The role of paper remains important in the digital age, with this ubiquitous material still used daily for many purposes worldwide. In fact, the global production of paper and cardboard totals more than 400 million metric tons each year. The most produced type of paper is packaging paper and board, which has been growing in demand in recent years due to the online shopping boom. The two largest paper producing countries in the world are China and the United States.
Steps involved in turning pulp to paper
Raw material preparation
Step 1: Pulp preparation: Wood reaches to a pulp manufacturing factory in many forms. Depending on the condition of the raw material, professionals prepare it for pulping. Round-wood logs get debarked with the use of huge tumbling steel drums. Then, they get turned into small-sized chips, which get cleaned and stored for a while. Sometimes the factories also receive small chips in large quantities, which allows exclusion of debarking process.
Step 2: Fiber preparation: This stage involves a number of pulping technologies. High-temperature steam is used for fiber separation. This is called the digestion of chips. A digester or huge pressure cooker is used with appropriate chemicals to complete the process of chemical pulping. The digestion process allows wood chips to dissolve partially into various extractives including lignin.
Step 3: Bleaching: After fiber separation, the pulp contains several discoloration materials. Hence, bleaching is necessary to acquire the white, clean appeal. Sodium hydroxide is a pretty strong alkali chemical compound that helps in the dissolution of lignin discoloration from the surface of fibers.
A paper machine is typically 10 – 25 feet wide and can range in length anywhere from half a football field (50 yards) to the full length (100 yards). The machine has two main components; a wet end and a dry end.
Step 1: The pulp mixture and water are spread across a forming fabric which resembles a large mesh screen on the front end, or wet end, of the paper machine.
Step 2: At this point, the pulp is 99.5 % water to 0.5 % pulp. Gravity, suction, and a mechanized vibration of the screen quickly begin to remove 20% of the water, much of which is re-used at the mill. Strict environmental guidelines are followed for the volume and quality.
Step 3: The next step on the wet end is the press section where even more water is removed. With the water still at 60%, the mix gets squeezed through large cloth wrapped cylinders called wet felts. It’s at this point in the paper making process that the product starts to get its smoothness and thickness.
Step 4: Once through the felts, the paper enters the dryer section. Here, it passes through steam heated dryers that leave the product with a moisture content of 2-6%.
Step 5: The next stop is the size press; at this point, a starch solution is applied to both sides of the sheet. If you have ever used starch while ironing your clothes, you can get an idea of what is happening to the paper. The starch solution enhances its ability to resist water and ink penetration during the offset printing process. This step of the paper making process also gives the paper the ability to receive toners and liquid inks used in various types of digital printing.
Step 6: As it passes through this process, big, heavy cylinders called the calendar stack await the roll that is forming. These cylinders apply pressure to the sheet to form the desired smoothness and caliper of the paper. The pressure on the calendar stack can be adjusted. More pressure yields smoother paper with less thickness.
Step 7: At the end of the machine, the paper is wound on a large reel called the winder. As the reel of paper goes to the winder, it is slit into manageable size rolls.
Step 8: Each finished roll, carton, or skid is stamped with a run number. The run number displays the date, time, paper machine and which part of the large reel the paper came from.
Need for lubrication
Heavy-duty and continuous operation of machinery is prone to fatigue damage. Mechanical rotation and bearings are directly linked to the rotating roller-based paper machine. There are hundreds of types of bearings which makes lubrication management extremely necessary. Another reason is due to the frequent movement of parts between parts, such as into the steam header, vacuum pressure roller, scraper, cylinder and vacuum pump.
Common lubricant performance requirement for wet end and dry end
- Water separation
- Antifriction and antiwear
- Anticorrosion and antirust
- Low volatility
Lubrication system for paper mill
The physical location or type of machine often makes this approach to lubrication impossible. As a result, production cycles, machine availability and manpower availability dictate the intervals at which machinery is lubricated which is not optimal for the point requiring lubrication. Automatic lubrication systems are installed on machinery to address this problem.
There are several different types of automatic lubrication systems. The most commonly used systems are as follows.
- Single line parallel
- Dual-line parallel
- Single line progressive
- Mist lubrication
- Multi-port direct lubricators
Need of oil analysis
The main paper manufacturing machine is a system of rollers, motors, and drives, so paper machine bowsers, bearings, hydraulics, and motor bearings should all be considered for oil analysis. These machines generally run clean, so one of the big issues is water ingression, due to the wet and humid environment of the paper mill. Viscosity, oxidation and acidity are important because they indicate and affect the potential for corrosion and film strength. Particle count is an important parameter due to the tight clearances in several of these valve filtration systems. Similarly, ferrous particle count can catch any possible wear, especially on geared systems at the back end of the machine. Supporting equipment includes gear drives, conveyor belts, stackers and grease bearings which mostly rely on grease hence measurement of grease penetration and consistency is required to check on a scheduled basis. These machines require consistent monitoring, and most mills perform monthly testing.
References and further reading
Cover image credits: Chris Allen/Fourdrinier paper machine, Frogmore Mills/
Harshvardhan Singh works as a Senior Service Engineer at a mining firm in India. He is currently working into oil analysis field. Has worked in the filed of tribology and lubrication and loves to write about the same.