As shown in Figure 1, a typical high-production coating line has equipment for pretreatment, application and curing of coatings. This line, like most high-production lines, has a conveyor that carries parts through the processes.
In the pretreatment section, surfaces of parts are prepared for coating by pickling, blasting, abrading, solvent cleaning, detergent or alkaline cleaning and application of phosphate and other conversion coatings. If aqueous pretreatment is used, an oven dries the parts prior to coatings application.
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Figure 1: Finishing
Line Elements
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After coatings application, time is allowed for water or solvent to begin evaporating before parts enter the curing area. Coatings are generally baked and/or cured in gas-fired ovens, although oil, electricity and steam are also used as energy sources.
Whether the finishing equipment is simple or complex and highly automated, the same components are used. These components, their ancillary equipment and their controls must be sized to accommodate variations in product configuration and operating speed.
Planning New Layouts
Locating loading and unloading stations, determining the types of finishing equipment required, dwell time required in each section and time between them are important items. Also important are aisles for accessibility and serviceability and deciding the conveyor height over work areas. Designing excess capacity into a new line rather than modifying it later is wise.
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Typical overhead power-and
free-conveyor system
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Select the major components (for pretreatment, application and curing) to apply the coating specified. Next, select the ancillary equipment. Put loading stations near sheet-metal-bending and stamping departments. Locate unloading near final assembly and shipping.
Production output, product size and shape establish conveyor speed. Conveyor speed and dwell time determine distances between components. Solvent flash-off time (to prevent solvent popping of the coating film in the oven) is also important. While their flash-off zones are the same length, dip coating and flow coating operations require more conveyor length than spray painting operations because of their drip zones.
Multi-pass ovens use less space than ovens with straight-through conveyors. In many cases, you can place ovens on the roof to conserve floor space. Build in cooling time after dry-off, so that products are not too hot to coat, and after oven curing, so that they are not too hot to handle at the unload station.
Modifying Existing Lines
When updating an existing line you can often replace old equipment with new, using the same space, with little or no modification. Chemical pretreatment for one finish is often adequate for another. Oven burners and alternative heat sources often can be changed without replacing the housings. Conveyors can be lengthened, shortened and rerouted as needed.
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Power-and-free-conveyor has carriers that allow the product to rotate in the powder booth, ensuring an even coating.
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Conveyors
Although available in many weights, sizes and shapes, conveyors can be specially built to move products through any finishing process. They can be configured as simple loops or serpentines. They can turn corners, change elevations, transfer parts from one route to another, accumulate parts, stop, resume travel and even rotate the parts where required. Ancillary equipment such as overspray protectors, hook strippers and controls help them perform these functions. Chain cleaners remove coating residues from conveyor chains. Dirt and scale are removed by using air or water blasts, heat, rotating brushes and other mechanical means. These contaminants accumulate on chains and can fall onto coated products, causing rejects. Most conveyor lines also have a lubricator that applies oil or grease to the chain on a prescribed schedule.
Overhead conveyors used most often for painting lines are monorails. They can go up and down inclines and around curves. The drive mechanism pulls a continuous chain of connected links along the monorail. It has a motor and gearing unit, speed regulator, drive sprockets and, in some cases, a special chain with fingers that engage the conveyor chain and transmit the drive force to a number of links simultaneously.
Long conveyors often use several drive units with synchronized speeds. Since conveyor chains increase in length as link connections wear—causing film defects because of uneven motion—coating lines have slack take-up devices. These are typically springs or weights and pulleys on a U-turn. When properly adjusted they reduce chain jerking.
The “I” beam, varying in size depending on the load it must carry, is the most commonly used monorail. For smooth operation, its sections are continuously welded, except for switching and expansion joints. The chain riding on the “I” beam varies in size, depending on the force required to pull the parts through the system, up and down the inclines and around the corners. Each chain link is held on the beam by pulleys, and positioned so that hangers and load bars that hold products can be attached. Product weight determines pulley spacing. It is often distributed among four, six or eight pulleys by load bars.
