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Thales turns to photochemical etching for an extreme application

09 November 2014

Photochemical etching is still a relatively low profile process that remains one of industry’s best kept secrets. Bonny Van Geel describes how one major global player adopted the technique for a special application.

Commonly misrepresented as merely prototyping technology, photochemical etching is in fact a versatile and increasingly sophisticated metal machining technology for mass manufacturing 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 metal stamping, pressing, CNC punching, and laser and water-jet cutting.

Traditional machining technologies can produce less than perfect effects in metal at the cut line, often deforming the material being worked, and leaving burrs, heat-affected zones, and recast layers. In addition, they struggle to meet the detail resolution required in the ever smaller, more complex, and more precise metal parts that many industry sectors require.

There are instances — typically when an application requires multiple millions of parts and absolute precision is not a priority — when these traditional processes may be the most cost-effective. However, if manufacturers require runs up to a few million, and precision is key, then photochemical etching with its lower tooling costs is often by far the most economic and accurate process available.

Another factor to consider in process selection is the thickness of the material to be worked. Traditional processes tend to struggle when applied to the working of thin metals, stamping and punching being inappropriate in many instances, and laser and water cutting causing disproportionate and unacceptable degrees of heat distortion and material shredding respectively. While photochemical etching can be used on a variety of metal thicknesses, one key attribute is that it can also work on ultra-thin sheet metal, even as low as 10 micron foil.

It is in the manufacture of intensely complex and feature-rich precision parts that photochemical etching really finds its perfect application, as it is agnostic when it comes to shapes and unusual features in the products to be manufactured. The nature of the process means that feature complexity is not an issue, and in many instances, photochemical etching is the only manufacturing process that can accommodate certain part geometries.

A good example of this is a recent partnership between Eindhoven based Thales Cryogenics and the Birmingham based photochemical etching specialist, Precision Micro that involved the manufacture of a critical flexure component for a satellite cryogenic cooler. There is no more extreme environment than space, which demands that parts and components not only work as intended, but do so over prolonged periods of time. The photochemical etching process was thus chosen as the preferred manufacturing method for this critical component.

As a manufacturing process, photochemical etching has one key attribute when it comes to part integrity, and that is that during the production process, it does not affect or degrade material properties.

The specific flexure used in the cryogenic cooler has been made since the 1990s. In early assessments, traditional machining processes were found to leave small burrs on the parts and recast layers that would ultimately compromise their performance – hence the move to photochemical etching, which induces no tension in the material being worked, and leaves no burrs.

While cost is an ever present concern when assessing alternative manufacturing technologies, in niche and highly critical part manufacture like the flexures for the satellite application, its importance is matched by the location and use of a process that guarantees accuracy, repeatability, and conformance with extremely exacting tolerances and maintenance of material integrity.

Such considerations were of especial concern for Thales Cryogenics as the flexure component was essential to the reliability and system life-expectancy of the company’s LSF and LPT coolers, which are sold to a variety of customers that use them to cool high-sensitivity sensor systems. Examples include gamma ray detectors, detectors for thermal night vision, and infrared spectroscopy detectors for earth observation satellites.

As these linear coolers contain pistons moving back and forth at around 50Hz, mounting them on a flexure spring with a radial stiffness and a low axial stiffness is essential for contact-free operation, which ensures that little or no wear occurs during use. Thales Cryogenics found that flexures made using the photochemical etching process met these demanding requirements and retained their properties even after many years of continuous operation.

As Thales Cryogenics assessed the nature of the flexure required, it looked at a number of manufacturing processes and suppliers before choosing Precision Micro and photochemical etching. The purity of the metal used in the flexure, however, was ultimately the key to supplier selection, as it was vital that potential fracture sites in the grain were eliminated, and this was only achievable with the consistency required through the use of chemical etching in combination with Precision Micro’s expertise and manufacturing experience.

Photochemical etching’s use of digital tooling ensures that multiple tooling iterations that are often necessary to perfect the precise nature of such intricate metal parts are not costly in terms of time or money. In addition, the consistency of the process means no time consuming and potentially costly retooling and revalidation are necessary. In the case of the flexure produced for Thales Cryogenics satellite cooler, all these attributes combined to make photochemical etching the manufacturing process of choice for this especially critical and exacting application.

Precision Micro has pioneered photo chemical etching, a manufacturing technology using subtractive chemical erosion to produce burr- and stress-free precision metal components, for over 50 years. The company is able to meet a wide range of engineering challenges using a 2D process to create 3D components that cannot be created with other technologies (and, in some instances, by another competitor).

Bonny Van Geel is technical sales manager, Precision Micro

Precision Micro has recently purchased a four-chamber etch machine - the first in a series of investments that the company believes will help maintain its position as Europe’s leading supplier of precision etched components.

The new machine — built to Precision Micro’s specification — comes with comprehensive data logging functionality allowing for full machine control and energy monitoring.

Photo-etching is precisely suited to applications where the requirement is for small, precise, complex, feature-rich parts with no burrs, and no stress-related changes in the metal, which can occur using alternative metal forming technologies. 

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