In the second of this two-part series on hot melt coating, Mark Miller says the newest technology is not always the best, but the best techniques applied to good technology provide the best product outcomes.
TECHNOLOGY AND TECHNIQUE
So how does a modern hot melt coater keep on the cutting edge of technology? The key is to balance the economic concerns of the product and process with increased productivity while maintaining or improving quality. Hot melt adhesive will have an internal rheological strength and “breaking point” at which the adhesive will be stretched and not recover, causing film split. This “breaking point” can only be improved through reformulation (mostly) and some process changes.
Reformulation is outside the context of this discussion, but is a major contributor to the processability of the adhesive. Process considerations include both the technology and technique. The major contributing factors in the process technology include the coating head, the backing roll, the adhesive delivery system, the substrate in question, and the overall control system. In the current state of the art, a rotary rod slot die is utilized in combination with a rubber backing roll to provide high precision, streak-free coating. The technique involved in the art of choosing, utilizing, and positioning the rod to the roll is what makes hot melt coating fast and thin possible.
When hot melt die coating first started out, compliant rubber back-up rollers were used. Because the coated film consistency was directly related to the geometry of the back-up roll, the industry began to move toward precision ground chrome back up rollers. This move was made possible by advances in die positioning systems that prevented die collisions with the non-compliant metallic back-up roller. The hard surfaced chrome rollers provided excellent geometry for consistent hot melt coatings.
As hot melt adhesives began to be used in more applications, the demands for thinner and clearing constructions also began to rise. As hot melt coatings are getting thinner, fluid dynamics require the die to get closer to the coated substrate. As this proximity goes to zero, gels present in hot melt coatings begin to get trapped in the feed gap and cannot exit the die. When this happens, the gels drag on the surface of the substrate and create intermittent and random streaking. The ironic solution to this problem is to utilize the older technology of rubber back-up rolls. The rubber back-up rolls, when used with a rotating rod slot die, is compliant enough to let the gels escape, therefore preventing streaks.
While the rubber roll helped to solve the streaking problem, the issue of roller geometry still exists. Fortunately, technology also has progressed allowing rubber rollers to be manufactured with tolerances the same as chrome rollers. It is now possible to manufacture and verify rubber cover tolerances of TIR < 0.0003 in. and Cylindricities < 0.0005 in. These advances now make it feasible to use rubber covers where chrome used to be the only choice. Ultimately, the technology need for thin and fast hot melt coating requires “compliant steel” rollers or rollers made of very hard rubber. A rule of thumb is that the thinner the adhesive coating, the harder durometer rubber roll is required (80-90 Shore A Durometer for sub-1 mil coatings). This harder roll also is easier to maintain the physical properties of roll runout and straightness.
How the precision rubber roll interacts with the rotating rod in the slot die is the magic that makes thin adhesive coating work. The physics of the rod to roll interaction improves the behavior of the adhesive and contributes to the removal of streak defects in the coated product. Streaks are caused when gels (solid particles) get stuck in the feed gap of the die and cannot escape. The rod intersects the roll at a tangent point and imbeds the rod up to 1 mil (0.0254 mm) into the rubber roll. This roll deflection causes the web to slip past the rod. As the rod turns and the roll moves the substrate past the rod, the adhesive has to catch up to the substrate “slip.” The slip and rotation of the rod allows the gels (solid particles) that are inherent to hot melt adhesive to pass as point defects.
Some key techniques to utilize when troubleshooting a thin hot melt coating include making sure the rod is tangent to the roll, rotating in the direction of coating, the rod is imbedded in the roll so deflection is occurring, and the attack angle of the die to the substrate creates a coating bead (“rolling bank”) of adhesive on the dry side of the coating station.
These detailed presentations on the physics behind hot melt coating do not end the story on how to successfully coat hot melt adhesive. The constant, pulse-free delivery of adhesive by a positive displacement pump is necessary. Consistent temperature profiles from adhesive melting to coating head delivery is critical to maintaining rheological flow characteristics. A control system that maintains substrate tension and process speeds is also critical.
In the end, the newest technology is not always the best, but the best techniques applied to good technology provide the best product outcomes. Utilizing a rotary rod die and rubber backing roll to precision coat hot melt adhesives provides the most efficient and controllable method for coating in the current state of the art.
As the pressures to reduce emissions and maximize productivity, economy, and quality escalate, we need to look at the best operating procedures to gain an edge. To this gain, the precision of the manufacturing of the equipment in interface is important to the outcome of the product. The adhesive will “remember” the last surface it sees, and if the roll does not have a precision surface, neither will the product.
Improvements in dimensional tolerances made possible by precision machining of these surfaces may seem minuscule, but in applications involving high volumes or costly coating materials, the economic gains can be great. The effect of variations becomes progressively greater as coating weights become smaller.