Paint baking

 

Paint baking is one of the biggest application areas of infrared heating. Infrared results in uniform heating and reduces the baking time. Long and medium wave infrared is almost blind to colour difference.

 

The length of the oven and the conveyor speed will depend on a few parameters. 

 

1)Thickness of the product : In the case of paint baking on thin metal sheets, the metal as well as the paint film rise in temperature simultaneously. The heated metal sheet by conduction helps in heating the shadowed areas, if any. In the case of thicker sheets or products, infrared helps in saving power as the paint film reaches the stoving temperature quickly and it is not necessary to raise the temperature of the metal.

2)Shape of the product : The heat intensity and the speed will be governed by the shape of the product. Flat metals will be cured quickly as it is possible to achieve uniform heating because of the linear shape of the product and as the paint film is of uniform thickness. 

3) Paint thickness : In the case of thick paint films, the power intensity has to be optimised to make sure bubbling and scorching of the paint surface does not occur. In the case of thin paint films, there is no problem.

4)Paint colour : Long wave radiation is less sensitive to colour difference. Short wave radiation is comparatively more colour sensitive. 

5)Paint type : In the case of solvent based paint, care needs to be taken to remove the evaporated solvent. 

 

For isothermal stoving, the temperature of the painted product is raised to the minimum necessary for curing in the first part of the oven and maintained as the product passes through the rest of the oven. Less heat is needed in the isothermal part of the oven. If an oven is used for painted products of different shapes and sizes, there will be a tendency for the product temperature to rise as it passes through the isothermal zone. In comparison with a convection heating system, the product is heated to the curing temperature more quickly resulting in a saving of a few minutes in the curing time.

 

Super heat stoving raises the temperature of the painted product to the maximum, which provides a complete cure in the time available without causing degradation. Reductions in curing time upto 90% of the convection curing time can be obtained.

 

For achieving maximum speed and efficiency, the oven for paint baking may be zoned.

Flash-off zone: to remove the solvents without bubbling. Usually achieved in ½ to 1 minute.

Heat-up zone: to bring the work pieces to the desired temperature.

Holding zone: to maintain the temperature of the work pieces.

 

Powder coating

 

Powder is applied electrostatically on the objects which then move on a conveyor into the infrared oven. 

 

Infrared helps in reducing the powder curing time and also helps in achieving a smooth glossy finish by rapidly flowing the powder before it has time to set. As there are no solvents involved, flass off zone is not required. As a result heat losses are minimal as no air needs to be extracted. 

 

In the continuous system used for powder coating, the heating time using IR would be a fraction of that required in a hot air oven because it will bring the powder upto the required temperature within a minute or two without the necessity of raising the temperature of whole mass of the object. A considerable amount of heat will go into the object and this will help the powder to cure. The powder first of all must be able to flow. Once it has flown, a certain amount of time is required for it to cure. This is usually quite short. The reason it takes so long in the hot air oven is because it is necessary to heat the full load to the temperature before the powder flows. 

 

With IR, because the powder flows very rapidly under the intense heat, a far better finish is obtained.

 

 

The advantages of using infrared heating elements with respect to conventional heating ovens for paint baking are:.

 

1.Since infrared heaters and lamps work on the principle of radiation, there is no need to blow air to dry the product. Thus, besides saving the expense of installing and operating the air blower, you avoid the possibility of dust sticking on to the paint.

2.The IR elements are much more energy efficient, you could in some cases take 2-3 minutes to bake paint on a product which would normally take 20 minutes in a conventional oven.

3.These elements have a long life of over 5000 hours.

4.Because of uniform heat, the finish obtained on the product is much better.

5.Because of the modular nature of the elements, you can save power by designing zones, the initial zone with elements of higher wattage and then the next zone with lower wattage. 

 

EXAMPLES:

Modern vehicle finishes are typically baked in the factory at approximately 130 C for 30 minutes for full hardening to take place. Powder coatings are about 40 microns thick and have a coverage of between 10 and 15 m2/kg. A suitable infrared panel placed at the recommended distance from the painted surface can provide fast heating without discoloration or solvent boil. The ability of IR energy to penetrate through the paint layer to the underlying metal means that they both heat up more uniformly. Thus, solvents are driven off before the outer skin is formed, removing the possibility of blisters. Moreover, blooming is prevented by ensuring that the metal temperature is not lowered below ambient by the process of solvent evaporation.