carbon fiber reinforcement and architecture-(2)

Woven spread tow: Patented processes have been developed to spread 12 or 24k carbon fiber tow to about four or five times its original width to give products which can be woven, giving low areal weights. They show increased surface smoothness with less pronounced cross-over defects when prepregged and molded, due to fewer interlacing points, increased fiber floats and less crimp. Basically, there are two manufacturing techniques. One of the techniques uses an air knife, employing hot air to soften certain proprietary sizes, which allows easier spreading. Careful control of the fiber tension on either side of the air knife gives a fiber catenary permitting adequate spreading, operating up to 10 m/ min. The other process uses a wter flume with circulating hot water, which extracts some of the water based fiber size and permits easier spreading, operating up to 10 m/min. The water is continually replenished to avoid resin build-up and the spread fiber is then dried on an electrically heated drum. The spread fiber from either process can then be woven, normally utilizing a plain weave.
Oxeon AB, in their weaving process, utilize 6g/m2 polyester to stabilize the fabric.
Knitted fabrics: A knitted fabric is produced by interloping flexible yarns of one yarn set. The yarn is formed into loops and the newly formed loops are inter-meshed with previous knitted loops. Each loop in a knitted fabric is called a stitch. The knitted fabrics vary in complexity and are highly conformable and drapeable, available as a flat, tubular or sandwiched construction. The fabrics may be with or without stretch and with a closed or open structure. Fabrics are generally resistant to raveling at cut edges, thus simplifying cutting and handing operations. The technology is divided into weft and warp knitting. A course is a horizontal row of knitted stitches running widthwise and a wale is a vertical column of stitches running lengthwise, with the appearance of a ridge of raised threads. Each wale is associated with its own needle. Gage is used to express the closeness of the needles- the higher the gage number, finer the fabric.
Weft knitting: As the name implies, yarns run horizontally across the width of the fabric. The knitting needles form the loops in horizontal courses, with one loop built on top of the other, and are generally made from a single yarn. Weft knits can be made flat on a flat bed machine, or tubular on a circular knitting machine. Weft knits are basic loop structures and can be stretched in both directions. There are three basic types of weft knitting- plain, rib and purl.
Warp knitting: Warp knitting is a very flexible system and involves a set of yarns running parallel. The yarns form a vertical loop in one course and then move diagonally to the next wale to make a loop in the following course, zig-zagging from side to side along the length. Each stitch in a course is made by a different yarn and each needle is provided with at least one yarn via a yarn guide. Warp knitting will produce a flat fabric with straight edges and once the machine has been set up, will produce at a fast rate. The two primary types of warp knitting are tricot and Raschel.


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