Why High-Stalk Bubble Geometry Is Preferred for Thin-Gauge Retail Bags

At gauge targets between 8 µm and 18 µm, high-density polyethylene blown film stability is controlled primarily by the distance from the die lip to the frost line. The high-stalk configuration, with a neck height of 6 to 9 die diameters, is selected for Sinopec Fujian HDPE DGDA6097 because it permits the melt to strain-harden before expansion and reduces the likelihood of draw resonance at blow-up ratios of 3.5:1 to 4.5:1. On monolayer lines constructed with 45 mm to 65 mm grooved-feed extruders and L/D ratios from 24:1 to 30:1, the melt temperature at the die adapter is normally held between 210 °C and 230 °C. The die gap is set at 0.8 mm to 1.2 mm because a wider gap at this gauge encourages excessive transverse direction orientation and produces film with lower handle tear resistance. Dual-lip air ring cooling is preferred over single-lip cooling at outputs above 0.8 kg/h per millimetre of die circumference, and internal bubble cooling is added when die diameters exceed 200 mm. The principal failure mode in this process is not bubble burst but cyclic gauge-band formation: local die lip temperature differences greater than 5 °C generate alternate thick and thin bands that become visible after gusseting and block separation on high-speed bag machines. Finished retail carrier film is tested under ASTM D1709-16a, Method A, for dart impact, and under ISO 527-3:2018 for tensile properties. Elongation at break below 350 % in the transverse direction is usually rejected because it produces handle splitting when the vest-style bag is loaded.

Formulation for this segment is kept intentionally simple. Typical mixtures contain 100 wt% DGDA6097, plus 0.5 wt% to 1.5 wt% silica-based antiblock masterbatch and 0.2 wt% to 0.5 wt% erucamide slip concentrate. Adding more than 2 wt% low-density polyethylene to soften the film lowers melt tension enough to force lower blow-up ratios and changes the high-stalk bubble from a tall neck to a less stable low-stalk geometry. The end product is a die-cut or heat-sealed vest carrier bag in the 10 µm to 15 µm range after conversion. Storage before dart impact testing must be performed at 23 °C ± 2 °C for at least 24 h because polyethylene impact strength is sensitive to conditioning temperature and strain rate.

Dry food liner extrusion with DGDA6097 is governed by organoleptic neutrality rather than film toughness. Cereal, cracker, and powdered beverage liners are typically converted at melt temperatures no higher than 220 °C because oxidised low-molecular-weight fractions generated in the barrel can transfer taste and odour to packaged food even when the base resin meets compositional requirements. The olefin polymer is covered under FDA 21 CFR 177.1520(c) as an olefin homopolymer, and the finished food-contact film must also satisfy the overall migration limit of 10 mg/dm² in Commission Regulation (EU) No 10/2011 when tested with food simulants assigned to the intended food type. For dry cereal liners, the common converter specification is a film gauge of 25 µm to 50 µm, haze below 15 % under ASTM D1003-21, and gloss above 60 GU at a 60° measurement angle. Because DGDA6097 is a high-molecular-weight film grade, gel formation is aggravated by dead spots in screen changers and by residence times above 230 °C. The recommended conversion practice is to use a continuous screen changer rather than a manual breaker plate, and to purge for 15 min to 30 min after any shutdown that exceeds 3 h. The primary end products are heat-sealed inner liners for bag-in-box dry food packaging and block-bottom liners for dry pet food. On horizontal form-fill-seal machines, jaw temperatures between 140 °C and 160 °C are generally required to achieve hermetic seals at production speed without burn-through at seal corners.

Heavy-Duty Industrial Liners and the Dart Impact Trade-Off

Construction debris sacks and chemical drum liners in the 50 µm to 80 µm range expose a direct trade-off between dart impact and secant modulus. Blending 10 wt% to 25 wt% of an octene-based mLLDPE with a melt index near 1.0 g/10 min before the feed throat raises dart impact measured under ASTM D1709-16a, Method B, but reduces stiffness measured under ISO 527-3:2018. On a blown film line using a 75 mm grooved-feed extruder with 28:1 L/D and a 250 mm die, the die gap is increased to 1.5 mm to 2.0 mm to reduce orientation of the high-molecular-weight fraction and preserve machine-direction Elmendorf tear strength under ASTM D1922-15. A blow-up ratio below 3.0:1 is selected to shift orientation toward the machine direction; this improves bottom-seal bag making but lowers transverse direction dart impact. Frost line height is maintained between 7 and 9 die diameters because a lower frost line raises bubble instability when the blend contains a low-viscosity mLLDPE fraction. The processing window is narrower than in single-component HDPE film: a melt temperature above 225 °C reduces bubble stability, while a melt temperature below 200 °C raises extruder backpressure and increases the chance of melt fracture at the die lip.

Environmental stress cracking resistance is tested under ASTM D1693-15, Condition B, in 10 % Igepal CO-630 at 50 °C. Liners intended for agricultural chemical intermediates often require failure time above 500 h. Published data for the exact DGDA6097/mLLDPE blend at these thicknesses is limited, so converter trials must verify the drop in dart impact at low storage temperatures. In cold-climate service, the minimum handling temperature is typically set at 0 °C unless the film is qualified by ISO 8570:1998 puncture resistance at the intended service temperature. The end products are self-standing construction waste sacks, drum liners, and heavy-gauge shipbuilding lay-up films; each requires a different antiblock package because the coefficient of friction under ISO 8295:2004 must be adjusted for either manual opening or automated bag insertion.

