THE ANGLE OF TWILL | FACTORS DETERMINING THE PROMINENCE OF TWILL WEAVES | COMPARISON OF THE FIRMNESS OF TWILLS

THE ANGLE OF TWILL:

The angle of twill is the angle between the diagonal twill line and an imaginary horizontal line or axis parallel to the weft. This angle is dependent on the ratio between the ends/inch and picks/inch in the cloth. When the warp ends/inch is equal to the weft picks/inch, the twill angle will be 45°. When the warp ends/inch exceeds the weft picks/inch the twill angle will be an obtuse angle i.e., >45° (high angle or steep twill). When the weft picks/inch exceeds the warp ends/inch, the twill angle will be an acute angle i.e., <45° (low angle or flat twill).

FACTORS DETERMINING THE PROMINENCE OF TWILL WEAVES:

The following factors determine the relative prominence of twill weaves

• Nature of the yarn
• Nature of the weave
• The warp and weft threads/inch, and
• The relative direction of twill and yarn twist

Nature of the yarn:

The fineness of yarn and the amount of twist given to it influence the prominece of the twill. A coarse yarn of lower twist produces a greater effect on the twill as compared to a fine yarn of higher twist. On the other hand doubled or ply yarns have a stronger effect on the twill as compared to single yarns.

Nature of the weave:

Twills with longer floats will give more prominence as compared to those with shorter floats. For example, a 3/1 twill will be more prominent as compared to a 2/2 twill. It is to be noted that an increase in float length has to be balanced by proportionately increasing the corresponding threads/inch.

The warp and weft threads/inch:

The twill prominence increases proportionately with the increase in warp and weft threads/inch.

The relative direction of twill and yarn twist:

Another important factor that influences the prominence of twill is the direction of twist in the yarn. When the direction of yarn twist is same as the twill direction, the prominence is reduced and when them direction of the the yarn twist is opposite to the twill direction, the prominence of the twill is increased. In other words a Z twill with Z twist yarn or an S twill with S twist yarn shows less prominence. On the other hand a Z twill with S twist yarn or S twill with Z twist yarn gives more prominence.

COMPARISON OF THE FIRMNESS OF TWILLS:

The firmness of a weave depends on the number of intersections. The greater the number of intersections, the better the firmness. On the other hand lesser the number of intersections, greater will be the cover of the cloth. This is due to the increase in the floats of the threads. Increase in the number of intersections restrict the thread density and vice versa.

The illustration in Fig. shows three cases in which an 8 thread repeat of twill weave is chosen.

Case 1: In this case shown at A, a floating weave of 4/4 twill is chosen.

Case 2: In this case shown at B, a combination of 3/2 and 1 /2 twill is chosen

Case 3: In this case shown at C, a combination of 3/1, 1/1 and 1/1 weave is chosen

The corresponding cross sections are shown at D, E and F, respectively. The distance between the dotted lines represent a distance equal to the diameter of a single thread. It can be seen that design A occupies a space of ten threads (D), design B occupies a space of twelve threads (E) , and design C occupies a space of fourteen threads (F). It can be seen that a spacing of 10 threads in G is occupied as against the spacing of 14 threads at F.

The following conclusion can be drawn on observing the interlacings in the three above cases:

Relative firmness of Twill Weaves

In case 1, the cloth will have the minimum level of firmness and maximum cover.

In case 2, the cloth will have firmness and cover intermediate between 1 and 2.

In case 3, the cloth will have maximum firmness and minimum cover.