Photochemical machining, variously known as photo-etching, chemical machining, chemical milling and metal etching, is a manufacturing process to fabricate metal parts through precision photolithography and chemistry. The technology’s unique methods make it ideally suited to the demands and requirements of modern manufacturing including low costs and quick turns on tooling that, nevertheless, still incorporate accuracy and complexity of design; rapid prototyping; quick turnaround on production parts orders; the use of very thin gauged metals and foils, and the production of clean, precise, unstressed and burr free parts.
Tooling for Photochemical Machining: the Photo-tool or Photomask
Known as a photo-tool or photomask, tooling for photochemical machining consists of the negative image of the desired part or parts printed or plotted onto a piece of stable Mylar film. Once made the photo-tool appears as a film with clear areas representing the desired parts and opaque or black areas. Designed with computer drafting programs and subsequently output on high resolution photo-plotters, tooling generation is quick, inexpensive and accurate. And, just as expeditiously as the original tool is produced, subsequent revisions, changes or modifications can be incorporated. Typically the tooling consists of top and bottom halves accurately aligned either through a pin registration system or through optical targeting. In subsequent processing, this two piece or double sided tooling will allow for double sided etching or etching the metal panel from both sides simultaneously.
First Processing Steps: Metal Cleaning, Photoresist Lamination and Printing
Metal part fabrication with photochemical machining is a multi-step process. The metal sheets or panels from which the part will ultimately be cut or etched is chemically cleaned and then roll laminated or coated with a UV light sensitive polymer known as photoresist. At this point the coated metal and the photo-tooling meet. In a vacuum frame, the coated metal panel is sandwiched between the aligned top and bottom halves of the photo-mask. Vacuum is pulled in the frame forcing the tooling into intimate contact with the metal panel. Then, while still under vacuum, the photo-tool and metal sandwich is exposed to high energy UV light. In this step, known in the industry as printing, the photoresist coating on the metal is selectively exposed to UV light – exposed where the photomask is clear and unexposed where it is opaque. Where exposed to UV light, the photoresist cures or hardens while the unexposed area remains in its original uncured state.
The Advantages of Creating Parts with UV Light
In essence, by shining UV light through a mask, the image of the desired parts are transferred through the tooling to the photoresist and, by association, to the metal panel below. In practice, there are numerous important implications. Since only light is used in this imaging process, there is no tool wear which ensures part repeatability over long production runs and eliminates all need for and the costs associated with traditional tooling maintenance. Moreover, as the UV light shines through the photomask, the complete and total design and geometry of the part is instantly transmitted; consequently, any complexity of design and all features that carry tight geometric tolerances, e.g. true position or concentricity, pose no extraordinary difficulties and add no significant additional costs to photochemically machined parts.
Next Steps: Developing and Metal Etching
In the next step, known in the industry as developing, the metal panel coated with selectively exposed photoresist is sprayed with a chemical solution that removes all unexposed, uncured or unhardened photoresist. During this process, parameters such as spray pressures and spray distribution, temperature, dwell time and pH are all tightly monitored and controlled. At the end, what remains are coated areas of cured photoresist that define the desired parts and bare metal stock.
Now photoresist earns the second half of its name. Its “photo” or light properties are already documented. In this next step, the metal sheet is sprayed with an acid or an acid salt that etches the exposed, unprotected metal panel while the “resist” prevents any metal removal of the desired metal parts. As all the exposed, bare metal is all removed, only the desired “resist” coated parts remain. Again, like the photoresist removal process previously, so also are the metal removal processing parameters including spray pressures and spray distribution, temperature, dwell time, pH, conductivity and oxidation reduction potential tightly monitored and controlled.
The Benefits of Metal Etching
Unlike other metal parts fabrication methods such as stamping that uses shearing force; conventional machining, grinding and water-jet that rely on abrasive cutting ; or laser, electrical discharge and plasma that employ heat and melting; photochemical machining removes metal through oxidation-reduction, a chemical reaction where the metal removal occurs at an atomic level. This type of metal removal has many noteworthy consequences. First, all the base metal’s intrinsic properties such as temper, hardness, spring properties or magnetic permeability remain unaffected. And just as etching does not affect the metal, the reverse is also true – the metal has no effect on etching. Soft annealed thin metals etch as well as thicker extremely hard tempered metals. In addition, a part’s etched edge is clean and clear of any recast and all burrs. Finally, as the imaged panel is sprayed with etchant, metal is removed from all uncoated areas of the panel at the same time. So much like the imaging process, in the etching process all points defining the parts are etched away simultaneously. The consequences are the same as well: any complexity of design poses no extraordinary difficulties and adds no significant additional costs to photochemically machined parts.
After etching, only the resist coated part remains. Photo-resist, though invulnerable to the acidic etchants, is readily removed or stripped from the metal parts with basic solutions. Either through soaking or spraying these basic, stripping solutions, photo-resist is completely removed from the metal parts or panels. Once stripped, the metal parts are water rinsed and dried.
Some Processing and Design Parameters
This, then, is the process from metal sheet to metal parts. Because of the nature of chemical removal, thin gauge metals, i.e. less than .040” or 1.00 mm thick, are best suited for manufacture with this technique. Additionally, in photochemical machining, process capability and processing parameters are ultimately a function of metal thickness. Typically feature size tolerance is +/-10% material thickness with a practical limit of +/-.00025” or +/-.006 mm. Additional parameters are illustrated in the following diagram.
At Lancaster Metals Science, we operate our photochemical machining across three platforms: standard sized panels up to 18” x 24”, large sized panels up to 24” x 144” and in continuous reel to reel format with a maximum width of 18” and up to 1000 foot lengths.
Any industry that uses precision metal parts can turn to photochemical machining for their metal parts requirements. That said, there are some common applications that we list here:
- Shielding: Electromagnetic and Radio Frequency
- Bus bars
- Flexible circuits
- Heat sinks
- Flat Springs
Battery Technology / Energy / Motors
- Current collectors
- Perforated metal screens
- Fuel Cells
Medical / Surgical / Dental
- Surgical blades
- Metal seals
- Metal gaskets
- Heater elements