How's Corrugated Boxes Made?

Corrugated board is manufactured on large high-precision machinery lines called Corrugators running at 500 lineal feet per minute or faster.

The corrugated medium is usually a 26 lb/1000 sq ft (127 g/m^2) paperboard; higher grades are also available. It arrives to the corrugator on large rolls. At the single-facer, it is heated, moistened, and formed into a fluted pattern on geared wheels. This is joined to a flat linerboard with a starch based adhesive to form single face board. At the double-backer, a second flat linerboard is adherred to the other side of the fluted medium to form single wall corrugated board. Linerboards are often kraft paperboard (of various grades) but may be bleached white, mottled white, colored, or preprinted.

Common flute sizes are "A", "B", "C", "E" and "F" or microflute. The letter designation relates to the order that the flutes were invented, not the relative sizes. Flute size refers to the number of flutes per lineal foot. For example, "B" flute is approximately 1/4 inch from the top of one flute to the next, or 50 flutes per foot. "C" Flute is 5/16 inch from flute to flute or 42 flutes per lineal foot. "E" flute is 1/8 inch flute to flute or 90 flutes per lineal foot. Board thickness is an unreliable metric, due to various manufacturing conditions. However, a rough guide is: "C" flute=5/32 inch thick, "B" flute=1/8 inch thick, "E" flute=1/16 inch thick. The most common flute size in corrugated boxes is "C" flute.

Corrugated board is often graded by the basis weights of the linerboards, burst or mullen strength, edge crush test, or flat crush test. TAPPI [[1]] and ASTM test methods for these are standardized.

The choice of corrugated medium, flute size, combining adhesive, and linerboards can be varied to engineer a corrugated board with specific properties to match a wide variety of potential uses. Double and triple-wall corrugated board is also produced for high stacking strength and puncture resistance.


Box Manufacture
Boxes can be formed in the same plant as the corrugator. Alternitively, sheets of corrugated board may be sent to a different manufacturing facility for box fabrication.

The corrugated board is creased or scored to provide controlled bending of the board. Most often, slots are cut to provide flaps on the box. Scoring and slotting can also be accoplished by die-cutting.

The "Flexo Folder Gluer" is a machine that in one single pass prints, cuts, folds, and glues flat sheets of board to convert them to boxes for any application, from storing old family pictures to shipping the biggest of plasma TV sets to the global market. The most advanced of FFG's can run at speeds of up to 26,000 boxes per hour.

The most common box style is the Regular Slotted Container. All flaps are the same length and the major flaps meet in the center of the box.


Box blank showing score lines, slots, and manufacturer's jointThe manufacturer's joint is most often joined with adhesive but may also be taped or stitched. The box is shipped flat (knocked down) to the packager who sets up the box, fills it, and closes it for shipment. Box closure may be by tape, adhesive, staples, strapping, etc.

Cellulosic Ethanol

IN FEBRUARY 2007, the Department of Energy selected six cellulosic ethanol projects to receive up to $385 million in grants. Authorized by the Energy Policy Act of 2005, the funding was part of an effort by the Bush Administration to end the U.S.'s "addiction to oil" and enhance the nation's energy security.

The money was intended to further two of President George W. Bush's goals: to make ethanol out of nonfood biomass, including billions of pounds of agricultural waste, at a cost competitive with gasoline by 2012 and to increase the use of renewable and alternative fuels to 35 billion gal per year by 2017. In all, more than $1.2 billion was to be invested in the six biorefineries.

Two years later, none of the projects has been built, although one is under construction. Two were canceled right out of the gate. Hitches in the plans have turned up in numerous places. From securing feedstock to financing construction to finding a ready market, the experiences of the awardees illustrate that the nascent cellulosic ethanol industry faces several daunting hurdles.

The chosen projects represent technologies including enzyme hydrolysis, acid hydrolysis, and gasification. They were to be located in the Midwest, Southeast, and West and were planning to use feedstocks ranging from corncobs to wood chips. The companies advancing the projects had little in common other than having a plan to turn cellulosic waste into ethanol.

The designers of the DOE program envisioned that the grant money would be invested over four years, with the companies contributing 60% of the plant costs. When fully operational, the six facilities were expected to produce more than 130 million gal of cellulosic ethanol per year.

B.C.T. compression load of a corrugated board box

When a corrugated board box manufacturer tries to develop a new box, one of the design problems he runs into is selecting the correct cross section of the corrugated board. This includes the type of paper and the shape and number of the different layers. The board should meet different design requirements such as the maximum number of stacked boxes. Sometimes the capacity of a box to resist the vertical loads derived from stacking is expressed in terms of the so called "Box Compression Test" (BCT).

