Heavy-Duty Industrial Sack Lines: Extruder L/D Requirements and Dart Impact Thresholds

In heavy-duty industrial sack conversion, Marlex® 7109 is processed as a monolayer or three-layer A/B/A structure at total film thicknesses from 100 µm to 220 µm. Grade certification supplied with the resin typically lists a nominal density of 0.918 g/cm³ according to ASTM D1505 and a nominal melt index of 1.0 g/10 min according to ASTM D1238 at 190°C and 2.16 kg. The terminal products are 25 kg, 40 kg, and 50 kg sacks for resin pellets, cementitious dry mixes, fertilizers, and industrial salts. On a 70 mm single-screw extruder with a 26:1 L/D barrier screw and a Maddock mixing tip, the melt temperature window is held at 190°C to 215°C at the die, while adapter and die zones are maintained within ±5°C of the target to avoid localized melt fracture. A die gap of 1.8 mm to 2.5 mm is used with a blow-up ratio from 2.2:1 to 3.2:1 and a frost line height of 5 to 8 die diameters. At line speeds above 45 m/min, bubble respiration at the collapsing frame is stabilized by adding 10 wt% to 20 wt% of LDPE with a melt index of 0.25 g/10 min to 0.8 g/10 min as measured by ASTM D1238. Without that LDPE addition, the low melt tension of the LLDPE phase allows gauge banding of ±12% to ±18% around the circumference, which is outside the ±5% to ±8% tolerance demanded by automatic sack indexers.

For aggressive dry product exposure, a formulation containing 2 wt% to 4 wt% carbon black masterbatch with a 45% to 50% carbon black loading, 1 wt% to 3 wt% silica-based anti-block masterbatch, and 500 ppm to 1,500 ppm erucamide slip additive is typical. The erucamide addition creates a coefficient of friction target of 0.25 to 0.45 as measured by ASTM D1894-14 after 24 h ageing. Polymer processing aid is added at 300 ppm to 800 ppm to suppress melt fracture in the die land. The seal initiation temperature of the 7109-rich layer is in the range of 105°C to 115°C under a 0.4 MPa seal bar pressure and 0.5 s dwell. Seal strength is targeted at 10 N/15 mm to 16 N/15 mm when tested according to ASTM F88/F88M-21. For dimensional stability, the film is gusseted to 90 mm to 150 mm and converted on a bottom/back sealer at 40 to 60 sacks per minute. The critical limitation is hot-tack performance: 7109 exhibits a relatively narrow hot-tack plateau, and packaging lines with fill temperatures above 60°C require a 5 wt% to 10 wt% plastomer or EVA blend to prevent seal creep during drop loading. No pre-drying is required for the resin itself because polyethylene is non-hydroscopic, but condensation on cold pellets transferred from outdoor silos at relative humidity above 75% can create surface moisture defects. Therefore, hopper heating at 50°C to 60°C is applied when ambient dew point approaches pellet temperature.

Agricultural silage wrapping specifications rarely rely on a single dart impact value because film failure occurs by slow puncture during bale compression and retraction. Marlex® 7109 is used in 25 µm to 45 µm blown structures as the stiffening layer in a three-layer coextrusion with metallocene-catalyzed LLDPE skins to lift dart impact above 600 g at 30 µm, measured by ASTM D1709-16a Method A. Total gauge variation must remain within ±8% because bale wrapping over round bales of 1.2 m to 1.5 m diameter exerts intense local stress at the film overlap. A typical layer distribution is 15% to 25% metallocene LLDPE in the outer surfaces and 50% to 70% Marlex® 7109 in the core. Tack is generated by 1 wt% to 3 wt% polyisobutene concentrate of 20% active content, while ultraviolet stabilization is provided by 0.5 wt% to 1.0 wt% HALS masterbatch and 0.2 wt% to 0.5 wt% antioxidant. The bubble is run at a blow-up ratio of 2.0:1 to 3.0:1 with a die gap of 1.5 mm to 2.0 mm and a melt temperature of 195°C to 215°C. During conversion to 750 mm rolls, edge trim generated by slitting 2,000 mm layflat is recycled at up to 20 wt%. Regrind levels above this threshold reduce tear propagation resistance by 15% to 25% because of gel formation and loss of the high-molecular-weight fraction during reprocessing.

