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ELECTROSTATIC PHENOMENA

IN POWDER COATINGNew methods of improving Faraday-cage coating,

finish quality and uniformity, and recoating operations.

Sergey Guskov

Powder Systems Group

Nordson Corporation

Published by

Nordson Corporation

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This paper was presented at the Powder Coating 96 Conference in Indianapolis, Indiana, September 18, 1996. It is published in

the conference proceedings under the original title,Application of Powder Coatings: Old Problems, New Findings, New Developments,

and is covered by copyright protection.

Introduction

Over the past decade, worldwide popularity ofpowder coating technology has enjoyed steady,double-digit growth. One of the forces drivingthis tremendous success has been continuousimprovements in application equipment used in thepowder coating process.

Since the early days of powder coating, powder

coaters and application equipment manufacturershave faced several challenges, including:maximization of first-pass transfer efficiency;effective coating of Faraday-cage areas; betterfinish quality and uniformity; and recoating ofrejected parts. Recent technological developments,however, have allowed leading equipmentmanufacturers to offer users new equipmentfeatures that more closely meet these challenges.

An understanding of the electrostatic phenomenainvolved in the powder coating process is equallyimportant to equipment manufacturers and users. Asequipment becomes more and more sophisticatedto offer more capabilities previously unavailable,

it is important that powder coaters understandwhat features are worth the investment, in whichapplications, and why.

This paper focuses on corona-charging applicationsystems, and presents an overview of electrostaticprocesses utilized in powder coating technology inlight of new equipment features now available.

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Corona Development

In corona-charging systems, a sharply non-uniformelectric field is created between a gun and part byapplying high (usually negative) voltage potential toa pointed electrode. Non-uniformity of this field isimperative since the field lines converge on sharppoints and the density of field lines in any arearepresents the strength of the electric field.Therefore, if we apply a high-voltage potential to a

single-point electrode and position a larger-sizegrounded object before the electrode, we will createan electric field whose strength is greatest at the tipof the pointed electrode.

There are always free electrons or ions present in theair. If an electron passes through a strong electricfield, it will start moving in this field along the fieldlines and be accelerated by the field force. As theelectron accelerates along the field lines, it willultimately run into an air molecule (see Figure 1). Ifthe field strength is adequate and the electron hasgathered sufficient kinetic energy while travelingalong the field lines, its impact on the air molecule

will be strong enough to split that molecule to formtwo secondary electrons and one positive ion (theremainder of the molecule). Secondary electrons willinstantly be accelerated in the electric field. Movingalong the field lines, they will split new moleculesand create more ions and electrons.

Figure 1

Because opposite charges attract, positive ionsremaining from each split molecule will also beaccelerated by the field force and move alongthe field lines. Their motion, however, will be in theopposite direction toward the guns negativeelectrode. Once again, if the electric field issufficient, positive ions can either split moleculeson their way toward the negative electrode or, ifthey reach the electrode, impact it so hard that theywill split new ions from the metal surface of theelectrode.

1 Free ions are negative ions, produced by the corona ionization process,which do not get captured by powder particles and remain free in thespace between the gun and grounded part traveling towards the closestground along the field lines.

2 The distortion of an external electric field by an uncharged particle iscaused by the particles own field of polarization, a detailed discussion ofwhich exceeds the scope of this paper.

This process is corona discharge. It is self-sustaining at field strengths equal to, or greater than,some starting level. Immediately after the ionizationprocess begins, the space between the spray gunand grounded part becomes filled with millions ofions and free electrons. Henceforth in this paper,the term free electrons will be replaced with themore commonly used term free ions1.

Charging Powder ParticlesFigure 2 illustrates a powder particle in an electricfield. The uncharged dielectric particle will distort theexternal electric field so that some field lines willextend toward the particles surface, enter at a 90angle, pass through it, and exit at a 90 angle. If freeions are present in the external electric field, theywill follow field lines towards the uncharged particleand ultimately be captured by the particles field ofpolarization2, thus increasing the particles owncharge.

This process will continue until the chargeaccumulated on the particle, as a result of capturingmultiple ions, is sufficient to create the particlesown electric field. This field will again change thealignment of the external field lines. This time, theexternal field lines will be pushed away from theparticle (see Figure 3). When this happens, ions fromthe external field can no longer reach the particlebecause its own field repels them. In other words,the particle has reached maximum charge, given theexternal field strength, particle size, and material.

