REDUCING PAINT OVERSPRAY

Overspray refers to that portion of sprayed paint that does not land on the target.  It does not benefit the painting process in any way.  On the contrary, in the amount of wasted material, increased filter loading, affect on booth uncleanliness, and by the higher levels of emissions, overspray is a substantial burden on paint processes and it pays to minimize it.  Moreover, paint overspray that settles onto painted parts can seriously degrade the quality of the finished product, reducing gloss and contributing to orange peel.  If your application has excessive overspray from electrostatic or conventional spray guns, bells, disks, or other rotary atomizers, operator and maintenance personnel alike may find the discussion below useful for reducing levels of overspray and management may deem these discussions an aid in the decision making process.

North American Paint Applications is the world leader in the implementation of technology and process control for the reduction of overspray in every type of paint application.  

CALCULATION COST OF WASTE

It is easy to determine the volume of material sprayed by an applicator that is overspray.  First, calculate the amount of paint that lands on the target part.  Second, subtract that amount from the total sprayed paint.

Overspray _V = Total Volume of Sprayed Paint - Transfer Efficiency X Total Volume of Sprayed Paint

Two practical methods for calculating the paint transfer efficiency are shown at the bottom of this page.  To calculate the material cost of overspray, multiply the volume of overspray material times the cost of the material.

Overspray _$ = Overspray _V X Material Cost

This calculation can be done on a per part basis or a time basis, whichever is convenient.

OTHER COSTS OF OVERSPRAY

Filter loading and sludge are created as a direct result of overspray.  Increased levels of overspray require more frequent filter changes and higher volumes of sludge disposal.  The filter loading, in particular, is troublesome because not only does it create imbalances in the flow of air through the booth, but filter loading generally acts to reduce the level of booth air flow, which can create significantly higher concentrations of booth vapors, booth airborne particulate, and contamination.

Overspray is a source of increased levels of dirt build-up in the booth.  As filters restrict the flow of exhaust air from the booth, overspray lingers to create still higher levels of contamination in and around the spray booth.  This is particularly so with electrostatic applications.  In addition to creating the need for more frequent maintenance, all this contamination can create a major dirt problem, severely affecting the quality of the finished product.

Ecological cost is a more abstract measurement, but not a less important one.  Minimizing the emissions of airborne solid and vapor pollutants into the local community is the duty of every corporate citizen.  And in the case of paint overspray, responsible manufacturing and efficiency are synonymous; efficient systems are truly poorer polluters.

CAUSES OF OVERSPRAY

Triggering paint off the part or triggering when no part is present is a common source of overspray.  Improper head to target distances are a cause of increased overspray as are improper angles of application.  Turbulent air flow conditions can create overspray.  Mechanical force such as the turn around points of vertical or horizontal reciprocators can cause consistent bursts of directional overspray.  Insufficient levels of electrostatic force contributes to large amounts of overspray.  NA Paint can implement technology and process control to significantly reduce or eliminate overspray by all of the above causes.

Excessive atomization is another prevalent cause of overspray.  Creating tiny paint droplets, they can dry out in flight to the target and get swept away by the flow of air.  Often, spray guns have their atomizing and pattern air volumes so high that they are more efficient at fogging than painting.

Of course, choosing the most efficient applicator for your application can contribute to the reduction of overspray.  And, applying that applicator efficiently is very important.  For example, a 16 inch wide spray pattern is not necessary to paint a 4 inch wide part.  NA Paint can help you get your process under control at peak efficiencies.

North American Paint Applications is the world leader in the implementation of methods for the reduction and elimination of overspray.  Not only can we significantly reduce the amount of overspray, we can implement technology that adjusts air flows under changing filter load conditions to help maintain a constant flow of air through the booth.  

PAINT TRANSFER EFFICIENCY

Transfer efficiency refers to a ratio representing that portion of sprayed paint that does land on the target part.  This number is widely used in calculations involving paint application economics.  It is common for manufacturers to advertise transfer efficiency ratings for their applicators which do not correspond with the actual efficiency that is obtainable in your paint process.  Because this is an important measure of efficiency, two practical methods for calculating the actual transfer efficiency of installed systems are shown below.  These methods are for single component material applications.

TE by Weight

The most accurate method of calculating transfer efficiency is by weighing the part prior to painting and after painting.  This should be done when the part is completely dry.  This method will include in its result all film build variations, heavy edges, and wrap (paint on the reverse surface).  

Paint Solids Weight on Part = Part Weight After Painting - Part Weight Before Painting

This yields the weight of the solids in the coating.  Then, we must determine the weight of the paint solids dispensed from the applicator.

Paint Solids Weight Dispensed = Weight of Dispensed Paint X Paint % Solid Content by Weight

The weight of the dispensed paint can be calculated by multiplying the volume of the dispensed paint by its density.

Finally, the transfer efficiency by the weight method can be found.

Transfer Efficiency _W = Paint Solids Weight on Part / Paint Solids Weight Dispensed

TE by Volume

If it is impractical to weigh parts, a very accurate method of calculating transfer efficiency is by measuring the volume of solids on the part after painting and comparing that to the volume of solids that was dispensed.  This should also be done when the part is dry.  This method will be as accurate as your model of film build distribution across the part.

Transfer Efficiency _V = Paint Solids Volume on Part / Paint Solids Volume Dispensed

The volume of dispensed solids is easily determined in the manner similar to the method above.

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