Poor optical properties |
Change compound, or run hotter. If cast film, decrease gap between die and cooling surface. If tubular, increase blow ratio raise frost line. |
Improper Slip |
More slip agent and/or higher melt Temperature to increase slip. |
Film too weak in one direction (splitting) |
If tubular, adjust relation of linear speed and blow ratio to correct balance. If cast minimize draw down, run hotter use shorter gap, smaller lip opening. In both methods, check for die lines which may weaken film. |
Film too weak all over |
Both methods, change to stronger resin check for possible degradation polymer If tubular use greater blow ratio and greater linear speed or lower frost line. |
Film discolored or streaked |
Check condition and colour of feed. Run cooler to inhibit Decomposition. |
Too many fish eyes (Gala of other particles) |
These impair appearance and may cause bubble breaks. Review entire operation looking for source of contamination, decomposition inside the system, a major cause. Use more streamlined equipment. Follow recommended start-up/shut-down procedures. In opening bags, avoid pick up of outside dirt or paper fibers and thread. Reduce or eliminate addition of reprocessed trim, check cleanliness of the air. Keep hopper covered and full. Check plant ventilation, as dirty air must not enter extruder. Clean die often. Check raw material; if fish eye producing material there, change to another material or use fine screens (200 mesh) and/or continuous changer. Burnt plastic on Die lips may break and cause fish eyes and bubble breaks. |
Bubble breaks ( not form fish eye) |
Draw-down limits may be exceeded, increase temperature, use different compound, get better mixing and more uniform melt. Moisture in hopper either absorbed into Plastic or introduced as contaminant can cause tiny bubbles and roughness. |
Die lines (grooved) |
Clean die, follow proper start-up shut down procedures, avoid internal decomposition, check for nicks or burns on die lips. |
Varying thickness |
This is evidence by an asymmetrical bubble and/or heavy spots on the finished roll. Adjust die opening. Check for defective heaters or other hot/cold spots on die. Use uniform, jet-free cooling air. In tubular film thin bends require increased cooling air flow and/or reduction in gap opening at corresponding positions on die and vice-versa for thick ends. To get neat roll, use rotating/oscillating die. Uneven roll is also possible if wound too hot. |
Bubble instability vibrating or shaky |
Decrease blow ratio (use large die, or make smaller film) check temperature and thickness uniformity of melt at the die. If extruder is surging (watch pressure and power fluctuations) reduce screw speed increase pressure, use different screw, cool screw (last resort, as it reduce output) Check constancy of air pressure inside bubble. Improve air ring construction, aiming air more in an upward (vertical) direction. Use iris, expandable mandrel or other stabilizing device. Avoid outside drafts, shield whole system from doors, windows, other air currents. |
Wrinkless |
These are often caused by bubble instability (see proceeding item) Also check for improper alignment of take-off, skewed rolls etc. Check for smooth drag-free passage through flattener and nip rolls. Use auxiliary expander or bowed rollers having Helical pattern diverging from center out to ends. |
Blocking |
Improve cooling or run at lower temperature, inflate a secondary bubble just after the nip rolls. Use anti- block additives, reduce nip rolls pressure, check for excessive electrical discharge treatment for printing which can cause blocking. Use nitrogen instead of air in bubble. |
Poor cooling |
Tubular: Increase height of nip rolls from die, use more air, better doffing inside air; ring better direction of air (smaller angle with vertical), cooling sleeves, fog sprays metal mandrels, expandable mandrels extra air ring; refrigerated air. Put cooling section in air conditioned enclosure. For cast film; use colder cooling water, large chill roll, air knife. Both methods run at lower temperature. |
Puckering on cast film |
Run chill roll warmer. |