With the development and progress of knitting dyeing and finishing industrial technology, the finishing process of cylindrical knitted fabrics has become very mature. The purpose of post-finishing is to eliminate the shrinkage of the fabric caused by stretching during the dyeing process through post-finishing, and shrink it through drying to achieve the required physical indicators such as width, shrinkage, gram weight, and cloth surface flatness; the most important of which The purpose is to eliminate tensile deformation, reduce the residual shrinkage of the blank, and minimize the shrinkage of the finished product. To meet consumer demand for dimensional stability of finished products. Therefore, this article will focus on analyzing the shrinkage mechanism of fabrics during processing, and how to eliminate shrinkage and reduce residual shrinkage from a technical perspective.
1 Shrinkage mechanism of knitted fabrics
1. Shrinkage and its causes
Shrinkage is when knitted fabrics return to their original size after being stretched during dyeing, finishing and use, that is, a relaxed state without external force. The internal stress is determined by the structure of the knitted fabric.
Knitted fabrics are made of different yarns with different loop structures and woven by different knitting machines, such as ribs, cotton wool, jersey, etc. During the process of weaving, dyeing and finishing, as well as when consumers wear and wash garments, the coils of the fabric are stretched to varying degrees, forming varying degrees of shrinkage stress on the fabric. When the fabric or garment is fully washed and dried, , the shrinkage stress of the fabric will cause the fabric to return to a relaxed state, and its vertical and transverse dimensions will shrink due to shrinkage, and the shrinkage process is shrinkage.
The shrinkage of the fabric is caused by shrinkage stress. When the residual shrinkage of the fabric is in a lower range, the shrinkage stress becomes smaller. This is because the shrinkage between the fabric loops Frictional resistance offsets shrinkage stress. The size of the frictional force between fabric coils depends on the structure of the fabric and the elasticity of the yarn itself. When the elasticity of the fabric is not enough to overcome the frictional resistance, it can be achieved by increasing the kinetic energy of the fabric or compressing the coils. Improve the fabric’s ability to offset shrinkage pressure and minimize the residual shrinkage of the fabric.
2. Vertical and transverse shrinkage of fabrics
During processing During the process, the knitted fabric shrinks both vertically and transversely. Practical experience shows that the transverse shrinkage of circular knitted fabrics has a basically determined range according to different fabric structures. Once the transverse size of the fabric being processed is determined, the knitting process cylinder diameter is determined. The transverse size and shrinkage of the plain fabric can be easily accomplished through post-finishing.
The vertical shrinkage of the fabric is extremely unstable and is affected by many factors during the processing. The same fabric will also produce different shrinkage effects under different production process conditions. This requires minimizing the vertical stretch of the fabric in each production process in order to achieve the ideal vertical shrinkage. The main purpose of the finishing processing of circular knitted fabrics is to eliminate the longitudinal shrinkage of the fabric to the greatest extent so as to obtain a good longitudinal shrinkage of the plain fabric.
3. Types of shrinkage of knitted fabrics
①Elastic shrinkage
Elastic shrinkage occurs during various processes when the fabric is free to relax. Theoretically, the fabric has elastic shrinkage in both wet and dry states, but the fabric is less stable in the wet state and is more likely to be stretched. Therefore, in most cases, elastic contraction can be caused by applying mechanical kinetic energy (such as expansion, vibration) to the fabric. Since the friction between the coils is very high when the dry fabric is stretched and is prone to retraction, there is almost no elastic retraction effect when mechanical kinetic energy is applied to the dry fabric. It can be seen that elastic shrinkage mainly occurs in the first half of the drying process of wet cloth.
② Drying shrinkage
When the residual moisture content of the fabric reaches 30 % ~ 40% and then dried, the shrinkage produced is drying shrinkage; when the fabric moisture content is 6% ~ 8%, the shrinkage ends. The size of drying shrinkage depends on the amount of mechanical kinetic energy acting on the fabric during the re-drying process. Drying shrinkage occurs in the second half of the fabric drying process.
③Pre-shrinkage
Dry or slightly damp fabrics After the coil is compressed, pre-shrinkage will occur, that is, the fabric will produce pre-shrinkage when it is pre-shrunk by the felt. After pre-shrinking, the fabric will obtain a better residual shrinkage.
