Painting

 

Article summary:

I - Modern Application Methods

• Rolling
• Dipping
• Flow Coating
• Continuous Coaters
• Dip-Spin Coaters
• Curtain Coating
• Roll Coating
• Coil Coating

­­II - Electrocoating and Autodeposition

• E-Coat Solution Constituents
• Rinsing
• Pigments
• Bath Parameters
• Tank Details
• E-coat Curing Cycle
• Autodeposition

III - Air-Atomizing Spray Guns

• Compressed Air Supply
• Paint Supply
• Gun Operation
• Low-Volume High-pressure (Conventional)
• High-Volume Low-Pressure (HVLP)
• Spraying Techniques for Air-Atomized Guns

IV - Airless Spray and Air-Assisted Airless-Spray

• Airless Spray
• Paint Supply
• Gun Operation
• Spraying Techniques for Airless Guns
• Skin Injection Danger
• Skin Injection Danger
• Air-Assisted Airless Spray

V - Electrostatic Paint Application

• Electrostatics in Painting

VI - Rotary Atomizers

• Mounting Configuration
• Rotational Speed and Atomizational Fineness
• Paint Application
• Rotary System Operation
• Hand-Held Bell

 

Modern Application Methods

A number of application techniques have been developed in more recent times to improve the efficiency of paint application and the appearance of the cured coating.

Rolling

Painting with a roller is faster than painting with a brush, but it, too, has drawbacks. A roller cannot get into hard-to-reach areas and the application often requires touchup with a brush. In addition, rollers generally absorb-or “load up”-a substantial amount of paint, most of which cannot be saved when the job is done.

Dipping

Painting by dipping, is much faster than brushing or rolling and much less labor-intensive. Dipping, however, is extremely dependent on the viscosity of the paint, very messy and is high in hazard. The viscosity of the paint in a dip tank must remain practically constant if the deposited film quality is to remain high.

The film thickness in dipping can be controlled by varying the viscosity and by the rate that parts are withdrawn from the dip tank. Thin paint films are produced by a slow withdrawal; an increased film thickness results from an abrupt withdrawal.

Dipping is not suitable for many hollow-shaped items. They tend to float as they contact the paint and often become disengaged from their hangers or hooks.

Color change is very slow and generally not feasible for most dip operations, although sometimes multiple tanks mounted on rollers are used. As needed, the various color tanks are rolled into position under the conveyor line. This technique is not done in many plants.

A flammable paint can constitute a major fire hazard and sometimes safety dump tanks may be required by local fire codes. For all such tanks, an efficient fire-extinguishing system must be installed as a safety measure. Paint drippage from parts exiting the dip tank creates a mess that adds to the fire hazard.

Flow Coating

In a flow coat system, separate streams of paint are directed to impinge on the parts. The streams may flow out of holes drilled in pipes or through short, crimped pipe outlets. The paint headers are arranged so that all surfaces of parts carried through the flow coater on the conveyor are painted. Paint is liberally applied to all surfaces of the parts. The paint flowing off the parts drains into a sump, from where it is pumped through filters and recirculated to the flow coat paint reservoir.

Paint film thickness in flow coating is controlled by adjusting viscosity. Flow coating also needs a drippage zone, which adds to cleanup maintenance.

Both dipping and flow coating tend to be used to coat items whose appearance is not vitally important and to apply primers. Topcoats are not commonly applied by dipping or flow coating. Coatings applied by these methods have only poor to fair appearance unless parts are rotated during the drippage period.

Both dipping and flow coating have the disadvantage that the principal control of dry-film thickness is the viscosity of the paint. Automatic viscosity controllers that add solvent as required are frequently used to maintain tight control of paint viscosity. The amount of paint that drains from the parts depends on the viscosity. If viscosity is allowed to go too low, insufficient paint build will result. This could cause a large number of parts to need repainting. On the other hand, if the paint viscosity should rise too high, then extra paint will be applied. This can increase paint costs and can also plug small holes in the part.

When items are dipped or flow coated, the drain-off pattern may contribute runs or heavy lines on the parts. This can occur around through-holes that are large enough to avoid being filled with paint. Sometimes extra paint builds up as “fatty edges” along the bottoms of parts. If excessive, dried paint “tears” can form that resemble icicles.

A significant disadvantage of these two systems is that the appearance of the finish is not as attractive as those produced by more sophisticated methods. Despite the limited flexibility, the advantages make them highly desirable methods for painting certain kinds of parts.

Both methods achieve very high paint transfer efficiencies, often 90% and higher. The techniques are well suited for automation with conveyorized paint lines. Manual flow coating is nonexistent and hand dipping of parts is rare. The latter is suitable only when a very limited number of parts is to be painted.

Painting by dipping or flow coating is fast and easy, involves a relatively low installation cost, requires little maintenance and has a low labor requirement. In addition to a paint attendant, the only other labor requirement is that workers hang parts to be painted onto the conveyor line and remove them after they have been cured. Thus in addition to the low labor requirement, the skill required is also low.

Continuous Coaters

 A variation of the flow coat process is the “continuous coater”. An enclosed tunnel-like painting machine captures and reuses the excess paint that runs off coated parts. It employ directed sprays rather than streams of paint. Tennis racquets, metal panels and engines have been coated by this technique. Reuse of the paint within a confined space contributes to high paint transfer efficiency and reduced VOC emissions because of the solvent vapor capture capability of continuous coaters. One drawback to the method is that all parts being coated must be roughly the same size and shape. The gun-to-part distance should be kept uniform for best part appearance.

Dip-Spin Coaters

Dip-spin (centrifugal) coaters are designed to paint large-quantity batch loads of small parts, such as hairpins, clips and fasteners. A wire basket containing up to 50 Ib of parts is immersed in a reservoir of paint. The basket is raised, the parts are allowed to drain, the basket is spun to remove excess paint, the parts are dumped onto a mesh conveyor belt and moved through a bake oven. The entire process is automatic. The main advantage is the extremely high productivity. A disadvantage is that some of the painted parts dumped on the conveyor may stick together in the oven, leaving paint voids when they are separated. These parts may be run through the process a second time to eliminate the defects.

Curtain Coating

In curtain coating, paint flows at a controlled rate from a reservoir through a wide variable slot onto flat work being conveyed horizontally through the waterfall-like flow of paint. Curtain coating is suitable only for items that are flat or at least relatively so. The fixed width of the coating pattern is about 18 to 48 in., depending on the size of the machine in use.The volume of paint released and the speed of the conveyed flat work determine the thickness of the applied coating.

Because the paint that drops between the parts from the continuously falling curtain of paint is captured and recirculated, virtually no coating material is wasted. Extremely uniform coating thicknesses are possible, however, only one side of a flat item can be painted at a time.

 Curtain coating also is highly viscosity-dependent. Because it is simpler and uses less complex equipment, curtain coating may be preferred for lower production runs over roll coating.

Roll Coating

Roll coating is also limited to flat work. Paint is applied by one or more auxiliary rolls onto an application roll, which rolls across the conveyed flat work. The application roll can either turn in the same direction as the conveyed work (direct roll coating), or turn in the opposite direction (reverse roll coating). It too is extremely viscositydependent. It also is a clean process that carries a great deal of fire hazard due to 96 the large surface area of the paint-covered rollers, which contributes to heavy solvent evaporation.

“Precision roll coating” is a version of direct roll coating that employs engraving on the “doctor” roll running tightly against the “coating” roll. By engraving the doctor roll, it is possible to regulate accurately the thickness of paint that will be transfered to the coating roll and then to the surface being painted. A scraper blade on the coating roll removes excess paint so that the coating roll always applies only the exact amount of coating released to it by the doctor roll, Reverse roll coating can be used to apply fillers to wood substrates to smooth the surface, eliminating open pores and low spots.

Coil Coating

The roll coater is employed as the means of paint application in coil coating. The term “coil coating” refers to the painting of flat metal (steel or aluminum) sheet up to 72 in. in width. A coil coating line, automatically unwinds a roll of metal, cleans it, applies a conversion coating, paints it, bakes it and rewinds the painted metal into another coil. Some coil coating lines operate at speeds in excess of 600 fpm. The painted metal is sent to a fabricating plant for shearing, forming and assembly into various products. The main advantage in fabricating from prepainted metal is that no paint line is required for the comdeted Darts.

The problems associated with VOC emissions have made it attractive for some manufacturers to stop painting in their own plants and to manufacture their products with parts fabricated from prepainted metal. Because the metal is cleaned, conversion coated and painted while flat, parts made from this material are therefore finished uniformly.

When fabricating parts from precoated coil, precautions must be taken to hide the uncoated cut edges. This must be done to avoid visible corrosion that can readily form on unprotected metal. Edges can often be hidden by folded seams, but if they cannot be hidden, they need to be painted. Should painting of exposed edges be required, much of the advantage in using prepainted metal is lost.

