Unit Load Distribution

Typical distribution environment hazards.

Basic Hazard Typical Circumstances
Shock Drops during manual handling; package thrown, rolled, or tipped over; mechanical shocks (chutes, conveyors, palletizers); vehicle shocks (rail shunting, potholes, curbs).
Vibration Roadbed patterns (rail joints, tar strips), suspension-generated vibration, out-of-balance wheels, drive-train vibrations.
Static compression Warehouse stacking, bracing, and other restraints.
Dynamic compression Clamp trucks, arrests on conveyors and chutes, rail shunting.
Piercing, puncturing Equipment misuse, projections, hooks, shifting cargo, damaged pallets.
Racking, deformation Uneven support, uneven lifting.
Elevated temperature High ambient temperatures, direct sun exposure, proximity to boilers.
Reduced temperature Cold climate, unheated transport vehicles.
Low pressure Unpressurised aircraft holds, high elevations
Light Direct exposure to sunlight.
Moisture, water High ambient humidity, rain on unprotected cargo, condensation, bilgewater and seawater.
Biological hazards Micro-organisms, fungi, mould, insects, rodents.
Time Long storage.
Contamination Dust, dirt, rust, adjacent product leakage, other external materials, malicious tampering.
Pilfering Can occur at any stage.

By far the greatest damage is caused by the "physical' events: damage during warehousing, transport, and handling and damage caused by compression, shock, and vibration.

In order to protect against the dangers listed in Table above, their nature and magnitude must be thoroughly understood. It is not possible to exactly identify the, hazards encountered for any particular journey; however, statistical descriptions of what typically happens are available.

Tracking Distribution Losses

Insurance Companies estimate that 75% of international cargo losses are preventable, much of them through better packaging systems. In less-developed countries, food loss between producer and consumer can be as high as 45 %; unfortunately, in some instances this represents the difference between self-sufficiency and starvation.

It is vital to the development of good distribution packaging that the cause and nature of any loss be examined and understood. It is not enough to say, "the cases fell over." The exact events leading up to the loss must be analyzed, and the loss quantified. It is only through careful attention to detail that the basis for new packaging systems, or work toward reducing losses with existing systems, can be logically approached. Detailed data on losses can also reveal over- or underpackaging.

The impact of distribution loss and damage is frequently underestimated, the costs of reacting to damage being typically five times the cost of cargo replacement.

Consider the impact of a loss on net profit. After all the materials, salaries, and overheads have been paid, any product loss is subtracted from net profit. How much more product is going to have to be sold to make up this profit loss? If net profit is 5%, a claim for £500 means that an additional £10,000 in sales will have to be generated to restore the profit.

Since money is invested in distribution packaging in order to avoid loss, it is logical that product loss due to inadequate packaging should be accounted as a packaging cost. Generally, increasing packaging will reduce damage. However, there is an optimum balance between packaging costs and damage losses. To find the minimum system cost, the total distribution cost, including packaging cost and damage cost, must be established. The relationship of total cost helps to determine the optimum investment in distribution packaging. Keeping packaging costs down can be false economy if the result is a high damage rate.

The total cost can be the same for over or underpackaging. In one instance the principal expense is packaging cost; in the other it is the cost of product loss. Increasing packaging will reduce damage rates until at some point the additional packaging costs are greater than the damage being prevented. The addition of fur then packaging would be overpackaging. Some overpackaging can be justified if it provides additional value such as goodwill or an impression of high quality. One International Safe Transit Association study in the U.S.A. determined that product losses were higher in summer than in winter.  The difference is attributed mostly to higher humidity.

The extent to which inadequate packaging contributes to losses can be debated. The Table would certainly indicate that significant damage is caused by faulty transport equipment and improper handling and loading procedures. On the other hand, it must also be accepted that this is the real world and that packs must survive this test. Damaged product has your name on it. That the fault can be put on someone else is minimal consolation.


