Equipment Description:Press Fabrics
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The Press fabric, or Felt, is used to carry the sheet through the Press Section of the paper machine. It is also important in the process of water removal from the sheet. It has to be capable of carrying high volumes of water on fast machines, therefore many of the fabric structures and materials are complex and multi functional in their design. Large void volumes are needed in the fabric structure which have to be resilient enough to withstand going through the press nips repeatedly, then able to spring out again to carry away the water. It is important to keep the felt clean and free from fines which can block up the voids reducing felt efficiency. This is done using high pressure needle jet sprays. Fabrics are either seamless or seamed. Seam technology has advanced rapidly so that many mills opt for seamed felts which are quicker to change, therefore reducing machine downtime. Early seams used to leave a mark on the sheet which was unacceptable on lighter weight grades. |
Nip Pressure: This is the loading between two rolls. Edge guides: These units control the run of the felt to make sure they run straight. Shoe Press: This is an extended nip press. Draw: This is the tension between each section of the paper machine. Roll hardness: Press rolls can have different hardness covers. |
 Examples of Press Fabric designs. |
Present day fabrics use fibres in monofilament and multifilament form which have evolved to give the batt-on-mesh structures, so-called because the base layer resembled a mesh. Single layer or multi-layer structures can be used, depending on the press position, also, the type of batt can be chosen, for example, a fine batt for a top surface felt avoiding any sheet marking. Because the yarns are interwoven, this used to cause ‘knuckles’ which were prone to sheet marking, so alternate base constructions were developed where a grid of warp(MD) and shute(CD) yarns were laid perpendicular to each other without any interweaving. These are called Planar press fabrics and because they offered more top side support the problem of sheet marking was virtually eliminated. Different types of base constructions can then be needled together to form multilayer fabrics. The advantage of these fabrics is the high void volume in the non-compressible part of the felt. This means that in some cases the felt can carry all the water expressed from the sheet as it exits the nip. |
Press Fabrics was also mentioned in the following Fact Sheets (34):
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