Why would you use a roller instead of a drawdown blade? Wire sticks instead of scrapers? There are many methods of fluid coating, and there are many techniques for using equipment. Aside from space constraints, original equipment manufacturers (OEMs) have made almost any fluid coating equipment slide in to replace existing equipment. So what are your options? Check out the table below:
| Roll application | pre-metered applications |
| direct gravure printing | slot die |
| Offset gravure | curtain |
| reverse gravure | rotating rod |
| meyer stick | slide |
| knife | multi-layer tank |
| comma | multi-layer slideshow |
| dip | |
| direct roll | |
| reverse scroll | |
| flexo printing |
Coating methods fall into two main areas: roller coating and pre-metered applications. Pre-metered applications typically require a slot die where the line speed of the pump and substrate determine the weight of the fluid coating. Roll application requires the fluid to flow between two rotating rolls, which determines the weight of the fluid coating and the uniformity of the coating fluid on the substrate. The above table does not cover all possible coating methods or variations of these methods. Many coating facilities have developed unique methods and techniques for individual products.
With so many options available, a good place to start is the product specifications. Let's take an example of an optically clear coating that needs to reduce handling and coating debris. This low level of contamination tipped the decision in favor of pre-metered coatings included in the slot die. If the coating does not require contamination control, but rather a quick changeover from one product to another, a self-metering unit with a comma coater may suffice.
Another consideration should be the scale of the fluid coating. When choosing a method, a distinction is made between laboratory, pilot and production scale. Many times, it is possible to use Mayer rods (wire rod coaters) in the lab in order to accomplish multiple handholds in a short period of time. This is fine if you are expanding to produce a Mayer bar fabrication 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 (transverse uniformity required, volume of fluid to be applied, 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 already has an understanding and skills 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. But don't just look at the list price! Make sure to keep in mind the requirements for changeovers, replacement parts, precision rolls and process constraints (line speed and rheological design of the fluid). The decision of which fluid coating method to use is multifaceted and requires some thought.
