Drippings in industrial liquid coating: why they occur and how to eliminate them

It is often attributed to operational errors, although this is a partial truth: the formation of drippings is in fact the result of an unachieved balance between paint behavior, application mode and process conditions. Understanding why they form therefore means approaching coating as an integrated system, in which paint viscosity, application parameters and plant configuration combine to determine film stability. It is on this front that defect prevention is played out.

At the time it is deposited, the paint is in a fluid state(wet film) and must pass through a transient phase in which the atomized droplets coalesce (coalescence), solvents begin to evaporate, and viscosity gradually increases. If this process is not balanced, the force of gravity prevails over the surface tension and viscosity of the product, generating:

• vertical drippings;
• Extensive drips;
• Localized accumulations;
• Nonuniform thickening.

The risk increases on vertical surfaces, edges and complex geometries, where the material naturally tends to concentrate.

In industrial liquid paint booths, paint is dispensed through different types of automatic or manual dispensing guns.

The spray gun atomizes the paint, that is, it controls the droplet size, the rate of impact on the part, and the uniformity of the deposit. A suboptimal gun configuration causes poor atomization and can generate an unstable film. Below are the main critical issues to be monitored.

• Insufficient pressure: generates larger droplets, making the film more prone to gravity dripping.
• Excessive flow rate: causes paint buildup.
• Incorrect fan: results in uneven distribution of the product.
• Wrong distance between gun and workpiece: produces local overloads.

Added to these variables is the substantial difference between manual and automatic application:

Manual application

In manual spray booths, the operator manages distance, speed and overlapping of passes in real time. This provides greater process flexibility, but introduces a margin of variability related to human intervention: inconsistent movements or excessive dwell times on an area can generate buildup and dripping.

Automatic application

In automated systems, guns are mounted on reciprocators or robots and work with programmed parameters. This allows for high repeatability and consistency of results, but exposes the process to setting errors such as improper speeds, non-optimized trajectories, and dispensing rates that are not calibrated to part geometries.

In industry, coating quality depends not only on the correctness of the parameters, but on their ability to remain constant over time. Repeatability of the process is therefore a key factor in avoiding the systematic occurrence of drippings along the production.

Paint that is too fluid tends to flow easily, while proper viscosity allows the film to solidify more evenly as solvents evaporate.

Dilution is one of the most frequent causes of film instability: too much solvent reduces viscosity and increases the risk of leaking. Uncontrolled dilution is responsible for the product’s nonrepeatable behavior.

The choice of solvent directly affects how long the film remains fluid. Slow solvents slow evaporation, i.e., the film stays wet longer, with greater risk of dripping. Fast solvents promote faster setting: the likelihood of dripping is lowered, other defects, such as orange peel, are increased.

Leakage prevention requires maintaining a stable process window, that is, a controlled range of parameters within which film behavior is predictable. Even minor deviations from this window, in terms of viscosity, temperature, flow rate or ventilation, can compromise coating stability and generate surface defects.

In addition to parameters related to paint formulation and application, liquid paint drip is strongly influenced by environmental conditions and process variables along the paint line.

For example, a change in ambient temperature changes evaporation times, in turn affecting apparent film viscosity and response to spraying parameters. Managing drippings therefore requires an integrated reading of the process, rather than isolated interventions on individual variables.

Avoiding paint dripping in the context of industrial liquid processes therefore means governing a complex balance of physical, chemical and operational variables.

• The paint must maintain predictable and repeatable behavior, and it is therefore important to check viscosity and dilution regularly.
• The application must ensure uniformity of deposition, so pressure and flow rate of the spray guns must be adjusted to the geometry of the part and the intended passes.
• Environmental conditions are an integral part of the process, so temperature, humidity and airflow in the cabin must be stabilized.
• Integrating automated systems for process control and monitoring optimizes the efficiency of the entire system.