April 2000

Producing Nonwoven Fabrics by
Melt Extrusion: An Introduction to Meltblown and Spunbond Processes

By Alex James, CEO, Alex James & Associates

Nonwoven fabrics are produced by several different methods. The earliest fabrics were created from staple fibers, which were carded and formed into webs of loosely laid short fibers. The webs were stacked to give desired thickness by laying several layers onto a traveling belt, using a cross-lapping machine. The web was then fed into and through a needling machine which, by its up and down motion of specially designed needles, tacked the layers into a cohesive fabric. This method of producing nonwoven fabrics is still widely used today to produce heavy nonwovens.
Another similar method of forming a nonwoven fabric is hydroentangling, whereby the cross-lapped layers are entangled using high velocity streams of water. In the 1950s, Exxon Chemical introduced a new method of making nonwovens called meltblown. In the 1960s, another method, called spunbond, appeared. This article examines these latter two methods of producing nonwovens.

This original Exxon apparatus is still in existence and is located at the University of Tennessee. Click Here to See Sketch 1

The Meltblown Process
Exxon invented and patented the first meltblown nonwoven apparatus. Being a producer of polypropylene, the company introduced this apparatus to the textile industry in effort to sell polypropylene. Companies that were producing lightweight scrims from cotton for use in sanitary products, such as diapers and some medical fabrics, saw this as a way of producing cloth very inexpensively. These companies built very inexpensive machines consisting of a polymer extruder; a film die fitted with a long, narrow, spinneret, a hot-air plenum and an apparatus for taking up the fabric. Sketch 1. show such apparatus.
All of the original meltblown machines depended on the extruder screw speed for metering the desired quantity of polymer. In the early and mid 1980s, many of these machines were rebuilt incorporating filters and metering pumps. This made a significant improvement in fabric quality. These early machines produced fabrics one to 1.5 meters in width and operated at 25 to 50 meters per minute. Modern day meltblown machines are now operating at much higher speeds and making wider fabrics. A typical state-of-the-art meltblown machine is illustrated in Sketch 2.

• Extruder – Extruders for melt blown are basically the same as these used for producing thin films and fibers. Screw details and length to diameter ratios are selected for the polymer being extruded. A 24/1 length to diameter ratio is typical for polyester. Some machines are equipped with static mixers and some are not. This generally depends on screw design and opinion of the designers.

• Polymer Filtration – Care must be taken in selecting the proper filtration equipment for the polymer before it enters the metering pump. There are several manufacturers of good automatic screen-changing filtering systems available. The ones that do not interrupt the polymer flow are the best. In the case of the continuous screen filters, extra care must be taken if polymers of different melting points are fed one after the other. This is because of possible leaking of polymer in the sealing zone when a lower temperature polymer follows a higher temperature polymer.

• Metering Pump – The metering pumps used in meltblown are generally larger than those used for textile fibers and normally the sizes used in the film industry. Inlet pressure to these pumps is regulated using a feedback control loop, which directs the extruder motor drive to turn the extruder screw at a speed to maintain the specified pressure. Extreme care should be taken to keep the pump outlet pressure below the maximum rating of the spinneret — normally no more than 400-500 psig.

• The Melt Blown Die – This is the most critical part of the meltblown fabric forming system. It consists of the die body — which in most cases is similar to coat-hanger film dies — the spinneret plate and the hot-air plenum.
The die body is usually made from a heat-treated tool steel. Maintaining close machining tolerances and polishing the interior of the die body are required. In the lower section of the die-body, where the coat hanger design ends, there should be a 1” or 2” vertical section designed to create a good back pressure in the polymer flow, thus evening out flow to the spinneret.
The spinneret plate is a one-piece machined device attached to the die body with bolts. If the bottom of the die body and the top of the spinneret are properly lapped it will not be necessary to use a gasket. The cavity leading down to the spinneret holes will contain a breaker plate and a set of filter screens. Most spinnerets will contain from 15 to 25 holes per inch, and a few have been known to be considerably higher. Most spinnerets will tolerate pressures of up to 300-400 psig, which must be closely maintained. Spinnerets have been known to split like a zipper opening and cannot be repaired. Also, they are very expensive. As an example, a spinneret having 25 holes per inch that are 0.4 mm in diameter and with a capillary length of 4.0 mm, will cost about $1000 per inch.
The hot-air plenum, sometimes referred to as the air knife, is an enclosure that covers the sides of the die body along the length of the spinneret. The requirement is that it receives hot air, up to 500ºC, from one or two sides and equally distributes it to the tip of the spinneret. At this point, the spinneret is sharply pointed. The bottom of the plenum is fitted with a sliding closure that can be adjusted to create desired airflow conditions. Airflows of 300 to 500 meters per second through a 1.0 mm wide air-gap are often used. Electric heaters in the plenum maintain the heated air being supplied. When the polymer is pumped through the spinneret the, hot, high-velocity air impinges the fibers coming out of the holes and propels them in the direction of airflow. The air velocity is such that the fibers are stretched and break into small fibers that are then sucked onto a fabric-forming belt.