Prepainted metal is used on autos, appliances, cans and related containers, architectural panels and on numerous other products. The large volume output that is possible per line is an important factor in the economics of the coil coating industry. Coil coating was developed in about 1930 to paint steel in a thin, narrow, continuous strip for venetian blind manufacturing. In 1988 about 200 coil coating lines were operating in North America.

Because coil coating lines may be over 1000 ft in length, “threading” a strip of metal through a coil coating line is a fairly time-consuming process. If every coil had to be “threaded,” the process would be very inefficient. To avoid this, a scrap coil is always the first and last coil in a run. As the line is started, the leading edge of the new coil is fastened to the trailing edge of the scrap coil. When the scrap coil is wound, its trailing edge is sheared, and the leading edge of the production coil is threaded into a new take-up roll.

An “accumulator” at the line entry and exit allows new coils to be started, and painted coils to be moved out without stopping the lines. The entry accumulator stores some 300 ft of strip to feed the line while a new coil is being loaded and fastened to the end of the in-process coil. The exit accumulator runs empty until time to change coils at the painted end. That accumulator then collects the painted stock until the painted coil is moved out and a new take-up roll is started.

The process begins with a spray application of hot alkaline cleaner to both sides of the strip to remove oil and grease. Next is a rinse with Farm water, followed by abrasive brushing (cleaning) on both sides of the strip (8 needed). Then alkaline cleaning is repeated. A final immersion rinse in hot water dissolves any alkaline residues and completes the cleaning operations.

The strip is then ready for dip, spray or roll-on of the chemical conversion solutions. The conversion coat improves the adhesion of the paint to the metal. The type of pretreatment that is applied depends largely on the metal substrate and the kinds of parts to be manufactured from the prepainted stock. For cold-rolled steel, conversion coatings are microcrystalline iron phosphate or zinc phosphate. Iron phosphate is thinner and more flexible, but zinc phosphate offers superior corrosion protection.

Two conversion coatings used on hot-dipped or electrogalvanized steel are zinc phosphate and chromate-sealed oxide. The oxide coating is formed in three steps. First the zinc surface is treated in an alkaline solution, resulting in formation of the layer of oxide. A cold-water rinse follows. Unless this rinse is thorough, blisters and paint peeling can result. Any salts left from the first step can react with the metal under high humidity conditions. The last step is a dilute chromic acid rinse that yields a complex oxide to promote paint adhesion and flexibility. It does not have as good corrosion resistance as zinc phosphate. Hot-dipped aluminum-zinc coated steel requires a chromate conversion treatment for the best corrosion protection. Chromates can be used as well on galvanized steels. “React-in-place” and “nonrinsed” conversion coating processes avoid the costs and problems objectionable substances. Pretreatments that eliminate rinsing have not equalled associated with waste water treatment for removal of environmentally the anticorrosion properties of the phosphate or chromate coatings.

The metal strip then passes through an air knife drier (high-intensity air blowoff) before entering the coating section. A roll coater applies paint by forward or reverse coating to one or both sides of the strip, depending on the specific product requirements.

Epoxies, vinyl plastisols, fluorocarbons, polyesters and silicone-modified polyesters are often used. From the coating rolls the strip goes to the bake oven to drive out solvents and to cross-link (cure) the paint. To shorten oven dwell time and reduce the length of the oven, a very high temperature (about 750°F) is maintained. A 15-minute bake for strip traveling at 10 to 12 feet per second would require an oven roughly two-thirds of a mile long. For this reason bake times are normally held to about 5 minutes. This still will normally require an oven over 1000 feet in length. After baking, the strip is aircooled and water-quenched. Should a second coat be necessary, the coating and curing are repeated.

The coating is inspected visually (and sometimes instrumentally) as the painted strip winds onto the takeup roll. The finished coil is taken off the machine and moved from the area for shipment, storage, embossing or slitting. Samples are cut from the completed coil at periodic intervals for quality control tests. If required, samples are sent to customers for preshipment approval.

The coil can be slit to any width, cut into flat sheets of various size or transported in coil form as desired. Lines may also be equipped to print patterns such as wood graining. Thick-film coatings can be embossed into various textures. Many coil lines are able to laminate decorative or functional films onto the metal as well. Acrylic film can be laminated onto galvanized steel or onto aluminum for building applications; solid-color and decoratively printed vinyls are being used on appliances, cabinets, lighting fixtures and many related items. Even cork, paper and rubber are being laminated onto specialty goods.

Two problems for the coil coating industry are fastening methods for parts made from prepainted coil and protecting the bare metal edges where the strip is cut. Many potential users of prepainted stock depend on welding for joining parts. Although some paints (weldable zinc-rich primers, for example) do not interfere with welding, most paints prevent effective welding. Joining methods that have been employed for coil-coated parts are adhesives, self-tapping screws, rivets and spring clips. Bare edges can sometimes be located in nonvisible areas with proper product design. Hem folds, lock seams, plastic caps or plugs and plastic beading are common ways to hide cut edges from view.

Electrocoating and Autodeposition

Both electrocoating and autodeposition involve immersing the part to be painted into a waterborne solution containing the “paint.” Immersion of the part is the only thing these techniques have in common, however. Electrocoating involves the use of electricity; autodeposition does not.

Electrocoating Electrocoating-also called electrodeposition, electrophoretic coating, electropainting, the electrodeposition of polymers (Elpo) or, most commonly, Ecoat-is the electrical deposition of a paint film from a waterborne organic solution onto a part-a process in which the part is one of two electrodes forming an electrical cell. A source of electricity, the two electrodes and the solution comprise the electrical cell. The part to be painted, the other electrode and the solution must be electrically conductive to complete the cell requirements.

E-coat application is very similar to the electroplating of metals; a process which also involves a source of electricity, two electrodes and an electrolyte (instead of an organic solution) but which deposits metal instead of paint. Unlike the electrodeposition of metal in electroplating, the electrodeposited E-coat paint must be baked and cured.

The E-coat process is extremely efficient, depositing a mostly uniform coating on all surfaces that can be reached by electricity. This includes all surfaces except those in confining Faraday cage areas, such as inside a long, narrow-diameter pipe. In such a configuration the electrodeposition occurs at the high-current-density areas; e.g., conductive surfaces closest to the source of electricity.

Parts to be painted are usually conveyed in and out of an E-coat bath with an overhead conveyor. Sometimes, however, parts are lowered into an E-coat bath “elevator style”. Some systems raise the entire E-coat tank to the parts, a process which allows changing colors by switching tanks.

E-Coat Solution Constituents

An E-coat solution, or bath is similar in content to conventional waterborne paint, but the resin molecules are chemically modified to enhance their water solubility. For this reason E-coat paint cannot be used in ordinary dipping; it must be electrically deposited or film appearance and durability suffer dramatically.

Like conventional paint, an E-coat bath contains resin, pigments (unless a How Paint Is Applied - Part 2 105 clearcoat), solvent (water and a cosolvent) and additives. It is the chemical structure of the resin that makes an E-coat bath different from other paints. Most of the solvent in an E-coat bath is water. The water content puts E-coat in the family of waterborne coatings. A typical E-coat bath will contain from 2 to 6% of a high-boiling water-soluble solvent, such as butyl cellosolve or hexyl cellosolve (ethyleneglycol monobutyl ether or ethyleneglycol monohexyl ether). Moderately higher cosolvent levels do not affect the coating process, but if excess cosolvent is added, VOC emissions may become unacceptably high.

Not just any waterborne coating can be used as an E-coat bath; the resins must first be modified to contain certain chemical groups. The resins are treated with appropriate solubilizer chemicals and form positively charged (cathodic) or negatively charged (anodic) molecules called polymer ions. In their ionic form these resins are very polar and are soluble in water because water is also highly polar. They are, however, not interchangeable.

For anodic resins, carboxylic groups are chemically attached to resin molecules. The resin is mixed with potassium hydroxide, sodium hydroxide, triethyl amine or other amine compounds to solubilize the resin molecules.

Cathodic E-coat resin molecules are chemically modified so that tertiary amine groups are located along the backbone carbon chains of the molecules. Without ionizable groups such as amines, the resin cannot be used in an E-coat formulation. The tertiary amine groups, when treated with dilute acid solubilizers, such as acetic acid or formic acid, will form the necessary positively charged ions needed for cathodic E-coat.

In theory any type of resin can be modified for use with E-coat. The most commonly used resins, however are epoxies and acrylics. Epoxy E-coat paint films, like any other epoxy, tend to chalk when exposed to ultraviolet (UV) light. This is generally not a problem if the epoxy E-coat film is to be a primer. If the epoxy is to function as a topcoat, then it shouldn’t be exposed to UV light unless chalking does not present a problem.

E-coat paint films that require high appearance standards employ acrylic resins. These are resistant to attack by UV light and have outstanding weatherability.

