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5 Selection of the Application Package

There are several factors that must be considered in determining theproper application package.

The size and shape of the part

The variety of parts to be run

The variety of powder chemistry and colors to be run

Desired film thickness

Production output

Design line speed

Size of the product opening

Available capital

The part size and configuration will determine the product openinginto the booth. The maximum part size is also needed in the selectionof the number of automatic guns and their placement, the selection ofgun movers and gun attachments, or the determination of the numberof manual operators in a particular system.

The correct number of guns for a particular application depends largely

on the vertical surface to be coated but the shape of the part and theline speed are also factors. Basically, the number of guns should con-sider how much surface will be coated in a given time period andselect enough guns to coat it without above average output per gun.

A rule of thumb formula for determining the number of guns re-quired considers:

Design Line Speed

Coating Thickness Required

Part Size

Product height x line speed x mil thickness x the number of sidesneeding guns, divided by difficulty factor, rounded up to an even number.Difficulty Factor: 240 extreme 300 medium 360 simple

Example:

60" x 10 FPM x 1.5 mils x 2= 6 guns

300

Selection of the Application Package

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Powder Coaters Manual 1/98 III/33

One pound of powder covers 192.3 sq. ft. at 1 mil thickness at 1.0 specificgravity and 100 % utilization. Factoring the variables of actual thickness,specific gravity of a particular powder and the actual transfer efficiencywill provide an idea of the coverage that can be expected from a pound ofpowder.

Testing has shown that low outputs provide higher transfer efficiency.The powder charges better and is less likely to blow by the part or reboundoff the surface. So in the selection of guns it is important to avoid usingtoo few guns with high output per gun. Efficiency is improved with thecorrect number of guns that will allow the system to operate below 30

pounds of output per hour, per gun.

Proper performance in powder coating depends upon the powder particleshaving an electrostatic charge and the substrate being properly grounded.

The charge on the powder particles will determine how well the powderadheres to the part and the rate of film build. As a charged layer of powderaccumulates on the surface it develops a force which resists furtherdeposition.

The rate of film build is also related to delivery pressure, gun to targetdistance, particle size, the time of exposure in the spray zone, gun voltage

and the volume of the powder delivered.

Generally, higher voltage will produce heavier films

Gun to target distance should be 6" to 12". Generally, a closerposition will produce a heavier film and possible back ionization

If delivery pressure is too high it will blow powder past the part, ifit is too low the powder will not get to the part, it will be pulledinto the reclaim system.

A coarser grind of powder will produce a thicker film

The rate of film build diminishes with longer exposure, howeverthere is still an increase of film with prolonged exposure

Typically, a larger volume of powder delivered will result in quickerfilm build

This does not necessarily mean that there is greater transfer efficiency.If a larger volume is delivered the percentage of efficiency may remainthe same or decline, applying more powder but also over-sprayingmore powder. In addition, since a larger volume of powder sprayedwill mean more cumulative charge on the part surface in less timethere is a greater risk of back ionization when larger volumes of pow-der are sprayed.

Selection of the Application Package