coating materials and application methods
SPRAY APPLICATION PROCESSES
BY JERRY P. HUND
JPH FINISHING CONSULTANTS, WEST CHICAGO, ILL.
The spray application of coatings to protect and decorate products has undergoneconsiderable change since its inception some hundred years ago. TheIndustrial Revolution has given us an unlimited supply of new products to finish,along with an ever-increasing variety of coating materials to use on thoseparts to provide them with protection and decoration.Presently, we have four major processes of spray applications: (1) air atomization-conventional air spray and high-volume, low-pressure (HVLP) atomization,
(2) airless atomization,
(3) air-assisted airless atomization, and (4) electrostaticatomization, which can be combined with any of the three previously mentionedforms, or used with rotational atomizers. Regardless of the finishing system, allhave their advantages as well as their limitations. What may be suitable for onesituation may not be suitable in another.To select properly which system is best suited for your needs, a review of theadvantages and limitations of each process is in order. Table I lists advantagesand limitations for each process (see also Figs.1–3).
CONVENTIONAL AIR SPRAY
Conventional air spray (siphon/gravity/pressure feed) is the oldest system, cominginto prominence during the 1920s with the growth of the automobile andfurniture industries. It remains today as the finishing system most widely usedby industry.
Advantages
Conventional air spray lays claim to two basic advantages over the other methodsas follows.Control. This is the most controllable process available. The spray operator,when properly trained, can control the spray pattern from a fine dot to a largeproduction-type spray pattern. This permits the spraying of small or large areaswithout changing guns or nozzles.Also, the degree of atomization can be controlled. This process provides thefinest degree of atomization available in a hand-held system. Needless to say, itis the choice for those who want the best quality finish possible.Versatility.This process affords the operator the ability to spray the widest rangeof coating materials. It is also the easiest system to operate and maintain. Just thefact that the process has been around so long has resulted in a huge inventory ofequipment, plus volumes of knowledge regarding application techniques.
Disadvantages
On the other hand, conventional air spray has a low level of transfer efficiency.Often, more material is wasted than is actually deposited on the part.This condition is usually aggravated by excessive pressure and poor operatortechnique. Air spray also consumes large amounts of compressed air (7–35cfm at 100 psi).
How the System Works
The material to be sprayed is supplied to the spray gun by either a siphon orgravity cup mounted to the gun or by a pressure feed device such as a pressuretank or pump. When the gun is triggered, the material is discharged throughthe fluid nozzle of the gun in the form of a liquid stream. Upon exiting the gun,this stream is immediately surrounded by a hollow column of compressed air,usually under high pressure, emitted from the center of the air nozzle of thegun. The action of this column of air on the fluid stream converts it into smalldroplets and imparts forward velocity to them. Additional jets of high-pressurecompressed air from the face and horns of the air nozzle are directed into thedroplets, forming even smaller droplets and an elliptical or fan spray pattern.The ability to control these forces at work at the head of an air spray gun isthe key to a successful spray application. The proof that you have this controlis in the quality of the spray pattern produced.
HIGH-VOLUME LOW-PRESSURE ATOMIZATION
Worldwide concern over increased air pollution has necessitated numerouschanges, including in how we finish our products. HVLP air atomization andelectrostatics are now the only accepted methods of production spraying incertain parts of the country.Although all HVLP spray guns operate with the same objective in mind, howthey accomplish this goal may differ. First, air used in the atomization processreaches the HVLP spray gun’s nozzle in one of four ways:
(1) standard highpressurecompressed air, which has its pressure restricted within the gun body;
(2) standard high-pressure compressed air, which is assisted with a venturi feedand then filtered ambient air prior to its pressure restriction within the gun’sbody;
(3) standard externally fed HVLP turbine air; and (4) compressor-assistedexternally fed turbine air. Items 1 and 3 have seen the most growth and ultimateacceptance in recent years.All HVLP spray guns should operate at air pressures between 0.1 and 10 psi (atthe air nozzle) and consume air volumes of 15–30 cfm to be considered true HVLPspray guns. Although some HVLP guns with internal restrictors can exceed 10 psiair pressure, it is up to the operator to follow local regulations when necessary.