In formulating E-coat resins, chemists select cross-linking agents, such as aromatic and aliphatic isocyanates, that are appropriate to the end use of the paint film. The aromatic cross-linkers, however, tend to cause yellowing in the paint film upon outdoor exposure; aliphatic cross-linkers do not. To avoid darkening, acrylic resins are cured with the aliphatic cross-linkers.

Since isocyanates react with water, and E-coat baths contain water, chemists “block” the isocyanates to prevent water reaction. The blocking is done chemically by “tying up” the water-sensitive ends of the isocyanate molecule with a blocking agent. The blocked isocyanates deposit into the E-coat film but the blocking agent is driven off by the high E-coat bake oven temperatures, thereby allowing the isocyanate group to cross-link with the E-coat resin molecules.

Rinsing

A part emerging from an E-coat bath has a deposited paint film that is covered by a wet solution consisting of bath solids (dragout). An elaborate rinse system removes the dragout before the coated parts are baked. Extremely large quantities of rinse are required to remove all dragout materials that may cling to the part. If dragout were not thoroughly rinsed away, the E-coat film quality would suffer.

Poor rinsing causes pronounced E-coat roughness that can require sanding of the film after curing. Sanding E-coat films is possible, but it is not easy to do because they are extremely hard. The slow, tedious sanding increases labor costs. Thorough E-coat rinsing is, therefore, absolutely essential.

The need for such thorough rinsing, however, causes a major dilemma. The rinse water volume must be large, and the amount of rinsed dragout is very low. Discharging all of the rinsed-off bath solids to drain can be cost-prohibitive and would cause an excessively high “biological oxygen demand” for the sewer system. The problem is complicated because the small amount of solids and the large quantity of rinse water cannot economically be separated. Filtration, distillation and related methods are much too costly for taking bath solids out of the rinse. If this problem could not be resolved, nearly all E-coat processes would be stymied because most plants could not afford to discard the important volume of solids along with the rinse water. Continually adding fresh deionized water to rinses to remove the dragout and discarding all ultrafiltration solution is cost-prohibitive.

A possible answer might be to allow the rinse water and the small amount of dragout to run directly back into the E-coat tank. But if continuous fresh water were used for rinsing, the E-coat tank would rapidly be diluted and accumulate such a large amount of extra water that the tank would quickly overflow.

The remedy to the problem is found by using ultrafiltration to withdraw (temporarily) a small portion of water-like “permeate” from the E-coat bath. The permeate is “squeezed” out of the E-coat bath by ultrafiltration. This “borrowed” permeate is then counterflowed through the rinse system. All of the rinse permeate is collected and reused in several (usually three) separate counterflowing rinse stages. The used rinse permeate is returned to the bath from an initial rinse located directly over the exit end of the E-coat tank.

Thus the ultrafilter system temporarily appropriates a small portion of the water in the bath, uses it to rinse off the parts and finally allows the permeate plus the rinsed-off dragout to flow back into the E-coat tank. This closed-loop filtration system continually provides enough permeate for rinsing without diluting the bath. The permeate that is extracted from the bath by ultrafiltration is also refered to as “ultrafiltrate” and “flux.”

Factors that can affect the amount of permeate that is produced for E-coat rinsing include: 1) the concentration of paint resin and pigment, 2) the pressure of the bath flowing through the ultrafilters, 3) the flow rates of bath through the ultrafilters, 4) the bath temperature, 5) the pH, 6) the cosolvent concentration, 7) the pigment-to-binder ratio of the bath solids and 8) the presence of foulants such as chromates or phosphates from pretreatment carryover and dissolved solids from the use of tap water rather than deionized water.

Under optimum conditions the permeate production rate is about 1.0% of the total bath flow through the ultrafilters. As the filters gradually become fouled, this percentage slowly decreases until cleaning of the filter membranes becomes necessary.

Types of ultrafilter construction used in E-coat systems include spiral-wound, shell-and-tube, plate-and-frame and hollow-fiber multiple-tube. Each has advantages and disadvantages that should be examined by those considering Ecoat systems. Individual preferences and preventive maintenance patterns seem to be as significant as operational and performance factors.

Ultrafilters must be properly maintained because good ultrafiltration requires a sufficient flow rate through the filter elements. In many instances manufacturers suggest flow rates of at least 35 gallons a minute to prevent fouling of the thin filter membranes. A thorough rinse of the parts after pretreatment will help avoid bringing contaminants into the E-coat bath because the contaminants reduce permeate generation. It is inevitable that some insolubilized resin molecules will not deposit but remain in the bath. These will gradually clog the pores of the ultrafilter membranes.

Depending on throughput rates, normal cleaning of cathodic E-coat ultrafilters may be required at intervals of six to 20 weeks. In normal production the ultrafilter permeate output slowly decreases. When permeate generation falls to 80% (approximately) of initial output levels, the ultrafilters must be cleaned with a mixture of solvents and concentrated solubilizer solution. Cleaning ultrafilters frequently minimizes rinsing problems. Extending the interval between cleaning is an invitation for E-coat trouble. If cleaning is postponed too long, permanent impairment of ultrafilter function can result, especially with hollow-fiber and multiple-tube ultrafilters. Flat-plate and spiral-wound types may survive this neglect. However, even these ultrafilters should have regular cleaning as soon as their output diminishes to a designated level.

 After the ultrafilter rinse, the parts may undergo further rinsing with deionized water to ensure that all contaminants are removed prior to baking. The deionized water rinsing is usually in two stages: a recirculating rinse and a final virgin rinse. The final fresh deionized water rinse of about 2 gallons a minute can flow into the recirculating deionized rinse tank, which can overflow either to drain or into a permeate rinse tank. A large amount of deionized water is needed to prepare the E-coat bath and provide the final rinses. Many companies insist that the drain-off deionized rinse in the pretreatment section have a maximum conductance of 30 micromhos to prevent contamination of the E-coat bath and of the system’s ultrafilters.

Solubilizer-deficient makeup resin is added to the E-coat tank through bath recirculation entry injection ports called “eductors” located along the sloped exit end of the tank.  Adding the makeup resin in this way helps prevent fouling the ultrafilters, compared to when makeup is introduced into the intake of the circulation pump. The eductor entry method ensures thorough mixing of the makeup material.

Before the solubilizer-deficient makeup enters the bath, it must already have been fully solubilized. This is essential because even tiny amounts of unsolubilized resin will quickly plug the ultrafilters and sharply curtail the production of permeate rinse.

The bath content must be monitored closely for the buildup of excessive excess concentration of ions in the bath can raise conductivity and interfere with chemicals that can interfere with efficient E-coat operation. For example, an excess concentration of ions in the bath can raise conductivity and interfere with electrical efficiency. To prevent such buildups, a small amount of permeate is normally released to drain continuously and replaced with an equal amount of deionized water. Excess solubilizer, contaminants from pretreatment baths and water hardness salts are released to drain also in the discharged permeate. Excess solubilizer in the permeate rinse can resolubilize the freshly deposited paint film, in effect stripping off the film immediately after application.

Pigments

The pigment particles are mixed uniformly throughout the bath by vigorous circulation. During the E-coat process the pigment particles become entrapped among resin molecules as the resin is attracted toward and being electrodeposited onto the parts. E-coat pigments never undergo any change in electrical charge; they remain uncharged throughout the process. Only the resins are converted from a soluble ionic state to an insoluble neutral form.

Bath Parameters

An E-coat bath needs to be monitored closely to ensure quality coating deposition. Meticulous records should be kept of the following parameters:

• Percent total solids
• PH
• Solubilizer concentration
• Solution conductivity
• Temperature
• Relative amounts of pigment and binder
• Film build levels
• Voltage and amperage

The pH for many cathodic systems is held at a slightly acidic level of 6.0 to 6.5. Some other systems, especially those that eliminate excess solubilizer primarily by flushing ultrafiltrate to drain, may have a pH as low as 3.0 to 3.5.

A bath sample must be titrated periodically to check the concentration of acetic acid solubilizer. The concentration is usually expressed in units of “milliequivalents per liter of bath material.” A typical operating solubilizer concentration is 90 milliequivalents per liter.

Overall conductivity affects the electrical efficiency. If it is too high, efficiency drops. Normally the maximum of roughly 900 micromhos bath conductivity is appropriate.

The substantial amount of direct current used tends to raise the temperature of the bath because of the conversion of electrical energy to thermal energy. Chillers and heat exchangers are used in the circulation system to maintain bath temperature in a normal range of 70 to 95°F. Excessively high temperatures can cause difficulty with the ultrafiltration process and deteriorate the bath.

