Blown Film Extrusion (Film Blowing)

Blown film extrusion is the process by which most commodity and specialized plastic films are made for the packaging industry.

The Process

A typical set-up for blown film extrusion is shown in figure-2. In this instance the molten polymer from the extruder enters the die from the side but entry can also be effected from the bottom of the die. Once in the die, the molten polymer is made to flow round a mandrel and emerges through a ring shaped die opening, in the form of a tube. The tube is expanded into a bubble of the required diameter by an air pressure maintained through the centre of the mandrel. The expansion of the bubble is accompanied by a corresponding reduction in thickness. Extrusion of the tube is usually upwards but it can be extruded downwards, or even sideways, the bubble pressure is maintained by pinch rolls at one end and by the die at the other. It is important that the pressure of the air is kept constant in order to ensure uniform thickness and width of film. Other factors that effect film thickness are extruder output, haul-off speed and temperatures of the die and along the barrel. These must also be strictly controlled.As with any extrusion process, film blowing becomes more economical as speeds are increased. The limiting factor here is the rate at which the tubular extrudate can be cooled. Cooling is usually achieved by blowing air against the outside surface of the bubble. Under constant air flow

conditions an increase in extrusion speed result in a higher 'frost' line (the line where solidification of the extrudate commences) and this leads to bubble instability. Increasing the air flow gives more rapid cooling and lowers the 'frost' its application because too high a velocity of the air stream will distort the bubble. Various designs of air cooling rings have been worked out in order to produce improved cooling without these attendant difficulties. On higher output lines, the air inside the bubble is also exchanged. This is known as IBS (Internal Bubble Cooling). In the cooling step of blown film extrusion, the amorphous, transparent melt crystallizes to form a translucent, hazy, or opaque film. The point where opacity begins in the bubble is known as the frost line. The lay-flat film is then either kept as such or the edges of the lay-flat are slit off to produce two flat film sheets and wound up onto reels. If kept as lay-flat, the tube of film is made into bags by sealing across the width of film and cutting or perforating to make each bag. This is done either in line with the blown film process or at a later stage. Blown film extrusion is an extremely complex subject and there are many problems associated with the production of good quality film.

The frost line height is controlled by several parameters: the air flow, film speed, and temperature difference between the film and the surroundings.

Properties of the film, such as tensile strength, flexural strength, toughness, and optical properties, drastically change depending on the orientation of the molecules. As the transverse or hoop direction properties increase, the machine or longitudinal direction properties decrease. For instance, if all the molecules were aligned in the machine direction, it would be easy to tear the film in that direction, and very difficult in the transverse direction.

Typically, the expansion ratio between die and blown tube of film would be 1.5 to 4 times the die diameter. The drawdown between the melt wall thickness and the cooled film thickness occurs in both radial and longitudinal directions and is easily controlled by changing the volume of air inside the bubble and by altering the haul off speed. This gives blown film a better balance of properties than traditional cast or extruded film which is drawn down along the extrusion direction only.

Fig. 2 : BLOWN FILM EXTRUSION

Common Problems

The film bubble may be established by supporting it with horizontal stationary guides or the whole extruder may be protected from stray air currents by a film curtain. Other causes include non-alignment of the guide roll and the pinch rolls, or non-uniformity of pressure across the face of the pinch rolls. Among the surface defects mentioned earlier, 'fish eyes' are due to imperfect mixing in the extruder or to contamination. Both of these factors are controlled by the screen pack which not only screens out contaminating particles but improves homogeneity by increasing the back pressure in the extruder. 'Orange peel' or 'apple sauce' are also surface defects caused by inhomogeneity of the molten polymer. Since low density polyethylene forms by far the greatest percentage of all film made, it will be useful to consider the influence of the various polymer parameters such as metal flow index and molecular weight on the film properties. Impact strength, for instance, increases with molecular weight (i.e. decreasing melt index and with decreasing density. Heavy duty sacks, for instance, are normally made from polyethylene grades having densities between 0.916 and 0.922 g/cm3 and melt indices between 0.2 and 0.5. For thinner technical film as used in building applications or waterproof lining of ponds, higher melt indices have to be used because of the difficulty of drawing down very viscous melts to thin film. Melt indices of between 1 and 2.5 are more useful, therefore, and impact strengths are less than for heavy duty sacks. Clarity is, however, improved. Where a good balance of properties is