Advantages
The benefits of HVLP atomization are improved transfer efficiency, oftenapproaching 65–75%, compliance with local finishing regulations, a softer spraythat penetrates easily into recesses or cavities, reduced material (costs) consumptionas well as reduced spray booth maintenance and reduced hazardous waste.Turbine-operated HVLP systems enjoy great portability and ease of operationwhere compressed air is not available. HVLP spray guns with internal restrictorsuse existing air supplies, are easy to operate, and are low in cost.
Disadvantages
The most notable limitation to the HVLP process is that the finish quality fromsome HVLP spray guns is not as fine as conventional air spray. This may not posea problem to some finishers, but to others it may mean additional polishing,a change in the material formulation, or switching to electrostatics. TurbinegeneratedHVLP systems may be expensive to purchase and to operate. HVLPguns using internal restrictors must have an adequate supply of clean and dry airto operate efficiently. Turbine guns use a larger air hose, which may be difficultto work with. Some high-volume production lines may find HVLP to be too slow.Fluid deliveries that exceed 20 oz/min. may sacrifice finish quality for speed.
AIRLESS ATOMIZATION
In the 1960s, attention turned to another process of atomization known todayas airless spraying. Airless spraying is a method of spray application that doesnot directly use compressed air to atomize the coating material. Hydraulic pressureis used to atomize the fluid by pumping it at high pressure (500–4,500 psi)through a small orifice (0.007–0.072 in.) spray nozzle tip located at the front ofthe airless gun. As the fluid is released at these high pressures, it is separated intosmall droplets, resulting in a finely atomized spray. The fluid is dischargedatsuch a high velocity that it tears itself apart and sufficient momentum remainsto carry the minute particles to the surface. The spray pattern size, or fan angle(3–21 in.) and orifice are usually preselected, but different spray angles depositthe same amount of paint over a different area. A good rule is to determine thelargest fan angle and the smallest orifice that is practical for your needs
Advantages
The main benefit of airless atomization is speed of application. It is the fastestmethod of spray atomization. This process can deliver twice the amount of materialas a compressed air system. Other advantages include improve transfer efficiencydue to a reduction in fog and overspray, the ability to spray into recesses and cavitieswith a minimum of material bounce-back, and reduced spray booth maintenance.
Disadvantages
The airless process has had to cope with several limitations over the years and oneis coarse atomization. Although some users, such as maintenance painters, shipbuilders, and highway stripers, find airless atomization acceptable, others may not.Automotive finishers and refinishers, for example, would not select airless atomization.Other limitations include less control of spray pattern when compared toair spray or HVLP.An airless gun is either on or off; there is no “feathering” capability. The tendencyis to flood the surface, causing runs or sags if the spray gun movement is momentarilydelayed. Also, one must provide strict maintenance with this system. Any foreignobject in the fluid supply that is larger than the spray-tip orifice will cause thesystem to block or shut off. Equipment maintenance on pumps is also high becauseof the high pressures used. Nozzle tips will wear and may prove costly to replace.Finally, the high velocity of the fluid stream and spray pattern, as it immediatelyexits the gun or hose, is a potential hazard. Never allow any part of your body tocome into close contact with this high-pressure material. Failure to keep severalinches away from the coating as it exits the gun or hose will cause serious injury.
AIR-ASSISTED AIRLESS ATOMIZATION
In the 1970s, various combinations of spraying systems emerged. Air-assistedairless was one that developed to fill a need that resulted from escalated materialcosts and newly enacted Environmental Protection Agency (EPA) regulations.Air-assisted airless, under ideal conditions, combines the best features of airspray along with the best of airless.Air-assisted airless spray guns first partially atomize the fluid with a specialfluid nozzle tip similar to a standard airless tip. Second, they complete theatomization with small amounts of compressed air from the face and/or thehorns of the air nozzle that they use. The result is a finely atomized spray patternclosely resembling that of a compressed air system. Newly designed low-pressure,air-assisted airless systems are also available. Some systems restrict the atomizingair pressure to comply with various EPA guidelines; as a result, these systems canbe considered HVLP air-assisted airless.
