Corrugated

Materials

1.1 Structural elements

A corrugated box consists of two structural elements: the facings (linerboard) and the fluting structure (corrugated medium). Linerboard can be of several types.

1.1.1 Types of Liner

Fourdrinier Kraft Liner - "Fourdrinier" is the name of the man who invented the machine on which the liner is made. "Kraft", the German word for strength, is at­tributed to the strength applied to pulp, paper, or pa­perboard produced from wood fibers by the sulfate process. The Kraft liner is produced from a high per­centage of pinewood (softwood) fibers which imparts toughness as softwood fibers are longer in length than hardwood fibers and allows for a greater interlocking effect. The sheet is made by randomly orienting the fibers and is formed on a traveling endless wire screen which may also be vibrated. This Kraft liner provides the best Tear and Tensile strength properties available in the paperboard industry today. The liner is used to package products in all areas of the corrugated pack­aging and is widely known as one of the best packaging materials available.

Cylinder Recycled Linerboard - This liner is often called "Jute," a misleading term since it never contains any jute fiber. The correct reference name is "test liner" and is produced from reclaimed fibers on a cylinder-type paper machine. Although test liner often contains a certain proportion of virgin kraft fiber, it is inherently weaker than kraft linerboard. Due to the recycled fiber content, the basis weight of the recycled liner may be increased to meet the Mullen test of comparable virgin linerboard. The advantages of test liner is that it is gen­erally less expensive to manufacture than kraft liner and it is an environmentally sound means of recycling used packaging. The two recycled materials most often used to provide a furnish for the test liner are:(1) dou­ble line kraft clippings: and(2) old corrugated contain­ers.

Double lined kraft clipping(DLK) waste is comprised of the trim and cutouts generated during the process of man­ufacturing corrugated boxes. It is highly desirable due to its cleanliness and consistency.

Old corrugated container(OCC) waste is generated by collecting corrugated boxes from stores, supermarkets, etc. after they have been emptied. Although OCC ma­terial requires extensive cleaning, it is an important source of recycled fiber throughout the world.

Kraft linerboard can have up to 25% recycled fiber.  The average recycled content of linerboard in 1990 was about 12%.  There is a problem citing figures.  The numbers change rapidly and the recycled fiber content of containerboard is increasing each year.

Schrenz - In many parts of the world, there are pack­aging economies which lack extensive forests of soft­wood or the foreign exchange to import softwood fiber, In these countries the use of recycled fibers means "whatever fiber is locally available," down to and in­cluding "old newspaper." The "Schrenz" is of German origin but is understood elsewhere. Liners of this kind are often seen in the US in boxes bringing in exports. Unless colored or covered by a layer of other fiber de­posited on the surface from a secondary headbox, the sheet has a gray appearance and is inferior in strength characteristics.

1.1.2 Types of Medium

Semichemical - This type consists of hardwood fibers made by a neutral sulfite or comparable process. "Semichemical" refers to the fact that hardwoods are difficult to defiber, and so the defibering process is partly mechanical and partly chemical. This  variety is the most prevalent in the US. Appearance and other characteristics may vary, depending on the species of wood, the method of cooking and washing, and other production techniques. Most of this is produced on Fourdrinier machines. Some is produced on other forming machines. All methods of forming can make a good medium; quality depends on furnish and stock preparation, not on formation. Semichemical medium is "virgin" if 75% or more of the furnish is new fibers. All-hardwood sheets are rare since the addition of some softwood, although recycled, is desirable to give better "runnability."

Bogus - This type of medium is based on recycled kraft waste, generally old corrugated. The term "bogus" is unjustified since it denotes imitation which is incorrect. When properly made, bogus can be of high quality and equivalent to semichemical medium.

Kraft - The use of kraft medium is limited to situations when high tear or puncture resistance is needed. The total consumption for this purpose is so small that the equivalent weight kraft liner board is substituted, but it doesn't corrugate well.

Other - This covers any fibers not described in the foregoing paragraphs and corresponds to the com­ments made under the "Schrenz" heading for liner board.

