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The disc is capable of powder flow rates up to 180 lbs./ hr, whichprovides a theoretical coverage of over 30,000 ft2/ hr. It can be anexcellent retrofit for existing Omega-loop liquid disc system.

At least one manufacturer offers a powder bell that does not rotate.This bell uses a shaping disk that produces a 360 conical pattern

extending up to 35 inches in diameter. A special edge charging mate-rial is sandwiched within the disk to provide an expanded area ofcharge.

With a large powder pattern and charging surface, this device pro-vides good transfer efficiency over a broad surface. Since the conicalbell shape is non-rotating, it can be retrofitted to an existing powderspray gun.

4 Determining the Application Package

Choosing the right application package will depend on the nature ofthe task. The corona gun, tribo gun, the bell, and the disc are all tools

that can meet certain requirements. No single tool is perfect for everypowder application.

A corona-charging gun offers more methods of adjustment for differ-ences in part size and geometry. Air flow patterns, powder volume,velocity, and voltage level can be adjusted to accommodate different

Figure 3-15 Powder Disc

Gun Body

Disc DiscBooth Shell

Conveyor Path

PowderPattern

Product tobe Coated

Side View of Disc & P arts Plan View of the Omega-loop

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parts and powders. This gives the operator a lot of flexibility in con-trolling film thickness, dealing with Faraday cage and other processvariables that are common in production facilities. The use of dis-charge voltage creates field lines and a highly ionized electrical fieldthat can cause a variety of application challenges.

Generally, a tribo system will work best in situations where there isnot a lot of variety of parts and powders. With a tribo gun, the powdervolume can be adjusted and the gun to target distance can be ad-justed. The best application for tribo guns is one where there is little

or no need for adjustment. One powder and a fairly consistent mix ofparts can be run very effectively with a tribo gun. Used with the cor-rect powder material, the tribo gun can be very effective. Testing shouldbe conducted to be sure that the powder material is compatible withthis positive charging device.

Most powder operations in North America use the corona-charginggun to take advantage of the flexibility of voltage control and ability tospray any thermoset powder effectively. In Europe more tribo sprayguns are used to take advantage of the penetration characteristics.

The powder bell is well suited to high volume lines with a lot of sur-face. It would be great for shelving for example but not very good for

picture frames where there is a lot of empty area and very little sur-face. An air spray gun will likely provide better penetration into Fara-day cage areas than a bell.

The disc is a specialized piece of equipment for product that is denselyhung and long in the vertical dimension with very little thickness.Aluminum extrusions, large batches of small parts and wire goods aregood examples.

Any purchase of powder application equipment should involve test-ing, particularly when the equipment will be automatic. Testing can beused to compare manufacturers and to try out and compare differenttools. Processes can be tested, proven, documented, and used to as-sist with start-up when the installation is ready for production.

Application System Automation

Many powder systems are equipped with manual application equip-ment only. With an average sized product opening, say 30 wide by

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54 high, two manual operators should be able to keep up at a 6 feetper minute (1.8 meters/ minute) conveyor line speed. As the volumeof product to be coated increases, either in the size of the work pack-age, line speed or both, automatic application equipment becomesnecessary.

In addition to the ability to handle more volume, automation can pro-vide some advantages in reduced labor and improved efficiency. Au-tomatic guns can cover 80 to 100% of the parts. Manual operatorsmay be needed to cover the remaining bare areas and they can also

serve as inspectors. In some cases, the parts are fully covered withautomatic guns and there is no need for manual reinforcement.

Many automated systems use fixed position gun mounts but in manycases, gun movers and part recognition systems can be used to im-prove efficiency and reduce waste. In most cases, it will require moreautomatic guns to coat a given vertical work area with fixed guns thanit will if the guns are moved up and down. Also, gun movement willhelp to prevent the development of excess ionization on the part sur-face.

The equipment supplier will help to determine the precise packagethat is best in each situation. The need for automation will depend on

the volume of product to be coated, the expectations and goals of theapplicator and budget for the project.

Gun Movers

Machines that provide a straight up and down motion are called oscil-lators or reciprocators. An oscillator is typically a crank arm and fly-wheel design that can provide a vertical stroke of 6 to 24 (15 to 61centimeters). A reciprocator is a more complex device with a revers-ing or reciprocating drive and adjustment from 16 to 144 (41 to 366centimeters). Stroke speed is also adjustable. With both devices, gunscan be mounted in a straight vertical line, a horizontal line or a combi-nation of both, depending on the needs of the operation.

