1、 The process principle of melt spraying


Melt blown nonwoven technology uses high-speed hot air to stretch the polymer melt flow extruded from the die head wire hole, forming ultrafine fibers that condense on the screen curtain or roller and become nonwoven fabric by self bonding


Melt spraying devices are divided into horizontal and vertical according to their placement methods



2、 Process flow and equipment


1. Process flow of melt spraying


Polymer preparation → Melt extrusion → Metering pump → Melt spray die assembly → Melt fine flow stretching → Cooling → Receiving device


2. Meltblown equipment


Main equipment: feeding machine, screw extruder, metering pump, melt blown mold head assembly, air compressor, air heater, receiving device, winding device.


Production auxiliary equipment: mold cleaning furnace, electrostatic application device, etc.



3、 Melt blown mold head assembly


The mold head assembly is the most critical part of the melt spraying equipment, among which the most important parts include: polymer melt distribution system and mold head system


1. Polymer melt distribution system


Ensure that the polymer melt flows uniformly along the entire length direction of the melt blown mold head and has a uniform residence time, thereby ensuring that the melt blown nonwoven fabric has relatively uniform properties across the entire width.


At present, coat hanger polymer melt distribution systems are mainly used in melt spraying processes (T-shaped distribution systems cannot evenly distribute fluids).


2. Die system (die die)


It is composed of spinneret, air plate, heating and insulation elements, etc.


The uniformity of melt blown products is closely related to the mold head. Usually, the machining accuracy requirements of melt blown die heads are high, so the manufacturing cost of die heads is expensive.


The spinneret holes are often arranged in a single row with a aspect ratio greater than 10.


In the early development of melt blown mold heads, micro grooves are machined on the joint surfaces of the upper and lower molds, and then the upper and lower molds are bonded to form a row of spray holes.


This structure can achieve a larger aspect ratio of the spinneret hole, and the cleaning of the die head is more convenient. However, it requires high processing and assembly accuracy, and is currently less widely used.



4、 Air heater


The melt blown process requires a large amount of hot air. The compressed air output by the air compressor is dehumidified and filtered before being transported to the air heater for heating, and then sent to the melt blown mold head assembly. The air heater is a pressure vessel that must resist the oxidation of high-temperature air, so the material must be stainless steel.



5、 Auxiliary equipment


The main auxiliary equipment of the melt blown production line is the mold cleaning furnace. After a period of production, the melt blown mold head may experience hole blockage, and it is necessary to replace the melt blown mold head.


The replaced melt blown mold head needs to be roasted to remove residual polymers and impurities inside the mold head. Screw and spinneret are usually roasted to remove residual polymer and impurities.



6、 Raw materials for melt spraying


In theory, any thermoplastic (high-temperature melting, low-temperature curing) polymer chip raw material can be used for the melt blown process. Polypropylene is the most commonly used slicing material in melt blown processes. In addition, the commonly used polymer slicing materials in melt blown processes include polyester, polyamide, polyethylene, polytetrafluoroethylene, polystyrene, PBT, EMA, EVA, etc.


The type of polymer determines its melting point and rheological properties. For each polymer raw material, there are corresponding melt spraying processes, such as differences in heating temperature, screw length to diameter ratio, screw form, and raw material drying process.


Differences in Melt Blowing Processes between Olefin and Ester Polymer Raw Materials


Olefin polymer raw materials (such as polypropylene) have a high degree of polymerization, so the heating temperature above its melting point of 100 ℃ can smoothly melt and spray, while polyester can melt and spray when the heating temperature is slightly higher than its melting point. Olefin raw materials generally do not require drying. Polyester must be sliced and dried.



7、 Structure and properties of melt blown nonwoven fabric


One of the characteristics of melt blown nonwoven fabrics is that the fiber fineness is small, usually less than 10 μ m. Most fibers have a fineness of 1-4 μ m。


The various forces on the entire spinning line from the spinneret hole of the melt blown mold head to the receiving device cannot be balanced (such as the fluctuation of tensile force caused by high-temperature and high-speed airflow, the speed and temperature of cooling air, etc.), resulting in varying fineness of the melt blown fibers.


The uniformity of fiber diameter in the spunbonded nonwoven fabric mesh is significantly better than that of melt blown fibers, as the spinning process conditions in the spunbonded process are steady-state, with little fluctuation in drafting and cooling conditions.



8、 Factors affecting the performance of melt blown products


The performance of melt blown products mainly refers to physical and mechanical properties, such as product strength, breathability, fiber diameter, etc. Due to the complex melt blown process, there are many influencing factors.


Online parameters: Parameters that can be adjusted as needed during the melt blown production process, such as melt extrusion amount, melt temperature, temperature and initial speed of stretching hot air, receiving distance, etc.


Offline parameters: parameters that can only be adjusted when the device is not running, such as the shape of the spinneret hole and the size of the stretching hot air channel.


In actual production, should we choose polypropylene with a larger MFI or a smaller one?


MFI small: can produce high strength melt blown nonwoven fabrics.


Large MFI: high production and low energy consumption. Therefore, the current trend is to use higher MFI raw materials.


The earliest applied polypropylene has a high molecular weight and low MFI (12g/10min).


With the advancement of technology, polypropylene with an MFI of 12 was quickly replaced by MFI35, and polypropylene specifically designed for melt blown processes emerged with an MFI of up to 1500.



9、 Filtering principle


Filtration is the separation of granular substances dispersed in gases or liquids.


Filtering mechanism: sieve deposition, electrostatic deposition, diffusion deposition, etc.


Is the particle size larger than the pore size of the filtering material necessary for screening?


Research has shown that filter materials with pore sizes ranging from ten to tens of micrometers can capture 1 µ m of dust.


To improve the sedimentation effect of sieve filtration, it is necessary to reduce the pore size of the filter material, that is, reduce the fiber fineness and increase the density of the material.


Melt blown nonwoven fabrics with electret treatment have the advantages of high filtration efficiency and low filtration resistance due to their persistent static electricity, which can rely on the electrostatic effect to capture fine dust.


Polypropylene has a high electrical resistivity (7 × 1010 Ω · cm), with a large injected charge capacity, is an ideal material for manufacturing electret fibers. The experiment shows that the filtration efficiency of polypropylene melt blown nonwoven fabric treated with electret finishing remains unchanged after being stored in natural state for 1440 hours.