As resin and pigment are removed from the bath and deposited as a paint film onto the parts being coated, additional resin (presolubilized with acetic acid) and pigment must be added. Sometimes the resin and pigment are replenished in a single concentrate; however, since the resin and pigment may be used up at different rates as they deposit on the parts, it may be necessary to add each separately. Paint supplier assistance in monitoring the resin and pigment levels and in establishing the correct pigment-to-binder ratio is usually available. Improper ratios can quickly and easily be readjusted by the appropriate makeup composition.

Continuous circulation and filtering of the bath are necessary to prevent pigment settling and to keep the bath clean to avoid the deposition of foreign contaminants along with the resin and pigment. Pumps circulate the bath at a rate of four to six turnovers per hour. The bath is taken from the area behind the weir (an adjustable dam to control the level of the paint in the tank) and is then filtered. The paint is next fed back into the tank along the bottom through a series of pipes fitted with eductor nozzles. The jet force out of the nozzle orifices and the venturi action of the eductors on the surrounding bath help prevent pigments from settling and maintain good pigment dispersal in the bath. Eductors produce a mild scouring action that also reduces the amount of residue that can accumulate on the bottom of the tank. If adequate circulation is not maintained, extremely hard resin/pigment aggregates can build up on the tank bottom. As much as 14 inches of deposit have been found in E-coat tanks due to grossly neglected circulation problems. Parts that fall off the conveyor should be removed immediately from the bottom of the tank so that dead circulation spots are not formed that would contribute to residue.

Tank Details

The E-coat tank must be large enough to hold the parts to be coated and the necessary quantity of bath. Some tanks hold as much as 120,000 gallons. To paint continuously moving parts on a conveyor line, the tank must be considerably longer than for parts held stationary. A conveyor line operating at 14 feet per minute requires a total tank length of roughly 50 feet, including the portions used for gradual part immersion and withdrawal.

Anodes are located vertically along both sides of the E-coat tank. In some systems each anode is enclosed within a flushable cell. These enable continual flushing to remove excess solubilizer produced by electrochemical reaction at the anodes. Tubular (5 inches in diameter) or rectangular (6 by 24 inches) membrane cells of the needed length can be employed, but the ease of handling makes the tubular type increasingly popular.

E-coat Curing Cycle

The final E-coat step is oven curing of the deposited coating. Although lowtemperature curing and even air-curing are possible, the most durable films require a moderately high-temperature bake to cross-link the resins in the binder. This is often in the order of 300 to 350°F for 12 to 25 minutes, depending on the design, configuration and heat capacity of the part being coated. Convection ovens are used almost exclusively for curing E-coat paints, although infrared ovens can be used as well.

Autodeposition

Organic paint films can be deposited onto iron and steel parts by an oxidationreduction precipitation process known as autodeposition, chemiphoretic and Autophoretic coating. The process, which uses no external source of electricity as with E-coat, is available primarily in black, although several colors such as brown, orange and light blue have been reported. The paint film has a dull or low-gloss appearance and is primarily protective and not decorative. The largest application areas for autodeposition coatings have been nonappearance and under-hood parts for cars and trucks. Excellent anticorrosion properties and the black color make it highly appropriate for this application. It is also used on drawer slides for office furniture, replacing zinc-plating.

An important advantage of autodeposition is its 100 percent coverage of all surfaces of a part that are wetted by the coating bath. Faraday cage areas, which hinder E-coat deposition, are nonexistent with autodeposition.

The main process stages in an autodeposition system begins with a heated aqueous alkaline spray cleaning for about 1 minute at 160°F. Then a dip cleaner follows for 1 to 3 minutes at 185°F. Thorough cleaning is extremely important because the process tends to be especially intolerant of contaminants. These cleaners are the only stages in the entire process that are heated; the others operate at ambient conditions. After multiple rinses, ending with a deionized water rinse, the parts go into the autodeposition bath for about 2 minutes. The bath, typically held at 68 to 72"F, is a waterborne material containing about 10 percent of a vinyl emulsion, hydrofluoric acid and hydrogen peroxide. The coating deposition reaction is nonexothermic (does not give off heat), and the rate of deposition slows as increased coating covers the steel surface.

Coating deposition begins immediately upon immersion of the parts into the bath, which is maintained at a pH of 2.5 to 3.5. After about 2 minutes, 0.75 to 1.0 mil of coating is deposited. Hydrofluoric acid etches and removes iron ions from the part by chemically attacking the steel surface. Hydrogen peroxide converts the solubilized iron ions from the +2 to the +3 (ferrous ion to ferric ion) oxidation state. The iron +3 ion combines chemically with the vinyl emulsion polymer to form an insoluble resin that precipitates onto the surface of the steel. Even after parts are removed from the bath, unreacted resin is soon insolubilized by the continuing chemical action. Thus no unreacted paint needs to be rinsed from the parts. The inorganic chemicals remaining on the parts are removed from the paint film by a 30-second water immersion rinse.

After the coating is deposited, several options exist. One option is the use of a dilute chromic acid rinse, followed by an oven bake at about 250°F for 15 minutes. Another option is to cure the coating in water at 180°F for about 8 minutes. The water cure tends to yield a coating with less corrosion resistance than a coating preceded by a chromic acid rinse. cured in the hotter bake oven temperatures. A water cure may or may not be preceded by a chromic acid rinse.

An autodeposition coating cannot be recoated by a second autodeposition process because no exposed steel is left for the acid to attack. A major advantage of autodeposition is that no organic solvents are needed and that no VOC is emitted. Autodeposition is used both by product manufacturers and a few customcoating shops. The current process works only on ferrous metals, but a new autodeposition coating for zinc is in the late development stage.

Air-Atomizing Spray Guns

The methods of paint application described in the two previous chapters rely on various forms of spreading or dipping. Each has unique advantages and disadvantages, and one or the other can be a perfect choice for a particular end use.

Another method of applying paint is by atomizing the paint into tiny mist-like particles and depositing them onto the surface to be coated. If a sufficient number of the particles is applied, they will create a continuous coating. Paint can be atomized in various ways. The most common is with the air-atomized spray gun.

The essential components of an air-atomizing spray gun are:

• Gunbody Air inlet
• Fluid inlet Air nozzle
• Fluid nozzle Air valve
• Fluid needle assembly
• Fluid control assembly
• Fan control
• Trigger

Gun body. The gun body consists of the handle for the operator to grip and the barrel. The main thing that the almost countless models of air-atomizing spray guns have in common is a handle designed for operator hand comfort. The handle and barrel house the gun’s various components.

Fluid inlet. The fluid inlet is an opening, usually below the tip of the barrel, that allows the paint to flow into the gun. The opening is threaded to allow attaching either a siphon cup or a paint hose attachment.

Fluid nozzle. The fluid nozzle is a small device with a precision opening to permit the paint to flow out of the gun at a determined rate. One end of the nozzle is externally threaded and screws into the internally threaded barrel tip. An assortment of nozzles is available with different diameter openings.

Fluid needle assembly. This assembly serves as a needle valve to stop and start the flow of paint through the fluid nozzle.

Fluid control assembly. This allows the operator to start and stop the flow of paint through the gun by operating the trigger.

Air inlet. This is a threaded opening at the bottom of the gun handle to allow attaching a hose connected to a source of atomizing air.

Air nozzle. The air nozzle is a small device with precision openings that allows compressed air to be directed at the paint for optimum atomization. The nozzle, also called the “air cap,” is internally threaded to attach to the externally threaded gun barrel tip. Air nozzles are available with many different configurations of openings to allow various atomization patterns. Air nozzles typically have “horns” with precision openings that can be directed to vary the spray pattern into a fan shape.

Air valve. The air valve gives the operator a means of controlling the flow of air through the gun.

Fan control. This control permits the operator to regulate air flow through the air nozzle horn openings to vary the spray pattern.

Trigger. The trigger is connected to the air and fluid flow controls in the most commonly used “non-bleeder” guns. Partial triggering activates just the air valve, and enables the operator to blow dust off parts before painting. In “bleeder” guns the trigger activates only the fluid controls; air flows even when the trigger is released. The trigger is conveniently located on the gun to allow finger control.

Compressed Air Supply

To prevent paint contamination, the air supplied to a spray gun must meet a number of requirements. The air must be:

Dirt-free. The air must be filtered to remove dust, lint and other dirt-type contaminants. The air is generally filtered at the inlet port of the air-compressing device, and again after oil and moisture removal.

Oil-free. Air compressors usually send out oil vapor with the compressed air because oil is used to lubricate compressor rotors. Such oil vapor is generally removed with an oil-absorbing coalescing filter.

Moisture-free. A phenomenon of air, moisture and temperature is that warm air contains more moisture than cool air, and that when air is cooled, moisture tends to be wrung out of the air in the form of condensation or water. When air is compressed it increases in temperature. Enroute to the spray gun, the air temperature drops, producing moisture condensation that must be removed from the air line. This can be accomplished with air cooling systems and water traps, or by chemicals that hold water by absorption and adsorption.

