There are lots ways to develop light-gauge metal parts: die cutting, turret press, and photochemical milling are all options. Photo-etching is a quick and economical method that can hold good tolerances in light-gauge metals during the prototyping phase.
Photo-etching is a chemical milling process that uses similar principles to printed circuit manufacturing technology. The shape of the desired part is printed onto optically clear photographic film. This “phototool” is a two sheet negative of the required shape, which is then laminated onto a metal sheet top and bottom with an ultraviolet-sensitive photoresistant material.
Exposure to UV light then hardens the areas of the resist layer that are in the clear part of the film. Afterwards, the unexposed resist is washed away, leaving the areas that are to be chemically etched unprotected. An acid treatment then eats away the unprotected metal. Post bath, the acid is neutralized and the part sheet is rinsed, cleaned and dried. Finished parts are then removed from the skeleton.
Why Photo-Etch?
When a product delivery team is starting a new project, it’s typically under pressure. There are delivery dates, quality expectations, and cost targets, often with inflexible and demanding specifications. That pressure can lead the team to release a metal proof-of-concept component that’s not ready for a high-speed stamping press. If the end goal of manufacturability is not addressed early, that team can look forward to an expensive redesign in the near future. The low cost nature of photo-etching lets engineers validate small part designs before the hard tooling stage.
Determining Part Function
The first step to selecting a manufacturing process for a light-gauge part is determining what the part function will be. Typical parts that respond well to the photo-etching process include springs, battery holders, grounding clips, tabs and connectors, among many others.
Next on the decision list is what material will function best in the application? Are there any special mechanical properties needed such as spring force, wear, flexibility, solderablity or corrosion resistance? Unfortunately, metals are often specified without first checking if that material can be worked into a usable final part.
Solving Metal Forming Issues
All design is an iterative process, and changes are inevitable. Photo-etching gives a part designer room for adjustment to yield a functioning part. Photo-etching also has the unique capability to address both sides of a part. Lines, logos and pockets can be etched in one side but not the other.
It's quick and cost effective, but photo etching is a very different process; for example, the photo-etching process requires a defined area to hold the part in a large sheet as it is being processed. This tab area always leaves an artifact in the metal when parts are removed from the sheet. The location of the tab may be incidental during prototyping; however, as volumes increase and production goes from hand assembly to automated production, the tab mark location often becomes an issue.
There are two ways to deal with the tab mark. First, you can locate the tab in an area where it will not affect the final product during automated production. The second option is to hard-tool the part as soon as the design is deemed matured. The latter is always preferable but may not be an option in early-stage production. For moderate volumes, many engineers “design around” the small projection or specify a simple second operation to deburr the part.
Few engineers have the luxury of part design with no timeline. Every engineer that has “gone through the alphabet” when drawing part revisions knows the frustration and cost escalation of multiple changes. Photo-etching/forming offers the flexibility for a quick, reasonable cost revision. In comparison, hard tooling has an extended lead-time, from development to first article approval. It’s also notoriously expensive to change. Of course there are advantages to stamping with progressive dies: tolerances are tighter and the unit cost over large volumes is much lower than the photo-etch and form method, but for prototype pilot and short run production, low-cost, easy to implement part revisions are too useful for any engineer to ignore.
Why can’t these advantages be enjoyed in longer runs? In many cases, they can, especially where many revisions are expected. Every engineer knows that demanding customers combined with demanding specifications can generate multiple small part headaches. Photo-etch and form can be a great way to stop redesign pain before it starts.
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