When Melt Fracture Appears Above 120 kg/h, What Changes in Die Gap and Processing Aid Dose?

Heavy-duty sack production on 70 mm grooved-feed blown film lines running FB2310 at melt temperatures between 195 °C and 215 °C frequently produces sharkskin melt fracture when die-lip shear stress enters the 0.13–0.18 MPa range and the die gap is held below 1.6 mm. The first corrective intervention on production-scale equipment is widening the die gap to 1.8–2.2 mm while maintaining a blow-up ratio of 2.5–3.0 and a frost line height of 8–10 die diameters. If surface melt fracture persists after a gap increase, a fluoroelastomer processing aid masterbatch is added at 300–800 ppm by total polymer mass, with adjustment made batch-to-batch by monitoring die pressure fluctuation on a 0–400 bar transducer; fluctuation above ±8 bar after dosing indicates incomplete die-land coating. A second variable is the addition of 15–25 wt% LDPE with a melt flow rate of 0.25–0.5 g/10 min according to ISO 1133-1:2022 at 190 °C/2.16 kg, which increases melt tension and reduces bubble sag at 100–120 µm thickness. Below 190 °C the internal bubble cooling air cannot remove sufficient heat from the collapsing film and blocking increases; above 220 °C oxidative degradation accelerates gel formation. For food-contact heavy-duty sacks, the finished film is subject to EU Regulation (EU) No 10/2011 Article 12 overall migration limit of 10 mg/dm² and FDA 21 CFR 177.1520(c), with specific migration testing of antioxidant and slip additives under the intended food simulant. Industrial sacks for polymer granules, fertilisers, or construction materials commonly require REACH Article 33 SVHC declarations rather than food migration, but sacks used to transport crop protection active substances must additionally meet national dangerous goods packaging provisions. Terminal products include form-fill-seal heavy-duty sacks of 70–120 µm, valve bags, and woven outer bags with an inner PE liner.

Blown film lines running FB2310 at an MFR of 0.9 g/10 min per ISO 1133-1:2022 and a density of 0.931 g/cm³ per ISO 1183-1:2019 are used to produce frozen food packaging in the 35–50 µm thickness range because the C4 comonomer distribution in the Borstar bimodal process retains dart impact at −25 °C better than a unimodal HDPE of equivalent stiffness. The standard formulation on three-layer coextrusion lines uses a skin of 15 wt% LDPE and a core of 85 wt% FB2310, or alternatively a monolayer blend of 80 wt% FB2310 and 20 wt% LDPE with an MFR of 0.3 g/10 min; antiblock masterbatch is added at 2–3 wt% based on a 20 wt% synthetic silica concentrate with median particle size 8–10 µm, while slip masterbatch is limited to 0.5–1.0 wt% erucamide unless the film is subsequently laminated, because bloomed erucamide above this level lowers adhesive bond strength. Processing parameters on a 60 mm extruder with a 180 mm die are melt temperature 190–205 °C, die gap 1.6–2.0 mm, blow-up ratio 2.8–3.5, frost line height 6–8 die diameters, and output 90–110 kg/h; a lower frost line improves clarity but increases blocking at the contact surfaces of the collapsed film. The controlling seal requirements for frozen food lines are a hot tack force of at least 1.5 N/25 mm at 0.1 s dwell and a seal initiation temperature below 105 °C, both measured on the packaging machine rather than on laboratory heat sealers because film temperature drop at the jaw is typically 5–10 °C. Compliance is governed by EU Regulation (EU) No 10/2011 for overall migration and specific migration into aqueous frozen food simulants, FDA 21 CFR 177.1520 for frozen food contact under conditions of use E or F, and Regulation (EC) No 2023/2006 for good manufacturing practice in food-contact material production. Terminal applications include IQF vegetable and fruit pillow packs, frozen seafood pouches, and freezer-ready food service packs, all requiring retention of seal integrity after flex-cracking at minimum storage temperatures of −30 °C.

Lamination Sealant Webs and the SIT Plateau

FB2310 is converted into a 40–60 µm sealant web for adhesive lamination against PET, OPP, or aluminium foil, where the seal initiation temperature plateau between 95 °C and 105 °C determines converter acceptance. The formulation for a sealant web is typically 70 wt% FB2310, 25 wt% LDPE with an MFR of 0.3–0.7 g/10 min, and 5 wt% mLLDPE with a density of 0.918 g/cm³ and an MFR of 1.0 g/10 min; the mLLDPE portion lowers seal initiation by 2–4 °C but does not replace the C4 comonomer toughness contribution in the centre of the sealant layer. Slip additive must not exceed 0.2 wt% erucamide because migration to the sealant surface above this level causes adhesion loss to solventless two-component polyurethane adhesives applied at 2.0–2.5 g/m²; if a kinetic coefficient of friction below 0.20 is required, a high-molecular-weight silicone or non-migratory slip should be substituted. Blown film processing for the sealant web uses a die gap of 1.8–2.0 mm, a blow-up ratio of 2.2–2.8, and internal bubble cooling set to maintain film web temperature below 35 °C at the collapsing frame; the film is corona treated to 38–42 mN/m and laminated within 48 h because treatment decay on butene LLDPE proceeds faster than on LDPE. Seal strength on the finished laminate should be measured to ASTM F88/F88M-23 at 180 °C jaw temperature and 0.5 s dwell, with acceptance thresholds derived from pouch weight and headspace volume rather than from a single universal value. The terminal structures include 12 µm PET/ink/adhesive/45 µm FB2310 film pouches for coffee and confectionery, and 18 µm OPP/8 µm Al/60 µm FB2310 film packs for dry beverage mixes. In these structures the FB2310 layer does not provide oxygen or moisture barrier; the barrier contribution comes from the foil or metallised layer. Operational boundaries include storage below 60% RH and lamination within 48 h after surface treatment; pre-drying of the resin before extrusion is not normally required because moisture uptake of LLDPE is below 0.01 wt%, but regrind from printed film edges must be dried at 60 °C for 4 h if exposed to water-based inks. Solventless adhesives must comply with FDA 21 CFR 175.105 for indirect food additive use and with EU Regulation (EU) No 10/2011 for multi-layer food contact, while the finished pouch is subject to specific migration testing for primary aromatic amines if the adhesive is aromatic isocyanate-based.