Radial Oscill ator

A radial oscillator uses two vertical bars, one that moves up and downwith the drive and one that is stationary. The gun body is mounted onboth bars so that as the one bar moves it produces an arcing motion.

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Figure 3-16 Reciprocator

Figure 3-17 Radial Oscillator

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The arcing motion has its advantages and disadvantages. At the topof the stroke, the gun is aimed slightly up where it can help to coat thebottom of the part; and at the bottom of the stroke, the gun is aimeddown, helping to cover the top of the part. The gun to target distancechanges as the unit moves and this can cause some inconsistencies infilm build.

Radial gun movers are relatively inexpensive and they can be veryeffective. Like the reciprocator, the radial oscillator does increase thevertical area that one gun can cover.

Figure 3-18 Multi-Axis Gun Mover

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Mul ti-Axis Machines

A multi-axis machine, commonly called an XYZ machine, moves theparts in more than one direction. In addition to the vertical movementof a reciprocator, they also move in direction of conveyor travel, andin and out. This provides a lot of flexibility to coat complex shapesand maximize system efficiency.

Robots

The advantages of using a robot are replacement of manual labor and

more consistent, repeatable application. It is also capable of makingadjustments in voltage and flow rates in the middle of the spray opera-tion through the controlling PLC. There are hydraulic and electricrobots used for coating. The hydraulic robot is less expensive but theelectric is preferred because it is smoother, more accurate, faster, andrequires less maintenance.

Gun Triggering

In addition to the gun movers, it has become increasingly common touse automatic gun triggering. A photo-eye or bank of photo-eyes ismounted on a stand on one side of the conveyor, in line with reflectorson the other side of the conveyor. As parts pass through the photo-

eyes, they break the beam of light to the reflector and send a signal toa microprocessor to provide part size information. An encoder is usedto sense conveyor movement and provides information on part posi-tion for timing of gun triggering. The output from the microprocessorcontrols what guns come on and when.

Gun triggering is used to identify part presence and line gaps. It is alsoused to determine the size of the part and react to the length, trigger-ing only the number of guns that are required. It can also be used toreduce or increase the electrostatic wrap, helping to reduce fatty edgeson simple flat product and enhance edge coverage on the side of theparts.

Liquid coaters have done this for many years to reduce overspray. Forthem, the reduced overspray means less paint waste and lower emis-sion levels. For the powder coater, gun triggering can provide signifi-cant savings in several areas.

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Better Overall Powder Material Utilization - A percentage of ma-terial is lost when it is sprayed, to the racks, from handling, in theclean up process, etc. It can be as small a percentage as 5% or, inthe case of spray-to-waste systems, 100%. It pays to reduce thetotal amount sprayed.

Powder materials are delivered in a particular range of grind size,measured in microns. Analysis shows that after reclaiming pow-der, the range of grind size shifts and the blend has a higher per-centage of fine particles and/or coarse particles. Usually, the per-

formance and appearance of reclaimed powder is inferior to thevirgin material. As a result, the virgin material and reclaim materi-als are blended to provide satisfactory results. The blend ratio canbe 50/ 50 or it may be a higher percentage of virgin, dependingon the performance characteristics of a particular material. In somecases, after the same material has been recirculated several times,the reclaim powder can become saturated with fines and the per-formance becomes so poor that the material must be scraped.

Compressed Air Consumption - Compressed air cost money togenerate. Having the guns on only when parts are present re-duces air consumption.

Wear on Gun Parts - Surfaces in the powder delivery path are

abraded by the sharp edges of the particles. Less spraying meansless wear and less replacement of parts.

Other Gun Control Features

The same microprocessor based system that controls the gun trigger-ing can be used to provide a variety of other functions. Specific gunsettings and gun movement can be altered to provide precise adjust-ment for the needs of a particular part. Features that can be controlledinclude:

Voltage

Air pressure

Stroke speed of the gun mover Stroke distance (up/ down, in/ out, side to side)

Control over these variables can provide a much higher degree ofefficiency from one part to another, reducing waste and improvingproduct quality.

Determining the Application Package