Resin selection for three-layer blown stretch wrap begins with control of melt index at 0.8–1.0 g/10 min under ISO 1133-1:2022 condition 190 °C/2.16 kg and density in the range 0.917–0.920 g/cm³. Octene-based linear low density polyethylene is preferred over butene-based LLDPE because longer short-chain branching produces a broader shear-thinning response and delays sharkskin melt fracture at the die lip. On a production-scale line with 55 mm grooved-feed extruders, 30:1 L/D screws, and a 250 mm spiral mandrel die, barrel set points of 180–220 °C and adapter/die set points of 220–240 °C produce a die melt temperature of 215–235 °C. Melt pressure at the die entry with a clean screen pack is typically 25–35 MPa; a partially blinded screen pack raises pressure by 8–15 MPa, altering spiral mandrel distribution and increasing transverse gauge variation from ±1.5% to ±3.5%. If density falls below 0.915 g/cm³, the solidified film at the frost line has lower secant modulus, and bubble stability becomes sensitive to collapsing frame misalignment, air turbulence from the building, and haul-off speed oscillations above 2% of setpoint.

What Limits High-Stalk Bubble Stability When the Blow-Up Ratio Exceeds 3:1?

The high-stalk process separates extrudate drawdown from circumferential expansion by maintaining a molten neck height of 5–9 die diameters before transverse stretching begins. With an octene LLDPE of MI 0.8 g/10 min at a die melt temperature of 225 °C, stable operation is generally reported at blow-up ratios between 2.2:1 and 3.0:1; excursions above 3.2:1 increase hoop stress in the expanding bubble and require higher melt strength. Frost line height after expansion is normally maintained at 3.5–5.5 die diameters; if the frost line is too low, the film quenches before full expansion and generates excessive transverse direction orientation, while a frost line above 6 die diameters creates bubble flutter because the unsupported molten web responds to ambient air currents. Dual-lip air ring chilled air at 8–15 °C raises quench rate but lowers bubble symmetry if one lip is fouled with low-molecular-weight additives. Output on a 250 mm die with 1.8 mm die gap and blow-up ratio 2.5:1 is typically 0.50–0.60 kg/h/mm of die circumference; beyond 0.65 kg/h/mm, butene-based LLDPE exhibits transverse sharkskin ridges at 5–20 mm intervals and narrows the stability window.

Migration of polyisobutylene cling additive in blown stretch wrap is retarded compared with cast film because the frost line quench freezes the film surface before full exudation occurs. Polyisobutylene with number-average molecular weight 1,500–3,000 Da is dosed into the cling layer at 1.5–4.0 wt%; cling force measured under ASTM D5458 on a 25 µm film typically reaches 150–250 g after room-temperature ageing for 48 h, whereas immediately after winding the value may be below 80 g. Storage below 5 °C delays migration and yields lower initial cling; storage above 35 °C accelerates migration and increases the rate of low-molecular-weight deposit formation on air ring lips and collapsing frames. Polyisobutylene levels above 4.5 wt% plasticize the cling layer and reduce tensile strength, while also increasing the risk of roll blocking under prolonged compression.

When Die Gap Narrows Below 1.5 mm in Thin-Gauge Stretch Wrap

For film gauges below 15 µm, die gap selection controls the shear history and the level of unresolved melt orientation. A die gap of 1.2 mm with final film thickness 12 µm and blow-up ratio 2.5:1 forces the melt through a gap-to-final-thickness ratio on the order of 100:1; high-stalk LLDPE tolerates this because transverse expansion occurs above the neck, but die lip shear is increased. Viscous dissipation in the spiral mandrel raises melt temperature by 5–10 °C. If die temperature is not reduced accordingly, the bubble elongates and the frost line moves upward; the air ring air volume is then increased, which raises machine direction orientation and reduces elongation at break. Gauge control through a segmented air ring maintains film thickness at ±1.5% at haul-off speeds up to 120 m/min; above 180 m/min, edge weave and collapsing frame bounce become limiting. Optical gauge systems identify transverse bands of ±0.8 µm thickness oscillation when the screw speed harmonic matches the haul-off frequency.

