coating materials and application methods

POWDER COATING EQUIPMENT

BY NICHOLAS P. LIBERTO

POWDER COATING CONSULTANTS DIV. OF NINAN INC., BRIDGEPORT, CONN.

There are many ways to apply powder coating materials; however, the materialthat is to be applied must be of a compatible type. For instance, if the applicationmethod is fluidized bed, the powder coating material must be a fluidized bedgrade. Conversely, if the method of application is electrostatic spray, the powdermaterial must be an electrostatic spray grade.Once the material is correctly selected, the application method is chosen bypart design and production goals. There are two forms of application methods:fluidized bed application and spray application. These vary as widely as theapplications they suit.

FLUIDIZED BED

This application method was the first one used to apply powder coating materials.It is still used on many applications where the cured-film thickness is above5.0 mils. Typical items are wire products, electrical bus bars, etc.The fluidized bed application method can be performed in two ways. One wayis the nonelectrostatic fluidized bed. This process requires preheating the partso that powder will melt and adhere to it. The hot part is placed into a fluidizedbed of powder for coating. The amount of powder that is applied to the partis a function of how hot the part is and how long it is in the bed. It should beobvious that tight film-thickness control is not of primary concern when thismethod is used, as the total coating thickness often exceeds 10 mils.To gain more control of film thickness on the part, with a fluidized bed system,the principles of electrostatics are introduced. As shown in Fig. 1, the part is transported above the fluidized bed and the powder is attracted to it. The partrequires no preheating prior to being placed above the bed. Powder is attractedto the part by an electrostatic charge on the powder particle. This electrostaticcharge is developed in an electrostatic field either above or in the fluidized bed.Film thickness on the part now is controlled within tighter tolerances notonly by the amount of time the part is in the fluidized bed but also accordingto how much electrostatic charge is on the powder particle. Sometimes, heatstill is used in this process to overcome Faraday cage problems caused by partconfiguration. This process routinely applies powder from 5 to 10 mils thick.Electrostatic fluid bed application is used for coating electrical motor armatures.These require a high dielectric strength coating with close film-thicknesscontrol to allow the wire to be wound properly.

SPRAY APPLICATION

Applying powder coating with electrostatic spray equipment is broken downinto two types. In each case electrostatics must be used to attract powder to thepart. There is no mechanical attraction or adhesion to hold powder to the partas seen in liquid spray systems. The two types of electrostatic spray equipmentare corona-charged spray guns and tribo-charged spray guns.

Corona Guns

This device uses an electrostatic generator to create an electrostatic field betweenthe gun and a grounded part. Powder is sprayed through the field, picks up anelectrostatic charge, and is attracted to the part. The amount of charge thatis transferred on the surface of the powder is a function of electrostatic fieldstrength and the amount of time the powder particle is in the field. Also ofimportance is the surface area of the powder particle, as finer powder particleshold less electrostatic charge. The following equations (see Fig. 2) best explainhow the powder is charged:

Field Strength: E=V/d

Charge on Particle: Q= 1/2 CEt2

Notice that some factors are more important that others. For instance, electrostaticfield strength is directly proportional to applicator electrode voltage.Also, the distance between the part and the applicator (sometimes called thetarget distance) will directly affect electrostatic field strength.The charge on the powder particle (which causes the attraction) is most affectedby the amount of time the particle is in the field (by its square). The time andfield strength will determine how much powder is attracted to the part (i.e., firstpass transfer efficiency). The time the powder particle is within the electrostaticfield is most easily controlled by adjusting the velocity of the powder pumpedthrough the gun, or applicator, and reducing the speed of the applicator motion.It is a known fact that systems that use reduced powder velocity and slow gunmotion will provide the best coating efficiency with the least effort.The powder coating process is most often used to apply a charged dielectricmaterial (powder coating) and onto a conductive (grounded) part. However,electrostatic powder coating on nonconductive materials (i.e. plastics, rubber,glass, etc.) can be performed using a conductive primer or aiding powder attractionby heating the surface to be coated. Additionally, electrostatic charging ofconductive materials (i.e., blended metallic powders) can be difficult since theycan short-circuit theapplicator’s charging circuit. However, most equipmentmanufacturers provide electrodes outside the powder path to overcome thisproblem.Both positive and negative polarity electrostatic guns are available frommost manufacturers to provide efficient charging of widely divergent coatingmaterials. It is worth noting that 98% of all applicators used in powder coatingoperations are negative polarity devices.Code requirements insist that certain protection circuits be part of the system.Among these are current limitation to control arcing and grounding of all equipmentand products that are coated to dissipate stored charges. System interlocksare required for automaticequipment. Guidelines for this equipment are listedin National Fire Protection Association Code (NFPA) 33.