Compliance for silage wrap is defined by EN 13207:2018, which requires minimum total thickness and specifies elongation at break after ageing. Under that standard, films must demonstrate at least 400% elongation at break in both machine and transverse directions, tested according to ISO 527-3. The 7109-containing structure typically exceeds that threshold at 25 µm, but aged films stored for more than 18 months under high UV irradiance can fall below the limit if the HALS concentration drops below 0.5 wt%. The terminal product is wrapped in 2 to 4 layers on round bales and 6 to 8 layers on square bales. Field loss is reported when the film contains more than 5 wt% inorganic anti-block filler because that filler has a disproportionate effect on slow puncture energy under ISO 7765-1:2020.

What Limits Seal Strength Retention in Blown Drum Liners After Contact with Aggressive Powders?

Blown drum liners produced from Marlex® 7109 at 100 µm to 250 µm are specified for dry-flowable chemical powders, pigments, and oil-contaminated metal swarf. The failure mode that controls conversion yield is not the seal initiation temperature but the retention of seal strength after the liner is filled and stored for 30 days at 40°C with a fill head adjacent to the seal. On a multi-layer blown line with an internal bubble cooling die of 200 mm diameter and die gap of 2.0 mm to 2.5 mm, the liner is run at a blow-up ratio of 1.8:1 to 2.5:1 to minimize transverse direction curl. Drop impact resistance after a 1.8 m fall at −20°C is specified at 12 kg to 16 kg for a 125 µm structure when tested according to ASTM D2463-15. That value degrades by 20% to 30% if extruder backpressure exceeds 250 bar and resin shear history generates micro-gels. To preserve seal integrity, the seal layer blend is formulated with 70 wt% 7109, 20 wt% LDPE, and 10 wt% EVA with a vinyl acetate content of 9% to 12%. The EVA lowers hot-tack initiation to 85°C and widens the sealing window from 100°C to 150°C.

The seal strength target is 20 N/15 mm to 25 N/15 mm according to ASTM F88/F88M-21, but powders with particle sizes below 10 µm can migrate to the seal area and reduce strength by 30% to 40% if the liner is filled at a drop height exceeding 2.0 m. The corrective measure on the packaging line is to reduce fill head velocity and to hold the liner mouth with vacuum-assisted opening rather than air-jet blowing. For food-contact liners, the olefin polymer is covered by FDA 21 CFR 177.1520(c) under conditions of use C through G, and EU Regulation (EU) No 10/2011 Annex I applies with an overall migration limit of 10 mg/dm². If the liner is used for retort or hot-fill above 80°C, the converter must switch to a liner grade with a higher Vicat softening point because 7109 loses dimensional stability above that threshold.

When Coextruded Surface Protection Films Require Low-Gel Filtration Below 100 µm

Surface protection films for polycarbonate sheet, stainless steel skid plates, and pre-painted aluminium are converted from Marlex® 7109 as the core layer in a three-layer structure with a total thickness of 40 µm to 80 µm. A typical skin/core/skin distribution is 15/70/15 by weight. The core provides stiffness and tear resistance, while polyolefin elastomer skins provide controlled adhesion from 0.05 N/25 mm to 0.50 N/25 mm as measured by ASTM D3330/D3330M-21 Method A. The conversion line is a 90 mm main extruder with a 30:1 L/D screw and a 120 mm annular die; melt temperature is held at 200°C to 220°C, and the die gap is 1.2 mm to 1.8 mm. The blow-up ratio is set at 2.2:1 to 2.8:1 because the three-layer melt curtain must be collapsed without blocking on polished surfaces. Because the film is stretched over polished metal and optical substrates, any inclusion above 80 µm produces a visible defect. Filtration through a screen pack of 60/80/100 mesh is used, but for optical-grade products the screen pack is replaced with a 100/120/150 mesh stack and the backpressure limit is set at 300 bar. Above that limit, the screen pack is changed every 4 h to 6 h rather than risk bypass leakage through the screen changer.

