Karl Hollis, engineering director, Precision Micro
Engineering is all about problem solving but, as the profession develops, the methods for solving problems can become overcomplicated, thus necessitating a simpler solution. These problems are encountered in the machining of complex high-precision components that are today required by almost every industry, including aerospace, automotive, electronics and healthcare. The most commonly used techniques for shaping and cutting detailed design features into these components are laser cutting and stamping, but their limitations and the need for others to compensate have led to a rock-paper-scissors approach to solving three key issues, namely thermal and cutting-edge stress, burring and material limitations. Furthermore, there are R&D to production process hurdles that need to be considered. This article takes a closer look at the limitations of laser cutting and stamping, and how they can be better overcome using photochemical etching.
Photochemical etching versus conventional processes
Laser cutting and stamping are the two most commonly used traditional sheet metal machining technologies but there are several other options. All have their uses and applications, but how does photochemical etching compare for flexibility, usage and performance? Below is an at-a-glance overview.
Chemically etched location plates used in the manufacture of X-ray beam collimators.
The photochemical etching process
Photochemical etching (often shortened to chemical etching or photo etching) is a subtractive sheet metal machining technology. It is a cost-effective means of manufacturing complex, high-precision metal components in almost any metal for numerous and, in some cases, extreme applications.
The main steps of the photochemical etching process can be summarised as follows:
- A photoresist is laminated onto the sheet metal.
- A CAD image of the component is laid over the sheet metal and printed via exposure to ultraviolet (UV) light. Those areas not covered by the image are cured and those covered remain unexposed.
- The sheet metal is developed, meaning that the unexposed photoresist is removed to leave those areas unprotected.
- A safe and sustainable chemical etchant is sprayed onto the developed sheet metal to remove the unprotected material and thus etch the component design.
- The remaining photoresist is removed to reveal the component.
This process offers tighter tolerances than those afforded by laser cutting and stamping, and is capable of producing the engraved features needed for specialist components such as bipolar fuel cells and heat exchanger plates.
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A chemically etched flexure spring used in automotive fuel injection systems.
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The complete, nine-step photochemical etching process.
Thermal and cutting-edge stress
In metal machining, stress is a constant factor limiting effectiveness. It primarily takes two forms: thermal stress and cutting-edge (or cutting-section) stress, the former a concern for laser cutting, and the latter an enduring issue for stamping.
Thermal stress
Heat is always a by-product, particularly in the case of laser cutting sheet metal. The laser cutting of sheet metal generates a continuous heat source, moving at constant speed along the cutting edge. This can build up to high levels of thermal stress that has the potential to deform the material at the micron level. This is a huge disadvantage as sheet metal components are typically used in critical applications and even the slightest deformation can inhibit their effectiveness.
Numerous academic studies have attempted to account for thermal stress, model its effects and propose counteractive methodologies. As a result, thermal stress fields can, to a large extent, be predicted and accounted for. However, heat generation is a basic principle of laser cutting and therefore thermal stress is always going to be a significant obstacle.
Cutting-edge stress
The aforementioned limitations of laser cutting often lead engineers to choose stamping as an alternative. However, the obstacle of heat generation is replaced by physical impact on the cutting edge. This is often due to shearing, a process that uses cutting steels to cut sheet metal to create desired components, and can have a detrimental effect on the flatness of the blank and compromise the quality of the component, particularly in the case of high-precision components.
Although post-machining methods such as overbending and applying additional force can reduce the impact on flatness, they add further time and expense to the production process.
Eliminating stress through photochemical etching
A degree of overengineering is required to overcome the challenges of laser cutting and stamping. A far simpler solution is to employ photochemical etching, since it does not involve heat generation or physical impact and thus eliminates the thermal stress of laser cutting and cutting-edge stress of stamping, respectively. This, in turn, allows for a significant reduction in the risk of deformation and therefore quality control issues.
Burring
Both laser cutting and stamping are capable of machining components in fine detail, but as heat generation and physical impact processes, they inevitably cause burring.
Accounting for the impact of the burred edge
Burring has the potential to compromise the accuracy, dimensions and therefore end-uses of high-precision components. Even the smallest burred edge can have a major impact on the performance of a component, particularly if said component needs to be exactly crafted and configured to perform reliably in a critical application.
