Coating The technician's corner

The most relevant ones are:

• Surface condition (cleanliness, surface energy)
• Type of pre-treatment
• Environmental conditions (temperature and humidity)
• Application parameters (flow rate, distance, speed)
• Film thickness
• Curing curve (time and temperature).

A poorly prepared surface is one of the leading causes of adhesion failure, even when application is carried out correctly. Unstable parameters, on the other hand, can cause film non-uniformity and compromise both mechanical resistance and protection.

The curing stage is equally critical. Insufficient curing produces a weak film; overcuring can alter its properties.

Adhesion and resistance are not the result of a single adjustment. They reflect the stability and consistency of the entire coating cycle.

The four main ones are described below.

Ionized air blowoff is the starting point of any process. It removes dust, particles, and electrostatic charges that build up on plastics during moulding. It does not chemically activate the surface, but it is an essential preliminary step: applying paint to a charged part means attracting fresh contamination during application itself.

CO₂ snow cleaning uses a jet of solid-phase CO₂ to remove thin organic contaminants such as release agents, mould residues, and additive films. It leaves no residue, does not wet the part, and works well on complex geometries and solvent-sensitive materials. Like blowoff, it does not chemically activate the surface, but it brings it to a very high level of cleanliness before subsequent stages.

Plasma treatment is the standard activation method. It exposes the surface to an ionized gas flow that breaks superficial molecular bonds and creates polar functional groups, raising surface energy in a measurable way. The result is a surface that paint can wet uniformly and bond to reliably. The effect is temporary: activation decays over time, typically within hours or days, so coating must be applied within a defined window. Variants include flame, arc, and cold atmospheric plasma, each with different degrees of selectivity and suitability for automation.

Laser treatment uses high-energy pulses to ablate contaminated surface layers and, depending on the parameters, modify the surface microstructure by creating a controlled roughness that improves mechanical anchoring of the coating. It is the most precise and selective method, allowing work on specific areas without affecting the rest of the part. It is particularly suited to high-performance technical plastics such as peek, pa, and pps, or where localised treatment is required. Integration with automated systems is more complex than plasma, but process traceability (energy, spot size, speed) is very accurate.

In industrial practice these treatments are not mutually exclusive. They are often used in sequence. A typical cycle for a technical plastic component may include ionized blowoff, CO₂ cleaning, and plasma activation immediately before application. The choice depends on the type of plastic, the paint, adhesion requirements, and the line architecture.

The reference measure for verifying treatment effectiveness is surface energy, detectable with calibrated test inks or a contact angle goniometer. An adequate value before application is the first guarantee of lasting adhesion.

The key point is that paint needs to wet the surface uniformly. If a thin contaminating layer sits between the substrate and the coating, that wettability is compromised. The resulting film loses continuity and adhesion, producing defects that show up in production as:

• Cratering or fish eyes
• Poor adhesion or film delamination
• Delayed delamination during use
• Cosmetic defects and non-uniformity.

From a physical standpoint, the phenomenon relates to surface tension. A properly cleaned surface allows paint to bond stably, while a contaminated one lowers surface energy and produces weak or irregular adhesion. The problem is not always immediate: parts may appear compliant initially but develop issues later, during use or in durability testing.

Parameters such as the chemistry used, temperature, contact time, and rinse quality must be managed precisely. Bath cleanliness is also fundamental: a contaminated bath can reintroduce dirt rather than remove it.

When washing is under control, coating becomes stable and predictable. When it is not, defects arise that are hard to explain and are often attributed to the paint when the real cause is upstream. Adhesion is not built in the spray booth. It starts in the pre-treatment stage.

Thickness is built during application. Flow rate, part travel speed, nozzle-to-surface distance, and in electrostatic systems, voltage and charge, all determine how much paint is deposited and how uniformly. If these values are not stable, thickness will vary even with the same product.

Measurement after application is essential. The most common methods use non-destructive instruments such as magnetic or eddy-current gauges, which allow fast checks during production. The point is not only to measure, but to define clear tolerances based on the application: insufficient thickness reduces protection, while excessive thickness can cause defects or material waste.

Distribution is an aspect that is often underestimated. Having the correct average value is not enough; uniformity is needed across the entire geometry, especially on edges, cavities, and complex surfaces. This is where system design and application strategies come into play.

Film thickness control ultimately does not stop at measurement. It is exercised by designing and stabilising the entire process that generates it.

Defects such as cratering, poor adhesion, runs, or non-uniformity are typically a sign that the process is not stable.

