Paint Shop Report (Repaired)

8/12/2019 Paint Shop Report (Repaired)

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OBJECTIVE:

To protect the body surface and enhance visual appeal by

adding color and gloss with efficient and environmentfriendly techniques.

General steps for painting and coating applications

include:

1.Substrate surface preparation2.Application of coating3.Drying of coating

Substrate surface preparation include following sequence

of pretreatment processes:

1.precleaning (optional)2.degreasing3.activation4.phosphating5.passivation

Each process is followed by DM rinse to avoid carryover

of reagents in the tank.


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Depending on the degree of cleanliness of the body

entering the coating line,

The legislation demands with respect to cascade rinsing,

and the existence of apostrinsing stage, the pretreatment line can have up to 14

stages for a safe and

reproducible process.


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PROCESS SEQUENCE FOR FERROUS METALS IN PAINT SHOP


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LOADING

Here,the work piece is loaded on the hangers of the

conveying system with mechanical cleaning by

brushing with wire brush if it is scaled or rusted.

DEGREASING

This unit comprises 2 steps :

1.KOD (knock off degreasing)2.Degreasing

The task of degreasing is to remove all kind of

contaminations from the metal surface, to achieve a

water-break free surface, that is, a continuous water

film on the surface after rinsing off excessive

degreasing chemicals with water and to obtain a

reactive surface which is able to build up the

phosphate coating that is, the conversion layer,

within a reasonable period of time.

Liquid- or powder-type alkaline degreasers are used

to feed the tanks. Liquid cleaners are typically two-pack products with the builder and the surfactant

separate, whereas powder products are typically one-

pack products.


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Alkaline degreasers are composed of inorganic salts,

the builder,and organic compounds, the

surfactants. The predominant task of the builder is

to remove inorganic and pigment contaminants like

metal grains and welding pearls. The task of the

surfactant is to remove oils,

lubricants, soaps, and other organic contaminants.

Typical builders used in alkaline cleaners are the

following: NaOH, KOH, Na2CO3, K2CO3 to maintain

alkalinity

silicates to particle removal, inhibitor, buffer

orthophosphates to degreasing

condensed phosphates degreasing to complexing

complexing agents to complexing.

Surfactants contain a hydrophilic group, that is, a

long chain of ethoxy (EO) and/or chain of propoxy

(PO) molecules, and a hydrophobic group, which is

typically a long chain alkyl. They are classified into

anionic, cationic, nonionic, and amphotericsurfactants. Typically nonionic surfactants are in use

today owing to their better environmental

compliance. The mechanism of particle removal


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from surfaces is shown in Figure 1. Owing to their

amphiphilic characteristics, surfactants in solutions

reduce interfacial tensions, absorb at solid interfaces,

and form micelles. The mechanism of oil removal

from surfaces is

shown in Figure 2. The surfactants first adsorb on

the oil surface, and then try to reduce surface

tensions and remove separated droplets containing

the oil from the surface.

Nonionic surfactants are solubilized through

hydrogen bonds between water molecules and the

ether group. As the hydrogen bond breaks at

increasing temperature, the surfactant becomes

insoluble and forms a second phase. This is called

the cloud point and the corresponding temperature isspecific for each surfactant, depending on the

number of EO and PO groups. Nonionic surfactants

bove the cloud point have a low tendency to build

foam, but are still surface active.


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Fig.1 Mechanism of particle removal from surfaces.

Fig.2 Mechanism of oil removal from surfaces.

Degreasing solutions are applied both by spray and

immersion applications.The advantages of spray application are as follows:

short treating times

used for parts with simple geometry


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excellent for particle removal

restricted in terms of low-temperature application

(surfactants tend to foam at low temperatures)

requires only small bath volume for less

investment, less

space, low capital cost

canbe combined with brushing operations, and so

on.

The advantages of immersion application are asfollows:

excellent cleaning of areas difficult to access (box

sections)

requires higher concentration and treatment times

compared to spray application

higher stability due to greater bath volume

Control of degreasing baths is accomplished mostly

by titration methods:

total alkalinity

free alkalinity (optional)

conductivity pH (optional).

