I I I I I I I I I

Center for Unit Load Design

Center Tech Note No. 2

What Pallet Manufacturers Should Know About Corrugated Boxes!

by John W. Clarke and Jorge A. Marcondes

September 18, 1998

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Figure 1: Types of Corrugated     Board Constructions ©Fiber Box Association

"Pallets Move the World!" is the slogan of the National Wooden Pallet & Container Association. Pallet manufacturers know that properly designed pallets are important in the transport, storage, and protection of unitized products. Pallet manufacturers are often frustrated, however, by their customers' lack of knowledge or concern about pallet quality and performance. How often are you asked to cut pallet quality to compete on price? How many pallet users know that GMA-type grocery pallets are called "48x40's," rather than "40x48's?" Although pallets are used to ship 90% of the US domestic product, the average pallet user and packaging engineer lacks a basic understanding of pallet quality and performance.

As a pallet supplier, however, how much do you understand about corrugated boxes? Corrugated boxes are the most common transport container used on your pallets. Pallet manufacturers are often called in after a failure to solve "pallet problems." If you don't understand corrugated boxes, you won't be able to effectively diagnose packaging and pallet related interactions, and your customer may feel that you aren't qualified to solve their packaging related problems. Pallet designers need to understand box design and performance to more effectively design pallets. Do you say "cardboard" boxes rather than the correct "Corrugated" boxes? Do you know how to describe a box's size? Do you understand the common box types and how they perform on pallets? Do you know how to design your pallets so that the performance of both pallets and boxes are enhanced?

This article is intended to introduce pallet manufacturers to the structure, design, and performance of corrugated boxes. More detailed information may be found in the references listed at the end of this article.

Corrugated Board Construction

According to the Fibre Box Handbook, the largest single product produced by the American paper industry is containerboard (35% of production), most of which ends up in corrugated boxes. Note that "corrugated", and not "cardboard," is the correct term. Cardboard is a thin stiff pasteboard often used for playing cards and is a much different material than the containerboard used to manufacture corrugated panels. Like pallets, corrugated is a very versatile material, and many raw material and structural design options are possible.

Corrugated panels, or "boards," are manufactured of 2 forms of containerboard: flat sheets of paper called linerboards ("liners") and corrugated sheets of paper called "medium." Flat linerboards are glued to one or both sides of a corrugated medium on "corrugators," or large press and gluing machines. The most common structural corrugated board configuration is single wall, manufactured using one layer of medium glued between two liners. Single wall corrugated is used for 80% of corrugated box production. Figure 1 illustrates single wall and other common board configurations, such as single face, double wall, and triple wall. Extra sturdy multiwall containers, such as IBC's (Intermediate bulk containers) may be manufactured by combining triple and double wall corrugated panels.

Table 1: Common Basis Weights of Containerboard used to make Liners and Mediums
Liners Basis Weight (lb/1000ft²)	Medium Basis Weight (lb/1000ft²)
26 33 38 42 69 90	26 33 40
Source: The Wiley Encyclopedia in Packaging Technology

As with pallets, softwood and hardwood containerboards exhibit very different properties. Softwoods, composed of longer and stronger fibers, are used to manufacture linerboard sheets. The fibers in hardwoods are much shorter, and are used to produce the fluted mediums. Recycled papers are also used in the medium. "Basis weight" is used to describe the weight of both liners and mediums. Basis weight is the weight of paper in pounds per 1000 square feet of material, listed as lbs/msf or just pound. Heavier basis weights generally indicate better performance. Table 1 lists the common basis weights. Today, the most common basis weights are 42 lbs/msf for liners and 26 lbs/msf for mediums.

	Table 2: Standard U.S. Corrugated Flutes
	Flute Designation*	Flutes per Linear Foot	Flute Thickness (in.)		Flute Cross Section
	A Flute	33±3	3/16
	B Flute	47±3	1/8
	C Flute	39±3	5/32
	E Flute	90±4	1/16
	*  Other specialized flute sizes, including K, F, and N, are less common but also available. Source: The Wiley Encyclopedia in Packaging Technology

Corrugated manufacturers may also vary performance of boards by adjusting the "flute size," or size of the corrugations in the medium. Flutes vary by height and the number of corrugations per foot. The 4 most common flute sizes, designated by letter codes, are listed in Table 2. Flutes typically run perpendicular to box height, and supply rigidity to the board liners. In general, larger flutes create wider vertical columns, therefore increasing stacking strength in the vertical direction and softer cushioning in the horizontal direction. Smaller flutes give better resistance to flat crushing and use less material. Therefore, A-flute is stronger than B-flute in stacking compression, while B- flute is stronger in flat compression. The C-flute size is a compromise between A and B, and today C is the most common flute size in use. For stronger boards, double and triple-wall combining different flutes (AB double-wall, ACA triple-wall, etc.) further increases possibilities.

