Hot Melt Adhesive Pumping and Filtration: Gear Pump Selection and Filter Design
The pumping and filtration system is the heart of a hot melt coating machine, responsible for delivering molten adhesive from the melter to the coating die at a precise, consistent flow rate while removing contaminants that would cause coating defects. The two key components are the gear pump (also called a melt pump) and the filter (screen changer). The pumping system must handle adhesive viscosities from 500 to 50,000 mPa·s, temperatures from 100-200°C, and pressures up to 15 MPa. Gear pumps are overwhelmingly preferred because they provide positive displacement, low pulsation, and high accuracy. In hot melt applications, gear pumps are commonly used because of their ability to handle higher viscosity fluids, their resistance to higher temperatures, and their ability to accurately meter adhesive. When delivering adhesive at controlled rates, the accuracy of the pump is critical. A gear pump can increase and decrease proportionally to web speed and required coat weight, making it the essential component for closed-loop coat weight control.
Gear pump operating principles: Two intermeshing gears rotate in opposite directions within a tight-fitting housing. As the gears unmesh on the inlet side, they create a vacuum that draws molten adhesive into the pump. The adhesive is carried around the gear teeth in the cavities and expelled on the outlet side as the teeth re-engage. The theoretical flow rate Q = displacement (cc/rev) × pump speed (rpm). Displacement depends on gear geometry (tooth pitch, width). Actual flow rate is slightly lower due to slip (internal leakage) past the gear tips. Slip increases with viscosity decrease and pressure increase. For precision coat weight control, the pump must maintain volumetric efficiency above 90%. Advanced coating systems equipped with high-precision gear metering pumps achieve flow control accuracy of ±0.25%. Specially designed involute tooth profiles and nitriding processes stabilize gear pair clearances within 5μm at 280°C. Integrated with PID temperature control modules, the system confines adhesive viscosity fluctuations to ±3%, ensuring coating uniformity with a CV value ≤1.8%. 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
Pump sizing is critical. The required pump output (kg/h) = coat weight (g/m²) × width (m) × speed (m/min) × density (kg/m³) × 60 / 1,000,000. For a 1600mm line coating 20 gsm at 200 m/min with density 0.95 kg/L, output = 20 × 1.6 × 200 × 0.95 × 60 / 1,000,000 = 0.365 kg/min = 21.9 kg/h. A pump with displacement of 10-15 cc/rev would be suitable. Oversizing the pump forces it to run at very low rpm (e.g., 20 rpm), increasing slip and reducing accuracy. Undersizing forces high rpm (e.g., 300 rpm), causing excessive shear heating and potential degradation. A pump with 24 teeth running at lower RPM (20-80 rpm) creates less shear than a 12-tooth pump at high RPM (200-600 rpm). The pump should be sized so its normal operating speed is in the mid-range of its capability (e.g., 100-200 rpm). For demanding processes, a MAAG Therminox pump with special bearings and special clearances can be selected to achieve maximum pump efficiency and the correct lubrication. The first filtration unit was designed to transfer 15,000 litres per hour from a mixing vessel to a holding vessel, requiring a DSC148 single piston screen charger with 148.3mm screen diameter and oil heating channels within the body.
Filtration systems: Filters protect the die from gels, carbonized particles, and foreign debris. Without filtration, these particles would clog the narrow slot (0.1-0.5mm) and cause streaks or missing coating. Common filter types: (1) Screen pack in the pump inlet or discharge: mesh size 50-400 mesh (depending on adhesive cleanliness). (2) Screen changer (manual or automatic): allows filter replacement without stopping the line. (3) Cartridge filter: large filtration area, suitable for high-flow or high-contamination applications. The dual-column large filter cartridge changer 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 hot melt adhesive (e.g., unmelted particles, gels), preventing clogging that could cause poor adhesive flow or uneven curing, ensuring product purity. The mesh changer filters impurities while the melt pump homogenizes the melt, providing dual assurance for adhesive purity and performance consistency. This reduces product defect rates to below 0.5% compared to approximately 2%-3% in traditional processes.
Filter selection based on adhesive type: For clean adhesives (e.g., EVA for labeling), 100-200 mesh filters are used. For filled adhesives (with calcium carbonate or other minerals), coarser mesh (50-80) is used to avoid premature clogging. For optical-grade PSAs, 300-400 mesh may be used, but the pump must generate higher pressure (up to 15 MPa) to push the adhesive through the fine mesh. In all cases, monitor differential pressure across the filter: typical clean ΔP = 0.5-1.5 MPa. When ΔP reaches 2-3× baseline, change the filter. Ignoring rising pressure risks filter rupture, sending all accumulated debris into the die. For PUR adhesives, the dual-column large filter cartridge changer supports selection of different mesh sizes based on adhesive viscosity and impurity content, balancing filtration effectiveness with production efficiency. The filter block captures particles contained in the adhesive as the adhesive flows through the pump, with the removable filter positioned between the pump inlet and outlet. The systems in the HB 6000 series are equipped with a pump filter (50 mesh) as standard to prevent entry of impurities and burnt hot-melt adhesive residues conveyed by the pump into the heatable hose and the application head or handgun.
Maintenance best practices: Clean or replace filters at scheduled intervals; for screen packs, a common schedule is weekly for clean EVA, daily for filled or high-gel adhesives. For duplex filters (two parallel units), the operator can switch to the clean side while replacing the dirty filter without stopping the line—essential for 24/7 operation. Keep spare filters in stock. For PUR lines, never let the filter become completely clogged; the adhesive may crosslink inside the filter, making it irreplaceable. For lines producing optical products, use a magnetic filter before the fine screen to capture ferrous particles that may have originated from gear pump wear. Document filter change dates and differential pressure readings to predict clogging trends. A rise in ΔP over time indicates filter nearing end of life; a sudden drop indicates filter rupture (hole in screen) requiring immediate shutdown. The collaborative application of dual-column screen changers and melt pumps reduces downtime by 10%-20%, significantly improving production line utilization. The screen changer filters impurities while the melt pump homogenizes the melt, and this arrangement minimizes adhesive waste while stabilizing delivery to prevent spillage, with filter screen replacement frequency decreasing by 50% and annual raw material cost savings reaching approximately ¥200,000 based on a medium-sized production line. By properly sizing and maintaining the pumping and filtration system, manufacturers achieve consistent coat weight, long die life, and minimal defects, ensuring that the hot melt coating line operates at peak efficiency.