Total and free alkalinity are defined by different

titration methods, which are


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developed by the suppliers of the pretreatment

chemicals.

WATER RINSE

The main task of rinsing is to remove excessive

chemical from the metal surface and hence to avoid

contamination of the following stages, which may

cause severe problems in the chemistry of the

process. It is done by ordinary water.

ACTIVATION

Activation increases the number of crystallization

nuclei on the metal surface. This results in an

increased number of phosphate crystals per unit

surface area and a reduced coating weight for the

applied conversion layer. As the surface will be

uniformly covered with crystals in a shorter time,

activation treatment also has an accelerating effect in

the phosphate process. For activation prior to zinc

phosphating, aqueous dispersions of titanium ortho

phosphates with a pH between 7 and 11 are typically

used.The performance of activating baths

diminishes with time, independent of the throughput

of parts. This phenomenon varies to a great extent


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and depends on the specific product formulation.

This can be explained in terms of titanium phosphate

colloids being negatively charged and being

precipitated out by divalent or trivalent cations,

especially Ca(II) and Mg(II) ions, which are

contained in hard water. In order to reduce

degradation by divalent ions, most activator products

contain condensed polyphosphates to form

complexes with the aforementioned cations.

It is strongly recommended that activator baths be

made up using demineralized water and losses be

compensated with water of the same quality.

Depending on the activator product, the level of

contamination in the bath, and the specifications

with regard to crystal size and coating weight,activating baths are dumped at fixed intervals. These

time intervals may range from about 1 week to about

6 months. Liquid activator products are normally fed

directly into the supply tank, although special pumps

are required to cope with the high viscosity of these

products. When powder products are used, a 5%slurry is typically prepared in a premixing tank and

used for product dosage in the working tank. It is

recommended that activator baths are stirred


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continuously in order to prevent settling of the

titanium phosphate dispersion.

PHOSPHATING

Low-zinc phosphate is the standard phosphate

conversion layer worldwide for processing. Othertypes of pretreatment like iron phosphate or

conventional zinc phosphate, which are used for

steel surfaces, and chromating or chrome-free

processes on basis titanium/zirconium compounds

for aluminum surfaces have been tried in the past,

but neither provides the required quality onall substrates, nor are they suitable for multimetal

treatment.

The initial step in all conversion treatments is a

pickling attack of the metal surface by free

phosphoric acid. The metal loss on cold-rolled steelsheet, zinc-coated steel sheet, and aluminum is

typically in the range of 0.5 to 2 g m-2 The

consumption of hydrogen ions leads to a shift of pH


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in the diffusion layer adjacent to the metal surface,

exceeding the solubility limits, and consequent

precipitation of zinc phosphate. Accelerators are

therefore added to the zinc phosphate chemicals to

speed up the pickling reaction by replacing the

unwanted H2 evolution by chemical reactions that

take place more easily, like reduction of the

accelerators themselves. Zinc phosphate solutions

typically contain dihydrogen phosphates of zinc,

nickel, manganese, free phosphoric acid, sodiumnitrate, fluorosilicic acid, one or several oxidizing

compounds like sodium nitrite, hydrogen peroxide,

hydroxylamine,sodium chlorate, nitroguanidine

(CN4),N-methylmorpholine-N-oxide (NMMO),

acetaldoxime, and sodium nitrobenzenesulfonate

(SNIBS). The Zn, Ni, Mn compounds and thephosphoric acid along with the ferrous ion (FeII)

from the steel surface are the layer forming

compounds. All other chemicals have supporting

functions like acceleration, oxidation, etching, and

stabilization of the bath and the film.

Processes for phosphating steel and aluminum

surfaces at the same time additionally contain

fluoride (hydrofluoric acid, alkali fluoride, or alkali


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bifluoride) to generate amounts of about 50250

ppm free fluoride, depending on the type of process

and application, that is, spraying or dipping. In

patent literature, low-zinc phosphate processes are

defined by the low Zn : PO4 ratio in the working

solution. This ratio had been set to 1:12 to 1:110

originally, but has been changed to 1:20 to 1:100 as

of today.Pickling reaction:

Coating formation:

Hopeite

Phosphophyllite

ZnMn-phosphate

Sludge formation:

The sludge generated during the phosphate process

has to be continuously removed by filtration


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techniques in order to maintain constant

performance of the conversion layer.