Air can be supplied to an air-atomizing spray gun by either an air compressor or an air turbine. Air compressors are of various types, and may include diaphragm, rotary and reciprocating construction. The compressed air from one air compressor can be further compressed in a two-stage system. An air turbine is a fan-like device that is used to supply large volumes of air at low pressure.

Compressed air from air compressors is generally passed through an air regulator to maintain a steady pressure for the gun. For example, the air line pressure in a manufacturing plant may be 100 psi, but perhaps only 50 psi is desired at the spray gun. The air regulator would drop the pressure to 50 psi and maintain this figure even though the plant air pressure fluctuated.

Paint Supply

Paint to be fed to an air-atomized spray gun can be fed by gravity, siphon or pressure systems. In gravity-feed, as the name suggests, the paint supply is above the gun and feeds by gravity. The paint container must be covered to keep out dust and needs to be vented to allow paint to flow.

Paint in a siphon cup is drawn upward into the gun because of a negative pressure produced in the siphon tube by the flow of compressed air. The cups come in various sizes ranging up to a quart. They need to be vented.

Because of the limitations of gravity- and siphon-feed, the paint supply system in predominant use is pressure-feed. Pressure-feed is of two types: 1) compressed air is applied to paint, in a pressurized container, forcing paint to the gun; and 2) paint is pumped to the gun, either in a “dead-end” system or in a recirculating system, by various types of pumps.  

The pressurized container may be a cup that is mounted at the bottom of the gun, but is usually a “pressure pot” (tank) connected by a 5-10 foot flexible hose to the spray gun. Pressure pot sizes range from about 2 quarts to 50 gallons.

Paint can also be pumped to the spray gun out of any un-pressurized container such as a pail, drum, or tote tank. Paint pumps that supply air-atomizing spray guns are of three basic categories: reciprocating piston (single- or double-acting), rotary (cam or gear) and centrifugal. The pumps may deliver paint directly to a gun or to a circulating system from which paint can be piped to one or more guns.

The paint delivered to a gun is generally filtered to remove contaminants that might cause rejects if allowed to be sprayed onto a product. The micron rating of the filter must be high enough to permit passage of pigments and metallic particles.

Gun Operation

The rate of flow of paint through a spray gun is a function of the fluid pressure (driving force), paint viscosity, size of the fluid nozzle opening and the setting of the gun’s fluid control valve. The fluid pressure is set to deliver the correct amount of paint through an appropriately selected fluid nozzle. High-production requirements need high rates of paint flow to be able to apply sufficient coating on parts moving past the spray gun at conveyor speeds.

The degree of atomization in an air-atomized gun depends on how efficiently the atomizing air breaks up the paint particles. It isn’t the amount of air pressure alone that breaks up the paint particles, but instead is a summation of air pressure, air volume and the precise merging configuration of the air and fluid streams.

Air-atomizing spray guns can be categorized into two general types according to the volume and pressure of the atomizing air:

• Low-volume high-pressure (conventional)
• High-volume low-pressure (HVLP)

Low-Volume High-pressure (Conventional)

This category of air spray gun has been known as “conventional” to distinguish it from the modified gun verions that have appeared over the last 30 years. The gun was developed early in the 1900’s and has remained essentially the same over the years except for refinement in gun construction materials and in the design of air and fluid nozzles.

These spray guns use compressed air from an air compressor. The air pressure may range to about 100 psi, and the air volume from about 3 to 25 cfm. The air volume tends to be low when using low air pressures and rises somewhat proportionately as the pressure is increased. The air pressure selected is tied in closely with the air nozzle that is used. The size of the openings in the air nozzle orifices must not deplete the air compressor capacity. The relatively high air pressure typically used with these guns gives exceptionally fine atomization and allows high rates of paint flow to meet high production requirements.

If the atomizing air pressure is too high, droplets as small as 5 microns in diameter may result and may create a fog that can decrease application efficiency. The greater surface area of tiny droplets will increase solvent evaporation, producing “dry” spray, giving the part being painted a dusty look.

High-Volume Low-Pressure (HVLP)

This category of air spray gun uses minimal atomizing air pressures, often below 10.0 psi. because certain regulations stipulate that air-atomized guns may not exceed this pressure.

HVLP guns are classified into two categories, depending on whether the air is supplied by an air compressor or a turbine. The type of gun that uses an air compressor typically works in conjunction with an air regulating device located in the air line or inside the spray gun itself to ensure that no more than 10.0 psi of air pressure reaches the gun tip. The HVLP gun can also be supplied by an air compressor. In this case a non-bleeder HVLP gun is used.

Both the air compressor and turbine types of HVLP guns are characterized by an air nozzle with a relatively large-diameter opening for atomizing air. At 10.0 psi the air compressor typically supplies 15 to 30 cfm. Turbine type HVLP guns are recognizable by the large-diameter air hose connecting to the gun. Compressor type HVLP guns are usually not easily distinguishable from conventional (LVHP) guns.

An air turbine can typically put out about 200 cfm of air at 10.0 psi. This means that up to eight HVLP guns can operate off a single turbine. An advantage of a turbine is that its air output can be heated to as high as 180"F, which helps provide easier atomization by heating the paint in the end of the gun to lower the viscosity. An air heater would need to be used with the air compressor type of HVLP gun to provide the same heated air. Bleeder guns are used with heated air so that the tip of the gun remains warm.

The low atomizing air pressure of an HVLP gun tends to minimize the amount of “bounce-off’ paint fog and reduces the amount of atomized paint that is blown past a part to be painted as overspray. The improved transfer efficiency helps hold down operating costs by reducing paint waste. As the solids percent of paint is increased, the need to minimize overspray increases accordingly to hold down costs. However, such reduced atomizing air pressure tends to decrease the fineness of atomization, which reduces the finish smoothness capability. The low atomizing air pressure of HVLP guns also tends to require reduced paint flow to the gun, which limits production speeds.

Spraying Techniques for Air-Atomized Guns

Proper spraying techniques with air-atomized guns is extremely important because of the cost of the paint being applied and the expense of having to rework reject parts. Good spraying techniques will minimize overspray and result in the correct paint application for optimum appearance.

Before a person begins to spray paint onto a product, a number of points should be reviewed. The spray gun should be clean and in perfect working order. It should yield a spray pattern with clearly defined boundaries.

A proper spray pattern is achieved with the least amount of air pressure for correct atomization and the minimum amount of fluid pressure to provide enough paint to meet production requirements.

Good spraying technique requires adhering to the following basic principles:

• The gun should be put in motion before the trigger is squeezed
• The gun should be kept a uniform distance from the surface being coated
• The gun should be moved across the surface to be coated at a uniform speed
• The gun should be triggered at the beginning and end of each stroke
• Each spraying stroke should be started at the same vertical or horizontal location on a particular product
• Each previous stroke should be overlapped by the same amount
• The same number of strokes should be used on identical product surfaces
• The final stroke on identical products should be ended at the same surface location
• For an optimum coating, the gun distance from the product surface being coated should be 6 to 8 inches. Moving the gun closer will increase the wetness and film build. Backing the gun away will decrease wetness and minimize film build.

Some general principles should be followed when spraying products with different shapes. For example, the ends of vertical flat panels should be sprayed first, followed by back-and-forth horizontal spraying, beginning at the top. Long panels should be sprayed this way in strokes up to about 5 feet wide. When edges are being sprayed, the gun should be aimed so that as much overspray as possible lands on uncoated surfaces. Exterior edges should be sprayed first. Spray should not be directed straight into internal corners; each side of the corner should be sprayed instead.

Gun variables should be monitored closely while spraying. These variables are the paint flow rate, fluid pressure, paint viscosity, air pressure, fan pattern and distance of the gun from the work.

he evaporation rate of the solvent from the atomized paint particles moving from the gun to the product being painted needs to be considered. Excessively slow solvent evaporation will yield an applied coating that might be excessively wet and cause paint to run or sag on vertical surfaces. Excessively fast solvent evaporation can produce a coating that is too dry.

Airless Spray and Air-Assisted Airless-Spray

Airless Spray

 In addition to using compressed air as a driving force in spray painting, another method of atomizing paint is to increase the paint’s fluid pressure in a spray gun and redesign the fluid nozzle so the paint is atomized without introducing a pressurized air flow. This type of spray gun is termed an airless spray gun.

The design of the airless spray gun is much the same as that of the air-atomized spray gun. The main differences are the elimination of the air inlet, air nozzle, air valve and fan control, all of which are unnecessary because of the absence of an air supply to the gun. Like the air-atomized gun, the airless gun also has a:

• Handle and barrel
• Fluid inlet
• Trigger
• Fluid nozzle

Handle and barrel. These perform the same function as in the air-atomized gun: The handle allows the operator to grip the gun, and the barrel provides a support for the fluid nozzle and trigger.