Figure 2

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Figure 3

Figure 4

Figure 5

In electrostatic powder coating, we spray powderthrough an area of strong electric field and high

free-ion concentration. Passing through this area,the particles are charged as discussed earlier.The process of powder particles charging in theelectric field of corona discharge is governed byPautheniers equation (see Figure 4). Charging ismost strongly affected by field strength, powderparticle size and shape, and the length of time theparticle spends in the charge area.

Powder Deposition and Layer Formation

Figure 5 illustrates forces affecting a chargedpowder particle as it travels from the spray gun tothe grounded part. It should be noted, however,

that the only force pushing the particle towards the

grounded substrate is the electric force equal to thecharge of the particle, multiplied by the strength ofthe electric field.

The air stream delivers a particle to the part.However, if the particle is not charged or the fieldstrength is not sufficient, the particle will bounce offthe metal substrate and either be carried away bythe air stream or fall down under gravity forces.The electric force helps the particle overcome

aerodynamic and gravity forces, and remain on theparts surface to allow yet another force to beestablished. This new force is the attraction betweenthe charged particle and grounded metal surface.

Most materials used for powder coatings are strongdielectrics. Once charged, they do not allow charge

to bleed off quickly. In fact, most materials usedfor powder coating retain their charge for at leastseveral hours, even if small particles of thematerial are placed on the grounded metalsurface. When a charged powder particleis positioned next to the metal surface,it induces a charge of equal value butopposite polarity inside the metal(see Figure 6). In simple terms, this resultsbecause the conduction electrons insidethe metal vacate the area close to thepoint of contact of the powder particleand metal surface. As electrons move out,what remains is the area with an excess

positive charge equal in value to thenegative charge on the powder particle.This positive charge is commonly calledmirror charge.

As soon as the mirror charge is inducedinside the metal, two charges of equalvalue and opposite polarity exist next toeach other, separated by the metal surface.These two charges will not only attracteach other and hold the powder particle tothe metal surface, but also create anotherelectric field between themselves.

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The larger the powder particle on the metal surfaceand higher its charge, the stronger the electric fieldis between the particle and its mirror image. Thus,the stronger the electrostatic attraction is betweenthem (see Figure 6). The fact that larger particlesexperience a stronger attraction to the groundedmetal provides one explanation as to why we aremore likely to observe orange peel effect on thickerlayers of powder coatings.

smaller particles may not be sufficient to create anattraction force strong enough to retain the particleson top of the already deposited powder coatinglayer.

Larger powder particles usually accumulate strongercharges and, therefore, the attraction force betweenthem and their induced mirror reflection is alsostronger. As a result, larger powder particles aremore likely to be deposited on top of the existinguncured coating. If one were to look at a cross-sectional view of uncured powder coating layer, thebottom portion (closer to the metal) would likely havea smaller average particle size than the top portion.

If a powder coating material does not flow wellduring the curing process, the larger particlescomprising the upper coating layer may not flowout completely and will retain some of the surfaceprofile of an uncured coating layer. This will result inlower-gloss, bumpy finishes and orange peel due toinsufficient flow properties of coating material.

Back Ionization, Finish Quality

and Transfer Efficiency

We have analyzed the process of powder particlesdeposition on a grounded metal surface. If wecontinue spraying charged powder on the samesurface, back ionization will ultimately occur(see Figures 7a, 7b, and 7c).

When charged powder coating is applied to a metalsurface, the strength of the electric field insidethe layer of powder coating increases. Every newpowder particle deposited increases: 1) cumulativecharge of the powder coating layer; 2) cumulativemirror charge inside the metal; and 3) strength of theelectric field inside the layer of powder coating.

As we continue applying charged powder, thestrength of the electric field inside the powdercoating layer ultimately becomes sufficient to ionizeair trapped between powder particles. The ionizationprocess inside the coating layer is very similar to theone that develops around the guns electrode whenhigh voltage is applied to it. Stray electrons, presentin the air, accelerate in the electric field, split airmolecules, and generate a great number of negative

electrons and positive ions. Becauseopposite charges attract, negative electronsrush toward the relatively positive ground,and positive ions try to escape from the

coating layer toward the guns negativeelectrode. As a result of this intensive flowof electrons and ions, streamers developthrough the layer of powder coating.