DM WATER RINSE

It is used to remove leftovers of phosphating and

passivation tank.

PASSIVATION

In order to improve the corrosion resistance of

phosphated and coated metal sheet, the conversion

layer can be given a passivation as a postrinse with

chrome(VI), Cr(III), or chrome-free solutions.

Today, hexavalent chrome has been replaced by

zirconium-based solution due to the toxic danger of

chromium VI-compounds. Although the mechanism

of passivation by zirconium-based solutions is not

fully understood, it is generally accepted that the

effect of improved corrosion protection is mostly

associated with reduction of the pore size by

precipitation of insoluble compounds and removal of

secondary phosphate crystals from the surface of the

phosphate layer by the acidic solution of the

passivating agent.


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WDO

WDO is water drying oven.Dry-off ovens are used

to ensure that the body surfaces are absolutely dry

before paint application. Sufficient time is provided

to allow the body to be heated to about 125-150 0C

at which all water is evaporated. Ventilation is

necessary to take away the evaporated water. With

direct gas heaters, ventilation must be sufficient toprovide the amount of oxygen required for the

combustion as well to take away the flue gas

produced.

3 COAT-3 BAKE OR 2 COAT-2 BAKE

PAINTING PROCESS :

The outer surface of the body is prepared and

painted through 3 coat (i.e. primer

surfacer, base coat, top coat) - 3 bake or 2 coat

(primer surfacer, top coat) - 2 bakes

process.

The primer surfacer application is carried out in

number of sections of the booth such as


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1.wiping off contaminants and an ionised blow-offto eliminate static electricity

2.application of anti-chip coating to lower bodyarea

3.application of primer surfacer to entire bodyexterior

4.inspection and repair of primer surfacer, ifrequired

PRIMER

It is used to level the structure of substrate. Firstly,

any shot-through to the bare metal must be

prevented because perforations inevitably lead to

corrosion. At the same time, good adhesion of the

top coat is required to ensure minimal detraction

from the visual appearance in the event of anychipping.

BASE COAT

It is the function of the base coat to provide color

and durability. A pigment will retain its crystal or

particulate structure throughout the colorationprocess. It will alter the appearance of an object by

the selective absorption and/or scattering of light.


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TOP COAT

To protect these pigments from the environment, a

clear coat is applied over the colored base coat. The

clear coats offer protection from extensive sunlight,

scratches, and all types of chemical attack.

After a flash off, the body is moved through an oven

for about 30 minutes at about 1700C for baking of

primer surfacer. The bodies before moving to base

coat and/or clear coat wet sanded, rinsed, and driedoff. A preparation area is used for manual wipe

down, compressed air blow-off and deionised air

application. Depending on the manufacturers

process plans either the body moves through a clear

coat application booth or a two-coat (basecoat/clear

coat) booth. Between each coat, a flash off isnecessary.

FLASH OFF ZONES

Drying as a flash-off step between two painting

areas like the waterborne base coat and the clearcoat; in this intermediate drying process, only the

solvent in the paint film is removed by evaporating

to a certain degree


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PAINT BOOTHS

A uniform coat of paint is applied manually or

automatically in booths. A stable painting

environment is the basic necessity for a good booth.

Controls in air supply system maintain the desired

temperature, humidity and cleanliness of the air in

the different portions of the booth. Air from the

atmosphere drawn by supply fans placed at outer end

of the air supply system passes through air intake

grilles, air intake dampers, primary air filters, andthe air supply fan. Depending on the climatic

conditions of the plant location and the

specifications of paints; systems for heating, cooling

or air conditioning with humidity control are

incorporated in the air supply line. The air supplied

by the fan passes through the duct to the plenumchamber, from where the air is divided and passes

through the secondary filters placed in the ceilings of

the different sections of the spray booth, the entrance

and exit vestibules. The air entering the vestibules

travels towards the outside ends, and precludes the

entry of dust from the outside. It also carries awaythe dust that might have been brought by the bodies

on the entrance vestibules. The quantity of the air

supplied must be sufficient to obtain an outward air


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water and air are separated. Air that is now clean and

free of water droplets moves through exhaust and is

generally reused. As the paint mist is killed when it

passes through the orifice, no paint buildup takes

place.