In the US and Canada, freight rule requirements have led to the development of standard grades of linerboard and medium. For linerboard, "standard" is de­fined in terms of basis weight and burst strength, based on kraft facings. Caliper is not specified for liners used to make corrugated; mills allow caliper to "float." This permits them to run their machines to optimize other and more important characteristics.

For corrugated medium, "standard" is defined in terms of caliper and basis weight. The rule calls for a mini­mum caliper of 0.009 inch (2.29 mm) and a minimum basis weight of 26 pounds per thousand square feet (lb./MSF) (127 g/m2). medium is thus referred to in the trade as "nine point".

Information on the different properties that medium can add to the board combina­tion can be found in the "Compression Guideline" available from the address and phone number on the bottom of the page.

1.2 Combined Board Structures

There are four basic combined board structures in the corrugated industry; single faced, single wall (double faced), double wall, and triple wall.

Single faced corrugated is made up of a single corrugated medium and a liner.  It is mainly used as a protective wrap.

Single wall or double faced board is made up of a single corrugated medium between two liners. The medium material can be formed into any flute height depending on the properties desired of the board or the end use of the container.  It is used in 90% of all corrugated con­tainers. It can be used to make box liners, pads, shells, tubes, partitions, die cut interior forms and display units. single wall lends itself to use in automated high speed packaging lines because of easy bending and lower board memory and relative dimensional stability. The advantages of this material that it can have tailored construction to meet package use and requirements. It has superior die cut workability and dimensional preci­sion and high strength to material ratio.

Double wall is made up of three liner elements and two corrugated mediums. the medium, like single wall, can be formed into any combination of flute configurations. The combination of fluting and the weights of the liners used is determined by the use of the board or the de­mands to be made on the container in performance.  It is used in packaging of heavy products, bulk material, items which require the extra strength and puncture resistance. The high compres­sion strength and flexural stiffness in relation to single wall, make the material outstanding for inner packing, corner posts and bulge resistant applications.

For reasons of economy, symmetrical flute construc­tions in double wall boards, such as AA or CC are in­frequently used. Mainly for production flexibility, cor­rugated box plants run both fluting rollers (A, B, and/or C) on the corrugator to produce two different flutes in the final sheet.

Triple wall is the most complex and least used of the three. It consists of 4 liners and 3 corrugated mediums. Using various strength liners and mediums, this mate­rial can be tailored to any packaging demand.  It is used where maximum stack strength and sidewall strength is required.

1.3 Flute Orientation

Boxes are normally made with the flute direction run­ning vertically.  This provides better stacking strength on the vertical direction.

For a few commodities, shipping containers may be made with the flutes running horizontally.

Such boxes are usually end-opening. They are stacked, stored or shipped laying on their side because this is the direction of their greatest strength.

1.4 Flute Structure

There are four basic fluted board heights available in conventional corrugated board...A, B, C, and E flutes.

2. Corrugated Manufacturing

2.1 Formation

The corrugator machine flutes the medium, places liquid starch on the peaks of the flutes, then combines it with the liners into a continuous web of combined board.  The material is then passed through hot "irons" causing the water to be dried out of the starch gluing the liners and medium together.  Once combined the web passes through a slitter/scorer where the web is cut and scores are applied in the machine direction.  The last operation on the corrugator is the cut off knife where the slit/scored  web is cut into blanks.

2.1.1 Die-cut

Corrugated that is to be die-cut does not get scored on the corrugator and is printed before being die-cut.  Some die-cutting machines can print the corrugated before die-cutting it, others need the corrugated to be printed on a separate machine.  The process is simple, a plain pad of corrugated is feed into a machine where a die (like a cookie cutter) stamps out the item.  The die can either be rotary or flat depending the item's size and quantity being run.  The die can cut, slit and scores all at the same time depending on what type of knife or rule is used.  The cutting die's tooling cost is based on how many inches of knife or rule is used to make the die and the type of die (i.e. rotary dies are more expensive than flat dies).

2.1.2 Non Die-cut

Non die-cut items are run on machines that can be modified for each order and do not require tooling.  The most common example is a RSC (regular slotted container) below.

The flap scores are put on by the slitter/scorer at the corrugator.  The resulting blank is next processed on a printer-slotter, which prints, puts in the panel or body scores, and slots the box.