A streamer can be viewed as miniaturelightning or a spark shooting through thepowder coating layer. Inside a streamernumerous electrons and positive ionstraveling in opposite directions. Oncedeveloped, streamers can be seen throughimage intensifying equipment as glowingdots on the surface of powder coating.

Figure 7

Figure 6

After the initial layer of powder coating is depositedon a metal surface, the particles of subsequentlayers have to induce mirror image charges over thealready existing layer of dielectric material to bedeposited. The presence of the existing layer ofdielectric powder coating dampens the inductionprocess (since there is no direct contact of powderparticles with the metal surface). Lower charges of

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The process of streamer development through thelayer of powder coating is virtually identical to coro-na ionization around the high-voltage electrode of aspray gun. Thus, it is commonly referred to as backionization.

Back ionization is another common cause of orangepeel effect on the surface of powder coatings. Italso is the driving force behind what is often calledthe self-limiting property of the powder coatingprocess, because it greatly reduces transferefficiency.

As positive ions produced by back ionization insidethe powder coating layer move out of the coatinglayer, they neutralize the charge of powder particlesadjacent to the streamer channel. Active, directedmotion of positive ions along streamer channels alsoengages air molecules, resulting in a phenomenoncalled electric wind. Electric wind rips powderparticles that have been neutralized by positive ionsoff the powder coating layer. This action createsmicro craters that can easily be seen on the surfaceof uncured powder coating in the form of starring.

If powder coating material does not flow wellduring the curing process, craters formed by backionization will not flow over completely, resulting ina wavy surface appearance of cured powdercoating.

Another important effect of back ionization is illus-trated in Figure 7c. When positive ions find their wayoutside the powder coating layer, they becomeattracted to negatively charged powder particleswhich are continuously arriving on the surface of thegrounded part. The collisions of positive ions andnegatively charged powder particles result in powderparticles losing their charge and, therefore, the ability

for deposition. As back ionization develops, theability to continue building the layer of powdercoating is significantly diminished by the presence ofpositive ions in front of the grounded part. Transferefficiency of the powder application declinesdramatically with the onset of back ionization.

During the analysis of corona development at the tipof the gun, it was established that the space betweenthe gun and part becomes filled with billions of freeions which, together with charged powder particles,form the space charge. Until now, our analysis ofpowder deposition and back ionization developmenthas ignored the presence of free ions in anyconventional corona-charging powder coatingapplication. Lets analyze their effect.

We can compare the build-up of cumulative chargeon the powder coating and ultimate development ofback ionization to filling a bucket with water. If youtake a bucket with a small hole in the bottom and tryto fill it with water from a faucet, it will take sometime for the bucket to overflow. In this analogy, thestream of water from the faucet represents thestream of charged powder particles building apowder coating layer, water in the bucket is thecharge accumulating on this layer, and water leaking

Figure 8

through the hole in the bottom of the bucket isthe small amount of charge that can bleed off thecoating. The overflowing bucket represents theonset of back ionization.

Following the same analogy, the presence of freeions in the space between the gun and part can berepresented by adding a fire hose to fill the bucket.Just as the bucket would overflow almost instantlywith the addition of a large hose, back ionization onthe layer of powder coating develops much fasterwhen free ions are present in the powder coatingprocess.

Free ions are attracted to, and travel toward, thegrounded part along electric field lines. As long asthe parts surface is not covered with a layer ofdielectric powder coating, free ions arrive on themetal surface and flow off to the ground. However, ifthe metal substrate already has a layer of powdercoating, this layer partially insulates the metalsurface, restricting the flow of the charge deliveredby free ions to the ground. The charge that doesnot bleed off to ground dramatically increases the

cumulative charge of the coating layer, resulting inrapid development of back ionization, significantreduction in powder transfer efficiency, anddeterioration of finish quality and uniformity.

Faraday-Cage Effect

Lets look at what occurs in the space between thegun and part during the electrostatic powder coatingprocess. In Figure 8, the high-voltage potential appliedto the tip of the guns charging electrode creates anelectric field (shown by red lines) between the gun andgrounded part. This leads to the development ofcorona discharge. A great number of free ions

generated by the corona discharge fills the spacebetween the gun and part. Some of the ions arecaptured by powder particles, resulting in theparticles being charged. However, multiple ionsremain free and travel along the electric field linestoward the grounded metal part, mixing with powderparticles propelled by the air stream.