The water enters the spray booth at the top of the

water flow plates and is distributed along the whole

width of the respective water flow plates. The water

is drawn into the longitudinally placed venturi

orifice where it mixes with the paint-laden air,extracts the paint out of the air. The water with paint

particles is separated from the air in the scrubber.

Colour booths may be in various designs: with or

without glazing, as sheet metal

construction with 2-component interior coating or inaluminium or stainless steel.

The water system in spray booth washes the paint

overspray out of the paint laden air. The water

carries the paint deadner in solution and finally

carries the deadened paint in the sludge pit. In sludge

pit, the paint rises to the surface and can becollected, while the water is drawn through a filter

by recirculation pump and recycled to the booth.


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Different methods may be utilised for sludge

removal.

Conventional method -floatation or sedimentation

in a tank

Floatation overflow method (based on the

principle of coagulation) - The paint-laden water is

mixed with an alkaline or chemo-physical coagulant.

The denatured paint is separated from a floatationtank. The paint sludge is periodically collected in

filter bags.

External partial circuit waste treatment (based on

the principle of adsorption) - The paint is denatured

by the withdrawal of the solvent surrounding thepaint particles through the reaction of the specific

adsorbing agents, such as aluminium oxide. The

reaction is almost spontaneous. Paint sludge

separation is from the outside coagulation tank.

Precipitating tank can be small.

The exhaust air volumes from spray booths are very

high and the solvent proportion is low. A process to

increase the concentration of the solvent content in


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Paint circulation systemcovers paint delivery, paint

mix, paint transfer, paint filtration and

the final delivery to the point of paint application

equipment. Paint circulation system

maintains stable temperature, viscosity and pressure

of the paint for the effective functioning of the

painting equipment. A fixed rate of paint fluidity and

viscosity is preserved with no sedimentation of

solids to achieve uniform paint thickness. Optimum

combination of paint temperature, car bodytemperature, and the temperature and relative

humidity inside the spray booth facilitates

smoothness and gloss finish for the body.

Colour sprayingmay be executed by air atomising,

airless spray, electrostatic sprayingthrough manual or automatic equipment or by

spraying robot. Conventional air spray guns are still

in use, particularly for retouching work. However,

the disadvantages are the excessive overspray and

resulting waste requiring extremely expensive

ventilation system to meet VOC legislation.

In air less spraying, a special high pressure hydraulic

pump delivers the paint to a spray gun with a


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tungsten carbide nozzle- the spray tip. The energy

required to atomise the paint comes from the sudden

change in pressure as the paint emerges from the

spray tip. The absence of air helps the spray

penetrate corners and cervices better and

substantially reduces overspray. However, heavy

wear of nozzles caused by the high speed of the

paint leaving the gun and often frequent blockage

due to the fine hole of the spray tip; are the

problems.

Air assisted air less sprayingis a synthesis of the

conventional air spray gun and airless spraying. A

small pump partially atomises the paint by hydraulic

pressure. With the Kremlin Airmix system, the paint

fan at the gun nozzle is then uniformed andenveloped by the air jets incorporated in the spraying

head to give fine atomisation. Overspray is reduced

upto 30% in comparison with conventional air spray.

It requires a compressed air volume of only one

twentieth of that needed by a conventional spray

gun, and a paint feed pressure which is lower than anairless gun.


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as against the outside routing on conventional

machines. With no open, loosely suspended cables

and conduits above the body, the flaws on the paint

finish caused by falling dirt is avoided. Paint and

solvent losses during automatic colour changes are

considerably reduced, as the colour change devices

are located close to the spray nozzles. Quantity of

cleansing liquid required to flush the system before

the colour

change is also reduced considerably. Further,modularity in construction is another feature of these

machines that provides flexibility for model change.