In another type of overhead conveyor, pulleys are mounted inside a continuous hollow tube. Hanger brackets extend through the slot. These are used for small, lightweight parts and often use a steel cable in place of a chain.
Belt conveyors made of continuous metal screen or open mesh serve lines that paint large numbers of small parts. Because of their configurations, they do not traverse the entire line. Instead, a number of short belt-conveyor units carry products through different sections of the finishing line.
Floor conveyors move large objects such as automobiles, truck cabs and appliances through finishing sequences. Tables, platforms or dollies with wheels or casters often carry such items. Chains or cables mounted on or below the floor pull the carriers through various finishing stages, guided by tracks.
Power-and-free conveyors carry parts through finishing sequences when dwell time required in one section of a line is markedly different from that in others, or a change in routing is necessary. One design uses a powered conveyor on an upper rail and a “free” conveyor or driven trolleys on a lower rail. The upper conveyor’s chain has dogs that engage the lower conveyor. It pushes them to each station or coating line component, where the dogs disengage, leaving the work for the required time.
Crossbar conveyors carry large numbers of long products crosswise through wide pretreatment, application and curing zones. Two parallel chains move the crossbars.
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Typical power spray washer for cleaning and phosphating
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Pallet conveyors do not always have a moving chain. Instead, the products are placed on pallets that ride on tracks having built-in rollers or wheels. Pallet conveyors are often used in conjunction with floor conveyors at loading, unloading and transfer points.
Pretreatment
In chemical pretreatment, aqueous solutions of cleaners, phosphates and chromates are applied by spraying, dipping or combinations of both, to clean and apply conversion coatings. The choice of immersion or spray depends on the product and its requirements. On one hand, spraying will involve less floor space and weight loading, use smaller tanks and less solution than dipping or immersion. On the other, spray washers will require more heat, electrical power and exhaust.
For cleaning inaccessible areas, large sizes and odd shapes, immersion pretreatment is better, despite the fact that it requires larger tanks, higher floor weight loadings and more floor space. Immersion pretreatment tanks have heaters, some means of agitation, overflow weirs, filters and oil separators.
Power washers spray chemical solutions on parts traveling through tunnel-like enclosures, using a separate chamber for spraying each chemical. Each chamber has headers, risers and nozzles aimed to spray chemical solutions on the entire work surface. Pumps feed solutions from supply tanks to spray nozzles at a prescribed rate. In each chamber, solution run-off drains onto watertight, sloped floors and back into the supply tank. These tanks typically hold two to five times the volume pumped per minute.
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Truck body enters immersion cleaning stage of a nine-stage pretreatment machine.
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Tubes in which gas or oil is burned often heat supply tanks. The resulting heat is transferred through tube walls to the solution. The hot exhaust gases may be channeled to heat the dry-off oven, conserving energy.
Ancillary equipment in the pretreatment area includes liquid-chemical-supply circulating systems, strainers and filters, pumps, sludge-removal devices, heat exchangers, solution-control units, air make-up systems and reverse-osmosis units.
Ultrafiltration often removes emulsified oils from the pretreatment section of high-volume production lines. The oil is continuously removed as a concentrated emulsion from the cleaning solution, which is returned to the wash stage. These devices simplify disposal of oil concentrates and extend the life of cleaning solutions.
Vapor phosphating equipment uses specially formulated chlorinated solvents in equipment resembling a vapor degreaser to apply iron phosphates on steel surfaces. Application is normally by dipping, although there are often hoses and nozzles for spraying the solution over the tank. Exhaust is required, but no drain is necessary.
Steam or hot water pretreatment is for manually spraying cleaning and phosphating chemicals. Low-pressure steam or hot water mixes with chemicals and is applied at about 200 psi. These sprayed solutions remove heavy soils and apply a light phosphate coating on products too large for conventional pretreatment systems. Floor drains carry away oversprayed solutions for recycling and/or treatment and disposal. Exhaust fans remove moist, chemical-laden air.
