What is Melt-Blown Extrusion and How is it Used for Making Masks?
Meltblown extrusion is a manufacturing process that is used for creating a type offabric called non-woven fabric, which is made from polymers such aspolypropylene. Traditional fabrics made of natural materials like cotton arewoven together, meaning that the material is first formed into a yarn and theninterlaced using a weaving or knitting process that results in the creation ofa sheet of fabric from the yarn. Non-woven fabrics do not involve joining yarnby weaving or knitting; instead, they mechanically, thermally, or chemicallybound together material created from separate fibers of molten polymers forminga web-like fabric. The resulting fabric has a number of desirable propertiesthat include:
Non-wovenfabrics are used in a variety of applications, creating products that are usedin 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 usingnon-woven fiber include:
Filtration,such as HEPA air filters or liquid and gas filter products
Masks and respirators formedical and industrial use
Disposablemedical garbs, such as gowns, drapes, shoe coverings, and head coverings
Sanitaryproducts, such as those for feminine hygiene and disposable diapers
Oiland liquid adsorbents, which are products that contain spills and pick up oilfrom the water
Coffeefilters and tea bags
Meatand vegetable packing trays
Thisarticle will focus on describing how the melt-blown extrusion process is usedto make non-woven fabric, and will then describe how this fabric is then usedto create masks such as medical masks, surgical masks and N95 respirators thatare critical pieces of personal protection equipment (PPE) used by medicalprofessionals who face exposure on a daily basis to hazardous airborne andaerosolized pathogens.
Tolearn about key companies that produce non-woven fabrics, see our related guide TopManufacturers and Suppliers of Non-Woven Fabrics. To see other guides andarticles related to PPE such as masks, respirators, goggles, gloves, gowns, orPAPRs, a list appears at the end of this article.
Themelt-blown extrusion process is a single-step process that uses a stream ofhigh-velocity air to blow a molten thermoplastic resin from an extruder die tiponto a conveyor or what is called a take-up screen. The process has been inexistence since the 1950s and has grown in significance since its origins. Thebasic process is illustrated in Figure 1 and is performed using Melt BlownFabric Extruder Machinery that is specially designed to manage and controlthe process.
Figure1 - The components of a typical melt-blown extrusion process.
Thebasic components of the process are the resin feed system, the extruderassembly, the metering pump, the melt-blown die assembly, the collector, andthe winder unit.
Theraw material for the melt-blown process is a thermoplastic resin in the form ofpellets 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 inmelt-blown extrusion. These polymers include:
Theextruder assembly receives the feed of pellets from the resin feed system. A screw impeller similar to an Archimedean screw moves the pellets througha heated barrel of the extruder assembly, where they contact the heated wallsand 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 theimpeller where the material enters the extruder and begins to melt. Thetransition zone features a decreasing depth and serves to homogenize thepolymer feed and compress it. Once the polymer has reached a molten state, itis fed to the metering zone which increases the pressure to prepare the materialfor discharge through the melt-blown die assembly. At the output of themetering zone of the impeller screw is a screen pack that acts as a filter totrap any dirt or lumps of the polymer from reaching the metering pump.
Theoutput of molten polymer, which is now at 250oC – 300oC and pressurized, is fedto the metering pump. The metering pump is a positive displacement pump that isdesigned to deliver a constant volume of clean polymer mix to the die assemblyand accounts for process variations in temperature, pressure, or viscosity ofthe molten polymer. Within the pump are two intermeshed, counter rotatinggears. As the gears rotate, they draw the molten polymer from the intake orsuction side of the pump and deliver it to the discharge side of the pump. Themetering pump output then feeds to the die assembly.
MeltBlown Die Assembly
Withinthe die assembly are three key components – the feed distribution, the dienosepiece, and the air manifolds. Two types of feed distribution are commonlyused; these are the T-type, which may be tapered or untampered, and the coathanger type. The coat hanger distribution is more common owing to its evenpolymer flow.
Thedie nosepiece is a critical component for determining the uniformity of theresulting web of melt-blown material produced from the machine. The dienosepiece is a tight tolerance wide, hollow, tapered metal part that contains alarge number of orifices in it through which the molten polymer will pass toform the melt-blown non-woven fabric.
Theair manifolds supply high velocity heated air to the extruded fibers that areoutputted from the die nosepiece. An air compressor supplies the pressurizedair flow, which is first passed through a heat exchanger drive off a gas orelectric furnace to raise the air temperature to a range of between 230oC – 360oCat a velocity of between 0.5 – 0.8 the speed of sound (560 – 900 feet persecond).
Themolten polymer that is extruded through the die nosepiece orifices is thendriven by the high-velocity hot air stream from the air manifolds and causesthe polymer to form microfibers as they further extend in the air stream (SeeFigure 2). These microfibers have diameters that range from 0.1 microns to 15microns. (By comparison, cellulose fibers have a diameter of around 50 micronsand a human hair 120 microns.) At the same time the fibers are extending, theyare being blown together while in a semi-molten state and directed towards acollector screen. The hot air stream also causes secondary air to be drawn fromthe surrounding ambient air and helps to cool and solidify the collected web ofmaterial that forms on the collector, which is a take-up metal screen attachedto 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 toone another. By varying the collector speed and the separation distance betweenthe die nosepiece and the collector, variations in the web fabric density canbe achieved to suit different applications. A vacuum pump is often used to drawa vacuum on the inside of the collector screen. This serves to remove the hotair stream and enhances the web-laying process on the collector.