From a designer's standpoint, it would be very useful to have a predictive tool that allowed the computation of the BCT number of a box, from the geometry of the box and the cross section of the boards, without the need to make a prototype. Such a tool would speed up the design process and would produce boxes close to the optimum for each particular application. The difficulties in developing a tool like this come from the anisotropic nature of paper, together with the geometrical nonlinearities of an assembly of thin sheets of paper.

The BCT is obtained by simulation of the compression test, with a speed of 10-13 mm/min in the press.

In our methodology, we work at two levels: the global (complete box) level and the local (board) level. The local level analyses the micromechanics of the board and produces macromechanical properties for the study of the box at the global level.

The flowchart describes our methodology. We have created a library with the macromechanical behaviour of different board configurations (different thicknesses and geometries). Using this library, we assign to each box prototype the properties of a corrugated board, so we can easily obtain the BCT number of the prototype.

Corrugated Printing

Printing on corrugated to many customers has become as important an element in package design as protection. There are a variety of different ways to apply graphics to corrugated board. The main three are flexography, lithography, and digital printing.
Flexography
Flexible printing plates are mounted to a cylinder and pick up a fast drying water based ink from an anilox cylinder that is metered by a rubber roll or doctor blade system. Sheets are fed through the cylinders and graphics are transferred to the corrugated board. This is also known as direct print. There are a number of machinery alternatives available to converters and every converter is equipped a little differently.
Flexography is also used in preprint applications where the printing is applied to the outside liner prior to being converted into a corrugated sheet. This process offers printing advantages but requires certain levels of volume before it can be considered an option.
Lithography
Litho quality graphics can be incorporated into corrugated packaging in 2 ways, either through single-face laminating or labeling the combined board. Both result in high quality graphics with bright colors and sharp resolution.
Single face laminating
A printed “top sheet” can be laminated directly to the medium as the outside liner. The printed top sheet covers the entire sheet and is then ready to be converted.
Labeling combined board
Also known as litho labeling, a printed sheet or label is laminated to corrugated board. The label can be full sized or “spot”. Full sized labels are laminated prior to converting whereas spot labeling can be done after the box has been converted.
Digital Printing
Digital printing is the most recently developed printing technology. Graphics from a digital computer file are transmitted directly to a printer that plots the graphics onto the corrugated board. This eliminates the need for printing plates. However because it is a time consuming process digital printing is used primarily for short runs of high graphics. As speeds pick up this method of printing is sure to gain popularity.

Cassava Adhesives

Cassava-based adhesives, like the cereal starch adhesives, are of three main types:

1. Liquid starch adhesives
Liquid starch adhesives are supplied by the manufacturer in liquid forms usually in plastic or lined metal drums or in jerricans and bottles. They are used by consumers as supplied or diluted further, depending on specification, with a suitable solvent (usually water). They have a limited shelf-life (6 to 18 months) and are not usually considered for export. The high solvent/water content and the relatively low cost of these adhesives do not also recommend them for export.

2. Pre-gel starch adhesives
Pre-gel starch adhesives are produced in dry flakes and milled to specific adequate particle sizes. They are packed in waterproof, lined, multi-wall paper bags/sacks. They are very suitable for export.

3. Dextrin-based adhesives
Dextrin-based adhesives are delivered to consumers either in liquid and dry forms depending on specification or requirement. The liquid dextrin adhesives are packed in the same way as the liquid starch adhesives while the dry dextrin adhesives are packed in the same way as the milled pre-gel adhesives. Dry dextrin adhesives are very suitable for export, especially to Europe and America where they are used in very large quantities.

Machinery and equipment for the manufacture of starch adhesives
The main equipments used for manufacture of starch based Adhesives are:

For liquid starch adhesive

• Baurme (specific gravity tube)
• Reactor
• Temperature gauges/thermometers
• pH meters
• Packaging materials
• Steam boiler or source

For pregelatinized starch glue

• All mentioned above plus:
• Roller dryer
• Dry milling machine
• Packaging material—bags and sealers

For dextrin adhesives

• Reactor
• Dextrin pans
• pH meters
• Temperature gauges/thermometers
• Sifters

Cold Corrugating Process

There are quite a few basic differences between old and hot (regular) processes, but extremely important differences are in:

1) Type of adhesive, its application and setting temperature.

In conventional hot-corrugation process uncooked starch is dispersed in semi cooked solvent with additives. At elevated temperatures, the starch granules swell by water absorption and gelatinisation occurs. This slurry gets to the pores and voids of paper and solidifies there by making a bond. In cold corrugation process something like the reverse happens; i.e. the adhesive develops high viscosity during cooling process (instead of gel formation in heating during hot-corrugation). The adhesive is made up of fully cooked starch which is applied at high temperature to the cold paper, with very low viscosity; then it undergoes cooling. The adhesive is chemically made by thermo chemical modification of corn-starch. Cooking is carried out at a temperature of about 130/150⁰ C in the presence of ammonium persulfate. Sodium Hydroxide is then added at a high pH (9-10) which helps to increase alkalinity, stabilise the solution and bonding. Boric acid can also be added to improve viscosity.

