CURING METHODS (2024)

EPA Self-Audit and Inspection Guide
Organic Finishing of Metals

Table of Contents

CURING METHODS

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 stage occurs.

Regardless of the type of technology used, curing equipmentgenerates thermal energy that is absorbed by the coating and part.The curing stage elevates the workpiece and coatingto a particular temperature and holdsthat temperature for a set period of time. The combination oftemperature and time serves to evaporate solvents and setthe coating. Temperature and exposure times are carefullymonitored to ensure proper curing and di-ying.Extended baking or exposure to heat sources may impairthe coating characteristics.

If ambient air conditions permit, curing of low-solventcoatings can be completed in open areas. Noheat is generated or supplied to the area, but aircirculation may be enhanced by blowers and fans. Open aircuring is often done during the warmer summer months.

The type of curing method employed is often dictated bythe coatings materials used. Air dried coatingsare defined by the EPA as those that cure at roomtemperatures, while those that cure at temperatures up to 194 F are classified as forced-air dried. Baked coatings require a curingstage at temperatures above 250 F. Others coatings are classifiedas radiation curable. The curing occurs whenthe part and coating are exposed to infrared, ultraviolet,microwave, or other radiation.