In a typical metallic spray booth using solvent borne

paint, the process sequence may be as follows:

1.Tac rag manual or using ostrich feather machine2.Inside painting of base coat with robots3.Outside first base coat with electrostatic paint

spraying machine

4.Outside second base coat with paint machines5.Intermediate blowoff6.Inside painting -clear coat with robots7.Outside clear coat with electrostatic paint

spraying machine.

The paint shop operators are provided a control

system that graphically displays and


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simulates the operating parameters. For operator, it

is easier to set the enormous number of coating

parameters and to optimise the system.

Paint spray booth is thus a totally integrated system

with paint supply system, colour

changing system, and painting equipment. Air

turbine driven minibells with high voltage block and

efficient paint atomisation system are capable of

flexible positioning through a 3-axis control.Controls for the conveyors, application equipment,

robots and the safety installations are integrated

through a control panel mounted into the side wall

construction of spray booth.

ELECTRO-DEPOSITION COATING

Electro-deposition are of two types:

Anodic Electro-Deposition (AED) and Cathodic

Electro-Deposition (CED).

In electro-deposition, charged particles from thepaint emulsion move to Anode (AED) or Cathode

(CED) under electrical forces (because of voltage

created between the electrodes - one of which is


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work piece). It happens during the period the body

remains completely submerged in the paint bath. In

anodic system, the work piece is made positive

electrode. In cathodic system, the work piece is

made negative electrode. The direct current

established through the bath makes the pigment and

resin base of the paint wander towards the body

surfaces. Ultrafiltrate (UF) equipment is used for

condensation of ED paint ingredients to form film

and separation of those not forming film. Filtrates ofthe ingredients that do not form film are used in a

multiple cleaning process as a solution to clean out

surplus paint adhered to electro-coated body. As the

film formed has high resistance, so further

deposition ceases. Coating reaches all the recessed

area. Deposited film does not redissolve. However,the undeposited material is rinsed. Neutraliser in the

system is controlled. The film is compact almost

touch dry with high solids. Deposited film after

stoving becomes hard, durable polymeric film.

Major advantages of electro-deposition overconventional solvent borne dip primer are:

1. Fully automatic operation, including the facilityof controlling film thickness


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2. More uniform coating3. Better coverage in box and interior surfaces4. Deposition of film of comparable thickness onthe inner and outer surfaces of box section

5. Good coverage of sharp corners6. Better penetration between spot welded surfaces7. No runs and sag8. No solvent boil9. micron dry film buildup10.Better chip resistance11.Better corrosion resistance12.Nearly 100% paint utilisation because of closedloop system

13.Superior in anti-pollution, safety, health hazardDisadvantage of Electro-deposition system ofcoating is its high capital investment and operating

cost. While mild steel is good enough as material for

alkali resistant anodic plant bath, acid resistant

stainless steel or PVC is used for cathodic tank.

Cathodic plant is almost 80% or so costlier than

anodic. Electro-deposition was initially anodic.However, over the years cathodic electro-deposition

has become universally preferred system in

automotive industry. Some clear advantages of


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cathodic over anodic systems are the resulting

superior quality characteristics. A typical

comparison of the results is as follows:

Other advantages of the cathodic elecro-deposition

are:

1.Increased throwing power(about 50% more)with better capability for the paint to reach

recessed and critical corrosion prone areas,which might not be possible with anodic system

2.Smoother film deposit as the film flowssmoothly during baking, and so lesser necessity

of rectification work that ensures a smoother

final finish

3.With its better corrosion resistance, the filmthickness can be reduced(upto half in

comparison with the film thickness of anodic

paint).


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4.Less problem in maintenance of the quality ofpaint as the cathodic paint has a greater

resistance to bacteria

5.Electrodepositing can also be done by spraying.

Electrostatic sprayinghas increasingly been

accepted as the best method. The method of

atomising the paint may be:

1. By centrifugal force off rotating sharp edge as in

high speed rotating bell.

The high voltage imparts the maximum electrostatic

charge at the edge.