Silage bale wrap and silage pit covers impose different mechanical requirements than pallet stretch film. A 25 µm bale wrap fabricated from LLDPE/EVA with EVA vinyl acetate content 8–18 wt% and MI 0.3–0.7 g/10 min provides extensibility and puncture resistance; puncture resistance is evaluated under ASTM D5748, and tensile properties are determined under ISO 527-3:2018 at 23±2 °C and 50±5% RH. UV stabilization with a hindered amine light stabilizer package at 0.2–0.5 wt% and a UV absorber at 0.1–0.3 wt% is required for outdoor storage. Oxygen transmission of 25 µm LLDPE measured under ASTM D3985 at 23 °C/0% RH is approximately 2,000 cm³/(m²·day·bar); pit covers of 150–200 µm reduce this value to 300–400 cm³/(m²·day·bar) because transmission is inversely proportional to thickness. Silage preservation therefore relies on multiple wraps and compaction rather than on barrier polymer alone; when high-barrier coextrusion is required, a core of polyamide or EVOH reduces OTR below 50 cm³/(m²·day·bar) but requires anhydride tie layers and increases melt pressure by 10–25%. Published data for blown silage film with EVOH core at blow-up ratios above 3.0:1 is limited, and resin supplier technical data should govern the processing window.

Coextruded Silage Film Viscosity Hierarchy and Layer Stability

Coextrusion of silage film often places an EVA-rich layer on the outer surface for tack and puncture resistance, a LLDPE core for tear resistance, and a white or black pigmented layer for UV absorption and light reflection. Viscosity matching at 220 °C is required to prevent interfacial instability; a polyamide or EVOH barrier layer with high viscosity at low shear can divert melt into the low-viscosity LLDPE layer if the feedblock is not designed with layer-specific melt pumps. Layer thickness ratios of 20/60/20 or 30/50/20 are common, with the pigmented layer kept at or below 20% to reduce die lip build-up from titanium dioxide. Die gap for silage film is generally 2.0–2.5 mm, and blow-up ratios between 2.0:1 and 2.8:1 balance machine direction tear resistance and transverse direction dart impact. The bubble is often run with a lower stalk height of 3–5 die diameters to reduce melt exposure and limit oxidative gel formation. Melt temperature above 240 °C in the presence of HALS and titanium dioxide can cause yellowing and gel formation; the resin supplier’s maximum processing temperature of 230–235 °C should not be exceeded.

Internal Bubble Cooling and Frost Line Height Are Not Independent Variables

Internal bubble cooling changes the thermal gradient between the die and the frost line. When IBC exhaust air temperature is held at 35–55 °C, cooling rate increases by 15–25% relative to external air alone, and frost line height drops by 0.5–1.5 die diameters. This is not a pure temperature effect; positive internal pressure inside the bubble alters the geometry of the molten stalk and can suppress bubble breathing if the IBC blower speed is not synchronized with the external air ring. On a 250 mm die with stalk height 6 die diameters, increasing IBC pressure from 0.2 kPa to 0.5 kPa reduces frost line height enough to increase MD tensile strength by 15–20% but can increase gauge variation from ±1.5% to ±2.5% if the collapsing frame is misaligned by 1°. The external air ring and IBC must therefore be treated as coupled control loops; changing one without the other moves the frost line and alters the MD/TD tensile ratio.

Polyamide and EVOH barrier layers used in silage coextrusion require drying to below 0.1% moisture before extrusion; polyamide at 80 °C for 4–6 h and EVOH at 90 °C for 4 h are typical starting conditions, but resin manufacturer data supersede these values. Failure to dry EVOH produces hydrolytic degradation, gel particles, and interfacial voids with adhesion loss. PIB-containing cling layers are incompatible with EVOH in the same layer and can reduce interlayer adhesion when placed adjacent to polar tie layers without adequate separation. Processing temperatures above 240 °C in the presence of HALS and titanium dioxide should be avoided because yellowing and gel formation increase, and the die lip accumulates oxidative deposits. The operating window for polyamide-containing silage film is narrower than for LLDPE stretch wrap; melt temperature must be held at 225–235 °C to balance polyamide viscosity and avoid thermal degradation of the tie layer, and screw speed should not exceed 80–100 rpm on a 55 mm barrier screw if residence time exceeds 3 min.