Hot melt adhesive melting process in coating machine
The hot melt adhesive melting process is the foundational first step in any hot melt coating operation. Unlike solvent-based or water-based adhesives, a hot melt is a 100% solid thermoplastic polymer. It has no viscosity at room temperature and must be transformed into a workable liquid before it can be pumped and applied. This transformation occurs within the machine's melting unit or "melt tank," which is designed to efficiently and uniformly raise the temperature of the adhesive to its precise application point, typically between 100°C and 220°C, without causing thermal degradation. The quality of this melting process directly determines the uniformity of the final coated layer and the long-term performance of the equipment.
The process begins when solid hot melt adhesive, which comes in various forms like pellets, blocks, or pillows, is loaded into the heated melting grid or tank. Modern melt tanks, such as those with an aluminum alloy cast body and a two-layer structure, are designed for even heat distribution. The inner surface is often coated with a non-stick material like Teflon to prevent the molten adhesive from charring and sticking to the walls. The melting is usually done in stages: the lower section of the tank acts as a reservoir for the molten adhesive, while the upper section and grid are responsible for melting the newly introduced solid material. Some systems use a progressive melting concept to maintain the integrity of the adhesive, ensuring it melts at the correct rate and does not overheat. For very high production, large capacity tanks (e.g., 60 kg) with high melt rates (e.g., 60 kg/h for EVA) are used.
Hot Melt Coating Machine - Hot Melt Adhesive Coating Machine
Precision temperature control is the most critical aspect of the melting process. The adhesive's viscosity is an inverse function of its temperature; a small change in temperature results in a large change in viscosity, which affects coat weight. Therefore, hot melt coaters employ advanced PID (Proportional-Integral-Derivative) temperature controllers on the melting tank, hoses, and coating head. These controllers, often with an accuracy of ±1°C, maintain a stable temperature profile. Many machines feature independently controlled zones—like top and bottom layers in the tank—to prevent overheating of the adhesive at the bottom while ensuring solid pellets at the top melt sufficiently. This multi-zone approach prevents "hot spots" that can cause the adhesive to char and form troublesome gels, which can clog the die and create coating defects. The entire system is designed to bring the adhesive up to temperature quickly but controllably.
The melting process is more than just heating; it is about careful thermal management. Solid adhesive is a poor conductor of heat; if too much is added at once to an underpowered tank, the temperature can dip significantly, leading to incomplete melting and high viscosity, which can starve the pump. Conversely, if the tank is over-heated or the adhesive is left at a high temperature for too long, the polymers can undergo oxidation and cross-linking, leading to charring and gelling. Many modern machines address this by having a "standby" mode that significantly lowers the temperature during production pauses, preventing degradation while keeping the adhesive molten. For specialized adhesives like PUR (polyurethane reactive), which are moisture-sensitive, the melting process is performed under a protective gas blanket, and the machine must be purged at the end of each shift to prevent the adhesive from cross-linking and solidifying inside the equipment.
From a workflow perspective, the melting stage is the beginning of a synchronized system. Once the adhesive in the tank reaches its set temperature and becomes a low-viscosity liquid, it is ready to be pulled into the gear pump. The pump then moves this precisely heated adhesive through a heated hose, which maintains the temperature during transit, to the final distribution point: the coating die. The entire system from the melting tank to the die lip must be maintained at a consistent temperature to ensure the adhesive's viscosity is uniform as it exits the die. A failure in any one heating zone will have an immediate impact on coating quality, underscoring that the melting process is not just a preparatory step but an integrated part of the precision coating system. This solvent-free process is what makes hot melt coating a clean and environmentally friendly technology.