2. By high pressure fluid feed and airless

atomisation through a small orifice3. By air-assisted airless feed with certain clear

advantages over 2.

4. By compressed air as in conventional spray gun

In the case of centrifugal atomisation, the particles

are charged as they leave the rotating bell. Othersystems ionise the air at the tip of a sharply pointed

external electrode and depend on partial transfer of

the charge from air ions to the atomised paint


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particles. Paints are generally charged with negative

polarity. Close tolerance of electrical resistance is

the most critical feature of the paint formulations.

The bodies to be coated are earthed to attract all the

paint and create a wrap around effect of the paint

particles. The method has many advantages over

conventional spray finishing:

1.Faster painting2.Better uniformity once the parameters are

established3.Considerable saving in paint through absence of

overspray

4.Less need for comprehensive exhaust facilitiesDifferent methods of providing the high potential are

used by paint equipment manufacturers and the

equipment are available for both manual, automatic,or robotised application. While manual painting can

hardly attain 25~35% paint transfer efficiency,

automatic painting machines achieve a transfer

efficiency of 80~90% with electrostatic bells, and

60~70% with air atomising reciprocating paint

machinery. Automated painting also achievesuniform paint deposition over all the surfaces, and

the quality is consistent unlike skill-dependent

manual spray painting. Water borne paints are


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replacing solvent borne ones for better emission

Control

Application of water borne paints by electrostatic

methodsrequire special

precautions, as the higher conductivity of these

paints provides an earth path via the

distribution system. Some major systems have been

developed by the paint application

equipment manufacturers to meet this requirement ofwater borne paint.

TheInternal Charge System uses a paint reservoir

situated within the machine and sized to provide

sufficient paint for one application. This is filled

from the distribution system and then physicallyisolated before the high voltage applied. The

reservoir is finished and refilled for every body

painted. The transfer efficiency is same as one with

solvent-based paint. Application does not depend on

the booth conditions. The paint finishing is

excellent.However, the system is not suitable for very fast

colour changes.


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TheElectrode System uses external electrodes. The

atomised paint from the high speed bells passes

through a corona field created by a system of

electrodes arranged around the rotating bell but

electrically isolated from the main body of the

machine. The system can make very fast colour

changes, but the environment in the booth must be

perfectly controlled in terms of humidity,

temperature and ventilation to achieve the optimum

efficiency. In a new system calledAccubell, a singletank is fitted as close as possible to the bell. The

tank can be filled with the quantity of paint

necessary for one job. To change colour, the

atomiser travels to a docking station which is

connected to the colour changing block for

colour change cycle to begin. The tank is firstcleaned with water as are the bell and the paint

injector, and then all are flushed with compressed

air. Finally, the tank is filled with the new colour.

The colour change time is short. It prevents paint

waste and water consumption is low. The principle

for Accubell has also been applied with electrostaticguns. The tank in this case is bigger as the flow rate

for a gun is usually higher for a bell and the system

is called Accustat. It has to correspond to the multi-


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axis robots. The high voltage power unit is

integrated into the gun so that only the low voltage

cable runs through the arm of the robot.

Baking Oven

After every paint coating, ovens are required to curethe coating in addition to drying off the wet surfaces.

It also polymerize with the help of cross linking

agents. The type of oven system generally depends

on the type of paint coat used.

Typically in a paint shop, the ovens are:

pretreatment water dry-off oven, electro-depositionoven, underbody sealer oven, surfacer coat oven,

dry-off oven after wet sanding, finish coat oven,

metallic finish coat oven, repair oven.

Almost directly after the final rinsing step in the

rinsing zone the body enters the baking oven. Some

ovens use multistages in continuous lines. Theyconsist of an IR (Infra Red)-heating zone and two or

three stages of convection zones with circulating air.

Some only have circulating air stages. Even the


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convection ovens in general are not very energy

efficient compared to IR-ovens, but are mandatory

for the baking process because of the complex shape

of a other bodies.

