Introduction To Melt Spinning
What is a Fibre?
A fibre or a filament (a continuous form of fibre) is the fundamental unit of textile materials. It has high strength (tensile, bending, torsional, or compression), high flexibility (i.e. low moduli), extensibility, and shows recover ability on deformation. To know more about tensile properties please refer my article on tensile properties of textiles. Most of these properties are observed about one principal direction, which is known as the axis of the fibre. Since all textile structures -one to three dimensional (yarn, fabric, or braids etc.), are built using this basic structural unit, these structures also possess such unique properties.
In order to possess such properties, the fibre or filament has a unique micro structure (morphology) in which majority of the polymer chains are oriented in the direction of the axis of the fibre. The more oriented the polymer molecules are in this direction, the better properties the resulting fibre/filament is considered to have.
How are fibres made?
Nature has its own ways of arranging these polymer chains in unidirectional orientation to make natural fibres such as cotton, jute and wool, etc. This process possibly involves constructing the natural polymers, such as cellulose and proteins, bond by bond while it is being simultaneously organized in an oriented form. Spiders and silk worm also have a unique methodology of spinning, where the hydrophilic parts of different polymer chains are forced to come together in an oriented form inside the duct of the insect.
As this material is extruded, stretched and dried outside the insect’s body, it converts into a stable, water insoluble highly oriented structure. However, at present, we are unable to adopt such methodologies for producing man-made fibres/filaments. For producing man-made fibres, the polymers (either natural or synthetic) must be unfolded and extended uni-directionally to extremely large dimension to get high aspect (length to diameter) ratio and high orientation. This is known as spinning.
In spinning, a small amount of polymer (say ~1 g) may be elongated to over 9000 km of length while the other dimension (diameter) is only in microns. This requires a precise control over the spinning process to enable such unidirectional extensions in melt form. In fibre formation, all efforts are directed in controlling the microstructure of the polymer so that properties as mentioned above are obtained with respect to the principal axis of the fibre. This is unlike other methods of polymer processing such as injection molding, compression molding, extrusion and blowing etc., where mostly isotropic properties are desired and only a little attention is directed in making a controlled microstructure in terms of molecular orientation and their spatial arrangement. The engineering complexity of the spinning operation is evident from the fact that, in modern spinning plants, large amount of polymer is expected to be continuously converted (spun) into filaments without any breaks.
FIBER SPINNING CONCEPT
What is spinning?
In a typical spinning process, polymer melt or solution is extruded from a fine hole and is elongated by applying a tensile external force on the extruded portion. As the polymer melt or solution is pulled, it is cooled or precipitated, respectively, to form a solid filament. This filament is then usually subjected to post spinning operations such as drawing, which is unidirectional stretching in a semi solid form, and heat-setting, which is crystallization to equilibrium. Other post spinning processes such as texturing is simply a variation of the drawing and heat-setting processes to impart curvilinear shape to an otherwise straight filament. This gives physical bulk to the filaments. The process of fibre formation is complete only when both spinning and post spinning operations are carried out. In this module, an attempt has been made to present the fundamental principles involved in fibre formation
What is melt spinning?
Melt spinning is a process for producing filaments. However, only those polymers that can be melted without undergoing thermal degradation can be spun into fibres/filaments using this process. Some of the typical examples are nylon-6, nylon 66, poly(ethylene terephthalate) and poly(propylene). Though, melting is the essential criteria for carrying out melt spinning, it is not the sufficient condition. There are other requirements that a polymer should meet before it can be regarded as spinnable. These are discussed in detail later in this section.
What are the various components of a melt spinning line?
A typical melt spinning setup consists of a melting device-normally an extruder, a manifold- distribution arrangement for the melt, a metering pump- device to regulate polymer flow rate, a spin-pack- arrangement to filter and extrude the polymer through fine holes, a quench duct- cooling zone for the extruded polymer filament to turn solid, and a winder- a device to pull and wind the solidified filament. The entire line from extruder output to the spin pack are maintained at a constant temperature, which is called the spinning temperature. This arrangement is shown schematically in Figure.
