In this article we will discuss about:- 1. Meaning of Photo Chemical Machining (PCM) 2. Tooling and Development of PCM 3. Production 4. Photo Resist Coating 5. Exposing 6. Developing 7. Etching Process 8. Advantages and Limitations.
Meaning of Photo Chemical Machining (PCM):
Photo Chemical Machining (PCM) is used for the manufacture of micro-components, used widely in electronic and electrical industries (and, in particular, in electronic instruments, measuring tools and A.C/D.C. motors). It is an intricate process requiring considerable precision. Photo Chemical Machining is an unconventional way of milling thin, intricate, burr-free and accurate micro-components.
It offers many advantages over the conventional press tool process (which is complex, not very accurate, and involves high capital investment in tooling) like: It requires little capital investment and is more economical; whatever is drawn on paper can indeed be manufactured under PCM because the process involves nothing more than chemical ‘etching’ out of the unwanted material from a metal sheet to be able to get the specified component.
Tooling and Development of PCM:
It is necessary, first of all, to develop accurate tooling to get the right results from PCM. The component to be manufactured should be neatly and accurately drawn in the form of what are generally known as Art Works. These are done in several sizes and in multiples of the original component size, depending upon the tolerances required and the thickness of the material to be cut.
ADVERTISEMENTS:
The artworks are made keeping in view a major consideration— the correction factor in the dimensions, known, as the Etching Factor. The artwork is then reduced in size through a reduction camera. The reduction camera is usually a sophisticated and computer controlled instrument. The artwork is reduced to the normal specified size of the component.
The purpose of reducing a multiple size artwork to a normal size is mainly to minimise any possible cumulative errors in drawing. The actual sized negative and positive photo tools are thus made. The negative and the positive are both checked on a profile projector and then led to a repeat camera for gang photography.
Production of PCM:
Once the basic tool designs are ready, production is possible. The first step in production in a PCM job is to procure the right kind of raw material which is normally in the form of sheets. The thickness, the width and the length of the sheets are thoroughly checked before taking them into the production line.
Since the sheets are all rolled and usually in a coil form, they require straightening without of course causing dent marks or wrinkles on them. For this purpose, 3 roller straightening or 5 roller straightening machine is employed. The sheets are subjected to shearing, depending upon the size of the production negative and the direction of the grain lines. They are then degreased by a method which varies with the job and the raw materials. Rinsing in hot caustic soda 10% solution and, thereafter, washing it with a detergent is the commonly used method.
ADVERTISEMENTS:
Others include chemical process, mechanical process, an electrical process or a combination of any two or three processes. An important requirement for degreasing is a neat and clean plate. This ensures that sufficient adhesive of photoresist remains during further processing. The process plates are then washed with water thoroughly and dried in a dust free room. The dried plates are now ready for what is called Photo Resist Coating.
Photo Resist Coating of PCM:
There are two types of photo resist coatings—
i. Liquid photo resist and
ii. Dry photo resist.
ADVERTISEMENTS:
The selection of the type of photo resist depends, again, upon the base metal used and the type of job to be done. No doubt, dry photo resist is known widely to give better results but there are many cases where liquid photo resist is preferred. This decision is normally based on one’s own practical experience.
The liquid photo resist is applied by a sponge roller. It can also be dip-coated in which case the plates have to be later dried in room temperature or slightly heated in a low temperature oven. Dry photo resist is applied usually under the effect of heat and pressure between two rollers.
Exposing of PCM:
After drying, the photo resist coated plates are subjected to an exposer. Most of the photo resists are not very sensitive to light in the range of a light spectrum but are certainly sensitive to an ultra violet light which has a higher wave length.
The exposer is the result of a catalytic process, i.e. polymerisation of molecules, which leads to a complicated chain or chemical cohesive forces. Consequently the photo resists chemical becomes resistive to certain etchants or solvents.
ADVERTISEMENTS:
The plates are then inserted in photo tool pockets and exposed to ultra violet light under vacuum. This secures a thorough contact of the negative to the metal, leading to the formation of an image on the metal. The area which is exposed by light becomes resistive to etchants and also unsoluble. However, the dark area of the negative remains soft and could be etched by a solvent.