The distribution warehouse is a central collecting point. Finished goods are for warded to and held at the warehouse until selected and assembled into a customer order. The warehouse environment is not well understood by many carriers. A typical dry groceries warehouse may contain twenty thousand individual stock items. A hardware chain warehouse holds upwards of forty thousand stock items. Product arrives at the central warehouse in bulk or unitized, is broken down or reunitized according to the warehouse's needs, and then is arranged for stock-picking. Stock-picking is the process of selecting individual items to fill an order for a particular store or destination. Central warehouses serve large

customer areas; in some instances one or two warehouses may essentially serve the entire country. Certain major retailers have taken this a step further and reduced or almost eliminated stockroom space at the rear of the store. Deliveries 4 are made up to three times a day from a warehouse that serves up to 25 stores depending on the geographical concentration of outlets and their relative sizes. In some cases the warehouses, which are sometimes known as regional distribution centers (RDCs), are owned and staffed by the retailer, but there is an increasing trend for these to be sub-contracted.

Product may be routed through more than one warehouse. For example, an export product may be moved from a local warehouse to dockside storage, to the cargo ship, and back to a receiving warehouse. A product must fit the warehouse's material handling system. This often means palletizing loose loads or repalletizing loads from nonstandard pallets. Depending on the operation, anywhere from 33 to 70% of product received at a warehouse must be handled manually before an order is placed in stock. Manual handling, in addition to being costly, is also a primary source of damage from dropping and personnel injury.

Warehouse damage rate by cause per 100,000 containers. (Source: U.S. Department of Agriculture.)

Cause Number
Hitting bars at back of racks 24.5
Cases dropped in aisles 16.1
Protruding nails in pallets 15.8
Fork tine damage 14.4
Unidentified storage damage 14.4
Pallet edges 13.0
Damaged while filling racks 12.8
Damaged while removing from second-level location 8.6
Hitting merchandise on pallet below 7.8
Ramming by hand truck 5.1
Crushed during stacking 5.0
Leaning stacks 4.8
Corner cases hit by truck or tractor 4.6
53 other identified causes 39.0
Total cases damaged in warehousing per 100,000 185.9

In the picking aisles, stock must be clearly identifiable from every side. Multicolor graphic displays serve only to obscure vital information from the picker. A box labeled "Golden Triangle Farms" does not inform the stock-picker of the contents. Containers should be strong enough to be dragged off the pallet by one end, and stiff enough that they don't distort and release their contents when handled in less than ideal fashion. Glue flaps must have enough adhesive to resist abusive handling.

An assembled order may contain many differently-sized and packaged items. These are assembled on a mixed pallet for transport to the retail outlet or may be packed into an outer. Containers must be easily handled by the picker and should be readily packed onto a mixed-order pallet. Container orientation on mixed-load pallets will tend to be on a "best fit' basis, regardless of "this side up" and "do not stack" labels. It may be possible to pack a trapezoidal container efficiently on your pallet, but odd shapes do not pack well in a mixed-product pallet load. Use boxes with a rectangular cross section wherever possible.

Storing Palletized Loads

There is not always a need to invest in special racking for pallet storage. They can often be stored economically on the warehouse floor if quantities are moderate, throughput rapid and if adequate space is available. The versatility of floor storage can be demonstrated in the following ways:

Block Stacking

Flat topped loads are stacked on flat bottomed pallets, these can then be stacked directly on top of one another. The block stack makes good use of the area available, has low equipment costs and allows for rapid throughput. However, the height of the stack (and therefore the use of available headroom) is limited by the weight and stacking strength of the palletized load and only the pallets at the top of the stacks are immediately accessible. Stack height is also dependent on load stability, and it is sometimes necessary to use load spreader boards to prevent damage to the individual packs on the top layer of each pallet.

Pallet Converters

Pallet converters are supports fitted around the corners of the pallet loads enabling them to be stacked without the goods bearing the load. This method is clearly more suitable where crushable or less stable loads are involved. Order picking from any pallet is possible (where aisle access is available), but individual pallet movement is not.

Post Pallets

These stack on top of each other securely without transferring any of the load to the goods. Box and cage versions can give easy storage and movement of small loose items, irregular shapes and unstable loads. Order picking is again possible, where aisle access is provided, but as with pallet converters, individual pallet movement is restricted. This method is clearly more expensive than block stacking.

