Shipping Polyethylene: 25 kg Bags, Jumbo Bags and Bulk — Loading Maths begins with pellet bulk density and load-bearing constraints rather than commercial invoice mass. Polyethylene resins move as non-hazardous solid cargo under the IMO/ILO/UNECE CTU Code, but pellet flowability, package footprint, stack height, floor pressure, and low surface friction govern safe loading. For HDPE and LLDPE pellet grades, loose bulk density typically falls between 0.52 g/cm³ and 0.60 g/cm³; LDPE powder grades can be lower, and filled compounds can exceed 0.75 g/cm³. Pellets are usually cylindrical or lenticular with nominal diameter 3 mm to 5 mm, and fines below 500 µm may segregate during pneumatic transfer, altering local bulk density. Loading calculations therefore change by packaging form: 25 kg bags multiply unit-load count and compression height, jumbo bags introduce point loads and loop-rated lifting stresses, and bulk shipments move by volume-to-mass conversion inside closed tanks, hopper containers, silo trucks, or vessel holds.

The governing arithmetic is expressed as follows: dry bulk mass equals usable volume multiplied by settled bulk density; unit-load payload equals unit count multiplied by gross unit mass; and fill ratio equals occupied volume divided by internal cargo space. The controlling capacity is the lowest of structural payload, floor bearing limit, usable volume, and road or rail axle mass. Two standards frame the arithmetic: ISO 1496-1:2013 defines series 1 freight container dimensions and ratings, while EN 12195-1:2010 supplies acceleration coefficients for load securing calculations. The IMO/ILO/UNECE CTU Code requires that all cargo be packed, blocked, and secured to withstand inertial forces in all directions; the non-dangerous classification of polyethylene does not waive restraint duties.

How Does Pellet Bulk Density Alter Container Payload and Floor Load Limits?

For a 20 ft general purpose container built to ISO 1496-1:2013 with nominal internal dimensions 5.898 m × 2.352 m × 2.393 m, the internal volume is approximately 33.2 m³. With a nominal maximum gross mass of 30,480 kg and a tare mass near 2,230 kg, the available payload is close to 28,250 kg. Filling the same container completely with loose PE pellets at 0.56 t/m³ yields only 18.6 t; at 0.60 t/m³, the figure reaches 19.9 t. The 20 ft box is therefore volume-limited for loose PE pellets because the loaded mass cannot reach the structural payload. In a 40 ft high-cube container with internal volume near 76.4 m³, the same bulk density range produces 39.7 t to 45.8 t, exceeding a typical payload near 26.5 t; the 40 ft configuration becomes mass-limited. The first loading check is therefore bulk density multiplied by usable volume, followed by comparison with rated payload and allowable floor load. Published data for specific pneumatic loading configurations is limited, but container geometry and resin bulk density data support these calculations. Floor load becomes particularly binding when FIBCs are used because the entire mass may pass through a small base footprint. The table below shows representative volume-to-mass conversions for common PE pellet bulk densities; actual hopper container volumes vary by manufacturer and hatch position.

Palletized 25 kg bags follow ISO 6780:2003 flat pallet dimensions but not a universal bag footprint, so the loading arithmetic must be built from supplier measurements. If a filled bag has a footprint of 600 mm × 400 mm and a closed height of 130 mm, a 1200 mm × 800 mm EUR pallet accepts four bags per layer with alternating row orientation. A twelve-layer stack, excluding a pallet height of 150 mm, yields 48 bags per unit, a gross unit mass near 1,225 kg, and a block height of 1,710 mm. In a standard 20 ft container, two pallet rows across the 2,352 mm internal width and five pallet rows along the 5,898 mm internal length produce ten pallet units. The resulting payload is 12,250 kg, well below the structural payload, while the occupied volume is approximately 16.4 m³. Palletized bags on EUR pallets are therefore floor-area-limited and stack-height-limited, not volume-limited or mass-limited. Interlocking the bag layers with alternating collar patterns is required to maintain unit load shear stability under road transport acceleration; EN 12195-1:2010 uses longitudinal acceleration of 0.8 g and lateral acceleration of 0.5 g for load securing calculations. A 1,225 kg unit load at 0.8 g requires 9.6 kN of horizontal restraint, which stretch-film alone typically cannot provide; the pallet must be blocked against the container sidewalls or faceboard. Polyethylene outer surfaces present low static friction, so anti-slip interlayer sheets or high-friction pallet tops are required when the coefficient of friction falls below 0.3. Static charges generated during bagging require grounding of the filling spout, and the fill area is typically dehumidified when the dew point exceeds 10 °C to prevent condensation on film surfaces.

When Jumbo Bags Are Stacked Below Deck, Lashing Arithmetic Replaces Nominal Capacity