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As stated earlier, a cloud of charged powderparticles and free ions created in the space betweenthe gun and part has some cumulative potentialcalled space charge. Much like a thunder cloudcreating an electric field between itself and the earth(which ultimately leads to lightning development),a cloud of charged powder particles and free ionscreates an electric field between itself and agrounded part. Therefore, in a conventional

corona-charging system, the electric field in closevicinity to the parts surface is comprised of fieldscreated by the guns charging electrode and thespace charge. The combination of these two fieldsfacilitates powder deposition on the groundedsubstrate, resulting in high transfer efficiencies.

Positive effects of the strong electric fields createdby conventional corona-charging systems are mostpronounced when coating parts with large, flatsurfaces at high conveyor speeds. Unfortunately,stronger electric fields of corona-charging systemscan have negative effects in some applications. Forexample, when coating parts with deep recessesand channels, one encounters Faraday-cage effect(see Figure 9).

When a part has a recess or a channel on itssurface, the electric field will follow the path of thelowest resistivity to ground (i.e. the edges of such arecess). Therefore, with most of the electric field(from both the gun and space charge) concentratingon the edges of a channel, powder deposition will begreatly enhanced in these areas and the powdercoating layer will build up very rapidly.

Figure 9

Figure 10

Unfortunately, two negative effects will accompanythis process. First, fewer particles have a chance togo inside the recess since powder particles arestrongly pushed by the electric field towards theedges of Faraday cage. Second, free ions generatedby the corona discharge will follow field lines towardthe edges, quickly saturate the existing coating withextra charge, and lead to very rapid development ofback ionization.

It has been established earlier that for powderparticles to overcome aerodynamic and gravity

forces and be deposited on the substrate, there hasto be a sufficiently strong electric field to assist in theprocess. In Figure 9, it is clear that neither the fieldcreated by the guns electrode, nor the field of spacecharge between the gun and the part penetrateinside the Faraday cage. Therefore, the only sourceof assistance in coating the insides of recessedareas is the field created by the space charge ofpowder particles delivered by the air stream inside

the recess (see Figure 10).If a channel or recess is narrow, back ionizationrapidly developing on its edges will generate positiveions which will reduce the charge of powderparticles trying to pass between the Faraday-cageedges to deposit themselves inside the channel.Once this occurs, even if we continue sprayingpowder at the channel, the cumulative space charge

of powder particles delivered inside the channel bythe air stream will not be sufficient to create a strongenough electric force to overcome the air turbulenceand deposit the powder.

Therefore, the configuration of the electric field and itsconcentration on the edges of Faraday-cage areas isnot the only problem when coating recessed areas. Ifit were it would only be necessary to spray a recessfor a sufficient length of time. We would expect thatonce the edges are coated with a thick layer ofpowder, other particles would be unable to depositthere, with the only logical place for powder to gobeing the inside of the recess. Unfortunately this doesnot happen due, in part, to back ionization. There aremany examples of Faraday-cage areas which cannotbe coated regardless of how long powder is sprayed.In some cases, this happens because of the geometryof the recess and problems with air turbulence, butoften times it is due to back ionization.

Back Ionization and Recoating of Parts

The phenomena of back ionization complicates theprocess of powder coating with conventionalcorona-charging equipment by: 1) reducing transferefficiency; and 2) restricting the ability to effectively

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coat Faraday-cage areas. Free ions are the majorcause of back ionization in corona applications and,therefore, the excessive number of free ions is thereason for the above mentioned problems.

Recoating parts is another common in the powderapplication challenge where free ions have anegative impact. In this case, we try to apply asecond layer of coating on top of the cured one.The difficulty arises because free ions generated bycorona discharge travel between the gun and partwith velocities much greater than those of powderparticles. Free ions very quickly make their way tothe part and increase the charge of the layer of theexisting cured coating. A cured coating is a muchbetter dielectric than an uncured one. So, the chargedelivered by free ions to the surface of a coated parthas no way to bleed off.