Parts washers, originally intended to remove machining residues, also can apply pretreatment chemicals on small parts. The parts are loaded into mesh baskets and cleaned and/or conversion coated by immersion.
Emulsion cleaning removes heavy oils and greases, often as a precleaning step. Aqueous emulsions of hydrocarbon solvents preclean surfaces prior to rinsing with hot water and then cleaning with solvent or alkaline cleaners to remove oily residues.
Dry-off ovens use heat and forced air to evaporate water remaining on products after pretreatment. Enclosures should be long enough to allow 4–6 min at 150–200°F. If there is sufficient exhaust heat from the pretreatment unit, the dry-off oven can be heated with it. Otherwise, separate gas, oil, steam or electric heat sources are required.
In conveyorized coating lines, dry-off ovens are just far enough from the final rinse to allow most of the water to drain from the parts being finished. Dryoff ovens are designed to remove surface water, not puddles. Therefore, products must be hung properly and have holes provided for drainage. Sometimes an air blow-off prior to the dry-off oven is necessary.
Solvent cleaning by hand wiping is labor intensive and has fallen into disfavor because of safety problems. In vapor degreasing, another form of solvent cleaning, cold parts are immersed in the vapors of a boiling chlorinated solvent. Solvent vapors condense on the cold part, dissolve the oily soils and wash them down into a sump.
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Painting robots have found such applications as painting small trucks
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Blast cleaning equipment uses a stream of pressurized air or water to propel abrasive particles that impinge on a surface, abrading away oxides and scale. Other forms of blast cleaning involve mechanical equipment that hurls media against surfaces. These and other methods that depend on media impingement for cleaning can be performed in automatic, totally enclosed units. There are also blast-cleaning rooms where workers wearing protective clothing and a fresh-air breathing mask manipulate nozzles to clean large parts.
Tumbling is another abrasive-cleaning method. Small parts and abrasive media are loaded into a barrel or drum. When the drum rotates, the parts and the media rub against one another, causing oxide and scale to be abraded away. Although primarily a deburring tool, tumblers can be used for painting pretreatment.
Vibratory finishing is a variation of tumbling. Parts are loaded into a media-filled drum or tub. The tub vibrates, and the media rubs against the parts, abrasively cleaning them.
Oxides and scale can be removed manually by sanding and wire brushing as well as mechanically by hand-held power sanders and rotary brushes. Enclosed automatic machines employing rotating brushes and abrasive pads can clean regularly shaped parts. To prevent air-borne dust contamination they must be isolated from the coating areas.
Coating Application
Three basic methods apply coatings: spraying; dipping; and flow coating. Spray application equipment includes conventional air, airless, air electrostatic, airless electrostatic, high-volume low-pressure and electrostatic bell and disc equipment. Dip coating is done in simple dip tanks or complex electrocoating tanks. Flow coating also can be simple or complex. Roller coating, centrifugal coating and tumbling also apply paints. For more information on coating application methods, read "Today's Paint Application Methods," starting on page 18.
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Bake oven has two conveyorized lines passing through it.
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Spray-Painting Booths
Spray booths should confine overspray, remove solvent-laden air and eliminate airborne dirt from surrounding areas. They can be totally enclosed or open on one or more sides. The two basic types—dry booths, and water-wash—are further classified as down draft and side draft. “Face velocities,”—rate of air movement through the booth opening measured in fpm—are dictated by regulations.
To protect workers, one must provide more exhaust and make-up air when applying paint manually than when coatings are applied automatically. Fire prevention also influences the exhaust rates required.
In older automatic-spray systems, spray guns are mounted on vertical or horizontal reciprocators that move up and down or side to side as products pass before them. Gun triggering can be controlled by electric eyes, trip switches and mechanical signaling devices, working in conjunction with conveyors and timers.