required as in the medium clarity/medium impact grades, slightly higher densities are used (0.920 to 0,925 g/cm3) and the melt index is varied between 0.75 and 2.5. For high clarity, a high density and a high melt index are required since increases in both these properties cause an increase in see-through clarity, a decrease in haze and an increase in gloss. High clarity film will, of course, have a relatively poor impact strength because of the high melt index and such film should not be used for packaging heavy items. Among the many other defects low tensile strength, low impact strength, hazy film, blocking and wrinkling. Air entrapment between film layers and rollers – this may cause film scratching or wrinkling, or processing problems when winding up the film due to reduced friction. Wrinkling is a particularly annoying problems because it can be costly, leading to scrapping of a roll of film, and because it can arise from such a wide variety of causes that it is likely to occur even in the best regulated extrusion shop. If the film is too cold when it reaches the pinch rolls, for instance it will be stiff and this may cause crimping at the nip and wrinkling. One way of raising the film, temperature at the nip rolls is to raise the melt temperature but this can lead to other troubles such 'as blocking. In fact, this is illustrative of the whole subject of film blowing inasmuch as compromises are often necessary to achieve the best balance of properties. Wrinkling can also be caused by the die gap being out of adjustment. This causes variations in film thickness and can lead to uneven pull at the pinch rolls. Another cause of wrinkling may be surging from the extruder or air currents in the extruder shop. Both of these factors can cause wobbling of the film bubble and thus wrinkling at the wind-up stage. Another possible solutions to this is using a vacuum to remove eentrapped air or by using winding rolls with a diamond shaped groove in the rubber cover to increase surface area and decrease amount of entrapped air in the film.

Large output fluctuations from the die – this causes thickness variations, and can be prevented by keeping the extruder clean and by using more consistently shaped pellets in the extruder.

Melt fractures – these appear as roughness or wavy lines on the film surface, and can be eliminated by lowering the viscosity of the polymer melt. This can be done by increasing the melting temperature or by adding an internal lubricant to the material composition.

Thickness variations in the film – this can be avoided by centering the die in the extrusion line before every run, adjusting the air speed of the cooling system, or by using heated die lips.

Die lines on the surface of the film – this defect reduces the aesthetic appeal of the film, reduces optical properties, and weakens mechanical properties such as tear strength. This can usually be avoided by routinely cleaning the inner surfaces of the die and by refinishing scratched or roughened flow surfaces.

Gels – these defects are small, hard globules encapsulated in the film or stuck on the film surface and reduce the aesthetic appeal of the film and cause stress concentration points which may result in premature failure. These are caused by overheating to the point of polymer degradation in the die, and can therefore be avoided by cleaning the inner surfaces of the die on a regular basis.

Optimization of the Process

Coextrusion

One way to improve the line efficiency of blown film extrusion is to implement coextrusion. This is the process of extruding two or more materials simultaneously through a single die. The orifices in the die are arranged such that the layers merge together before cooling. This process saves time because it extrudes two or more layers at the same time, and it provides a method with fewer steps to produce multilayer films. The production rate for a coextruded multilayer film of three layers is about 65m/min, and the production rate for a single layer of blown film is about 130m/min[11]. Thus, in order to produce 10 000m of a three layer multilayer film, it would take almost 4 hours using a single layer blown film process, and only 2 and a half hours using the coextrusion process. Furthermore, the film produced from the single layer process would require

an extra step to glue the layers together using some sort of adhesive. Coextrusion is the least expensive means of producing layered films and the coextrusion system is capable of quick changeovers to minimize production line down time.

Minimizing the Melt Temperature

The efficiency of blown film extrusion can be improved by minimizing the temperature of the polymer melt. Reduction of the melt temperature causes the melt to require less heating in the extruder. Normal extrusion conditions have a melting temperature at about 190° C despite the fact that the temperature of the melt only needs to be about 135°C. However, it is not always practical to decrease the melting temperature by that much. By decreasing the melt temperature 2 to 20°C, the motor load can be decreased by about 1 to 10%. Furthermore, reduction of the melt temperature causes less need for cooling, so there is a reduced use of the cooling system. Moreover, removing heat from the bubble is usually the rate-limiting factor in this extrusion process, so by having less heat in the polymer to remove, the rate of the process can be increased, thus yielding higher productivity. A way to maintain the melt temperature at a minimum is to choose an extruder that is matched to the specific processing conditions, such as the material of the melt, pressure, and throughput.