On high-speed bottom-seal bag converters running 120 cycles/min, a 70:30 blend of FB2310 and LDPE at 30 µm requires seal bar temperature modulation between 125 °C and 145 °C to prevent edge tear while maintaining a pad seal width of 1.5–2.0 mm. The 30 µm film is produced on a 65 mm single-screw extruder with a die gap of 1.6–1.8 mm and a blow-up ratio of 2.0–2.5, which yields sufficient machine-direction tensile modulus for high-speed wicket bags while avoiding the anisotropic tear propagation found when blow-up ratio falls below 1.8. A slip and antiblock masterbatch combination is added at 1.5–2.0 wt% total, targeting a kinetic coefficient of friction of 0.15–0.25 according to ASTM D1894-24 and a blocking force below 50 g/cm² at 60 °C after 24 h compression; roll-stored converted bags develop surface bonding if the masterbatch silica particle size is below 5 µm. For produce bags or direct food-contact carrier bags, the blend is covered by FDA 21 CFR 177.1520 and EU Regulation (EU) No 10/2011, with the finished bag tested for overall migration using food simulant A for wet produce and food simulant E for dry goods. The EU Packaging Directive 94/62/EC heavy metal limit applies to the final printed package: the sum of lead, cadmium, mercury, and hexavalent chromium must not exceed 100 mg/kg in the packaging or packaging component, and the converter must retain declarations of compliance from ink and masterbatch suppliers. Terminal products include T-shirt bags, produce rolls, bakery bags, and bin liners where puncture resistance and bottom gusset weld integrity determine the lower practical thickness limit.

Greenhouse Layer Selection: Why C4 LLDPE Requires Coextruded HALS Stabilization

Three-layer greenhouse cover films of 150–200 µm total thickness use FB2310 in the inner layer at a layer ratio of 20–30% of total thickness because its 0.931 g/cm³ density and butene comonomer content provide puncture resistance at trellis contact points, but the resin alone does not survive more than 12 months under direct UV exposure. The inner layer formulation is 75 wt% FB2310, 20 wt% LDPE, and 5 wt% UV masterbatch containing a hindered amine light stabiliser loading of 20 wt% and a benzotriazole UV absorber at 10 wt%; the outer layers are typically 80 wt% LDPE or EVA with a separate UV package and an inorganic infrared absorber for thermal retention. Coextrusion is carried out on a three-layer blown film line with a 250 mm spiral mandrel die, layer distribution 1:2:1 or 1:1:1, melt temperature 190–205 °C, die gap 2.0–2.4 mm, and blow-up ratio 2.0–2.5; the frost line is held at 12–14 die diameters to balance bubble cooling and stress relaxation during high-thickness extrusion. Pesticide exposure, particularly sulfur-containing fungicides, attacks unstabilised polyethylene and accelerates film failure at greenhouse structure edges; converters therefore specify a minimum 0.4 wt% HALS in the outer layer and avoid transitional metal stearate concentrations above 0.1 wt%. Long-term UV degradation data for FB2310 in this specific greenhouse configuration are limited, so field trials under the intended pesticide programme and solar exposure remain the only reliable validation method. Regulatory requirements for agricultural film are not food-contact rules, but REACH Article 33 SVHC declarations apply to additives above 0.1 wt%, and disposal is governed by EU Waste Framework Directive 2008/98/EC with agricultural plastics collection schemes applicable in several member states. Terminal products include greenhouse tunnel films, low-tunnel crop covers, and silage cover sheets, where the FB2310 inner layer improves puncture resistance at structural contact points but is not the chief UV barrier.

A 45 µm dry-food form-fill-seal web produced with a 75:25 blend of FB2310 and LDPE on a vertical form-fill-seal line requires a hot-tack window that overlaps with jaw release timing, typically 1.0–1.5 N/25 mm at 0.1 s dwell and 115 °C jaw temperature. The web is produced on a 60 mm blown film extruder with a die gap of 1.8–2.0 mm, a blow-up ratio of 2.5–3.0, and a frost line height of 7–9 die diameters; extruder output is 80–100 kg/h at melt temperature 185–205 °C. Slip masterbatch is added at 0.3–0.5 wt% erucamide and antiblock at 0.5–1.0 wt% synthetic silica to prevent blocking during roll storage at 40 °C ambient warehouse conditions; the kinetic coefficient of friction target is 0.10–0.20 according to ASTM D1894-24 if the pouch is to be filled on a high-slip transport conveyor. For dry food contact, FDA 21 CFR 177.1520 conditions of use E or F apply depending on fill temperature, and EU Regulation (EU) No 10/2011 requires overall migration testing according to Article 12 with food simulant E for dry foods; specific migration of primary antioxidants and slip additives must be checked against Annex I restrictions. The main operational limitation of mono-layer FB2310 is its high oxygen transmission rate, so the material is used for short shelf-life dry goods or as the inner sealant layer of a laminated structure where a barrier layer is present. Terminal products include single-serve sugar and salt pouches, dried soup packs, tea pouches, and spice packs, where seal integrity at product-filled creases is the primary driver of film downgauging.