Each of these components in the spinning process has a critical function to perform, which will become clear as the discussion continues. The polymer is taken in chip form, which is a cylindrical form having dimensions in the range of 1-3 mm. A polymer chip is first subjected to an extruder
1. What is an Extruder and what does it do?
In earlier days, the polymer chips were melted using heating grids, however, now, extruders have completely replaced other melting arrangements. Even when polymer is fed to spinning section in melted form directly from a continuous polymerization line, the extruder is often used as the first device in the spinning line. This is because an extruder performs multiple functions. Apart from melting solid polymer chips, an extruder homogenizes the melt by mixing it at various stages. Homogenization is an important aspect in order to ensure continuous spinning without any breaks or non-uniformity in the spun yarns
Extruder compresses the polymer fluid to remove any trapped gasses including air/nitrogen that is drawn along with the chips as they enter the extruder. It also helps in metering the flow rate in the spinning line. It is the first control of flow rate. Finally it acts as a polymer fluid pump and provides the necessary pressure that is required by the polymer to flow from the extruder to the metering pump
What is the design of the extruder?
In order for the extruder to carry out all these functions effectively, extruder design is very critical. Figure Given Below shows the schematic diagram of a typical extruder. It has a cooled feeding zone- where the chips enter the extruder, a melting zone- where chips melt with the heat supplied by the heaters and the heat generated by the dissipation of the polymeric viscous forces, a compression zone- where the material is compressed into a smaller volume to push out trapped gasses. This is followed by a metering zone, which is the narrowest part of the extruder channels. Because of its constricted size, only a limited amount of polymer melt may be dragged through this zone depending upon the screw rpm. Since the metering zone does not allow the entire material coming to the compression zone to pass through, the excess polymer is pushed back resulting in continuous mixing. At the end of the metering zone is the head of the extruder screw, which may have spikes (optional feature) for further mixing/homogenization
The design of extruder and its screw may vary considerably depending upon the material being processed and the principal functions required for processing that material. The readers may refer to other literature for details about the various extruder design and their functions
The design of the screw, which includes the length to diameter ratio (aspect ratio of 1:25 or 1:40), angle of the flanges, etc. is kept in accordance to the specific heat capacity and rheological behaviour of the polymer it is meant to process. Higher the amount of heat required to be transferred to the polymer, longer is the residence time. For example for PP, since the heat capacity of the polymer is large and the molecular weight is often high, it needs longer residence time in an extruder to attain lower melt viscosity. A considerable amount of heat has to be transferred to the polymer to melt it and bring it to the temperature of spinning. Similarly, if a polymer is to be blended with an additive, the extruder design should allow effective mixing and homogenization, which is again the function of the residence time and the shear rate
The design of the screw also has significant effect on the heat generated during shear melting of the polymer, and the energy needed for melting the polymer chips is provided by both the heaters and the mechanical action of the screw.
2. What is a manifold?
Polymer flows from the extruder to the metering pump and spin packs through a manifold, which is a simple network of cylindrical pipes. Each pipe is connected to one metering pump. The manifold is designed in such a way, that polymer takes the same amount of time from the extruder outlet to any of the metering pump whether it is located near or far away from the extruder. This allows the polymers to have same thermal history, hence the same rheological properties, at all positions of the spinning. The same residence time is achieved by keeping the length and bends of each pipe same. Also, the pressure drop across each pipe is kept same so that the polymer gets divided equally. When the distribution lines are long, at times, static mixers are also installed inside distribution pipes. These allow shear mixing of polymer melt within a pipe to keep it homogenized
3. What is a static mixer?
A static mixer is a network of channels interconnected with each other like a honey comb web, which takes the polymeric fluid from the periphery to the centre and that from the centre to the periphery. Also, in this process it induces shear mixing among the various fluid elements
Such mixing becomes necessary because in a pipe flow, fluid travels in a parabolic velocity profile, which means the fluid velocity is maximum at the center and is much lower near the pipe walls. This nonuniform flow profile across the cross-section of the pipe develops due to the stresses exerted on the fluid by the stationary walls. The shear fluid flow has been explained later in this module.
4. What is a metering pump?
Metering pump has a very important function in spinning as it regulates the through put of the polymer from the spinneret. Throughput rate and the winding speed (i.e. take-up speed) together decide the denier of the spun filament. A metering pump must deliver constant throughput irrespective of the back pressure felt by it from the choking filters in a spin pack. Therefore, only the positive displacement pumps are used for metering polymer melt in spinning.
