Bubble Instability In Lldpe Blown Film Five Causes And The Fixes
Bubble Instability in LLDPE Blown Film: Five Causes and the Fixes
Linear low-density polyethylene (LLDPE) produced by low-pressure gas-phase, solution, or slurry copolymerization of ethylene with 1-butene, 1-hexene, or 1-octene spans a density range of 0.915–0.940 g/cm³ under ASTM D792-20. Blown-film grades typically carry a melt flow index between 0.5 g/10 min and 2.0 g/10 min at 190°C and 2.16 kg per ASTM D1238-20. LLDPE delivers high dart impact and Elmendorf tear resistance, but the absence of long-chain branching and the relatively narrow molecular weight distribution lower melt strength compared with LDPE. Under process stress, that molecular architecture produces five distinct bubble instability classes in blown film: die-lip shear roughness or sharkskin, draw resonance, frost line height oscillation, bubble breathing or helical instability, and die-lip deposit formation. Each class has a different process signature, root cause, and corrective action.
On a commercial blown film line, bubble stability is governed by the interaction of die-lip shear history, extensional deformation in the stalk, and cooling air dynamics. Typical LLDPE extrusion conditions include a die gap of 0.8–2.5 mm, blow-up ratio between 2.0:1 and 4.0:1, frost line height from 300 mm to 900 mm, and melt temperature between 190°C and 230°C. The bubble is monitored using optical pyrometers, capacitance thickness scanners, and internal pressure transducers. A stable bubble exhibits steady frost line position, consistent bubble diameter, and uniform web gauge. Instability appears when one or more of the five mechanisms exceeds the available process window.
| Cause | Process signature | Corrective action |
|---|---|---|
| Die-lip shear stress/sharkskin | Fine transverse surface ridges spaced 0.1–0.5 mm; onset when wall shear stress exceeds 0.12–0.18 MPa; reduced clarity | Open die gap from 1.0 mm to 2.0–2.5 mm; reduce output to lower die-lip shear rate below 300–500 s⁻¹; add fluoropolymer processing aid at 200–800 ppm; blend 10–20 wt% LDPE |
| Draw resonance/low melt strength | Periodic thickness and neck diameter oscillation of 5–30 s; amplitude increases with take-up ratio | Reduce blow-up ratio to 2.0:1–2.5:1; lower melt temperature by 5–15°C; blend 15–30 wt% LDPE; select melt flow index 2.0 g/10 min grade instead of 1.0 g/10 min |
| Frost line height oscillation/cooling air imbalance | Frost line variation greater than ±25 mm; gauge bands at the winder | Center the die in the air ring; set air lip gap to 3–8 mm; balance plenum pressure; add internal bubble cooling with pressure control at ±0.2 kPa; reduce output by 5–10% |
| Bubble breathing/helical instability | Bubble diameter variation above ±5%; slow rotation or pulsing of the stalk | Lower melt temperature to 195–205°C; adjust internal bubble cooling exhaust and air ring damper; reduce blow-up ratio to 2.2:1; verify air ring centering and venturi height |
| Die-lip deposits/gels | Local thin bands, streak lines, or rotation-locked defects; amber or oxidized deposit at the die lip | Purge with HDPE or a purge compound at 10–20°C above normal melt temperature; reduce melt temperature if yellowness index increases; install melt filtration at 60–120 mesh; eliminate adapter dead spots |
Die-lip sharkskin in LLDPE appears as a fine, transverse surface roughness that reduces film clarity and can initiate local gauge variation. The defect originates at the die exit when wall shear stress exceeds the critical value for slip. In narrow-MWD LLDPE of melt flow index 1.0 g/10 min at 190°C, sharkskin onset is frequently observed near 0.14 MPa. Because LLDPE has higher shear viscosity at a given melt index than LDPE, the same die gap and output rate can produce melt fracture in LLDPE while LDPE remains stable. For an annular die with a small gap h relative to die diameter D, apparent die-lip shear rate scales with 6Q/(πD h2), where Q is volumetric throughput. Increasing the die gap from 1.0 mm to 2.2 mm therefore reduces shear rate quadratically for a fixed output. Fluoropolymer processing aids added as masterbatch at 200–800 ppm migrate to the die-lip surface and form a low-shear slip layer. However, they require a conditioning period to coat the die lip, and they can be rendered ineffective when competing silicone-based additives are used at the same time. Reducing melt temperature is not always a stable fix for sharkskin because lower temperature raises viscosity and can push wall shear stress farther above the critical threshold.
