Paper making and fiber liberation – Apparatus – Running or indefinite length product forming and/or treating...
Patent
1994-10-24
1997-01-28
Hastings, Karen M.
Paper making and fiber liberation
Apparatus
Running or indefinite length product forming and/or treating...
162902, 162903, 139383A, D21F 712
Patent
active
055974503
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to fabrics intended for use in the manufacture of paper and like products, in which hollow monofilaments replace at least a portion of the wefts, also known as cross-machine directions strands. The invention is particularly applicable to paper machine dryer fabrics.
DESCRIPTION OF THE PRIOR ART
The primary function of a dryer fabric is to hold the paper web in contact with the heated surfaces of the dryer cylinders. This increases the efficiency of heat transfer and improves the flatness of the paper.
An important property of dryer fabrics intended for use in modern, high-speed paper making machines is low permeability to air flow. Dryer fabrics must have low air permeabilities so as to prevent sheet flutter and, ultimately, breakage of the sheet (as documented by Race, Wheeldon, et al. in TAPPI, vol. 51, no. 7, July 1968). Low air permeability values may be considered to be those in the range of 127 cm.sup.3 /cm.sup.2 .multidot.s (250 ft.sup.3 /min/ft.sup.2) or below. It is also desirable that the air permeability of the fabric be constant throughout both the fabric itself, and its operational life.
It is difficult to obtain low air permeabilities in woven dryer fabrics when solid monofilaments are used as the weft strands. Manufacturers of dryer fabrics have thus traditionally resorted to incorporating spun yarns, multifilament yarns or plied monofilaments in order to obtain low air permeabilities in conventional dryer fabric designs. These types of yarns, however, make it difficult to accurately control fabric air permeability during manufacture. They also allow foreign matter to become entrapped in the fabric, which changes the air permeability of the fabric throughout its life on the paper making machine. Trapped contaminants are usually distributed unevenly in the fabric, and will cause uneven drying of the paper web. The use of spun yarns, multifilament yarns, or plied monofilaments in dryer fabrics also reduces the efficiency with which water is evaporated out of the paper web, because water tends to condense and be retained within such yarns.
Another method of lowering fabric air permeability is to use machine direction strands that are essentially rectangular in cross-section. Such a method is disclosed by Buchanan et al. in U.S. Pat. No. 4,290,209. This patent also discloses the use of shaped or hollow monofilaments as weft strands to further reduce dryer fabric air permeability. However, it does not teach the critical physical parameters required for the hollow monofilaments, such as strand diameter, or solidity of cross-sectional area. No data is disclosed as to the effectiveness of hollow monofilament weft strands in reducing fabric air permeability.
Goetemann, et al., in U.S. Pat. No. 4,251,588 teach the use of hollow monofilaments to improve dimensional stability and flex life in paper machine clothing. The range of void fractions in the yarn cross-sectional area disclosed is from 0.03 to 0.15 (3% to 15%), or a range of solidities of from 97% to 85%. Solidities less than 85% were not recommended because such monofilaments would flatten from a circular cross section to a void-free filament. Goetmann et al. also teach that conventional techniques may be used to weave these hollow monofilaments into papermaking fabrics without collapsing them. No consideration is given to any interrelationship between the strand diameter of the hollow monofilament, its solidity, and the space available within the woven structure to accommodate the yarns. The use of these hollow monofilaments for the purpose of reducing fabric air permeability is not taught.
It is also difficult to obtain low fabric air permeabilities in spiral fabrics. These fabrics are assembled from a multiplicity of helical coils which are intermeshed and connected together in a hinged relationship by hinge yarns, substantially as described by Kerber in DE 2,419,751, Leuvelink in U.S. Pat. No. 4,345,730, and Dawes in U.S. Pat. No. 4,481,079. The air permeability of these fabrics is typically
REFERENCES:
patent: 4251588 (1981-02-01), Goetemann et al.
patent: 4290209 (1981-09-01), Buchanan et al.
patent: 4345730 (1982-09-01), Leuvelink
patent: 4359501 (1982-11-01), Ditullio
patent: 4481079 (1984-11-01), Dawes
patent: 4567007 (1986-01-01), Gauthier
patent: 4632716 (1986-12-01), Smith
patent: 4755420 (1988-07-01), Baker
patent: 4820571 (1989-04-01), Searfass
patent: 5103874 (1992-04-01), Lee
patent: 5368696 (1994-11-01), Cunnane, III et al.
ASTM Designation: D737-75 "Standard Test Method for Air Permeability of Textile Fabrics".
TAPPI, Jul. 1968, vol. 51, No. 7 "Air Movement Induced by Felts and Fabrics and Its Ventilation Effect on Dryer Pockets".
Baker Samuel
Harrison James
Hastings Karen M.
JWI Ltd
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