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PRE-PROCESSESSURFACE PREPARATION

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SequenceMechanical Preparation

Blast Finishing Shot Peening Mass Finishing Polishing Buffing

Chemical PreparationPre-process BakingPreheatingEffects of Discontinuity in processEffects of Poor Cleaning process

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

Mechanical PreparationProcesses

– 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

Mechanical PreparationProcesses

– 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

Chemical PreparationParts 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

Chemical PreparationParts 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

Chemical PreparationImportant 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

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

Chemical PreparationChemical Cleaning Processes

• Alkaline cleaning

• Solvent cleaning

• Acid cleaning• Emulsion cleaning

• Ultrasonic cleaning

Ultrasonic cleaning uses high-frequency mechanical vibrations combined with chemical

Chemical PreparationAlkaline CleaningUses 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

Chemical PreparationSolvent CleaningOrganic 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)

Chemical PreparationAcid CleaningRemoves 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)

Chemical PreparationAcid PicklingA 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

Chemical PreparationEmulsion CleaningUses 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

Chemical PreparationUltrasonic CleaningMechanical 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

Chemical PreparationDuring Cleaning process design, the substrate material is critically considered, so that damaging reactions are not caused by 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

Pre-process Baking

Baking is carried out as a pre-process so that the induced stresses (due to earlier operations of machining, grinding etc) are relieved

These stresses, if not removed, can reach to part’s critical stress level due to further stress aggravation during plating processes (the embrittlement phenomenon)

Preheating

Preheating, high temperature cleaning, hot water rinse/dip is also a critical prior to certain plating baths which have high temperature solution

Many platers are not aware of how important preheating can be to the quality of the final deposit. If parts enter the high temperature plating bath not at approximately the same temperature; non-adhesion and many other defects can result.

Effects of Discontinuity in ProcessThe 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

Effects of Discontinuity in ProcessDiscontinuity 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

Effects of Poor Cleaning ProcessPlating defects

Non-adherent depositsDiscolored depositsPin 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”

Effects of Poor Cleaning ProcessIt is expensive and wasteful to find out the reasons, only

after the whole plating sequence has been accomplishedCleanliness tests which can be applied before the part

enters plating sequence are helpfulWater 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