When Draw Resonance Overcomes the Bubble Cooling Envelope
Draw resonance in LLDPE blown film is a periodic thickness and neck diameter oscillation with cycle times commonly between 5 s and 30 s. It originates downstream of the die in the extensional deformation zone, not at the die lip. LLDPE exhibits lower strain hardening than long-chain branched LDPE because the linear chains lack the high-molecular-weight tail and branching needed to resist necking under high draw. Melt strength measured on a Rheotens-type apparatus is commonly 3–8 cN for LLDPE at 190°C, whereas tubular LDPE may exhibit 15–25 cN. When take-up ratio or stalk height exceeds the strain-hardening capacity of the grade, minor disturbances in haul-off speed or die pressure amplify into sustained oscillation. A narrow processing window can develop: lowering melt temperature by 5–15°C raises melt strength and stabilizes draw resonance, but it may simultaneously increase die-lip shear stress and induce sharkskin. On some 90 mm grooved-feed extruders running 1.0 g/10 min octene-LLDPE through a 300 mm spiral mandrel die, the stable melt temperature band may be no wider than 10–15°C. Blending 15–30 wt% LDPE introduces long-chain branching and is generally more effective than temperature adjustment alone. Grade selection also matters: a 2.0 g/10 min LLDPE produces lower die pressure but may reduce melt strength further; therefore, the melt index change should be paired with a reduced draw-down ratio.
Air Ring Aerodynamics, Internal Bubble Pressure, and Frost Line Oscillation
Frost line height oscillation and bubble breathing are aerodynamic problems that become visible as cyclic changes in bubble diameter, stalk rotation, or frost line movement greater than ±25 mm. A dual-lip air ring with a venturi insert creates a pressure profile around the bubble; if the die is not centered in the air ring, the melt is cooled unevenly and the bubble can rotate or pulse. Air lip gaps outside the range of 3–8 mm often generate asymmetric velocity jets that disturb the bubble surface. The venturi height should be set so that the low-pressure zone stabilizes the stalk without pulling the bubble against the air ring. Internal bubble cooling systems regulate bubble diameter through controlled internal air exchange. A pressure control tolerance of ±0.2 kPa around a setpoint of 1.0–2.5 kPa is common; drift beyond this band changes bubble diameter enough to alter frost line height and local cooling rate. When frost line oscillation persists after air ring centering, output reduction of 5–10% is an effective temporary measure while the air ring is balanced. Bubble breathing can also be reduced by lowering melt temperature to 195–205°C, reducing blow-up ratio to 2.2:1, and confirming that the internal bubble cooling exhaust is not cycling against the air supply.
Die-lip plate-out and gel-induced instability typically appear as localized thin bands or streak lines that follow bubble rotation, distinct from uniform sharkskin. Oxidized or crosslinked gel fractions originating from high-temperature zones, dead spots, or contaminated feedstock accumulate at the die lip and locally alter the annular gap. A deposit thickness of 0.05–0.10 mm at one circumferential location is sufficient to create visible gauge deviation on a 300 mm die. The corrective action is mechanical and thermal: purge the die with HDPE or a purge compound at 10–20°C above normal melt temperature, reduce melt temperature if the film yellowness index rises, install or clean melt filtration screens at 60–120 mesh, and inspect the adapter and spiral mandrel for stagnation points. This defect is often misdiagnosed as melt fracture because both reduce clarity, but die-lip deposits are rotation-locked and selective to one side of the bubble, whereas sharkskin is distributed around the full circumference. Published data for specific gel-to-gauge deviation correlations in LLDPE is limited; the diagnosis is therefore verified by direct die-lip inspection during a brief line stop.
What Happens When All Five Instabilities Are Superimposed on a Coextrusion Line?
In coextruded LLDPE/LDPE structures, the LDPE core layer increases overall melt strength, but LLDPE skin layers can still exhibit die-lip sharkskin at the outer die lip. The interaction of draw resonance, frost line oscillation, and die-lip deposits creates a combined signature that cannot be corrected by a single change. The troubleshooting sequence should begin with die centering and air ring alignment, followed by melt temperature and die pressure verification, then die gap adjustment, and finally material changes such as LDPE blending or fluoropolymer processing aid addition. This sequence separates aerodynamic causes from shear-history causes and avoids increasing melt temperature in response to draw resonance, which would worsen die-lip buildup. Melt flow index measured under ASTM D1238-20 is not sufficient to predict bubble stability because it does not capture extensional strain hardening. A Rheotens measurement or extensional rheometer is required when draw resonance is suspected. Film property testing should accompany process fixes: tensile properties under ASTM D882-18, Elmendorf tear under ASTM D1922-15, and dart impact under ASTM D1709-16a confirm that the corrective action did not sacrifice mechanical performance. When a blown film line operates near the edge of the LLDPE processing window, bubble instability should be treated as a system-level conflict among shear stress, melt strength, cooling air balance, and die cleanliness rather than as a single resin defect.
