Low-density polyethylene (LDPE) polybag resin choices—which density and MFI fit which bag—are resolved on the blown-film line before laboratory tensile data are returned. Density, recorded by ASTM D1505 in a density-gradient column at 23 °C, and melt flow index, recorded by ASTM D1238 at 190 °C under 2.16 kg load, define the processing envelope. Commercial polybag grades cluster between 0.916 g/cm³ and 0.930 g/cm³ density and between 0.2 g/10 min and 2.5 g/10 min MFI, although specialty films may occupy a narrower band. A 75 mm grooved-feed extruder with L/D 30:1 and a 250 mm spiral mandrel die fitted with a 1.0 mm die gap converts these resins into film from 12 µm to 150 µm. The selection criterion is not the single-point datasheet value but the combined effect of density and MFI on bubble stability, melt pressure, heat-seal response, and the bag failure mode observed in the filling operation.

On production-scale lines, batch-to-batch variation of MFI within ±0.15 g/10 min and density within ±0.001 g/cm³ is sufficient to move the frost line or change the stable bubble diameter. Resin lots that are within specification may still be unsuitable for a thin-gauge line operated at the top of its output window. Polybag resin selection therefore tracks the limiting process condition, not the arithmetic average of a certificate of analysis. The equivalent ISO methods are ISO 1183-1:2019 for density and ISO 1133-1:2022 for melt mass-flow rate.

LDPE Polybag Resin Choices: Which Density and MFI Fit Which Bag

At the resin selection stage, the converter must identify the controlling variable. A bag filled with sharp, frozen, or heavy products requires dart impact and tear resistance, pulling selection toward low MFI and lower density. A bag produced at narrow gauge for high-speed packaging requires drawdown and bubble stability, pulling selection toward higher MFI and density near 0.922 g/cm³. The matrix below is a starting point, not a final specification.

Density differences of 0.003 g/cm³ within the matrix alter seal initiation temperature and film modulus. Increasing density from 0.918 g/cm³ to 0.924 g/cm³ typically raises 1% secant modulus measured under ASTM D882 and narrows the heat-seal temperature window, while lowering density improves Elmendorf tear propagation measured under ASTM D1922. MFI differences of 0.2 g/10 min shift head pressure and draw-resonance sensitivity more strongly than density because MFI is an inverse function of melt viscosity at the test shear rate.

For thin-gauge produce bags below 25 µm, the practical LDPE choice lies at 0.921–0.924 g/cm³ and 1.0–2.0 g/10 min. A high-clarity grade with 1.8 g/10 min MFI can be run on a 250 mm die with a dual-lip air ring at melt temperature 175 °C, frost line 180 mm, and blow-up ratio 2.8. At these settings, a 20 µm film often produces haze below 8% when tested under ASTM D1003 and 60° gloss above 60 GU when tested under ASTM D2457. Lowering density to 0.918 g/cm³ improves tear propagation but reduces stiffness and can increase blocking in humid storage. Raising density to 0.925 g/cm³ improves clarity and seal bar release but narrows the heat-seal window and may cause automatic bag openers to misfeed because the film becomes too stiff.

Resin suppliers’ technical bulletins for high-clarity film grades reflect these trends; published data for a specific converter line configuration is limited and requires a film trial. The same grade may fail on a line with a worn air ring or uneven die temperature zones, producing gauge variation and lower puncture resistance.

Heavy-Duty Sack Grade Selection and the Extrusion Pressure Boundary

Industrial sack films from 80 µm to 150 µm are extruded from LDPE homopolymers with MFI between 0.2 g/10 min and 0.4 g/10 min and density between 0.918 g/cm³ and 0.922 g/cm³. These grades carry enough melt strength to sustain a bubble at blow-up ratio 2.0 to 3.0 with a deep frost line, without excessive sag before the nip rolls. Dart impact resistance measured by ASTM D1709 Method A and Elmendorf tear measured by ASTM D1922 improve as MFI decreases and as density decreases, but head pressure rises. On a 75 mm L/D 30:1 barrier screw operating at 80 rpm with an 80 mm adapter and 1.0 mm die gap, a 0.30 g/10 min LDPE can move melt pressure above 30 MPa at melt temperature 185 °C; the same configuration running a 0.80 g/10 min grade may remain below 25 MPa. Production-scale failures in this segment are often pressure-related: screen pack blinding, die lip freeze-off, and breaker plate stagnation. If the selected MFI is too low for the available heater capacity, screw speed must be reduced and output falls below the line’s breakeven throughput. Published data for exact pressure-versus-output relationships for every screw and resin combination is limited; plant-scale trials are required.