The programmable controller (PC) is being used increasingly to trigger spray guns. In a typical system, an operator either enters product part numbers by keyboard into the memory of a mini-computer, which provides triggering instructions for the spray guns, or this information is fed into the programmable controller by electronic and other signaling devices. This information is based on the size and shape of products to be coated.
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Electric infrared oven dries waterborne enamel on dust pans.
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In the case of PC-controlled vertical reciprocators moving an array of spray guns, the guns start spraying just before the leading edge of the object passes in front of the first gun. They stop spraying just above the top of the object on the up stroke, start spraying just before reaching the edge on the down stroke, stop spraying just below the bottom edge of the down stroke and restart below the bottom edge on the up stroke. They shut off immediately after the trailing edge passes the last spray gun in the array.
Spray painting robots perform repetitive coating jobs, reducing labor costs and insuring reproducible quality. An experienced spray painter programs the robots by manipulating a “teaching arm” or the actual robot arm, which is called the manipulator. In this manner the skilled painter “teaches” the robot’s electronic memory how to coat a specific product efficiently. This information is recorded and electronically stored for use by the robot’s manipulator, which duplicates the painter’s motions every time a specific product is to be coated. Some robots now use optical scanning devices to recognize specific products, search their memories for the right program and start coating in a few seconds.
Ancillary equipment for spray-painting includes air-make-up systems, sludge removal, and fire protection and controls. Since heat is generated during pumping of paints, cooling is required.
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Dry-off oven evaporates water left by pretreatment before painting.
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Dry booths have enclosed bottoms, tops and sides and either horizontal or side exhausts. The openings through which air is exhausted are equipped with disposable filters, rolls of filter media, or staggered plates to catch most of the paint overspray. Exhaust rates must be high enough to pull the overspray away from the operator at a rate specified by health and safety regulations.
Special dry booths having curved sides enclosing a loop in the conveyor are used with automatic electrostatic rotating-disc applicators. These “shrouds” have smooth walls. Sometimes oversprayed paint that runs down the walls is collected and reused.
Waterwash booths have their bottoms, tops and two sides enclosed. The sprayer works either in an open-face or in front of a dry-filter-covered air-intake wall, which removes dust and lint from the make-up air. If the booth is totally enclosed, its lighting is by fluorescent lamps behind windows in the walls and ceilings.
In side-draft waterwash booths, air (and overspray) is exhausted through a water curtain running from an upper trough, down over a surface facing the sprayer and finally into a lower trough.
In down-draft booths, exhaust air is drawn through a metal floor grating into a pan of water below the floor grate or to the nozzles that spray water into the air flow zone.
Water that has been chemically treated to either sink or float the entrapped over-spray is pumped throughout these systems. Skimming, decanting and filtering remove the resultant “sludge.” This waste must be packaged in approved containers and processed in accordance with local and federal waste-disposal regulations. Sometimes overspray is collected, washed, homogenized, dewatered and blended with virgin coating material to be reused, lowering paint consumption.
Curing
On some finishing lines, coatings are air dried rather than baked. On others, auxiliary energy sources raise ambient temperatures, enhancing air drying in cold weather. Most factory-applied liquid paint is converted to solid films by several curing mechanisms, including solvent or water evaporation, and polymerization. Heat accelerates curing.
In conventional paint-baking ovens, organic solvents and water evaporate and paint films dry and cure under carefully controlled conditions. These ovens use gas, oil, steam or electricity to heat the air. Up to 45 min dwell time may be required in thermal ovens, depending on the chemistry of the coatings, oven temperature, configuration of the product, and curing equipment.
Direct-fired gas ovens may have the burners inside the enclosure, where combustion products contact the coating film. On the other hand, the burners may be in separate combustion chambers equipped with recirculating blowers to push oven air through the burner box and back to all places where heat must be supplied. They also have blowers to exhaust fumes.