Bulk rice, salt, and mineral fertilizer sacks assembled from woven polypropylene use a blown HDPE inner ply of 30 µm to 50 µm to provide moisture resistance and clean discharge. The film is produced from DGDA6097 at a blow-up ratio of 3.0:1 to 3.5:1, with a die gap of 1.0 mm to 1.4 mm, to keep the coefficient of friction low enough for automated bag filling. Where the liner is pre-sealed into a tube, seal strength is measured under ASTM F88/F88M-21. The critical processing constraint is thickness uniformity: gauge variation greater than ±8 % across the web causes the inner ply to bridge or fold during lamination, and pinholes appear after the filled sack is dropped. Drop testing follows ISO 7965-1:2022 at a mass selected by the converter, while tear resistance is assessed by ASTM D1922-15. Because the woven outer fabric carries most of the tensile load, the HDPE film does not need to retain high machine-direction stiffness, but it must resist flex cracking during repeated handling. The Gelbo flex test under ASTM F392/F392M-21 is frequently specified for 1000 cycles, with a post-flex pinhole count below 5 holes per 300 cm².

Compliance for this segment is determined by the filled product rather than the film alone. Packaging for food-grade rice and salt must use film that meets FDA 21 CFR 177.1520(c) or China GB 4806.7-2016, while packaging for chemical fertilizers falls under national transport and safety regulations. Heavy metals in the packaging material are limited to 100 mg/kg total for lead, cadmium, mercury and hexavalent chromium under EU 94/62/EC. The laminate structure also requires an adhesive or extrusion coating layer between the woven fabric and the HDPE film; without adequate adhesion the inner ply can delaminate during moisture expansion of the fill.

When Three-Layer Coextrusion Positions DGDA6097 as the Stiff Core

Multi-layer film for dry pet food pouches and bag-in-box beverage liners uses DGDA6097 as a 10 µm to 25 µm core layer between LLDPE or LDPE skin layers. The objective is to increase the secant modulus of the total film without moving the sealing layer to a higher initiation temperature. The core layer is extruded at a melt temperature of 210 °C to 230 °C, while the skin layers are usually run 10 °C to 15 °C lower to limit surface oxidation. A three-layer die with adjustable layer distribution is required; the core proportion is kept between 30 wt% and 50 wt% of total film mass. If the core layer exceeds 50 wt%, the film loses puncture resistance and the heat-seal strength under ASTM F88/F88M-21 declines because the high-modulus layer reduces deformation at the sealing interface. Film blocking between layers is controlled by adding 0.3 wt% to 0.8 wt% of silica or talc antiblock to the skin layers only; adding antiblock to the core increases haze without improving blocking. The end structure is often printed and laminated to PET or BOPP, so film surface tension must be maintained above 38 mN/m before corona treatment, with treated retention above 38 mN/m for at least 30 days under controlled storage.

The main process risk in this structure is interfacial instability between the core and skin layers when the viscosity ratio is too high. If the skin resin has a melt index more than 1.5 g/10 min above the core resin, flow disturbance inside the feedblock can produce wavy interlayers and visible optical defects. Layer thickness measurement after the die is performed on pressed cross-sections under optical microscopy, and the total film gauge is monitored with a scanning capacitance gauge to maintain a variance below ±5 %. Puncture resistance of the final structure is measured under ISO 7765-1:1988, and tear resistance under ASTM D1922-15.

Deadfold Retention Drops at Blow-Up Ratios Above 3.0:1

Courier envelopes, document sleeves, and map sleeves require film that retains crease memory after folding. The high modulus of DGDA6097 contributes deadfold, but the blown film process must avoid excessive transverse orientation. A blow-up ratio of 2.5:1 to 3.2:1 is selected, combined with a die gap of 1.2 mm to 1.6 mm and a gauge of 25 µm to 60 µm. In this configuration, the machine-direction secant modulus is higher than in general-purpose packaging film, but tear initiation resistance measured by ASTM D1004-21 may be lower in the crease area because localized yielding occurs after repeated folding. The film is corona treated in line to a wetting tension of 40 mN/m to 44 mN/m. Ink adhesion for flexographic printing is evaluated by a tape test after 24 h aging. Because the material is paper-like only when pigmented, converters add 4 wt% to 8 wt% titanium dioxide white masterbatch; this reduces tensile strength and requires verification of the coefficient of friction under ISO 8295:2004 before envelope inserting machines can run at rated speed. The end products include self-adhesive mailers, document pouches, and map sleeves, each of which must be folded manually or by a plough folder without spring-back.

The process conflict is that increasing the blow-up ratio above 3.0:1 may improve film gauge uniformity but reduces deadfold by orienting more polymer chains in the transverse direction. Converters that require both deadfold and high gauge uniformity often use a rotating die or counter-rotating die centre rather than increasing blow-up ratio. This preserves the orientation balance and avoids the haze increase associated with higher TiO₂ loading. Published data for the specific deadfold retention of DGDA6097 after flexo printing is limited, so trial measurements are necessary before commercial mailer conversion.