Tribo Guns

Tribo-charged spray equipment uses the principle of frictional electrostaticcharging. This type of charging is best explained by the following analogy: Whenyou shuffle your shoes on a carpet in the winter, you create an electrostaticcharge that is stored in your body. This charge is usually dissipated when youcome into contact with a ground, such as a light switch. This phenomenon willonly occur in a dry (not humid) environment. This is why we are not botheredby static electricity in the humid summertime, but only in the dry air of winter.Tribo-charge spray equipment will direct the powder stream through a paththat it will tumble and rub against a dielectric surface within the applicator,yielding a frictional electrostatic charge on the powder particle. This path isaccomplished by lengthening the powder route through the spray equipment ineither a straight, radial, or oscillating path. The amount of electrostatic chargethat builds up on the surface of the powder particle is a function of several variables,including (1) the amount of time the powder particle is subjected to thefrictional charging apparatus; (2) surface area of the powder particle; (3) drynessof air the powder is transported with or comes into contact with; and (4) thetype of resin material from which the powder is made Controlling these variables is important to assure that the powder particle willbe properly charged. Remember: if the powder is not charged, it will not adhereto the part unless the part is hot enough for the powder to stick on contact.The amount of electrostatic charge that typically is developed by this apparatusis less than that produced by corona equipment. The polarity of the tribocharge is a function of the material being sprayed and the material that it isrubbed against. If the same two materials are used, the polarity will always bethe same.Tribo-charge applicators can often be used to overcome Faraday areas ondifficult-to-coat parts, as there is no electrostatic field used to charge the powder.This flexibility, however, is often overshadowed by the additional processand coating materials controls that are required to ensure successful coating.

Powder Bells

This device uses an air turbine to rotate a conical cup used to atomize thepowder coating. Powder is pumped to the cup where the rotational forces causecomplete powder atomization. The feed system used to support this device issimilar to that of spray guns. These devices employ the corona charging method,described earlier in this article.Powder bells are capable of dispersing a large quantity of powder coatingover a large area. Therefore, the typical applications for this device are large flatcomponents, such as appliances and automobile bodies.

POWDER DELIVERY

All spray application equipment requires a delivery system (see Fig. 3). This. delivery system consists of a feed hopper, a powder pump, and a powder feed hose.The feed hopper can be one of two types (see Fig. 4). The first type is called agravity feed hopper. As the name suggests, this feed hopper uses gravity to movepowder to the powder pump located at the bottom. This hopper usually is conicalin shape to funnel powder to the pump. Sometimes a mechanical stirrer orvibratory assist is used to maintain an even powder flow. Frequently, without amechanical assist, powder will bridge across the bottom of the funnel causinguneven feed to the pump. Since there is no air mixed with the powder in the hopper,this device is often employed when spraying blended metallic powders thatcan be stratified within a fluidized hopper.The second type of powder feed hopper uses a fluidized bed. It is the same as thefluidized bed system described previously. A compressed-air supply is connectedto the plenum chamber below the fluidizing plate. The fluidizing plate causes theair to fluff powder in the hopper to a state resembling water. Now the powder canbe drawn out by the powder pump. Since powder is mixed with compressed airfrom the plenum, the powder within this device is very homogeneous in nature.Powder pumps are mounted on the hopper and are connected to a pick-up tubeto draw powder out of the hopper. These pick-up tubes usually are positioned anappropriate distance into the fluidized bed to assure that the turbulence usuallypresent on the surface of the fluidized plate is not drawn up into the powderpump. This turbulence can cause inconsistent powder feed to the applicators.

Box Feeders

Powder equipment manufacturers also provide methods of pumping powdercoatings directly from their shipping containers (box or bag) to the spray gun.This method is called the box feeder and utilizes a tilted vibrating table to supportthe box of powder. A powder pump connected to a pick-up tube is insertedin the lowest portion of the box. A compressed air jet is employed at the end of this tube to assistpowder flow into the tube. Powder is then pumped directlyfrom the box to the spray gun without the need of a feed hopper. This approachmakes color change cleanup quick and easy, as only the pick-up tube, pump,and hose need to be cleaned. Changing the powder box completes the colorchange task.

