Melt blown extrusion is a manufacturing process that is used for creating a type of fabric called non-woven fabric which is made from polymers such as polypropylene. Traditional fabrics made of natural materials like cotton are woven together, meaning that the material is first formed into a yarn and then interlaced using a weaving or knitting process that results in the creation of a sheet of fabric from the yarn. Non-woven fabrics do not involve joining yarn by weaving or knitting; instead, they mechanically, thermally, or chemically bound together material created from separate fibers of molten polymers forming a web-like fabric. The resulting fabric has a number of desirable properties that include:
- Bacterial barrier
- Flame retardancy
- Liquid repellency
Non-woven fabrics are used in a variety of applications, creating products that are used in agricultural, automotive, construction, personal hygiene, roofing, carpeting, upholstery, and medical products, to name just a few examples. Specific examples of the types of products that can be fabricated using non-woven fiber include:
- Filtration, such as HEPA air filters or liquid and gas filter products
- Masks and respirators for medical and industrial use
- Disposable medical garbs, such as gowns, drapes, shoe coverings, and head coverings
- Sanitary products, such as those for feminine hygiene and disposable diapers
- Oil and liquid adsorbents, which are products that contain spills and pick up oil from the water
- Coffee filters and tea bags
- Artificial turf
- Insulating products
- Meat and vegetable packing trays
- Disposable disinfectant wipes
This article will focus on describing how the melt-blown extrusion process is used to make non-woven fabric, and will then describe how this fabric is then used to create masks such as medical masks, surgical masks and N95 respirators that are critical pieces of personal protection equipment (PPE) used by medical professionals who face exposure on a daily basis to hazardous airborne and aerosolized pathogens.
To learn about key companies that produce non-woven fabrics, see our related guide Top Manufacturers and Suppliers of Non-Woven Fabrics. To see other guides and articles related to PPE such as masks, respirators, goggles, gloves, gowns, or PAPRs, a list appears at the end of this article.
Melt-Blown Extrusion Process
The melt-blown extrusion process is a single-step process that uses a stream of high-velocity air to blow a molten thermoplastic resin from an extruder die tip onto a conveyor or what is called a take-up screen. The process has been in existence since the 1950s and has grown in significance since its origins. The basic process is illustrated in Figure 1 and is performed using Melt Blown Fabric Extruder Machinery that is specially designed to manage and control the process.
The basic components of the process are the resin feed system, the extruder assembly, the metering pump, the melt-blown die assembly, the collector, and the winder unit.
Resin Feed System
The raw material for the melt-blown process is a thermoplastic resin in the form of pellets that are stored in a resin bag and gravity-fed to the extruder hopper. There are a number of different polymers that are adaptable for use in melt-blown extrusion. These polymers include:
- Polypropylene [PP]
- Polycarbonate [PC]
- Polybutylene terephthalate [PBT]
- Polyamide [PA]
- Thermo-plastic Polyurathane [TPU]
- Elastic Polypropylene [ePP]
The extruder assembly receives the feed of pellets from the resin feed system. A screw impeller similar to an Archimedean screw moves the pellets through a heated barrel of the extruder assembly, where they contact the heated walls and being to melt. There are three zones in the screw impeller – the feed zone, transition zone, and metering zone. The feed zone is the section of the impeller where the material enters the extruder and begins to melt. The transition zone features a decreasing depth and serves to homogenize the polymer feed and compress it. Once the polymer has reached a molten state, it is fed to the metering zone which increases the pressure to prepare the material for discharge through the melt-blown die assembly. At the output of the metering zone of the impeller screw is a screen pack that acts as a filter to trap any dirt or lumps of the polymer from reaching the metering pump.
The output of molten polymer which is now at 250oC – 300oC and pressurized, is fed to the metering pump. The metering pump is a positive displacement pump that is designed to deliver a constant volume of clean polymer mix to the die assembly and accounts for process variations in temperature, pressure, or viscosity of the molten polymer. Within the pump are two intermeshed, counterrotating gears. As the gears rotate, they draw the molten polymer from the intake or suction side of the pump and deliver it to the discharge side of the pump. The metering pump output then feeds to the die assembly.
Melt Blown Die Assembly
Within the die assembly are three key components – the feed distribution, the die nosepiece, and the air manifolds. Two types of feed distribution are commonly used; these are the T-type, which may be tapered or untampered, and the coat hanger type. The coat hanger distribution is more common owing to its even polymer flow.
The die nosepiece is a critical component for determining the uniformity of the resulting web of melt-blown material produced from the machine. The die nosepiece is a tight tolerance wide, hollow, tapered metal part that contains a large number of orifices in it through which the molten polymer will pass to form the melt-blown non-woven fabric.
The air manifolds supply high velocity heated air to the extruded fibers that are outputted from the die nosepiece. An air compressor supplies the pressurized air flow, which is first passed through a heat exchanger drive off a gas or electric furnace to raise the air temperature to a range of between 230oC – 360oC at a velocity of between 0.5 – 0.8 the speed of sound (560 – 900 feet per second).
