The production process of meltblown nonwoven fabric includes: polymer preparation → melt extrusion → metering pump → meltblown die assembly → melt stream stretching → cooling → receiving device.
The meltblown nonwoven process uses high-speed hot air to stretch the polymer melt stream extruded from the spinneret of the die, forming ultrafine fibers that condense on a wire mesh or roller and bond together to become nonwoven fabric. In simpler terms, it involves blowing molten polypropylene with high-pressure hot air, allowing it to cool naturally and solidify into fibers to form fabric. This is the origin of the name "meltblown nonwoven fabric."
In essence, it's similar to the process of making cotton candy. Cotton candy is made by placing molten sugar syrup in a container and throwing it out using centrifugal force, where it solidifies naturally under air cooling. The high viscosity of sugar allows it to be drawn into threads at the right speed. However, while the principle of meltblown nonwoven fabric is similar to that of cotton candy, the manufacturing processes are not exactly the same. The main equipment components of meltblown nonwoven fabric production include: a feeding machine, a screw extruder, a metering pump, a meltblown die assembly, a screw blower, an air heater, a receiving device, and a winding device. For producing raw materials such as polyester, a chip drying device is also required. Auxiliary equipment mainly includes a die cleaning furnace, an electrostatic application device, and a spraying device.
Within this entire equipment, we see a crucial element, a significant technical challenge. It determines whether the meltblown nonwoven fabric produced will meet standards: the high-pressure, heated airflow that blows out the polypropylene solution. This brings us to the key component: the screw blower.
The role of the screw blower in meltblown nonwoven fabric production. The screw blower is used to stably deliver heated, high-pressure gas with precise flow control, high purity, and high pressure. Why did I use so many adjectives when describing the role of the screw blower? Let's explain the reasons one by one.
1. Precise Flow Control
In industrial production, we generally don't talk about airflow velocity, but rather flow rate. Because flow rate corresponds to velocity and nozzle cross-sectional area. Since the length of the meltblown fabric fibers is adjustable, if the flow rate is too high, the sprayed polypropylene will be blown into dots. If the flow rate is too low, the fibers will be too long, potentially failing to achieve the dust-proof effect. Therefore, this hot air delivery method cannot simply use a household electric motor with an impeller to directly deliver air.
2. High Pressure
During the meltblown fabric manufacturing process, we see that a certain pressure is required to deliver the polypropylene to a distant forming belt for solidification. Simultaneously, stable high pressure also corresponds to better temperature control. Everyone knows that a breath of air cools quickly after it leaves the fabric. However, under higher pressure, it can effectively ensure that the polypropylene liquid does not solidify prematurely at a certain stage, allowing it to cool and solidify properly upon landing on the forming belt.
This is equivalent to a process of hot air "escorting" the process.
Among all current blower concepts, there are three types of blowers capable of providing high pressure: reciprocating compressors (also known as air compressors), Roots blowers, and three-lobe Roots blowers, as well as centrifugal compressors. Reciprocating compressors are not used primarily because of their slow speed and low flow rate, making them unsuitable for meltblown fabric production. While centrifugal compressors can indeed achieve high pressure output with advancements in technology, their high cost makes them economically unsuitable.
Roots blowers are superior to the other two because they employ a rotary mechanical air compression method. A dynamic diagram illustrates the gas flow pattern of a Roots blower.
The gas compression principle of a three-lobe Roots blower (also known as a three-lobe Roots blower) explains the entire gas process. This mechanical rotary type of suction has significant advantages: the intake volume is fixed, and to increase the flow rate, simply increase the blower speed. The biggest advantage is that there's no "air leakage"—it's a sealed environment.
Therefore, from a high-pressure perspective, the Roots blower itself is a mechanical pressurization process. There's no need for secondary pressurization of the hot air.
3. High Purity
This is no joke. The production of meltblown fabric cannot have even a trace of oil. We all know that air compressors rely on pistons to compress air within a sealed chamber to output airflow. The piston and casing need lubricating oil for lubrication, otherwise it will overheat and burn out. Then, the oil is filtered after the gas exits, but this only filters out most of the oil mist; a small amount will still be sprayed out with the air, carrying the molten polymer to form the meltblown fabric. It's easy to imagine that this meltblown fabric would definitely have an oily odor. However, there's no way around it; oil is absolutely impossible during the production of meltblown fabric. Because if there's oil in the polypropylene solution before it forms a web, it will easily prevent the web from forming. After a period of practical application in actual production, customers, in order to improve fabric quality, have replaced their air compressors with Roots blowers. It's now safe to say that the air compressor was the first hurdle to overcome in the meltblown fabric industry.
Currently, only Roots blowers can achieve such high purity. Therefore, Roots blowers are widely used in the preparation of special gases, such as coal gas, carbon dioxide, and sulfur dioxide. Thus, in the production of meltblown fabric, Roots blowers are used to deliver high-temperature gas to the meltblown die.