Overhead Storage Areas

These are often built of structural steel, and operate as mezzanines. The increased use of available headroom at relatively low cost is an obvious advantage. Pallet handling on the overhead area is limited to hand trucks or overhead travelling cranes and there may be restrictions placed on the design of such storage areas by some local authorities and insurance companies.

Pallet Racking

Apart from block stacking, pallet racking is the simplest system of pallet storage in terms of equipment, and the most economical in capital costs. The load supporting beams can be easily moved to different levels to accommodate new load sizes, and installation or dismantling is rapidly achieved. Throughput and handling are relatively rapid, and goods are protected from compression and other damage - the racking, rather than the goods being stored, providing the strength and stability.

Adjustable racking can make good use of the available headroom in the warehouse, limited only by the handling equipment and roof height. When adjustable pallet racking is built to heights above 8 meters, special high reach handling equipment is required, and this in turn may require carefully engineered floors. The storage density (pallets per given area of warehouse floor) is less than with other systems because of the amount of gangway space needed for trucks to maneuver.

A development of standard racking allows for double-deep pallet stacking, where one pallet stands behind another in the rack. The clear advantage is that more economical use is made of the floor area available, but access to the rear pallet in each rack can only be achieved by removing the load nearest the aisle. It may also be necessary to use specialized long-reach trucks.

Narrow aisle racking has come to mean more than simply racking with narrow gangways. It describes a number of methods of providing high density storage while still allowing access to individual pallets. The aisles are only marginally wider than the handling equipment. If trucks are used, they move on fixed paths within guide rails, and do not turn in the gangways. They are designed to pick up or set down on the rack on either side as required. Installations served by stacker cranes often use computer control and automatic location systems.

The principal advantage of narrow aisle racking is the combination of relatively high density with 100% individual selectivity. Throughput speeds can be more rapid than with conventional trucks and layouts. Narrow aisle trucks can operate at greater heights than conventional trucks - up to 12 meters. Stacker cranes can go higher still.

Drive-in and Drive-through Racking

This consists of a continuous block of racking not divided by aisles. Reach or counter balance trucks can drive right into the center of the block between any uprights on the front face to deposit or pick up pallets. This is possible because there are no cross beams to block entry. Instead the pallets are supported on the front-to-back edges by continuous cantilevered rails on the uprights at each pallet level.

High density storage is possible using this system. Compared with conventional pallet racking, drive-in/drive-through racking can store a given number of pallets in about half the normal space or can roughly double the storage capacity of a given area. Heights of up to 10 meters are possible, and there is no crushing of goods. Capital costs are relatively low compared with other high density systems, and standard trucks can normally be used.

It is not possible to select every pallet without moving others. With a drive-in layout stock is only accessible on a last-in, first-out basis.

Powered Mobile Racking

This is a system where the mobile pallet racks are individually power driven and run on p n laid steel rails. With this system it is possible to serve six, eight, ten or more racks, the racks being moved to open up an aisle where required. Control is by push-button, located at the rack ends or at a central console. The electric motors move the racks smoothly, taking about 30 seconds for a gangway to open.

Great space savings are possible. With 80% or more of the floorspace being used for storage it is often possible to double the storage capacity or halve storage space. Direct access is available to every pallet load. Racks can be up to 10 meters high and of almost unlimited length.

Pallet Live Storage

Pallet live storage consists of a number of gravity conveyor lines ranged side by side and one above the other. Pallets are loaded at the upper end of the slope and travel through the block on guide wheels. They queue up in order at the lower end of the lanes. This lower end constitutes the picking face of the block. When a pallet is the remaining pallets in the lane move forward bringing the next in line to the picking face. The speed of movement is controlled by a braking mechanism.

Pallet live storage combines excellent space utilization with automatic stock rotation within each lane - pallets come out of each lane in the same sequence as they went in. This system is suitable where large stocks of the same products require storage over short periods. Generally a standard counter balance truck is satisfactory - for loading and unloading.

