In low-density polyethylene blown film extrusion, the base resin typically exhibits a density of 0.918–0.925 g/cm³ (ISO 1183-1:2019) and a melt mass-flow rate of 0.2–2.0 g/10 min (ISO 1133-1:2022). On single-screw blown film lines with 24:1–30:1 L/D ratios, 0.8–2.0 mm die gaps, and blow-up ratios from 2.0:1 to 3.5:1, the recurring production triad is bubble instability, gels, and output loss. These failures are not independent; a gel lodged at a die lip can create a gauge band that destabilises the bubble, and a partially blocked screen pack can elevate melt temperature enough to form oxidative gels while reducing throughput. The following sections separate the three phenomena but maintain the process relationships.

What Causes Bubble Instability in Low-Density Polyethylene Blown Film Lines?

When a low-density polyethylene film bubble becomes unstable, the visible mode may be low-frequency vertical breathing, high-frequency frost-line flutter, helical twist, or asymmetric neck curvature. The stability window is governed by melt tension, extensional strain hardening, die-flow uniformity, and cooling-air momentum. LDPE is a long-chain branched polyolefin; its strain hardening supports the molten tube between the die exit and frost line. That support is lost when the melt exits the die above 200°C or below 160°C. On a representative 90 mm single-screw extruder with a 1.2 mm die gap, die-zone temperatures above 200°C reduce extensional viscosity and produce slow bubble sag, while temperatures below 160°C raise die pressure and can generate helical instability from non-uniform melt flow. The die-zone set point should therefore be adjusted only after confirming melt temperature with a needle thermocouple at the die exit, not solely from the control set point.

Die-gap concentricity is a first mechanical check. Eccentricity beyond 0.05 mm around the die circumference creates oriented gauge bands; these bands alter local air drag and initiate flutter. Feeler-gauge measurements at 8 or 12 positions around the die lip are standard practice. Frost line height is the second variable. A frost line above 8 die diameters leaves a long molten tube that is sensitive to ambient plant drafts; a frost line below 4 die diameters quenches the film before bubble curvature stabilises. For general-purpose LDPE films, a frost line height of 5–7 die diameters is a practical initial setting, with fine adjustments made in 10–20 mm increments.

Blow-up ratio and die gap interact with melt relaxation. At blow-up ratios above 3.0:1, a die gap below 0.8 mm can amplify gauge variation because the polymer carries large elastic strain out of the die. Widening the gap to 1.5–2.0 mm increases residence time and allows stress relaxation before inflation. Internal bubble cooling systems add another control loop: they decouple internal bubble pressure from external air-ring flow and can reduce low-frequency breathing, but the improvement is resin-dependent. On lines without IBC, dual-lip air rings with independently adjustable upper and lower lips are preferred over single-lip air rings because the operator can move the cooling-air impingement point without changing total blower output.

Die melt pressure fluctuation is an additional diagnostic signal. A fluctuation amplitude above ±2 bar at a 100–150 bar baseline suggests irregular solids conveying or melting instability; this can produce bubble breathing even with stable air-ring conditions. Online capacitance or beta gauges after the collapsing frame should be used to confirm whether the instability produces a measurable gauge oscillation exceeding ±5% of target thickness. Resin selection should not be reduced to a single MFR value. ISO 1133-1:2022 reports the melt mass-flow rate but does not capture long-chain branching density or molecular weight distribution, which control extensional viscosity. A batch change to an LDPE of the same MFR but lower long-chain branching can destabilise the bubble even when all extruder settings remain unchanged. When bubble instability begins after a resin lot change, comparative melt tension testing or a controlled lab-scale blown film trial is more informative than MFR alone. Published data for the exact numerical melt-tension threshold is limited because the critical extensional viscosity is line-specific.

Gel Formation Mechanisms and Melt Filtration Limits

Visually, gels in LDPE blown film are localised inclusions that differ from unmelts by hardness, colour, and response to heating. The four practical classes are oxidatively crosslinked polymer, high-molecular-weight resin fractions, additive agglomerates, and external contamination. Oxidative gel formation is primarily a temperature–residence-time reaction. LDPE retained in stagnant zones—screw root, screen-pack edges, adapter gaps, or die lips—above 200°C can form crosslinked material through free-radical processes. The resulting inclusions are often yellow or brown and are associated with a reduction in elongation at break when tested according to ISO 527-3. The stagnant zone is not always visible during production; it is inferred from gel counts that increase after a shutdown and decline after purging.

