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
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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.