Figure2 - Polymer fibers forming and being passed to the collector.
Thecooled fabric from the collector is wound onto a cardboard core in the winderunit. For many types of melt-blown non-woven fabrics, there is sufficientcohesion achieved between fibers so that the material is suitable for usewithout any need for additional bonding. In some applications, furtherprocessing of the material may be necessary to alter the materialcharacteristics. Thermal bonding is a commonly used technique when additionalbonding is needed, which can increase the material’s strength but with aresulting increase in stiffness and loss of a fabric feel.
Afterany needed bonding, the production process for melt-blown extrusion ofnon-woven fabrics is complete. Additional postproduction processes may beapplied as needed, such as the addition of flame retardant chemicals, dependingon the end-use for the material. The non-woven fabric is then sold toconverters who use it as raw material to make filtration products, coffeefilters, insulations, or as will be discussed below, medical and surgicalmasks.
Thecharacteristics of the melt-blown non-woven fabric produced can be influencedand controlled to some degree by varying some of the operational conditions andinputs to the process. These include factors such as:
Thetype of polymer used and its material characteristics such as molecular weight
Theextruder operating conditions such as temperature
Thegeometry of the die nosepiece such as the orifice size and number of orifices
Thehot air stream conditions (temperature, velocity)
Thedistance between the die nosepiece and the collector screen
Thespeed of the collector
MedicalMask Construction and Mask Making Machinery
Non-wovenfabric is a primary material used in the manufacturing of medical and surgicalmasks. As with the process for non-woven fabric production, specialized FaceMask Production Machinery is utilized to mass-produce large quantities ofdisposable surgical masks medical masks (See Figure 3). To understand how thesemachines function, it is necessary to first learn about how these types ofmasks are constructed.
Figure3 - Automated mask-making machinery
Medicalmasks typically are created from stacking together three layers of non-wovenmaterial. An inner layer that comes in contact with the wearer’s face is usedto absorb moisture that is created during normal expiration. An outer layer ofnon-woven fabric serves as a waterproof barrier that precludes any liquidsexpelled by the patient while talking, coughing, or sneezing from beingtransmitted or absorbed by the mask. Sandwiched between the inner and outerlayer of the mask is a middle layer that serves as a filter. This middle layeris usually created from polypropylene (PP) melt-blown non-woven fabric and istreated to be an electret. The electret treatment adds electrostatic propertiesto the filter layer allowing for electrostatic adsorption which helps to trapaerosolized particles via electrostatic attraction.
Maskmaking machinery used to rapidly create disposable medical masks automates thesteps needed in the process. The basic process steps for creating flatdisposable medical or surgical masks are:
Combiningthe three layers of materials together to produce the multilayer mask fabric –The machine takes the different non-woven fabrics from their supports and feedsthem together into a layered structure.
Attachmentof the metal nose strip – the machine stitches the flat metal wire onto the3-layer fabric which will be used by the wearer to fit the mask to their noseand improve its seal to the face.
Addfolds and pleats – the machine uses a folding device to add folds and pleats tothe mask that will enable a standard mask to be adjusted to suit differentwearers.
Cutting& stitching – the three-layer material is cut to individual size masks andthe edges are stitched to join the layers.
Attachmentof ear loops – ear rope is attached, and adhesive is applied, followed by athermal press to secure the loops in place. Other methods of attachment includethe use of ultrasonic welding.
Disinfection– medical-grade masks are subjected to a sterilization process using ethyleneoxide to render any microbial contamination inactive. Following this treatment,masks must be allowed to stand for a period of 7 days until the ethylene oxidelevel dissipates, as the material is toxic to the human body as well as beingflammable.
Packaging– following the waiting period, completed masks are packaged for shipment.
Cup-shapedmasks and respirators are created using a similar process, but differentmachinery 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 andcoverweb, a polypropylene filter (middle layer), polyurethane nose form, aluminum nose clip, and thermoplastic elastomer straps that secure the respiratorfor a tight fit to the wearer’s face.
Aswith the medical mask, the non-woven polypropylene filter layer is key to thefiltration performance of the respirator. The random orientation of the fibersfrom the melt-blown extrusion process that was described earlier combine withthe density and fine fiber size to produce a material that can filter out thesmallest of particles with high efficiency. These characteristics make thematerial essential for filtering viruses and other pathogens in medical settingsand help explain why non-woven fabrics play a key role in filtration productsfor various uses.
Facemask production machines are expensive to purchase, representing an investmentin the hundreds of thousands of dollars. However, they can produce hundreds ofthousands of masks per day with a consistency in quality that makes theinvestment pay for itself in a short period of time. And in a crisis situationsuch as the coronavirus pandemic, and automated process is the only way to keepup with the demand for essential PPE such as medical and surgical masks toprotect the health of front line workers such as doctors, nurses, and EMTs.