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Surface treatment   preprocesses
Surface treatment   preprocesses
Surface treatment   preprocesses
Surface treatment   preprocesses
Surface treatment   preprocesses
Surface treatment   preprocesses
Surface treatment   preprocesses
Surface treatment   preprocesses
Surface treatment   preprocesses
Surface treatment   preprocesses
Surface treatment   preprocesses
Surface treatment   preprocesses
Surface treatment   preprocesses
Surface treatment   preprocesses
Surface treatment   preprocesses
Surface treatment   preprocesses
Surface treatment   preprocesses
Surface treatment   preprocesses
Surface treatment   preprocesses
Surface treatment   preprocesses
Surface treatment   preprocesses
Surface treatment   preprocesses
Surface treatment   preprocesses
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Surface treatment preprocesses

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discusses criticalities associated with surface treatment pre -rocesses of

discusses criticalities associated with surface treatment pre -rocesses of

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  • -one can make a poor coating perform with excellent pretreatment, but one cannot make an excellent coating perform with poor pretreatment. -Surface pretreatment by chemical and/or mechanical means is important -methods are designed to ensure good adhesion of the coating or paint to the surface.
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    • 1. ahmadkhan_99@hotmail.com 1
    • 2. Sequence  Mechanical Preparation  Blast Finishing  Shot Peening  Mass Finishing  Polishing  Buffing  Chemical Preparation  Pre-process Baking  Preheating  Effects of Discontinuity in process  Effects of Poor Cleaning process
    • 3. Mechanical Preparation Physical removal of soils, scales, or films from the work surface by means of abrasives or similar mechanical action It serves other functions such as deburring, improving surface finish
    • 4. Mechanical Preparation Processes – Blast finishing High velocity impact of particulate media (Hard: Al2O3 and SiC and Soft: nylon beads) propelled by pressurized air or centrifugal force to clean and finish a surface • Most well-known method, sand blasting, uses grits of sand as the blasting media – Shot peening High velocity stream of small cast steel pellets (called shot) is directed at a metallic surface to cold work and induce compressive stresses into surface layers • Used primarily to improve fatigue strength of metal parts surface cleaning is accomplished as a byproduct of the operation
    • 5. Mechanical Preparation Processes – Mass finishing processes Finishing parts in bulk by a mixing action in a container with an abrasive media for the desired finishing action. Usually for small parts – Polishing It involves removal of small amounts of metal by means of abrasives. It produces a surface that is free of larger imperfections left by grinding etc and is preliminary to buffing – Buffing It is similar to polishing except it employs finer abrasives and can produce extremely smooth, even mirror surface
    • 6. Chemical Preparation Parts as received in plating section simply cannot be introduced into an electroplating solution without pretreatment of some kind The electroplat (finish) is expected to be adherent to the substrate, and such adhesion will not be attained unless the parts are reasonably cleaned before entering the plating baths
    • 7. Chemical Preparation Parts must be cleaned chemically/mechanically many times to remove films, oil, dirt and contaminants during production Reasons for cleaning • Prepare the surface for subsequent processing (coating or adhesive bonding) • Improve hygiene conditions for workers and customers • Remove contaminants (shop soil and oxide films, quenching oils, heat scales, stamping & die lubricants) that may react with the surface • Enhance appearance and performance of a product
    • 8. Chemical Preparation Important Factors in Selecting a Cleaning Method • Contaminant to be removed • Degree of cleanliness required • Substrate material to be cleaned • Purpose of the cleaning • Size and geometry of the part The design of a process is linked with / dependent upon various pre & post activities THUS modifying / Omitting any step in the process can have adverse effects on entire process
    • 9. Chemical PreparationContaminants’ Origin Various contaminants build up on part surfaces either due to previous processing or factory environment • Principal surface contaminants are found in the factory: − Oil and grease, e.g., lubricants in metalworking − Solid particles such as metal chips, abrasive grits, shop dirt, dust, etc. − Residual of buffing and polishing compounds − Oxide films, rust, and scale present in shop environment
    • 10. Chemical PreparationChemical Cleaning Processes• Alkaline cleaning• Solvent cleaning• Acid cleaning• Emulsion cleaning• Ultrasonic cleaning Ultrasonic cleaning uses high-frequency mechanical vibrations combined with chemical
