6. CURING

For convection curing, several types of ovens may be used. For solvent-based basecoats, the time between application and start of curing should not exceed 5 minutes, as these coatings are sensitive to humidity (details are in the respective technical data sheet). Large frame parts or components with thick walls, large diameters, or high mass require additional time to reach the desired part temperature. Excessive stacking heights on conveyor belts or trays should be avoided. Higher bulk density of parts requires lower stacking heights, generally below 10 cm.

Conveyor belt oven (continuous oven): Parts are unloaded from baskets onto a slowly moving conveyor belt. Drop height should be minimal to avoid damage. The first section of the oven is the pre-curing area, where solvents (or water for water-based coatings) are evaporated at approximately 80–120 °C. Parts then fall onto a second, faster belt for better distribution and reduced sticking. Alternatively, slightly wavy belts or gentle vibratory movement can be used. Parts are moved gently during pre-curing or throughout the entire curing process. The next step is curing at product-specific temperature and duration according to the technical data sheet. Note that the specified temperature refers to the part temperature, not the oven temperature. Curing time is the time parts must remain at the target temperature.

Tray oven (continuous oven): Each load of parts is placed in a tray and pushed into the oven. Existing trays are shifted forward. Heating is the same as for conveyor belt ovens. Vibration stations may prevent sticking. Pre-drying on a conveyor belt and coordinated speed transfer to trays is also possible, particularly for flat parts that are difficult to redistribute by shaking.

Chain conveyor oven: Continuous ovens for rack-mounted parts operate the same way, with the entire rack passing through the oven.

Chamber oven: Suitable for small batches and low loading volumes, as reaching the required part temperature can take a long time after opening the door. Pre-curing and curing can occur in separate chamber ovens. Regardless of oven type, temperature distribution must be uniform, with deviations not exceeding 5 K. Verification should occur at least every four weeks using an oven temperature profile (see chapter '10. Inspection Plans'). Heating is usually gas-fired, with direct or indirect heating. Direct heating may cause contamination; Doerken recommends indirect heating.

• IR drying/curing (e.g., partial coating of screw heads)
• Induction drying/curing (e.g., gentle curing of grease-filled bearing housings)

These specialized processes must be adapted to each individual part.

Some water-based coatings cure at room temperature. This process can be accelerated by applying higher temperatures (forced drying). See the technical data sheet for details.

After curing, parts are cooled to room temperature (30 °C or lower). Cooling usually occurs in the oven using cold air. Care must be taken to ensure the dew point is not reached (minimum 3 K above dew point), to avoid moisture condensation on coated surfaces.

Volatile organic compounds (VOC) are organic substances that evaporate easily or are present as gas at low temperatures. Organic solvents used to thin coatings have a high VOC content. As these can negatively affect health and the environment, exhaust air purification is crucial. There are three main purification methods:

• Thermal: All emissions are incinerated.
• Biological: Microorganisms decompose harmful emissions.
• Photocatalytic: Harmful emissions are broken down by UV light and captured in activated carbon filters.