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.

Starch and Modified Starch

Starch is a polysaccharide, a chain of many glucose molecules. It is the main carbohydrate store in roots and seeds.

There are two types of glucose chains in starch. One is a simple chain called amylose, and the other is a complex branched form called amylopectin.

In the starch grains in a plant, amylopectin makes up the bulk of the material, between 50 to 80 percent by weight, made up of several million amylopectin molecules per starch grain. The rest is a much larger number of the smaller amylose chains, made up of 500 to 20,000 glucose units in each chain. Amylopectin molecules are made of several million glucose units.

Amylopectin forms branched structures with about 30 glucose units in a chain between branches. This makes the molecule somewhat striped in appearance, with the knotted branch points all in a row, and the smooth chains separating them. These molecules are so large that this striped appearance shows up under a light microscope, forming what appear to be "growth rings" in the starch grain.

Uses

Starch is a major source of calories in grains and tubers, and foods made from them.

When starch is added to products as an ingredient, however, it is the functional properties of the starch that are usually important, not the calories.

Starch is the main thickener in gravies, sauces, and puddings. It absorbs water, and becomes a gel when cooked. As the starch swells up with water, the amylose leaches out, and the amylopectin forms the gel. Some starches have higher amylopectin content, and make better gels than those containing lots of amylose.

As a thickener (as opposed to a gel), it is the amylose that has the main function. The long water-soluble chains increase the viscosity, and that viscosity doesn't change much with temperature.

Amylose chains tend to curl up into helices (spirals) with the hydrophobic parts inside. This allows them to trap oils and fats inside the helix, as well as aroma molecules.

Because starches are so good at absorbing water and bulking up, they are important in the "mouth feel" of many food products, and are used as fat substitutes.

Not all of the starch in a food ends up being digested. The starch that is not absorbed by the body is called "resistant starch", and is consider dietary fiber. It is also a source of nutrition for intestinal flora, which make important vitamins (and intestinal gas).

Starches are added to processed meats (lunch meats, hot dogs, sausages, etc.) as a filler, binder, moisture retainer, and fat substitute. They are added to soups, sauces and gravies as thickeners. They are used in extruded cereals and snacks to hold the shape of the material.

Modified starch

Starches can be modified in several ways to change their function as additives in products. They can be cross-linked, where the chains get stuck together into a mesh. They can be heated to break the long chains down into simpler molecules like dextrin, polydextrin, and malto-dextrin. These are simply short starches.

Starches can have a hydrogen replaced by something else, such as a carboxymethyl group, making carboxymethyl starch.

Adding the carboxymethyl group makes the starch less prone to damage by heat and bacteria. Carboxymethyl starch is used as an additive in oil drilling mud, and is used in the goo that makes ultrasound examinations messy. Carboxymethyl starch is also called a starch ether, or misnamed starch glycolate due to a historical misunderstanding.

Carboxymethyl groups make the starch more hydrophilic (water loving), and aid in cross-linking. This makes carboxymethyl starch useful in aspirin and other tablets to make them disintegrate quickly.

Longer carbon chains can also be added, such as carboxyethyl groups, or carboxypropyl groups.

Adding bulky functional groups like carboxymethyl and carboxyethyl groups reduces the tendency of the starch to recrystallize. When the starch stays as a gel, a product is softer, and we say it is "fresh". When the starch regains its crystalline form, the product becomes firmer, and we say it is "stale". The technical term for this is starch retrogradation.

Starches can be esterified by modifications with an acid. An ester is the result of reacting an alcohol with an acid. The starch loses a hydroxyl group, and the acid loses a hydrogen. These combine to form water as the other product of the reaction.

Using acetic acid, starch acetates are formed, which are used as film-forming polymers for pharmaceutical products, and as the polymer in biodegradable packing foam "peanuts". Starch acetates have a lower tendency to create gels than unmodified starch.

Acids can also break the long chains into shorter molecules, much like heat does, to form polydextrins, malto-dextrin, or dextrin. Enzymes are also used to do the same thing.

Cross-linking occurs when a hydroxyl group (OH) on one chain bonds with a hydroxyl group on an adjacent chain. This toughens the starch, and helps it resist heat and acids.

Cross-linking can be done by heating, or by reacting with compounds such as phosphates, or glycerol.

Starches are also sometimes "pregelatinized" to make them easier to dissolve during product manufacture.

Starches, especially modified starches, are also used as glues in cardboard manufacturing. Starches such as Gum Arabic and Gum Tragacanth are used as the glue for stamps and postal envelopes.

Oxidized starch, usually oxidized with sodium hypochlorite, is whiter than unmodified starch, has increased clarity, and a lower viscosity.