Board construction is specified by basis weight and flute size, separated by slashes. Therefore, a double wall corrugated board could be specified as 42/33/42/33/42/AC. This indicates that the 3 liners are 42 pound basis weight containerboard, the corrugated mediums are 33 pound basis weight containerboard, and the flute sizes are A and C.

Box Manufacture

The proper name for a fiber shipping container is "box," although carton or case are often used. Boxes may be assembled by an integrated corrugated manufacturer, but they are often assembled by a downstream box manufacturer, or "sheet plant," located at another facility. There are an estimated 600 corrugated manufacturers and 900 sheet plants in the U.S. Sheet plants receive flat corrugated boards as their raw material. The rigid boards must be "scored," or indented, to make folding possible. "Slots" are cut for flaps and edges. The sheet plant has the equipment needed to score, slot, print, and join corrugated sheets into boxes. The manufacturer will usually join two sides of each box at the manufacturers joint, a 1¼-inch tab on one side secured the adjacent side by staples, glue, or tape. Boxes are usually delivered to the user in the knock-down condition to save on shipping and storage costs. They are formed into their final box shape by the user, usually just prior to loading with product.

	Box 1		Box 2
	Figure 2: Box dimensions are specified in this order: Length x Width x Depth. Box 1 measures 10x5x5" and Box 2 measures 5x5x10"

The correct order when specifying box dimensions is length x width x depth. Box length and width are measured at the box opening. Box length is the longest of the side panels at the opening. Box width is the shorter dimension. Depth is perpendicular to the length and width, and typically parallel to the flute direction. Therefore, box 1 in Figure 2 measures 10 x 5 x 5 inches, while box 2 measures 5 x 5 x 10 inches. The acceptable dimensional tolerance in the box industry is ± 1/16-inch. Box size indicates the inside dimensions (ID), unless otherwise specified. Pallet manufacturers need to know the outside dimensions (OD) to properly specify the pallet size. The OD footprint may be approximated by adding 3/8" to single wall ID box dimensions, 3/4" for double wall boxes, and 1-1/8" for triple wall boxes.

'GMA' of Boxes Figure 3: The Regular Slotted Container, or RSC is the 'GMA' of the Box industry -- an economical, efficient use of corrugated material. All flaps are the same depth, and the two outer flaps are 1/2 the container's width so that they meet at the center of the box when folded. ©Fiber Box Association

The most common box style is a regular slotted container, or RSC. The RSC box blank is cut with all flaps the same depth. The 2 outer flaps meet at the middle of the box when assembled. The RSC is the "GMA" of the box industry, and is an economical and efficient use of corrugated material. An RSC assembled box is illustrated in Figure 3. The most economical RSC box size for a given cubic volume has Length:Width:Depth proportions of 2:1:2. For example, an RSC that measures 10 x 5 x 10 inches is the most efficient size to obtain 500 cubic inches of box volume. An RSC that measures 25 x 5 x 4 inches (again 500 cubic inches) will use 20% more corrugated board. Of course, other less efficient box dimensions are often needed to satisfy product requirements or market strategies. Many existing boxes, however, could be resized for better efficiency.

A Box Manufacturers Certificate, or BMC, is often printed on an outside bottom flap. Freight carriers maintain minimum regulations for shipping containers. Corrugated boxes are covered by Rule 222 for LTL motor freight and Rule 41 for Rail Freight. These rules regulate the maximum box size and gross weight for the various "grades" of corrugated board. Grades are defined in 2 ways: the bursting strength and combined face weights of the linerboards, or the Edge Crush Test (ECT) strength. The rules require boxes to display a Box Manufacturer's Certificate (BMC) to identify compliance with one of the 2 criteria. Examples of the 2 types of BMC's are given in Figure 4.

Box Performance

The performance of corrugated boxes in service is a factor of many variables. These variables can often cause premature failure of stacked boxes and/or product damage. If you understand the effect of these variables on boxes, you may better assist your customer in diagnosing packaging and pallet damage issues.
Product Type: Some products, such as glass bottles, are resistant to crushing and the box merely unitizes the contents. Most products, however, require a box that resists some or all of the compressive stacking stresses.
Stacking Time: A 30-day storage reduces box strength by 40 percent. One year of storage reduces box strength by 50%.
Humidity: At 95% relative humidity, a box is 71% weaker than at 50% humidity. Rain and extreme humidity will cause corrugated boxes to delaminate. Special coatings may be required if high moisture conditions are expected.
Stacking patterns: Approximately 75% of box compression strength is located at the 4 corners. Therefore, interlocked stacks (box layers crossed or not in columns) are more stable during shipping, but offer 40% less compression strength than column stacked boxes.
Stack misalignment: Even when column stacked, if box corners are not properly aligned, compression strength is reduced. A 1-inch misalignment in box corners is estimated to reduce compression strength by 43%.
Box Location: The boxes under most stress in a stack is typically the layer of boxes adjacent to the top deck of the bottom pallet. These boxes often fail before upper boxes.
Shock and Vibration: Rough roads and stiffer trailers can magnify the compressive forces exerted on boxes by a factor of 10! Smoother roads and air ride suspension trailers transfer much less stress to the boxes.

