WARP KNITTING
Warp Knitting Technology
* Warp knitting machines--needles are mounted collectively and rigidly in a horizontal metal bar (the needle bar that runs the full knitting width of the machine).
* Equally the yarn guides are also set rigidly into a horizontal metal bar (the guide bar that runs the full width of the machine).
Knitting Element Displacements
* The diagram summarizes the somewhat confusing displacements made by the guide bar. The front of the machine lies to the right of the diagram.
* The diagram shows the individual yarn guides set in a solid bar. The front-to-back movements are called swings. The first swing from front to back is followed by a lateral shog: the overlap, which wraps the yarn in the needle hook.
* The next movement is a swing from back to front followed by the underlap that may be from 0 to 8 needle spaces depending on the fabric structure being knitted.
Warp Knit Structure
* Warp knitting is defined as a stitch forming process in which the yarns are supplied to the knitting zone parallel to the selvage of the fabric, i.e. in the direction of the wales.
* In warp knitting, every knitting needle is supplied with at least one separate yarn.
* In order to connect the stitches to form a fabric, the yarns are deflected laterally between the needles.
* In this manner a knitting needle often draws the new yarn loop through the knitted loop formed by another end of yarn in the previous knitting cycle.
* A warp knitted structure is made up of two parts. The first is the stitch itself, which is formed by wrapping the yarn around the needle and drawing it through the previously knitted loop.
* This wrapping of the yarn is called an overlap. The diagram shows the path taken by the eyelet of one yarn guide traveling through the needle line, making a lateral overlap (shog) and making a return swing. This movement wraps the yarn around the needle ready for the knock-over displacement.
* The second part of stitch formation is the length of yarn linking together the stitches and this is termed the under-lap, which is formed by the lateral movement of
* The length of the under-lap is defined in terms of needle spaces.
* The longer the under-lap, the more it lies at right angles to the fabric length axis.
* The longer the under-lap for a given warp the greater the increase in lateral fabric stability, conversely a shorter under-lap reduces the width-wise stability and strength and increases the length-ways stability of the fabric.
* The length of the under-lap also influences the fabric weight.
* When knitting with a longer under-lap, more yarn has to be supplied to the knitting needles.
* The under-lap crosses and covers more wales on its way, with the result that the fabric becomes heavier, thicker and denser.
* Since the under-lap is connected to the root of the stitch, it causes a lateral displacement in the root of the stitch due to the warp tension.
* The reciprocating movements of the yarn, therefore, cause the stitch of each knitted course to incline in the same direction, alternately to the left and to the right.
* In order to control both the lateral and longitudinal properties, as well as to produce an improved fabric appearance with erect loops, a second set of yarns is usually employed. The second set is usually moved in the opposite direction to the first in order to help balance the lateral forces on the needles. The length of the under-lap need not necessarily be the same for both sets of yarns.
* Run-in: the yarn consumption during 480 knitted courses
* Rack: a working cycle of 480 knitted courses
* The run-in
………….is the yarn consumption for one rack.
* For a given machine with a given warp:
* A longer run-in produces bigger stitches and a generally slacker, looser fabric
* A shorter run-in produces smaller and tighter stitches
* With more than one guide bar the ratio of the amount of yarn fed from each warp is termed the run- in ratio
Lapping Diagrams
With the exception of the very simplest structures, it is too time consuming to represent warp knitted fabric using stitch or loop diagrams. For this reason two methods of fabric representation are commonly used.
Actual Guide Move
* This is the symbolic image of the technological process of lapping. This diagram can also be derived from a stitch chart by not drawing in the stitch legs but only the head and feet of the stitches.
Numerical Notation Related to Chain Link Height
* The numerical notation is best understood in relation to the mechanical system that is used to generate the lateral displacements (shogs) of the guide bars.
* If the pattern drive is on the right hand side of the machine, then the movement of the guide bar from the smallest chain link height (0) is only possible towards the left. With a chain link (1), the guide bar is moved to the left by one needle space (division), with a chain link (2) by two needle spaces, etc.
* If the pattern drive is on the right hand side of the machine, then the movement of the guide bar from the smallest chain link height (0) is only possible towards the left. With a chain link (1), the guide bar is moved to the left by one needle space (division), with a chain link (2) by two needle spaces, etc.
Chain Link Arrangement
* The guide bar is positioned with the follower roller on chain link 0'; it swings through, then moves to the left as the roller moves to chain link 1'. It swings back and returns to its starting position (chain link 0').
The chain should read: 0
1
In the opposite direction: 1
0
The smallest repeating unit (repeat) extends over one course: height repeat = 1 stitch, width repeat = 1 stitch.
Application
* Pillar stitch construction can be employed in the production of outerwear and for ribbed velour fabrics (corduroy). Even in these fabrics, the open pillar stitch is more popular as it provides the necessary longitudinal stability and runs freely. It is used in conjunction with the binding element in-lay' in laces and curtains, though always with a second guide bar.
Open and Closed St
* The stitch formed has an open or closed character according to the direction of the underlap and overlap motions.
The underlaps can be of differing magnitudes and directions:
If the underlap and overlap are in opposite directions then the stitch formed would have a closed character.
If the underlap and overlap are in the same direction, then the stitch formed will have an open character.
* The stitch is open when the feet do not cross and closed when the feet cross. The structure of a warp knitted fabric depends on the lapping motion of the guide bars, and therefore the structure could be represented by:
Drawing a stitch or stitch chart diagram, which takes time and is difficult
Lapping diagram
Yarn Threading Plan
* In warp knitting a yarn guide wraps the yarn around the needle hook, thus forming a loop. However, to form a fabric, the yarn guide must wrap the yarn around a different needle during the next course. The yarn guides, therefore, must be displaced laterally during knitting. Different warp knitted structures are produced by varying the magnitude of their lateral displacement. Therefore warp knitted structures can be described by noting the guide bar displacement.
