Paper Making

Paper is made on a paper machine. Paper making involves refining various types of pulps (mechanical work to improve bonding tendencies) individually, and then blending them together in combination with specific additives. This stock is then screened and cleaned to remove any dirt and is diluted to very lean pulp slurry before being ejected onto a moving drainage medium, known as "the wire," Fourdrinier, or former, where a sheet of wet paper is formed. This wet paper is then pressed, dried, and calendered (pressed between smooth polished surfaces) to produce a smooth final product with a specified basis weight (weight per unit area), moisture content, caliper (thickness), smoothness, brightness, color, ash content, and many other properties. As the stock is pumped from the High Density (HD) storage tanks, it is diluted to 4% and refined. Paper machine refiners have rotating discs which fibrillate or "fluff" the fiber thereby giving it better bonding strength. The hardwood and softwood stocks are then blended together in a desired ratio in the blend chest and sent to the machine chest, the final source of stock for the paper machine. From here the stock is diluted with "white water" (filtrate which has drained through the forming section) and is sent to the headbox (a pressurized compartment with a wide orifice or slice lip which allows the stock to be ejected as a wide jet). The stock discharges from the headbox at the required speed onto the wire a wide porous conveyor belt consisting of synthetic woven material. At this stage the pulp is about 99 % water.

The wire mesh, which is commonly called "wire," has about 6,000 meshes per square inch. Constant side-to-side vibration of the wire causes the cellulose fiber to interlace. In this process much of the water falls through the wire, leaving a very wet sheet of pulp. It is further dried by suction which pulls water down through the wire into tanks below.

Paperboard is made in cylinder machines instead of on moving flat wire. Wire-covered cylinders revolve in vats containing fibers suspended in water. The fibers collect on the screen as the water flows through. By using a number of cylinders a multiply sheet is formed.

From the wire or from the wire covered cylinders the wet sheet of pulp passes to a belt of woolen felt which carries it through many pairs of heavy rollers. These rollers press out more water and the felt absorb much of the remaining moisture.

Now strong enough to sustain (выдержать) its own weight, the fast moving sheet of paper or paper-board leaves the woolen felt and enters the dry end of the paper machine. Here it travels through a long series of rollers, may be as short as 20 f. for cigarette papers, or as long as 350 f. for some types of wrapping papers. Paper moves over these rollers at high speed. Here nearly all the water is removed.

Bark Removal. Barker

The equipment designed to remove bark from the logs are called barkers. They are of two types: hydraulic and mechanical. Hydraulic barkers require the use of water to remove the bark from the tree. The water is pumped by high-pressure pumps into jet squirters which impinge on the surface of the tree or log and blast off the bark.

The hydraulic barker does a very efficient job with very little damage to the wood and is most suited to the barking of logs that are large in diameter and very long. This type of barker has therefore found application in operations where a lumber mill and a pulp mill are operating together or in other applications where treelength logs need to be handled.

 Mechanical barkers can be divided into major categories: those that handle a large number of logs at the same time and those that treat individual logs. The type that treats a large number of logs at a time is called a drum barker. The drum barker is a large cylinder open at both ends. Logs with bark on their surface are loaded into the higher end of the drum. The drum is rotated causing the logs to tumble over one another, creating a rubbing action that stripes the bark from the logs. The initial part of the drum barker may be closed to retain water, soften the bark, and remove it from the tree.

Next section may be open to allow the bark that is removed from the logs to fall out through the openings and be transported away. In some cases the drum barker will be partially submerged in water to keep the logs moist and to carry the bark away from the barker.

The individual-log mechanical barker has different varieties. The wood is carried through a ring of knives that rotates around the log stripping bark from its surface. This barker can also remove good fibers with the bark if the wood is soft. Therefore chains are used which will tangle in the bark and tear the bark off the surface of the log.

Sometimes rough-surfaced wheels are utilized that rub on the surface of the log.

In any case all mechanical barkers have two disadvantages: they remove good wood along with the bark if the wood is soft and they cannot remove all the bark if the bark is too hard. For this reason, barking operations function better if the wood is wet and green.

Pulping and Refining

All the operations necessary for preparing the fibers begin with pulping and proceed through refining, metering and blending to dilution and cleaning.

The pulping operation has to liberate the fibers from the raw material being used by the process. The fibers liberated from the wood by a pulpmill are formed into thick sheets of paper and bundled into bales. The pulp is not dried completely but retains about 25% moisture to prevent the fibers from bonding to one another or collapsing.

The machine used for pulping is called pulper. The pulper is equipped with rotating blades in the bottom which serve to break the pulp into individual fibers. It may be operated as a continuous pulper, vats and bales of pulp being added to maintain the desired volume and defibered stock being removed through the strainer plate in the bottom. However, the pulper is more likely to operate as s batch operation because it is used to different types of pulp and sends them to separate storage tanks.

The refining is carried out by the refiners. Modern mills use different types of refiners. The original refiner, the beater, had a rotating wheel or roll with bars that beat against bars in a stationary plate. In the conical refiner the roll has been replaced by a cone with bars on its surface. The stationary plate has been replaced by the second cone with bars on its inside surface. In the conical refiner, the stock is not carried through by a rotation of the roll but is forced through by a pump in a direction parallel to the axis of the cone. Actually the combined action of the rotation and pumping causes the fibers to follow a spiral route through the refiner. The continuous refiner requires the use of a pulper. The function of the pulper is to deliver defibered pulp in water suspension.

The disc refiner has two discs with raised bars on their facing surfaces. One of the discs rotates and the other is stationary. The stock is pumped in at the center of one of the discs and flows out between them. Some disc refiners are designed, with both discs driven in opposite directions, to increase the amount of action on the fibers. These refiners with both plates driven are called double disc which has one driven disc with bars on both sides sandwiched between two stationary discs.

ГРАММАТИЧЕСКИЕ ТАБЛИЦЫ

Таблица 1

Глагол “to be”

Таблица 2

Глагол “to have”

Таблица 3

Страдательный (пассивный) залог ( to be + Participle II )

Окончание табл. 3

Таблица 4

Причастия

Продолжение табл. 4

Окончание табл. 4

Таблица 5