Fluid inlet. On some guns this is located under the end of the barrel, and on other guns at the bottom of the handle.

Fluid nozzle. The fluid nozzle on an airless gun differs substantially from the fluid nozzle on an air-atomized gun. The airless fluid nozzle orifice is elliptical in shape, but is rated in equivalent circular diameter, typically from 0.007 to 0.072 in. The orifice is beveled or fanned out at various angles, typically in increments from 10 to 80 degrees.

Fluid control (off-on) assembly. This starts and stops the flow of paint through the fluid nozzle. Some guns use a tungsten carbide ball and seat.

 Trigger. This gives the operator a convenient means of operating the fluid control (off-on) assembly.

A duckbill device, shaped like its name, is mounted on the end of the barrel as a safety device. It is designed to prevent the operator from accidentally touching the high-pressure paint emerging from the fluid nozzle. The paint stream exits the nozzle with great force and can penetrate the skin and cause serious injury.

Paint Supply

Paint is supplied to an airless spray gun typically by an air-driven reciprocating fluid pump. The pressure exerted by the pump is in proportion to the ratio of the area of the pump piston and the area of the air piston. For example, if the pump piston is 20 times as large as the air piston, and if 100 psi is applied to the air piston, then 100 X 20 or 2000 psi will be applied to the pump piston, or to the paint. Frictional losses lower the pressure at the gun tip, however.

Airless spray paint systems are of two types: dead end and circulating. In the dead end type, paint is pumped directly from a container to the gun. In a circulating system, paint is circulated from the paint container through a paint line continually, and the gun is connected to the circulating line. A fluid filter may be located just ahead of gun or elsewhere in the paint line.

Gun Operation

Paint is pumped to airless guns typically at about 1200 psi, although this may range anywhere from 500 to 6,500psi. When the paint exits the fluid nozzle at these high pressures, it expands slightly and is atomized into tiny droplets without the impingement of atomizing air pressure. The high velocity of the exiting paint propels the droplets toward the work being painted.

 The width of the spray fan from the fluid nozzle is determined by the nozzle’s fan angle. With the gun tip 12 inches from the part being sprayed, the spray width (at the part being painted) may vary from about 5 to 17 inches, depending on the fan angle of the fluid nozzle being used.

 It is the size of the orifice that determines the quantity of fluid sprayed, and the fan angle that determines the thickness of the coating. The same amount of paint, but over a different area, will be deposited by two nozzle tips having the same orifice size but different spray angles. Airless fluid delivery is high, ranging from about 25 to 75 ounces per minute.

Spraying Techniques for Airless Guns

Recommended spraying techniques for airless spray guns are nearly the same as for air-atomized spray guns. The basic difference relates to the high paint flow capability of airless guns and their absence of blowing air pressure.

The high paint flow requires a consistent spray procedure to prevent uneven paint film build and the associated problems of runs and sagging. It is extremely important in spraying to point the gun directly perpendicular to a surface and to move the gun laterally or vertically in motions that are parallel to the surface. Overlapping of the spray pattern between spraying strokes must be consistent to prevent fluctuations in film thickness.

The absence of pressurized air flow in the vicinity of the target allows airless guns to spray into corners and hard-to-reach areas. When air-atomized spray is directed into these restricted areas, the air flow builds a cushion of air turbulence that tends to repel the movement of atomized paint particles toward the target. The paint sprayed from an airless gun nicely penetrates the cavity; the airless spray gun has considerable atomized paint blown out of the cavity. The high air pressure associated with air-atomized spray (conventional) creates air turbulence and atomized paint bounceback. The absence of air pressure eliminates such turbulence and bounceback.

Skin Injection Danger

Probably the biggest disadvantage of airless spray is the considerable danger of injecting paint through the gun operator's skin. The high fluid pressure creates a paint stream that can easily penetrate the skin if the gun is triggered directly against or close to the skin. Sprayers have accidentally injected paint into their fingers, hands, arms and other parts of their bodies. As a consequence, some have required amputation of fingers, hands and arms. Some have even died after their bodies went into shock from the injection.

When paint injection occurs, only a tiny opening may be noticeable in the skin. This can be deceptive, for the injury can be severe, despite the minor-appearing wound.  

Total removal of such injected paint is extremely difficult. The body reacts to the injection by the formation of considerable amounts of fluid that may cause further tissue damage if the pressure is not relieved. Note the small entrance wound and that the entire finger is swollen and turning blue. To remove the material, the finger will need to be surgically opened throughout its length, possibly causing further nerve and tissue damage.

Physicians knowledgeable in airless spray paint injection recommend immediate surgical examination. Keeping the affected area immobile after an injection will minimize the spread of the injected paint deeper into the body. Physicians report that injections into a finger can work their way past the wrist rather quickly with physical manipulations. This obviously complicates the surgery and the recuperation period for the patient.

As an aid to preventing injection accidents, devices such as the duckbill have been helpful. The extremely high pressure at the gun tip decreases rapidly with distance from the fluid nozzle. A pressure of 3000 psi at the tip will decrease to 200 to 25 psi an inch or two away. Safety tip guards should always be used. The safety on the gun trigger should always be activated when the spray operator moves the gun to a new location (and at similarly appropriate times).

Some manual airless guns are designed to be inoperable without the duckbill. However, sometimes operators will cut off the safety device because the tips of the duckbill sometimes collect paint that then drips onto newly painted parts.  

Using the duckbill is a small inconvenience for the greatly added safety that it brings. Even with the duckbill, operators of airless spray guns still need to use extreme caution. Spray operators should be reminded frequently that fluids under great pressure can be dangerous, as evidenced by the use of high-pressure water jets to cut thick steel plates.

Air-Assisted Airless Spray

The air-assisted-airless gun spray gun looks almost exactly like an air-atomized spray gun. The handle, barrel, trigger and tip look the same. An air hose attaches to the handle and a paint hose connects to the bottom of the end of the barrel.

But beyond the similarity in looks, the air-assisted gun is much different in operation. The difference lies in the amount of fluid and air pressures that are used. An air-assisted airless gun uses from about 150 to 800 psi of fluid pressure and only 5 to 30 psi of air pressure. The fluid pressure is far more than an airatomized gun but considerably less than an airless gun. The air pressure is far less than a high-pressure low-volume (conventional) air-atomized gun, and, of course, higher than the airless gun, which uses no atomizing air pressure at all.

The major difference in gun construction between an air-assisted airless gun and an air-atomized gun is in the atomizing tip. The air-atomized tip incorporates a fluid nozzle and an air nozzle. The fluid orifice in the center of the tip is surrounded by a concentric atomizing ring of air. The air-assisted fluid tip delivers a flat fan spray of partially atomized paint. Jets of atomizing air, exiting from ports in small projections on each side of the tip (similar to the wings of airatomizing guns), impact at a ninety degree angle into the spray. The air jets break up the large droplets and complete the atomization, “assisting” the airless spray process.

The tips are available with various size fluid orifices and fan angles. These may range from about 0.009 to 0.036 in. and 15 to 80 degrees. With only fluid exiting the tip, an air-assisted airless spray gun has a spray pattern with heavy fluid “tails” on each side of the pattern. The tails are eliminated by gradually increasing the air atomizing pressure. The overall pattern can be refined by varying the air flow from the shaping air ports located adjacent to the ends of the slotted fluid orifice. The choice of tips and air pressure selection variation permits achieving a spray width of from about 4 to 19 inches at a distance of 12 inches from the target.

The paint flow rate can be varied from about 5 to 50 ounces per minute by varying the fluid pressure. The selection of tip and fluid pressure would be determined by production requirements. Paint is typically delivered to the gun from an air-driven reciprocating pump with an 8:l ratio of air piston and fluid piston size. Fluid control typically is with a ball and seat-type valve in the end of the gun.

Electrostatic Paint Application

In spray application, the driving force that pushes the atomized paint to the part to be coated includes various combinations of fluid pressure and air pressure. In the electrostatic application of coatings, the small coating particles are given an additional driving force: an electrostatic attraction that is made possible by electrically charging the coating particles.

All types of coatings can be electrostatically charged. Each of the previously discussed spray methods-air-atomized, airless and air-assisted airless spray can be applied with or without electrostatic charging. In addition paint can be applied by electrostatically charged disk or bell rotary applicators.

The principles of electrostatic charging apply equally to liquid or to powder coatings; however, this chapter will deal only with the electrostatic charging of liquid coatings. Electrostatic powder charging will be covered in the chapter on powder coatings.

Electrostatics in Painting

In electrostatic painting, the atomized paint droplets pick up an excess of electrons, becoming negatively charged. The part to be painted is electrically neutral, making the part positive in respect to the negative paint droplets. The opposite charges set up an electrostatic attraction between the charged droplets of paint and the part to be painted.

