Root Cause Analysis of Streaks in Hot Melt Coating: Machine Direction Streaks
Streaks are linear defects parallel to the machine direction (MD) in hot melt coated webs. They are the most common coating defect and can appear as thin lines (no adhesive), thick lines (excess adhesive), or variable lines. Understanding the root cause is essential for rapid correction. Streaks are almost always caused by a localized variation in the die lip gap or adhesive flow. This article categorizes streak types by appearance and provides a step-by-step diagnostic procedure. Use a magnifying glass or low-power microscope to examine the streak pattern. Also, note whether the streak is fixed in position (same location across multiple rolls) or wandering (changes position). Fixed streaks indicate a permanent defect on the die lip or backup roll. Wandering streaks may be caused by particles moving or web defects.
Type 1: Thin or missing adhesive streak (dry streak). Appearance: a clear line where the substrate is visible. Causes: (a) Die lip contamination – a small particle (carbon, gel, dust) lodged on the lip prevents adhesive from exiting. Solution: Clean the die lip with a brass scraper while hot. If particle is stubborn, use a soft brass wire to gently dislodge. (b) Die lip scratch – a groove on the lip creates a gap that is too small or allows air. Requires die regrinding. (c) Clogged manifold slot – a foreign object partially blocks a section. Purge die with cleaning compound or disassemble. (d) Backup roll defect – a low spot on the roll reduces pressure, causing incomplete transfer. Check roll runout. Action: Inspect the die lip under magnification. If a particle is visible, remove it. If a scratch is found, note its location. For temporary workaround, you can sometimes mask the streak by adjusting die bolts on either side to increase flow, but permanent fix requires grinding.
Hot Melt Coating Machine - Hot Melt Adhesive Coating Machine
Type 2: Thick adhesive streak (heavy line). Appearance: a raised line of adhesive, often glossy, with possible build-up. Causes: (a) Die lip nick or burr – causes adhesive to accumulate and release intermittently. Polish the lip with fine abrasive (e.g., 2000 grit) if minor. (b) Local low temperature – a cold spot on the die (e.g., dead heater zone) increases viscosity, causing more adhesive to be picked? Actually, higher viscosity leads to thicker coating for the same gap? In slot die, the flow is pump-controlled, not gap-controlled. But a cold spot can cause the adhesive to be stiffer, potentially increasing local pressure and causing flow non-uniformity. The result is often a thick streak. Check zone temperatures. (c) Die bolt too loose – a loose flexure bolt increases local gap, allowing more adhesive? No, pump flow is fixed, so a larger gap reduces pressure but not steady-state thickness. Actually, thickness is determined by pump flow, not gap, so a larger gap alone does not increase thickness. However, a loose bolt can cause the lip to deflect under pressure, creating a thicker spot. Tighten bolt. (d) Backup roll high spot – a bump on the roll increases nip pressure, causing more adhesive to be squeezed? In slot die, the coating weight is set before the nip, so roll high spots usually cause thin areas, not thick. So thick streaks are more likely from die lip buildup. Action: Inspect die lip for adhesive “whiskers” or carbon stalagmites; scrape off. Use a thermal camera to find cold spots.
Type 3: Intermittent or wandering streaks. Appearance: streaks that appear and disappear or move across the web. Causes: (a) Particles traveling from the melt tank – periodic clogs and clearing. Check filter condition; replace if needed. (b) Gear pump wear – particles from pump gears. Rebuild pump. (c) Web debris – a piece of tape or fiber stuck to the web upstream. Inspect web cleaning system. (d) Air bubbles – intermittent voids that look like missing spots, not continuous lines. Air entrainment. (e) Web slitting dust – if slitting is done inline, dust can fall onto the die lip. Add a vacuum dust collector. Action: Examine the streak pattern over time; if the streak location changes each roll, it’s likely a moving particle. Install a magnetic filter or finer filtration. For air bubbles, install a vacuum degasser.
Diagnostic procedure: Step 1 – Stop the line and visually inspect the die lip under bright light. Use a magnifying mirror to see the slot. Most particles are visible. Step 2 – If nothing found, run a short test with a different substrate (e.g., paper instead of film) to see if streaks persist. If they disappear, the problem is substrate-related. Step 3 – Measure the streak’s lateral position relative to die bolts. Mark the position. Step 4 – Loosen the two bolts nearest the streak, then tighten them slightly to create a local pressure change. If the streak moves or changes, the die is the source. Step 5 – If still unresolved, remove the die and disassemble. Inspect the manifold for blockages. Step 6 – Check the backup roll: rotate it slowly and feel for bumps or dips with a dial indicator. If runout >0.01 mm, schedule regrinding. Step 7 – Review temperature data for the die zones. A zone with low power output may indicate a failed heater causing a cold spot. By following this systematic approach, operators can quickly identify the root cause of streaks and take corrective action, minimizing waste and downtime. Keep a streak log to identify recurring issues (e.g., streaks always appear after 6 hours of running, indicating a cleaning interval is needed).
Case example: A converter experienced a fixed thin streak at 300 mm from the left edge. Inspection revealed a small carbon particle on the die lip. After cleaning, the streak disappeared. However, the streak returned after 4 hours. This indicated a source of carbon: the melt tank had a large charred area. Cleaning the tank resolved the issue permanently. By understanding why hot melt coating leaves streaks, and using systematic root cause analysis, manufacturers can maintain high coating quality. Regular die lip cleaning (every shift) and tank hygiene (weekly) prevent most streak-related defects.