3. General Styles

Box dimensions are always inside dimensions (I.D.) and are stated in sequence of Length x Width x Depth (LxWxD).  The first two dimensions mentioned are always considered the size of the opening.

Common drawings of box styles can be found in the format drawings at the end of this section or in the Fibre Box Handbook which can be ordered through PPO order or call Corporate Packaging Programs at the number on the bottom of this page.

4. Design Concepts

4.1 Optimum Design

Every Packaging Engineer endeavors to arrive at a design which will be "optimum." The question is, optimum with respect to what? At what point does the process of optimizing begin?

The answers to these questions involve an understanding of what the function of the package is to be, the conditions and circumstances of its use, and balance between sometimes conflicting goals. Circumstances also dictate where the optimizing process begins. If the package is designed by the Packaging Engineer, the option of selecting the best material will most likely not be part of the process. The Packaging Engineer usually has neither the means to control that process nor the information needed to implement it. Therefore, the more collaboration there is between the Packaging Engineer and the package maker, the earlier in the chain the process of optimizing can start.

4.2 Constraints

The number of choices and options available is large, but only within certain constraints. Regulations, such as freight rules, the Hazardous Materials Regulations, and specification requirements may mandate limits with respect to box size, test criteria, contents weight, board combination, etc. Your equipment and the storage, handling and shipping environment must be considered.

The nature, shape and fragility of the product help to define what is optimum.

Before going on to the specifics, it should be clear that it is not suggested that all steps outlined in this section are part of the design process of every package.

4.3 Board Structure

The basic fact to remember is the way corrugated functions as a structure under load.

When a corrugated column is under load, the structure bends until it buckles. In a box with contents this bending generates compression stresses in the inside liner, while the outside liner is in tension.

Therefore, if the board combination is unbalanced, the heavier or rather, the stiffer liner (the two usually go together) should be the inside liner.  For printing it is best to have the heaviest on the outside.

On double wall board there is an additional factor to be considered. The center liner, even if it is not commonly at the exact center - because most double wall construction do not have identical flute contours - is near the neutral axis where the compression contribution is minimal. Therefore, the optimum board combination is the one where the inside liner is the stiffest, the weakest liner is in the center, and the second stiffest liner is on the outside. The center liner needs only to be strong enough to hold the flute structure in place. The more weight per stiffness one can concentrate on the inside, the better, with due regard for other characteristics, of course. In other words, when it comes to double wall constructions, one can juggle board combinations at will, the only constraint being that the minimum combined weight of facings must comply with the freight rules.

Schematic of stresses in a compressed corrugated panel.

Along the same lines, a thinner column bends more easily than a thicker one. Therefore, all things being equal, any step towards maximizing caliper is a step towards optimizing the box. The chief mechanism for achieving this is the flute structure.

In single wall boards the progression from best to worst in (top to bottom) compression is from A to C to B, except that caliper loss sustained in fabrication tends to negate the advantage of A-flute, a fact of increasingly greater importance as medium becomes softer. For general good performance with regular 26 lb. medium, C flute is better, combining as it does the advantages of B flute with those of A flute.

4.4 Die Cut Holes

A box is only as strong as its weakest column. Die cut holes, slots and slits are normally the weakest part of any corrugated shipping container. The removal of material, or crushing of the corrugated structure, causes the surrounding area to take up the additional stress causing it to fail before the rest of the container. This is multiplied when the die cut is placed near the strongest part of the box, a corner or an edge.

Die cut holes can only weaken a shipping container's design and should be eliminated whenever possible.  However, sometimes the need for localization holes (the customization of products for international requirements) is unavoidable.  The Packaging Engineer should strive to design a structurally sound method for localization.

4.5 Printing

Printing can reduce the strength of a shipping container just as efficiently as a die cut hole can. Printing words and/or graphics on the outside of a container can cause damage. The pressure from the printing rollers needed to obtain a good print will crush the flute structure weakening the board.

A study conducted by Colgate Palmolive Peet Company shows that a corrugated container starts losing compression strength when printing exceeds 20% coverage.

