FLOW COATING APPLICATIONS
Flow coating is a useful application when it is desirable to coat only one side or when it is desirable to coat different sides of a part with different coatings. Flow coating can be used on large or small parts, although the technique has some limitations on parts that contain holes, protrusions, or other surface irregularities. Herein, we are not referring to a slot-die flow coating that the part is moving below a stationary die.
Flow coating is an automatic process, used for optically clear, abrasion-resistant coatings for thermoplastics used on displays, sheets, automotive interiors, exteriors, and many other parts. Flow coating consists of controlled gravity flow of a coating solution over the substrate. Excess coating is collected, filtered, and combined with a fresh coating and make-up solvent, and then re-used. This results in less coating waste and lower usage. Flow Coating can achieve smooth surface of high thickness in one operation, as opposed to applying several coats. Flow coating overcomes some of the limitations of dip coating, as the volume of coating required in the system is lower than with dipping. Flow coating can be done with parts on a chain on edge conveyor that takes them through a flow system enclosure. Flow coating, like dip coating, has problems with the wedge effect and solvent reflux. The process requires careful control of evaporation to obtain uniform coatings. In both the dipping and flow coating methods, product orientation is important to its final appearance. While the process is surprisingly simple, flexible and easy to maintain, there are many considerations in part design, coating and equipment selection which must be addressed to determine whether flow coating is the optimal process for a given application.
The basic equipment for a flow coating application consists of a support fixture and a delivery system comprising a drip trough, a coating reservoir, a circulation pump, a filtering element, and a coating delivery nozzle. In a typical flow coating operation, such as coating of sheets, the coating is applied by moving the coating delivery system from the bottom left-hand side of the part, up along the left-hand side, then slowly across the top of the part, and down the right-hand side. As the coating is deposited on the part, it flows down the vertical surface and eventually covers the entire part. The excess coating solution that drips off the part is collected in the coating reservoir where it is fed back through the filter elements to the delivery nozzle.
It is desirable in a flow coating application to be able to control the coating delivery rate. In general, it is desirable to “flood” the part with coating, which allows the coating to carry particular debris off the part and helps with the adhesion by increasing the coating’s “wet time” on the part.
The circulation system should provide a sufficient turnover rate to adequately filter the coating solution. Ideally, the filtration system should consist of two or more filters in series, a configuration that allows for the use of a coarse prefilter and finer subsequent filters. The optimum filtration configuration will depend on the coating being used and the rate of circulation through the filters. In a typical flow coating application, the circulation rate is limited by the allowable delivery rate. In general, the delivery rate should be set as high as possible, while maintaining flow control and avoiding bubble generation or splatters as the coating flows onto the part. The flow rate should also be adjusted to avoid creating a large pressure differential (>10 psi) across the filter elements. A high-pressure differential across the filters can cause coating gels to extrude through the filter elements. The size of the filter elements can be adjusted to reduce a high-pressure differential, although this may also affect the cleanliness of the system. In any coating application, the amount of coating that is deposited on the part, i.e., the coating thickness is a function of the coating solids, solvent composition, temperature, and viscosity. The coating thickness can also be influenced by the coating environment. Anything that causes the coating to dry quickly, e.g., high coating or environmental temperature, low humidity, high airflow over the part, part high temperature, etc. all can contribute to higher coating thickness. In a flow coating application these factors will also add to the normal “wedge effect” which results in a coating thickness gradient. The coating thickness gradient increases from the top to the bottom of a coated part. This “wedge effect” can be significant on a large part.
It is generally most effective to modify the coating solids to adjust the coating thickness, while keeping the other coating properties and environmental factors constant. Controlling the coating thickness is important in achieving optimum performance of PCI Vueguard 901™ coatings. The coating thickness will affect the abrasion resistance and possibly other performance properties of a product. An optimized coating thickness can be determined empirically, according to the desired performance features of the final coated product.
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