On application of the adhesive which is at a high temperature, 90⁰ C, the molecules of starch get into the pores and voids. This happens by absorption process due to the paper (at room temperature) trying to establish equilibrium with hot glue and in the process heat transfer, water absorption and molecule migration into paper occurs. Further, the process is aided by high pressure (about 25% more than what is applied in hot process); applied on corrugating rolls. The absorption of glue under high pressure takes place in a slip second time as the production process operates at 300-350 m/min. The small portion of glue that goes into paper develops "green bond" which is just adequate to take the paper lines and fluting off the labyrinth. Subsequently a few seconds later, as the glue cools down, its viscosity increases more and more and the bond solidifies.

2) Tension control

If paper enters labyrinth at a very high speed the subsequent speed of its travel through the nip line needs to be much more. Higher speeds than desirable levels at labyrinth would lead to poor corrugation due to inadequate time available for paper to form flutes and such flutes to pick up gum. This would also result in poor bonding and resultant delamination.

Web tension is much more critical. Under high tension, the corrugations would develop cracking at the tips because in cold process the paper is not heated and moistened to make cellulose and lignin more pliable. In cold condition the paper is rather brittle or less plastic in nature and when such paper is fluted under high tension it is bound to crack on flute tips. Hence in the cold process it would be necessary to provide pre feeder to control tension and keep speed. Adjustments of regular break tension may also be given as required, to keep medium entering the fluting nip line at constant tension. The pre-feeder and break-tension adjustments would also help to even out any piping in paper which if continues up to labyrinth, will surely develop creasing and high-low flutes. Tension adjustment of medium is also required to even out wobbling of paper due to bad winding of reel, besides providing initial tension required to feed paper properly just at the entrance of labyrinth along upper corrugating roll.

3) Frictional coefficient of paper

Co-efficient of friction can be reduced by use of chemical agents (like paraffin wax), by keeping higher moisture in medium, by less wrap of medium on corrugating roll and by adjustments on pre-feeders.

The latest in metallurgical sciences can help to manufacture rolls with less co-efficient of friction and very high smoothness so that paper under tension and speed, can move smoothly on such rolls. Nevertheless it is important to see that friction is not very low as to avoid slippage of paper which is much more dangerous.

4) Picking and its control

Picking relates to accumulation of loose fibres detached from paper surface. In hot process such dust or fluff is reduced by release properties of paper roll-surface due to high heat. In the cold process the loose fibres can be blown off by air-jet web cleaner suitably designed. Certain chemical agents can be also used to treat medium against picking.

5) Wrap on top corrugating roll

As the flutes are continuously formed, the medium develops wrap and it wraps itself around top corrugating roll. This needs to be reduced so that fluff-out, i.e. detachment form the roll and movement to take liner is easy. This may be done by finger guide adjustment or by vacuum pull.

6) Medium moisture

7) Forming pressure

Forming pressure is one important factor which helps in glue absorption mentioned above as well as flute forming. In hot process the paper is flexible and therefore greater nip pressures are not required to bend the paper to corrugate. In cold process, the paper is less flexible and hence a 25% more pressure than usual would greatly help to form perfect flutes.

8) Pre-treatment of paper

9) Finger guides and/or vacuum handling of web

10) Double-backer and bond formation there.

Adhesive application at the double facer is rather demanding. It has to be put on the fluted paper flute-tips when it is not supported by any back-up roll. There is a much open time between glue application and double-facer joining. Hence the double facer-glue needs to be slightly different so as to take into account the transport time. If the adhesive solidifies before joining of liner then there is no bonding. If may help, to pre-heat the liner at about 40-50⁰C (or slightly above room temp.) so as to allow glue to penetrate liner easily. This preheating is required only for double-backer.

Adhesive compositions for corrugated boxes

A process for preparing an adhesive composition, comprising the steps of:

1. preparing a gelled carrier portion, by adding from 1.5 to 10% by weight of a starch, based on the weight of said adhesive composition, to 20 to 40% by weight of water, mixing to disperse the starch in the water, then adding from 0.25 to 2.5% by weight of NaOH or KOH and mixing to constant viscosity to produce the gelled carrier portion

2. adding to said gelled carrier portion from 10 to 30% by weight of a water-soluble alkali metal silicate characterized by a molar ratio SiO₂ :M₂ O in the range from 1.5 to 4.0, where M is Na or K, then mixing the whole until substantially homogeneous;

3. adding a further 3 to 20% by weight of water and mixing until homogeneous; and

4. adding a further 2 to 25% by weight of starch and mixing until viscosity is substantially constant to produce a final adhesive composition having a solids content between 31.2% and 45% by weight.