By the time the powder particles arrive at the surfaceof the part being recoated, the existing coatingalready has a lot of charge on it. Arriving powderparticles and additional free ions rapidly drive up thecumulative charge on the coating layer almost

instantaneously, resulting in back ionization. In fact,back ionization may already exist on the surface ofthe part before the first powder particles arrive there.

As has already been shown, once back ionizationdevelops, transfer efficiency declines quitedramatically. Thats why we often experiencedifficulties when trying to recoat parts.

A traditional method of facilitating recoatingoperations and improving penetration of Faraday-cage areas is to turn down the gun voltage.Reduction in the gun voltage reduces: 1) strengthof the electric field in the vicinity of the partssurface; and 2) gun current.

Reduction of the field strength in the vicinity of theparts surface results in easier Faraday-cagepenetration because the electric force pushingpowder particles toward the edges of recessedareas becomes weaker. A lower gun currenttranslates into a lower number of free ions in thespace between the gun and part. This delays thedevelopment of back ionization and leads to easierrecoating operations and Faraday-cage penetration,thicker film builds and better finish quality.

Unfortunately, manually turning down gun voltage isnot always an acceptable solution. For example, itwould not be an easy task in automatic applications.

Additionally, how low should gun voltage be toaccomplish our coating goals and still optimizeprocess efficiency?

Difficulties associated with manual adjustment ofgun voltage have led to the development of moreadvanced techniques of combating back ionizationand achieving greater uniformity and quality offinishes. These techniques are: 1) automatic controlof the gun current; and 2) free-ion collecting devices.Both techniques allow coaters to improve theirfinishing operations by eliminating or reducing strayion current from the gun to the part.

Figure 11

3Although the curves in Figure 12 are not the exact graphs constructed asa result of a series of experiments, they very closely represent the actualcurves.

Automatic Current Control

The principle of operation of automatic current control(ACC) is the automatic adjustment of gun voltage tomaintain the gun current and field strength betweenthe gun and part at some optimum level. To betterunderstand why ACC brings improvements to thepowder coating process it is important to comprehendOhms law ( U = I x R) and the concept of load line ofpowder application equipment.

The load line is the relationship between the guncurrent and actual voltage at the tip of the gunselectrode. A load line of a conventional corona gun isshown in Figure 11. The closer the distance betweenthe gun and part, the higher the current flowingthrough the gun and the space between the gun andpart. What is important in our discussion is that as wemove the gun closer to the part, the resistivity of thespace between the gun and part becomes lower andthe gun current higher. Given that the gun currentdirectly translates into the number of free ionsgenerated by the corona discharge, the number of freeions flowing to the part for a gun-to-part distance of

3 inches will be significantly greater than the numberof ions flowing to the part at a gun-to-part distanceof 10 inches.

Figure 12 shows a family of curves3 approximatelyrepresenting the relationship between the transferefficiency of the powder coating process and guncurrent for a fixed powder flow rate and conveyorspeed, and gun-to-part distances of 3, 6, and12 inches. It is clear from the graph that for gun-to-part distances of 6 to 12 inches, maximum transferefficiency is achieved at some current level. At the3-inch distance between the gun and part, the

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maximum transfer efficiency was reached at a lowercurrent setting. This difference can be easily explainedusing, once again, the example of a bucket of water.

If we take an empty bucket and try to fill it from awater hose positioned at a distance of 10 feet, thewater spray coming out of the hose would fan out andonly a portion of the water would get into the bucket.This way, it would take us some time to overfill thebucket. If we now try to fill the same bucket from thehose positioned three feet away, a much greaterportion of sprayed water would get in and we wouldoverfill the bucket more quickly.

to the free ion flow for a certain period of time(depending on conveyor speed). Maximum applicationefficiency is achieved if we apply powder at thehighest transfer efficiency possible throughout theentire time. If the current is too high, we may reachback ionization so quickly that only a part of theallotted coating time is used with maximum efficiency.

Results of experiments show that for a broad range ofgun-to-part distances there is some current level thatprovides for maximum transfer efficiency. Therefore, ifwe had a tool that would allow us to automaticallymaintain gun current at this optimum level (regardlessof the gun-to-part distance), we would optimize overallefficiency of the powder coating process. This tool isautomatic current control (ACC).

