Roll coater systems are utilized to coat flat substrates by employing a combination of various rollers to achieve the desired coated surface. These coating systems, utilizing rollers, can accommodate different coating fluid viscosities and thicknesses through the integration and specialization of numerous technologies. Consequently, roll coaters have found applications across various industrial sectors, including electronic components, optical and LCD products, food, and medicine.
Typically, their use is restricted to coating relatively thin flat substrates such as films and sheets. However, wide and long substrates can also be continuously and rapidly coated. Primarily used for wet coating, in some instances, the process is conducted within a cleanroom environment to prevent foreign particle adherence to the wet surface prior to solidification.
Types of Roll Coater Methods
Roll coater methods can be categorized based on the state of the coating fluid before coating and whether the coating amount is determined before or after coating. Although some systems employ several methods, the following section outlines examples of typical coating methods.
The classifications below serve as examples. There are various classification methods, and some may deviate from the descriptions provided below.
Open Coating
This method typically involves a coating head and a coating reservoir with exposed coating fluid to the atmosphere. Gravure coaters and reverse coaters are typical examples.
Gravure Coaters
A gravure roll with an unevenly engraved surface is immersed in the coating fluid in the reservoir. After wiping off the coating fluid on the gravure roll's surface with a drawdown blade, the remaining fluid in the pits is transferred onto the substrate web. Altering the coating thickness necessitates changing the gravure roll to one engraved with a different pattern. Offset gravure coaters deliver a smooth and uniform coated surface at high speeds.
Reverse Coaters
Reverse coaters typically comprise an applicator (coating) roll, a backup roll, and a metering roll. The backup and applicator rolls rotate in the same direction to transfer the coating fluid onto the substrate web. The coating thickness is determined by the gap between the applicator roll and the metering roll, as well as their speed.
Closed Coating
This method prevents the coating fluid from exposure to the atmosphere before coating. Slot die coaters and lip coaters adopt this approach.
Slot Die Coaters
Coating fluid is passed through a die to create a uniform film, which is then applied to the substrate to achieve coating. Since the coating fluid is pressure-supplied to the die from an enclosed tank, it remains unaffected by air, resulting in stable coating.
Lip Coaters
Lip coaters feature a numerically-controlled coating head for easier quality control. Coating fluid is supplied to the lip nozzle from a lower cavity to the upper cavity at the end of the lip nozzle, where it is applied to a moving substrate.
Post-Metering Coating
"Pre-metering" coaters determine the coating amount at the time of application. Conversely, "post-metering" coaters apply additional coating and then remove excess to achieve the target amount. Knife coaters and blade roll coaters fall into this category.
Knife Coaters
Knife coaters consist of a knife roll, backup roll, and liquid dam. The coating fluid in the liquid dam is transferred onto the substrate web through the gap between the turning backup roll and the fixed knife roll.
Blade Roll Coaters (with Comma-Type Circular Blades)
Blade roll coaters are characterized by an applicator roll with a blade, referred to as a circular blade or comma blade, due to its shape. Coating fluid is pooled between the applicator roll and the substrate, and the blade on the roll wipes off excessive fluid to ensure an optimum coating thickness.
Other Examples of Methods
Apart from the aforementioned methods, various other methods are available based on the coating purpose or the properties of the substrate or coating fluid. These include blade coaters, chamber doctor coaters, Bar Coaters, and air knife (air doctor) coaters.
