Curing is the final stage of the organic finishing process.
Curing has two stages. The first is the removal of the solvent
or diluent through evaporation so that the coating is no longer wet to
the touch. This is often called drying. The second stage is
the actual curing, during which the resins or binders in the coating material
are undergoing a chemical reaction. The reaction causes crosslinking
between the resin molecules and renders the coating film relatively inert
to the environment. In the curing of powder coating materials, because
no solvent or diluent is there to be removed, only the second
Regardless of the type of technology used, curing equipment
generates thermal energy that is absorbed by the coating and part.
The curing stage elevates the workpiece and coating
to a particular temperature and holds
that temperature for a set period of time. The combination of
temperature and time serves to evaporate solvents and set
the coating. Temperature and exposure times are carefully
monitored to ensure proper curing and di-ying.
Extended baking or exposure to heat sources may impair
the coating characteristics.
If ambient air conditions permit, curing of low-solvent
coatings can be completed in open areas. No
heat is generated or supplied to the area, but air
circulation may be enhanced by blowers and fans. Open air
curing is often done during the warmer summer months.
The type of curing method employed is often dictated by
the coatings materials used. Air dried coatings
are defined by the EPA as those that cure at room
temperatures, while those that cure at temperatures up to 194
F are classified as forced-air dried. Baked coatings require a curing
stage at temperatures above 250 F. Others coatings are classified
as radiation curable. The curing occurs when
the part and coating are exposed to infrared, ultraviolet,
microwave, or other radiation.
Two common curing technologies are convection ovens and
infrared radiation systems. Both provide consistent
curing of many different coatings. This chapter
describes each of these systems and their environmental