Which LDPE Film Thickness for Heavy-Duty Bags? Micron-to-Load Guide

Low-density polyethylene (LDPE) film employed in heavy-duty industrial bags normally falls within a thickness range of 100 µm to 250 µm, although compacted-waste liners are sometimes supplied at 300 µm. Film density for this application is commonly 0.918 g/cm³ to 0.924 g/cm³ when determined by ISO 1183-1 or ASTM D1505. The selection of film thickness cannot be reduced to a single guaranteed load rating because LDPE bag failure proceeds through competing mechanisms: tensile yield of the film web, heat-seal peel, dart impact rupture, and tear propagation. A measured machine-direction tensile yield of 12 MPa under ISO 527-3 does not automatically mean that a 150 µm bag carries twice the load of a 75 µm bag, because seal strength, gusset folding, and thickness-induced stiffness changes alter the failure sequence.

For a flat, unwelded LDPE web, the ultimate tensile load per metre width can be estimated by multiplying the tensile stress at yield by the cross-sectional area. A 150 µm film with an assumed 12 MPa machine-direction yield strength therefore has an ultimate capacity of 1800 N/m. A 3:1 design factor reduces the permissible working value to 600 N/m, equivalent to approximately 61.2 kgf/m. This flat-web calculation does not include gusset folding, corner stress, handle cut-outs, or seal peel. It serves only as a starting point for the more restrictive conversion factors applied to fabricated bags.

What Parameters Control Film Load Capacity Besides Thickness?

Heat-seal strength measured under ASTM F88/F88M is frequently the limiting factor in heavy-duty bags, particularly when the seal width is below 10 mm. LDPE film sealed at a jaw temperature of 120 °C to 150 °C can produce seal strengths of 8 N/15 mm to 15 N/15 mm, but the conversion to bag capacity depends on the total seal length and the failure mode at the seal. Tear resistance is measured by ISO 6383-2 or ASTM D1922; LDPE normally exhibits high Elmendorf tear values because of its branched molecular architecture and high elongation at break. Dart impact, tested under ISO 7765-1 method A or ASTM D1709, becomes critical when filled bags are dropped or thrown from loading docks. Gauge variation, recorded by scanning capacitance or beta-transmission thickness profiling per ISO 4593, determines how much of the nominal thickness is actually available at fold lines and side seals.

Deriving the Micron-to-Load Reference From Tensile Yield

The reference table below uses an isotropic flat-film assumption with machine-direction tensile yield of 12 MPa. The equation is:

F_ult = σ_y × t × w

where w is 1 m of web width. The safe load column applies a 3:1 design factor to the calculated ultimate load. This is not a universal bag rating; actual bags must be tested under ISO 527-3 and ASTM F88/F88M in final fabricated form.

Creep and sustained-load conditions require an additional reduction. LDPE has a time-dependent modulus; a static load held for more than 48 h may produce creep rupture at stress levels below the short-term yield value. Published data for specific bag configurations under creep loading is limited, so controlled static load tests should be performed before selecting thickness for long-term storage applications.

In blown film extrusion of heavy LDPE webs, a 150 µm film is typically produced on a single-screw extruder with a screw diameter of 60 mm to 90 mm and an L/D ratio between 24:1 and 30:1. The die gap is normally widened to 2.0 mm to 2.8 mm when target thickness exceeds 100 µm; narrower die gaps raise shear rate and may reduce output stability. A blow-up ratio of 1.8:1 to 2.5:1 is common for heavy-duty bags because high blow-up improves transverse-direction toughness but can increase film gauge variation. Frost-line height is held high enough to stabilize the bubble but low enough to avoid excessive crystallinity; LDPE film toughness typically benefits from a lower melt temperature of 160 °C to 180 °C at the die.

When Gauge Variation Reduces Safe Working Load

Gauge uniformity is a prime determinant of actual bag failure load. A nominal 200 µm film with a gauge spread of ±10% has local thickness that may fall to 180 µm at the fold line or near the gusset. Because the flat-web tensile capacity is directly proportional to thickness, the local ultimate load at 180 µm under the same 12 MPa yield assumption is 2160 N/m instead of 2400 N/m. If a 3:1 safety factor is applied to the nominal value, the design load is 800 N/m, but the local safe load is only 720 N/m. Thus, the effective load rating of a fabricated bag is governed by the thinnest section that crosses the load path. Random thickness monitoring is inadequate; scanning gauges should be used to map the full width of the layflat web before conversion.

Heat-Seal Integrity in Heavy LDPE Bag Construction

Heat sealing is a thermally activated melt-fusion process. For LDPE film above 150 µm, seal-bar temperature must be raised or dwell time extended to ensure adequate heat penetration through the full film cross-section. A seal-bar temperature of 135 °C and dwell of 1.0 s may be sufficient for 75 µm film, but 200 µm film often requires 145 °C to 155 °C and dwell of 1.5 s to 2.5 s on impulse sealers. Seal cooling under pressure is critical; hot tearing can occur if the molten seal is released before solidification. Seal strength is measured according to ASTM F88/F88M, and the failure mode should be recorded. Cohesive film failure adjacent to the seal is preferred over interfacial peel because it indicates fusion across the interface. Interfacial peel at values below 8 N/15 mm typically indicates contamination or insufficient heat.

Heavy-duty LDPE bag formulations often contain an antiblock masterbatch added at 2% to 5% and a slip additive at 500 ppm to 1500 ppm to maintain coefficient of friction measured under ISO 8295 below 0.6. These additives do not change the tensile yield used in the flat-web calculation, but they can alter seal initiation temperature. Antimicrobial or UV-stabilized films must be evaluated for seal strength because some inorganic antiblock systems can reduce hot-tack performance. Recycling content above 20% post-industrial rework can increase gel count and reduce tear resistance; film for heavy-duty bags should be inspected for gels larger than 0.6 mm by camera-based surface inspection when high recycled content is used. Where load-bearing performance is critical, a filled-bag drop test following an internal procedure derived from ASTM D5276 should be conducted because flat-film tensile values do not capture dynamic stress concentration.

At service temperatures below -10 °C, LDPE retains more flexibility than many lower-cost polyolefin films, but the measured Elmendorf tear value decreases. Brittleness temperature tested by ASTM D746 is commonly below -40 °C for LDPE, yet heavy-gauge film can still fail at seal notches under dynamic loading. Selecting the next higher micron step is a conservative approach when cold-storage bags are handled at temperatures below -10 °C. However, no thickness increase substitutes for verification of dart impact and seal strength on the finished bag under the actual drop mass and height. Published data for specific thickness-load combinations in cold-storage heavy-duty LDPE bags is limited; controlled drop testing and load retention measurements on the production line are therefore required.