Flexible intermediate bulk containers, described by ISO 21898:2004 and commonly called jumbo bags or bulk bags, are rated by safe working load and safety factor. A FIBC marked for 1,000 kg may have a 5:1 safety factor and a loop-specific lifting limit, but the footprint and filled height are not fixed by the standard. For PE pellet loading, the fill volume required to reach 1,000 kg at a bulk density of 0.56 t/m³ is 1.79 m³. If the same bag is used for a higher-density compound or masterbatch with a bulk density above 0.75 t/m³, the fill level must be reduced to avoid exceeding the safe working load. Stacking FIBCs below deck is performed only when the lower bag loop height, top shape, and internal frictional stability allow. The vertical load on the lower bag is the sum of the upper unit weights; for two high at 1,000 kg gross per unit, the base of the lower FIBC receives 19.6 kN of static force plus dynamic factor. If the lower bag base footprint is 0.9 m × 0.9 m, the static pressure is 24.2 kPa; if the footprint narrows to 0.8 m × 0.8 m, pressure rises to 30.6 kPa. These values can exceed the floor-bearing limits of some corrugated container flooring when combined with point loads and should be distributed with steel or plywood load spreaders. The IMO/ILO/UNECE CTU Code requires blocking, lashing, or a combination when FIBCs cannot be loaded tight to the container walls. Polyethylene FIBCs are inherently low-friction and may creep under repeated vibration; lashing angles below 45° from horizontal lose vertical hold-down force, and the effective tension in polyester webbing must be derated for knot efficiency and edge abrasion. When form closure is not possible, the blocking strength is calculated as the cargo weight multiplied by the residual horizontal acceleration after friction is subtracted, using acceleration coefficients from EN 12195-1:2010. Lifting operations require a spreader bar or equalizing frame; asymmetric loop loading can overload individual seams even though the bag is marked for the gross mass. Single-trip FIBCs exposed to ultraviolet radiation beyond the supplier’s stabilization limit lose seam strength and should not be restacked or re-lifted.

Bulk shipments of PE pellets in hopper containers, silo trucks, and railcars are quantified by volume-to-mass conversion and the angle of repose. The angle of repose for dry, free-flowing PE pellets is in the range of 30° to 40°, depending on pellet shape, size distribution, and surface lubricant content. A hopper container filled through a top hatch may not achieve full rectangular volume at the hatch plane because the natural repose cone leaves an air gap unless the loader uses a spreader or vibration. For a top-hatch fill, the missing cone height above the hatch is estimated as h = r tan α, where r is the hatch radius and α is the angle of repose; a wide hatch or movable spreader reduces this loss. If a 40 ft hopper container has an effective volume of 45 m³ and the product bulk density is 0.56 t/m³, the load is 25.2 t; with a 20 ft hopper at 30 m³, the load is 16.8 t. Discharge is lateral through a hopper slide, so the container must be tipped or the pellet must be fluidized with dry air. Bridging can occur when pellet blends contain excessive fines or irregular regrind; the flow function of the blend, not the mean particle size, controls discharge. This is a critical boundary: PE pellet bulk density measured by ASTM D1895-17 method A or B can vary by 5% between a loose fill and a vibrated fill. Filling systems that use dense-phase conveying may aerate the resin and produce transient bulk densities lower than settled density; a silo truck load calculated at aerated density may exceed the legal axle load after settling if freeboard is not adjusted. Road weight limits therefore require a 5% aerated-density correction on first fill calibration. Moisture absorption is not a primary concern for HDPE and LLDPE pellets, but condensation on cold silo walls can create wall-sticking layers, and air dryers should be specified for conveying air with a dew point above 5 °C to prevent biofouling in long idle lines. The stowage factor for settled PE pellets is approximately 1.6 m³/t to 1.8 m³/t, the reciprocal of the bulk density range; this low stowage factor means that PE occupies less volume per tonne than many agricultural dry bulk cargoes but can impose greater floor pressure when trimmed to a level block in a vessel hold.

For floor-loaded 25 kg bags, stack compression governs height

When 25 kg bags are shipped without pallets, the container floor area becomes the constraint. With a bag footprint of 0.6 m × 0.4 m (0.24 m²), a 20 ft general purpose container with a floor area of 13.87 m² accepts approximately 54 bags per layer after allowing for ingress/egress gaps and wall corrugation. Each layer is 1,350 kg. A sixteen-layer cross-tied stack reaches 21,600 kg and a total height of 2.08 m, leaving 0.31 m of headspace. Increasing to eighteen layers raises the mass to 24,300 kg but reduces headspace to approximately 0.05 m, which is insufficient for door closure tolerance. The limiting parameter is therefore not the container payload but the stack compression and the ability to close the door. The bottom bag carries 425 kg of vertical load in a seventeen-over-one stack at eighteen layers; although the average compressive pressure is only 17.7 kPa, local point loads at pellet corners may puncture the bag film if slip sheets or honeycomb boards are not inserted every 5 layers. The IMO/ILO/UNECE CTU Code requires a doorway bulkhead capable of withstanding the full cargo mass multiplied by the forward acceleration coefficient; for road transport under EN 12195-1:2010, the longitudinal acceleration is 0.8 g, so a 21,600 kg stack requires a bulkhead restraint of 172.8 kN if friction is ignored. This explains why floor-loaded PE bag stacks are generally limited to full-container block stow with wooden dunnage and lashing belts rather than mixed-cargo consolidation.

Segregation between food-contact and industrial grades introduces another boundary. Polyethylene homopolymers and copolymers intended for food contact are controlled under FDA 21 CFR 177.1520 and Regulation (EU) No 10/2011, while recycled post-consumer resin may carry broad molecular weight distribution and contaminant load. Shipping units that carry both grades in the same container require impermeable divider curtains or dedicated packaging because pellet spillage can commingle at floor level. The loading arithmetic for a compliance hold includes empty chute inspection, residual pellet count, and dust suppression. Bulk truck carriers require a prior-cargo certificate when shipping food-contact resin; the absence of this document can reject the shipment at receiving. For CTU packing certificates, the recorded verified gross mass must align with the actually secured block mass, because wheel and axle overload enforcement is based on the certified mass rather than the nominal product weight.