After finishing, the moisture content of non-woven fabrics exceeds the allowable range of quality requirements. In addition to affecting the quality, shelf life, measurement accuracy and packaging quality of the product, the excess moisture can also cause the product to breed bacteria and become moldy and deteriorate. Therefore, drying is an important link in the nonwoven finishing production process. Its function is to remove excess moisture in the product and ensure product quality. For some special products, the drying process also activates the treatment agent function (such as oil repellency, alcohol repellency , anti-blood penetration function) necessary process. In the production of nonwoven fabrics, heating drying is mostly used. There are four commonly used heating drying methods as follows.
01 Far-infrared drying
Since infrared rays emit electromagnetic waves, when irradiating materials with infrared rays When the frequency is within the same range as the natural vibration frequency of the molecules of the material to be dried, electromagnetic resonance is caused, and the energy of infrared rays can be effectively absorbed to dry the object. Far infrared dryer is a drying equipment designed based on the principle of infrared radiation. The infrared generating device of far infrared dryer includes infrared lamp, infrared electric heating tube, electric heating plate, far infrared electric heating tube, electric heating plate, etc. In order to improve the efficiency of infrared drying, dryers are often equipped with a forced convection ventilation system to take away the hot air attached to the surface, reduce the humidity on the product surface, and accelerate water evaporation. Because far-infrared drying does not require an intermediate medium, the energy penetrates into the interior of the product and directly heats the product, so its efficiency is higher than hot air drying.
The infrared dryer has the advantages of compact structure, easy installation, high output energy, high drying efficiency, and good sanitary conditions. However, since the surface temperature of the heating element far exceeds the melting point of the non-woven fabric, if the product is in a stationary state or is in direct contact with the heating device, the product may be melted or a fire accident may occur. Therefore, corresponding security measures should be set up in the equipment.
02 Hot air drying
Hot air drying uses the energy of hot air flow to cause the moisture in the product to be The process of heating, evaporating, and being carried away by airflow. Products dried by hot air are limited by the temperature of the hot air flow. The method of increasing the contact time between the hot air and the product is usually used to improve the heat energy utilization rate and drying efficiency of the hot air. Therefore, the products must be wound in a roundabout way in the drying equipment, and the size of the drying box is larger.
Hot air drying mainly relies on hot air flow convection heat exchange. Although the drying efficiency is not high, the drying process is gentle and the temperature is uniform. , the dried product still maintains a good feel. Therefore, hot air drying is also a commonly used process in the finishing and drying process of nonwoven fabrics.
03 Cylinder drying
Cylinder drying is to make the products with higher moisture content The non-woven fabric is directly attached to the surface of the high-temperature drying cylinder, and the non-woven fabric is heated up through heat conduction, thereby vaporizing and evaporating the moisture in it to achieve drying purposes. Cylinder drying is a drying process widely used in the papermaking industry and is also a drying method used in nonwoven finishing production lines.
In the drum dryer, the drum is made of copper with good thermal conductivity. When the finishing agent is corrosive or the product has high hygienic requirements, it needs to be made of stainless steel. . Dryers are generally equipped with multiple drying cylinders, and the drying cylinders can be arranged horizontally or vertically. The nonwoven fabric and the drying cylinder are often wound along the route with the largest wrapping angle (such as Ω type) and the largest contact area, so that both front and back sides of the product can be dried.
The drying efficiency of the drum dryer is related to the temperature and ambient humidity of the drum surface. The drying cylinder can be heated with steam or thermal oil. The higher the temperature, the higher the drying efficiency, but the higher the energy consumption of processing products. During the drying process of the product, there will be a large temperature difference between the temperature of the side that is in direct contact with the surface of the drying cylinder and the other surface. After drying, the hand feel of the product will become worse, and some light-colored products will also have uneven spots. Yellow color affects the quality of the product.
In order to adapt to the requirements of high-speed operation of the production line, the drum dryer will be equipped with a larger number of The drying cylinder takes up a lot of site and space on site, and its drying efficiency is not as good as the hot air penetration dryer. Therefore, it is rarely used in the drying process of spunbond products and spunbond melt-blown composite products.
04 Hot air penetration drying
Hot air penetration drying uses the principle of air convection. Let the hot air directly penetrate the non-woven fabric, and at the same time transfer the heat to the non-woven fabric to evaporate the moisture and take the moisture away, achieving an efficient heat and mass transfer process and making the product dry quickly.
The core component of the hot air penetration dryer is the drying drum. The surface of the drum has a porous structure with high air permeability. During operation, the non-woven fabric is wrapped on the surface of the drum with a large wrapping angle. On the top, the negative pressure generated by the thermal circulation fan draws the hot air flow outside the drum into the drum through the non-woven fabric. The hot air heats the cloth. The moisture in the cloth is evaporated by heat and is then taken away by the air flow in the drum. The product Then it was dried. The wet material extracted from the drumA small part of the air flow is discharged into the atmosphere through the dehumidification valve, and most of it enters the main circulation pipe, and then enters the air heater again to heat up and continuously dry the product. The air heater can use steam, gas or thermal oil as energy source, and adjust the temperature of the hot air by changing the flow rate of steam, gas or thermal oil to control the drying effect.
The hot air penetration dryer not only has the advantages of accurate temperature control, uniform temperature distribution, efficient drying process, and energy saving, but also the dried products have good hand feel and The sanitary conditions are controllable and the additional tension is small. The hot air penetration dryer occupies a small area and can not only be used for offline post-finishing of products, but can also be configured as the main process equipment for online drying in the production line. </p