Developing of PCM:
The photo resist coated plates are then subjected to developing where the unexposed surface area is washed off the photoresist by a suitable developer solution. To ensure pre-inspection of the plates after developing, the plates may be subjected to dyeing process. This ensures securing clear images of the components. But more importantly, it helps to carry out any required corrections. Or, if they have not been properly exposed, the plates may be rejected.
Etching Process of PCM:
The most acceptable etching process is Spray Etching. This gives optimum results in etch rate and minimum undercut. The etching process progresses both vertically and laterally under the coating and this is therefore known as ‘Undercut’.
The elliptical shape and dimensions vary with the kind of raw material used and its thickness. Ferrous metals have more elliptical shape than non-ferrous metals. The elliptical shape could be reduced by over etching but it cannot be eliminated totally.
ADVERTISEMENTS:
This is what is meant by the compensation factor or the etching factor. This must be taken care of right at the stage of making the art-works. Etching factor varies from metal to metal and is directly proportional to its thickness but one should go by experience in this regard.
After etching, the components are washed with water. The etchant solution is washed away. The plates are dried and then photo resist coating is removed by any good and economical solvent like trichloroethylene.
For different kinds of raw materials, etchants also differ. Etching is basically an oxidation process and so it is important to select the right etchant. The frequently used etchants are fabric chloride, nitric acid, hydrochloric acid, hydrofluoric acid, chromic sulphuric acid or a combination of any two etchants. Alkaline etchants are also sometimes used.
Etching being a complicated acid oxidation process, an appropriate machine is necessary in order to obtain an accurate, fast and a continuous etch rate.
All etching machines are equipped with the following arrangements:
1. Spray etching nozzle (top and bottom),
2. Conveyor system variable speed,
3. Water wash module,
4. Inspection access module.
But the latest developments in these machines are provisions for a hot dry air chamber, computer compatibility, specific gravity control system and concentration control system.
After etching, batch inspection of components is carried out. Since the products are miniature, one requires a profile projector for inspection. Suitable enlargement is selected and then the components are checked for accuracy.
They are then packed carefully so that they do not suffer any distortion in transit. The steel components which tend to get rusty should be packed in moisture-free polythene bags after applying a thin oil coating.
Advantages and Limitations of PCM:
Photo Chemical Machining has the following advantages:
1. The tooling cost is low compared to any mechanical process for such blanking.
2. Correction can be done on the artwork itself if samples are not approved. PCM is therefore suitable for development of blanks for components which are required in R and D experiments.
3. Making master drawings, reducing and photographing them all take little time. The process is therefore useful where the time factor is critical and results are required urgently.
4. Parts produced by this method are free from stresses which are common in the conventional mechanical method.
5. Storage space required for tooling is negligible. Tooling’s can be stored in envelops and filed in a cupboard.
6. Maintenance cost of tooling’s is totally eliminated since they are not subject to any wear and tear.
7. Components are produced in a burr-free condition, thus saving costs on burr removal.
8. Hard and brittle metals can also be etched by this process, which is impossible in any other conventional method.
9. Components do not loss their original material properties.
10. Very thin metal parts could be done by this process without any distortion or stress.
11. Magnetic properties of the metal are not lost during the process of manufacturing.
12. Blanking layout could be more economical in this process since to mechanical stress is employed.
Some certain limitations in PCM which one should bear in mind:
a. The operators must be skilled.
b. Chemicals used in this process are highly corrosive. So care must be taken to install the machines either in a closed room or away from other machines to prevent them from corrosion.
c. The process is slower than the other mechanical process of blanking. A feasibility study on the time factor is therefore necessary.
d. Accuracy is directly proportional to the size of the artwork and inversely proportional to the thickness of the metal.
e. Certain stainless steel materials do not show consistency in regular production like AISI 304 which has a thickness of over 0.2 mm.
But, inspite of these limitations, Photo Chemical Machining is still an ideal mechanism for all small, intricate and complex shapes.