Extrusion of high-molecular-weight high-density polyethylene into thin-gauge blown film for T-shirt sacks and merchandise bags follows a low-output, low-die-gap process. On a 65 mm grooved-feed extruder with L/D 30:1 and a barrier screw, the melt enters the die at 210–225°C while the die gap is maintained at 0.8–1.2 mm. Because the melt index is 0.18 g/10 min under ASTM D1238 at 190°C / 2.16 kg, bubble support depends on melt strength rather than melt flow. Typical blow-up ratios of 3.5:1 to 4.5:1 and frost line heights of 4–6 die diameters are used to balance machine-direction and transverse-direction tear anisotropy. A starting formulation for 12 µm film blends 90 wt% TR-144 with 10 wt% LDPE having density 0.923 g/cm³ and melt index 0.25 g/10 min to reduce machine-direction split tendencies during bag conversion. Film tensile properties are measured according to ASTM D882; dart drop resistance is measured under ASTM D1709 Method A. Compliance with food-contact regulations for grocery contact is based on FDA 21 CFR 177.1520(c) and, where applicable, Commission Regulation (EU) No 10/2011, Annex I. The finished article is a high-stalk T-shirt sack with seal strength at the bottom hem and side welds, typically processed on a bag converter at seal temperatures between 150°C and 175°C with seal dwell 0.2–0.4 s. The practical lower thickness boundary is 8 µm, below which dart impact becomes too variable for automatic bagging lines; the upper boundary for this segment is 15 µm, above which the sack loses the thin-gauge cost position relative to LDPE.

What Limits Bubble Stability at Blow-Up Ratios Above 4:1?

At blow-up ratios above 4:1 on HMW-HDPE film lines, the limiting variable is not extruder torque but bubble geometry. For institutional can liners, film thickness is increased to 18–25 µm, and formulations incorporate 10–20 wt% LLDPE with hexene comonomer to improve dart impact and Elmendorf tear in the transverse direction. The TR-144 phase contributes density and stiffness, but if the LLDPE fraction exceeds 30 wt%, melt strength declines enough to cause bubble snaking unless the internal bubble cooling air flow is increased by 10–15%. In practice, a grooved-feed extruder running 80 kg/h on a 300 mm die with a 1.2 mm die gap shows surface melt temperature at the die lip of 215°C; raising the temperature above 230°C reduces melt strength and produces gauge bands at the frost line. Frost line height is kept at 5–7 die diameters, and low frost line settings below 4 die diameters increase blocking in the collapsed film. The can liner is tested under ASTM D1709 and ASTM D1922; the failure mode shifts from dart puncture to tear propagation when the LLDPE fraction is above 15 wt%. Non-food can liners do not require food-contact clearance, but heavy metals and substances of very high concern are controlled under REACH Regulation (EC) No 1907/2006, Annex XVII. The terminal article is a 55-gallon drum liner or institutional trash can liner with a folded bottom seal and optional perforation for tear-off dispensing.

When Coextruded HDPE Stiffening Layers Replace LDPE in Dry-Food Lamination Webs

Dry-food lamination webs built with an HDPE stiffening layer differ from monolayer sack film in that the bubble is quenched more aggressively to limit haze and the HDPE layer is coextruded against a tie layer and an EVOH or metallized barrier layer. In this structure, TR-144 functions as the outer stiffening web, not as the sealant. A coextruded three-layer line with a 200 mm die and 1.4 mm die gap runs the HDPE phase at 200–215°C, the tie phase at 190–205°C, and the barrier phase at 205–220°C. The HDPE phase may be 70–80 wt% TR-144 blended with 20–30 wt% metallocene LLDPE to reduce water vapor transmission and maintain flex-crack resistance during pouch forming. The blown film is then slit and laminated to a printed web, and the final structure is used as a dry-food liner or stand-up pouch outer web. Migration compliance is evaluated under FDA 21 CFR 177.1520(c) and Commission Regulation (EU) No 10/2011, with overall migration measured in food simulants A, B, D2 as applicable. The practical processing window for the HDPE layer is narrow: raising the HDPE melt temperature above 225°C oxidizes the outer surface and increases gel count, while lowering it below 195°C increases melt pressure and disrupts the coextruded layer uniformity. The terminal product is a 15–20 µm HDPE layer inside a lamination that replaces a 25 µm LDPE-rich web, maintaining stiffness while reducing total package weight.

