Troubleshooting Hot Melt Coating Weight (gsm) Variations: Causes and Corrective Actions
Even with modern hot melt coating machines, variations in coating weight (gsm) can occur. Understanding the pattern of variation helps identify the root cause. Random weight fluctuations that have no clear periodicity are often due to temperature instability (check PID tuning and sensor placement) or inconsistent adhesive feed (check hopper bridging or pre-melter ramp). A slow drift of weight over several minutes is typically caused by filter clogging (pressure rises, pump slip increases, weight drops) or gradual temperature change (ambient or heater failure). Correct by replacing filter or checking heater contactors.
Periodic weight variations that repeat every few seconds or per roll revolution are often mechanical. For example, if the variation frequency matches the gear pump’s tooth engagement frequency (pump rpm times number of teeth), the pump is the source. Gear pump pulsation is inherent, but can be minimized by using a pump with more teeth (e.g., 24 instead of 12) and installing a hydraulic damper. If the variation matches the backup roll rotation speed, check the roll runout; a bent roll or worn bearing causes the gap to oscillate, modulating weight. The solution is to re-grind the roll or replace bearings. If the variation matches the cooling roll or other idler, they may be out of round or have a flat spot.
Hot Melt Coating Machine - Hot Melt Adhesive Coating Machine
Cross-web coating weight variation (thicker on one side or edges vs center) is a different class of problem. If the weight is consistently higher on the operator side, check if the die flexure bolts on that side are too loose, or if the temperature on that side is lower (higher viscosity picks up more). Use a thermal imager and tighten/loosen bolts accordingly. If the center is heavier, the backup roll may be crowned too much; if edges are heavier, the crown is insufficient. Also, check for die bolt torque uniformity. For slot dies, a “smile” or “frown” profile of the lip gap can be measured with a feeler gauge; adjust the die’s main flexure bolts to flatten the gap.
Gsm variations that occur only at high speeds indicate air entrainment or cavitation. At high speeds, the pump may require more inlet pressure to fill the gear teeth; if the tank is not pressurized (e.g., 0.5-1 bar of nitrogen), cavitation causes random weight drops. Install a pressure gauge at pump inlet; maintain at least 2 bar positive pressure for high viscosity. Alternatively, increase the temperature to lower viscosity. If there are bubbles in the coating, also check the die’s internal degassing port.
Variations related to web tension changes are common at splice points or when accelerating. When the unwind roll diameter decreases, if the tension controller does not compensate quickly, tension may spike and stretch the web, reducing gsm (since the same adhesive mass spreads over a larger area). Use a dancer roll with low inertia and fast loop response. Additionally, if the web is slipping on the pull roll, erratic tension will cause weight variation. Clean the pull roll and increase wrap angle or use a vacuum roll.
Finally, a systematic troubleshooting approach for gsm precision: first, verify the accuracy of the online gauge by taking offline sample cuts and weighing (5 samples per roll). If gauge is correct but variations persist, log key parameters (temperature zones, pump speed, line speed, tension, and filter pressure) at 1-second intervals. Overlay these with the weight measurement. A correlation will often reveal the cause. For example, weight peaks align with pump speed peaks; then the pump drive encoder may be faulty. Or weight drops as filter pressure rises; then schedule filter change more frequently. By applying such analysis, operators can pinpoint and resolve gsm precision issues, achieving consistent product quality.
