surface treatments

Chemical cleaning methods

• use chemicals to remove unwanted oils and soils from workpiece surface

mechanical cleaning

• removal of substances from surface by mechanical operations

• removing burrs, improving smoothness, adding lustre, enhancing surface properties

why clean the surface?

• prepare surface for subsequent industrial processing

• improve hygiene conditions for workers and customers

• remove contaminants that might chemically react with surface

• enhance appearance and performance

factors in selecting cleaning method (7)

• contaminant

• degree of cleanliness required

• substrate material to be cleaned

• purpose of cleaning

• environmental and safety factors

• size and geometry

• production and cost requirements

types of chemical cleaning (5)

• alkaline cleaning

• emulsion cleaning

• solvent cleaning

• acid cleaning

• ultrasonic cleaning

mechanical cleaning

• physical removal of soils, scales, or films from work surface

• by abrasives

• deburring and improving surface finish

Blast finishing

• high-velocity impact of particulate media

• cleans and finishes surface

• most well known is sand blasting

• propelled by centrifugal force

• wet slurry is sometimes fired

shot peening

• high velocity of small cast steel pellets (shot) directed at metallic surface

• improves fatigue strength of metal parts due to cold working

• purpose is different from blast finishing

tumbling and other mass finishing

mixing action inside container in the presence of an abrasive media. causes parts to rub against media and each other to achieve the desired finishing action

mass finishing uses

• deburring

• descaling

• deflashing

• polishing

• radiusing

• burnishing

• cleaning

Tumbling

• use of horizontally oriented barrel of hexagonal or octagonal cross-section in which parts are mixed by rotating barrel

• finishing by landslide action

• cycle of rising and tumbling occurs continuously

• barrel finishing is slower - several hours

• noisy

electroplating

• metal ions in electrolyte solution are deposited onto a cathode workpart

• aqueous solution of acids, bases or salts

• movement of plate metal ions in solution

• MUST make structure electrically conductive by applying conductive paint

electroforming

• electrolytic deposition of metal onto a pattern until the required thickness is achieved

• pattern removed to leave formed part

• electroformed parts are thicker so cycle is longer

patterns used in electroforming

• solid or expandable

• solid patterns have a taper or other geometry that permits removal of the electroplated part

• Expendable patterns are fusible or soluble

• Fusible is made of low-melting alloys, plastic, wax or other material that can be removed by melting

applications of electroforming

fine molds for lenses, CD, DVD, embossing and printing plates

electroless plating

• process driven entirely by chemical reactions

• deposition of metal onto part surface occurs in aqueous solution

• greater cost

advantages of electroless plating

• uniform plate thickness on complex part geometries

• process can be used on both metallic and non-metallic substrates

• no need for a DC power supply

anodizing

• for decorative purposes

• corrosion protection

electroplating vs anodizing

• workpart to be coated is cathode in electroplating, anode in anodizing

• in EP, coating is grown by adhesion of ions of a second metal to base metal

• in anodizing, surface coating is formed through chemical reaction of substrate metal into oxide layer

hard anodizing

thick coatings up to 0.25 mm can be formed on aluminium

• for high wear and corrosion resistance

organic coatings

• polymers and resins

• produced naturally or synthetically

• applied as liquids that dry or harden as thin surface films on substrates

advantage of organic coatings

• variety of colours and textures

• protects substrate surface

• cheap

• easy to apply

what do organic coatings contain

• binders

• dyes or pigments

• solvents

• additives

binders

• polymers and resins that determine the strength, physical properties and adhesion to surface

• holds pigments and other ingredients in coating during and after application

• most common binders are natural oils and resins

Dyes

• soluble chemicals that colour the coating liquid but do not conceal surface beneath

Pigments

solid particles of uniform size that are dispersed in the coating liquid but insoluble

• they colour the coating and hide surface below

• they strengthen the coating as they are particulate

solvents

• dissolve the binder and other ingredients

• common are alcohols, esters, ketones

additives

biocides, fungicides, defoamers, catalysts to promote cross-links

transfer efficiency

proportion of paint supplied to the process that is actually deposited onto the work surface

car finishes

1) phosphate coat applied by dipping

2) primer coat applied by dipping

3) colour paint coat by spray coating

4) clear coat by spraying

methods of application

brushing and rolling, spray coating, immersion, flow coating

brushing and rolling

• high transfer efficiency

• not suited to mass production

• brushing is versatile, rolling is only for flat surfaces

spray coating

• forces liquid to atomize into fine mist

• form a uniform coating when spread

• manually in spray painting booths

• or automated process

• transfer efficiency is low

electrostatic spraying

• improves transfer efficiency of spray coating

• workpart is grounded electrically and atomized droplets are electrostatically charged

• droplets are drawn to the part surfaces so that transfer efficiencies reach 90%

immersion

applies large amounts of liquid coating to the workpart, allowing excess to drain off and be recycled.

• dip coating is the simplest, immersed in an open tank of liquid coating material

electrocoating

variation of dip coating where the part is electrically charged and dipped into paint bath of the opposite charge

• improves adhesion and permits use of water-based paints

flow coating

workparts are moved through a paint booth where a series of nozzles shower the coating liquid onto the part surfaces.

• excess drains into sump for recycling

• organic coating converts from liquid to solid

curing

chemical change in organic resin where polymerisation/cross-linking occurs to harden coating

type of coating determines type of chemical reaction

ambient temperature curing

evaporation of solvent and oxidation of resin

elevated temperature curing

elevated temperatures are used to accelerate solvent evaporation as well as polymerisation of resin

catalytic curing

starting resins require reactive agents ‘

for polymerisation and cross-linking

radiation curing

various forms of radiation such as microwaves, UV light, and electron beams are required to cure

eletroplating formula

V= volume of metal deposited

E= Mass deposited per unit charge

C= current efficiency

t= time