High Bay storage

High bay storage with automatic placement and retrieval systems is used where space is at a premium and a large volume of stock is being handled. High bay installations invariably purpose-designed and can be costly, although the objective is to off-set the high initial costs by low running costs.



It is simpler to move one 1,000-kilogram load than it is to move a thousand 1-kilogram loads are most commonly unitized on pallets, a platform that can be picked up by the tines of a forklift truck. Another technique uses slip sheets, tough fiber board or plastic sheets on which the load is stacked. The truck used with slip sheets has a clamp mechanism that grasps a protruding edge of the sheet and pulls the sheet and load onto a platform attached to the truck. A third method of handling a large group of assembled objects is with a clamp truck, a mechanism that picks up loads by exerting pressure from two sides.

Each method has its advantages and disadvantages. Slip sheets are economical, take up little space, and are light. However, the equipment is not universally

available, is more expensive, and is slower to operate. Pallets are universally adaptable to a variety of handling situations and locations. However, pallets are costly, take up space, and can be difficult to dispose of. Clamp trucks use no added materials, but the geometry and character of the load must be such that it can be squeezed between the truck's clamps, without being damaged.

Most pallets are currently made of wood, and choice of wood species has a great impact on cost and durability. The denser and stiffer the wood, the greater the pallet's durability and usually the greater its cost. Other materials used for pallet construction are:

Fiberboard, which is used for low-cost, low-strength end uses, where a cheap, disposable and lightweight pallet is sufficient to meet requirements. They can be made by suction molding, or by assembling the various components using high performance adhesives.

Metal, used for long-life pallets, generally within a controlled area of a factory, e.g. handling fresh meat, where they can be readily cleaned and sterilized if required. Usually fabricated from sheet materials.

Plastic, again, used for long-life pallets, especially in a controlled area where the use of wood is considered an unacceptable contamination risk. Developments in manufacturing techniques and the use of recycled materials, have made plastic pallets more affordable than was previously the case. They can be injection and/or compression molded, may be reinforced with metal rods or can be fabricated from plastic components in the same way as wooden pallets. The development of plastic pallets is a subject worthy of interest, especially for the food and pharmaceutical industries.

There are many possible pallet sizes and designs; however, for the sake of standardized distribution, certain sizes and designs predominate. In the UK the majority of pallets used in the food industry conform to the ISO standard of 1000 x 1200 mm. While in continental Europe the ISO standard 800 x 1200 mm dominates. Many industrial packaging applications use non-standard pallet sizes, usually sized to the product, and/or intended for block stacking rather than racked storage in warehouses.

Pallet styles

There are several different pallet styles, the most common being shown in the illustraion below. Pallets can be two-way entry, which means they can be lifted from two opposite sides, the remaining two sides being closed by solid bearers. This gives a strong pallet of simple construction, which is relatively economical to produce, although the solid bearers restrict fork lift truck access, which may be unacceptable in some warehouses and in vehicle loading. (A variation of this style, commonly used in America, has notched bearers through which fork lift tine access can be gained).

Four-way entry pallets have corner blocks instead of solid bearers, allowing access by fork lift tines from all four sides. There are usually nine blocks in total: one at each corner, one at the mid-point of each side and one in the center. The loss of strength in moving from solid bearers to blocks is compensated for by the use of stringers (boards bridging the tops of the blocks in three rows, to which the top deck boards are attached) and base deck boards which bridge the blocks at the bottom. The number of components, complexity of manufacture and grade of timber required to maintain strength, results in a more expensive pallet than the two-way entry, although versatility can outweigh the increased purchase price.

Reversible pallets have a base or bottom deck which is the same as the top deck, giving even distribution of load when filled pallets are block stacked, particularly if the product is of an uneven density e.g. sacks, drums. However, these pallets cannot be moved using hand pallet trucks, for which a non-reversible construction is required.

Pallets can be close boarded, where the top deck boards are butted together with no gaps between them. This gives even load distribution and minimum product damage, but obviously uses more timber than open-boarded pallets and hence means more cost. The standard gap for open-boarded pallets is approx. 60MM, which may be unacceptable for small items. Top boards for both constructions can be planed for a smoother surface finish and to minimize product damage and contamination. Top boards for open-boarded pallets can be edge-chamfered for additional product protection.

