Board machines

Another development is the Inverform process and its more modem version, the Bel Bond (Fig. 8). In the Inverform unit, several pHes are formed on top of each other by consecutive, twin-wke forming units above a long carrying fabric. The Inverform process also is used for the forming of paper grades and is capable of moderately high speeds. Other versions of board machines involve mini-Fourdriniers and/or twin wkes which are placed on top of a carrying fabric. 

Dry Form Process, The advent of the pressurized refiner led eventually to dry-formed wood fiberboards because high-consistency pulps can be produced. Green wood chips contain equal parts water and dry wood matter and can be pressure refined in saturated steam with little change in moisture content. Fiber from the refiner at fifty percent consistency can be made fluffy and readily suspendable in air. This enables vapor-phase dewatering in hot-air driers to produce a fine, dry fiber that can be formed in air and dry pressed. Without liquid-phase dewatering, solubles formed by the steam cooking of chips remain in the fiber furnish going to the board machine. 

Used for some rigid boxes, e.g. solid board enema boxes where the slight odour and high chlorine content are of little importance. Fibre is provided by straw and is made on a conventional board machine. Lining is achieved by pasting paper, of... 

Paper/board produced Fine paper machines generally have a greater need for retention than drainage, whereas board machines are the opposite. Newsprint/LWC/ SC machines require both retention and drainage. Tissue machines are notoriously difficult in terms of applying retention aids due to the low basis weights produced and effects on cylinder adhesion etc. 

All aspects of the stock flow system and the paper or board machine equipment will have an effect on the retention and drainage aid. 

Depending on the paper/board machine, the grade(s) produced and the chemistries involved, choice of retention/drainage aid may be limited by other factors - sheet formation requirements, sheet porosity specifications, sheet linting propensity, sheet bulk density, etc. Due care is required to maintain sheet quality at aU times. 

Paperboard This mill produces board for tubes, eores, furniture backing and roll covering on a 160 t/day 7-vat cyUnder-board machine. The system pH is neutral, and the temperature is 60°C. Water systans in this mill are nearly 100% closed. Additives used in the wet-end include alum, rosin size, cationic retention aid and various dyes. Poor foam control results in high entrained air measuronents in the vats (>6%) and... 

Rosin is a non-reaclive prodnct and is retained on the anionic fibre by either attaching itself to a cationic source in the case of rosin soap, or anionic rosin emulsion or by having a cationic surface charge. The main requirement is that it requires a source of aluminium species to form the actual sizing agent, aluminium rosinate. Rosin is normally modified with maleic anhydride or fumaric acid to increase its reactivity with aluminium species and improve its efficiency at higher pH. The method by which this aluminium species is formed and retained in the wet-end of the paper/ board machine differs between anionic and cationic rosin sizes. 

The two major types of synthetic size employed at the wet-end of a paper or board machine, AKD and ASA, will be dealt with jointly as in many respects the wet-end chemistry is the same. Individual problems related to tlie sizes thanselves will be dealt with later in this section. 

The following effects may be seen on a paper or board machine at these pH values ... 

If the build-up situation occurs, it can be easily characterised in a paper or board machine by a series of symptoms ... 

An example of a mill experiencing slow cure was a board machine converting to alkaline size. It ran for some time with AKD size, before converting to ASA, as the board did not cure quickly enough. 

Deposits are a natural result of problems at the wet-end of a paper or board machine. They are normally related to problems of retention of the sizing agent although they can be caused by incompatibility. They may occur in the stock approach system, on the wire, on the presses or in the drying section. Many of the reasons for deposits have already been covered, but this section will try to summarise them. 

Of course, multiple layer machines make wet-end control more complex. The need to focus on major layers to achieve the best possible payback is obvious. To find out the best online control strategy for a certain board machine all aspects of the process should be analysed. 

One application area between 1962 and 1979 that has an impact on today s problems is the manufacture of wall-board. Wallboard was prepared by sandwiching a gypsum mixture between heavy papers. A typical core consists of gypsum, starch, potash, a pulp slurry, an asphalt or rosin-size emulsion, and gauging water (i.e., a colloidal silica plus polymer binder system). The mixture is blended together and placed between liners made on liner-board machines. 

There are two commercial installations of Condebelt dryers on board machines. According to recent reports [75,76], the minor operational issues have been resolved with modified design and expected improvements in product properties are realized. 

The systems for primary fiber preparation are less complex than a recovered paper processing system due to the much lower level of impurities and contamination in virgin fiber pulps. Depending on the paper type produced and the availability of the various fiber types, a paper machine, or especially a multi-layer board machine, may be fed by several different fiber qualities, each of them treated in separate lines with different process steps. 

Different types of rolls are used throughout the paper or board machine as well as in off-line machines and fulfill a variety of functions ... 

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