Continuous Filtration Systems for High-Purity Hot Melt Coating: Screen Changers and Pump Integration
For high-purity applications such as medical tapes, optical films, and electronic components, standard filter packs are insufficient. These demanding applications require continuous filtration systems that remove particles down to 10-50 μm without interrupting production. The solution is a screen changer—a device that holds two (or more) filter screens and allows switching between them while the line is running. A continuous filtration and dosing solution for hot melt adhesives with a heatable gear pump and control system allows for the production of highly viscous hot melt adhesives. For demanding processes, MAAG gear pumps and MAAG screen changers are the optimal solution. The filtration unit can transfer 15,000 litres per hour from a mixing vessel to a holding vessel. The DSC148 single piston screen charger has a 148.3mm screen diameter and oil heating channels within the body, selected to ensure the right level of filtration area at the given flow rate, resulting in an acceptable lifetime.
Types of screen changers: (1) Single-piston discontinuous: when the screen is clogged, the operator activates a hydraulic cylinder that pushes the piston, moving the screen out of the flow path. The line must stop briefly during this change (or the flow is diverted). Suitable for lower-purity applications or where occasional stops are acceptable. (2) Double-piston continuous: two screens in parallel; one can be removed and replaced while the other remains in the flow path. A rotating valve plate directs the flow to the active screen. This allows continuous operation with no pressure interruption. (3) Dual-column large cartridge: features a dual-column, dual-station design with one screen in operation and one on standby. Rapid switching via hydraulic/pneumatic systems (within seconds) ensures continuous production, eliminating efficiency losses caused by downtime associated with traditional single-column screen changers. The large filter cartridge design increases filtration area, effectively capturing impurities in PUR hot melt adhesive such as unmelted particles and gels. This prevents clogging that could cause poor adhesive flow or uneven curing, ensuring product purity. For a filtration system to transfer 2,000 to 2,500 litres per hour from a holding vessel to a cooling belt at a constant pressure and flow rate determined by the adhesive formulation, a MAAG Therminox 56/56 pump was selected. The MAAG gear pump design allows for pulsation free delivery of the adhesive to a very high degree of accuracy, resulting in better quality of the end product.
Hot Melt Coating Machine - Hot Melt Adhesive Coating Machine
Integration with gear pumps: In a continuous filtration system, the gear pump is typically placed after the filter. This ensures that any pressure drop across the filter is isolated from the pump’s outlet, and the pump can maintain a constant pressure to the die regardless of filter clogging. The pump is equipped with two helical gears; one is driven by a motor unit. The extruder fills the gears from the suction side, and the rotating gears carry the adhesive to the discharge side. A Siemens control system can be programmed to control the pump speed, monitor pressure, temperature, heating control, hydraulic power pack, safety interlocks, and alarm settings to inform operators when filter change is needed and if any of the various parameters fall above or below their limits. The integrated application of dual-column large-cartridge screen changers and melt pumps delivers three core functions: non-stop screen changing, precise flow control, and efficient impurity filtration. The melt pump compensates for extruder pressure fluctuations, preventing adhesive flow interruptions or overflow caused by unstable pressure to ensure production stability. For PUR hot melt adhesive extrusion lines, the combined application of dual-column large filter cartridge changers and melt pumps represents an optimal solution for enhancing production efficiency and ensuring product quality. Through coordinated filtration, conveying, and regulation functions, they form a complete chain of “impurity interception - flow stabilization - performance optimization,” significantly improving production continuity and product consistency.
Filtration area and pressure drop: The filter area must be sufficient to achieve an acceptable pressure drop and filter lifetime. If the filtration area is too small, the increase of productivity will lead to higher pressure growth, whereas the appropriate size of filter area will result in the opposite. For a given flow rate Q (L/h) and adhesive viscosity η (mPa·s), the pressure drop across a screen with area A (cm²) and mesh size is approximately ΔP ∝ η × Q / A. To keep ΔP below 5 MPa at maximum flow, the filter area must be large enough. The dual-column large cartridge design maximizes area within a compact footprint. In addition to filter contamination, mixing viscosity, sieve size, and usable filter area are the decisive factors of melt gear pump output pressure. A strainer or screen with large openings will prevent egregious errors (such as weld slag, nuts, bolts, or large clumps of material) from entering the pump and causing damage or malfunction. For finer filtration, systems can achieve particle retention down to 37 microns or smaller depending on the pump model.
For PUR (reactive) hot melts, filtration is especially critical because any cured particle will cause a permanent streak if it lodges in the die. Dual-column screen changers are preferred for PUR because they allow screen changes without exposing the adhesive to air, which would cause moisture-induced crosslinking. The screen changer body is heated (oil heating channels) to keep the adhesive molten. Corrosion-resistant materials: constructed from stainless steel or specialized alloys to withstand PUR hot melt‘s high temperatures (typically 120-180°C) and high-viscosity environments, reducing equipment wear and maintenance costs. When processing recycled PUR hot melt adhesive, screen changers efficiently filter impurities such as paper scraps and plastic fragments, and melt pumps homogenize the recycled melt, reducing raw material costs and supporting eco-friendly adhesive production. The entire system for PUR must be purged with a cleaning agent (e.g., wax) before any screen change to prevent curing. For high-volume lines, a automatic screen changer with a timer or pressure-actuated change can run unattended for days.
Maintenance and troubleshooting: For continuous filtration systems, monitor the hydraulic pressure that advances the screen. If the frequency of screen changes increases over time, the upstream melter may have excessive char buildup. Clean the melter thoroughly. If the pressure drop across the new screen is unusually high, the filter mesh may be too fine for the adhesive; use coarser mesh or a larger area filter. If particles appear in the coating despite filtration, the filter may have a hole or the seals may be leaking. Inspect the filter assembly. For the second unit that doses 2,000 to 2,500 litres per hour of mixed adhesive, a MAAG Therminox 56 size pump was selected as a lower flow was required. For optimal filter performance, the control system must monitor pressure, temperature, and safety interlocks to inform operators when filter change is needed. Regular maintenance includes cleaning the screen changer body, inspecting hydraulic cylinders for leaks, and replacing seals annually.
