than aluminium, it has a relatively low fatigue limit, which makes aluminium the material of choice in numerous aerospace applications, and the automotive, transportation and building sectors.
Its low density is the key, coupled with another characteristic, the fact that it is an excellent conductor of heat and electricity, making it a preferred material for cabling and heat exchangers.
But this characteristic is also its Achilles heel when it comes to working the metal. For example, aluminium in the photochemical etching process reacts with the corrosive chemistry and becomes exothermic, releasing heat energy. This has a knock on effect to the efficiency and accuracy of the photochemical etching process.
Working with Aluminium
So it is its exothermic nature that introduces hurdles for the efficient application of photochemical etching processes to aluminium, as well as the fact that aluminium is a corrosive resistant metal, and photochemical etching is a machining process based around selective corrosion.
Aluminium is amphoteric in nature meaning that it will react with acids or alkalines. As such it can be photochemically machined using either acidic or alkaline etchants, but use of either presents challenges.
The key to successful photochemical etching is strict process controls. Due to the amount of research and experience in this field, Precision Micro is today perhaps the most adept company in Europe at the application or the photochemical etching process to aluminium.
The highly exothermic reaction of aluminium to the etching process can in some instances destroy the photo-tool, and creates a rough etch. Effectively, the exothermic reaction causes the resist to pull away from the aluminium sheet, allowing the etchant to seep under the resist and compromising the etch. This leads to a rough, jagged, granular edge, a long way away from the smooth and straight etched profiles typical of the photochemical etching process.
Some research into the efficient etching of aluminium has focused on the etchants used, some on temperature control, and still more on the cleaning and preparation of the metal.
Due to the exceptional demand for aluminium in a variety of industrial applications, and the requirement for a machining process that is both repeatable, consistent and overcomes such issues as already highlighted with laser cutting and punching, Precision Micro has invested considerable resource (some £500,000) in lamination equipment to improve the adhesion of the resist to the aluminium.
In addition, the company has utilised its five-decade long experience of photochemical etching to refine and adjust the concentrations of the corrosive chemistry used on aluminium to overcome some of the inherent issues involved with etching an amphoteric material.
Such adaptations of chemistry have proved satisfactory for Precision Micro’s numerous client projects with additional emphasis on a range of other factors including process speed adjustment and temperature control. All these factors are inter-related, and one can have a negative impact upon the other. For example, adjusting speed of etching can lead to resist lift, so the optimisation of the process is a balancing act, success based on considerable expertise honed over many years and numerous projects.
Practical Applications
Through the reconfiguration of machines, chemistry and process parameters, Precision Micro has been involved in countless successful projects involving photochemical etching of aluminium.
One such was work was undertaken for a leading helicopter manufacturer and involved the etching of aluminium air intake grilles. This was a not a mass volume project by any means, and the parts were certainly not as feature rich as many that Precision Micro works with (photochemical etching typically being associated with high volume runs of complex parts). However, etching was the chosen manufacturing process as other alternative technologies introduced stress into the components. As this was an aerospace application, weight was a key factor, so the client demanded the use of aluminium due to inherent weight savings, and the project was successfully completed thanks to Precision Micro’s combination of chemistry, process, and machine adaptations.
In another aerospace application, Precision Micro successfully applied similar techniques to the manufacture of numerous different aluminium parts that made up a dehumidifier used in aircraft cabins. For this application, aluminium was used for a variety of reasons, key among which was its low weight, but also its thermal characteristics. Precision Micro produced etched channels and precision apertures in many of these parts, photochemical etching being the manufacturing process of choice as use of stamping would have been prohibitively expensive due to the need for 36 different stamping tools. In addition, stamping would not have been able to achieve the necessary cross section of the profiles which were essential to achieve the flow and thermal profile specified. Also, photo-chemical etching meant that there were no re-cast layers which are typical of some machining processes such as EDM.
One final application was for an audio equipment manufacturer that required planar ribbon tweeters for high-end recording studio speakers. On this project, Precision Micro etched 20 micron aluminium from a 40 micron kapton backing which needed to be kept intact. This material would have been too thin to stamp successfully, and stamping would also have compromised the kapton backing.
Conclusion
The fact that across industry, aluminium is now so often the material of choice requires that any metal manufacturing process is able to efficiently and precisely work the material. Photochemical etching is one of today’s most cost-effective and accurate manufacturing technologies, especially for the high volume manufacture of complex and feature-rich products and components. The inherent accuracy of the process, the fact that it induces no stress in worked materials, and the fact that the use of the process provides massive tooling costs savings, means that OEMs are clamouring for the process in a variety of metals.
The nature of aluminium is such that it has until recently provided significant challenges for photochemical etching practitioners. However, Precision Micro has developed and successfully applied photochemical etching to aluminium in a variety of real industrial applications. Through a mixture of five decades of experience with the technology, massive investment in equipment and research into the etching chemistry, and optimisation of process control, Precision Micro is one of the only companies able to provide consistent, accurate and replicable manufacturing of aluminium parts through photochemical etching.
Albert Tsang is the Technical Manager at Precision Micro, Birmingham, UK, with 13 years’ experience at the company. For over 50 years, Precision Micro has pioneered photo chemical etching, a manufacturing technology using subtractive chemical erosion to produce burr- and stress-free precision metal components. The company creates highly innovative solutions to a wide range of engineering challenges using a 2D process to create 3D components that cannot be created with other technologies and, often, by any other competitor. That is why Precision Micro has won a reputation as an industry innovator, trusted to deliver by major global manufacturing concerns across multiple markets.