Heated Extrusion Die Lips

Typically, solutions to melt fractures involve decreasing the output or increasing the melt temperature to decrease the shear stress in the extruder. Both of these methods are not ideal because they both reduce the efficiency of the blown film line. Heated extrusion die lips can solve this problem. This targeted heating method allows for film extruders to be run at higher production rates with narrower die gaps while eliminating melt fractures. Direct heat is applied to the surface of the polymer melt as it exits the die so that viscosity is reduced. Therefore, melt fractures, which are caused when trying to extrude too much of the polymer at one time, will no longer act as a limiting factor to increasing the production rate. Furthermore, heated die lips use less energy than increasing the melting temperature because only the surface of the melt is heated and not the bulk of the liquid. Another benefit of using heated die lips is that thickness variations can be controlled by adding heat to certain areas along the die circumference to make the film at that position thinner. This would ensure that no excess material is used.

Disadvantages

Blown film has a less effective cooling process than flat film. Flat film cooling is done by means of chill rolls or water, which have significantly higher specific heat capacities than the air that is used in the blown film cooling process. The higher specific heat capacity allows the substance to absorb more heat with less change in the substance temperature. Compared to cast film, blown film has a more complicated and less accurate method to control film thickness; cast film has a thickness variation of 1 to 2% versus the 3 to 4% for blown film. The resins used for casting typically have a lower melt flow index, which is the amount of polymer that can be forced through a standard die in 10 minutes according to a standard procedure. The melt flow index for cast film is about 5.0 g/10 min where as for blown film it is about 1.0 g/10 min. Consequently, the production rates for cast film are higher: cast film lines can reach production rates of up to 300m/min where as blown film lines are usually less than half this value. And finally, cast film has better optical properties, including transparency, haze, and gloss.

Materials: Polyethylenes (HDPE, LDPE and LLDPE) are the most common resins in use, but a wide variety of other materials can be used as blends with these resins or as single layers in a multi-layer film structure. these include pp, pa, evoh. In some cases, these materials do not gel together, so a multi-layer film would delaminate. To overcome this, small layers of special adhesive resins are used in between. These are known as “tie layers”.

Advantages:

Produce tubing (both flat and gussetted) in a single operation Regulation of film width and thichness by control of the volume of air in the bubble, the

output of the extruder and the speed of the haul-off Eliminate end effects such as edge bead trim and non uniform temperature that can result

from flat die film extrusion capability of biaxial orientation (allowing uniformity of mechanical properties) Very high productivity Permits the combination of a number of different materials and properties

Applications:Blown film can be used either in tube form (e.g. for plastic bags and sacks) or the tube can be slit to form a sheet. Typical applications include Industry packaging (e.g. shrink film, stretch film, bag film or container liners), Consumer packaging (e.g. packaging film for frozen products, shrink film for transport packaging, food wrap film, packaging bags, or form, fill and seal packaging film), Laminating film (e.g. laminating of aluminium or paper used for packaging for example milk or coffee), Barrier film (e.g. film made of raw materials such as polyamides and EVOH acting as an aroma or oxygen barrier used for packaging food, e. g. cold meats and cheese), films for the packaging of medical products, Agricultural film (e.g. greenhouse film, crop forcing film, silage film, silage stretch film).

