Melt blown extrusion process is a one-step process, which uses high-speed air flow to blow molten thermoplastic resin from the extruder die to the conveyor or so-called coiling screen. This process has existed since the 1950s, and its significance has become increasingly significant since its origin. The basic process is shown in Figure 1, using a melt blown fabric extruder specially designed to manage and control the process.
The basic components of the process are resin feeding system, extruder assembly, metering pump, melt blown die assembly, collector and winder unit.
Resin supply system
The raw material of melt blowing process is thermoplastic resin in the form of particles, which is stored in resin bags and transported to the extruder hopper by gravity. There are many different polymers available for melt blown extrusion. These polymers include:
Polypropylene PP
polycarbonate
Polybutylene terephthalate
Polyamide PA
Thermoplastic polyurethanes TPU
Elastic polypropylene EPP
Extruder assembly
The extruder assembly receives particle feed from the resin feed system. A helical impeller similar to the Archimedean spiral passes the particles through the heating cylinder of the extruder assembly, where the particles contact the heating wall and are melted. There are three areas in the helical impeller - feed area, transition area and metering area. The feed zone is the part of the impeller where the material enters the extruder and begins to melt. The transition zone is characterized by reduced depth and is used to homogenize and compress the polymer feed. Once the polymer reaches the molten state, it is sent to the metering area, which increases the pressure to prepare the material for discharge through the melt blown die assembly. There is a filter screen group at the output end of the impeller screw metering area, which is used as a filter to intercept any dirt or polymer block reaching the metering pump.
Metering pump
The output temperature of the molten polymer is 250oC – 300oC and pressurized, and then transmitted to the metering pump. The metering pump is a positive displacement pump designed to deliver a constant volume of clean polymer mixture to the mold assembly, taking into account the process changes in temperature, pressure or viscosity of the molten polymer. There are two counter rotating gears meshing with each other in the pump. When the gears rotate, they draw molten polymer from the suction side or suction side of the pump and deliver it to the discharge side of the pump. The output of the metering pump is then sent to the die assembly.
Melt blown mold assembly
There are three key components in the die assembly - feed distribution, die head and air manifold. Two types of feed distribution are usually used; These are T-type (can be tapered or undamped) and hanger type. Due to the uniform polymer flow, the hanger distribution is more common.
The die head is the key component to determine the uniformity of melt blown material mesh produced by the machine. The die head is a tight tolerance wide, hollow, conical metal part, which contains a large number of small holes through which the molten polymer will form melt blown nonwovens.
The air manifold provides high{{0}}speed heated air to the extruded fibers, which are output from the head. The air compressor provides compressed air flow, which first drives the gas or electric furnace through the heat exchanger to raise the air temperature to 230 degree C – 360 degree C at a speed of 0.5 – 0.8 sonic speed (560 – 900 ft / s).
collector
Then, the molten polymer extruded through the die head hole is driven by a high{{0}}speed hot air flow from the air manifold and forms micro fibers when the polymer is further extended in the air flow (see Fig. 2). The diameter of these microfibers ranges from 0.1 microns to 15 microns. (in contrast, cellulose fibers are about 50 microns in diameter and human hair is 120 microns in diameter.) As the fibers extend, they are blown together in a semi molten state and towards the collector screen. The hot air flow also causes the suction of secondary air from the surrounding ambient air and helps to cool and solidify the collection material net formed on the collector, which is a tensioned metal net connected to the conveyor. After the fibers are cured, they are randomly laid on the collector, wound and bonded with each other to form a net. By changing the collector speed and the separation distance between the die head and the collector, the change of mesh density can be realized to adapt to different applications. The vacuum pump is usually used to vacuum inside the collector panel. This helps to remove hot air flow and enhance the netting process on the collector.
Winder
The cooling fabric from the collector is wound around the cardboard core in the winder unit. For many types of melt blown nonwovens, there is sufficient adhesion between fibers, so the material is suitable for use without additional adhesion. In some applications, further processing of the material may be required to change the material properties. When additional bonding is required, thermal bonding is a common technology, which can increase the strength of the material, but will lead to increased stiffness and loss of fabric feel.
After any required bonding, the production process of melt blown extrusion of non-woven fabric is completed. Depending on the end use of the material, additional post production processes can be used as needed, such as the addition of flame retardant chemicals. The non-woven fabric is then sold to the converter, which uses the non-woven fabric as raw material to manufacture filter products, coffee filters, insulating materials or medical and surgical masks discussed below.
Process variable
By changing some operating conditions and process inputs, the characteristics of melt blown nonwovens can be affected and controlled to a certain extent. These factors include:
The type of polymer used and its material properties, such as molecular weight
Operating conditions of extruder, such as temperature
The geometry of the die head, such as orifice size and number of orifices
Hot air flow conditions (temperature, speed)
Distance between die head and collector screen
Collector speed