Low-density polyethylene (LDPE) is defined by ASTM D792-20 and ISO 1183-1:2019 density limits of 0.910 g/cm³ to 0.940 g/cm³. High-pressure autoclave and tubular reactors generate long-chain branching, which controls melt strength, extensional flow, and neck-in. The comparison LDPE vs LLDPE Cost per Ton: Why the Cheaper Resin Isn't Always Cheaper is not resolved by spot price per metric ton; it is resolved by mass per square metre, line speed, scrap rate, minimum functional thickness, and the cost of equipment modification required to process linear resins.

When Does a Lower Price per Metric Ton Fail to Reduce Finished Part Cost?

The cost per 1000 m² of monolayer film is calculated as thickness in micrometres multiplied by melt density in grams per cubic centimetre multiplied by resin price in USD per metric ton, divided by 10⁶. At 25 µm, an LDPE grade with density 0.923 g/cm³ yields 23.1 g/m², while a C8 LLDPE grade with density 0.918 g/cm³ yields 22.95 g/m². If LDPE is cheaper by 10% per metric ton, finished film cost at equal thickness falls only 6.5%. That differential becomes comparatively small when LLDPE enables conversion from 38 µm LDPE to 25 µm LLDPE: mass per square metre drops from 35.1 g/m² to 22.95 g/m², a reduction of 34.6%, before line speed, scrap rate, and transport mass are considered.

Across monolayer blown film lines with a 75 mm screw and 30:1 L/D, neat LLDPE raises die discharge pressure by 15–25% relative to LDPE at equal mass throughput and melt temperature. A general-purpose screw designed for LDPE commonly loses 20–30% output when switched to neat butene LLDPE, because the linear resin consumes higher specific energy and reaches the drive torque limit before the LDPE setpoint is achieved. The reduction is not a rheological defect; it is torque-limited operation. Processors selecting LDPE because of a lower price per ton may avoid LLDPE output losses but retain a finished film that is 25–40% heavier than a downgauged LLDPE alternative at equivalent dart impact resistance. Published data for exact output loss on every screw design is limited, but the trend is documented across barrier-screw and smooth-bore extruders.

Die Gap, Melt Fracture, and Torque Response in Neat LLDPE Film Production

Transferring an LDPE formulation to a neat C8 LLDPE on a 250 mm spiral mandrel die with an 0.8 mm die gap can produce visible sharkskin at moderate line speeds unless the die gap is widened to 1.8–2.2 mm. The wider gap reduces residence time at the lip and lowers wall shear stress below the critical onset of melt fracture. LDPE’s long-chain branching suppresses sharkskin across a wider die-lip shear-rate window, while LLDPE requires deliberate die-gap and blow-up-ratio adjustment. Field measurements on a 65 mm extruder show that LLDPE at 100 kg/h may exceed the drive rating of a machine sized for LDPE at the same throughput. Retrofitting with a barrier-flighted screw and blending 5–8% LDPE can reduce melt pressure while maintaining bubble stability, but the modification adds capital cost that must be charged against any per-ton resin saving.

If Downgauging from 38 µm to 25 µm Is Attempted Without Resin Substitution

Maintaining LDPE while reducing thickness from 38 µm to 25 µm lowers dart impact in a nonlinear manner. A film grade with density 0.923 g/cm³ and melt flow rate 1.0 g/10 min at 190 °C/2.16 kg per ASTM D1238-20 can show a ductile-to-brittle transition below 25 µm in cold-storage bag applications. The same caliper reduction with a C8 LLDPE often retains dart impact above 150 g under ASTM D1709-16a Method A, while the LDPE film may fall below 70 g. Distribution specifications for pallet wrap and frozen food bags commonly require 100–150 g dart impact. Under those conditions, LDPE downgauging fails specification before any purchase-price saving is realised.

A comparative property survey for 25 µm monolayer blown films, using commercial film grades, illustrates the mechanical performance gap relevant to downgauging decisions. Ranges are aggregated from commercial datasheets and vary with comonomer type, melt flow rate, and film fabrication conditions.

Heat-Seal Initiation Temperature and Seal Strength Decay Are Not Equally Balanced

LDPE retains a sealing advantage because its melting range of approximately 105–115 °C and long-chain branched structure permit seal initiation at 88–95 °C under ASTM F88/F88M-21 jaw-temperature profiling. LLDPE seals at 100–115 °C, requiring higher jaw temperatures, longer dwell, and tighter temperature control. However, hot-tack and seal-through-contamination performance of C8 LLDPE can exceed LDPE after 7 days at 40 °C, because the linear resin’s higher tie-chain population reduces seal delamination under load. On a horizontal form-fill-seal line running 80 packs/min, a 5 °C shift in seal initiation can alter seal integrity more than a USD 50/ton resin price difference. LDPE may be selected where heat-seal windows are narrow, but the same resin can generate higher reject rates when seal strength after abrasive filling falls below 8 N/15 mm.

Because environmental stress crack resistance under ASTM D1693-15 Method B with 10% Igepal CO-630 is often 10–100 h for LDPE and can exceed 500 h for C8 LLDPE, closure and container manufacturers evaluate more than pellet price. A 3.2 mm wall injection-molded cap may require an LDPE melt index near 4.0 g/10 min to fill a multi-cavity tool under ISO 1133-1:2022, while LLDPE at 2.0 g/10 min may generate short shots at the same melt temperature. The lower-priced LDPE grade then becomes the technically necessary choice even though part mass is higher. Published data for this specific configuration is limited, and each closure geometry must be validated with spiral-flow measurements and tensile yield data per ASTM D638-14.

Food-contact compliance under 21 CFR 177.1520 and EU 10/2011 does not distinguish resin cost per metric ton; it restricts migration and extraction. LDPE and LLDPE base polymers must meet the same overall migration limit of 10 mg/dm² under EU 10/2011, but the additive package, density, and conversion history affect compliance. Under REACH and RoHS, both base polymers are non-restricted, so the regulatory burden shifts to slip agents, antiblocks, and processing aids.

In extrusion coating of 12 g/m² LDPE onto 50 g/m² kraft paper at 300 m/min, LDPE is specified for melt strength and neck-in control below 25 mm per side. Substituting neat LLDPE on the same line raises neck-in to 50–70 mm unless die repositioning or edge encapsulation is used. Because LDPE is often lower priced per ton than C8 LLDPE, coating lines may appear to save raw-material cost. However, if the only industrially stable route to LLDPE requires a 20% LDPE blend, the blend cost is the weighted average of the two resins plus the added edge trim. The functional selection is governed by ISO 1133-1:2022 melt flow rate, ASTM D792-20 density, and coating-line neck-in, not by resin purchase price alone.

Collation shrink lines running at 0.7 m/s require high shrink tension and bubble stability. LDPE with high long-chain branching provides shrink tension of 0.8–1.2 MPa after orientation, while LLDPE may produce 0.4–0.6 MPa unless blended. A 70:30 LDPE:LLDPE blend balances shrink force and puncture resistance. The lower price per ton of LDPE reduces raw-material cost, but a neat LDPE film can fail puncture resistance under ASTM D5748-19 at 25 µm. Shrink percentage and shrink tension measured with ASTM D2732-20 therefore determine the blend ratio more directly than the per-ton price gap.