Paper-machine felts in paper production
In the forming, pressing and drying section of paper production, so-called paper-machine clothing or paper-machine felts, known as "PMFs" for short, are used. They are used in paper transport as well as production, and represent the only interface between the paper and the machine. The quality of the clothing is largely responsible for the quality of the end product. The wet felt is used in the pressing section, to press as much water as possible out of the paper pulp.
The drying section is clothed with drying felts. Here, pressure and heat remove the last of the water from the paper. The third and last felt is for the paper surface. The following diagram shows a forming, pressing and drying section during paper production:
The use of needle felts in a paper machine
High demands placed on paper-machine felts
High technological requirements are placed on paper machine felts. Very high tear strength in the longitudinal direction is essential, especially because of the high production speeds of paper machines, as is dimensional stability in both longitudinal and transverse directions. Moreover the felts should guarantee an efficient drying process without destroying the sensitive cellulose. Maximum evenness with regard to surface, thickness and weight as well as heat resistance, wear resistance, good fiber integration into the surface and a very smooth paper page round off the highly stringent requirements placed on a paper-machine felt. Here, the surface consistency plays a very decisive role where later evaluation of the paper's surface quality is concerned. In sort, the quality of the paper-machine felt contributes significantly to sophisticated paper quality - and simultaneously ensures that a high degree of productivity is guaranteed.
Fulfilment of all the technical requirements demands the use of a complex structure consisting of layers of fiber and base materials. Here, combinations of different materials and textile surfaces such as scrim, woven or knitted fabrics are used as a base material for the felt layers. With paper-machine felts this principally involves nonwovens whereby one felt layer after another is needled onto a very compact base material.
Where needling of paper-machine felts is concerned the main requirement is to keep damage to the base material down to the absolute minimum. For this, Groz-Beckert offers special needles that are optimally designed for needling paper-machine felts with regard to shape of working part, tip and barb geometry.
During the needling process - since the base material is subjected to very high tear forces, especially in the machine direction - the needle must not destroy the monofilaments aligned in the direction of the warp thread. This is where a special shape of working part, the so-called teardrop shape - is used especially frequently. In comparison to the standard working part, only one edge has barbs, while the other two are rounded.
The needle tip can also damage the base material, however. This is why the shape of the tip on the needle being used is also important. If the tip is too sharp, it can cut the base. If it is too round, this can result in snarling or even needle breakage because of the increased bending pressure.
The barb makes the difference
Also very important during the needling of paper-machine felts are the size, position and shape of barbs. Overly large barbs damage the base, but if the barb is too small, this has a detrimental effect on needling efficiency. As a consequence, the best option here is a needle with relatively small barbs, whereby the distance from barb to barb per edge is kept as short as possible. Fiber transport is thus effected by several barbs, without the base fabric being damaged. Moreover, the shorter the distance between the tip and the first barb, the shorter the time the needle remains inside the product, which in turn reduces the risk of base damage even further. Finally, three-dimensional rounded barbs, so-called HL barbs, are often used. In combination with a teardrop-shaped working-part cross-section, and depending on the alignment of the needle, it is possible to steer the attack points on the filaments of the base in such a way that base damage is minimised. In this regard, positioning of the working part in the needleboard plays a very decisive role.
As already mentioned, during production of felts for the paper industry even the tiniest dimensional deviations lead to sharply fluctuating results in the paper later on. For constant quality here, Groz-Beckert provides needles with especially strict tolerances.