Chemical Machining : Advanced Manufacturing Method........

• Nontraditional machining processes are widely used to manufacture geometrically complex and precision parts for aerospace, electronics and automotive industries. There are different geometrically designed parts, such as deep internal cavities, miniaturized microelectronics and fine quality components may only be produced by nontraditional machining processes.
• Chemical Machining is a type of material removal process for the production of desired shapes and dimensions through selective or overall removal of material by controlled chemical attack with acids or alkalis often called as etchant solutions.
 
• There are mainly two types of Chemical Machining

1. Chemical Milling
2. Chemical Blanking

Chemical Milling

mainly used to produce shapes by selective or overall removal of metal parts from relatively large surface areas. The main purpose is to produce shallow cavities with complex profiles on plates, sheets, forgings, generally for the overall reduction of weight. This process has been used on a wide variety of metals with depths of metal removal as large as 12 mm.  Chemical milling entails four important steps:

1. Cleaning.
2. Masking.
3. Etching.
4. De-masking.

The stresses in the parts should be relieved in order to prevent warping after chemical milling. The surfaces are degreased and cleaned thoroughly to ensure both good adhesion of the masking material and uniform material removal.  Then the masking material is applied. Masking with tapes or paints (maskants) is a common practice, although elastomers (rubber and neoprene) and plastics (polyvinyl chloride, polyethylene, and polystyrene) are also used. The maskant material should not react with the chemical reagent. If required, the maskant that covers various regions that require etching is peeled off by the scribe-and-peel technique.  The exposed surfaces are machined chemically with etchants, such as sodium hydroxide (for aluminium), solutions of hydrochloric and nitric acids (steels), or iron chloride (for stainless steels). Temperature control and agitation (stirring) during chemical milling is important in order to obtain a uniform depth from the material removed. After machining, the parts should be washed thoroughly to prevent further reactions with or exposure to any etchant residues. The rest of the masking material is removed and the part is cleaned and inspected. The masking material is unaffected by the reagent but usually is dissolved by a different type of solvent. Additional finishing operations may be performed on chemically milled parts. This sequence of operations can be repeated to produce stepped cavities and various contours. Schematic sketches of chemical milling process are shown in the Figure-1.

Chemical milling is used in the aerospace industry to remove shallow layers of material from large aircraft components, missile skin panels, and extruded parts for airframes. Tank capacities for reagents are as large as 3.7 m X 15 m. This process is used to fabricate microelectronic devices and often is referred to as wet etching for these products.  Some surface damage may result from chemical milling because of preferential etching and intergranular attack, which adversely affect surface properties. The chemical milling of welded and brazed structures may result in uneven material removal. The chemical milling of castings may result in uneven surfaces caused by porosity and non-uniformity of the material.  With optimum time, temperature and solution control, accuracies of the range of plus or minus 0.01 mm can be achieved on relatively shallow depths of cut. The surface finish obtained may be around 5 microns. Aluminium alloys show better surface finish of the order of 1.6 microns. The metal removal rate on an aluminium component is reported to be about 140 cubic centimeters per minute.

Chemical Blanking
Chemical Blanking is similar to the blanking of sheet metals and it is applied to produce features, which penetrate through the thickness of the material, with the exception that the material is removed by chemical dissolution rather than by shearing. Typical applications for chemical blanking are the burr-free etching of printed-circuit boards, decorative panels, and thin sheet metal stampings, as well as the production of complex or small shapes. It is otherwise called as Chem-blanking, Photo forming, Photo fabrication, or Photo etching. In this process, the metal is totally removed from certain areas by chemical action. The process is used chiefly on the sheets and foils. This process can work almost any metal, however, it is not recommended for material thinner than 2 mm. A Schematic sketch of the chemical blanking process is shown in Figure-2

The work piece is cleaned, degreased and pickled by acid or alkalis. The cleaned metal is dried and photo resist material is applied to the work piece by dipping, whirl coating or spraying. It is then dried and cured. The technique of photography has been suitably employed to produce etchant resistant images in photo resist materials. This type of maskant is sensitive to light of a particular frequency, usually ultraviolet light, and not to room light. This surface is now exposed to the light through the negative, actually a photographic plate of the required design, just as in developing pictures. After exposure, the image is developed. The unexposed portions are dissolved out during the developing process exposing the bare metal. The treated metal is next put into a machine, which sprays it with a chemical etchant, or it may be dipped into the solution. The etching solution may be hydrofluoric acid (for titanium), or one of the several other chemicals. After 1 to 15 minute, the unwanted metal has been eaten away, and the finished part is ready for immediate rising to remove the etchant.  Chemical blanking by using photo resist maskants can suitably make printed circuit boards and blanking of intricate designs.
The advantages of this process are summarized below:

1. Very thin material (0.005 mm) can be suitably etched.
2. High accuracy of the order of plus or minus 0.015 mm can be maintained.
3. High production rate can be met by using automatic photographic technique.