Headbox forming section

Even if all stock components including broke were previously fine-screened, bundles and lumps can be aeated thereafter by deposition. Secondary stickies and pitch particles may also form in the paper machine system. Final screening is therefore done directly before the headbox with pressure saeens (see Section 4.2.3). In most cases the screen baskets are slotted. Due to their position in the process, the screens exhibit special characteristics, i. e. very low pulsation generation by using special rotary elements, polished surfaces to avoid deposits, high availability by simple design and special design to prevent air pockets [1]. 

In the wire section, a fiber web is formed out of the suspension supplied by the headbox. The kind and quality of suspension delivery from the headbox to the vdre has a strong impact on the quality of the paper web formed. Therefore headbox and tvire section - together -with the approach flow system - have always to be regarded as one unit. The main objectives of the vdre section are ... 

The Fourdrinier former is likely the most widely used forming method employed in modem papermaking. It has proven uniquely versatile in the variety of paper grades it can produce. Wire speeds up to 5000 feet per minute are possible with some lightweight grades. The following description follows the pulp flow fi-om the headbox to the couch roll just prior to the press section. A Fourdrinier wet end is shown in Figure 3. 

In the headbox, the pulp suspension is spread across the entire width of the web and passed onto the wire mesh at the correct speed. The sheet is formed as the water drains from the mesh and the fibers are fixed into their final orientation while still in the wet mat stage. In the pressing section, water is driven out of the wet mat by applying pressure to the web. The web enters the pressing section with a dry content of about 20 % which increases to 40-50 % as the web leaves this section of the machine. 

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