The adhesive application nozzle is either an integral part of the jetting element or is screwed onto a jetting element. You can easily replace screw-in nozzles yourself, which brings us to the heart of the topic of this blog post.
Each product or format sets a different demand on the quality of the adhesive application. Gluing primary packaging for pralines, for example, requires only one tiny adhesive dot placed precisely and a cut-off without stringing. With transport packaging made of corrugated cardboard, the situation is quite different. There, millimeter-thick, long beads are required to ensure that the cardboard boxes remain securely closed despite high return forces. The quality of the cut-off plays a secondary role.
When applying adhesive, the application nozzle is responsible for the following parameters:
There is a complex interaction between the diameter and geometry of the application nozzle and the channel length. However, the following rules of thumb offer welcome assistance:
✔ The smaller the nozzle diameter, the less adhesive applied and the more precise the adhesive cut-off.
✔ The better the heat conduction through the geometry of the nozzle and the shorter the nozzle channel, the cleaner the adhesive cut-off.
✔ The channel length determines the distance at which the jetting head can be positioned to the substrate.
Hot melt adhesives differ in many respects. The viscosity and temperature sensitivity of the adhesive are of particular interest when choosing the application nozzle. The following applies here:
✔ The higher the viscosity and/or the faster the hot melt adhesive tends to coke, the larger the nozzle diameter should be.
Larger nozzles have the advantage that they are less prone to clogging. But they also automatically apply more adhesive, which proves a favorable setup for gluing corrugated cardboard, mattresses or wood.
However, for high-end applications such as gluing primary packaging, a small nozzle diameter needs to be selected. In this case high-quality hot melts, such as metallocene-based adhesives, should be used. These have a very clean, reliable cut-off behavior. As a result, small diameter nozzles remain clean for longer, which is not the case when using less thermostable adhesives.
This usually results in quality problems, such as stringing, too much adhesive on the substrate or too little. Obviously, too little adhesive application leads to a lack of adhesion and thus to production rejects. On the other hand, excessive adhesive application jeopardizes the safety of the bonding, as an increasing amount of adhesive prolongs the bonding time. That is the time it takes for the adhesive to develop a sufficient level of adhesion between two substrates. The nozzle diameter, adhesive application quantity, opening time, and pressing time are no longer optimally aligned. All in all, it is a scenario that has an adverse impact on operating costs because both rejects and adhesive are cost drivers.