Advantages
The primary advantage of air-assisted airless is its “soft” spray atomization.Atomization air pressures are usually low, and as a result, this system provides afine finish with most coatings approaching that of compressed-air atomization.With air-assisted airless we can see an approximate 30% improvement in transferefficiency over compressed air. This system allows us to increase flow rates, whilealso spraying into recesses and cavities, without excessive bounce-back of material.This means less booth maintenance and cleanup time. We also can expect a reductionin compressed-air supply requirements. Since fluid pressures generally rangefrom 200 to 800 psi, less wear on the pump and tip is to be expected.
Disadvantages
Tip plugging may still be a problem with air-assisted airless. Many operators feelthat air-assisted airless is too slow when compared to airless and that the finishis still not as good as compressed-air atomization. Many operators tend to useexcessive fluid and air pressures with this process. There are more controls tolearn to use it correctly.
ELECTROSTATIC ATOMIZATION
During World War II, paints and solvents were in short supply or very expensivewhen available. Thus, to fill that need and to maximize the use of these materials,electrostatic atomization was developed.The coating is first atomized using either the compressed air, airless, or airassistedairless methods previously discussed. Although these systems requireequipment designed for electrostatic use, the atomization principles are thesame as those for nonelectrostatic applications. The atomized paint particles,at this point, are provided with forward velocity and direction.The particles are made to pass through a cloud of free-floating electrons(negative charges) produced by a high-voltage source such as an independentpower supply, cable, and electrode at the gun, or a turbine-operated generatorlocated inside the spray gun. The basic principle of electrostatics is that “like”electrical charges repel one another and “unlike” electrical charges attract.Since each particle of material is negatively charged from 30–140 kV and0–200 mA, it will want to seek the closest grounded object (positive) to completethe electrical circuit. If the product is sufficiently grounded, particles thatwould have missed that part will now be drawn back or attracted to it. This iscommonly referred to as the “wrap effect.”
Rotational Atomizers
The first low-speed rotational atomizers premiered in the early 1950s. This typeis sometimes referred to as a “true electrostatic” system. The atomization is createdby the high voltage itself. The rotation of the electrically driven disk or bellevenly disperses the paint to the edge of the spinning disk or bell.When the unit is charged to approximately 120 kV DC, a current flow developsbetween it and the grounded object. This current flow pulls the particles ofcoating off the spinning disk or bell and carries them to the product.With the introduction of paints high in solids these atomizers had to use higherspeeds. Thus, electric motors were replaced with high-speed air-driven turbines.These high-speed rotational atomizers (10,000–60,000 rpm) use centrifugal forceto atomize the coating and carefully directed compressed air to give the atomizedpaint particles forward velocity and direction. A voltage differential (120 kV DC)then takes over and allows the charged particles to migrate to the grounded part.
Disk versus Bell
The spray pattern needed for the kind of parts being finished, the shape of theproduct, and racking, or the existing facility, determine whether a disk or bellis used. For long, thin parts and flat stock, the reciprocating disk directs thehorizontal spray in the thin, narrow plane that is needed to provide a fine evencoat in conjunction with the omega-style conveyor and booth. For shorter,wider parts, the donut-shaped cloud produced by a bell directs a fine even coatand is mounted horizontally or used with a vertical short-stroke oscillator in astandard overhead conveyor system and booth.
Advantages
The principal advantage of electrostatic spraying is the savings in material andlabor. This process can provide transfer efficiency ratings from 65 to 95% ifall conditions are favorable. Air velocity in the spray booth where electrostaticspraying is performed may be reduced from 100 to 60 ft/min. This means a40% reduction in air makeup costs as well as reduced emissions.
Disadvantages
The old saying “Where the current goes, so goes the paint” applies here. TheFaraday cage effect will make painting in some corners and recesses difficult,thus manual nonelectrostatic touch up will be necessary. Contrary to somebeliefs, electrostatics will not improve adhesion or provide superior gloss,nor will it always provide a complete wrap effect. Also, some users may findexcessive buildup on corners and edges unacceptable. The parts you want to spray electrostatically must alwaysbe conductive, either by virtue of thematerial from which it is made orthrough the application of a conductivepretreatment.Recent equipment developmentsnow make it relatively simple toapply water-reducible coatings withelectrostatic equipment. Both handheldand automatic systems are available.Since the material is highly conductive,it is necessary to isolate thematerial supply. The fluid supply willbe highly charged, and it is, therefore, necessary to erect barricades to preventphysical contact with this portion of the system.Sometimes the added expense to apply this coating may not justify the materialsavings. Also, added maintenance — cleaning the hangers, supports, or conveyors— to assure conductivity to ground is necessary. All electrically conductiveitems, such as containers and spray equipment, within 10 ft of the spray areashould be grounded to prevent static buildup. Operators grounding out anystatic buildup may experience a severe shock.