In other cases, hot air containing solvent evaporated from paint is exhausted to heat exchangers, providing heat for other manufacturing processes. Or vaporized solvent may be removed from the air before discharge to the atmosphere, be incinerated, using carbon filtration or other means, to comply with air-pollution-control regulations.
Direct-fired oil ovens are possible because of redesigned burners. They can bake a variety of colored paints without discoloration and soot accumulation.
Indirect-fired ovens are similar in design and operation to direct-fired ovens, except that air is heated as it passes over heat exchangers. Since combustion products do not contact the coating film, they cause no darkening, or no discoloration, or no haze.
Electrically heated ovens use resistance heaters. Otherwise, they are similar to indirect-fired ovens and are used where other fuels are unavailable.
Steam-heated ovens can be used in plants having a sufficient supply of high-temperature steam. Super-heated steam is used to heat ovens having the same configurations as indirect-fired ovens.
Electric and gas infrared ovens cure coatings requiring a rapid temperature rise at varying film temperatures and varying distances from the radiation source. These ovens use specially designed elements to produce infrared radiation. Some units have lightweight elements and reflectors that can be moved from one location to another or erected and suspended in various ways. They all require make-up and exhaust air.
Induction-curing ovens cure coatings on wire, rod or continuous strips. High-frequency induction coils excite the molecules in the substrate, generating heat that cures a variety of liquid and powder coatings from the inside out.
High-speed curing ovens are used in coil-coating lines where strip or tape travels through coating and curing processes at several hundred feet per minute. These ovens can cure coatings in extremely short times. In some cases, hot air jets support the long catenary curve of strip passing through the oven. In others, opposing air jets alternating with high-intensity heat sources float the strip in a horizontal position.
Radiation curing uses ultraviolet light or electron-beam radiation to cure coatings specially formulated to respond to these energy sources. These coatings are essentially free of volatile organic compounds (VOCs). Owing to their rapid cure rate and negligible surface heating, they can be used to cure coatings on temperature-sensitive substrates such as plastics.
Vapor curing is not one, but two technologies. In the first, called vapor- permeation curing, parts coated with specially formulated paint pass through a chamber containing air permeated with catalyst vapor. In the second, called vapor-injection curing, the catalyst requires no curing chamber. Instead, it is sprayed, along with the coating, using a specially designed spray gun.
Ancillary Equipment
Coating areas should be designed to supply slightly more air than is exhausted from the finishing area, resulting in a slight pressurization that keeps outside, dirty air from intruding. Air make-up systems supply clean, conditioned air to replace that exhausted by pretreatment equipment, dry-off ovens, spray booths and curing ovens. Oil-coated screens, fiberglass or paper filters remove airborne particles. Electrostatic precipitators also can be used.
Gas burners or exhaust heat from other units temper air for employee comfort. Chillers cool and dehumidify air in hot, humid climates. On the other hand, steam- or water-spray humidifiers raise the relative humidity when necessary for worker comfort or to reduce static charges. Humidity control is essential when using waterborne paints.
Coated rejects, hangers and hooks must be stripped occasionally. This can be done chemically using corrosives and solvents; pyrolytically using high-temperature ovens, hot fluidized beds and molten salt baths; or mechanically using abrasive media, high-pressure water and cryogenics.
To ensure the safety of personnel and property, fire protection devices, ranging from hand-held extinguishers to sophisticated automatic smoke- or temperature-sensing systems are required in all buildings housing coating operations. The major locations for concern are those involving high solvent concentrations, such as coating-application areas and paint-mixing and storage rooms.
A line designed and built to apply coatings within the limitations of safety as well as air- and water-quality standards will meet governmental regulations. When the line applies low-VOC-emitting coatings by methods having higher transfer efficiencies, paint overspray and solvent usage will decrease. This will result in decreased costs and fewer hazardous wastes.
