Why do spandex fabrics need to be pre setting? 

The main characteristics of spandex fabric

1. Spandex has very high elasticity. Generally, products do not use 100% polyurethane, and are often mixed in a ratio of 5-30% in fabrics. The resulting spandex fabrics have a comfortable elasticity of 15% -45%.

2. Spandex is usually not used alone. Spandex fabrics are often made of composite yarns, such as spandex as the core and other fibers (such as nylon, polyester, etc.) as the outer layer to make core spun yarn elastic fabrics. It has good adaptability to the body and is very suitable for making tight fitting clothes without pressure.

3. The appearance style and wearability of spandex elastic fabric are similar to those of similar products wrapped in outer fiber fabrics.

In the dyeing and finishing process of spandex, improper craftsmanship can cause problems such as loss of elasticity and wrinkling. So the dyeing and finishing of spandex must focus on these two major issues and adjust the process appropriately to meet customer requirements.

Due to the use of silicone oil lubricants and other additives in the weaving process of spandex to improve its spinning performance. These additives will naturally degrade over time, causing the fibers to turn yellow and the elasticity of the fabric to decrease. Meanwhile, fabrics containing spandex will form a "cold setting" during long-term storage, resulting in permanent wrinkles that cannot be eliminated during post-processing. So, in addition to reducing storage time, it is more important to immediately open and roll spandex knitted fabrics flat after weaving to avoid the opportunity for creases.

Knitted fabrics containing spandex have certain internal stresses formed during weaving due to the spandex inside the fibers after being taken off the machine. So, the purpose of preheating and shaping is to eliminate stress through the process of fabric tension and relaxation, make the size of the fabric fully stable, and eliminate permanent creases in the fabric during post-processing. If the coil formed during cylindrical weaving has the second coil on the left side of the first coil, then the feeding direction for the predetermined shape should also be 7% to 10% (relative width) ahead of the left side, so as to maintain consistency with the twist of the finished

Presetting

Therefore, the optimal predetermined shape scheme is:

Relaxation → Steaming (hot rolling water at 80 ℃ without steaming conditions) → Superfeeding——preshaping → (pre-treatment before entering the fabric → shaping → printing).

Most dyeing factories, due to the lack of steaming equipment, directly roll water into the shaping process. When the temperature is below 10 degrees Celsius (in winter), creases are easily formed on the fabric. The fabric will achieve satisfactory results by relaxing it in a steamer and then shaping it. The optimal heat setting temperature is between 185 and 190 ℃, and the setting time is between 45 and 60 seconds. If the setting temperature is too high, it will cause the fabric to turn yellow.

At the same time, if impurities and additives on the fabric are partially degraded due to the influence of baking, once their degradation substances migrate to the inside of the fibers and produce spots, these spots are difficult to remove in the subsequent processing. During dyeing, the oil spots inside the fibers will first absorb the dye, and during the insulation transfer process, they will not be able to run out in time and form deep spots. Even if they are stripped and re dyed, they will still remain on the fabric. Therefore, it is important to avoid high temperature pre shaping above 195 ℃ (note: the softening temperature of spandex is about 200 ° C or above), which will also affect the firmness of spandex. The width during shaping is 10% to 15% wider than that of the raw fabric, allowing for sufficient recovery during pre-treatment in the dyeing tank and completely eliminating internal stress in the fabric.

Pretreatment

Knitted fabrics that require printing also need to undergo pre-treatment to remove cotton stains, impurities, oil and wax, increase the whiteness of the fabric, and improve its wetting and water absorption properties, thereby achieving good printing results. Pre treatment processes can be divided into two categories, namely traditional alkali treatment and modern biological enzyme treatment. Biological enzyme treatment meets the requirements of environmentally friendly processing, but currently it is difficult for fabrics with high requirements for whitening or bleaching to meet the requirements. Therefore, conventional alkali treatment processes are still widely used. It is necessary to gradually cool down the water washing of spandex containing fabrics after pre-treatment to prevent sudden cooling from causing fiber shrinkage and creasing.

Requirements for semi-finished product shaping

The shaping of semi-finished products must be carried out according to the original design process. Generally speaking, the shaping width is about 3% to 5% wider than the finished product width, and the longitudinal shrinkage rate is controlled within the required shrinkage rate of the finished product. The elongation of the sticker is equal to the magnification ratio during the drawing process. If it is a single-sided knitted fabric, it is also necessary to control the blade cutting edge and optimal twist to reduce the printing weft deviation caused by twist. The width of the edge of the slurry is about lcm, with a break in the slurry. It is not advisable to be too thin or too thick. If it is too thin, it will not have the desired effect, and if it is too thick, it will affect the printing effect near the edge of the fabric. Paint printing, due to the solid paste remaining on the fabric surface, will relatively increase the square meter weight of the fabric. Therefore, under the conditions of width and shrinkage, the weight can be slightly lighter.

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