COMPONENTS OF A SPRAY SYSTEM
A spray finishing system consists of the following components: a spray gun, acompressed air supply, a fluid supply, and other necessary items such as hoses,fittings, regulators, and proper respiratory equipment
Spray Gun
Spray guns (see Fig. 4) are available in a wide variety of performance capabilitiesand costs. The three factors in order of importance are function, service,and price. As with anything, you pay for quality. Cheap spray guns, which maylook like the expensive guns they imitate, have no place in a production finishingshop. Consult your spray gun supplier for correct air and fluid nozzlerecommendations. After all, this is the part of the spray gun that develops theatomization desired.
Compressed Air Supply
The compressor you use does not make air, it only compresses it. The conditionof your air supply usually tells a lot about the compressor and the air-supplylines. A compressor is rated for its volume,measured in cubic feet per minute(cfm), and its pressure, measuredin pounds per square inch (psi). Toassure maximum performance of anyspray finishing system, the compressedair supply should always exceed therequired psi and cfm requirements ofits components. This is especially truewhen using HVLP spray guns. The ruleof thumb is to expect about 4 cfm forevery 1 horsepower at 100 psi with anelectric compressor. Gas compressorsare somewhat lower in performance.Do not ignore the necessity ofproperly selected, cleaned, and operatingair-supply components such asextractors, regulators, ball valves, andhoses. Failure to do so only adds tofinishing-room difficulties.Pulling back slightly on the triggeropens the air valve to allow use of thegun as a blow gun. In this position thetrigger does not actuate the fluid needleand no fluid flows. As the trigger isfurther retracted, it unseats the needlein the fluid nozzle, and the gun beginsto spray. The amount of paint leaving the gun is controlled by the pressure on thecontainer, the viscosity of the paint, the size of the fluid orifice, and the fluid needleadjustment. In industrial finishing where pressure tanks or pumps are used, thefluid needle adjustment should normally be fully opened. In a siphon cup operation,the needle valve controls the flow of paint.
Fluid Supply
The fluid supply can range from siphon cups, to pressure tanks, to materialhandling pumps for dead-end or circulating systems. Siphon and gravity cupsremain popular with refinishers, and those on touch-up lines. Pressure tanksremain popular with those on low- to medium-production lines, using compressedair or HVLP atomization. Pumps are generally used on medium- tohigh-volume production lines, circulating systems, and airless and air-assistedairless production lines. Pumps may be powered by compressed air, hydraulics,or electricity.
Paint Heaters
Paint heaters, when used correctly, provide viscosity control, reduced solventuse, sprays of higher solids, reduced flash time, reduced air and fluid pressures,and improved flow. Heaters may be used with pumps on compressed air,HVLP, airless, air-assisted airless, and all types of electrostatic systems. Paintheaters are becoming more prominentin the finishing industry as solvent useis restricted to meet compliance standards.
OPERATOR TECHNIQUES
More often than not, spray painters areusually improperly trained (if trainedat all), allowed to develop many badhabits, and, in many cases, frequentlychange positions or employers, takingtheir knowledge and skills with them.On the other hand, managementsometimes fails to recognize the importance of traininguntil problems exist. The organization may havethe best coating materials and spray equipment moneycan buy, but without skilled operators the entiresystem will fail.Examine the following variables that can lead toproblems in the finishing room. Failure to observethese rules may result in finishing problems. These arethe five operator variables that must be controlled toimprove application efficiency.
1. Distance of gun to work.
2. Stroking speed.
3. Pattern overlap.
4. Spray gun attitude (heeling, toeing, fanning).
5. Triggering.
Figures 5 through 8 illustrate proper versus improper spraying technique.
SPRAY PATTERNS AND PRESSURES
The general rule of thumb for setting pressures is to always use the lowest pressurethat will give a satisfactory pattern. This rule applies to all spraying processes.Failure to observe this rule, or using faulty or dirty nozzles, will usually result infaulty spray patterns. Typical patterns with suggested corrections are shown inFigures 9–13Keeping the spray gun too far from the substrate will increase the likelihood offallout. Fan patterns that are excessive in width will increase overspray. Excessivepressures will contribute to off-spray and rebound (see Fig.14).
