The Hot Melt Adhesive Melting Process: From Solid Pellets to Uniform Coating Fluid
The melting process is the first and most critical stage in hot melt coating, where solid adhesive—supplied as pellets, blocks, granules, or bulk drums—is transformed into a molten fluid with precise viscosity and temperature ready for application. Unlike solvent-based systems where the adhesive is already liquid, hot melt requires controlled heating to achieve flowable consistency without degrading the polymer. Generally speaking, the hot-melt adhesive is fed (manually or via automated systems) into the adhesive melter. As the adhesive melts, it is pumped through heated hoses from the melter to an application system such as a dispensing gun or slot die. The melting process must balance three competing demands: complete liquefaction, minimal thermal exposure time, and uniform temperature throughout the melt. Poor melting leads to unmelted particles that clog dies, inconsistent viscosity that affects coat weight, or degraded adhesive that causes gelation and char formation, all of which compromise product quality.
Tank-style melters are the most common design for industrial hot melt coating machines. These use cast-in heaters embedded in the tank walls, typically located at the bottom of the tanks, beginning melting at the tank floor. Heat is then conducted up the sides of the tanks, which helps maintain consistent adhesive temperature. The adhesive gradually softens as it contacts the heated surfaces and flows downward toward the pump inlet. Tank melters are simple and robust but have a significant disadvantage: they hold a large volume of molten adhesive (typically 5-20 kg or more). This large reservoir means that adhesive may remain in a molten state for extended periods—hours or even days—leading to thermal degradation, oxidation, and char formation. Degradation is one of the most visible signs of money loss in a melter. Each piece of char that breaks loose and clogs the system—causing downtime and maintenance—has a dollar amount attached to it. For applications where adhesive is used continuously, tank melters can be acceptable if the tank is drained and cleaned regularly (e.g., weekly). However, for intermittent production or sensitive adhesives like SBC-based PSAs, the large residence time in tank melters can degrade the adhesive, causing gelation and requiring more frequent filter changes.
Hot Melt Coating Machine - Hot Melt Adhesive Coating Machine
Grid and reservoir melters (also called grid melters or tankless systems) offer a more advanced melting solution that minimizes thermal degradation. These incorporate a heated grid and reservoir at the bottom of a hopper, accommodating higher melt rate demands than tank melters. A tankless system uses a melt-on-demand heated grid and a small reservoir to efficiently melt the adhesive granulate and then keep a small amount of molten adhesive ready for dispensing. The reservoir holds an estimated 1 lb (0.45 kg) of molten adhesive, compared to the 5-20 lbs (2.3-9 kg) held by a traditional tank melter. A sensor monitors the level of adhesive and sends a low-level signal to the system to add a small amount of granulate. This process occurs at frequent intervals to keep the amount of molten adhesive relatively consistent and minimize thermal shock. The bulk adhesive granulate is stored in a large container at ambient temperature until it is automatically fed into the melter using pneumatic transport technology. Melting grids in hopper type melter units are the primary devices for transforming the adhesive from its initial form into a molten or at least semi-molten state, consisting of heating fins and melt spaces that allow for close control of heat energy addition.
Bulk drum melters (or pail melters) are used for high-volume applications where adhesive is supplied in 200 liter (55 gallon) drums or smaller pails. A drum unloader uses a round, heated platen that fits into the bulk adhesive container. An axially finned surface design on the platen provides increased melt surface area and higher melt rates. The molten layer of adhesive is pumped through a port in the platen assembly and delivered through a heated hose to a melt tank (buffer) or directly to the coating head. If using a drum unloader, the tank acts as a buffer when changing out adhesive drums, allowing the coating line to continue without stoppage. A series of sensors communicates with the drum unloader to maintain correct adhesive levels in the tank, avoiding shortage or overflow. Drum melters are efficient for continuous high-volume production but require additional investment in the platen and control system. For PUR hot melts, a drum melter with a nitrogen blanket is used to prevent moisture exposure. The Robatech JumboFlex melters, designed for adhesive viscosities up to 65,000 mPas, can process high-viscosity adhesives with up to 100,000 mPas once melting performance is adjusted, making them suitable for demanding applications.
Temperature control during the melting process is essential. Conventional hot-melt adhesive dispensing systems typically operate at temperature ranges sufficient to melt the received adhesive and heat it to an elevated application temperature prior to dispensing. Single-stage melters maintain the adhesive within the melting chamber at the application temperature at all times, which can lead to degradation if the adhesive sits idle. Dual melt zones (or progressive melting) are a feature of advanced systems like Nordson’s melters, providing staged heating: a lower temperature zone for initial softening and a higher temperature zone for final viscosity reduction. This minimizes the time adhesive spends at peak temperature. The grid type melter includes one or more heating fins and melt spaces, allowing for close control of heat energy addition to the material to be melted. For PUR hot melts, the RobaPUR 20 MOD applies only minimal heating to the adhesive in the hopper, while high melting capacity from the press cylinder and efficient melting grid, along with rapid cooling, prevents premature crosslinking. UV hot melt systems (UVHotMelt.com) describe a melt tank that uses a series of heated grids at the bottom of the reservoir to maintain consistent temperature during delivery of the adhesive to the head.
Preventing degradation during melting requires disciplined operation. For tank melters, never let the tank level drop below 25% capacity—low levels increase oxidation and degradation. Drain and clean the tank weekly, using a brass scraper to remove char from walls. For tankless grid melters, the small reservoir can reduce energy usage by an average of 10-38%; some tankless melters can reach the “ready” state up to 50% faster than a traditional tank melter. This results in more production uptime and less energy wasted on keeping large volumes of adhesive melted. Advanced systems also have temperature setback settings, which can be used to keep adhesive warm during breaks without wasting energy holding the entire system at full temperature. Melting adhesive on demand keeps char and sludge from developing inside the system, which reduces nozzle clogs and damaged modules, and lowers the amount of filter changes and system flushes required to keep the system fully operational. For all melter types, use a nitrogen blanket (0.1-0.2 bar positive pressure) on the headspace for oxygen-sensitive adhesives (polyamide, PUR, some SBCs) to exclude oxygen and prevent oxidation. For PUR, the RobaPUR system’s minimal heating in the hopper and efficient melting grid with rapid cooling prevent thermal degradation. By selecting the appropriate melter technology for your production volume and adhesive sensitivity, and following strict operating and cleaning protocols, you maintain adhesive integrity from the melting stage through to the final coating, ensuring consistent product quality and minimizing waste.
