The present invention relates to a papermakers dryer fabric adapted to carry a paper web on one fabric surface thereof and woven from machine direction and cross machine direction yarns wherein the yarns cross over each other to create void volume both inside the structure of the fabric and at the surfaces of the fabric.
Generally speaking papermaking machines are made from up to three sections, namely forming, pressing and drying sections. By the time the paper web enters the drying section from the pressing section, as much as fifty percent of the water has been removed from the paper web. The remaining water removal is then completed in the dryer section. Here the paper web is carried by dryer fabrics transferring the paper web in succession from one to another of the rotating surfaces of sections of steel cylinders arranged along the length of the machine which are heated by high pressure steam.
The so far known papermakers dryer fabrics have the disadvantage that contaminants from the paper making process become trapped within the fabric structure. This reduces the permeability and effectiveness of the fabric: both sheet runnability and tail feeding are adversely affected due to reduced transmission of under pressure through the fabric to the paper web. Drying efficiency and uniformity of drying are adversely affected due to contaminants making the fabric surface uneven, or the lower permeability reduces ventilation and extraction of air/water vapour during the drying process.
High pressure water showers can be used to loosen the contaminants and remove them within the flow of water to a “save all” tray or by suction at an extraction zone. If the fabric is dirty then the papermaker can increase the water pressure. However too much water to clean the fabric will cause the fabric to run wet causing uneven moisture profiles and wet streaks in the paper. The removal of the water and contaminants is also difficult because the water and contaminants are held within the voids of the fabric structure.
It is therefore an object of the present invention to provide a papermakers dryer fabric where both excess water and suspended contaminants should no longer be trapped in the voids inside the structure of the fabric and the contaminants are more exposed to the fabric surfaces for ease of cleaning. The overall contamination of the fabric should therefore be significantly reduced.
In accordance with the invention, this object is satisfied by the provision that the machine direction yarns and the cross machine direction yarns are interlaced so that virtually all of the void volume is exposed to the paper web carrying and non-paper web carrying surfaces.
This will result in the effect that the water and suspended solids will be in the surface void volume areas facilitating easier removal to a save all tray or by extraction. Furthermore, excess water and suspended contaminants will no longer be trapped in void volume inside the structure of the fabric resulting in a higher dryness of the dryer fabric and therefore also in the paper web.
In a first preferred embodiment, if using a machine direction yarn of 0.67 mm wide, the internal void volume beneath at least one layer of a machine direction yarn and inside the structures ranges from 0 to 0.9 mm3, preferably from 0 to 0.5 mm3 and most preferably from 0 to 0.05 mm3 and/or from 0 to 3,000 mm3/100 mm2 and preferably from 0 to 250 mm3/100 mm2. The void volume inside the structure of the fabric is per definition defined as a void between machine direction yarns, cross machine direction yarns or both machine direction and cross machine direction yarns where there is space between two or more surfaces. These voids can be completely enclosed by one or more surfaces resp. planes or have an opening between these surfaces, where the said opening is narrower than the length of the shortest surface in the void.
In a second preferred embodiment, the void volume at the surfaces of the fabric ranges from 0 to 10,000 mm3/100 mm2 and preferably from 2,500 to 10,000 mm3/100 mm2. The void volume at the surfaces of the fabric is per definition defined as non-structural void between machine direction yarns, cross machine direction yarns or both machine direction and cross machine direction yarns where there is space between two or more surfaces. These voids have an opening between these surfaces, where the said opening is wider than the longest surface of the void.
Additionally, the total void volume as defined by addition of the structural and surface void volumes ranges preferably from 100 mm3/100 mm2 to 3,500 mm3/100 mm2.
Moreover, in another preferred embodiment of the present invention, at least one weave pattern of machine direction warp yarns with minimal yarn float lengths is woven side by side.
The cross machine direction yarns form preferably at least one plain weave with a single weft in the same shed.
Alternatively, the cross machine direction yarns form at least one plain weave with multiple wefts in the same shed. Thereby, the void volume inside the structure of the fabric can range from 0 to 0.9 mm3, preferably from 0 to 0.5 mm3, and most preferably from 0 to 0.15 mm3 for a path of warp yarn or a stacked warp yarn segment and/or the void volume at the surfaces of the fabric can range from 0.5 to 1.5 mm3 and preferably from 0.75 to 1.0 mm3. In summary, the void volume at the surfaces of the fabric and the void volume inside the structure of the fabric have preferably a ratio between 1:1 and 30:1 and preferably between 4:1 and 25:1.
