Nontraditional Chemical Processes
Fundamentals of Chemical Processes
The common tie among the various chemical material-removal processes is the use of acid or alkaline solutions to dissolve unwanted material, leaving the final desired pattern or part configuration. However, significant differences exist among the various chemical techniques, which make up the family of chemical material-removal processes. These differences are as follows:
Maskants and resists are workpiece coatings used to protect areas of the workpiece, which are not to be exposed to the etchant and to define exposed areas for etchant attack. The three classifications of maskants and resists in general use today are cut-and-peel maskants, photographic resists, and screen resists.
±Cut-and-peel maskants: Cut-and-peel maskants, which are used almost exclusively for chemical milling of aircraft, missile, and structural parts, are applied by flowing, dipping, or spraying a coating to thickness of 0.20-0.38 mm in dry-film form. The materials are removed from areas to be etched by cutting and hand peeling away the unwanted areas, generally with a template to aid accuracy. Etching depths of 12.7 mm or more can be attained, and after a certain area has been etched, additional maskant may be removed so that step etching is possible.
±Photographic resists: Photographic resists are materials, which produce etchant-resistant images by means of photographic techniques. When exposed through a high-contrast negative and then developed, these materials produce an image of the negative itself. Both positive and negative-working resists are available. Photoresists are generally characterized as extremely thin coatings, which produce high detail, are sensitive to light, require careful handling and a clean environment, and have more complicated processing steps than required by other resists.
±Screen resists: Screen resists are applied through a polyester or stainless steel mesh, which has an image stencil on it. Although the stencils are generally made photographically, printing accuracy does not approach that of photographic printing. Screen printing is a rapid method of producing a large number of parts to moderate accuracies.
ØEtchants and their Selection
Selection of an etchant is dependent upon numerous factors, some of which are:(1) material to be etched, (2) type of maskant or resist used, (3) depth of etch, (4) surface finish required, (5) potential damage to or alteration of metallurgical properties of the material, (6) speed of material removal, (7) permissible operating environment, (8) economics of material removal, and (9) heat-treated condition of material.
±Surface finish. Surface finish required in an etched area has a substantial impact on etchant selection. Certain etchants attach grain boundaries in a metallic surface and produce uneven etching. Others can produce smuts or scum on the surface of the part being etched, which often causes irregularities in the etching rate over the surface, which in turn causes surface unevenness.
±Etching rate. If all other factors are equal, the faster a part is etched, the more economically it can be produced. However faster etchants generally tend to have side effects, such as increased undercutting, more heating, a greater change of etching rate with temperature change, reduced capacity, and greater degradation of the resists image.
Chemical milling is the process used to shape metals to an exacting tolerance by the chemical removal of metal, or deep etching of parts, rather than by conventional mechanical milling machining operations. The amount of metal removed, or depth of etch, is controlled by the amount of immersion time in the etching solution. Masking or protecting these areas from the action of the etchant solution controls location of the unetched or unmilled areas on a part.
When chemical milling is used, castings can be uniformly designed oversized, heat-treated with little or no warpage, and then chemically milled to achieve the desired final dimensions. Surface finish can often be reduced from greater than 5.0 m m Ra to 1.0-1.5 mm Ra.
Materials to be chemically milled should be of the same heat treatment and, when possible, of the same mill run or from the same manufacturer for each group of parts run. This ensures uniformity of physical and chemical structure, and also ensures close tolerance control. Parts should be heat treated, when necessary, prior to chemical milling. The process consists of five main steps: cleaning, masking, scribing, etching, and demasking.
±Cleaning: Proper cleaning is necessary to ensure uniform adhesion of the maskant. After being cleaned, the parts are allowed to stand until dry.
±Masking: The mask is applied by dip, flow-coat, or airless-spray techniques, depending upon part size and configuration. Two or more coats are applied to obtain sufficient protection from the etchant. After the last coat is tack-free, air cured for 3-24 hours depending upon individual plant schedules and storage.
±Scribing: Patterns are placed on the masked and cured part by using a template as a guide and scribing the mask with a fine knife. In some cases, a so-called hot knife is employed in scribing the mask film to avoid damage or scratched on the underlying alloy surface. In some cases, lasers are used to scribe mask films.
±Stripping: After the part is scribed, the mask is hand stripped from the part, leaving the areas to be milled exposed to the etching solution. The parts are carefully measured with a micrometer or other device to determine the initial thickness. The initial thickness minus the final thickness gives the depth of cut. The exact immersion time in the etchant can then be determined by diving the depth of cut by the etching rate.
Photochemical machining is the process or producing metallic and nonmetallic parts by chemical action. Basically, the process consists of placing a chemical-resistant image of the part on a sheet of metal and exposing the sheet to chemical action, which dissolves all the metal except the desired part. Most parts produced in this way are similar to thin-gage stamping and are generally flat and of complex design.
ØPhotographic-Resist Processing Fundamentals
The photographic-resist process of photochemical machining is by far the most common one in use today. Figure 1 shows the process steps involved.
±Cleaning. Metal can be chemically cleaned in numerous ways including degreasing, electrocleaning, or chemical cleaning.
±Coating: The cleaned metal is coated with photographic material which, when exposed to light of the proper wavelength, will polymerize and remain on the panel as it goes through developing stage. This polymerized layer then acts as the barrier to the etching solution applied to the metal.
±Prebake: After being coated with resist, the panel must usually be baked prior to being exposed. This “prebake,” as it is called, is used to remove all solvents from the resist in a simple drying operation. Care must be taken not to over bake the photoresists, since most of them are sensitive to heat prior to exposure.
±Exposure: Artwork that has been drawn and photographically reduced is used to expose the photographic resist. The negatives are generally used in matched pairs so that a minimum amount of undercut is achieved and so that the final part has straight sidewalls. Typical exposure times of from 10-30 s are generally required.
±Postbake: Certain resists require an additional baking operation following development. This “postbake” is necessary to drive out residual solvents or cause further polymerization, which improves the chemical resistance of the resist image. Following postbaking, it is generally advisable to cool the resist prior to etching.
±Processing: The next step is etching to remove the unwanted metal that is not protected by the photoresists. Following etching, the workpiece is generally washed and dried if reremoval is not required. When removing the resist is necessary, the removal can be accomplished manually or by machines either spray-on removal compounds or with use of mechanical action in addition to chemical action.