In metal finishing industry fat or oil containing aids such as soluble oil, cutting emulsion, drawing grease, polishing / lapping paste and such similar materials are used during the many machining operations, which simultaneously form metal shavings such as swarf, filings, etc. from lesser degree to denser quantities. These contaminations and foreign materials are not acceptable for further finishing, particularly for electroplating, anodising, electroless plating, etc. which require excellent adhesion to finishes.

Chemistry for cleaning : Chosen between solvent and aqueous (detergent) based systems depending on the level of contamination and degree of cleaning. Conventionally, till a decade ago solvent cleaning was more popular because of its efficient and faster operation. However, due to disturbance in the ozone layer of atmosphere, solvent is being replaced by aqueous based systems.

You can view a typical chart of comparison between solvent based and aqueous based cleaning chemistries


Solvent


Commonly used pre-cleaning method is solvent degreasing mostly by chlorinated solvents like trichloroethylene, perchloroethylene, etc. for vapour, liquor, spray and other operations.
Vapour and liquor-vapour operated solvent degreasing plants are equipped with heaters, condensing coil, water jacket and trough to collect condensed solvent. Apart from collecting distilled solvent, condensing coil / chamber also prevents escape of vapour so as to maintain pollution free atmosphere in the surrounding region.
The soiled suspended work receives vapour of the liquor which condenses on the greasy matter and dissolves these, flowing them off the parts to the boiling solvent sump at the bottom of the tank. The cold components attain temperature of vapour in a matter of few seconds and normally the cleaning operation is completed almost immediately.
It should be noted that if parts covered with dense polishing compositions are initially exposed to vapour, the sticky material of such compositions would dissolve, leaving a hard solid residue which is very difficult to remove. In such cases, therefore, the parts are first to be immersed in hot liquid in order to facilitate mechanical removal of these solids. The articles should then be cooled slightly before exposing in the vapour chamber to obtain correct results.


Aqueous


Many developed countries avoid use of solvents due to pollution hazards and have now been favouring aqueous, i.e. alkali based cleaning systems. The latter, depending on the geometry of components as well as quantum and nature of contamination, provides various permutations and combinations such as:

Dip, Rinse
Dip, Spray, Rinse
Dip, Tumble, Rinse
Dip, Ultrasonic, Rinse

The main advantage of aqueous system is the availability of various options of multiple stations as also vigorous / pressurised spray which aids in removal of contamination.


Chart


A typical chart of comparison between solvent based and aqueous based cleaning chemistries is as under :

A typical chart for selection / comparison of type of cleaning systems. view chart


Ultrasonic


Ultrasonic means sound vibrations at frequencies above 20,000 cycles per second. The expansion and contraction of the transducer caused by the high frequency alternating current, transforms electrical energy into sound waves above the audible range, corresponding pressure fluctuations make the liquid expand, forming minute vapour bubbles which collapse on being compressed.

This cavitation action repeats itself several thousand times per second thus producing agitation which helps in cleaning. The cleaning medium is either detergent or solvent. The latter dissolves soluble solids and grease and the former holds in suspension the dirt torn free by the cavitation. Many solvents cavitate readily. High concentrations of alkaline solution dampen cavitation. Ultrasonic cleaning is necessary particularly in the use of precision components, jewellery, watch parts, ball bearing, parts which contain difficult-to-remove soils such as finger prints, magnetic metallic chips, polishing and buffing compositions, lapping and honing compounds, soldering fluxes, etc.


Vacuum


For stubborn contamination such as lapping paste entrapped in minute cavities, use of vacuum with or without ultrasonic is warranted. Though, solvent has been effectively used formerly, extensive multi chambered aqueous cleaning plants have also been under use in the recent past.


Continuous Strip Plating Plant and Continuous Tube Plating Plant


Demand for such plants have of late increased in steel industries who by practice have discovered that electrolytic galvanizing (Zinc Plating) has scored over Hot dip galvanizing in terms of film thickness, corrosion resistance, compatibility for end use etc.

The Plant is designed, based on the customers output and tailor made to suit size of the strip or tube, traverse speed, No. of passes etc. It normally demands more length to accommodate the Plant and is therefore laid in a two-tier system, based on the availability of space. A lot of mechanical engineering inputs are fed in, for designing such plants.

Plating on ABS Plastics

Its application is very much felt in automobile accessories and some of the domestic products, including cosmetic items for aesthetic, as well as corrosion proof properties. While Nickel chrome process is like a normal process on steel, ABS plastic undergoes; initially an elaborate pre-treatment process to become conductive and to receive plating deposits.