Equipment Description:Forming fabrics
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The Forming Fabric on paper machine plays a very important part in the forming of the fibre mat into a uniform and consistent web of fibres. The Forming Fabric is a finely woven fabric designed to support the fibrous suspension and assist with the drainage of the stock suspension. They are classified by surface structure, often 2 or 3 layers, and by open area. They normally have a finer mesh on the top of the structure with a coarse supportive mesh on the non-paper side. During the forming process the Forming Fabric allows the water to pass through the voids between the filament weaves of the wire while the fibre forms a mat on top of the Forming Fabric. The Forming fabric is a continuous belt which not only allows the water to be drained evenly through the fabric while forming a fibrous mat, it also transfers the web over the forming area (Forming Table) as part of the continuous paper making process. The Forming fabric design will influence the sheet formation. The forming fabric open area, the number of layers used in the fabric structure and the size of the filaments used will determine the drainage rate and the size of the fibrous and filler particles which are maintained within the sheet (Web) structure (Retention). Forming Fabrics require good continuous or semi continuous cleaning to maintain their drainage properties Traditionally Forming Fabrics were made from Bronze wire, interwoven to the correct mesh dependant on the grade to be produced, now days the Forming Fabrics are made from a combination of Polyester and other synthetic materials interwoven into single double and triple layered fabrics. |
Forming Board – is the leading forming unit under the forming fabric closest to the slice. The stock jet velocity, the impingement angle and the position of the impingement onto the forming board will determine the water removal and the activity produced at this point. Retention is the amount of fibrous and filler particles which are maintained within the sheet structure.. Formation is the degree of uniformity in which the fires and other ‘solids are dispersed within the sheet. Dandy Rolls and Top Former Units assist in the two-sided effect of dewatering on Fourdrinier Machines. High Pressure needle showers and low pressure high volume showers are critical for maintaining the Forming Fabrics clean. Forming Fabrics run as a continuous belt sometimes in excess of 2000 m/min and therefore the elasticity and cross directional stability properties of the Fabric are important features of Modern Fabrics. The natural drainage and forced drainage loads will cause significant pressure between the moving fabric and the stationary parts of the drainage table – high wear characterises are als |
 Image courtesy of CEPATEC AB |
Dewatering equipment is normally made from high density polythene (HDPE) or from ceramic material. The ceramic material has high wear resistance and is recommended for high speed applications. The impingement angle can be altered by moving either or both the horizontal slice or vertical slice on the Headbox / Flowbox. Vacuum levels are increased as the sheet moisture reduces down the length of the forming table. Vacuum requirements and dewater table layout vary by the grade being produced, the furnish and machine speed. Top Formers and Dandy Rolls are fitted to higher speed machines to even out the 1-sided dewatering effect and reduce the 2-sidedness of the sheet. Retention of the fibre and other ‘solids’ on the forming fabric is determined by the amount of fines and filler being used, the drainage efficiency of the forming table and the open area of the forming fabric. |
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