Typically, box designers lump these negative factors into one loss factor, called a safety factor. The safety factor for boxes used with pallets range from 4.5 to 10, depending on the product, storage conditions, and transportation mode. For example, assuming a safety factor of 4.5, a box that supports 1500 pounds in a standard compression test will support only 333 pounds in use. Safety factors for pallets are much lower, ranging from 1.5 to 3.5, and reflect a greater resistance to detrimental storage and shipping conditions.

Unitized Boxes on Pallets

Boxes must be designed with a knowledge of the above performance variables, plus pallet design. If pallets are undersized, box corners will overhang the pallet edges and up to 40% of box compression strength is lost. Pallet deckboards that do not support box corners reduce box strength by 20%. Boxes intended for palletized loads are designed based on the stresses predicted for boxes adjacent to the pallet. Therefore, in a 4 high stack of palletized product, the design for all boxes in that stack is based on the stress experienced by the bottom layer of boxes in the bottom pallet.

If box designers do not understand pallets, they make wrong or conservative assumptions to account for variations in pallet design. For example, a 48x40-inch GMA pallet does not provide a 1920 square inch uniform platform for boxes. A ¾" thick deckboard, GMA-type pallet will provide approximately 1140 square inches (2-5½ and 5-3½" wide boards, each 40" in length) of effective coverage with most boxed loads. If more flexible deckboards are used, such as ½-inch, the stresses on the boxes will be concentrated over the deckboard area located over the more rigid stringers. In extreme cases, the effective bearing area may be as low as 130 square inches (area of above boards directly over 1½" wide stringers). Clearly, if users request lower priced pallets or less stiff pallets, improvements to the box design may be required to offset increases in product damage. Other pallet variables, such as inconsistent board thickness and large gaps between boards, can also create concentrated stresses and cause premature box and product failures.

If pallet designers partner with box designers, there may be opportunities to minimize detrimental interactions between boxes and pallets. The costs of pallet improvements - such as sizing pallets to reduce box overhang, placing extra deckboards at known box corners, reducing gaps between pallet deckboards, using stiffer pallet boards, drying pallets to reduce moisture, etc. - should be evaluated for their savings on corrugated expenditures. In addition, lower cost and more variable pallets may be more economical if strong multi-wall corrugated boxes are required for certain products.

Figure 5: Palletization
		Left, two-thirds of the potential compressoin strength of a box is in the four vertical corners; they should be fully supported. Pallet overhang reduces compression strength. Above, the best stacking pattern for box compressoin strength is vertical columns. Interlocking stacke add stability but they reduce compression strength.

Pallets and boxes should work in unison to transport, store, and protect the product. If designed as a system, pallet and box performance can be maximized at a minimum cost. If you understand the requirements and performance of both pallets and corrugated boxes, you are much more valuable to your customer as a unit load supplier. The authors are John W. Clarke, Director of The Center for Unit Load Design at Virginia Tech and Dr. Jorge Marcondes, Associate Professor in the Packaging Program at San Jose State University. The Center for Unit Load Design researches interactions between packaging, pallets, and material handling equipment. If you would like to learn more about corrugated boxes or other activities at the Center, call John Clarke at (540) 231-5370.

References:
Brody, A.L. and K.S. Marsh, 1997. The Wiley Encyclopedia of Packaging Technology, 2nd Edition. New York: John Wiley & Sons, Inc.

Fibre Box Association, 1992. Fibre Box Handbook. 20th Edition. Rolling Meadows, IL. Hanlon, J.F., R.J. Kelsey, and H.E. Forcinio, 1998. Handbook of Packaging Engineering, 3rd Edition. Lancaster, PA: Technomic Publishing Co., Inc.

Marcondes, J.A., Fiberboard Packaging. Seminar taught at Clemson University, Clemson, SC. June 1998.

McKinlay, A.H. 1998. Transport Packaging, Herndon, VA: Institute of Packaging Professionals.

Soroka, Walter, 1995. Fundamentals of Packaging Technology, Herndon, VA: Institute of Packaging Professionals.

The authors are John W. Clarke, Director of The Center for Unit Load Design at Virginia Tech and Dr. Jorge Marcondes, Associate Professor in the Packaging Program at San Jose State University. The Center for Unit Load Design researches interactions between packaging, pallets, and material handling equipment. If you would like to learn more about corrugated boxes or other activities at the Center, contact John Clarke at unit.load@vt.edu or call the Center at (540) 231-5370.

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