Equipment Description:Headbox / Formers
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What is it for? The main function and objective of the Headbox is to distribute a suspension of stock evenly and uniformly on to the Forming Fabric – This is the most important part of papermaking. The headbox operational parameters will determine and influence many of the important end paper properties like formation, surface characteristics, sheet stability and strength. Jet Velocity and the jet speed relative to the Forming Fabric speed ([PL]Efflux ratio[/PL]) and the headbox consistency play a significant roll in the machine direction properties. The Cross directional properties ([PL]Profile[/PL]) are determined by the distribution of the stock across the width of the Headbox. There are many different types of Headbox namely hydraulic, pressurized air pad Headboxes, Open Headboxes, High Dilution Headboxes, Vat Formers, Gap Former Headboxes and Stratified Headboxes. |
Related terminology Headbox, Flowbox and Former are all terms used to describe the equipment used for distributing the fibre suspension onto the forming fabric. Distributor / Holey / Rectifier Rolls are used in air pad Headboxes to generate turbulence to assist formation. In Hydraulic Headboxes Turbulence and Micro Turbulence is generated by turbulence generators (step diffuser tubes). Formation is the evenness of the distribution of fibres within the sheet structure – this can be influenced by the Jet to wire ratio, the forming consistency, the amount of micro turbulence within the headbox, the jet angle out of the Headbox, and the ‘activity’ caused by the forming board and the dewatering foils. Web Stratification – is the creation of a multilayered product within a single sheet. |
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Some Technical Info The headbox consistency is normally around 1% Consistency, although on high dilution formers (Inclined Formers) the consistency can drop down to 0.01% consistency. The stock flow velocity in the tapered inlet header and approach flow pipe work and within the Headbox itself is critical to the formation. Any pressure pulsations within the Headbox will result in pulsation of stock exiting the slice – this will result in MD basis weight variability called barring. The Total Head Pressure within the headbox determines the velocity of the jet exiting the slice. Efflux ratio is the relationship between the wire speed and the jet velocity – this relationship will determine the fibre orientation within the sheet during the forming process, and therefore influence tensile ratio and MD / CD Strength properties. The tapered Inlet Header (Manifold) translates the flow into an evenly pressure distributed flow in the Machine direction. |
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