Photo Chemical Machining -
A Cost Effective Alternative
Using a process called photo fabrication, photochemical machining (PCM) companies turn your CAD file, blueprint or drawings into thin metal parts. The material specified is coated with a light sensitive polymer, imaged with a photo tool using UV light, developed much like film and then chemically etched. The results are precision components manufactured in most metals, including some exotics.
Designers, manufacturing engineers and purchasing groups are recognizing the advantages photochemical machining has to offer. Some of the benefits of this unique manufacturing process are the ability to make a complex part that is beyond hard tool capability and to fabricate the part in dead soft or full hard material without burrs or metal distortion. Another significant benefit is response time. An order may be filled in days, which allows the design to be proven without incurring the cost of a hard tool.
Photochemical machining is a growing industry, gaining experience and recognition as a viable method of working metal.
Accuracy is the byword of the chemical milling technique, which produces thin gauge metal parts utilizing photo tools, light sensitive coatings and etching reagents. . . eliminating the burrs left by standard machining methods.
PCM can etch a wide range of metals and alloys including Molybdenum, Hastelloy X, Nickel Silver, Silicon Steel, and Titanium, as well as the more common metals in any quantities.
The bulk metal is cut to an optimum size to reduce waste and allow for efficient use of the coating material used in later steps of the PCM process.
The importance of metal cleaning (prep) cannot be overemphasized. Proper metal preparation, degreasing, and the removal of surface contaminants is essential for proper adhesion between the raw material and photoresist. Most photoresists have been developed for the printed circuit industry and have adequate adhesion to copper and copper alloys. Care must be taken to promote adhesion when other materials are etched, especially stainless steel. Passivation or pre-etchings are considerations as well as abrasion by mechanical means to texture the surface to be coated. Modern PCM companies have invested in clean lines for improved material cleanliness. These conveyorized machines are typically multi-chambered and material is put through a series of clean and rinses to insure proper surface condition. The quality and quantity of the finished parts depends greatly on the care taken during this step.
In any PCM process, the material should be coated with photoresist directly after cleaning. This reduces the chance of surface contamination and oxidation. The coating is photosensitive and resistant to the etchant used to cut the chosen metal. Dry film or liquid resist is applied to both sides of the work piece to allow etching from both sides simultaneously. Photoresist can be either a positive or negative masking. In positive masking the areas exposed to light during the imaging process washes away during the development stage. In the more commonly used negative masking the areas exposed to the UV light are bonded to the material and the non exposed areas are rinsed away during the development process. Advances in resist technology combined with new and improved artwork have enabled PCM companies to expose, develop and etch features not thought possible a few years ago.
During this step material with the photosensitive coating is exposed to UV light. This is a straightforward process that transfers the photo tool image onto the coated material.
Imaging has been accomplished by exposing the coated material to sunlight however in the northeast it is more practical to utilize more sophisticated equipment that will control the desired exposure length and intensity. Exposure equipment commonly used in the PCM industry consists of a vacuum frame, high intensity UV light and multiple trays for more economical transfer of image. The coated material is sandwiched between the registered film tooling and exposed from both sides; this exposure polymerizes the areas of the filmtool.
After exposing the sheets of material need to be developed. This can be accomplished in trays filled with developer however most PCM companies opt for conveyor equipment. During the run through the developer a liquid developer is sprayed onto the sheets and this removes the unpolymerized areas of the image. What is left after developing is the sheet of metal with a likeness of their design protected by photoresist and bare material ready to be removed by chemical etching.
During the chemical etch process any material not protected by the resist is eroded. This means that as the chemicals cut through the metal they are also cutting laterally under the protective coating of photo resist. When the process has etched halfway from both sides they reach a breakout point. When breakout occurs the etchant can run through the holes and smooth the sidewalls. During the engineering of the film tool, compensation is required to produce parts within specific tolerances, this is called etch factor.
The next step in the manufacture of PCM parts is stripping off the protective coating of photo resist.
Many companies use machines that have a strip chamber on the etch machines. This is practical when running a product line, less practical when producing the varied parts most PCM companies offer.
The most common method of parts stripping is to soften and lift the resist coating using the batch method.
Other processes dissolve the protective coating completely. In either process care must be taken to insure no resist material is left on the parts and staining or surface corrosion does not occur.
Depending on your application, the parts may need to be formed after the etching process has been completed.
Before shipping, the finished parts are inspected. The final step in the photo etch process. From sampling to 100% inspection a wide variety of inspection equipment is utilized. Plug and pin gauges are common inspection tools as well as more sophisticated optical comparators with the ability to program inspection requirements.
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