Warp knitting & Turfing

By Mr. Yong Kok Swee

Warp knit fabrics are manufactured by preparing the equivalent of warp beam containing several hundred ends. Each end passes through its own needle and is formed into lops which intersect with adjacent loops. Thus, a flat looped fabric is knitted using only “warp yarns” without the necessity of “filling yarns” being interwoven.

The two major types of warp knits are tricot and Raschel. Based on the number of yarns and guide bars used, tricot knits are identified as single（figure 1.3-d）, two bar（as in figure 1.9-a）, three and four bar tricots.

Figure 1.12 A Two-bar tricot

(a) A two-bar tricot            (b) Simple Raschel crohet knit      (c) Weft inserted warp structure                                            

Raschel knitting is suitable for making highly patterned, lacy, crocheted or specialty knits（figure 1.9-b）. In general, Raschel machines are used for the production of knit structures for industrial applications. For increased structural support in the filling direction, additional filling yarns can be inserted as shown in figure 1.9-c.

The knitting elements required for a warp knitting machine include:
 l        Needles arranged in one or more solid bar to function as a unit, called a needle bar;
 l        Yarn guides, one for each warp yarn, arranged in solid bars, one for each different
           warp, to function as a unit, called guide bar;
 l        Sinkers arranged in a solid bar to function as a unit, called a sinker bar;
 l        Presser bar functioning on spring beard needle bars only;
 l        Pattern chains or cams to control the side-to-side motion of guide bars;
 l        In Raschel knitting, over the self-acting latch needle, no pressure bar is needed; in its
           place, a latch guard and a faceplate, over which the old loops are “knocked-over” 
           are used.

Stitch formation on warp knit machines differs from weft knitting is that a complete course of loops is formed by one circle of the needle bar(s) rather than individually acting needles forming loops within a course. 

Tufting
A tufted fabric involves a system of yarns that are sewn into a fabric (primary backing) leaving yarn loops below the primary backing. Sewing is accomplished by a solid bar of needles extending the full width of the primary backing with each needle threaded with a yarn. Because the needles are spaced with a certain distance apart (called the needle gauge), the yarns will be aligned in the structure in precise vertical rows across the width (or horizontal direction) of the fabric. The rate of reciprocal up and down motions by the needle bar is referred to as “stitch per minute” or the speed of the needle bar in revolutions per minute (rpm). Spacing of these needle bar stitch is controlled by the feeding rate of the primary backing fabric resulting in a specified number of “stitch per inch” along the machine (or vertical) direction of the fabric. The primary backing fabric is supported by a bed plated which may be lowered or raised to control the depth of needle penetration through the fabric carrying the yarn loops to a desired distance below the primary fabric (the fabric is made upside down). The distance of the loops below the primary fabric constitutes the “pile height” of the yarn tufts. These surface tufts represent the face or use side of the fabric after the fabric is removed and finished.

Different type tufting elements are used to produce various surface effects. Fabrics may have surfaces of level loops, multi-level loops, level cut and uncut loops. These variations are made possible by the types of loopers, knives and yarn controls used. A uncut loop surface is produced as shown on figure 1.10-a with the loopers take yarns from the needles and hold the loop formed while the needles retract.

Figure 1.13 Tufting elements

       (a) loop pile                                (b) Cut pile

Notice that the looper is facing in the direction of fabric motion to allow the formed loops to move off the looper after forming. The cutted pile surface is produced by changing the type of looper with corresponding knives. As shown in Figure 1.10-b, the loops are formed on the looper and must progress through the knives and be cut free of the looper which points opposite the direction of fabric flow. In all cases of level loop surface, the feeding rate of yarns must be adjusted to accomplish the desired pile height, in conjunction with the bed plate setting and to obtain a secure (tight) backstitch between surface stitch.  

Bonding, Nonwoven fabric

Opinions vary as to the range of fabrics to be classified as nonwovens. In general, they can be defined as textile structures made directly from fibre rather yarn. These fabrics are normally made from continuous filaments or from fibre webs or batts strengthened by bonding using various techniques; these include adhesive bonding, mechanical interlocking by needling or fluid jet entanglement, thermal bonding and stitch bonding. The controversial areas are:

  1. ） Wet –laid fabric, containing wood pulp, in which the boundary with paper is not clear;

  2. ） Stitch-bonded fabrics which contain some yarn for bonding purposes;

  3. ） Needled fabrics containing reinforcing fabric.

There are two standard definitions:

Nonwovens are defined under ISO 9092:1988 as: A manufactured sheet, web or batt of directionally or randomly orientated fibres, bonds by friction, and/or adhesion, excluding paper. And products which are woven, knitted, tufted, stitch-bonded incorporating binding yarns or filaments, or felted by wet-milling, whether or not additionally needled. The fibres may be of natural or manufactured origin. They may be staple or continuous filaments or to formed in situ.

Nonwoven fabric is defined under ASTM D 1117-80 as : A textile structure produced by bonding or interlocking of fibres, or both, accomplished by mechanical, chemical, thermal, or solvent means and combinations thereof. Discussion: The term does not include paper or fabrics that are woven, knitted, or tufted.

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