MAINTENANCE
The spray gun is a precision tool andwill perform best if kept clean andlubricated.Siphon Spraying:Wipe off the siphon tube with asolvent rag. Dip the siphon tube into a container ofclean solvent and spray. Trigger repeatedly to thoroughlyflush the passageway and clean the fluidnozzle and needle. (Consult local codes for restrictionson spraying solvent.)Gravity Spraying:Turn off atomization air to spraygun. Remove material from cup. Wipe interior ofcup clean with solvent rag. Pour solvent into cup.Pull trigger allowing gravity to flush all fluid passages.Repeat until clean.Pressure Spraying:Substitute clean solvent underpressure for the paint being sprayed, using low fluidpressure (no atomizing air is necessary) trigger thegun repeatedly to permit the solvent to flush out thepassageway. Do this until clean.Wipe off the gun body with a cloth wet with solventand lubricate the spray gun with a drop of lightmachine oil each day.
TROUBLESHOOTING SPRAY GUNS
If fluid is leaking from the needle packing nut, check fora loose packing nut or dried-out packing. Tighten the nutuntil leaking stops, or replace packing, if required.If air is leaking from the front of the gun, check forforeign matter on the air valve stem or seat, a brokenair-valve spring, or a bent air-valve stem; check the airvalve packing nut to see if it is too tight, the air-valveassembly to see if is tight enough, and the air valveassembly gasket for leaks; and check to see if the gun isa bleeder-type spray gun.If fluid is leaking from the front of gun, check fora worn or damaged needle or fluid nozzle, dirt in thefluid needle seat, and a broken fluid controlspring; check to see if the fluid packingnut is too tight, and if the wrong sizeneedle is being used.
FAULTY PATTERNS AND HOW TO
CORRECT THEM
Dried material in a side port of a spraynozzle restricts passage of air through theport on one side resulting in full air pressurecoming from the clean side of thepart in a “C” or fan pattern in the directionof the clogged side (see Fig. 9). Thissituation can be remedied by dissolvingthe material with thinner. Do not usemetal devices to probe into the air nozzleopenings.Dried material also gets stuck aroundthe outside ofthe fluid nozzletip restrictingpassage ofatomized air toa point at thecenter of the airnozzle openingcausing an inverted tear-drop-shaped spray pattern(see Fig. 10). This faulty pattern can also be caused bya loose air nozzle, or a bent fluid nozzle or needle tip.If dried material is causing the trouble, remove the airnozzle and wipe off the fluid tip using a rag soakedwith thinner. Tighten the air nozzle and replace thefluid nozzle and/or the needle, if it is bent.
؟؟؟
Fig. 13. Fan spray pattern
caused by low atomizing air
pressure or too much spray
material being fed through
the gun. To correct the problem,
increase air pressure
from air supply.
A split spray pattern — heavy oneach end of a fan pattern and weakin the middle — is usually causedby (1) too much atomizing air pressure;(2) attempting to get too widea spray with thin material; or (3)too little spray material available topump through the spray apparatus.This situation can be remedied by (1)reducing the air pressure; (2) openingthe fluid control knob to the full position;and (3) turning the spray patterncontrol down to narrow the spray patternwidth. This reduces the width ofspray but will correct the split pattern(see Fig. 11). Spitting is caused by airentering the fluid supply, and by dried or missing packing around the materialneedle valve that permits air to get into the fluid passageway. Dirt betweenthe fluid nozzle seat and body, or a loosely installed fluid nozzle, can alsocontribute to the problem, as well as a loose or defective swivel nut, siphoncup, or material hose. To stop the spitting, be sure all fittings and connectionsare tight.A fan spray pattern that is heavy in the middle, or a pattern that has anunatomized (salt-and-pepper) effect, indicates that the atomizing air pressureis not sufficiently high or that there is too much material being fed to the gun.This problem can be solved by increasing the pressure from the air supply.Correct air pressures are discussed elsewhere in this article.
؟؟؟
Fig. 14. Improper spraying creates paint losses
in the form of overspray, rebound, and fallout.
