Why would you apply with a roller instead of a knife; a rod instead of a slot die? There are many methods for fluid application and many techniques for utilizing the device. Outside of space constraints, Original Equipment Manufacturers (OEMs) make it possible to slide in virtually any fluid coating equipment to replace existing equipment. So what are your options? Take a look at the table below.
| Roller coating method | Pre-Measured Coating |
| direct gravure printing | slot die |
| gravure printing | the curtain |
| reverse gravure printing | rotating rod |
| meyer stick | slide |
| scraper | multi-layer slot |
| comma | multi-layer sliding |
| Dipping | |
| scroll directly | |
| reverse | |
| flexo printing |
Coating methods fall into two main areas: roller coating and pre-metered coating. Slot dies are often required for pre-metered applications, where the line speed of the pump and base plate determine the fluid coating weight. Roller coating applications require the fluid to flow between two rotating rollers, which determines the fluid coat weight and the uniformity of the coating fluid on the substrate. The above table does not cover all possible coating methods or variations of the coating methods offered. Many coating facilities have developed methods and techniques that are unique to individual products.
With so many options, a good place to start is the product specifications. For example, let's take an optically clear coating that needs to reduce handling and coating debris. This low level of contamination tends to dictate the pre-metered coating included in the slot die. If the coating does not require contamination control, but instead needs to be quickly changed from one product to another, then a self-metering arrangement with a comma coater may be sufficient.
Another consideration should be the scale of the fluid coating. The difference between laboratory, pilot and production scale is the choice of method. Many times, Mayer rods are used in laboratories to allow multiple manual operations to be performed in a short period of time. This is great if you are expanding into a Mayer bar production facility, but if the production facility is running gravure rolls, there are a number of process factors that will not scale properly. If possible, the method should be based on the physical parameters of the product (cross-web uniformity required, fluid volume to be coated, solution properties, number of coats and type of substrate).
Don't forget to remember expertise! Even if a product requires a specific type of coating method, a fluid coating operator has some knowledge and skill with the current coating methods used in your facility. Training for new coating methods requires some downtime, which needs to be factored into the return on investment of newly installed equipment.
However, the bottom line is always the bottom line. The cost of one fluid application method can vary significantly from the cost of another fluid application method. Don't just look at sticker prices! Make sure to keep in mind the requirements for changeovers, replacement parts, precision rolls and process constraints (flow rate and fluid rheology design). Deciding which fluid coating method to pursue is multifaceted and requires some thought.
