Why hot melt coating leaves streaks
Streaks in hot melt coating – also called die lines or longitudinal defects – are continuous or intermittent lines running in the machine direction where the coating thickness is different (usually thinner or absent) than the surrounding area. Streaks ruin product appearance and can cause functional failures such as poor adhesion or leaking. Understanding the root causes is essential for rapid correction. The main categories of causes are: die lip contamination, die lip damage, adhesive char or gels, non-uniform temperature across the die, pump pulsation, and substrate defects. This article explains each cause and provides step-by-step solutions to eliminate streaks.
Cause 1: Die lip contamination – the most common cause. A small piece of dried adhesive, char, or fiber sticks to the die lip, locally blocking or disturbing the adhesive flow. This creates a thin streak. Solution: Stop the line, raise the die, and use a brass scraper or a soft wooden stick to gently wipe the lip while the adhesive is still molten. For persistent contamination, perform a full die cleaning. Prevention: Use finer filtration (100-200 mesh), keep the plant clean, and install a web cleaner to remove dust before coating. Some machines have an automatic "lip cleaning blade" that traverses the die periodically.
Hot Melt Coating Machine - Hot Melt Adhesive Coating Machine
Cause 2: Die lip damage – a nick or scratch on the die lip from improper cleaning (using steel tools). The damage causes a local gap change, leading to a thick or thin streak. Solution: If the nick is small (<50 microns), it can sometimes be polished out using fine diamond lapping film. For larger nicks, the die must be removed and the lip reground by a precision machine shop. Prevention: Always use brass or soft plastic tools for cleaning; never scrape with steel blades or screwdrivers. Handle the die with care during removal and installation.
Cause 3: Adhesive char or gel particles. Overheated adhesive forms hard carbon particles or soft gels that travel through the pump and lodge in the die slot, creating a streak. Solution: Stop, purge the system with fresh adhesive, and clean the die. If the problem recurs, the melt tank may have accumulated char; perform a thorough tank cleaning. Reduce the tank temperature and avoid holding adhesive for extended periods. Use a finer filter (e.g., 100 mesh). For gel-prone adhesives, consider an in-line melt filtration system with a changeable filter cartridge.
Cause 4: Non-uniform temperature across the die width. If one zone of the die is cooler than the others, the adhesive viscosity increases locally, reducing flow and creating a thinner streak. Conversely, a hotter zone causes a thicker streak. Solution: Check the temperature readings of each die zone. Use an IR thermometer to measure the actual die lip temperature at several points. If a zone is off, calibrate or replace the thermocouple and heater. Ensure that the die cover is properly installed to prevent drafts. For wide dies, consider adding insulation.
Cause 5: Pump pulsation. Gear pumps produce small pressure fluctuations as teeth engage. If the pump is worn or has too few teeth (e.g., 6-tooth pump), the pulsation can cause periodic streaks (often appearing as repeating pattern every few centimeters). Solution: Replace with a pump having more teeth (12 or more) or install a pressure damper (hydraulic accumulator) in the line. Also, check if the pump shaft is bent or the coupling is misaligned. Reduce pump speed and increase line speed (requires adjusting die gap) – this reduces the frequency of pulsation relative to web travel.
Cause 6: Substrate defects or uneven tension. A wrinkle in the substrate or a depression in the backup roller can cause momentary separation from the die lip, creating a streak. Solution: Inspect the backup roller surface for flatness (runout <0.01mm). Check substrate for lumps or splices. Ensure tension is constant – a tension spike can lift the substrate. If the substrate has a slippery coating, use a vacuum backup roller. By systematically checking these six areas, most streak problems can be resolved. Keep a streak sample and photograph for your troubleshooting guide. For chronic streaks, consider upgrading to a higher-quality die with finer surface finish and better thermal control. Remember that a small investment in die maintenance saves large amounts of wasted material and downtime.