The state-of-the-art ovens are called A-ovens

characterizing the shape of the oven in which the

body is moved up to a certain height in the entry

area, conducted through the oven and moved down

back to the starting level at the exit of the oven. The

advantage of this shape is significantly reduced lossof energy due to the fact that warm air moves to the

top of the oven, in other words it cannot leave the

oven easily. The IR-zone also helps to save energy

because it heats up the outside of the body very

rapidly, so that the circulation zones can concentrate

their air flow specifically on the critical parts of thebody. Those are the inner parts and the parts with

high masses of steel like rocker panels or the B-

columns. This is provided by air fans in conjunction

with flow apertures and corresponding air speeds of

25m s1 or higher for special tasks. Nevertheless

measurements from the temperature/time curvesshow differences depending on the location of the

bodies (Figure 4).


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Paint bake ovens are divided in two parts: the heat-

up zone and the holding zones followed by the

cooler. Air seals are incorporated at oven ends. In

heating zone, the painted bodies are rapidly brought

up to the required baking temperature. The holding

zone then maintains the temperature of the bodies

for the time required for complete baking of the

paint. Layout of an oven with a cooler Convection

ovens, radiation heat ovens, or a combination of both

systems may be used in paint stoving process.

Convection ovens: The body is heated by means of

circulating air that sweeps over the surface thus

transferring its heat to the body by convection. For a

uniform metal temperature, the circulatiing air is

blown into the oven at floor level. The air then ismade to pass through the oven vertically towards the

ceiling from which it is extracted and conveyed back

to the heating unit.

Radiation ovens: Due to the natural air circulation

caused by the heat convection against the radiationpanels, there is a tendency of temperature increase

just below the oven ceiling in radiation oven. A

supplementary air circulation fan is used to exhaust


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the hot air under the ceiling and blow it again at

floor level to maintain an even temperature inside

the oven.

Combined radiation-convection oven: The

radiation zone is used as a heating zone, as the

radiation heating is expected to be quicker than

convection heating- particularly for the surfaces

exposed to the effects of radiation. Adequate

ventilation is provided to eliminate the evaporatingsolvent and the make-up air is filtered before entry

into the oven. Convection zones work as holding

zones in which the temperature of the bodies can be

kept constant during the full time required for

complete baking of the paint.

The number of heated zone is dependent onapplication and capacity. Heating may be

indirect or direct with electricity, gas or oil. Oven

heat transmission may be through

convection. Blast nozzles may be arranged at the top

or at the sides. It may be indirect

heating in infra-red oven. The bodies pass through acooling zone where fresh air is directed onto them at

high velocity. The fresh air thus heated up is carried

away via the exhaust air system.


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Detection of the measured values is by contact free

radiation thermometers, while the

radiator output is controlled electrically.

Advantages are:

1.An uniform and quick heat up of the car body,independent of sheet metal thickness and body

shape.

2.Optimum drying conditions despite shorter ovenlength

3.No overstoving due to over temperature reactione.g. in case of conveyor breakdown.

4.Excessive heating up of plastic, rubber electricalcomponents during repair curing is avoided due

to spot heating of the repaired surface. An

additional benefit is energy saving.

To guarantee the specified film performance it is

mandatory to control these data on a regular basis.

The minimum metal temperature has to be reached

at all points for example 10 minutes over 170 C.

Sensitivity to overbake has also to be considered

while this is not such a critical factor compared tounderbake. Owing to the already mentioned fact that

cathodic electrocoats based on their chemistry split

chemical components the air flow also has to


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manage this material load. Depending on the zone

the share of recirculating air changes in the different

stages. The last stage can have higher recirculating

air compared to the first and middle ones in case of a

three stage oven. Considering the weight loss of

between 8 and18% for most electrocoat materials

based on the type of coatings and air dried deposited

films this results in the concentration of organic

emissions in the oven air of several grams per cubic

meter. This is well above the value limits specifiedin the regulations of most countries in the world.