With ACC, as the gun-to-part distance changes (eitherdue to the complex part shapes or parts of differentdepths passing before the gun), the gun control unitautomatically adjusts voltage up or down to maintaingun current at a preset level, which is optimum for agiven operation.

For example, when a manual operator has to touch-upsome part with a difficult recess on its surface, theoperator almost inevitably will move the gun closer tothe part in an attempt to push powder inside theFaraday-cage area. Without ACC, this would lead toan increase in the gun current, higher density offree ion stream per unit of the parts surface, andfaster development of back ionization. ACC willautomatically reduce gun voltage as the gun ismoved closer to the part. As a result, ACC: 1) keepsthe current at an optimum level to prevent generationof an excessive number of free ions; and 2) controlsfield strength in the vicinity of the parts surface, andfacilitates penetration of Faraday-cage areas by

reducing voltage at the tip of the gun in proportion tothe reduction in gun-to-part distance.

Figure 13 shows two conventional corona guns coat-ing different areas of the same part. Because of theparts profile, one gun is spraying from a distance of6 inches and another from 10 inches. On the samefigure, one can also see the load lines and currentlevel for each gun. The gun spraying at a 6-inchdistance draws more than double the current ofthe gun spraying at 10 inches. This results in: 1) asignificantly greater number of free ions in the spacebetween the gun and part; 2) faster developmentof back ionization; and 3) lower transfer efficiency,film thickness and finish quality.

Figure 14 shows the same two guns spraying thesame part in the ACC mode. Notice that the currentlevels and number of free ions in the space betweenthe gun and part will be equal for both guns. Thisresults in optimum transfer efficiency and consistentfinish quality and uniformity.

To summarize, automatic control of the gun currentdelays the development of back ionization and:1) optimizes the powder coating process bycontrolling the number of free ions generated at thetip of the gun and the field strength at the parts

Figure 12

Using the analogy in which the overfilling bucketrepresents back ionization on the layer of powdercoating, we can say that when we apply powdercoating from a gun positioned 10 inches from thepart, field lines are distributed over a large area ofthe part, and the density of free-ion current per unitof the parts surface is low. Therefore, it takes longerto start back ionization. If we move the gun 3 inchesto the part, two things happen using a conventionalcorona gun: 1) free ions will flow through a narrowerchannel causing a higher current density per unit ofthe parts surface; and 2) a closer gun-to-part

distance will result in a higher current level which,following our analogy, would be equal to increasingthe volume of water flowing through the hose.Therefore, at a 3-inch gun-to-part distance,conditions are such that even a gun current whichis optimal for 6 inches, is enough to cause rapiddevelopment of back ionization and lower transferefficiency.

A good way to envision why overall transfer efficiencyis lower at a higher current level is to imagine thateach square inch of the parts surface gets exposed

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surface; 2) results in the maximum attainabletransfer efficiency over a broad range of gun-to-partdistances; 3) facilitates penetration of recessedareas; 4) improves finish quality and uniformity; and5) enhances recoating operations.

Although one current setting still may not beoptimum for all applications, modern PLC processcontrollers further enhance the efficiency of powdercoating operations by allowing automatic adjustmentof the current settings depending on the powderflow rate, conveyor speed and other operating

parameters.

Free-Ion Collecting Device

The principle behind the operation of ion collecting(IC) device is that it extracts free ions out of thespace between the gun and a part, and draws themto a grounded collector electrode positioned behindthe tip of the gun (see Figure 15).

When a conventional corona gun is equipped withan ion collector, a grounded electrode is positionedbehind the end of the gun at a distance shorter than

Figure 14

Figure 15

Figure 13 the one between the gun and part.The fact that a grounded IC deviceis closer to the tip of the gun thanthe surface of the part implies thatthe electric field, following theshortest path to the ground, willdevelop between the gunselectrode and ion collector, andnot between the gun and part. As

a result, the electric field in thevicinity of the parts surface will becreated only by the space chargeof the cloud of charged powderparticles. This field will be weakerthan what is achieved with aconventional corona gun becausewe eliminate (or greatly reduce theeffect of) the high voltage at the tipof the gun. However, if the powderis well charged, transfer efficiencywill not suffer while the ability topenetrate recessed areas will begreatly enhanced.

