When using Hot Melt Adhesives, one critical variable in slot die coating is the proper thermal conditioning of the adhesive. Thermal conditioning refers to heating the adhesive to the required temperature before applying it to the substrate. A very common misunderstanding is that production can start once the heating grid inside the melter indicates the desired temperature has been reached, but the grid temperature does not represent the actual temperature of the adhesive. The key to proper thermal conditioning is allowing sufficient time for all adhesive in the system to reach the same required temperature level. Since different types of adhesives vary in heat absorption and retention properties, some adhesives may require a longer "heat soak time" to stabilize the temperature before being applied to the substrate.

There is no simple formula to calculate the "heat soak time" of an adhesive, but one determining factor is the standard ring-and-ball test, used to determine the softening point of the adhesive, which is usually provided by the supplier. This can serve as an indicator of the adhesive's heat absorption capacity. The second factor is the adhesive's composition and whether it is amorphous or crystalline. Amorphous adhesives absorb heat slowly, while crystalline adhesives are highly heat-resistant until reaching a certain temperature. While it is important to introduce heat into the adhesive to achieve a consistent temperature, it is equally critical not to overheat the adhesive to the point where its bonding performance degrades or it even burns. Both conditions will result in unstable end-product quality.

To prevent any temperature inconsistencies or thermal degradation, the following guidelines should be followed:

"On-demand melting" is a proprietary equipment technology that melts only the amount of adhesive required to meet production demands.

Avoid "slow cooker" style melters, which only heat the perimeter of the melt tank and require extremely high external temperatures to transfer heat to the center of the tank, often leading to thermal degradation and/or charring of the adhesive.

Select melters with the highest power and largest internal heating surface area to maximize contact with the adhesive; the larger the heating surface area and the higher the power, the more consistent the adhesive's temperature profile will be.

To fully ensure temperature consistency before coating, an immersion sensor should be installed at the inlet of the coating head to continuously measure the adhesive temperature in the fluid stream. The immersion sensor should be connected to the power grid via closed-loop control to automatically adjust temperature settings and ensure consistent output.

The second key variable to ensure process consistency is system pressure, to guarantee process repeatability. The fluid delivery system must be able to provide constant pressure to the slot die for optimal production quality. Variables that can cause system pressure fluctuations are often related to the gear pumps on the adhesive supply unit. Accuracy decreases when fluid flow from a single pump to the slot die is diverted, or when multiple pumps feed a single slot die opening.

To address this issue, several different methods can achieve precise and repeatable pressure in your process. First is the pump itself; only precision gear pumps with 1–2% accuracy per revolution should be used. Additionally, a pressure sensor should be installed at the die inlet to continuously monitor inlet pressure. This reading should be connected to a closed-loop control system that feeds pressure back to the melter and automatically adjusts pump speed to ensure precise die pressure is maintained every time.