BackRemedies for High Density Polyethylene (HDPE) film production faults

Problem Possible Cause Remedy
Low Output Extruder screw speed too slow Increase screw speed
Melt temperature too low Increase temperature
Die too small Increase size of die
Die gap too narrow Widen die gap
Worn screw, slippage Replace screw
Surging Polymer not completely melted at screw end (Uneven back pressure) Increase temperature at end of barrel
Excessive back pressure Use screw with deeper flights
Worn nip-rolls Check nip-roll and repair
Poor Drawdown (Bubble breaks at thin gauge) Melt temperature too low Increase temperature
Lack of back pressure Use extra screen packs & use better mixing screws
Contamination of polymer Change batch of polymer
Attempting extrusion below minimum thickness Change to higher MFI polymer grade
Wandering Freeze line too high Reduce freeze line by decreasing melt temperature or increasing cooling air or improving cooling ring design
Uneven pressure from nip rolls Adjust nip rolls
No bubble support Use guide rolls or guide cage
Incorrect cooling air velocity Reduce air flow
Change cooling ring lip angle
Alter cooling ring design
Snatching (Grabbing) Incorrect design of collapsing frames Improve frame design by reducing contact area with bubble, using rollers rather than boards
Film too hot Increase film cooling
Bubble instability (Variation in bubble size or snaking of bubble) Blow ratio too high Decrease blow ratio by using larger die for same lay flat width
Freeze line too high Decrease freeze line by decreasing melt temperature or increasing cooling air
Cooling air impinges on bubble; air velocity too high Change cooling air ring design to give parallel air flow
Draughts near machine Eliminate draughts
Extruding LLDPE Blend in 10-20% LDPE
Poor thickness uniformity Uneven temperature in die Check heater bands, controller and thermocouples
Poor head and die design Check die dimensions
Worn die Check die for ovality and wear
Blow ratio too high Reduce blow ratio by using larger die
Air flow uneven Check uniformity of air ring aperture
Clean cooling ring fan and air pipes
Surging from extruder See "Surging" above
Telescoping rolls Insufficient winding tension, especially film too slippery Increase tension
Reduce slip level
Excessive winding tension Decrease tension
Furry edges of film roll Blunt slitting blades Replace or sharpen slitting blades
Film blocking Film too hot at nip rolls Increase nip roll height < br /> Decrease extrusion rate
Increase cooling capacity (Lower freeze line)
Decrease melt temperature
Inflate secondary bubbles after nip rolls
Excessive nip roll pressure Reduce nip roll pressure
Excessive wind-up tension Reduce wind-up tension
Excessive 'C' treatment Reduce treatment level
Not sufficient slip and antiblocking additive Use polymer with higher additive levels
Add extra antiblock masterbatch
Wrinkles in film Bubble instability See "Bubble Instability" above
Poor thickness uniformity See "Poor thickness uniformity" above
Rollers of takeup system not aligned Check rollers for alignment and parallelism
Nip rolls not vertically in line with die Check using plumbline
Uneven pressure or uneven wear on nip rolls Align nip rolls
Replace nip rolls
Rollers on collapsing frame not rotating Check that rollers are rotating
Winder tension too high or too low Adjust winder tension
Tape or dirt on idler rollers Clean rollers
Poor optical properties Inadequate melt homogenisation Increase screw cooling
Use mixing screw
Die gap too wide Use narrow die gap
Sharkskin/surface melt fracture (with LLDPE) Lower output rate
Increase melt temperature
Use wider die gap
Add processing aid
Use polymer with higher MFI
Poor roll flatness (thick bands) Uneven thickness of film See "Poor thickness uniformity"
Use rotating air ring
Use rotating or oscillating die