The pump has two gears whose teeth are intermeshed with each other at the center. The gear pair sits in a cavity made into a metal plate and with tip of the teeth in very close clearance from the wall of the cavity. The polymer enters from the one side of the intermeshed zone and fills the empty spaces between the two teeth of each gear as they emerge out from the intermesh(See Figure). This fluid is then taken around the gears by their teeth as shown in the figure. When the polymer reaches the other side of the intermesh, it is forced out of the spaces between the teeth as the teeth enter the intermesh zone. The emptied out or pushed out fluid then exits from the other side of the intermesh to the spin pack. The quantity of the fluid passing through the metering pump is given by the number of teeth getting filled and emptied in a unit time.
Mass flow rate = volume between teeth x no. of teeth in a gear x 2x rpm x density of melt.
The volume of gear pump and its efficiency of pumping may be estimated as per the description given in the literature.
5. What is a spin pack?
Spin pack is the heart of the spinning system (Figure Given Below) It has a reservoir of polymeric fluid-that dampens the pulsating effect of the gear metering pump, a filter pack- that removes the solid particles from the melt. These may be polymer gels, agglomerated additives, contamination, etc. Normally a filter is a set of filters containing 3-5 individual filter meshes, where the first filter is a coarser filter followed by the finer ones. The lowest filter is the finest of all which makes sure that no particle other than those desired (such as well dispersed micron sized particles of an additive such as TiO2, a delustering agent often used in fibres) is pushed through the spinneret hole. Improper filtration will clog the spinneret hole leading to a lower throughput and a lower denier of that filament, and eventually, result in a break of the spinning filament.
It is often believed that if a particle in polymer melt is bigger than 1/10 th of the diameter of the final filament, it will result in a catastrophic break in the filament during spinning or post spinning operations. Not only should the particles of the additives be smaller in size, they should not agglomerate to form bigger particles. Addition of additives in polymer melt is a challenging proposition and all care must be taken to properly disperse the additive particles.
If the polymer is being recycled, often a large amount of dust particles and gel particles are present in the melt. In such spinning lines, polymer is either filtered using a centralized filtration unit (CFU) located just after the extruder or placing additional filtration medium such as sand inside the spin pack cavity (melt reservoir) before the filter pack
Once the polymer is filtered, it reaches a distributor. Function of the distributor, as the name suggests is to ensure proper supply of the polymer melt to all spinneret holes. Also, it makes sure that there is no dead volume and all the polymer coming to the spin pack is being utilized in spinning.
6. The spinneret- an important component
The last but also the most important of all components in the spin-pack is a spinneret plate. It is simply a thick metal plate with fine holes drilled through them as shown in Figure Of Spin Pack. The hole has larger diameter towards the inside surface with conical entry. This allows entry of the polymer melt with less pressure drop. The conical entrance facilitates alignment of the molecules to some extent to enable them to enter without much force. However, the most important dimension in the spinneret is the length and diameter of the final cylindrical spinneret (hole). Polymer passes through it in a shear flow and comes out on the other end as an extruded strand. Spinneret plates for monofilaments have single spinneret hole with diameter of about 0.5-1 mm, while those for multifilament have several holes with diameters in a range of 0.5-0.05 mm. In case of monofilament spinning, spinneret plate with one hole, spins a single filament which is wound on a bobbin. The deniers used for such filaments are in excess of 20. However, for multi filament yarn, the spinneret plate has several holes arranged in a particular fashion and all the extruded filaments from this spin-pack are wound together on one bobbin to make a multi filament yarn. In a multi-hole spinneret plate, holes are placed is staggered configuration so as to allow enough separation from each other and to allow cooling air to be available to all the filaments in the quenching zone. Also, the space between the various holes allows filaments to spin independently without sticking to each other during extrudate swell (also known as die swell) phenomena (explained later).
The main role of a spinneret is to impart cross sectional shape to the extruded filaments. The cross section may vary from circular to trilobal, to hexalobal or hollow, etc. Usually it is thought that spinneret is able to orient polymer chains to make a fibre. It is not true. Spinneret is much bigger than the dimensions of a polymer chain, and hence, can not induce orientation in coiled polymers.
