Pallet construction

Pallet construction

Types

Although pallets come in all manner of sizes and configurations, all pallets fall into two very broad categories: "stringer" pallets and "block" pallets. Various software packages exist to assist the pallet maker in designing an appropriate pallet for a specific load, and to evaluate wood options to reduce costs.

Stringer pallet
Stringer pallets are one of the original models of wooden pallets. They use a frame of three or more parallel pieces of timber (called stringers). The top deckboards are then affixed to the stringers to create the pallet structure. Stringer pallets can have a notch cut into them allowing "four-way" entry. Forklifts can lift a stringer pallet from all four directions, though lifting by the stringers is more secure. Stringer pallets no longer need to be built in or permanently attached to the pallet making it possible customize the basic pallet. Stringer pallets can be made of both wood and plastic.

Block pallet
Block pallets are typically stronger than stringer pallets. Block pallets utilize both parallel and perpendicular stringers to better facilitate efficient handling. A block pallet is also known as a "four-way" pallet, since a pallet-jack may be used from any side to move it.

Carrier block
Carrier blocks are specialized pallets for lumber carriers

Flush pallet
Flush pallets are pallets with deck boards that are flush with the stringers and stringer boards along the ends and sides of the pallet.

Perimeter base pallet
All stringer and some block pallets have "unidirectional bases," i.e. bottom boards oriented in one direction. While automated handling equipment can be designed for this, often it can operate faster and more effectively if the bottom edges of a pallet have bottom boards oriented in both directions. For example, it may not need to turn a pallet to rack it, and operation is less sensitive to pallet orientation.

Quality improvements
The least expensive way to improve a pallet is usually to specify better nails. With non-wood pallets, a controlled coefficient of friction is often helpful to prevent the pallet from slipping from forks and racks. Stiffer pallets are more durable, and are handled more easily by automated equipment. If a pallet does not need to be lifted from all four sides, two-way pallets with unnotched stringers may be used, with the additional benefits of added rigidity and strength. Specifying tolerances on flatness and water content may help the supplier meet target requirements. Inspection of pallets, whether in person or by a third-party (such as "SPEQ" inspected pallets) offer additional assurance of quality.

Materials

The production of pallets accounts for 43% of hardwood and 15% of softwood usage in the U.S.

The cheapest pallets are made of softwood and are often considered expendable, to be discarded as trash along with other wrapping elements, at the end of the trip. These pallets are simple stringer pallets, and liftable from two sides.

Slightly more complex hardwood block pallets, plastic pallets and metal pallets can be lifted from all four sides. These costlier pallets usually require a deposit and are returned to the sender or resold as used. Many "four way" pallets are color-coded according to the loads they can bear, and other attributes. Wood pallets can pose serious bio-hazard risks as they are susceptible to bacterial and chemical contamination, such as E. coli problems in food and produce transportation, and even insect infestation, and thus the need for ISPM 15.

Wooden pallet construction specifications can depend on the pallet's intended use: general, FDA, storage, chemical, export; the expected load weight; type of wood desired: recycled, hard, soft, kiln dried or combo (new & recycle); and even the type of fasteners desired to hold the pallet together: staples or nails.

Paper pallets, also referred to as "ecopallets", are often used for light loads, but engineered paper pallets are increasingly used for loads that compare with wood. Paper pallets are also used where recycling and easy disposal is important. New designs of ecopallets have been made from just two flat pieces of corrugated board (no glue/staples) and weigh just 4.5 kg (9.9 lb), offering dramatic freight savings. Ecopallets are also ISPM 15 exempt, negating fumigation and barrier "slip" sheets. They are cleaner, safer, and provide a cost-saving eco-friendly alternative to other pallet materials. Some engineered Corrugated pallets offer a significant reduced height, providing substantial freight cost reduction. Low profile hand pallet trucks allow picking up pallets as low as 25 mm.

Steel pallets are strong and are used for heavy loads, high-stacking loads, long term dry storage, and loads moved by abusive logistic systems. They are often used for military ammunition. Metal pallets make up less than 1% of the market. Materials include carbon steel, stainless steel, and aluminum. Of these, carbon steel offers excellent durability at the lowest cost. Stainless steel doesn’t require a paint coating, and is preferred for such applications as clean room environments. Carbon steel units are expensive compared to wood, and stainless and aluminum cost about 2–3 times that of carbon steel. Long term costs, however, can be lower than wood. General advantages of metal pallets are high strength and stiffness, excellent durability, bug free, no splinters, sanitary, and recyclable. Disadvantages include a higher initial price, significant weight, low friction, and susceptibility to rusting (carbon steel). Metal is primarily used in captive or closed loop environments where durability and product protection are key performance requirements. Metal units today are increasingly price competitive and lighter in weight. Primary industries that use metal pallets include automotive, pharmaceutical, lawn tractors, motorcycles, and tires.

Aluminum pallets are stronger than wood or plastic, lighter than steel, and resist weather, rotting, plastic creep and corrosion. They are sometimes used for air-freight, long-term outdoor or at-sea storage, or military transport.

Plastic
Plastic pallets are often made of new HDPE or recycled PET (drink bottles). They are usually extremely durable, lasting for a hundred trips or more, and resist weathering, rot, chemicals and corrosion. The benefits of plastic pallets over wood pallets include the ability to be easily sanitized, resistance to odor, fire retardancy, longer service life span, durability and better product protection, non-splintering, and lighter weight, thus saving on transportation and labor costs and making them safer and more environmentally friendly.They often stack. Plastic pallets are exempt by inspection for biosafety concerns, and easily sanitize for international shipping. HDPE is impervious to most acids and toxic chemicals clean from them more easily. Some plastic pallets can collapse from plastic creep if used to store heavy loads for long periods. Plastic pallets cannot easily be repaired, and can be ten times as expensive as hardwood, so they are often used by logistics service providers who can profit from their durability and stackability. The large supply chains have increased the use of plastic pallets as many organisations seek to reduce costs through waste, transport and health & safety. Pallets and dollies can be combined (for example the Pally) ,eliminating pallet instability and the need for additional lifting equipment along with creating valuable space in busy operating environments. They also deliver significant time and cost savings by reducing supply chain handling. Plastics' reusability has contributed to an increase in usage of plastic pallets.

Plastic pallets are produced and used widely in the U.S. and Europe, spurred by the adoption of the ISPM 15. A full comparison of wood vs plastic can be made by a life cycle analysis. Plastic pallets can cost 10 times as much as hardwood pallets and even more expensive compared to cheap expendable softwood pallets. RFID chips can be molded into the pallets to monitor locations and track inventory.

Manufacture

There are six main types of plastic processes that are used to manufacture pallets:

High pressure injection molding
Structural foam molding
Thermoforming
Compression molding
Rotational molding
Profile extrusion
Here are the main processes that are used to manufacture wooden pallets:

Material selection
Cutting wood plate / block
*Chamfering (depends on the demand)
Notching
Nailing wooden pallet