Optimising painting in line manufacturing

The painting metal products – such as machines and equipment – is an important part of the line manufacturing production process. It is an essential element of the product’s brand and recognisability, but above all, a protection against corrosion, mechanical wear and chemicals. A well-executed surface treatment extends the product’s service life, facilitates cleaning and improves its resale value, but what should be considered when planning a line painting process, asks Nor-Maali?

The painting of industrially line-manufactured metal products – such as machines and equipment – is an important part of the production process

Accurate surface preparation is the basis for a successful painting process. As in all industrial painting, metal surfaces must be carefully cleaned and pre-treated before painting. Cleaning removes all impurities that weaken adhesion – solid particles, salts, greases and oils. A typical solution today is an alkaline, water-based detergent that effectively removes most impurities.

Steel surfaces are blast-cleaned to at least grade Sa2½. Certain special products may require even level Sa3. The blasting process can be automated. If the shape or material of the component does not withstand blasting, chemical pre-treatment is used instead. The most typical methods are iron and zinc phosphating. There are different types of phosphating agents and the storage passivation often used with them. Therefore, paint adhesion to the passivated surface must always be verified. Other types of chemical pre-treatment are also available, but they are less common.

The higher the corrosion category (e.g. ISO 12944) defined for the object to be painted, the more significant the role of pre-treatment. The quality of surface preparation is emphasised especially in the case of waterborne systems, as waterborne coatings tolerate impurities less effectively than solvent-based alternatives.

Selecting the right coating system for a line manufacturing process

The choice of coating system typically depends on environmental conditions: whether the equipment is exposed to corrosion, chemicals, mechanical wear or UV radiation.

The coating system can be selected in accordance with ISO 12944-5, but in product painting it is common that customers have their own experience-based painting specifications, which define the requirements and tests for surface treatment.

The following details the properties of the most common industrial coating types used on metal surfaces.

• Epoxies provide excellent chemical and corrosion protection. They are well suited particularly as primers and also as topcoats for indoor use. In outdoor applications, epoxies tend to chalk under UV radiation from sunlight, so they require a weather-resistant topcoat. If the painted product is exposed to corrosion categories C4–C5, epoxy coating is a recommended choice for the primer in a multi-coat system. Zinc-rich epoxies are particularly suitable for applications requiring very high corrosion protection (typically C5). Zinc acts as a sacrificial metal in the system and effectively prevents rust progression even in the case of surface damage.
• Polyurethanes offer excellent gloss and colour retention even under UV exposure. They are suitable both as topcoats and direct-to-metal coatings. As single coats, polyurethane coatings typically provide good protection up to corrosion category C3.
• Acrylics and alkyds are single-component options for lower corrosion categories (C1–C2). Their use has decreased in recent years due to environmental legislation (e.g. VOC content) and stricter customer quality requirements.

Single-coat or multi-coat system?

Painting in a line manufacturing process can be done as a single-coat or multi-coat system, with each having its own advantages.

Single-coat painting (DTM) saves time and costs, as it requires only one application. Modern direct-to-metal coatings can already achieve demanding corrosion categories, but high quality requires precise workmanship: film thickness control must be at a high level to ensure coverage and the desired corrosion protection.

A multi-coat system takes more time in the process than single-coat painting but is less sensitive to possible film thickness deviations and often allows for higher corrosion protection. If the primer and topcoat are based on the same chemistry (e.g. epoxy), the topcoat can often be applied after a short interval (wet-on-wet application), which speeds up the process.

Environment and user safety in a line manufacturing process

Companies may have defined policies regarding the environmental friendliness and user safety of paint products used in an in line manufacturing process. It should also be noted that EU Directive 1999/13 limits solvent emissions (VOC) from production facilities. VOC emissions can be reduced by choosing a waterborne or high-solids coating system, in which case emissions can be reduced to as little as one fifth compared to an equivalent solvent-based system.

Some companies may also require paint products to be isocyanate-free. EU Commission Regulation 2020/1149 sets restrictions on products containing more than 0.1% isocyanate monomers. In addition, companies may have their own chemical lists that prohibit or restrict the use of certain substances. Typical restricted substances include those on the REACH Candidate List (SVHC, Substances of Very High Concern), Category 1 CMR substances (carcinogenic, mutagenic or reproductive toxins), and respiratory sensitisers.

Appearance and cleanability

The degree of gloss has a major impact on the final appearance of the product. A glossy surface looks stylish and is easier to clean, but it also reveals possible surface defects in the coating more easily. A high gloss level therefore also requires a higher standard of painting quality.

Other considerations for a line manufacturing process 

In product painting in a line manufacturing process, it is typical that the line speed is high and there is limited time for drying of the painted parts.

The most common painting technique in product painting in an in line manufacturing process is still high-pressure airless spray. Painting can be performed automatically (traverse or robot), manually or with a combination of both. The choice of the correct nozzle tip size and spray angle is extremely important and is made according to the shape of the painted object and the product used. A smaller nozzle allows more precise film thickness control but slows down the work.

The use of electrostatic or air-assisted spray guns is common. The former reduces material waste, while the latter improves the surface quality. If the quantities are very large and the shapes complex, dip coating can also be used. In this case, it is important to monitor the stability of the product (viscosity, pH, solids content), often together with the paint supplier.

Drying can be accelerated by increasing the temperature of the drying line. Typical solutions include electric and gas-fired ovens. The use of infrared (IR) elements has also become more common due to rising energy costs. IR radiation is a more efficient way to transfer thermal energy to the object being dried.

Especially with waterborne products, the drying process must be carefully designed, as water evaporates relatively slowly compared to solvents and requires a great deal of energy for evaporation. The evaporation of water can alternatively be accelerated by reducing the humidity of the surrounding air. After the heating process, it is also important to consider the time required for the part to cool down and whether this should be accelerated, for example, with fans.

Final product quality control, both in terms of appearance and technical durability, is an essential part of product painting. The maximum number of surface defects is often defined, and adhesion is typically monitored through random testing, for example, using the cross-cut test (ISO 2409).

Durable and high-quality results

The optimal selection of surface preparation, coating system and drying conditions enhances the efficiency of the line manufacturing product painting process, reduces the need for maintenance painting and lowers the overall lifecycle costs of the product – even if the paint used is more expensive per unit.