Industrial liners for drums and FIBCs are converted from blown film that is intentionally downgauged to 30–50 µm while maintaining puncture resistance. The TR-144 level is kept at 85–95 wt%, with the balance a hexene LLDPE; as the LLDPE level rises to 15 wt%, the tensile yield stress decreases but the Elmendorf tear increases. Film is produced on a high-stalk line with a 250 mm die, 1.5 mm die gap, blow-up ratio of 3:1, and frost line height of 8 die diameters. The primary failure mode in drum liners is snag tear from sharp edges during installation; tear resistance is evaluated under ISO 6383-2 or ASTM D1922. For FIBC liners, the end article is tested as part of a certified flexible intermediate bulk container under UN 13H2 or ISO 21898:2004. Moisture exposure during warehouse storage at relative humidity above 60% is not a bulk hydrolysis risk, but surface condensation on pellets stored below dew point can produce blown film bubble defects; pellets should be brought to ambient temperature before entering the hopper. The processing boundary is set by drawdown: below 30 µm the liner loses snag resistance, while above 50 µm the economics favor a lower-density polyethylene.

Blown Film Die Design and Melt Fracture Boundaries for Narrow-MWD HDPE

Slot geometry influences the onset of sharkskin and melt fracture in TR-144 films at high throughput. The low melt index of 0.18 g/10 min produces high extruder head pressure, often above 30 MPa on a 90 mm extruder, which demands die gaps between 0.8 mm and 1.4 mm. If the die gap is closed below 0.8 mm to improve gauge uniformity, shear rates at the die lip exceed the critical value for sharkskin; surface roughness appears as fine transverse ridges at output rates over 120 kg/h. Conversely, widening the die gap beyond 1.5 mm reduces shear but produces a thicker melt web that requires higher drawdown, increasing machine-direction orientation and lowering dart impact measured under ASTM D1709. Die lip build-up from oxidized low-molecular-weight fractions is observed after 6–8 hours continuous running unless the die lip is cleaned and the antioxidant package in the masterbatch is matched to the residence time distribution. Published data for this specific configuration is limited; the ranges above are derived from HMW-HDPE film extrusion practice and should be validated on the target line. The terminal product for this processing segment is die-lip-limited thin film at 10–15 µm; the operational boundary is set by melt pressure and surface melt fracture, not by film mechanical properties.

Post-industrial regrind from TR-144 film edges and its use in non-food sacks

Post-industrial regrind from TR-144 film edges and start-up purge is not a drop-in substitute for virgin pellet. Regrind particles have lower bulk density 0.35–0.45 g/cm³ and irregular shape, and they can bridge in the feed throat of a smooth-bore extruder if added above 30 wt%. In non-food sack applications, up to 50 wt% post-industrial regrind is blended with virgin TR-144 and 5–10 wt% LLDPE, but the film loses dart impact by 10–20% at 50 wt% regrind due to gel formation and molecular weight reduction. The regrind stream must be controlled for melt index by ASTM D1238 and density by ASTM D1505; an increase in melt index above 0.22 g/10 min indicates shear-induced chain scission and should trigger a reduction in regrind fraction. The end product is a non-food can liner or industrial sack that is not cleared for direct food contact under FDA 21 CFR 177.1520(c) unless the regrind is derived from compliant virgin film and the converter documents traceability under FDA 21 CFR 177.1520 paragraph (b). Regrind use above 50 wt% is not recommended for blown film lines with smooth-bore feed sections because feeding instability produces gauge variation of ±5% or greater.