Cast Film Extrusion

Basic Concepts of Cast Extrusion In the cast film extrusion process, the molten polymer travels through a flat die system to adopt its final flat film shape. The die system is formed by the die and feedblock (if the process requires coextrusion) or simply the die, if the process is that of mono-layer extrusion. Figure 1 shows a coextrusion cast film line. The process starts with the feeding of plastic resins by means of a gravimetric feeding system to one or more extruders. The materials are then melted and mixed by the extruders, filtered and fed to the die system. Immediately after exiting the die, the molten curtain enters the cooling unit where its temperature is lowered with a water cooled chill roll to freeze the film. The rollers are highly polished turning roll, where it is quenched from one side. The speed of the roller controls the draw ratio and final film thickness. The film is then sent to a second roller for cooling on the other side. Finally it passes through a system of rollers and is wound onto a roll. The film is then passed downstream where the edges are trimmed, corona treatment is applied (if a fabrication process such as printing or coating is required) and the film is wound into rolls. A description of the main components of a typical cast film line is presented below. Most flat dies are of T-slot or coat hanger designs, which contain a manifold to spread the flowing polymer across the width of the die, followed downstream by alternating narrow and open slits to create the desired flow distribution and pressure drop. Most cast film lines manufactured today are coextrusion lines, combining layers from as many as 7 extruders into the product through multimanifold dies, or else single manifold dies with the aid of feedblocks.In a cast film extrusion process, a thin film is extruded through a slit onto a chilled, Thicker polymer sheets can be manufactured similarly. A sheet is distinguished from a film by its thickness; by definition a sheet has a thickness exceeding 250 m. Otherwise, it is called a film.

Cast Film Line Components:

Gravimetric Feeding System Gravimetric feeding systems control the amount of material that is fed into the extruders by weight, not volume. The system is more precise than its volumetric counterpart and features a reduced error tolerance in the order of 0.5%. In many cases, the film is fabricated with materials that are blends of a base polymer with one or more secondary components. In state of the art production lines, this blending is carried out inline. Special care is needed to prevent premature melting of the pellets, especially when materials with low melting temperatures are processed, or when the pellet size is small. Vibration and cooling of

the feeding hoppers are options recommended to alleviate this problem. It is also important to ensure that the material being fed carries no moisture that could give rise to the appearance of small bubbles, also known as fish eyes, in the final film. In some cases, drying of the material is required. This may be performed by a separate unit or by a highly sophisticated feeding system with built-in drying capabilities.

ExtruderThe main functions of an extruder are to melt the plastics pellets and mix the resulting molten polymer to achieve a homogeneous melt. This is done by conveying the material along a heated barrel with a rotating screw. Commercially used extruder barrels are typically 3 (90 mm) to 6 (150 mm) in diameter. The screws are tailored to the specific characteristics of the extruded materials and process parameters. The length of the screw is heavily influenced by their diameter. Screw length to diameter (L/D) ratios commonly lie in the range of 26:1 to 30:1. It is critical to ensure that the flow exiting the extruder is well controlled and constant with variations on the screw's rotational speed not exceeding 1%. A failure to accurately control the screw speed typically results in undesired pulsating flow that can cause periodic changes in film thickness in the machine direction. The metering section, or final section of the extruder, is designed to guarantee a precise dosing of material from the extruder. In order to achieve the above, the gap between the screw and the barrel is very small. This creates another challenge since it is difficult to maintain a constant gap between the rotating screw and the barrel. To overcome the above-mentioned potential problems, a melt pump is commonly employed downstream of the extruder. The pump is a positive displacement device that produces a consistent flow regardless of the discharge pressure of the extruder (Figures 2 and 3). The pump alleviates the workload on the extruder by taking on the job of generating pressure. The reduced extruder head pressure translates into energy consumption savings, a drop in the melt temperature and less wear between the barrel and the screw. In coextrusion lines, the number of extruders depends on the number of different materials being extruded and not necessarily on the number of layers. This is because the existing feedblock technology allows the flow from one extruder to be split into two or more layers in the final coextrudate.

Filtration System The objective of the filtration system is to prevent downstream passage of melt impurities and/or gels that are formed during the extrusion process. Proper control at this stage is imperative to prevent melt contamination. The most common filters are those containing a metallic mesh. The case hosting the filter media has to be capable of bearing the forces exerted by the polymer flow when subjected to the maximum pressure allowed by the extrusion process. It is highly recommended to use continuous screen changers, in which the mesh is continuously regenerated, to minimize the replacement time of the screen pack. Applications of Cast Extrusion

The cast film process is used for very tight tolerances of thin film, or for lowviscosity resins. Cast films are used for food and textiles packaging, flower wrapping, as photo album page protectors, as coating substrates in extrusion coating processes or laminated to other materials in the formation of more complex films, among others.