Gel counts are evaluated by ASTM D7310-20 on a 40 µm sample, with a target of fewer than 5 gels per 0.1 m² for gel size greater than 100 µm. If regrind from edge trim exceeds 15 wt%, the gel count rises above that target because the polyolefin core undergoes shear-induced degradation in the reclaim extruder. The terminal product is wound on 1,500 mm to 2,500 mm rolls and die-cut to customer-specific widths; edge smoothness is critical because a serrated slit edge triggers propagation when the film is peeled at a 180° angle. For adhesion to high-energy substrates, corona treatment of the skin layer to 38 dyn/cm to 42 dyn/cm is applied in-line, but the core 7109 remains untreated and is not rated for direct food-contact lamination unless covered by a functional barrier. Published data for the long-term outdoor weathering of this specific three-layer configuration is limited, and converters must conduct their own QUV ageing according to ASTM G154-23 before replacing a PVC-based surface protection film on outdoor profiles.

Palletizing Stretch-Hood Film Edge Tear and Viscous Damping in Vertical Form-Fill-Seal

Stretch-hood palletizing produces an 80 µm to 150 µm blown film hood that is stretched by 60% to 80% in the transverse direction before being released over a loaded pallet. Marlex® 7109 is combined with 20 wt% to 30 wt% LDPE and 10 wt% to 20 wt% metallocene-catalyzed LLDPE to achieve the necessary balance between extensional viscosity and tear propagation resistance. The blow-up ratio is run at 2.5:1 to 3.5:1. The process conflict arises at the film fold edges: after an automatic gusset is introduced, machine-direction tear resistance drops by 20% to 35% compared with the center web, as measured by ASTM D1922-15e1. On a line producing 1,200 mm layflat, edge tear is controlled by maintaining a frost line height of 7 to 9 die diameters and by running a dual-lip air ring with a chilled lower lip at 12°C to 15°C. This produces a more uniform transverse quench and reduces edge gauge thinning to below 10%.

The film is sealed on a vertical form-fill-seal unit with a hot-bar temperature of 140°C to 160°C and a seal pressure of 0.3 MPa to 0.5 MPa. Seal strength is measured at 15 N/15 mm to 20 N/15 mm according to ASTM F88/F88M-21. The 7109-containing formulation provides a dart impact of 350 g to 500 g at 100 µm by ASTM D1709-16a, but the addition of metallocene LLDPE raises dart impact by 20% to 35% while reducing modulus. That trade-off limits the stacking load on a pallet to 600 kg to 800 kg when a 100 µm hood is used; above this load, side gusset creep and film puncture at the pallet corners are observed. The terminal application is the replacement of shrink wrap for non-ventilated pallets carrying bagged cement, paper reams, and beverage cans. For outdoor storage, the film is UV-stabilized with 0.5 wt% HALS, but the product is not specified for exposure beyond 12 months because the low-density polyethylene matrix loses impact resistance after extended UV exposure.

At frozen-food forming temperatures below −25°C, the selection of a blown film grade with stable bubble geometry and low stress-whitening is dictated by line-surge behavior on vertical form-fill-seal machines. Marlex® 7109 is converted at 20 µm to 60 µm in laminates and monolayers for frozen vegetables, seafood, and ice-saturated poultry bags. A formulation for a 30 µm monolayer includes 1 wt% to 2 wt% silica anti-block and 500 ppm erucamide slip. The blown film process set includes a melt temperature of 195°C to 210°C, a blow-up ratio of 2.0:1 to 2.8:1, and a die gap of 1.5 mm to 2.0 mm. The terminal bags are run at 60 to 120 bags per minute on vertical form-fill-seal lines with a seal jaw temperature of 125°C to 145°C. Low-temperature drop impact is evaluated by freezing the filled bag for 24 h at −30°C and conducting a 1.0 m drop test; the failure rate is specified below 5%. Thicker films above 50 µm show lower stress-whitening and higher drop survival.

The selection of 7109 for this use is limited to applications where the package is not subject to sharp-edge puncture from ice crystals. In a monolayer structure, dart impact at −30°C is typically 30% to 40% lower than at 23°C, and the film should be downgauged only after line trials confirm seal integrity on vertical form-fill-seal machines. Compliance for direct frozen-food contact is governed by FDA 21 CFR 177.1520(c) and EU Regulation (EU) No 10/2011 Annex I, with any added slip or anti-block masterbatch requiring its own specific migration assessment. The film is corona-treated to 36 dyn/cm to 40 dyn/cm for flexographic printing, but surface treatment decays within 30 days under high-humidity storage above 70% relative humidity; therefore, in-line treatment immediately before printing is required.