The essential procedure of industrial papermaking has not really changed since the Fourdrinier brothers patented their steam-driven, continuous paper machine over two centuries ago.
Simply put, a slurry of wood fibre enters the ‘wet end’ of the machine and a roll of paper exits the ‘dry end’. Yet continual improvements over the intervening years have resulted in tremendous changes in the scale, speed and control of this process.
Pulp preparation
- Fresh fibre preparation
- Recovered fibre preparation
Papermaking
- Wire section
- Press section
- Drying section
- Finishing
- Real-time quality control
- Winding
- Paper testing
The SCA Containerboard difference
- Experience & commitment
- BAT network
Pulp preparation
Fresh fibre preparation
Bark is removed from wood in a de-barking drum and used as fuel to produce steam for paper drying. A chipper chops the wood into small fragments which are cooked under high pressure and temperature in a digester. Rotating grooved steel plates separate the cellulose fibres. The residue of lignin and other dissolved substances, known as black liquor, is concentrated and used as heating fuel, while the separated fibres are treated in refiners to toughen them. The wood fibre (now called pulp) leaves the pulp mill and is buffered in tanks for a continuous flow to the paper mill.
Recovered fibre preparation
Mixed with process water, recyclable paper is stirred in a giant stainless steel vat called a pulper to make a fibre suspension. Impurities, such as staples, sand, glass and plastic are removed using a series of screens. Ink can be removed during white recovered fibre production by mixing the pulp stock with soap and blowing air through it to create a foamy ink residue. (The residue is sometimes re-used as a secondary raw material.)
Papermaking
The papermaking machine can be up to two soccer pitches in length. The massive apparatus has the chief job of removing water from a solution comprised of roughly 1% fibre and 99% water. Only by diluting the stock in this way can it be used to make thin, uniform paper quickly.
Wire section
At the wet end of the paper machine sits the headbox, which distributes a uniform jet of watery stock the width of the paper to be produced – from two to twelve meters wide depending on the machine. The suspension of water and fibre emerges from the slice, a slit-like opening. The aperture is computer controlled to adjust the jet speed, the basis weight profile (paper thickness) and the amount of dilution water. Optimum settings ensure that the fibres will weave together in a tight mat during their journey through the paper machine.
The liquid falls onto the wire or forming fabric, a moving plastic screen travelling at up to 75 kilometres per hour. Beneath the wire, foils (short for hydrofoils) remove water and improve fibre uniformity. The wire passes over suction boxes that vacuum out the water, leaving a soft mat of pulp that forms the paper sheet, or web. By now the wire has travelled 30-40 meters. In a couple of seconds the water content has dropped to 75-80% and the web has lost its wet sheen.
To make classic two-ply containerboard, above the web a second layer of web is being formed, but heading in the opposite direction. Now its supporting wire makes a turn, looping upside down so that the upper web is pressed against the lower one, sticking them together, or couching them.
Press section
The next stage of water removal consists of passing the paper web through a series of nip rollers that squeeze the water out of the pulp mat. This pressure also compresses the fibres so they intertwine to form a dense, smooth sheet. This reduces the water content to around 45 to 55%.
At this point a few centimetres of uneven edges are trimmed off either side of the web. This unused liner passes directly into the recycled paper stream.
Drying section
The paper web now enters the drying section. Here, the web travels through an enclosed space containing numbers of large rotating cylinders. They are warmed up to 130º C using steam heat, often efficiently produced by burning the factory’s waste material. Due to the high use of thermal energy, the drying section is the most costly stage of the entire papermaking operation.
The paper is now 80 to 85% dry. But in the production of some recycled containerboard it is moistened again in the size press. Wet sizing solution is applied to the paper in order to add a thin layer of starch to the surface. Starch contributes to stiffness and the bonding of the fibres within the sheet of paper. (In the manufacture of some papers, the sizing is also added directly to the stock before it goes through the paper machine.) After the sizing is applied the paper passes through another set of steam-heated drying cylinders.
In all, the paper web may travel 400 meters through the dry end. In the process it will lose roughly 93% of its water. (In most everyday conditions all paper contains about 7% water absorbed from the air.)
Finishing
To give the containerboard a smooth and glossy surface, especially necessary for printing, the paper passes through the calender. This is a set of smooth rollers, which can be hard or soft, that press the paper, embossing a smooth face on the still-rough paper surface.
Real-time quality control
Producing high-quality paper depends on maintaining extremely fine control over countless variables in the manufacturing process. The sheet of paper is now inspected by an automated measuring device that detects imperfections.
These measurements are instantly analysed and the results used to control the headbox parameters at the other end of the paper machine. For example, if the paper starts getting a bit thin on one side, the pulp dilution or application parameters can be immediately adjusted. Because the entire process from liquid jet to calendered paper takes no more than 25 to 30 seconds, the quality is effectively controlled in real-time, ensuring the paper specifications remain within acceptable tolerances.
Winding
After completing its 500-metre journey the containerboard exits from the paper machine and is automatically wound onto a jumbo reel. Weighing in at up to 50 tonnes and with a diameter of up to five metres and width up to eleven metres, these giant reels are lifted by crane to a nearby winder. Here the paper is unwound from the jumbo reel, cut into smaller rolls as ordered by the customer, then labelled for shipment.
Paper testing
On a regular basis, samples of containerboard are taken to an adjacent testing room. The most advanced facilities have computer-controlled testing machines that automatically punch, pull, stretch and tear the paper samples in different directions. The force required to do these operations provides a precise indication of paper strength. Water absorption characteristics (the Cobb value) and thickness are also tested. All this data is stored along with reel identification information. Months later, if a customer has a question about a specific delivery of paper, technicians can retrieve and share the relevant testing data.
The SCA Containerboard difference
Experience & commitment
The real challenge for papermakers is maintaining production parameters at an optimum level. SCA Containerboard meets this challenge by investing in the most advanced monitoring and control technology, and making sure it is operated by the industry’s most qualified people.
This is where the experience, knowledge and commitment of SCA Containerboard personnel make all the difference. It takes real expertise to diagnose subtle variations in parameters and know which corrective action to take. This expertise is derived from our people’s long immersion in the world of containerboard production.
BAT network
Moreover, SCA Containerboard know-how is leveraged across the company through a systematic knowledge-sharing network, known as the BAT (Best Available Technology) Group. Collecting all the resources within our organisation, including the support of the R&D Centre, the BAT Group facilitates the exchange of best practices in order to increase performance in all mills and foster the SCA Containerboard culture of continuous development.
The network is linked to our mill improvement programme, referred to as ‘Lean’, which focuses on efficiency improvement and waste reduction. It consists of measuring and analysing key performance indicators, enhancing business structure and increasing employee involvement. It’s all part of our never-ending quest for containerboard quality.