3-Layer Coex Die Gap Adjustment

3-Layer Coex Blown Film Machines can produce films with excellent barrier and mechanical properties, and are widely used in food packaging, chemical packaging and other sectors. Accurate adjustment of die lip gap is critical to guarantee uniform film gauge and consistent performance. As the core forming component for molten plastic, the annular die lip directly determines melt extrusion rate. Uneven gaps lead to defects such as unilateral thickening and localized thinning of films, and in severe cases impair sealing, tensile and other properties of finished products. Therefore, mastering standardized die lip gap adjustment procedures is an essential operational skill for blown film production.

Three preparation steps shall be completed prior to gap adjustment. First, confirm machine status. Shut down the equipment and let it cool naturally until die temperature drops below 120°C to avoid high-temperature burns or die lip scratches. Cut off power supply to the die and extruder heaters, then clean residual melt around the die lips. Use a dedicated copper scraper to gently remove adhered polymer, followed by fine sandpaper of 800 grit or above to polish lip surfaces, eliminating burrs and buildup. Second, prepare tools: high-precision feeler gauges (accuracy: 0.01 mm), Allen wrenches matching adjustment screws, infrared thermometers, to ensure precise measurement. Third, preset process parameters. Set initial die gap according to target film specifications (thickness, width). Generally, die gap is 6–8 times the gauge of finished film. For instance, produce 100 μm film with an initial gap of 0.6–0.8 mm. Allow extruder and die heating temperatures to stabilize for 30 minutes before adjustment to guarantee steady melt fluidity.

After preparation, gap adjustment falls into coarse tuning and fine tuning. During coarse tuning, loosen locking nuts on the outer ring of the die to prevent overall die lip displacement during adjustment. The die is normally fitted with 3–6 symmetrically distributed adjustment screws corresponding to circumferential positions of the lip. Set uniform initial preload for all screws. Measure gaps at evenly spaced 12–16 points around the lip circumference with a feeler gauge. Fine-tune corresponding screws to control gap deviation within 0.05 mm for preliminary overall uniformity. After coarse tuning, start the extruder at low speed (approx. 1/3 of normal production speed). Observe film gauge once a stable bubble forms. If the film runs thicker in one direction, the local die gap is excessively narrow; tighten the corresponding screw clockwise to push the lip and reduce gap. If the film is thinner, loosen the screw counterclockwise to widen the gap. Follow the principle of small incremental adjustments: turn each screw no more than 1/8 circle at a time. Allow 1–2 minutes stabilization after each adjustment before rechecking, avoiding abrupt gap changes from over-adjustment. For 3-layer coextrusion applications, attention should also be paid to layer thickness distribution. Priority is given to overall die lip gap tuning, supported by monitoring melt pressure; abrupt pressure fluctuations indirectly indicate uneven gaps.

Once adjustment is finished, retighten die locking nuts and recheck all measuring points to confirm stable gap values. Raise extrusion speed to production level and monitor film gauge uniformity for more than 30 minutes before formal operation. Conduct periodic online monitoring with film gauges during routine production; recheck and trim die gap every two hours to offset lip thermal deformation and pressure-induced gap drift. In addition, never strike die lips with hard metal tools during adjustment. Fully clean the die upon production shutdown to prevent cured residual melt from damaging lips or undermining adjustment precision next time.

In conclusion, die lip gap adjustment for 3-Layer Coex Blown Film Machines demands patience and meticulous operation. Proper pre-work, standardized procedures and regular maintenance jointly sustain stable film quality.