Laboratory test results on printed and unprinted boxes will nearly always favor the unprinted box. Laboratory technicians who suspect that excessive printing pressure may have been the major cause of failure may dissect box panels after compression tests and caliper board thickness on the printed and unprinted areas. The difference will be "printing pressure." Typically, the results will be rated against the following standards: A-flute - .015", B-flute - .013" and C-flute - .011". If these maximums are exceeded, the box maker should be advised to make adjustments on their printer.

4.6 Size

It is important to control the size of the container so it can fit properly on to a 40" x 48" pallet. If the container is designed too large then there will be overhang. Overhang prevents the use of the strongest elements of the container, the corners and edges. The stress is placed on the bottom panel which fails.  Another possible problem with oversized container is that they are placed on their sides to obtain optimum cube on a pallet. This places the corrugation of the body panels parallel to the compressive forces reducing the strength of the container. The flap panels become the load bearing surface and quickly fail.

Most recent tests, under tight laboratory conditions and pre-conditioning of the packaging, showed a 50% difference in strength between a box with vertical fluting vs. horizontal fluting.

4.7 Inserts

Corrugated insert are mainly used to separate products but can be employed to increase the structural strength of a shipping container.  Information on the effects of some of the more common corrugated interiors on box compression can be found in the "Compression Guideline" available from Corporate Packaging Engineering (address and phone number on the bottom of the page).

5. Standards

5.1 United States Standards

The standards that control how corrugated boxes are made in the US are "Railroad Classification Rule 41" and "National Motor Freight Classification Item 222".  The most effective way to get a copy of these rules along with other useful information about corrugated (which some of this section came from) order the Fibre Box Handbook. 

These standards, Rule 41 & Item 222, are based upon some very basic tests that do not represent the corrugated container's actual strength.  These tests are the Mullen burst test (which tests to see how much pressure it takes to pop a hole in the corrugated) and minimum combined weight of facings (which looks for how much the corrugated weights per thousand square feet and determine if the correct weight of liners were used).  Since neither of these tests correlates to how the container will perform, the corrugated industry is investigating edge crush testing as a replacement for the Mullen burst test.  The edge crush test requirement will allow the designer to choose the type of corrugated bases on stacking strength requirements

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5.1.1 Box Maker's Certificate

A corrugated container must bear a legitimate box maker's certificate in order to qualify for shipment by common carrier.  The certificate guarantees that the box was made to specifications of the Fibre Box Association, and in conjunction with the requirements of the various freight classification agencies such as the American Trucking Association, Inc.

The certification must include the results of a burst, puncture, or edge crush test; the results of the minimum combined weight of facings; the size limit; and the gross weight.

For more information regarding the information contained within the Box Maker’s Certificate refer to the Fibre Box Handbook which can be ordered through PPO order or contact Corporate Packaging Programs at the phone number at the bottom of the page.

5.2 International Standards

Standards for corrugated in the international community differ from country to country.  Therefore, it is difficult to present that data here. 

It has been observed that the US Rule 41 and Item 222 will meet most other countries requirements.

6. Closure Requirements

If boxes are improperly or carelessly closed, their carrying qualities are lessened - an invitation to pilferage and to other loss or damage. 

6.1 Closure Performance

The package closure's primary function is that of containment.  In addition, the closure must:

·         Maintain interior cleanliness and ensure that contents remain intact during shipping and handling.

·         Permit access to the contents for inspection without destroying the usefulness of the container.

·         Not create a safety hazard when the package is opened or allowed contents to spill.

·         Conform to all applicable carrier regulations.

6.2 Preferred Closure Materials and Methods

Preferred materials for closure of corrugated cartons are reinforced (non-asphaltic) water-activated gummed paper tape (kraft color) or pressure-sensitive transparent plastic tape, minimum width of 2".

Strapping is an acceptable form of closure and is commonly used when securing a container to a pallet.

Other sealing techniques, such as staples and adhesives, usually cause significant structural damage when cartons are opened along with becoming a possible safety hazard and therefore should not be used.

Containers for contents weighing less than 70 lb. can be sealed with only two strips of tape on the top and bottom center seams.  The tape should extend a minimum of 2 1/2' over the edge of the container.  Heavier contents requires additional tape along the other top and bottom edge seams.