“Overspray” is the portion of sprayed paint that does not deposit on the surface to be coated. By its very nature, overspray does not bring any benefit to the coating process. On the contrary, it is a cost factor: it increases the amount of material that must be disposed of, adds load to the filters, soils the booth and raises emission levels to the atmosphere. Reducing overspray therefore leads directly to both economic and environmental savings.
In addition, overspray that settles on already painted parts has a negative impact on coating quality, reducing surface gloss and contributing to the typical “orange peel” effect.
The information below can offer a practical contribution both to operators and to production/maintenance managers in companies that use spray coating devices (conventional or electrostatic spray guns, bells, discs or other equivalent rotating atomisers).
CALCULATING THE DIRECT COST OF OVERSPRAY
Quantifying overspray is a simple operation.
First, calculate the amount of paint that actually deposits on the part to be coated, then subtract this value from the total amount of paint sprayed.
Overspray = VT − (ET × VT)
where:
VT = total volume of product sprayed
ET = transfer efficiency
To calculate the direct cost of overspray, simply multiply the value obtained from the formula above by the cost of the paint.
INDIRECT COST FACTORS
The two main consequences associated with overspray are filter saturation and the generation of waste to be disposed of.
The amount of overspray directly affects how often filters must be replaced or cleaned, with a clear impact on maintenance costs. In addition, excessive filter loading can cause imbalances in the airflow patterns inside the booth and, above all, a reduction in airflow. This leads to higher concentrations of solvent vapours, particulate and various contaminants in the booth.
Overspray is also a major source of contamination for the booth itself. The reduction in airflow across the booth, caused by overspray loading on the filters, generates more turbulence and promotes re-deposition of particulate both on painted parts and on booth surfaces, extraction ducts and other components (fans, dampers, etc.). All of this further increases maintenance costs and downtime.
Lastly, there is the environmental and energy cost: less immediate to measure, but no less important. Minimising emissions of pollutants (VOC, particulate, etc.) and reducing the consumption of materials and filters is a responsibility for any company using spray painting. Responsible production and efficiency go hand in hand: efficient systems and devices reduce pollution, consumption and operating costs.
CAUSES OF OVERSPRAY
One of the most common causes of overspray is triggering the spray gun when there are no parts in position, or delaying the shut-off of the paint flow.
Other frequent causes include:
- incorrect distance between gun and part and/or incorrect spray angle;
- improper gun setup or poor gun maintenance;
- turbulent airflow conditions inside the booth;
- insufficient electrostatic force;
- incorrect atomisation parameters.
Excessive atomisation can also cause an increase in overspray. If the paint droplets are too fine, they may dry in flight and, instead of depositing on the surface to be coated, are captured by the airflow and transported elsewhere.
In conclusion, to reduce overspray it is essential to:
- select the most suitable and efficient applicator for the type and size of part to be coated (for example, using a 16″ applicator to paint a 10 cm part is clearly counterproductive);
- set up and maintain the best operating techniques (correct distance, gun speed, spray angle, etc.);
- carry out appropriate and effective maintenance on both the coating system and the air filtration system.