Manufacturing of Papers

Preparation of the fibres

The material to be used for making paper is first converted into pulp, a concentrated mixture of fibres suspended in liquid. The source of fibre is often natural (softwood or hardwood trees or other plants) or recycled, such as old corrugated boxes, newsprint, or mixed paper.
When best sheets materials are used to make paper, it is usually necessary to break down the lignin inside of the plant's cell walls. This is done via a chemical process, such as the Kraft Process. These processes are not needed when breaking down recycled fibres, as the lignin has already been removed from the source material. If the lignin is retained in the pulp, the paper will yellow when exposed to air and light.
Pulp that has been broken down mechanically is often known as "groundwood pulp." The mechanical process to break down wood chips into pulp requires no chemicals. Since the lignin is not removed from mechanical pulp, yields are relatively high, approximately 90-98%. However, due to the aging issues indicated above, mechanical pulp is most often used for newspapers and other non-permanent goods.
Pulp that is broken down chemically is known as "chemical pulp." The main purpose of a chemical pulping process is to break down the chemical structure of lignin and render it soluble in a liquid (most often water) so it may be washed from the remaining fibres. Removing the lignin from wood chips also serves to break them apart into the fibres that compose pulp.
Recycled fibres do not need to be pulped in the conventional sense. These fibres have already been treated once, so instead they need a more gentle process to break the fibres apart while preserving their integrity.
Once the fibres have been extracted, they may also be bleached, dyed, or have additional ingredients added to alter the appearance of the final product. For example, Kaolin (or calcium carbonate) is added to produce the glossy papers typically used for magazines.

Sheet formation

The pulp mixture is further diluted with water resulting in a very thin slurry. This dilute slurry is drained through a fine-mesh moving screen to form a fibrous web. A watermark may be impressed into the paper at this stage of the process. This mark is used on paper currency and other things. This moving web is pressed and dried into a continuous sheet of paper.
In the mould process, a quantity of pulp is placed into a form, with a wire-mesh base, so that the fibres form a sheet on the mesh and excess water can drain away. Pressure may be applied to help remove additional water. The paper may then be removed from the mould, wet or dry, and go on to further processing.
Most mass-produced paper is made using the continuous Fourdrinier process to form a reel or web of fibres in a thin sheet. When dried, this continuous web may be cut into rectangular sheets by slicing the web vertically and horizontally to the desired size. Standard sheet sizes are prescribed by governing bodies such as the International Organization for Standardization (ISO).

Drying

After the paper web is produced, the water must be removed from it in order to create a usable product. This is accomplished through pressing and drying. The methods of doing so vary between the different processes used to make paper, but the concepts remain the same.
Pressing the sheet removes the water by force. Once the water is forced from the sheet, another absorbant material must be used to collect this water. On a paper machine this is called a felt (not to be confused with the traditional felt). When making paper by hand, a blotter sheet is used.Drying involves using air and or heat to remove water from the paper sheet. In the earliest days of papermaking this was done by hanging the paper sheets like laundry. In more modern times, various forms of heated drying mechanisms are used. On the paper machine, the most common is the steam-heated can dryer. These dryer cans heat to temperatures above 200ºF and are used in long sequences of more than 40 cans. The heat produced by these can easily dry the paper to less than 6% moisture.