Four-way entry pallets commonly have two extra base boards bridging the corner-blocks, producing what is known as a full perimeter base. This allows easier movement on roller conveyors. The base boards can be stop chamfered for easy access using pallet trucks. A further variation is the cruciform base, which increases pallet strength and provides more even weight distribution when loaded pallets are block-stacked. Winged pallets can be two- or four-way entry. Bearers or blocks are fixed inside the top/bottom deck boards, leaving protruding ends for use with slings. This construction also reduces the span of the unsupported top deck, increasing the load carrying capacity.

Pallet Stacking Patterns

In addition to providing a platform on which to assemble and move a collection of products, a pallet also acts as a buffer against the handling environment. It is not uncommon for a forklift driver, who cannot see his exact placement position, to stop when he hits something. Automatic handling systems may operate in the same way. The way in which pallets are stacked with products is crucial to the damage level which may be incurred. Excessive overhang, for instance, means the product is exposed to damage whenever the palletized load is moved (as well as during static storage).

Unit Load Efficiency

Warehouse floor space is rented by area, and the more product that can be put into that area the better. Trucks loaded with light product should have the available volume completely filled to carry the maximum amount of product per trip. Area and cube utilization should be every packer's concern.

Optimum area and cube utilization begins with the design of the primary pack. Primary dimensions should be considered in terms of possible packing orientations in the shipping container, impact on corrugated board usage in the shipping container, and palletization pattern and space utilization.

Traditionally, the problem was solved through intuition, experience, and a few nominal calculations. However, small cartons, packed 24 to a case, may have over a thousand possible orientation and palletization solutions. Computer programs calculate all the implications of size decisions in minutes. Typical input data required is:

Typical output data for such a program might provide the following information:

Distribution efficiencies of motor oils.

Bottle Type Case Blank Area (sq. meters.) Case Cost Bottles per Pallet Bottle Weight (grams)
A 0.59 .56 576 69
B 0.66 .62 576 65
C 0.65 .61 480 61
D 0.64 .58 600 64
E 0.69 .65 576 63
F 0.78 .74 384 56
F (MOD) 0.69 .65 432 56
G 0.54 .52 947 60
H 0.70 .67 432 73

The impact of cube and area utilization can be critical. Table 17.3 compares 8 competitive oil bottles packed 12 to a case. F(mod) is the theoretical outcome of changing one bottle dimension by 3 millimeters. It is obvious that some bottles are more competitive than others.

Figures such as 80% area utilization are difficult to visualize in concrete terms. Consider a product palletized in such a manner that 50 millimeters of space exists on all sides. This amounts to a pallet utilization of 82.5%. When compared on a large scale to a fully utilized (100%) pallet,

A thorough system analysis (including losses) can lead to substantial savings. A major business equipment manufacturer found that it had poor shipping experience because of the hundreds of different pack sizes in the product line. The company designed a modular system, and all products were designed to fit one of 17 standard case sizes. Besides significant inventory reduction, the company gained substantial transport savings, since larger, more stable pallet loads could be built with the modular system. More-secure pallet loads resulted in further savings through reduced product damage.

Stabilizing Palletised Loads

Palletized loads need to be stable enough during all stages of handling and shipping, to retain load geometry and avoid product loss/damage. Careful choice and specification of secondary packaging materials with a sufficiently high coefficient of friction to avoid pack-to-pack slippage, should be the first option, as this avoids the use - and therefore the cost and disposal - of further materials. This is not always feasible, however, and additional pallet stabilizing methods may be required.

Shrink-wrap, in the form of a shroud or hood of oriented plastic film, usually polyethylene, is one option. The shroud is placed over the loaded pallet and heat applied. This causes the molecules in the film to shrink back to their original unoriented state, and the film tries to shrink to its original dimensions, thus forming a tight closure around the pallet. The gauge of pallet shrink-wrap films is usually in the range of 100 - 200 microns, the heavier gauges being used to support heavier, less stable loads. The advantages of shrink-wrapped loads is that the product can clearly be seen through the film and they are protected against rainwater. Installation and running costs may be high, however.