• Forming Box – The forming box is an apparatus for accepting the fibers being blown from the spinneret. Assuming that a uniform stream of fibers is coming from the spinneret, the duty of the forming box is to gather these fibers in uniformly distributed manner. The control of airflow across the complete width of the forming box is the key to good fabric formation. Air velocities from 30 to 100 meters per minute are normally sufficient for good fabric formation. Most forming boxes use a woven wire mesh fabric as the collection surface rather than plastic belts as used in the paper industry.
This is because on startups, occasionally, molten polymer will fall onto the belt and, if it’s a plastic belt, it will be almost impossible to clean.
A suction box is located just below the forming belt. Most boxes have several compartments fitted with dampers to control the airflow pattern. The belt is supported and rides on a fluorocarbon polymer surface that minimizes wear on the belt. The belt travels around a set of pull rolls, which also contains a tensioning roll. Speed and belt alignment must be very precisely controlled.
Forming boxes are operated at speeds of a few meters per minute to as fast as 1000 meters per minute. Speeds of production machines normally run from 200 to 500 meters per minute. It is always wise to install some type of air filtration device before the suction fan.

• Calender Rolls – Calender rolls can be very simple or very complicated. This depends on how the fabric is pulled from the forming box, how much pressure is to be exerted on the fabric, what type of surface pattern is to be imparted in the fabric and whether the rolls must be heated. Most commercial calandering systems have heated rolls, and, in many cases, one of the rolls is embossed so that it leaves a distinct pattern in the fabric. The same forming and calandering systems are also used in making spunbonded fabrics

• Takeup – Takeups for collecting nonwovens are very similar to those used for high-speed take-up of other types of textiles and also films. These are commercially available units from a number of manufacturers.

• Air-Compressor – Most any air-compressing device that produces, non-pulsing, clean air can be used to produce the required airflow necessary in making meltblown products.
In the early days, high-pressure air compressors, up to 120 psig, were used. This air had to be cooled in order to remove the water being produced, and oil if the compressor was not the oil-less type. In today’s systems, actual air pressure required is in the order of 6 – 15 psig. This is easily obtained from a multi-stage blower. There are also some economic advantages. The air is clean, the water stays in solution with the air, the air is heated to around 65ºC and lower schedule piping can be used. A 75 Hp blower will generate 15 psig air that is suitable for a one-meter-wide spinneret with an air gap setting of 1 millimeter.

• Compressed Air Heater – There are several suitable types of air heaters available. The most commonly used heater consists of a section of pipe, which contains sufficient resistance heaters. The heaters can be sheathed with fins or bare-resistance wire. Since a considerable amount of power is usually packed in a short tube, extreme care must be taken to make sure that air is always flowing across the heating elements when they are on. Most meltblown systems have both low flow and over temperature detection with appropriate shut down controls.
A typical one-meter-wide meltblown die can be successfully operated using a 75 HP multi-stage blower and an 80 KW heater. This assumes that the compressor is reasonably close to the unit to minimize pressure losses in the piping. Normally 6 – 9 psig delivered to the air plenum is sufficient.

• Suction Blower – There are several items that must be considered when selecting a suction blower. The cross-sectional area of the forming box and the air velocity required will set the volume of air to be used. The other critical item is the pressure loss across the fabric being formed. Very thin fabrics may only require 1-2” (water column). This affects the size and horsepower required to drive the blower. A 5 – 7.5 HP blower is sufficient for a 1-meter-wide line. Heavier fabrics may produce higher pressure drops.
It is also desirable to have a filter system on the inlet of the fan. The self-cleaning or continual cleaning types are the best. This keeps the rotor and other internals of the blower from collecting small fibers. In any case a light filter on the discharge will collect any fibers passing through the blower into the room.

