What are the classifications of forming fabrics on paper machines

The forming wire is an important part of the forming device on the paper machine. It is driven by a flat roller or other driving rollers, so that the pulp is continuously filtered and dehydrated on the net to form a wet paper layer, and the wet paper layer is transported to the flat roller. In addition, the papermaking wire also has the function of pulling the various rollers of the wire section to rotate. Therefore, the water filterability of the papermaking net must be suitable for papermaking requirements; it can withstand greater tension; it has the toughness and wear resistance of repeated bending and stretching.

There are two types of paper machine forming nets: metal nets (mainly copper nets, but also stainless steel nets) and plastic nets.

(1) Copper mesh

1. Specifications and wires of copper mesh

The specification of copper mesh refers to mesh (namely mesh number) and weft density. The mesh refers to the number of warp threads (copper wires running through the length of the copper mesh) or the number of meshes in a width of 25.4 mm (per inch) parallel to the weft. It is also expressed by the number of warp threads per cm width. Weft density refers to the number of weft wires (copper wires that penetrate the full width of the copper mesh across the width of the copper wire) per inch of width parallel to the warp wires. In general, the weft density is 60%-70% of the mesh. For paper machines with higher speeds, a mesh with a larger weft density is used.

The copper mesh is woven with steel wires arranged in warp and weft. The vertical direction is called the warp, and the horizontal direction is called the weft. During the operation of the copper mesh, the warp and weft have different forces and operating conditions, so the material composition of the warp and weft is also different. The fourdrinier wire paper machine requires the copper wire warp to have greater tension resistance, bending force, abrasion resistance, not easy to elongate, and acid and alkali resistance. Phosphor bronze (89%-96% copper, 4%-10% tin, 0.1%-0.5% phosphorus) is suitable for the required properties of warp threads, so it is usually used as a material for warp threads. Appropriate bendability and straightness are required for the weft. This is because when the copper mesh is in operation, the warp threads are subject to considerable tension, which causes the weft to bend and the width of the copper mesh tends to shrink. If the warp thread tension is not uniform, wrinkles will occur, so copper wire thicker than the warp thread must be used as the weft thread. Brass (65% copper, 35% zinc) has the mechanical properties required by the weft, so the weft is generally made of brass.

2. Types of copper mesh

Copper nets can generally be divided into three types: single-woven nets, double-wire nets, and twisted nets.

(1) Single woven net (single wire net). There are plain weave nets and twill nets in single woven nets. The single weave plain weave is cross-woven with a single warp thread and a single weft thread, that is, two adjacent warp threads alternately pass through the upper and lower parts of the same weft thread, and the warp threads have the same spacing. The difference from the plain weave net is that the warp and weft threads of the twill weave are cross-woven through two weft threads or the same side of the warp threads. This kind of net has obvious diagonal diagonal lines. Semi-twill weave (also called three-wire twill) is characterized in that the warp threads alternately pass through the lower part of two weft threads and the upper part of one weft thread. Because the contact surface of the twill wire and the semi-twill wire and the rollers of the paper machine is larger than that of the single woven wire, their service life is longer.

(2) There are two-wire network and three-wire network in the double-wire network, which are generally woven with finer copper wires. The double wire net is woven with two warp threads and one weft thread, and the two warp threads are staggered up and down in the direction of the weft thread. Three-wire nets (also called three-warp nets, three-woven nets, and woven nets) are cross-woven by three sets of warp threads and one weft thread. One of the warp threads in the middle of each group of warp threads and the two warp threads on both sides pass through the weft threads alternately. Multi-wire nets are mostly used to make fine and smooth paper.

(3) Twisted woven nets can be divided into single-twisted woven nets and double-twisted woven nets. Single-twist woven net refers to a net that uses twisted yarns only in the warp yarns. For double-twisted nets, both warp and weft are twisted. The twisted wire is formed by twisting 5 to 7 copper wires around the metal core wire. Twisted yarn has greater flexibility, so double-twisted webs have a longer life than single-twisted webs. Twisted nets are mostly used on pulp board and paperboard machines. The two ends of the copper mesh are welded into endless loops, and there is a difference between a pair of wire mesh and a wrong wire mesh in the welding of the warp. Wrong wire mesh refers to the copper mesh butt welded at both ends into an endless loop, and the warp threads are welded by staggering several. For example, the warp wires are welded by three staggered wires, which are called staggered three-wire meshes.

From the perspective of water filtration performance, the single weave plain weave is better, the single weave semi-twill weave is second, the single weave twill weave is the second, and the three warp weave is the worst. From the perspective of the net marks of the finished paper, the three-warp net and the twill net are lighter, followed by the single-woven net, and the twist-woven net is the worst.

(2) Plastic net

The formed plastic net is a monofilament net with uniform evenness, uniform orientation and stable physical properties. Formed plastic nets are classified according to the form of the nets when they leave the factory, and there are two types of ring nets and plug-in nets. Ring net, also known as endless net, is an endless loop net which is woven directly on the loom. Its characteristic is that the extension of the net and the reduction of the net width are very small during operation. It is suitable for large high-speed paper machines. The disadvantage is that the net length is fixed and cannot be adapted to paper machines with various net lengths. The weaving method of the plug-in net is exactly the same as that of the copper net. It can be produced by a copper net loom. First, it is woven into a single piece, and then the two ends of the net are plugged together in a certain width to form a plug-in net with joints. Its advantage is that it can adapt to complex and variable lengths, but its disadvantage is that it is easily deformed during operation.

There are many types of plastic mesh materials, such as polyester, polyethylene, polypropylene, polyamide, etc. Among them, polyester plastic mesh has the best performance.