MFI is a single-point viscosity at a relatively high shear rate and does not capture the low-shear melt strength that governs bubble sag. Two 0.25 g/10 min LDPE grades from different reactor lines can differ in head pressure and dart impact because of variations in long-chain branching and molecular weight distribution. When changing suppliers, a film-grade qualification must include not only standard melt index and density but also motor load, die pressure, and bubble stability under the same frost line.

When post-industrial reclaimed LDPE is incorporated into medium-duty refuse liners, the resin choice shifts toward a slightly higher virgin MFI to compensate for viscosity scatter. A 45 µm can liner made from 70% virgin LDPE and 30% post-industrial reclaim with nominal MFI 0.6 g/10 min can show effective MFI across batches from 0.4 g/10 min to 0.9 g/10 min. Because reclaimed LDPE contributes gels and oxidative residues, tear strength measured by ASTM D1922 and puncture resistance measured by ASTM D1709 often fall below the values of the virgin base resin. Converters typically compensate by selecting a virgin resin with MFI 0.2–0.3 g/10 min higher than the target final MFI or by reducing reclaim to 20% for thin liners below 25 µm. Density is kept in the 0.919–0.924 g/cm³ range to retain seal bar release and to allow the film to be wound without excessive blocking.

In low-temperature storage applications, LDPE retains better low-temperature flexibility than high-density polyethylene, but the polybag film must still be selected for seal integrity after filling with frozen product. Freezer bags around 38 µm often use LDPE with density 0.920–0.923 g/cm³ and MFI 0.7–1.2 g/10 min because the film must run fast on a side-seal bag machine and still maintain tear resistance at −20 °C. Lower density reduces stiffening under frozen storage; however, it also increases water vapor transmission compared with higher-density LDPE. Oxygen and moisture barriers are not primary functions of single-resin LDPE polybags, so multi-layer structures with EVOH or nylon are outside the scope of density and MFI selection alone. For ice bags, the specified density is often kept at 0.918–0.920 g/cm³ to maximize puncture propagation resistance.

When Tubular Quench and Internal Bubble Cooling Shift the Practical MFI Window

After the bubble passes the frost line, the quench rate determines how far a given MFI can be pushed outside its typical gauge range. A 0.30 g/10 min LDPE with high melt strength can be drawn to 18 µm if internal bubble cooling and a high-velocity dual-lip air ring increase the quench rate, but the resulting film may contain higher frozen-in stress and show lower machine-direction Elmendorf tear after slitting. Conversely, a 1.8 g/10 min grade runs faster on the same line but is more sensitive to frost line movement and air ring turbulence; the bubble enters a narrow stable window and begins to oscillate when the frost line rises above 4 die diameters. Melt fracture and sharkskin are less common with LDPE than with linear low-density polyethylene because long-chain branching suppresses the critical shear-rate onset, but they can appear when melt temperature drops below 160 °C at a die gap below 0.8 mm. Internal bubble cooling reduces the effective melt temperature at the frost line and acts like a higher-viscosity resin, shifting the practical MFI window downward by approximately one resin grade.

The onset of bubble oscillation in high-MFI LDPE correlates with the ratio of draw momentum to melt strength; the condition is not identified by MFI alone. A resin with MFI 1.8 g/10 min and narrow molecular weight distribution oscillates more readily than one with a broader distribution, even though the MFI is identical.

What Seal Strength and Slip Additive Migration Limits Must Be Observed

Because migrating slip additives alter the seal-bar interface and the film surface, polybag LDPE selection must include additive type, loading, and migration time. Polybag films frequently contain erucamide or oleamide at 500 ppm to 1500 ppm to reduce coefficient of friction. After room-temperature aging, coefficient of friction measured by ASTM D1894 or ISO 8295 can drop below 0.2. However, the same additives plate out on seal bars and reduce heat-seal strength measured by ASTM F88. The effect is more severe in low-density LDPE because the amorphous fraction and free volume are higher, which accelerates additive migration. A 25 µm film at density 0.918 g/cm³ may reach equilibrium slip within 14 days; a 0.924 g/cm³ film with the same loading may take longer and produce a lower final slip. For food-contact polybags, the finished film must comply with FDA 21 CFR 177.1520 or European Commission Regulation (EU) No 10/2011, including overall migration limits of 10 mg/dm². High-slip formulations are not appropriate when heat-seal integrity is critical in liquid-filled heavy-duty sacks; a medium-slip skin layer or non-slip inner layer is required.

Seal strength is also rate dependent. Minimum seal strength for a 50 µm film is often set at 2.0 N/15 mm when tested at 150 °C, 0.2 s dwell, and 0.3 MPa jaw pressure under ASTM F88. High-speed packaging machines can peel and load the seal at much higher rates. A seal that passes a laboratory test can fail in automatic bagging because the heat-seal dwell time is under 0.1 s and the cooling time is shortened. Converters must specify maximum coefficient of friction and minimum seal strength together because additive packages optimized for one requirement commonly reduce the other.