Unit load design: designing the pallet, packaging and load together

A unit load is the pallet, the product packaging, the stacking pattern and whatever holds it together, moving as one piece. Traditionally each part is designed by a different person: the packaging engineer picks the box, the warehouse picks the pattern, purchasing picks the pallet. Unit load design treats them as one structure, because each part changes how the others perform. This guide explains the interactions and how to use them.

#The four parts of a unit load

#How the pallet affects the packaging

A box's compression strength is measured in a lab between two rigid, flat platens. On a pallet, the box sits on deckboards with gaps between them, on a deck that bends. That changes how the load reaches the box walls.

• Deckboard gaps: box walls or corners that land over a gap are unsupported and can fold into it. Wider gaps mean more of the box perimeter is unsupported.
• Deck stiffness: a flexible deck sags between stringers or blocks, especially in racking, so the boxes above the sag carry less and the ones over the supports carry more.
• Overhang: boxes that hang past the pallet edge lose support at their bottom edge. See pallet overhang.

Research at the Virginia Tech Center for Packaging and Unit Load Design has shown that these pallet features change the stacking strength boxes deliver on the pallet. The practical result: improving the pallet deck (thicker or more deckboards, smaller gaps) can let you use a lighter box, and a cheaper pallet may force a stronger, more expensive box. Look at both together.

#How the load affects the pallet

The interaction works the other way too.

• Load bridging: a stiff, tightly stacked load spans the gap between supports and carries part of the bending itself. A pallet in racking then deflects less than it would under the same weight of loose material.
• Flexible loads such as bags, bulk bags or liquids in flexible containers bridge very little, so the pallet does all the work.
• Concentrated loads such as drums, machinery feet or pails put the weight on small areas, so deckboards and stringers under those points govern the design.

Pallet design software can model some of these effects if you describe the load. A PDS analysis, for example, can be run with a specific box, pattern and stabilizer rather than a generic uniform load. The pallet load capacity guide explains why ratings assume uniform loads unless stated.

#The stacking pattern

Column stacking keeps box corners aligned and preserves compression strength. Interlocking improves stability but reduces it. Column stacking with stabilization often gives the best combination. See pallet stacking patterns.

The pattern also sets how the load bears on the pallet. A pattern that puts box walls over the stringers or blocks gets better support than one that puts them over gaps.

#Stabilization

Stretch wrap, straps and tier sheets hold the unit together during handling. They also add stiffness: a well-wrapped load bridges better and resists shifting in transport. Too much tension crushes box edges, so containment force should match the load. See stretch wrapping pallets.

#A unit load design process

1. Define the distribution environment. Every handling, storage and transport step, including racking type and beam span, stacking heights, truck or container, vibration and duration of storage.
2. Define the product and primary package. What load the product can carry and how fragile it is.
3. Choose the footprint. Match the case size to the pallet so the pattern fills the deck without overhang. Use the cases per pallet calculator.
4. Design the pallet for the supports and the load type. See how to spec a custom pallet.
5. Design the box for the load on the bottom layer, with factors for the pallet deck, pattern, humidity and time.
6. Choose stabilization to hold the load through the worst handling.
7. Test. Compression, vibration and handling tests, or trial shipments, confirm the design.
8. Iterate. Reduce material where there is margin.

#Testing a unit load

Calculations narrow the design; tests confirm it. Common test references:

A full unit load can also be tested as shipped: loaded, wrapped and put through vibration or a trial route. Trial shipments with inspection at the receiving end are cheap and catch problems no calculation will, such as a forklift driver's habits at a particular dock.

#Where unit load design saves money

The savings come from not over-designing every part to cover for the others. When each designer adds a margin without knowing the rest, the total is usually far stronger, and more expensive, than it needs to be.

#Information to share between pallet and packaging suppliers

• Case dimensions, weight and compression strength
• Pattern (Ti-Hi, column or interlock) and load height
• Pallet deck layout: board widths and gaps
• Racking direction and beam span
• Stacking heights in storage and transit
• Stabilization method

#Common unit load failures and causes

#Where to start

If you are starting from scratch, describe the load and distribution environment first, then design the pallet and box together. The pallet designer lets you set the footprint, deck layout and gaps so they can be matched to the case size; open the 48 × 40 template to begin. See also deckboard thickness and spacing.