PUMPS

Most powder pumps are designed to work by the venturi principle. Compressedair is directed perpendicular to the venturi pickup, causing a differential inpressure, or vacuum, that siphons powder out of the feed hopper or box feeder.When the powder enters the compressed air stream, it is pushed through thepowder hose toward the applicator. An additional compressed air supply isintroduced at the point where the powder enters this air stream (see Fig. 5) todilute the powder and increase its velocity. Increasing powder velocity ensuresthat the powder stays within the air stream as it proceeds through the hose,reducing surging or pulsing problems. Surging occurs when the powder lays atthe bottom of the hose until enough air pressure builds behind it to push it outwith a burst. Both air supplies have check valves to force the air to go throughthe powder hose, allowing independent control of both powder quantity andspeed through the feed hose.Powder hose can be made from several materials, including urethane, vinyl,and certain rubber compounds. Hose diameter and length are critical. Diameteris dictated by the powder pump used; it always should match the manufacturer’srecommendation. Length always should be as short as possible to reduce backpressure to the powder pump. This reduces surging of the powder stream to thegun. Avoid bends and kinks in the hose routing.The more powder you pump using venturi style pumps the faster it travelsthrough the electrostatic field. Consequently, transfer efficiency will be lowerat higher feed rates. Applications requiring highly controlled powder flow ata wide range of output rates use high density - low pressure (HDLP) powder pumps. These devices deliver a column of powder to the applicator withouthaving to mix it with compressed air. Reducing the compressed air within thepowder stream decreases the velocity of the powder delivered from the applicator,slowing powder speed, increasing powder density, and eliminating aerodynamicissues that may cause coating difficulties on box-shaped parts. Since thesepumps employ significantly smaller diameter feed hose, the hose is much easierto clean with compressed air purging, making these pump the preferred choicefor “fast color change” systems.

GUN MOTION

Automatic spray devices are often accompanied by some ancillary equipmentused to produce spray gun motion. Gun-motion equipment can be broken downinto four general categories: oscillators, reciprocators, multi-axis machines androbots.Each of these gun-motion systems has a different design and is used to fill aspecific coating requirement; however, all have one common feature. They aredesigned to move the spray gun(s) in one or more planes to coat a larger areathan a fixed spray gun. Thus, the number of spray guns required to coat a givenarea can be reduced. This makes for a more efficient and economical systemdesign

Oscillators & Wagglers

One type of gun-motion device is called an oscillator. This design is differentfrom other movers in that it usually has a fixed stroke and speed. Some unitshave adjustment of these parameters, but they cannot be used while the machineis running. The main component of this type of equipment is an eccentric wheeland lever as shown in Fig. 6. The motor rotates the eccentric wheel. The lever,which is attached to the wheel at some distance from the center, will translatethis rotary motion to a vertical motion.Stroke length is determined by the position at which the lever is attached tothe eccentric wheel and by the diameter of the wheel itself. It can be adjusted bylocating the lever at different points on the wheel radius. Speed is dictated bythe motor and gear reducer used in the design. Sometimes, there are clutchesand adjustable belt sheaves that will provide some speed adjustment; however,neither speed nor stroke adjustment can be changed while the unit is running.Wagglers (radial oscillators) pivot the gun through an arc, where straightoscillators provide vertical gun motion in a straight line. Gun-to-part targetdistance is affected with radial oscillators, while straight oscillators will nothave this problem.

Reciprocators

Reciprocators (see Fig. 6) use a variety of electronics to control both stroke andspeed. In these machines, the mechanical linkage between the motor and gunsis fixed; therefore, speed and stroke control must be adjusted electrically. Theseadjustments are sometimes made at the control panel and sometimes at theunit itself. For instance, stroke adjustment can be made by moving electricallimit switches in the unit or by adjusting an electronic feedback loop variablein the control panel.Speed control is accomplished by a variety of methods depending upon thetype of motor used. For instance, those designs that use a DC motor will providespeed control by varying voltage to the motor. Reciprocators that use AC motorshave variable speed-control circuits to adjust speed. Both types allow adjustmentduring operation. This offers some flexibility over the oscillator design whendifferent stroke lengths and speeds are required to coat different parts duringthe production cycle.

Multi-Axis Machines

Both oscillators and reciprocators provide movement in one plane only. Multiaxismachines were developed to provide increased coating flexibility and meeta demand for total automation. Multi-axis machines have been successful ineliminating some or all of the manual touch-up necessary on some products.Though costly, this increased automation often will pay for itself by providingconsistent part coating with minimal, if any, touch-up.The multi-axis machine design is made up of two or three reciprocators thatwill move the gun(s) in two or three planes. The convention used to label thethree axes of motion is as follows (see Fig. 7).

X = parallel to the conveyor travel

Y = up and down

Z = in and out

The design of these units is the same as reciprocators with respect to thecontrol of speed and stroke adjustment; however, because the units must trackparts moving along the conveyor, the addition of a programmable logic controller(PLC) is required.The PLC will accept inputs from encoders (that determine conveyor speed)and photo cells or limit switches (that determine part position). This informationis used to determine at what speed the multi-axis machine must run totrack the part and when the multi-axis motion program is to be executed. Thepurpose of this complex tracking and motion system is to provide gun dwelltime and powder pattern direction.

