PHOTOCHEMICAL MACHINING (SPRAY ETCHING)
Photochemical machining (spray etching) is a variation of the chemical milling in which the workpiece resistant mask is applied by photographic techniques.
The two processes are quite similar, because both use the etchant to eliminate the material.
Chemical milling is usually used in 3D parts originally formed by other production processes, such as forging and casting of irregular shapes.
However, Photochemical machining is a promising method for machining foils and sheets of thickness ranging from 0.013 to 1.5 mm to produce precise and micro shapes.
Therefore, the photochemical machining becomes a realistic alternative to the shear and punching operations performed by mechanical presses.
Furthermore, a main difference between the chemical milling and the photochemical machining is that in the chemical milling, the depth of the etch is controlled at the time the component is immersed, while in photochemical machining, the depth of etch is controlled when the component is sprayed.
For a fresh chart through higher and lower nozzles, thus improving the performance of the PCM process by activating the registration rate and improved quality.
Of course, in PCM, expensive highly developed equipment is required to provide high pressure / high temperature.
PHOTOCHEMICAL MACHINING EQUIPMENT
This machine is equipped with the following units:
1. System of upper and lower nozzles.
2. Multi logical conveyor to serve workpiece.
3. A unit to clean the water circulation sheet and dry with hot air.
4. A unit for measuring and control of density and concentration of etchant.
5. A unit for product inspection.
In photochemical machining, the following steps are performed:
PHOTOCHEMICAL MACHINING STEPS
1. The form of the requested part considered as a primary image for the photo tool is created by CAD.
2. Two photographic negatives, called illustrations, occur in the actual dimension of work.
3. The metal plate is chemically cleaned and covered with a highly sensitive photoprotection (called sensitive emulsion).
The coating is performed by spraying, dipping or rolling. The work is allowed to dry. Photoprotection adheres to the surface, protecting it during etching.
4. After the coating, the work is polished between the two negatives (illustrations), therefore exposed in vacumm, to an ultraviolet light (UV). The coating is solidified in areas exposed and is eliminated from the exposed area dissolving the developer.
5. The worksheet is once exposed to a powerful water jet.
The worksheet is rinsed by deionized water, then dried by nitrogen gas.
6. The worksheet is then spray etched into the upper and lower part.
This allows the material to be recorded on both sides, thus reducing the sub-cut, reducing processing and the production of lateral side walls.
7. After etching, the hard photoprotection is deleted and the worksheet is rinsed to avoid reactions with suspended etchant.
The phototooling is fast and economical to produce.
Most phototool costs less than $ 350 and can occur in two days or less.
Unlike “hard” tools, such as stamping and punching dies, phototools are exposed exclusively to light and, therefore, does not suffer from wear.
Due to the cost of hard presses for printing and fine cutting, a significant volume is required to justify the expense.
In PCM, the unit of work is the sheet.
Therefore, it is cheaper to schedule the size of the largest sheet as possible with the size and dimensional tolerances of the part.
As the parts per sheet increases, the costing decreases.
The thickness of the material affects the costs depending on the length of time to record.
In general, steel, copper or aluminum parts with thicknesses up to 0.020 in (0.51 mm), the costs of the parts will be approached at $ 0.15-0.20 per square inch.
Since the geometry of the piece becomes more complex, photochemical processing earns a greater economic advantage compared to sequential processes, such as CNC punching, laser cutting or electric discharge processing.
APPLICATIONS OF PHOTOCHEMICAL MACHINING PROCESS
Copper, zinc, steels, stainless steels, lead, nickel, titanium, molybdenum, glass, germanium, carbide, ceramic and some plastic materials are photochemically mechanized.
The process also works well for PEAK materials that are difficult to hit.
The materials must be flat so that they can be folded to shape and assemble in other components.
Products made by PCM are generally found in electronics, in the automotive, aerospace, telecommunications, computer and other industries.
Typical products like PCBs, fine screens, flat springs, etc.
The precision and the registration rate are considerably greater than those performed by the milling of the EC.
Since the tools are performed using photographic techniques, they can be easily stored, and the models can be easily reproduced.
Delivery times are small compared to those required by the processes that require hard tools.
The delicate and fragile parts of small thicknesses are produced by PCM without any deformation and wrapping.
Apart from the disadvantages of the disco, the PCM also has the following limitations:
Requires highly qualified operators.
Requires a more expensive team.
The machine must be protected by the corrosive action.