Slot-die coating is a relatively new precision coating technique, which is increasingly replacing standard roll-coating methods because of several technology advances that have increased the quality, productivity and range of products that can be coated. These advances, along with improvement in metal fabrication and metrology, have resulted in low cost, precise flexible die designs.
The slot-die coater is a pre-metered coating method in which all of the coating solution metered to the die is applied to the web. A key component of the application system is a precision pump to deliver the coating solution to the slot die. The flow line can also include filters, temperature controls, bubble removal and the ability to inject additives prior to the slot die.
The slot die has several advantages when compared to current roll-coating methods:
• The coating quality and appearance is excellent for a wider range of coating weight, and it is very reproducible.
• There is no exposure of coating solution to the open atmosphere. This improves emissions control and safety because no vapors are introduced into the coating room.
• Since all of the coating solution is applied to the web, there is no recirculation loop, as in roll coaters, that can introduce contaminants and bubbles into the solution. Thus, the coated solution is cleaner, thereby, reducing defects.
• Coating-weight transverse direction uniformity of 1-2% [(high c.w.-low c.w)/average] can be obtained and is better than most of roll-coating methods—typically 5-10%.
• The coating weight can be easily controlled by adjusting solution flow rate. Also, in long coating campaigns reproducibility is excellent because coating weight is not a function of viscosity as in roll coating.
• Three layers can be simultaneously applied.
• Rapid-acting coating ingredients can be injected prior to the slot die, thus eliminating aging effects.
• Boosting line speed increases the solution flow rate, which improves quality and increases the acceptable operating range for good quality.
• A vacuum source applied to the underside of the coating bead will improve coatability and quality.
• The slot die can be adapted for patch coating both MD lanes and shaped areas.
• Typical operating conditions are: Viscosity—5-20,000 centipoise; Wet thickness—50-340 microns; Line speed—20-1,700 fpm; Accuracy—2%; and Layers coated—three.