④ Washing Shrinkage
When the fabric is not completely shrunk, When garments made of this fabric are washed in a washing machine, washing shrinkage will occur. It appears in the form of elastic shrinkage and drying shrinkage. The shrinkage detection method of finishing fabrics is to detect the degree of shrinkage of fabrics after washing. Usually different in a dry way. There are two testing standards: the hanging drying standard in the form of static drying and the tumble dryer drying standard in the form of kinetic energy drying. Usually the shrinkage rates of these two standards differ by about 5%.
2 Technical analysis of finishing process
Cylinder finishing process The process is shown in Figure 1
1. Fabric wet finishing
① Fabric wet finishing Methods and purposes
The wet finishing of fabrics is the wet expansion finishing process after the fabrics are dehydrated (rolled). UsuallyThere are two finishing methods: centrifugal dehydration and humidification expansion machine expansion and padding car water expansion.
The purpose of wet finishing includes the following points:
a. To remove moisture from the fabric and keep the residual moisture content of the fabric as low as possible , in order to save energy and improve drying efficiency;
b. Expand the width as much as possible in the lateral direction and overfeed in the vertical direction to increase the vertical shrinkage stress of the fabric and increase the elasticity of the fabric. Shrink;
c. Make the fabric surface smooth to reduce drying wrinkles.
2. Fabric drying and finishing
①Commonly used drying Machine type
a. Four-wire rotary screen dryer
The four-wire rotary screen dryer was developed in the early 1970s. By the end of the 1980s, the main drying models used by knitting companies in my country were shown in Figure 2 for their basic drying principles.
As can be seen from Figure 2, the fabric passes through the 1st net to the 2nd net, 3rd net, and 4th net, and then comes out to the distributor, and the fabric passes through the circular net The mesh is adsorbed and passed on the net by negative pressure. There is hot air drying. The overfeeding between the four nets is achieved by decreasing the speed. The decline rate is a constant. Although there is overfeeding in this type of drying, the amount of overfeeding is small. The fabric is transferred by the rotary screen. During the transfer process, it is easy to slip and form stretching, resulting in elastic shrinkage and no drying shrinkage. The maximum elastic shrinkage is within 3%, which is close to the static drying effect.
b. Flat mesh relaxation dryer
Flat mesh relaxation The dryer was designed and developed abroad from the late 1980s to the early 1990s. The fabric is transferred by a mesh belt, and is fed into the mesh belt by an overfeed roller. The mesh belt lifts the fabric and passes through the drying box to complete the drying process. The basic drying principle is shown in Figure 3
As can be seen from Figure 3, the overfeed amount of this form of drying can be adjusted, and the fabric is transferred during drying There will be no additional straight stretch during the process, and the elastic shrinkage of the fabric can be greatly obtained. Generally, the elastic shrinkage can reach 5%. In addition, since the fabric has no additional kinetic energy during the drying process, no drying shrinkage occurs.
2. Selection of drying route
The drying process is It is an important process for fabrics to change from wet state to dry state. To achieve ideal shrinkage effect of plain fabrics, we must pay attention to the drying process; we must choose different drying routes and products according to different fabric varieties, shrinkage rates and finished product gram quality requirements. Process parameters (overfeeding amount, drying temperature, drying speed, etc.).
3. Blanket pre-shrinking and finishing
Blanket pre-shrinking Finishing is the last process of finishing circular knitted fabrics. Its main function is to overfeed, expand, steam and humidify the dried cylindrical knitted fabric through the support plate, and pre-shrink and iron the felt to achieve the required transverse size and obtain a smooth and flat fabric. .
The key to solving the shrinkage of fabrics lies in the drying process. The shrinkage caused by the pre-shrinking process of felts has little impact on the final shrinkage index of general products. The key is to finish the smooth fabric. The requirements for transverse dimensional stability and smooth surface must be met. When producing fabrics with a vertical shrinkage of less than 3%, an imported high-end blanket pre-shrinking machine must be used.
3 Postscript
Finishing process of circular knitted fabrics The technology is very mature. The main purpose of finishing is to complete the transformation of the fabric from wet state to dry state, and at the same time achieve the fabric physical indicators required by customers, namely shrinkage, width, gram weight, hand feel and cloth surface smoothness, etc. </p