7. Design and role of quench chamber
The extruded filament from the spinneret is allowed to pass through an air quench zone (or chamber). This has mild flow of cooling air at a low temperature (~15- 30 °C) with low to moderate relative humidity. The cooling air when comes in contact with the spun filaments, takes away their heat and facilitates their solidification. This involves cooling of the polymer past melt crystallization temperature and eventually to its glass transition temperature. As soon as the glass transition temperature is reached, the spinning is considered to be complete. This is because, below glass transition temperature, polymer is in glassy state and can not extend any further. The filament speed at which the polymer reaches glass transition temperature is also the spinning speed of the process. Thereafter, the speed of the filament does not change and the polymer is wound on a bobbin using a take-up winder.
The uniformity of airflow is extremely important in controlling the variation of filament diameter in a spun fibre. It has been estimated that a sudden but small change of 1% in quench air velocity may bring about a change of about 0.3% in cross sectional area of the filament.
The quench chambers may be of various configurations- cross flow or radial flow. In cross flow the cooling air flows from one side to the other side of the spinneret across the cross-section of the spinning path . This kind of quench chamber is used when a limited number of filaments are being spun in a filament yarn. This is because the filaments at the far end (in the direction of air flow) of the spinneret get air which has been heated by the filaments at the near end. This problem becomes acute when a very large number of spinneret holes are used in a staple fibre spinning line for making a tow. In such cases 10,000 or more holes are normally present in a single spinneret. For quenching this large number of filaments, radial flow –either of outflow ( or in-flow type is used. In outflow the cooling air enters at the centre of the spinneret and flows out radially while in the in-flow type, the air enters from the periphery of the spinneret and flows into the centre of the spinning path. This air then passes to the take up room along with the filament tow. The radial quenching can accommodate a large number of filaments as more number of spinneret holes can be arranged in concentric circles. Since filaments get more uniform cooling in radial flow than in cross flow, the structure and properties of the various filaments are closer to each other. Fibers with delicate dimensions are also spun in a radial type quench chamber.
8. Spin finish applicator:
Normally the spinning room and the take-up room are separated by a floor and the two have different atmospheric pressure from each other. The spinning room is at a slightly higher pressure (by 10 mm H2O) than take-up room. This allows part of the cooling air to flow along with the delicate freshly spun filaments. The filaments are given a spin finish at the end of the spinning line (just after the glass transition is reached) by one of the many techniques- kiss-roll or spray. The finish is normally sprayed onto the filaments in high speed spinning machines.
The roles of the spin finish are to provide (i) lubrication- to reduce friction between the yarn and the metallic/ceramic parts of the spinning line, (ii) antistatic property- to allow dissipation of static charge generated due to contact of yarn with the machine parts and (iii) cohesion- to keep the filaments of a yarn together, so that unwinding becomes easier from the spun cake. Lubrication is provided by aliphatic/alkyl molecules, which have very low van der waals attraction among them. Antistatic properties/cohesion is provided by polar molecules, which have strong hydrogen or ionic bonding and provide path for charge dissipation. Since a spin finish needs both types of molecules, it is generally made by emulsifying alkyl chain molecules with the help of surfactants in aqueous medium. A balance of the two ingredients is important to achieve an optimum of all properties needed in a spin finish.
Readers may refer to literature for information on spin finish functions and its formulation.
9. What is a take-up winder?
The next important device is the take-up winder. Usually, the yarn is not wound directly on the winder but is passed through a take-up godet, or a set of godet rollers. This breaks the vertical path of the spinning and allows the winder to be adjusted comfortably in the available space. Also, in an integrated system, the spun filaments may be subsequently drawn between the two godets before winding (also known as in-line spin-draw frames). The spinning speed is decided by the speed of the first rotating surface the filament comes in contact after the spinning chamber. This can be the first take-up godet or the take-up winder if the spun filaments are being directly wound onto the winder without breaking their vertical path.
The winders may be friction driven where the bobbin is driven by a friction roller so that the surface speed of the winder remains constant throughout the formation of the yarn package. However, now a days, godets and friction rollers are not being used in high speed spinning plants. This is because the yarn when comes in contact with such surfaces can be abraded and may result in poor quality or poor wind-up. Therefore, new winders are used that have bobbins which are directly driven by a motor. In order to compensate for the increasing speed as the diameter of the bobbin package changes, an auto feedback mechanism is installed where the speed of the winder is regulated to maintain constant tension in the spinning line.
Now you know very much about basics of melt spinning. Please feel free to comment or you can also raise questions, if you want to know anything else about textiles.