Warp Knit Basic Structure Jimmy K.C. Lam The Hong Kong Polytechnic University

Agenda Characteristics of Warp Knit Fabric Advantages of Warp Knit Fabric Disadvantages of Warp Knit Fabric Formation of Warp Knit Fabric Pattern notation and Guide Bar Movement Lapping movement Overlap only Underlap only overlap and underlap neither overlap nor underlap

Advantages of Warp Knit Fabric Dimensional Stability In general, warp knitted fabric are more stable than weft knitted fabric. By modifying its structure (by weft insertion), the warp knitted can be as good as woven fabric. Fabric Tightness The warp knitted fabrics are thinner than double knitted fabrics and the loops are smaller than double knitted fabric. Fabric appearance Most regular warp knitted fabrics give a nice, clean and balanced loops on surface. Normally the technical face and back for warp knitted are different.

Advantages (II) Yarn unroving Low Knitting cost Yarn cannot be unroved from a warp knit fabric from any edges; this is the clear difference between weft knit fabrics. Low Knitting cost The knitting cost is inversely proportional to the machine production speed

Disadvantages The warp knit is not suitable for apparel textile because of the following drawback: Raw Material Not all materials are suitable for warp knitting, especially those yarns with low yarn strength and irregular surface. Normally filament yarns are for warp knitting. Yarn count Only a fine yarn count (50-70 deniers for 2 guide bar) is used for warp knitting. The spun yarns are seldom made to these fine counts.

Disadvantages (II) Machine Stoppage Pattern preparation Fabric Faults Warp knitting machines must be stopped for changing warp beams and piecing the broken threads. These are normally take hours to complete Pattern preparation For a fancy structures using multi-bar, large amount of time and chain links have to prepare. For jacquard selection, it is only possible on electronic raschel machine. Fabric Faults One special feature on warp knitting machine is no stop motions is equipped. The modern machines today are installed with optical scanner to scan the fabric and stop the machine in case the faults occurred.

Disadvantages (III) Small orders DISCUSSION The warp knitted machines are built for mass production and is not suitable for small orders and frequently changes pattern. DISCUSSION Please explain why warp knitted fabrics are seldom used on apparel and fashion market.

Formation of warp knit fabrics Unlike the weft knitted fabrics which can be produced by even one single end of yarn for any number of courses and wales, the warp knitted fabrics required one warp yarn for a wale. The warp knitted fabric is formed by knitting the warp yarns on the adjacent needles course by course and intermesh the loops with the neighbouring yarns to form fabric.

From the diagrams below, it is clear that if a warp knit fabric required 1000 wales, then there must be 1000 warp yarns on the beam and slightly more than 1000 needles on machine

Pattern Control Unit The development of a pattern on warp knitting machine is a combination of colour yarn pattern on warp beams; warp thread setting on guide bars; and lapping movement of the guide bars For a simple single guide bar, single colour fabric, the structure is based on the lapping movement of the guide bar.

Guide Bar Shogging Mechanism The control of the lapping movement of guide bar is from the chain links on the pattern wheel

Chain Links Different Height of Links control the lapping movement

The guide bar movements There are three types of movements on the guide bar, namely forward and backward swing; lateral movement for overlaps; and lateral movement for underlaps The first one is controlled by the cam shaft, the next two are controlled by the chain links

Example Lapping movement of Half Tricot: 1-2/1-0 “1-2” indicates the guide bar is doing overlap from needle position “1” to “2” ie. From right to left for one needle “1-0” indicates the yarn is fed to the needles from the left to right for one needle space.

The last example only shows one warp thread on one guide bar for a repeat structure. When thousands of warp threads are on the same guide bar, and all the warp threads are doing the same movement, a knitted fabric is formed

Basic combination of overlap and underlaps All warp knit fabric structures are composed of both overlap and underlap

1) Overlap Only In this group, the guide bar only feed yarn to the same needle all the time. The result is that each needle knits a chain of stitches. Example: 1-0/01, known as pillar stitch A pillar stitch is not a fabric, but is commonly used with other lapping movements to form a fabric.

2) Underlaps Only Underlap alone cannot form into a fabric and is commonly used with other lapping movements. If a guide bar only made underlaps in a multi-guide structure, this guide bar is called inlay bar and the warp are called inlay yarn, which never form into loops but only “tie-in” at the back of the fabric.

3) Overlap with underlap When overlap and underlap are worked together, two type of fabrics can be formed The first one, when overlap and underlap are moving the same direction, a open lap fabric will be formed. The second one, when overlap and underlap are moving in opposite direction, closed lap will be produced

1 X 1 Open Lap Fabric 1 X 1 Closed Lap Fabric

4) Neither Overlap Nor Underlap This seems to be warp float in the fabric. The guide bars give no lateral movements for a few courses in the repeat, laying the warps straight in the fabric. For a multi guide bar fabric, it is used to hide colour warps at the back for a colour pattern.

5) Double Needle Overlaps Unlike the underlaps, which can go across many needles spaces; overlaps can be in form of one or two needles only However, double needle overlaps are not common because of the difficulty in the control of the warp tension and let off.

Discussion Write down the lapping movements, chain notations for the given loop diagrams