How is cardboard made? Everything you could need to know…

Chances are that you will have handled some cardboard today, without even thinking about it. And even if you haven’t, you will have definitely used a product that was at some stage transported using this material.

But have you ever thought about what cardboard is? Or how it is made?

This guide aims to give you a brief overview of this versatile material, along with manufacturing processes and benefits.

Despite the confusion over his nationality (either Swedish or German) Chemist Carl. F. Dahl developed the process of pulping wood, which was later to be used in the paper making process. He named the development the “Kraft process”, a term which, still stands in the present day.

Coincidently, the German word for strength is “Kraft”. The advanced method given its name for the strong and robust nature of the completed product turns woodchips into a paper strong enough not to split or tear.

The practice has also been said to be an expansion and modification of the soda process, but, with sodium sulphate being the primary chemical used for a cooking liquor instead of sodium hydroxide.

The process now used nationally, is created in Kraft or Pulp Mills. Dahl, produced the first Kraft Mill in 1885, located in Sweden.

Before delving too far into these details, if you are looking for a better understanding of pulp the first requirement is too acknowledge where the production initially commences.

A Pulp Mill is an industrial facility usually large in size which converts timber, woodchips and other wood products into pulp using the Kraft or other processes.

These should not be confused with a Paper Mill. A non-integrated Paper Mill will purchase pulp from a Pulp Mill which has been dried and transported in a bale form (named market pulp). The bales will be rehydrated with a solution at the Paper Mill before being made into paperboard.

The fibre’s from any plant or tree can be used to create paper, however, the strength and quality of these fibre’s varies among tree species.

Hardwood trees tend to have shorter fibres which produce weaker paper but, this tends to also produce a smoother and opaque finish, generally more suitable for printing.

On the other hand, softwood trees such as pines and firs, have longer, stronger fibres which produces the strength within corrugated packaging.

Pulp is not only produced from timber but can also be created in an environmentally friendly way by recycling woodchips and shavings leftover from lumber mill waste.

Before being recycled this waste used to be disposed of at landfills or was burnt.

The aim of the pulping process is to break down the structure of the fibre source into the fibres ready to be made into market pulp.

To achieve this the fibres are put through either of the two processes that are used to develop the pulp. The two process are mechanical pulping or chemical pulping (otherwise known as kraft).

The chemical process, as you may have previously read involves cooking the wood shavings in a sulphate solution to digest the wood. Both sulphate and sulphite can be used to separate the fibres from the lignin which, is a natural glue like substance that bonds the fibres.

Chemical pulping degrades the lignin into small water soluble molecules which can be washed away without weakening the fibres. The result of the chemical process is either a dark brown in colour or, it can be bleached during the procedure resulting in a white kraft.

Mechanical pulping however involves grinding debarked logs against a revolving stone or disk grinders in order to break down the fibres to make a pulp.

The stone gets sprayed with water to remove the fibres, this however results in very little removal of lignin meaning paper quality is reduced, which therefore also indicates that the strength of the fibres may have been impaired from the process.

Mechanical pulping however is a low cost solution which generates a higher output.

Considering the environmental impact, a lot of the produce is now made from recycled paper. The paper is shredded and the same chemical process is then processed to break the paper down into pulp again, its strength reduced each time it’s recycled.

These types of recycled waste can be found in the table below.

Material can also be manufactured from sustainable sources, including those that are FSC certified.

So now we know how the pulp process works the next stage is how the pulp is then made into the paper/board. As previously mentioned a paper mill will outsource the pulp for the next part of the paper making process (unless the mill is integrated).

Paper mills can have a single paper machine or several machines which make a single or variety of paper grades. The process of papermaking remains the same regardless of the type of paper manufactured or the size of the machine.

Before the pulp can be made into the finished paper product however, it must go through a process called “beating”. In effect, the pulp is squeezed and pounded by machine beaters in a large tub.

Filler materials such as chalks and clays can be added which influences the opacity of the final product. Sizing’s such as starch, rosins and gums can also be added at this stage, a sizing will affect the way inks react with the paper, the choice of sizing is dependent upon the intended use of the paper.

For the pulp to then be made into paper, the pulp is fed into a large automated machine, often a Fourdrinier. The machine has a moving fine mesh belt on which the pulp is squeezed through rollers to drain water while a suction device underneath drains the excess water.

The next stage is for the nearly made paper to be pressed between wool felt rollers, and then pass through a series of steam heated cylinders to remove any remaining water.

Paper with the intended use of corrugated paperboard is now wound onto a wheel and the process is finished.

Paper intended for other uses may go through other stages until completion, some of these stages include coating, winding and calendaring (smoothing of the surface). These are typically created off line to the Fourdrinier.

Completed paper for use of corrugated paperboard can now typically be used as liners for the inner and outer liner of paperboard which is supported by the flutes.

Typically, Kraft paper (chemically processed) is used for the outer liner of the board. Being made from softwood gives the paper the benefit of being smooth for print and provides some resistance to water penetration in comparison to test paper.

Test paper, typically made from hardwood or recycled paper have short fibres giving the paper a more abrasive finish. The rough finish is why it’s commonly used for the inner liner, printing on this surface would prove difficult.

So now we have both the inner and outer liners, the next addition to corrugated board is the flute. The flute used is dependent of the type of support that is required from the corrugated board.

Below is a table of the types of flutes which can be found made up within cardboard.

A very fine flute used for corrugated cartons.

A fine flute also used for corrugated cartons (less rigid than the F flute).

More often, mostly used, its robustness, compression strength and compactness makes it a good choice.

Larger than B and has a greater compression strength but, it can be crushed more easily.

A combination of two flute sizes. Often B & C. Ideal for compression strength.

Again a combination of different flutes is common to create these three walled corrugated boxes. These are more so used for heavy duty product shipping.

We’ve detailed the steps taken to create the paper ready for the production of cardboard, and if you’ve been reading from the beginning you’ll have picked up that a standard corrugated board is made up out of three parts; the outer liner, the inner liner and the flute. But, before we can discuss completion of the board there is one more step a manufacturer must take.

Creation of the flutes.

Machinery called a Corrugator will do both: A. create the flutes and B. glue the components to create the finished product.

Simply put, a set of machines in a row, which is designed to adhere three (a standard board), five or seven sheets of paper that shape single, double or triple walled corrugated boards. The corrugator is a continuous system which produces in bulk.

The system works in the following ways. First off, reels of paper will be fed into the corrugator, where at this point the paper is conditioned with heat and steam prior to being fed into the single facer.

The single facer is a section of the corrugator which, transforms the paper into the flute by creating a series of arches. These arches are created by large rotating cylinders with a corrugated profile which creates the grooves in corrugated paper. There are different profiles for each of the flute types previously listed.

Next still included as part of the previous stage, starch is applied to the tips of the flutes on one side, where an inner liner is then affixed to the fluting – this is called a single web.

The second part of production is a part of the machine called the double backer. In essence, the double backer glues the single web to the outer liner following a similar process to the step above. During the course the product is heated to ensure that the bonds are strong, gelling the glue and removing moisture.