Summary of 17 Common Blown Film Problems and Solutions

Preface: Blown film extrusion is a commonly used processing method in plastics processing. Familiar products such as vest bags and agricultural film are all the output of blown film. The blown film process appears simple, yet it conceals many subtleties. We have compiled 17 problems encountered in blown film production to share with you.

1. Difficulty in drawing the film

(1) Improper die head temperature control: Adjust the die head temperature appropriately.

(2) Uneven discharge from the die: Adjust the die gap while appropriately adjusting the temperature of the die connector.

(3) Burnt material and impurities contained in the melt: Purify the raw materials and clean the die head and screw.

(4) Improper control of extrusion process conditions: Adjust the process conditions appropriately.

1. Bubble skewing

(1) Barrel and die temperature too high: Lower them appropriately.

(2) Die connector temperature too high: Lower it appropriately.

(3) Uneven discharge from the die and uneven film thickness: Adjust the die gap and the die connector temperature appropriately.

1. Bubble takes on a reed-like shape

(1) The clamping force of the take-off rolls is too small, or the rotational speed of the take-off rolls is affected by regular changes in mechanical transmission resistance: Appropriately increase the clamping force of the take-off rolls, and service the mechanical transmission part of the take-off unit to keep the take-off roll speed stable.

(2) The take-off speed is unstable, and the air pressure of the cooling air ring is too high: For this, adjust the take-off speed to make it run stably; the air pressure of the air ring should be appropriately reduced.

1. Bubble oscillation

(1) Melt temperature too high: Lower the barrel and die head temperatures appropriately.

(2) Cooling efficiency of the cooling air ring too low: Increase the air pressure and air volume of the air ring appropriately.

(3) Friction between the bubble and the collapsing frame too high: Appropriately increase the included angle of the collapsing frame to reduce the contact area between the bubble and the frame.

(4) Die head temperature too low, making discharge difficult and causing the bubble to pulsate: Appropriately raise the die head temperature and increase the screw speed.

1. Mottled film surface

(1) Barrel or die head temperature too low, resulting in poor plastication of the melt: Appropriately raise the barrel or die head temperature.

(2) Screw speed too fast: Slow it down appropriately.

(3) Screw temperature too high or too low: Adjust the flow rate of the cooling medium inside the screw to keep the screw temperature within an appropriate range.

1. Visible hang-up lines

(1) Degraded material or impurities adhering to the die discharge port: Scrape off the impurities or burnt material with a copper scraper; in severe cases, push out the mandrel for cleaning.

(2) The die has been damaged by impact: Repair the die.

(3) Internal die pressure too low: Add a screen pack.

(4) Melt temperature too high: Lower the barrel and die head temperatures appropriately.

1. Visible seam lines

(1) Die head or die connector temperature too high: Lower it appropriately. However, the temperature must not be too low, otherwise seam lines will still occur.

(2) Burnt material adhering to the tip of the mandrel: Modify the shape of the mandrel tip to give it a smooth transition.

(3) Unreasonable design of the die head mechanism: Modify the die head design.

1. Poor transparency

(1) Barrel or die head temperature too low: Raise it appropriately.

(2) Insufficient bubble cooling: Improve the cooling efficiency of the cooling system, increase the air pressure and air volume of the air ring, and also appropriately reduce the extrusion speed.

1. Burnt particles or white spots in the film

(1) Impurities mixed into the raw materials: Screen out the impurities and purify the raw materials.

(2) Broken screen pack: Replace the screen pack.

(3) Overheating and degradation of the melt: Clean the die head and screw, and appropriately lower the forming temperature.

1. Hard lumps in the film

(1) The screen pack has been ruptured: Replace the screen pack.

(2) Improper control of forming temperature: Generally, if the forming temperature is too low, the melt is poorly plasticated, which easily produces cold-material lumps; the temperature needs to be adjusted appropriately.

1. Bubbles

(1) Raw materials not fully dried and moisture content too high: Perform pre-drying.

(2) Cooling water leaking at the barrel or hopper: Service the cooling pipes and devices to eliminate the leakage.

1. Uneven film thickness

(1) Improper adjustment of the die gap and uneven discharge: Adjust the die gap.

(2) Uneven air output from the cooling air ring: Adjust and clean the air ring so that its air output is uniform.

(3) Mandrel off-center and deformed: Replace the mandrel.

(4) Uneven temperature around the die head: Service the heating device of the die head.

(5) Blow-up ratio too large: Reduce it appropriately.

(6) Unstable compressed air pressure: Service the air compressor.

(7) Unreasonable die head structure design: Modify the process parameters of the die head.

(8) Output too low: Appropriately increase the extruder speed to increase output.

1. Film wrinkling

(1) Uneven film thickness: Adjust the film thickness.

(2) Insufficient or uneven cooling: Improve cooling efficiency or appropriately reduce the forming speed.

(3) The collapsing frame or take-off rolls are not aligned with the die head center: Recalibrate the center line.

(4) The included angle of the collapsing frame is too large: Reduce the angle appropriately. Sometimes the included angle of the collapsing frame does not match the bubble diameter and should be adjusted; the geometry of the frame should be symmetrical.

(5) Uneven clamping force of the take-off rolls, with inconsistent tightness: Adjust the clamping force appropriately so that it is evenly distributed across the roll surface.

(6) Winding tension not constant: Adjust the friction force of the winding friction disc to ensure constant winding tension.

(7) Die head installed unevenly: Correct the levelness of the die head.

(8) Unstable airflow outside the bubble: Set up windbreaks around the bubble to keep the airflow around the bubble stable.

1. Uneven winding

(1) Uneven film thickness: Adjust the film thickness.

(2) Insufficient or uneven cooling: Strengthen cooling and adjust the air volume of the air ring.

(3) Uneven clamping force of the take-off rolls, causing the film to wander: Adjust the clamping force so that it is uniform across the roll surface.

(4) Air in the bubble is entrained into the film, causing wrinkles: Adjust the clamping force of the nip rolls and expel the gas inside the film tube.

1. Poor film openability

(1) Barrel and die head temperature too high: Lower them appropriately.

(2) Take-off roll clamping force too high or take-off speed too fast, resulting in poor cooling: Reduce the clamping force or lower the take-off speed.

(3) Poor cooling: Improve the cooling efficiency of the cooling system.

(4) Too little lubricant: Add an appropriate amount of internal lubricant.

(5) Too much plasticizer: Reduce its amount and choose high-efficiency plasticizers where possible.

1. Frost line too high

(1) Die head temperature too high: Lower it appropriately.

(2) Output too large: Appropriately reduce the screw speed.

(3) Insufficient cooling: Increase the air volume of the cooling air ring.

1. Frost line too low

(1) Die head temperature too low: Raise it appropriately.

(2) Output too low: Appropriately increase the screw speed.

(3) Excessive cooling: Reduce the air volume of the cooling air ring.