After spinning and dyeing this is the longest and most complex process that weaves the yarns which then become fabrics.
Before the beam reaches the loom there are a few operations required to optimise the results of the process.
Warping prepares for weaving. It consists in winding a certain number of yarns, all of the same length and tension, around the beam - a cylinder fitted with flanges.
There are two warping techniques. Sectional warping is used for limited runs, especially for fancy warps where yarns are used with different colours, counts and twisting. It calls for great manual ability regardless of the fact it is not very productive but from which high quality beams are achieved that can already be loaded on the loom. Fractional warping is used for big runs, used for grey fabrics and fabrics with simple designs, with a limited colour ratio.
The purpose of sizing is to improve the resistance of warp yarns that are close to the mechanical stresses of weaving without creating problems for the following desizing, dyeing and finishing operations.
This is done using both natural and synthetic substances called "sizes". Preparing the sizes requires, besides a good dose of experience, a great deal of attention in dosing the ingredients in order to ensure the duration of the treatments. The warp yarns go through the tank containing the sizing product. They then go through drying cylinders to evaporate the water in the size leaving only the gelatine (or sticky) material.
When the yarns come out of the dryer they are separated by large bars or rods and then lastly through a "zig-zag" reducer comb that distributes them regularly at the height wanted before being wound around the loom beam.
This is the process where the warp yarns are threaded through the reeds of the warp guard, through the heddles and through the teeth of the comb. This is necessary when the characteristics of the beam, ready to be taken to the loom, are different from the one that is nearly finished. Threading is closely linked to the number and different evolutions of the threads inside the fabric.
Fabrics and products obtained almost always by weaving yarns or threads together in different ways.
The fabrics are divided into three macro classes:
- made with yarns: orthogonal fabrics, weaves and perforated fabrics.
- made with fibres: non woven (felts, needle punches).
- combinations: sewn and laminated fabrics.
Orthogonal fabrics, like those for shirts, are formed by the weaving of the warp and weft yarns, arranged crosswise.
The basic element for studying weaving is the principle exploited for forming the fabric.
The sequence of the operations has remained the same over the years, although loom mechanics have been modified enormously.
The parts that make up the loom are:
- the warp beam, consisting of a cylinder with two big flanges that prevent the yarn windings from dropping off at the ends, made according to the warping technology;
- the yarn holder roller, useful for arranging the warp on the horizontal work top and for keeping its tension constant during opening of the shedding motion (they can be fixed, swivel or swinging);
The reeds are fitted on the warp guard, metal surveillance devices that remain raised under normal working conditions. When a thread breaks the reed, which is no longer supported by the yarn, falls, it closes an electrical circuit and the loom stops.
The heddles are carried by aluminium frames that go by the name of healds and their job is to move the yarns up or down each time a weft is inserted. A heald has several heddles and a thread passes through each one. Moving, the heald controls movement of all the heddles. Since it is necessary for all the threads to move up and down at a certain frequency to make a weave, there are at least two healds on one loom.
The angle created by the series of raised yarns with the series of lowered yarns form the so called "warp opening" or “shedding motion”.
The width of the opening is such to allow the inserter element to pass through. This is one of the most important parts of a loom; in fact the construction characteristics of the machine and of the fabric to produce depend on it.
In the most common looms the inserter element can be:
- a projectile - a steel rod fitted with tweezers that, during insertion, holds the end of the weft and which, pushed violently, goes through the shedding motion at great speed (more than 200 km/h) in just one direction, sliding along guides
- two pairs of tweezers - "slides" in metal and synthetic resin fitted with special tweezers to hold the weft.
- a jet of water that, pushed out at high pressure from a nozzle located on one side, at the entrance to the opening, manages to drag the weft from one end to the other of the fabric at more than 300 km/hour;
- a jet of air that, coming out of several nozzles at a high pressure, pushes the weft at a very high speed into a guide that can be created in the combs.
Use of the shuttle requires preparation of the weft on bobbins (small diameter cylindrical packages with 20-30 grams of yarn) using weft winders. In all the other cases, the weft is unwound directly from big capacity packages (more than one kilo of yarn) or by means of feeders that modern machines are fitted with. The two different methods determine a structural difference of the edges of the fabric (selvedge). It is only with the shuttle, in fact, that the weft is inserted in both directions without being interrupted (real or good selvedges). In the other cases, after insertion, the weft is cut so its ends protrude from the fabric by about one centimetre and are simply tied by the selvedge threads (false selvedges). Or they can be folded in the next step by means of special hooks (tucked selvedges).
The comb is made of a series of suitably spaced thin reeds (teeth). The warp yarns coming from the heddles pass through the teeth of the comb following an established order: reed drawing-in Combining it with the reduction in teeth to a centimetre, the warp yarns are distributed evenly on the width that has been set, called "fabric width in the comb”.
The comb's job is to keep the yarns parallel and to bring the weft up. It is fixed to the breast beam on which the lowered yarns rest and the inserter element slides above them. When the breast beam is retracted and the heddles moved to form the opening or shedding motion, the weft inserter element makes the weft enter from one end to the other.
The principle of weaving
The warp yarns are made to pass above the yarn holder roller - through the reeds, heddles and comb - and wound on the front fabric roller. The warp pitch is formed by lifting and lowering the heddles, in which the weft yarn is inserted. After the weft has been inserted the comb knocks against the weft yarn, pushing it towards the fabric being made.
[Source: Cotonificio Albini]