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 Preparation
Contaminants’ 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 Preparation
Chemical Cleaning Processes
• Alkaline cleaning
• Solvent cleaning
• Acid cleaning
• Emulsion cleaning
• Ultrasonic cleaning
Ultrasonic cleaning uses high-frequency mechanical
vibrations combined with chemical
11. Chemical Preparation
Alkaline 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 Preparation
Solvent 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 Preparation
Acid 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 Preparation
Acid 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 Preparation
Emulsion 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 Preparation
Ultrasonic 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 Preparation
During 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
18. 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)
19. 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.
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
Hinweis der Redaktion
-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.