24-inch Reicofil Meltblown Line at University of Tennessee

•Machine Frame – The machine framework holds the extruder, the pump, manifolds, filtration and the melt-blown die. Sketch II illustrates a melt blown system wherein the distance from the spinneret to the forming box is set by lifting the frame away from the forming box. A typical lifting platform unit will have the gap between the spinneret and forming box set a 200mm minimum. The jacking device can then lift the platform an additional 450mm to create a larger gap of 650 mm. This system allows the top of the forming box to be fixed and is desired when the fabric is to be further treated on adjacent equipment.
Another common way for packaging meltblown systems is to have the extrusion equipment and spinneret fixed and then moving the forming box up or down. This system is desirable when more than one extruder and spinneret are feeding onto the forming box. Multilayered fabrics of several polymers can then produce a sandwich fabric.
The Spunbond Process
It is easy to visualize a fabric-making machine where an extruder is connected to a wide, multihole, spinneret that allows the extruder to pump polymer through it forming many strands of fibers which then fall onto a traveling conveyor. The fallacy is that the fabrics produced in this manner would be weak and easily stretched out of shape because the fibers would not have been stretched.
Stretching the fibers causes molecular orientation, which makes the fibers stronger. The degree of stretching controls the ultimate strength in the fibers. The earliest spunbond units were equipped with sets of draw rolls, which would stretch the filaments coming from a number of spinpacks that often contained spinnerets having as many as 500 holes each. Then, each filament bundle was pulled through a tube to a suction device and then blown down on a conveyor. These devices were mounted on a rail that was extended across the conveyor which could be as wide as 20¢.
Several different kinds of devices located at the end of the conveyor were used to compact and heat-set the fabric. Some of these were steam chambers and others were heated rolls. The steam chambers were used when the filaments were bicomponent, having an outer layer of a low melting polymer that would cause them to stick to each other. Single-component filament fabrics would be sprayed with a resin, which would hold the fabric together after passing between rolls.
The modern spunbond unit has replaced the draw rolls and, instead, uses a device wherein high-velocity air engages and draws the filaments. The filaments emerge from the spinneret and are cooled while passing through a quenching system before entering these devices.
These devices are often referred to as drawdown units and are either round tubes or long slotted fabrications. Collection boxes, calendering rolls and take-up units are similar or the same as used for making melt-blown fabrics. A typical spun-bond machine is illustrated in Sketch 3.

• Extruders – The extruder or extrusion system selected will depend on the end product being produced. The simplest, single-component system will require an extruder similar to those used to make single-ply film. When producing bicomponent filaments, two extruders will be required. In either case, a single screw extruder is normally satisfactory. However, a recent trend is to use twin-screw extruders.
A twin-screw will allow the producer to use polymers that are not completely dry. Such a case would be to use nylon or polyester polymers. In addition, polyester polymer would not have to be crystallized. PVA polymers would always require a twin-screw extruder. This could also be true when certain additives must be incorporated.

• Filters – There are a number of suitable filters on the market. The requirement for spunbond will be the same as for meltblown.

A typical spunbond machine is illustrated here. Click Here To See Sketch 3

•Metering – Metering systems with single pumps are also similar to those used in meltblown except that two pumps are required for bicomponents, which also require independent drive systems because one or the other of the polymer flows could be more or less than the other.
With units making fabric widths more than one meter wide, multiple pumps are used. The designer of such systems must be very careful in selecting the proper drive system. Motors that are sensitive to heat should be avoided. A good selection would be synchronous motors being driven with a single, high accuracy inverter. Pump pressure required for spunbond are normally considerably higher than meltblown and will require more horsepower.

• Spin-Block – Often known as a spinning beam, this unit must provide even heating to its components which are:
The Spin Block Body – This is a weldment designed to hold and heat the spinpack. It must accurately support the spinpack and allow for easy installation. The spin block nearly always has a cavity in which heat transfer fluids are circulated or condensed. Some spin blocks are internally heated using electric submersion heaters or are heated by fluid flowing from external heaters.
Spin Pack – This device, when seated in the spin block, receives molten polymer from the metering pump and distributes it evenly through a filter to the spinneret.
Spinneret – The design of the spinneret will depend upon the product to be produced. Some packs and spinnerets are round and limited to certain diameters because of heat and polymer distribution while others can be very long and not too wide. A typical round spinneret can have only a few holes, perhaps 36, or up to several hundred, but long rectangular spinnerets can have up to several thousand holes. Round and rectangular packs and spinnerets are used for producing both single and bicomponent fabrics. The difference being how the polymer or polymers are distributed to the spinneret.
The design of the spinneret holes depends upon the polymer being used. Olefins normally require a spinneret that has a longer length-to-diameter capillary. The total spinneret hole length depends on the pressure being exerted by the metering pump. Therefore, the spinneret plate has to be designed to withstand the pressure being exerted. For large diameter or wide spinnerets, the spinneret plates are much thicker. Since the length of the capillary is fixed, the extra thickness must also contain a longer lead hole down to the capillary. A typical capillary could have a hole diameter of 0.35 mm and be 2.8mm long.
Some very promising results have been developed using technology similar to that used in making multilayer film. This allows two or more polymers to be extruded through multirow spinnerets. This technique could have some advantages over sheath-core bicomponent in that the spinpacks are less likely to leak.