Therefore the waste air, that is replaced by preheated

fresh air, normally goes into an incineration unit to

fulfill the VOC-emission regulations. Other methods

like adsorption on activated carbon beds are used

rarely. Because the thermal energy generated in theincinerators can also be used for heating air, thermal

incineration systems are often an integral part of the

oven system. Gas- or oil-fired-boilers are normally

used for heat generation, which is then transferred

by a heat exchanger to the recirculating oven air. If

natural gas is used the heat exchanger can bedispensed and the hot fuel gases can go directly into

the oven. In some cases this can lead to film


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performance deficiencies like adhesion failures to

the primer surface

Figure 4

DEFECTS

As is usual in the coating technology processes,

paints have to blend together toprovide a stress- and defect-free coating result. If

defects occur it is not always easy

to identify the source of the problem immediately as

related to the process or to the

paint. Types of defect:

Dirt

Dirt is one of the most common recurring problems

of coating processes, however


43/48

this is not the case for electrocoats.

If dirt appears it may be caused by insufficient

cleaning cycles of the conveyor,

dirt or other particles in the body, broken filters, or

paint stability.

Craters

Craters in amorphous films are caused by surface

tension differences of inhomogenic

particles with very low surface tensions (


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Surface Roughness

Unusual surface roughness exceeding the

specification may have many different

causes. First rough steel surfaces result in rough and

structured electrocoat surfaces.

The countermeasure in these cases can only be

replacement of the respective coils.

The residual ripple of the recti.ers being too high

may be another reason for

rough surfaces. Normally this can be corrected byadjustment of the rectifier.

An application voltage that is too high and close to

the rupture voltage starts

the general deterioration of the surface smoothness

of the electrocoat before heavy

local defects and ruptures occur. Correction of bathtemperature and voltage to

.

Film Thickness/Throwing Power

The film thickness and the .lm thickness distribution

on a complex body for

cars and trucks are the most important qualitiesdefining the property of the

electrocoating. The film thickness of the inner

segments of the body is characterized


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by the throw power. It is often not possible to

compare the throwing power of the

different paints of different suppliers on cars, but in

lab tests the potential of an

electrocoat material can be judged. On the basis of

physical application data of the

respective paints and defined body sections

calculation programs exist which are

designed to forecast the .lm thickness and material

consumption of an automotivebody

From a practical point of view, to reach the specified

film thickness and the

throwing power, the anode to cathode ratio, the

appropriate bath temperature, the

solid content, and conductivity as well as the voltageprograms and sometimes

even the feed ratio of the resin and paste dispersions

have to be optimized for each

specific tank. The performance of the process

parameters has to be monitored at

least daily, in tanks with short turn over severaltimes a day. This includes the

appearance of the applied and cured film as well as

.lm thickness data on de.ned


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spots on the body, outside and inside. In cases where

the .lm thickness increases

or decreases without changing the process

parameters one should carefully analyze

the analytical bath parameter. Increasing solid and

solvent levels and decreasing

degrees of neutralization that is increasing pH may

be one of the reasons for .lm

thickness increase. Countermeasures have to be

discussed with the paint supplier.The pH is not a sensitive indicator for changes in the

degree of neutralization

because the electrocoat behaves like a pH buffer

system. So the acid and base

numbers have to be specified for best operation and

have to be checked frequently.

Other Defects

Related to the sensitivity of the paint formula to

process parameters there are

issues such as hash marks, water spotting, boil outs,and pinholes that may appear

causing a lot of manual work and related costs for

repairing those defects.


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POWDER CLEAR COAT

Powder clear coat is an environmental friendlytechnology, as it does not emit any organic solvent

during its application Besides this, powder clear

coat has the following other advantages:

1.direct recycling: the collected overspray powdercan be directly used for the original coating

process.

2.no waste, waste water, or paint sludge from theclear coat application.

3.no use of solvents for cleaning of applicationequipment o spray booth: just vacuuming.

4.reduction of total energy: air supply to spraybooth can be reduced by higher recycling rate,

no VOC, very low toxic aspects.


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5.same film thickness and similar appearance onhorizontals and verticals.

As in a standard application process using

waterborne base coat, the powder

clear coat is applied after a heated flash off of the

base coat at 6 minutes and 60 0C.

To achieve good flow and appearance, clear coat

film builds must be 6070 m.

The melting and cross-linking of the powder clearcoat is done at 20 minutes and

145 0C.

Date post:	03-Jun-2018
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