Melt filtration is the primary barrier to visible gels. Screen packs of 60/100 mesh, approximately 250/150 µm, are common for general LDPE film, whereas 100/200 mesh reduces smaller gels but increases shear heating and pressure drop. A melt pressure transducer upstream and downstream of the breaker plate is essential. A pressure-drop increase of 15–20% over the clean-pack baseline at constant screw speed indicates progressive blinding; continued operation forces degraded material through the pack and raises melt temperature by shear heating. The screen pack should be changed before the pressure excursion becomes the root cause of further gel generation.

FTIR microscopy can separate oxidised gels from high-molecular-weight resin gels. Oxidatively degraded LDPE exhibits a carbonyl stretching absorbance near 1715 cm⁻¹; unoxidised high-MW resin gels do not. This distinction prevents misdirection of the corrective action. Where full FTIR mapping is unavailable, a hot-stage test can soften a high-MW resin gel but not a crosslinked oxidised gel. The die lip itself can be a secondary source: oxidised LDPE at the lip periodically detaches into the melt stream and appears as intermittent fisheye gels. Cleaning or re-gapping the die lips is required when gels are concentrated in a single gauge band.

High-molecular-weight resin gels appear as soft or clear fisheyes and are more common when a lower-MFR LDPE is used without a sufficient melting zone. A film-grade LDPE with an MFR of 0.3–1.0 g/10 min (ISO 1133-1:2022) is generally suitable, but screw design must supply enough melting capacity. Additive agglomerates are controlled by feed-letdown accuracy and dispersion; a slip or antiblock masterbatch that has absorbed moisture or has a wide particle size distribution can produce soft gels even when the base resin is gel-free. The table below summarises the diagnostic distinctions.

The gel troubleshooting sequence should begin with the screen pack pressure differential, then purge, then die-lip cleaning, and only then resin lot evaluation. This order avoids unnecessary resin changes that can introduce a second variable and obscure the original defect.

When Output Loss Persists Without a Detectable Melt Fracture Signature

When throughput declines at constant screw speed but no sharkskin or melt fracture is visible, the fault is usually hydraulic, mechanical, or feed-related rather than rheological. The first diagnostic is specific output: kilogram per hour per screw revolution at a constant melt temperature. A falling specific output without a change in MFR indicates either screen-pack blockage, feed-zone instability, or screw/barrel wear. The extruder drive load should be recorded at the same time; a high motor load with falling output indicates excessive backpressure or viscosity, whereas a low motor load with falling output indicates feed starvation or severe screw wear.

Screen-pack blockage is confirmed by the pressure differential across the breaker plate. On a representative 90 mm extruder, an increase of 20–30 bar above the clean-pack baseline at constant screw speed is sufficient to reduce throughput. If changing the screen pack restores baseline pressure and output, the cause is gel or contamination loading. If pressure remains high after a screen change, the die gap or adapter should be inspected for carbonised LDPE, particularly at lip edges and thermocouple ports. A narrow die gap of 0.8 mm may be suitable for thin film but will limit maximum output if the extruder cannot generate sufficient pressure. Widening the die gap to 1.2–1.5 mm often restores output without changing film gauge, provided blow-up ratio and haul-off speed are compensated.

Excessive melt temperature is a separate route to output loss on fixed-speed extruders. At melt temperatures above 220°C, melt viscosity decreases, increasing pressure flow and leakage flow in the metering section. The result is reduced specific output even though the drive load falls. This condition is often accompanied by oxidative gel formation and should be corrected by reducing screw speed or barrel temperatures rather than increasing die temperature.

Screw and barrel wear reduce pumping capacity because increased flight clearance permits recirculation flow. An increase in radial screw clearance of 0.1 mm above the original equipment specification can measurably reduce specific output on a 90 mm extruder, especially in the metering section. The barrel temperature profile should also be checked with an external contact thermocouple because incorrect zone settings can mimic wear. For LDPE blown film, barrel zone settings from 150°C near the feed throat to 190°C at the metering zone, with die temperatures of 180–200°C, are typical starting values. A feed throat that is too hot reduces solids conveying and lowers output; a metering zone that is too cold increases melt viscosity and torque. Adjustments should be made in 5°C increments only after melt pressure and specific output have been logged for at least 30 min at each condition.

Feed-throat bridging or inconsistent pellet geometry can produce output loss without a change in melt pressure. LDPE pellets with high fines content or irregular shape can reduce solids conveying efficiency in a smooth-bore feed throat. A grooved feed section improves conveying but increases feed-zone pressure and may require additional cooling. When output loss appears after a resin lot change and no pressure, temperature, or wear anomaly is found, pellet bulk density should be measured according to ASTM D1895 Method A, along with sieve analysis using 2 mm and 0.5 mm screens. Published data for the exact specific-output penalty of a given fines fraction is limited because the effect is strongly influenced by screw geometry and feed-throat temperature.