* The actual guide bar motion consists of an underlap, swing-through, overlap and swing-back movement, and this motion is known as lapping.
*The yarn is wrapped around the needle hook due to the swing-through, overlap and swing-back movement of the yarn guide, and this forms a stitch. A warp knitted fabric is, therefore, made from stitches (overlap) and connecting underlaps.
The five basic overlap/under-lap variations:
All guide bar lapping movements are composed of one or more of the following lapping variations (Fig. next slide):
a). An overlap followed by an underlap in the opposite direction (closed lap) (Fig. a).
b). An overlap followed by an underlap in the same direction (open lap) (Fig. b).
c). Only overlaps and no underlaps (open laps) (Fig. c).
d). Only underlaps and no overlaps (laying-in) (Fig. d).
e). Neither overlaps nor underlaps (miss-lapping) (Fig. e).
Types of warp knitting machine:
In broad, there are two major types of warp knitting machine. Namely—
1. Tricot warp knitting machine
2. Raschel warp knitting machine.
Features of Tricot warp knitting machine:
The main features of a tricot warp knitting machine are mentioned below:-
1. In tricot machines were employed bearded needle with a pressure bar, but now a day compound needles are used.
2. In tricot machine gauge is expressed in needle per inch. It is finer gauge machine with 28-40 npi.
3. In tricot machine chain link numbering commences with 0, 1, 2, 3…….etc and generally there are 3 links per course.
4. The function of the sinkers are knocking over, holding down
and supporting the fabric loops
5. The fabric is drawn away towards the batching roller almost
at right angles to the needle bar.
6. The top warp beam supplies yarn to the front guide bar and
the bottom warp beam supplies yarn to the back guide bar.
7. The guide bars are numbered from the back towards the front
of the machine because of this threading sequence.
8. The maximum number of warp beams and guide bars are.
The main features of raschel warp knitting machine are mentioned below:-
1. In this machine latch or compound needle is used.
2. Raschel machines have a gauge expressed in needles per 2
inch. For example in a 36 gauge raschel machine their will be
18 needles per inch. The most common gauges are 30, 40
and 56.
3. Their chain links are usually numbered in even numbers 0, 2,
4, 6………… etc generally two links per course.
4. Raschel sinkers only perform the function of holding down
the loops when the needles rise.
5. The fabric is drawn downwards from the needles almost
parallel to the needle bar at an angle of 120°-160° degrees
by a series of take down rollers.
6. The guide bars are threaded commencing with the middle
bars.
7. The guide bars are numbered from the front of the machine.
8. In a raschel machine there is accommodation for at least 4,
32-inch diameter beams or larger number of small diameter
pattern beams ( vary between 4 and 36)
Single Bar Structures
* A plain warp knitted structure is produced on a single needle bar. The resulting structures are known as single face fabrics. Rib and interlock warp knitted structures are produced on double needle bars, and these structures are known as double face fabrics.
* In single face structures (plain), stitches are visible on one side, known as the technical face, and on the other side (known as the technical back) only under-laps are visible.
Pillar Lap
* A pillar stitch (or chain stitch) is a stitch construction where lapping of a yarn guide takes place over the same needle.
* As there are no lateral connections between the neighboring wales, the stitches are only interconnected in the direction of the wales.
* Due to the absence of under-laps, a fabric is not created, only chains of disconnected wales.
* Single bar pillar lap is technically possible only on Raschel machines where the trick plate acts a knock-over bed.
* On a tricot machine the sinkers are unable to control the position of the old loop when there is no under-lap (pillar stitch) and so the knitting of pillar stitch on its own is impossible.
* Open or closed pillar stitches can be produced depending on the guide bar movement.
1 and 1 Lap (Tricot Lap)
* The laps are executed in alternate overlap and underlap motions on two defined needles.
This stitch creates a textile fabric as the underlaps connect both the courses and the wales.
The simplest of this group of structures is made between two adjacent needles.
Guide bar motions:
First course:
| Under 1 needle to the right | ↓(UL) |
| swing through | 1 |
| over 1 needle to the right | ↓(OL) |
| Swing Back | 0 |
Second course:
| Under 1 needle to the left | ↓ |
| swing through | 1 |
| over 1 needle to the left | ↓ |
| Swing Through | 2 |
Result:
Therefore, the chain link arrangement is:
1
0
1
2 closed 1 and 1 stitch
As a result of the underlaps, the diagonal sinker loops are formed.
These pull the stitch heads of each alternate row into the same direction.
2 and 1 Lap
Swing through → 1
swing back → 0
swing through →2
swing back → 3
swing through → 1
swing back → 0
3 and 1 Lap
Swing through → 1
swing back → 0
swing through → 3
swing back → 4
swing through → 0
swing back → 1
swing through → 4
swing back → 3
4 and 1 Lap
Swing through → 1
swing back → 0
swing through → 4
swing back → 5
Atlas Lap
The atlas construction differs in that the laps are continued over two or more courses in one direction and then return in the other direction to the point where they started.
Lapping movement
0-1/2-1/3-2/4-3/5-4/3-4/2-3/1-2/
Two guide Bar Fabric
The use of two guide bars gives a wider scope for patterning than is available with single guide bar fabrics, and these fabrics form the basis of the commercial trade, using continuous filament materials in most cases.
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