Since the electric field created bythe gun no longer goes to the part,free ions generated by coronadischarge will follow the field linesto the grounded ion collector. Thismeans that, depending on how theion collector is set up, there will beeither virtually no free ions in thespace between the gun and part,or their number will be greatlyreduced.

The ability to adjust the distancebetween the tip of the gun and the

ion collecting device is veryimportant. The easiest rule of

thumb to follow in setting a free-ion collecting deviceis to place it behind the tip of the gun at no morethan half the distance between the gun and part.Following this rule ensures that most of the electricfield and free ions created by the guns electrode go

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produces a good blending of ions with powderparticles when an ion collector is set up properly.In addition, closer proximity of the grounded ioncollector to the charging electrode (compared tothe part) results in higher gun current level, greaternumber of ions, and greater likelihood of eachpowder particle being impacted by them. However,the time powder particles spend within the zonedensely populated by ions is reduced.

Figure 16 illustrates that the size of the denselypacked ions area around the charging electrode getssmaller when the ion collector is moved closer to thegun. In other words, at very close distances the sizeof the charge zone will be so small that the timepowder particles spend passing through it will notbe sufficient for optimum powder charging. This willresult in lower transfer efficiency.

Based on the arguments presented here, the useof free ion collectors may not be very effective atgun-to-part distances of less than 4 to 5 inches. Ingeneral, ion collecting devices are practical andhighly effective when gun-to-part distance is

relatively fixed and at least 5 inches.

The use of ion collecting device on manual guns isalso limited due to human nature. Because of thefact that manual operators usually move the guncloser to the part when trying to coat recessedareas, the likelihood of the gun-to-part distancebecoming shorter than the distance to the ioncollector is very high. This leads to a great reductionor even cancellation of any positive effect of the ioncollector. If the gun-to-part distance varies rathersignificantly either from part-to-part or within onepart, automatic control of the gun current mayrepresent a more effective and easy to use tool

because of its automation.

Conclusion

In this paper we have established that the excessivenumber of free ions generated by conventionalcorona charging equipment is the reason behindsuch powder coating challenges as Faraday-cagepenetration, recoating of rejects, and improvementof finish quality and uniformity. With the increasingdemands for greater coating efficiency and finishquality, powder coating equipment supplierscontinue advancing the technology by offering tothe market new equipment features. Unfortunately,a single solution to all challenges that would work

in every type of application has yet to be found.

Although automatic current control and ion collectingdevices provide finishers with powerful tools tooptimize their powder coating operations, suchoptimization can only be realized if the proper tool isused for the right application. Knowledge of thebasics of electrostatic technology will help finishersmake the correct decision about which features ofequipment will bring them closest to meeting theiroptimization goals.

to the ion collector. If the ion collector is properlyset up, it will often deliver impressive results inimproving Faraday-cage penetration, and finishquality and uniformity. Multiple field installationsalso report dramatic improvements in the ease ofrecoating operations.

In many applications the use of ion collectingdevices may be even more effective in reducingback ionization and coating recesses than the use of

ACC control. However, caution should be exercisedwhen deciding to use ion collectors since the rangeof gun-to-part distances at which ion collectors canbe used with maximum effectiveness is limited. Forthe maximum effectiveness in collecting free ions,the ion collector should be positioned behind the tipof the gun at no more than half the distance betweenthe gun and part.

Therefore, if the gun-to-part distance is 8 inches, theion collector will not only be effective in collecting freeions but also produce maximum transfer efficiencyif positioned at about 4 inches behind the gunselectrode (see Figure 16). For 10 inches between the

gun and part, such performance will be achieved at4 to 5 inches to the ion collector. But, if we are spray-ing parts with the gun-to-part distance of 4 inches,the ion collector would have to be placed only2 inches behind the end of the gun. Unfortunately, atsuch close distances between the charging electrodeand ion collector device, transfer efficiency of theapplication process is likely to suffer.

Reduction in transfer efficiency is likely to occur withion collectors placed too close to the tip of the gunbecause of changes in the size of the charge zone.In conventional corona applications, powderparticles and free ions travel in the same direction

from the gun to the part. This extends the timeduring which powder particles can be charged byions. With an ion collecting device in use, powderparticles travel toward the part while ions go backto the collector. This motion in opposite directions

Figure 16

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