Robots

Most robots provide six axes of gun motion by adding wrist movement. Roboticmachines can be electrically or hydraulically driven. Because of their cost andcomplexity, these units are rarely used in powder coating systems. Anotherdetriment to these units is that hydraulic fluid is not something you want tohave around powder. Also, powder coating material is very abrasive and can playhavoc with hydraulic seals and pistons.

POWDER RECOVERY

A powder booth/recovery system must accomplish two specific goals: containthe powder overspray within the booth and remove the powder from this containmentair so that it can be reused or disposed of properly. Powder booths aredesigned using several filtration techniques to separate the overspray powderfrom this containment air stream depending upon if the system will reclaim thispowder or employ a spray-to-waste strategy, the number of reclaimed powders,and the time available to perform the color change.

Cyclone Booth System

A cyclone powder booth system, as pictured in Fig. 8, is made up of a spraybooth, cyclone(s), a cartridge collector, and possibly ductwork.The spray booth can be made of metal, plastic, or composite sandwichdesigns. Metal booths provides strength and durability but attract more powderthat will prolong color change time. Plastic will allow more light into the boothand will attract less powder, reducing color change time. Composite sandwichdesigns offer strength and attract the least power, significantly improving colorchange time. All powder booths should provide a smooth interior to facilitateeasy and thorough cleaning.Ductwork connection(s) can be at one of several locations. The preferredmethod is to locate the ductwork connection in the base of the booth as thisprovides a down-draft air flow inside the booth helping to keep it clean.The booth may have devices, such as baffles, to help control air flow withinthe booth, touch-up openings to provide access for manual spraying, and gunslots to provide access for automatic equipment.The cyclone is designed to separate most of the powder from the airflowbefore entering the filtration section. This has several benefits. First, air enteringthe filter is “precleaned,” which will lower the loading on the filter media. Thistranslates to longer filter life. Second, the powder collected in the cyclone canbe easily recycled. Since the cyclone is a cleanable device, color change is attainablewithout additional equipment. Multiple cyclones are used when air flowis so high that one cyclone isn’t practical for a given plant ceiling height. Twincyclones are used in parallel before the filtration section. Cyclone efficiency canvary by manufacturer and design with some systems delivering in excess of 90%of the powder into the reclaim device.The filtration section used with a cyclone booth is a cartridge collector, givenits name for the cartridges used to separate powder from the air flow. Thesepaper cartridges are cleaned with a “back pulse” of compressed air to shockthe powder from the cartridge surface. The cartridges will separate most of thepowder out of the air flow from the booth (up to 99% efficiency). These are notcleanable devices for color change. The blower fan that produces the air flow inthe booth typically is located on the clean air side of the filtration device. Finalfilters are used after the fan to remove powder particles, down to 0.3 micron insize, before the air is returned to the work environment.All of these devices—booth, cyclone, collector, fans, and absolute filters—canbe connected by ductwork. The velocity of air within this ductwork usually isabove 4,000 fpm and the ductwork is designed to promote laminar flow toassure “self-cleaning” during operation.Some powder booth manufacturers have taken the approach of reducingthe ductwork in this type of booth. This design has numerous smaller cyclonesattached directly to the powder booth wall. The booth airflow enters the cyclonesdirectly and without ductwork. These cyclones are much smaller than those usedin standard cyclone booths, allowing for simpler cleanup. The blower, filter pack,and final filters are downstream from, and attached to, thecyclones, allowingthe air to be returned directly to the plant.

Cartridge Booth System

The cartridge booth system (see Fig. 9) answers the same technical needs thatall powder recovery systems must address: safe containment and separation ofpowder coating overspray. In a cartridge booth system, this is accomplishedby filtration of powder from the containment air using a cartridge collectorattached to the booth. There are no external filtration devices (or ductwork toconnect them) with this system.The cartridge collector is usually located in the wall of the booth (side draft)or in the base of the booth (down draft). The powder-laden air flow entersthe collector. The air passes through the cartridge filter and the powder isdeposited on the filter surface. Periodically, cartridges are back-pulsed withcompressed air to shock the powder from their surface and deposit it in thecollector base. Powder in the base is pumped to a reclaim stand for reuse orto a container for disposal.The cartridge filter pack can be removed from the blower pack for colorchange. A separate cartridge pack is required for each recoverable color.Cartridges are made of a paper filter media. The blower pack houses the blowerfan and filter assembly. The blower is on the clean-air side of the cartridges. Airfrom this powder booth system is returned to the plant.The booth may have touch-up openings and/or gun slots depending uponthe application for which it is used. The booth is typically of metalconstruction,though some manufacturers prefer plastic. This type of powder boothsystem is known for its compactness. Safety is another important benefit tothis design. Since there are no “enclosed” devices the need for explosion ventingis eliminated.