Both laser cutting and stamping require greater or lesser degrees of deburring, typically through mechanical processes such as linishing or barrelling, or electrochemical means, depending on the material in question and the nature of the component being created. This can account for a significant proportion of the overall manufacturing cost.
Eliminating burring through photochemical etching
By the same means that photochemical etching eliminates the potential for thermal and cutting-edge stress, it also eliminates the potential for burring. No heat generation or physical impact is involved, therefore there is no stress resulting in deformation, and no burred edges.
Photochemically etched stainless steel components exhibit no altered physical properties and are made to exacting specifications. They can be for mission-critical satellites, safety-critical anti-lock breaking systems (ABS), fuel-injection solutions or corrosion-resistant microfilters.
Material limitations
It is imperative to know the physical properties and limitations of chosen materials at the design stage of any manufacturing project.
Metal properties and parameters
Across the spectrum of metals, from stainless steel to aluminium, titanium and more specialist materials, there is a huge variance in hardness and integrity. Any machining process needs to account for these, so that the resulting components adhere to all relevant standards of quality and performance.
Stainless steel is something of an industry standard, used across applications thanks to its versatility and durability. Aluminium is lighter, more flexible and more desirable for aerospace and heat-exchange applications, but it is also significantly more difficult to machine than sheet metal staples. Aluminium and its alloys tend to work-harden when punched or stamped, and their high heat reflectivity makes them extremely challenging to laser cut. Titanium has low thermal conductivity and chemical reactivity, making it the material of choice for many medical applications from cranial and dental implants to pacemaker battery collector grids, but issues arise if profiling complex high-precision components. Titanium has the highest strength-to-density ratio of any metal and can be extremely tricky to manipulate without damaging tools and equipment.
This is a brief overview of the limitations of a select few materials, serving to highlight the shortcomings of traditional sheet metal machining technologies.
Eliminating material limitations through photochemical etching
As outlined, using traditional sheet metal machining technologies on a number of certain metals results in difficulties; but photochemical etching can be used on almost any metal. Thousands of types of complex high-precision components can be photochemically etched from metal sheets as thin as 0.010 mm and long as 1,500 mm. In addition, digital photo-tooling is inexpensive, easily reworked and affords high accuracy and repeatability, helping to ensure economical and efficient production.
The fact that achieving a finer finish on high-precision components is often hampered by the limitations of metals such as aluminium and titanium means it is necessary to look beyond traditional metal machining technologies.
R&D to production process hurdles
There is a clear thread from R&D to production, or from concept to completion. In an age that is seeing cost pressures becoming ever greater, it is important to consider how the machining of complex high-precision components is going to impact the two typical hurdles of a project, namely lead time uncertainty and cost.
Lead time uncertainty
The biggest factor affecting manufacturing supply chain performance is lead time uncertainty. It can result in increased inventory costs and an inconsistent service; the adoption of just-in-time logistics and ordering processes go some way to mitigating these issues, but the photochemical etching process is also able to play a part.
The unforeseen consequences of stress, need for additional machining processes such as deburring and inadequate consideration of material limitations are all significant contributors to lead time uncertainty. Photochemical etching eliminates stress, burring and material limitations, thus providing certainty in lead times that are typically days rather than the weeks or months expected using traditional sheet metal machining technologies.
Cost
As far as traditional sheet metal machining technologies are concerned, complexity is often a byword for cost if using non-standard materials, material grades or thicknesses and additional machining processes. On the other hand, photochemical etching allows for the creation of feature-rich, near-infinite geometrically complex components but is not subject to these limitations and their costs.
Furthermore, photochemical etching is a more economical approach to prototyping, since it allows engineers to pay by the sheet. This means components that have different geometries and almost limitless complexity can be processed using a single tool in one production run.
Summary
This article has explored some of the critical limitations of laser cutting and stamping and explained how photochemical etching can overcome these. To quickly recap, these are:
- thermal and cutting-edge stress—no heat generation or physical impact means that the issues of thermal and cutting-edge stress are completely eliminated;
- burring—no heat generation or physical impact prevents burred edges; and
- material limitations—the ability to use almost any metal means that the required density, hardness and strength of the finished component are not compromised.
Additionally, the elimination of stress, burring and material limitations means that photochemical etching provides certainty in lead times that are typically days and is low cost.
All of these advantages provide a fitting explanation as to why photochemical etching is proving the sheet metal machining technology of choice for an increasing number of engineers.
Precision Micro