And in another alternative, the machine direction yarns are vertically stacked in at least two systems.
Furthermore, in order to fill the void volume inside the structure of the fabric the dryer fabric preferably uses additional yarns with a yarn diameter between 0.10 and 0.40 mm.
In addition, the yarns possess preferably a high crimp level in the range of up to two times the vertical dimension of the warp yarn.
The yarns can also possess a high warp density, where the warp is not vertically stacked, by increasing the warp cover above 120%. The warp cover for two layers would then be 240%, whereas for three layers 360%. The warp cover is by definition the total number of ends per given width multiplied by the cross sectional width of the warp yarn, expressed as a percentage of the theoretical maximum attainable surface warp cover.
Aiming to increase crimp interchange between warp and weft to produce a denser fabric, the yarns possess preferably a high tension level due to crimp interchange from the effect of the heat setting machine of above 5 kg/cm.
Additionally, the yarns possess preferably a melting point below the material employed in a comparable parent fabric. The lower melting point material will flow into the interstices of the fabric structure while reducing the void volume inside the structure of the fabric. The melting point of the yarns is preferably between 90 and 240° C. This range is between the lowest running temperature of the fabric on the paper machine and the melting point of polyester.
The yarns can also be profiled, either singularly or in multiples, to fill the void volume inside the structure of the fabric.
And finally, preformed castellated wefts can be preferably used to fill the internal shape of the weave pattern.
In order that the present invention may be more readily understood several preferred embodiments of the papermakers dryer fabric structure according to the invention will now be described by way of example with reference to the accompanying drawings wherein:
The following
The void volume V.T inside the structure T of the fabric C ranges from 0 to 0.9 mm3, preferably from 0 to 0.5 mm3, and most preferably from 0 to 0.15 mm3 for a path of warp yarn or a stacked warp yarn segment and/or the void volume V.U1, V.U2 at the surfaces of the fabric C ranges from 0.5 to 1.5 mm3 and preferably from 0.75 to 1.0 mm3. The calculated value for the shown fabric C are: calculated void volume V.T inside the structure T of the fabric C of 0.023 mm3 and calculated void volume V.U1, V.U2 at the surfaces of the fabric C of 0.927 mm3.
Furthermore, the void volume V.U1, V.U2 at the surfaces of the fabric C and the void volume V.T inside the structure T of the fabric C have a ratio between 1:1 and 30:1 and preferably between 4:1 and 25:1. The ratio for the shown fabric C has been calculated at 4.03:1.
In this shown example, the fabric C has 8,100 segments in 100×100 mm area, therefore the void volume V.T inside the structure T of the fabric C is 186.3 mm3/100 mm2 and void volume V.U1, V.U2 at the surfaces of the fabric C is 7,508 mm3/100 mm2.
The inventive fabrics C to G are additionally in common that internal void volume V.T beneath at least one layer of a machine direction yarn 1 and inside the structure T of the fabric C to G ranges from 0 to 0.9 mm3, preferably from 0 to 0.5 mm3 and most preferably from 0 to 0.05 mm3 and/or that the void volume V.T inside the structure T of the fabric C to G ranges from 0 to 3,000 mm3/100 mm2 and preferably from 0 to 250 mm3/100 mm2. The void volume V.U1, V.U2 at the surfaces U.1, U.2 of the fabric C to G ranges from 0 to 10,000 mm3/100 mm2 and preferably from 2,500 to 10,000 mm3/100 mm2.
The additional weft yarns 2 have a yarn diameter between 0.10 and 0.40 mm in order to fill the void volume V.T inside the structure T of the fabric C to G and have a high crimp level in the range of up to two times the vertical dimension of the warp yarn 1. Furthermore, the yarns 1 possess a high warp density, where the warp is not vertically stacked, by increasing the warp cover above 120% and possess a high tension level due to crimp interchange from the effect of the heat setting machine of above 5 kg/cm. Thereby, the yarns possess a melting point below the material used in a comparable parent fabric, which is between 90 and 240° C.
Number | Date | Country | Kind |
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10 2006 022 235 | May 2006 | DE | national |
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