Chemical Etching: How to manufacture burr-free metal components

The process of chemical etching is often overlooked in favour of metal stamping, laser cutting or water jetting. However chemical etching, also known as photo chemical machining (PCM, )is a genuine and economical alternative producing burr-free parts. Neil Shorten, MD of Photofabrication Limited explains.

Put simply, chemical etching is a means of fabricating metal components that uses an acidic solution to dissolve unwanted metal. Some people refer to the process as photo etching; it is also known as chemical etching and photo chemical machining.

Just by thinking about the various metal cutting processes available, it is easy to visualise the differences. Stamping (sometimes referred to as blanking) and punching are processes which quite simply use force, cutting the metal using powerful presses. Plasma, laser and electrical discharge machining (EDM) rely on extreme energy, literally burning through metal. Water jet as you might expect, uses pressurised water to cut through metal.Photo etching, by contrast, is like standing a sheet of metal in a shower where the metal you don't need is washed away and after the water is turned off, a sheet of pristine metal parts remains.

The invention of PCM dates back to the discovery of hydrofluoric acid in the 19th century which enabled glass etching.By the early 20th century, the advances in production of stronger resists allowed more highly corrosive etchants to be used on stronger metals.And in 1944, John Snellman patented the process of using outlines in a photoresist template to batch produce uniform, flat components out of metal that was too hard to stamp or cut with traditional machining methods.In the 1960s, the design and manufacture of the printed circuit board further spearheaded the growth and development of the chemical etching process for commercial use as we understand it today.

The basics of the photo chemical etching process include creating a photomask, which is typically derived from CAD files and output on to film via a laser photo plotter. The metal to be etched is carefully cleaned and coated on both sides with a polymer film called photoresist. When applied, the photoresist film is unexposed and this must be done in a yellow safe-light environment. The coated metal and the phototool come together in the imaging process where the black regions on the phototool prevent the exposure of the resist under intense UV light. The unexposed resist is washed away in a developing solution, leaving bare metal in the areas to be etched.

In the etching process, the exposed photoresist is strong enough to withstand the effects of the etchant. But the unprotected metal is dissolved right up to the edge of the resist. The etchant is sprayed on to both sides of the sheet until it is cut through. After etching, the resist is washed away in a different solution which is referred to as stripping.The photoresist films are removed from the substrate without leaving residues.

Photo chemical etching technology can be used to manufacture a broad range of metals and alloys for the production of precise, thin and flat metal parts. The process has many benefits, since the internal structure of the metal remains unchanged. PCM does not affect the properties of the metal with regard to hardness, grain structure, or the ductility of the metal i.e. the metal does not suffer any stress. Where stamping, punching and die-cutting leave deformation due to shearing and laser and water-jet cutting can leave worn edges, photo chemical etching simply dissolves the unneeded metal, leaving a flat and burr-free part.This therefore eliminates the need for costly time-consuming de-burring.

Tooling and set-up costs are also extremely low compared to hard tooling.Therefore, PCM can be used to make one or one thousand parts, with identical tooling used every time.Initial quantities of newly designed parts can be produced at short notice - often within days after designs have been finalised.Revisions to part designs are also quick, inexpensive and simple to implement.This allows engineers and designers to develop concepts and prototypes through to full production quickly and easily.The photo tooling can also accommodate more than one design at a time. Therefore it is feasible to trial two or three options on the same sheet of metal at the prototyping stage.

Photo etching has similarities to a printing process where the component designs can be hugely intricate without having an impact on the tooling or production process. It can produce complex parts that would be either impossible or impractical to produce by stamping or laser cutting.Forming lines can also be half etched into the metal to allow components to be formed by hand.Parts can be made as thin as 10 microns as small as 1 millimetre in diameter or as large as sheet of metal measuring 333mm x 660mm.

Because of the benefits already discussed, it's no surprise that chemical etching is used to fabricate metal parts for many different industrial applications including sensors, shields, retainers, flat springs, strain gauges, filters, screens, grids, shims, gaskets and more.Industry sectors currently using PCM in their production processes are wide and varied including aerospace, automotive, telecommunications, defense, electronics, medical and dental instrumentation, renewable energy and many other high tech OEMs.