With an electrostatic spray gun, the droplets pick up the charge from an electrically charged electrode at the tip of the gun. The charged droplets are given their initial momentum from the fluid pressure/air pressure combination. As the charged droplets approach the electrically neutral part, the charge tends to attract the droplets toward the part. This attraction toward the part reduces the number of droplets hurled past the part, increasing transfer efficiency. The attraction is so strong that some charged particles hurled past the part will curve, turn around and come back to the part (wraparound). This tendency actually allows electrostatic painting to coat the edges of a flat part and a portion of the side of the part away from the spray gun.

Charging of the spray gun tip is achieved by an electrical power supply that continuously provides a source of electrons. The electrons are pushed by the power supply to a needle-like electrode at the gun tip. With the power supply enerergized, the excess of electrons leaks off the tip of the electrode to the air in the immediate vicinity, creating a charged cloud of air molecules. This is called the “ionized air cloud.”  

When paint begins to flow and atomized paint droplets are hurled from the gun tip they must pass through the charged cloud. In doing so, the previously neutral paint droplets pick up an excess of electrons.

The electrical circuit is typically completed as follows: The part to be painted is suspended from a metal hanger attached to an overhead metal conveyor, which is electrically grounded since it is connected to the building’s steel foundation. The negative side of the power supply is connected to the gun electrode, and the positive side of the power supply is connected to ground via the building’s steel foundation. Thus, the electrons drawn from ground by the negative side of the power supply go to the gun electrode; to the droplets; to the part to be painted; to the conveyor; to the building’s steel foundation; back to ground from where they started their journey. An electrical power supply can be viewed as a generator of excess electrons. No electrons will flow through the power supply until the external electrical circuit is complete back to electrical ground.

The gun’s electrode is in the shape of a needle to help the excess of electrons from the power supply drift to the charged cloud. It is the nature of a sharp point or edge to easily allow electrons to drain away.
The rate of flow of electrons from a gun’s electrode during electrostatic painting is small, in the order of 5 to 50 millionths of an ampere (5 to 50 microamperes). An ampere (or amp) is a unit of current flow and is a measure of the rate of electron flow. An energized 100-watt light bulb has about 1 amp flowing through it. A lightning bolt may have a flow of millions of amps. A part being electrocoated may draw 800 to 900 amps.

The force in an electrical power supply that provides the push for the electron flow is called voltage. A residential power outlet has about 110 volts. A bolt of lightning may have millions of volts. A typical electrostatic power supply for painting operates in the range of 30,000 to 120,000 volts (30 to 120 kV).

In electrostatic painting terminology, a building’s steel frame is said to be “grounded.” This is because the frame is either embedded into the ground or is physically connected to something that is. The term “grounded” means that the object is electrically neutral with respect to the ground (earth).

The secret of success in electrostatic painting is to be sure that the positive side of the power supply is grounded and that the part to be painted is grounded. The positive electrode of the power supply is hardly ever a problem because it is connected to ground by a tight mechanical connection such as a bolt or a soldered welded connection. The part to be painted may not be properly grounded, however, due to a poor electrical connection between the part and the hanger, the hanger and the conveyor and between the conveyor wheels and the conveyor I-beam. For efficient electrostatic painting, all of these connections must be electrically sound. If they are not, electrical current flow will be restricted, and poor electrostatic charging of paint droplets will result.

For top efficiency, the part to be coated should be the closest grounded object to the charging needle on the spray gun. The charged paint particles are attracted to attraction. - the nearest electrically grounded item; and the larger the item, the greater the attraction.

Ungrounded objects in the vicinity of the charged gun electrode can accumulate electrons and pick up a considerable electrical charge. The charge buildup can then arc over (spark) suddenly if a grounded object is brought near. The intense heat of the arc may be sufficient to ignite a solvent-containing atmosphere found in a typical spray booth.

Paint buildup on hooks and hangers can act as an insulator and block the flow of electric current in the electrostatic circuit. The greater the paint buildup, the more severe the problem. If the grounding loss is only slight, wraparound may be only partially reduced. When paint buildup on hooks is heavy, the paint droplets that hit the part do not lose their negative charges. From the repulsion of like charges, the incoming negatively charged particles are actually pushed away by the negative charges already on the part. The result is thin paint; parts are produced with film builds that are below specification. Should complete loss of grounding be experienced, the danger of arcing becomes large, and fires can result. Numerous instances can be cited where fires have occurred this way. To ensure good electrical connection between parts and hangers, some plants use square rather than round rods for hangers. The rod is formed into a hanger so that a sharp edge meets the hung part to be painted. The sharp edge tends to cut through paint that may accumulate on the rods. Hangers should be regularly stripped or otherwise cleaned of paint buildup to maintain good grounding contact to the parts and the conveyor.

People in the vicinity of the charged electrode or the spray gun should also be grounded. Spray paint operators must wear leather-sole shoes to drain charges to ground and have a gloveless (bare) hand on the grounded spray gun handle, or a glove with its palm removed. These precautions will ensure that the operator never accumulates an electrical charge.

To complete the electrical circuit satisfactorily, the part to be coated must be an electrical conductor. This is essential to maintain current flow and prevent a charge buildup on the part, which would repel additional charged paint droplets.

The conductivity of the paint can affect the path of the electron flow from the gun’s electrode to the part to ground and back to the electrical power supply. Solventborne paints tend to be extremely poor conductors; waterbornes and some metallic paints are excellent conductors. This difference in conductivity, however, introduces a new phenomenon: The conductive paint can carry the full supply of electrons away back through the paint line to the grounded paint tank. If this occurs electrostatic painting is not possible. To prevent shorting (grounding out) the system through the conductive paint and the paint pot, the paint supply is isolated from ground by non-conducting plastic supports. While this isolation of the paint supply (isolated system) enables electrostatic application of conductive paints, it still permits electrons to travel back to the pressure pot, turning it into a very dangerous source of a potential high voltage electrical shock to persons in the vicinity. As a safety precaution with waterborne electrostatic spray, the paint line and pressure pot are confined in a caged area. The poorly conductive solventborne paint does not transfer the charge back to the pressure pot.

To ensure that solventborne paint remains a poor conductor, paint formulators recommend using “nonpolar” solvents. “Polar” solvents tend to be conductive because of the nature of their covalent and ionic atomic bonds. Nonpolar solvents tend to be poor conductors. Examples of recommended nonpolar solvents are mineral spirits, VM&P naphtha, xylol, toluol and N-butyl acetate. Examples of high-polarity solvents are acetone, methyl ethyl ketone, isopropyl alcohol, methyl cellosolve, diacetone alcohol, ethyl alcohol, methyl alcohol and methyl acetate.

The electrical hazard associated with electrostatic conductive paint spray tends to cause some plants to limit it to automatic application where operators are not involved. A manual gun used with electrostatic conductive paint spray would carry the full power supply high voltage, but with the proper equipment it is possible to manually spray conductive coatings.

Recalling the examples of static electricity, the charging of the comb (after combing one’s hair) and a person’s body (after walking across a carpet) tends to be more pronounced in low humidity. This is because the highly polar water molecules in humid air readily accept electrons. Thus, humid air will tend to speed the leaking away of charges from the comb and from the person’s body. In the case of lightning, the air in the vicinity of a thunderstorm tends to be humid, increasing the tendency of a cloud’s charge to discharge to earth or to another cloud in the form of lightning.

Humid air in the vicinity of a gun’s electrode tends to increase electron flow also. This is why electrostatic painting is sensitive to humidity fluctuations.

The electrical shock and fire dangers inherent with high voltages used in electrostatic paint spraying have led to the development of devices to lower the voltage at the gun’s electrode when a person approaches or as the electrode approaches an electrical ground. This prevents electrical shock and eliminates the chance of a spark starting a solvent fire.

A number of patented protective systems are used by various equipment manufacturers in their power supplies as safety devices to restrict the total amount of current that can flow. Some of these current-limiting devices proportion voltage and current. If the current begins to rise, the voltage drops correspondingly. This is known as a “resistive” system. This occurs because the increasing current flow causes the device to decrease the voltage at the gun tip. A so-called “stiff” system maintains the voltage until a preset current flow limit is reached. At this point the circuit is tripped off. Generally stiff systems are not approved for use with manual spray guns but are appropriate for automatic electrostatic spray operations. Other sophisticated protective devices measure how fast the change in current draw occurs and can shut off all current almost instantly, even though current draw has barely begun to rise.

Rotary Atomizers

This chapter deals with rotary atomization, which involves the breaking up of paint into atomized droplets using a spinning device. Instead of air or fluid pressure to atomize the paint, as with spray guns, rotary atomizers use centrifugal force. (Centrifugal force tends to project an object outward from the center of rotation.)

Some spray guns use electrostatic charging, and others do not. Rotary atomizers, however, always use electrostatic charging. With spray guns the electrostatic charging is not believed to have much effect on the actual atomizating; with rotary atomizers, electrostatic charging can play a key role in the atomizing.