The molten polymer that is extruded through the die nosepiece orifices is then driven by the high-velocity hot air stream from the air manifolds and causes the polymer to form microfibers as they further extend in the air stream (See Figure 2). These microfibers have diameters that range from 0.1 microns to 15 microns. (By comparison, cellulose fibers have a diameter of around 50 microns and a human hair 120 microns.) At the same time the fibers are extending, they are being blown together while in a semi-molten state and directed towards a collector screen. The hot air stream also causes secondary air to be drawn from the surrounding ambient air and helps to cool and solidify the collected web of material that forms on the collector, which is a take-up metal screen attached to a conveyor. The fibers solidify and are randomly laid onto the collector, binding together to form a web by both entanglement and cohesion of fibers to one another. By varying the collector speed and the separation distance between the die nosepiece and the collector, variations in the web fabric density can be achieved to suit different applications. A vacuum pump is often used to draw a vacuum on the inside of the collector screen. This serves to remove the hot air stream and enhances the web-laying process on the collector.
The cooled fabric from the collector is wound onto a cardboard core in the winder unit. For many types of melt-blown non-woven fabrics, there is sufficient cohesion achieved between fibers so that the material is suitable for use without any need for additional bonding. In some applications, further processing of the material may be necessary to alter the material characteristics. Thermal bonding is a commonly used technique when additional bonding is needed, which can increase the material’s strength but with a resulting increase in stiffness and loss of a fabric feel.
After any needed bonding, the production process for melt-blown extrusion of non-woven fabrics is complete. Additional postproduction processes may be applied as needed, such as the addition of flame retardant chemicals, depending on the end-use for the material. The non-woven fabric is then sold to converters who use it as raw material to make filtration products, coffee filters, insulations, or as will be discussed below, medical and surgical masks.
The characteristics of the melt-blown non-woven fabric produced can be influenced and controlled to some degree by varying some of the operational conditions and inputs to the process. These include factors such as:
The type of polymer used and its material characteristics such as molecular weight
The extruder operating conditions such as temperature
The geometry of the die nosepiece such as the orifice size and number of orifices
The hot air stream conditions (temperature, velocity)
The distance between the die nosepiece and the collector screen
The speed of the collector
Medical Mask Construction and Mask Making Machinery
Non-woven fabric is a primary material used in the manufacturing of medical and surgical masks. As with the process for non-woven fabric production, specialized Face Mask Production Machinery is utilized to mass-produce large quantities of disposable surgical masks medical masks (See Figure 3). To understand how these machines function, it is necessary to first learn about how these types of masks are constructed.
Medical masks typically are created from stacking together three layers of non-woven material. An inner layer that comes in contact with the wearer’s face is used to absorb moisture that is created during normal expiration. An outer layer of non-woven fabric serves as a waterproof barrier that precludes any liquids expelled by the patient while talking, coughing, or sneezing from being transmitted or absorbed by the mask. Sandwiched between the inner and outer layer of the mask is a middle layer that serves as a filter. This middle layer is usually created from polypropylene (PP) melt-blown non-woven fabric and is treated to be an electret. The electret treatment adds electrostatic properties to the filter layer allowing for electrostatic adsorption which helps to trap aerosolized particles via electrostatic attraction.
Mask Making Process
Mask making machinery used to rapidly create disposable medical masks automates the steps needed in the process. The basic process steps for creating flat disposable medical or surgical masks are:
- Combining the three layers of materials together to produce the multilayer mask fabric – The machine takes the different non-woven fabrics from their supports and feeds them together into a layered structure.
- Attachment of the metal nose strip – the machine stitches the flat metal wire onto the 3-layer fabric which will be used by the wearer to fit the mask to their nose and improve its seal to the face.
- Add folds and pleats – the machine uses a folding device to add folds and pleats to the mask that will enable a standard mask to be adjusted to suit different wearers.
- Cutting & stitching – the three-layer material is cut to individual size masks and the edges are stitched to join the layers.
- Attachment of ear loops – ear rope is attached, and adhesive is applied, followed by a thermal press to secure the loops in place. Other methods of attachment include the use of ultrasonic welding.
- Disinfection – medical-grade masks are subjected to a sterilization process using ethylene oxide to render any microbial contamination inactive. Following this treatment, masks must be allowed to stand for a period of 7 days until the ethylene oxide level dissipates, as the material is toxic to the human body as well as being flammable.
- Packaging – following the waiting period, completed masks are packaged for shipment.
Cup-shaped masks and respirators are created using a similar process, but different machinery is employed, and other materials and steps are needed. As an example, the material composition of a 3M™ Particulate Respirator Model 8210, which is an N95 type respirator, calls for the use of a polyester shell and coverweb, a polypropylene filter (middle layer), polyurethane nose form, aluminum nose clip, and thermoplastic elastomer straps that secure the respirator for a tight fit to the wearer’s face.
As with the medical mask, the non-woven polypropylene filter layer is key to the filtration performance of the respirator. The random orientation of the fibers from the melt-blown extrusion process that was described earlier combine with the density and fine fiber size to produce a material that can filter out the smallest of particles with high efficiency. These characteristics make the material essential for filtering viruses and other pathogens in medical settings and help explain why non-woven fabrics play a key role in filtration products for various uses.
Face mask production machines are expensive to purchase, representing an investment in the hundreds of thousands of dollars. However, they can produce hundreds of thousands of masks per day with a consistency in quality that makes the investment pay for itself in a short period of time. And in a crisis situation such as the coronavirus pandemic, and automated process is the only way to keep up with the demand for essential PPE such as medical and surgical masks to protect the health of front line workers such as doctors, nurses, and EMTs.