The factors that matter most are:

• Recipe standardisation: for each product type, precise operating conditions must be defined and documented, covering application parameters, temperature, humidity, and curing times. Without fixed and documented recipes, the outcome depends on operator variability or the environmental conditions of the day.
• Continuous in-line monitoring: conditions need to be checked cycle by cycle, not just at the end of a shift. Small deviations, if not caught early, turn into systematic defects.
• Phase-to-phase consistency: inadequate surface preparation can compromise adhesion regardless of how well the application is controlled. Each stage depends on the one before it.
• Environmental condition control: temperature and humidity in the spray booth directly affect application and drying. Unmanaged fluctuations introduce variability even when machine parameters are correct.
• Anomaly management: clear procedures must be in place to identify and correct deviations quickly, without waiting until the end of production.

A process designed for quality on the first pass reduces operating costs, shortens cycle time, and increases effective production capacity. It is not a quality objective in the abstract sense: it is a direct indicator of how well the process is under control.

This approach allows the production history of every part to be reconstructed accurately, supporting quality control activities, audits, and non-conformance management. When a defect occurs, the conditions under which the part was processed can be identified quickly, making root-cause analysis faster and more reliable.

Traceability also has a preventive value. Monitoring data over time makes it possible to spot trends or deviations before they cause problems. The longer-term goal is predictive maintenance, which is becoming a standard expectation in manufacturing.

It is a tool that connects quality, control, and continuous improvement, and over time it also becomes a competitive advantage with customers who require certified process documentation.

• Precision and uniformity: speed, flow rate, and application mode can be defined and maintained within tight tolerances, producing more consistent coating distribution than manual application.
• Cycle-to-cycle repeatability: with the same recipe, an automated system delivers the same result regardless of the operator, shift, or time of day.
• Integration with monitoring: automated systems lend themselves to continuous process data acquisition, enabling full traceability and statistical process control.
• Productivity: faster, more consistent cycles increase volume without sacrificing quality. Rework decreases and throughput time shortens.
• Reduced operator dependency: repetitive tasks are handled by the machine, allowing operators to focus on supervision, setup, and anomaly management.

Automation does not replace human oversight; it raises its quality. The process becomes more reliable and predictable, and the operator's role shifts from manual execution to supervision and optimisation.

Components must always be presented in the same position and orientation for the system to operate accurately. This means more structured design of loading, handling, and transfer stages. Fixtures and gripping systems also become critical, because they must guarantee stability and surface accessibility throughout the process.

Production becomes less improvised but far more stable and predictable. The informal flexibility of an operator adjusting on the fly is reduced, but the gains in quality, repeatability, and long-term process control more than compensate. For many companies this is as much a cultural shift as a technical one: it requires discipline, documentation, and a structured process logic from the very beginning.

The central point is reducing waste and emissions by optimising material use and controlling airflow and process conditions. Overspray recovery technologies and emission abatement systems help limit environmental impact while maintaining high coating quality.

A sustainable approach also considers energy consumption and overall process efficiency. In some cases, well-optimised solvent-based solutions can outperform theoretically greener alternatives that are less controlled in practice.

Sustainability in this sector is the result of a balance between technical performance, consumption, and emissions management. It is a continuous optimisation effort, not a label to apply to a product choice.

One of the main areas is overspray management, the portion of paint that does not reach the part. Filtration and recovery systems reduce dispersion and improve material utilisation efficiency. At the same time, controlled airflow in spray booths limits the spread of contaminants.

Downstream of the process, emission treatment systems can further reduce volatile compounds. The most effective approach, however, remains prevention: optimising application parameters to minimise waste is always preferable to treating emissions that have already been generated.

Process stability plays a determining role. Fewer variations mean fewer rejected parts and therefore fewer overall emissions. In this respect, production efficiency and sustainability are not competing objectives. They reinforce each other.

At the same time, optimised airflow and work cycle management can lower energy consumption, especially in spray booths. Greater process stability also leads to fewer rejected parts and less rework, with a direct impact on both costs and environmental footprint.

A more controlled process uses fewer resources and generates fewer emissions while improving operational performance. In automated coating, efficiency and sustainability go hand in hand.

Fixtures play a fundamental role because they define the part's position throughout the process. They must be designed to ensure repeatability, surface accessibility, and where needed, protection of areas that should not be coated.

In recent years, advances in materials and manufacturing technologies have made it possible to develop more flexible and faster-to-produce fixtures, improving a system's adaptability to different geometries. Fixtures and gripping systems are not accessory components. They are structural elements of the process, and a careless approach to their design translates directly into defects or rework.

By using vision, it is possible to improve operational precision and reduce positioning errors. These systems help make the process more flexible, allowing different product types to be managed without significant structural modifications to the line.

The integration of vision and automation is an important step towards more intelligent systems, capable of adapting to real production conditions while maintaining high quality standards. Looking further ahead, it is also the starting point for adaptive control logic and in-line quality monitoring.