Although all rotary atomizers are alike in that they feed paint to the center and hurl paint droplets off the perimeter of a spinning device, they differ in three principal ways:

• Shape of the atomizing device
• Rotational speed
• Mounting configuration

Shape of the device. Rotary atomizers are usually made from a quality steel and come in two basic shapes: disks and bells (cups). The disks are, as their name suggests, thin, relatively flat and round. They come in two diameter ranges, depending if their rotational speed is low or high. Low-speed disks have a diameter ranging from about 10 to 26 inches, and the high-speed disks, about 5 to 8 inches. The bells are not, however shaped like a bell. Their shape more closely resembles a cup, truncated cone or shallow sauce dish. The bells range in diameter from about 1 to 5 inches.

Rotational speed. The rate of rotation of low-speed disks is typically 900 to 8000 rpm.  Summing up the difference between the low- and high-speed rotary atomizers, the low-speed units tend to be rather large and rotate fairly slowly; the high-speed rotaries tend to be small and spin very fast.

Mounting Configuration

Disks are mounted in a horizontal plane to a vertical shaft and rotational drive that is generally attached to a reciprocating device to move the rotating disk slowly up and down. Parts to be painted by a rotary disk are hung from an overhead the disk perimeter. The reciprocating stroke distance is determined by the length conveyor that is looped around the disk, the parts being about 16 to 20 inches from of the parts to be painted. Extremely long vertically hung parts, such as extrusions, may incorporate floor- and ceiling-mounted disks, each with a stroke about half the length of the parts being painted. Very small parts may require no reciprocation.

Bells can be mounted in a vertical or horizontal plane, or at any angle in between. They can be mounted in fixed positions, onto vertically or horizontally operating reciprocators or onto robots. If a reciprocator is used with bells, the stroke is determined by the size of the part being painted.

Whereas a disk requires parts to be conveyed around it, the bells can function somewhat like a spray gun and be aimed at the part being painted. This allows bells to be used with conveyors much like spray guns. The typical distance from the bell to the part being painted is about the same as with disks.

Rotational Speed and Atomizational Fineness

As a general rule of thumb, the faster the rotational speed, the greater the centrifugal force and the finer the atomization. However, two exceptions stand out: rotational speeds below 2500 rpm and between about 5000 and 15,000 rpm.

Atomization efficiency occurring with low speed rotaries is due both to centrifugal force and electrostatic charge repulsion. The centrifugal force isn’t great enough to hurl the paint off the perimeter in finely atomized droplets, but only as coarse particles of paint. It is the electrostatic charge that atomizes the paint to a finer droplet size. This is termed “electrostatic atomization.’’ Without the electrostatic charging, atomization is not effective enough to use low speed rotation to apply coatings.

Atomization quality on low-solids paints with low-speed electrostatic disks and bells is intermediate between the extremely fine atomization of air spray and that of airless spray. The low-speed rotary atomization was used very successfully on low-solids coatings for some years before EPA regulations brought about the development of high-solids coatings.

Atomization occurring at from about 5000 to 15,000 rpm tends to be extremely poor because the centrifugal force hurling paint off the disk forms string-like filaments instead of discreet droplets. The filaments tend to intertangle, further preventing good atomization. Above 15,000 rpm, long filament formation ceases, atomization efficiency increases steadily and atomization is accomplished almost totally by centrifugal force. The function of the electrostatic charge at these high rotational speeds is mostly to place a charge on the atomized droplets so they will be attracted to the grounded parts to be painted.

High-solids and waterborne paints are difficult to atomize at slow rotational speeds because of their high surface tension. High-speed disks and bells are almost always operated at above 20,000 rpm to ensure a fine atomization. The particular operating speed is selected for the product being coated, the characteristics of the paint being applied and the rate at which paint is fed to the rotor.

Paint Application

The atomized paint leaving a rotating disk is hurled outward in a 360-degree pattern. The droplets are guided to the parts to be painted only by electrostatic attraction. No directional air is forthcoming from the disk vicinity to guide the atomized particles. With electrostatic attraction as the only propelling force, the air in disk booths must be relatively calm. Strong air currents would tend to carry the atomized paint droplets away from the part to be painted.

With the bells, directional air concentric with the bell is used to reduce the circular size of the atomized paint particle cloud, helping the electrostatic attraction guide the particles to the parts to be painted. The air, varying in pressure from about 10 to 40 psi, is directed through a circular groove or series of holes around the bell perimeter. The air forces the atomized particle cloud into a smaller “doughnut” of droplets whenever a reduced pattern of paint delivery is needed. With small parts the shaping air helps considerably to guide the atomized particles to the target to avoid paint waste. The shaping air does not assist in atomization but only controls the size of the delivery pattern.

 The disks and bells are insulated to accept the electrostatic charging voltage (about 100,000). As with electrostatic spray guns, the parts to be painted should be electrically grounded to establish maximum electrostatic attraction. The parts to be coated, being the nearest electrical ground, attract the electrostatically charged particles. The electrical circuit is completed as with electrostatic spray: The paint particles give up their charge to the grounded parts.

Rotary System Operation

Paint is delivered to the spinning surface of rotary atomizers via delivery holes around the periphery of a smaller concentric inner cup.

Retractable flush shouds can be used to collect color-change purges and solvent rinses on rotary bells. Collection shrouds for side-mounted bells are fitted with gravity drain lines; overhead-mounted bells use siphon tubes. Capture of paint and solvent during these operations reduces the plant’s overall VOC output and in many instances lowers maintenance costs significantly. Internal dump valves are used for line purges.

The rotational speed of high-speed bells slows when going from an unloaded condition to when paint is delivered. At a typical fluid flow of 10 ounces per minute, the reduction in spin speed is roughly 10%. For a fairly high fluid flow of 20 ounces per minute, the speed dropoff is about 20%. Depending on the paint being used and the particular application, the slowing could be a significant problem. The bell spin rate is set high enough so that the rotational speed reduction when paint is flowing is not a factor in atomization. Governors have been used to maintain rotational speed irrespective of fluid loading, but such devices are not commonly employed..

Bell spin speed will often affect atomization, pattern size and the distribution of metallic flakes. Rotational speed can affect color until a certain speed is reached, above which virtually no color difference is noticed.

Early disks and bells had no convenient fast method to monitor rotational speeds. Systems that permit digital rpm readouts of spin speed are now almost standard.

The spin rate of the bell can be continuously monitored in several ways. Two systems involve electronic speed regulation: one with fiber optic pickup and the other with magnetic impulse pickup. The latter type offers the advantages of extra durability and low cable replacement cost. A third method uses a rather simple dual-air-pressure system with a “high/low” setup. In this type two different air pressures are applied to the turbine. A high pressure is used when paint is being applied; a low pressure is used during nonpainting or bell “idle-speed” times. A dual-pressure system works well with stable fluid-flow rates. Electronic speed control is preferred for painting operations that require frequent on/off triggering, those demanding extremely fine finish quality or whenever fluid flow rates are varied.

Serrated (mechanically grooved) edges on some bells eliminate air entrapment “microbubbles” in the wet paint film. Entrapment of air is most likely to occur with high-surface-tension coatings, including both waterbornes or high-solids. Serrated edges enhance atomization so that bells can be operated at slightly reduced rotational spin rates. Each serration line forms a paint filament to enhance atomization.

The fineness of atomization from of a high-speed rotary depends on the paint viscosity, paint flow rate, rotational speed and electrostatic charging voltage.

Paint films applied by spray guns can take on a different shade of color from those applied by high-speed rotary atomizers. This is due to the difference in particle size and the velocities at which they are delivered to the part.

Hand-Held Bell

The bell rotary atomizers described so far in this chapter have wide use in applying finely atomized paint on industrial painting lines. These bell systems are mounted as permanent capital equipment; the parts to be painted are brought to them.

A portable bell rotary applicator is used, however, in office decorating painting. One widely used application is in repainting metal office furniture inside their office locations.

The portable unit has a rather long configuration but is surprisingly well balanced and easily maneuvered. The operator holds the unit perhaps 6 inches or so away from the part being painted, moving the unit slowly back and forth or up and down in even strokes, or in circular strokes of 4-6 inches diameter all over the part. The bell electrostatically charges the atomized paint particles.

The operator connects a grounding clip to the part being painted, which enables the part to attract the charged paint. Protective canvas or plastic sheets are positioned around other parts in the vicinity and the floor to catch any overspray. The painting is usually done after hours. Air dry paint allows the parts to be dry and ready for use the next day.

The bell can only apply up to 5-6 ounces of paint per minute. This rate of flow is satisfactory for furniture repainting but is inadequate for industrial production line painting.

 

 

Source: Roobol, Norman R., Industrial Painting. Principles and practices, Hitchcock Publishing Company, United States of America, p.89 - 169