• Quench System – Molten fibers coming from the spinneret must be solidified and cooled before going into a pull-and-stretch device. Almost all systems use either a cross-flow or in-flow system to supply conditioned air to cool the fibers. A two-sided in-flow quench air supply box allows the fibers to be cooled in a shorter distance than a one-sided cross-flow quench box. Most quench boxes have internal dampered sections for airflow pattern and velocity control. In some systems, the quench air system serves as the attenuation air.
The most successful quench systems take advantage of back pressure and pressure drop generated as air passes through a series of screens. Equal air pressure behind the entire screen will generate equal airflow through the screen and onto the fibers. The quench system should be designed to provide from 15 – 90 meters per minute of air velocity.

• Suction Devices – A nonwoven suction device is used to pull the fiber from the spinneret and stretch it, causing molecular orientation thereby strengthening the fiber. The driving force in a suction device is clean compressed air. Air pressure requirements vary from 45 psig to 120 psig. Very early designs required 250 psig.
The original suction devices were all round and had a section at the top that distributed air around the circumference of a tube through a venturi and then through a section of tubing. A successful suction device had to receive the fibers, which were converged into a bundle and then propel the fiber bundle at a high speed through the tube in an orderly manner. This requires even airflow that does not tangle or twist the filament bundles. The better devices had internal dampering gadgets that would allow airflow adjustment while the system is running. Air velocities of 2000 to 5000 meters per minute are generally required.
Another very important requirement is that the suction device must exit in such a manner that is spread out over a determined width. This is done by adding a section to the bottom of the suction device that configures the air tube from round to a flat nozzle. Therefore, instead of blowing a round pattern, the fiber pattern is long and not too wide.
Over the years, engineers have developed a number of gadgets to accomplish even distribution of the fibers as they are laid on a collection device. These devices include air foils, piddlers and electrostatic chargers. It is not difficult to see that using a number of round devices in a row to make wide fabrics can produce inconsistencies in the fabric being made. In some cases, staggering these tubes in order to get some cross lapping has helped.
In order to solve problems experienced while using round suction devices, a new type suction device has been introduced. This new suction device has a continuous slot that is the same width as the fabric being produced. Compressed air is fed in at both sides of the slot. The slot is adjustable, normally from 2 to 4mm and is about 250 mm deep. The compressed air is introduced near the top of the slot through an adjustable air knife. Velocities from 3000 to 6000 meters and even to sonic speeds can be accomplished.
An adjustable guiding nozzle of 300 to 800mm long is mounted to the bottom of the slot body and the width can be adjusted from 3 to 6mm. It is now possible for fibers coming from a long rectangular spinneret to enter a suction device that does not require converging and then have to be spread out to cover the formed width of the fabric.
The slot-type suction device for making nonwovens appears to be the trend in newer as well as retrofitted nonwoven lines. Widths of up to 4 meters are in operation on lines making nonwovens at speeds of up to 600 meters per minute.

• Air Compressors – The air compressors for producing air for spunbond should be of the non-pulsing type. The air must be free of water and oil. The output pressure of the compressor should be adjustable from 40 – 120 psig. A typical 500mm spunbond line using a slot die will require a 75 HP compressor that will produce 10 to 20 cum of air.

• The forming box, calender rolls and take up for spun-bond are the same or similar to those used in meltblown.

• Lift – In making spunbond, the forming box can be stationary or mounted on a lift. The lift is used to raise or lower the suction box to form the desired gap between the slot or round suction devices. Other ways of adjusting this gap include flexible tubing from the extruder to the spin beam.

Mr. Scott Gessner, a nonwovens expert and industry consultant who is president of Gessner & Associates Inc., reviewed this article for content and authenticity. Mr. James can be reached at Alex James & Associates, Tel: 1 864 879 4224. Fax: 1 864 879 0117. Email: ajames@ajafibers.com