Stretchwrap, which uses very thin plastic film, tightly wrapped around the palletized load, is probably more common. The film is usually linear low density polyethylene or mixtures of low density polyethylene and ethylene vinyl acetate, the later migrating to the surface after extrusion, to provide the "cling" needed in this application. Such films can also be produced by coextrusion. Gauges of 17 to 25 microns are typical. Stretchwrap can be applied by hand, for securing mixed loads, for example, but to be really effective it must be applied using automatic equipment, where the film can be pre-stretched to give up to 300% stretch on application and a very tight wrap. This method is usually more cost effective and provides greater stability, than shrink-wrapping, although the product visibility and water protection are inferior. Stretch-hooding is another, more recent method of providing load stability, combining some of the advantages of both shrink- and stretchwrapping.

Adhesives (usually hot-melts) can be used to bond packs together. The adhesive is applied in a number of predetermined spots, preferably using an automatic application system. Adhesives are formulated to provide good pack-to-pack bonds in the horizontal direction (to prevent packs from sliding over one another) and poor bonds in the vertical direction, so the packs can be readily lifted off the palletized load without damaging the packaging.

Table 17.4

Typical properties: (13mrn wide x 0.5mm thick).

  Tensile Strength MPa Break Strength kN Elongation at break % Retained Tension %
Polypropylene 345-415 1.4-2.7 15-20 23-29
Polyester 415-550 2.7-3.6 9-14 65-75
Nylon 435 2.8 15-18 71

A variety of other systems can be used, such as heavy elasticized bands or nets, but the only other main category is the use of tensional strapping, which can be metal or plastics. This is a relatively easy method of securing palletized loads, requiring little in the way of equipment, other than a tensioning and crimping tool. Care must be taken to pass the strapping through and not under the pallet, thus not impeding the entrance of forklift tines or damaging the strapping.

The important properties of a strapping material are its tensile strength, break strength and elongation at break. Retained tension is important for plastic materials, which gradually lose their tension over a period of time when kept under stress. The most commonly used and the cheapest strapping material is polypropylene, at 13mrn wide x 0.5mrn thick. Other materials used, offering higher tensile strength and improved retained tension are polyester and nylon, both being more expensive than polypropylene. Typical properties are shown in Table 17.4.

Steel strapping is still used, predominantly in the shipping of heavy equipment, owing to its exceptionally high tensile strength and the fact that it does not lose tension in the same way as plastic materials. It is available in widths from 9 to 50mm, and tensile strength 500 to 1000 MPa.

Some precautions which must be taken when using all strapping materials are:

Caps and trays made of fiberboard or corrugated board are used to provide shape to unstable loads, to provide bottom protection against rough pallet surfaces, and, when used on top of a load, to increase the platform quality for the next pallet. Interleaving sheets may improve available compression strength and increase stability by distributing weight and encouraging layers to act as a unit.

Whichever method is used to stabilize palletized loads, it should be remembered that the pallet wrapping/strapping may be removed at a point of receipt into a warehouse, and the pallet then moved to its final storage location. Thus, even without additional stabilizing materials, palletized loads must be reasonably stable, for instance, they should withstand being tilted through 5'. (See above).

Bagged Product

Special consideration should be given to products packed in bags and sacks, which are more frequently damaged than other pack types. Some recommended ways to avoid damage are:

Shrink-wrapped Trays

Shrink-wrapped trays are an alternative to corrugated cases for distribution. The following considerations apply to this type of packaging:


Preshipment Testing

Knowledgeable Packaging Technologists have recognized for some time that preshipment testing can reveal inadequate product design or packaging long before it starts costing money through damages and customer dissatisfaction. Many procedures and devices have been developed for use in evaluating distribution packaging. Some of these are material tests (such as the Mullen burst test), but the technologist usually wishes to evaluate not a single material property, but the suitability of a system.

First, the product itself should be studied to ensure that it has no inherent design faults that will make distribution difficult. These should be remedied before packaging is considered. Once the product is as durable as is practically the testing should begin.