Warp KnitsOpen and Closed LapsTwo different lap forms are used in warp knitting, depending on the way the yarns are wrapped around theneedles to produce an overlap. When the overlap and the next underlap are made in the same direction, anopen lap (Diagram 5 a) is formed. If, however, the overlap and the following underlap are in opposition toone another, a closed lap (Diagram 5 b) is formed.The most common lap is the closed one, which is used in most warp knittedstructures. The open lap is used when special needs arise or when technicallimitations are imposed.Special attention must be paid to the overlap direction, as it affects the fabricproperties greatly.Diagram 5: Open and closed lap configurationsStitch DensityThe density of loops in the fabric is defined as the total number of loops in a square area. The area is usu-ally a square inch or centimetre and the density is obtained by multiplying the number of courses by thenumber of wales in that area. The number of wales per unit length is determined primarily by the numberof needles per unit length in the knitting machine. This factor is called the machine gauge and is measuredas the number of needles in one inch.Today all warp knitting machine gauges are measured in the same manner. Previously, however, theRaschel part of the industry used a length measure-ment of 2 inches. It is, therefore, of the utmost impor-tance to make sure, when discussing Raschel gauge, to which length measurement the gauge applies. Somewarp knitted structures have the tendency to shrink widthwise when leaving the knitting zone, so that thewale count in 1 inch will be greater than the number of needles in the same length. A locknit structure, forexample, knitted on 28 NPI (needles per inch), will measure in some cases 36 WPI (wales per inch), whentaken off the knitting machine.The count of fabric courses is determined by machine settings and knitting conditions. The loops can beknitted to be long - so that the fabric is slack, or small - so that the fabric is tight. If Diagram 2 is consid-ered as a section of fabric with an area of 1 square inch, then the course density is 5 courses per inch, waledensity is 5 wales per inch and the fabric stitch density is 25 loops per square inch. One has to rememberthat since the wale density is almost exclusively determined by machine gauge and fabric construction, theknitter can far more easily control the loop size, i.e. the course density. For this reason, in some parts of theindustry, the main fabric parameter considered is the course count
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