    • 11. Chemical PreparationAlkaline Cleaning Uses an alkali to remove oils, grease, wax, and various types of particles (metal chips, silica, light scale) from a metallic surface Cleaning methods: immersion or spraying, usually at temperatures of 50-95 C (120-200 F), followed by water rinse to remove residue
    • 12. Chemical PreparationSolvent Cleaning Organic soils such as oil and grease are removed from a metallic surface by means of chemicals that dissolve the soils • Common application techniques: hand-wiping, immersion, spraying, Vapor degreasing (discontinued)
    • 13. Chemical PreparationAcid Cleaning Removes oils and light oxides from metal surfaces using acid solutions combined with water-miscible solvents, wetting and emulsifying agents • Common application techniques: soaking, spraying, or manual brushing or wiping carried out at ambient or elevated temperatures • Cleaning acids include hydrochloric (HCl), nitric (HNO3), phosphoric (H3PO4), and sulfuric (H2SO4)
    • 14. Chemical PreparationAcid Pickling A more severe acid treatment to remove thicker oxides, rusts, and scales • The distinction between acid cleaning and acid pickling is a matter of degree • Generally results in some etching of the metallic surface which serves to improve organic paint adhesion
    • 15. Chemical PreparationEmulsion Cleaning Uses organic solvents (oils) dispersed in an aqueous solution • The use of suitable emulsifiers (soaps) results in a two-phase cleaning fluid (oil-in-water), which functions by dissolving or emulsifying the soils on the part surface • Can be used on either metal or nonmetallic parts • Must be followed by alkaline cleaning to eliminate all residues of the organic solvent prior to plating
    • 16. Chemical PreparationUltrasonic Cleaning Mechanical agitation of cleaning fluid by high-frequency vibrations (between 20 and 45 kHz) to cause cavitations (The sudden formation and collapse of low-pressure bubbles in liquids by means of mechanical forces) that scrub the surface • Combines chemical cleaning and mechanical agitation of the cleaning fluid • Cleaning fluid is generally an aqueous solution containing alkaline detergents • Highly effective for removing surface contaminants
    • 17. Chemical PreparationDuring Cleaning process design, the substrate material iscritically considered, so that damaging reactions are not causedby the cleaning chemicals − Examples: - Aluminum is dissolved by most acids and alkalis - Steels are resistant to alkalis but react with virtually all acids“Don’t clean because Mom told you to − Unless Mom runs the plating shop” CLEANING IS A PROCESS
    • 18. Pre-process BakingBaking is carried out as a pre-process so that theinduced stresses (due to earlier operations of machining,grinding etc) are relievedThese stresses, if not removed, can reach to part’scritical stress level due to further stress aggravationduring plating processes (the embrittlementphenomenon)
    • 19. PreheatingPreheating, high temperature cleaning, hot waterrinse/dip is also a critical prior to certain plating bathswhich have high temperature solutionMany platers are not aware of how important preheatingcan be to the quality of the final deposit. If parts enter thehigh temperature plating bath not at approximately thesame temperature; non-adhesion and many other defectscan result.
    • 20. Effects of Discontinuity in Process The cleaning and activation steps must account for the fact that surface oxide re-forms at different rates on different metals Specifically, in case of iron or nickel the oxide re-forms slowly enough that the part can be transferred from a cleaning solution to a plating bath at a normal rate In case of aluminum or magnesium the oxide re-forms very fast such that special processing steps are required to preserve the metal surface while it is being transferred to electroplating
    • 21. Effects of Discontinuity in Process Discontinuity in Intra-cleaning, Inter-cleaning, Cleaning and Plating processes has aggravated effects on metal substrate  During Mechanical cleaning (buffing, polishing), residual polishing wax, adhesives layer remain available on the substrate, if not subjected to next cleaning process within certain period of time, the residue layer will form oxides or hard scales in case of prolonged discontinuity e.g Indium plating of rotor 
    • 22. Effects of Poor Cleaning Process Plating defects  Non-adherent deposits  Discolored deposits  Pin holes, peeling off coating film Contamination of subsequent process step “When trouble arise in a plating operation, the experienced Electro-plater always tend to suspect that the root of problem is Pre-plating Cycle i-e Cleaning”
    • 23. Effects of Poor Cleaning Process It is expensive and wasteful to find out the reasons, only after the whole plating sequence has been accomplished Cleanliness tests which can be applied before the part enters plating sequence are helpful  Water Break Test Clean metals are hydrophilic and will shed water in an unbroken sheet, whereas if traces of soil remains on the surface, water will run off, leaving un-wetted areas, which are easily observed  Wiping Method The surface is wiped with a clean white cloth, amount of soil absorbed by the cloth is observed; Non-quantitative but easy

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