Typically, the cast film process involves the use of coextrusion, which is a simultaneous extrusion of two or more materials from a single die to form a multi-layered film. This is because in many cases the final application of the plastic film demands a performance that cannot be

achieved if the film is composed of only one material. For example, in many instances food packaging applications require the use of films with oxygen barrier capabilities. To meet the requirement a high oxygen barrier material like EVOH is combined with polyolefin materials in a multi-layered structure. Coextruded films typically contain up to seven layers; however, the use of more layers is becoming more common. The number of layers, their position in the coextrudate and their individual thickness are all variables that change depending on the particular application of the film. Benefits/Limitations of Cast Extrusion Unlike the blown film process, the cooling of the film with cast extrusion is highly efficient. This allows for higher production line speeds resulting in higher production rates with superior optical properties of the product. In chill roll or cast film extrusion, the molten polymer is cast through a narrow slot die and drawn down and formed onto a rotating chill roll with the aid of a web forming device. One or more additional chill rolls can be provided fro improved film cooling and higher outputs. A heated roll can also be provided for annealing the film. The degree of draw and orientation is significantly lower in the cast film process than in the blown film process. This is the reason why the thickness distribution in the machine cross direction is more uniform with cast processes (with variations that ould be as low as 1.5%). However, the film mechanical properties in the machine cross direction are lower when compared to those obtained with the blown film process due to the higher level of orientation that the film experiences in the blown process. The principal advantages of the film casting method compared with the quench-tank method are substantial improvements in the control of optical properties of the film, potentially increased output with larger diameter chill rolls, production of films with a higher modulus, improved thickness and profile control.  The orientation of the die can vary greatly depending on the machinery manufacturer or process conditions. A die that extrudes vertically downward is generally preferred. However, dies which extrude horizontally or at any angle between vertical and horizontal can be used. Horizontal orientation is typically used in the production of rigid sheet over ten mils in thickness. Some European producers use angled dies in an attempt to minimize the distance from the die to the casting roll and to prevent die lip drag. The most common film casting arrangement uses the die in the vertical position. In cast film extrusion, the molten polymer drops onto the chill roll and contacts tangentially or above the tangent line. The alignment or parallelism of this roll to the die is critical in relation to the falling film. Whenever wrinkling of the film occurs on the casting roll surface, the first roll must be carefully repositioned in relation to the die lips.In cast extrusion the edges of the film are trimmed due to dimensional irregularities and/or poor layer distribution. As a result, the process can be negatively affected if the trimmed material cannot be recycled. Recent flat die system technology has minimized this problem by significantly reducing the amount of wasted material in coextrusion processes.This subject will be covered to some extent in a subsequent section.

Sheet / Film ExtrusionSheet / Film Extrusion is the process which is used for the products like plastic sheets or films. During the extrusion of these plastic materials, the required cooling is achieved by the action of pulling which is done through a specific set of cooling rolls that are generally three or four in number. These cooling rolls are also called calender or “chill” rolls. Extremely fast running speed develops “nerve”, which is an undesirable condition. This condition occurs when insufficient contact time is permitted to dissipate the heat, which is available in the extruded plastic.

The units for the solid sheet extrusion comprises minimum one Extruder along with one sheet extrusion die. These are supported by polishing stack which commonly includes 3 calenders that calibrate & cool the sheet with the use of their calender nips or surfaces. Behind this, the draw-off rolls to cool air and roller conveyor are located. The flat sheet may then be wound onto continuous rolls, or "pre-sheared" into discrete lengths.

In the plastic sheet extrusion, extrusion of molten plastic materials occurs 0through a flat die onto casting rolls. The rolls deliver the required cooling in addition to determining the sheet thickness, width along with surface texture. This is done specially in the case of structured rolls; i.e. levant, smooth, haircell, etc).

Plastic sheet may be extruded by an annular die onto a sizing mandrel. The pipe-like cross section that is extruded will be slit in one or more places and then flattened and handled as sheet. Commonly, co-extrusion is used for applying one or more layers for acquiring various specific properties like soft touch or "grip", UV-absorption, matte surface or energy reflection. The extruder, consisting of a heated barrel with an internal rotating screw, pumps the melted resin into a flat sheet die which sizes the sheet (thickness and width).

Sheet Extrusion Characteristics Width more than 2 m Thicknesses range of 0.5 to 15 mm (approximately) No limitations of length Setup operates as multilayer sheets having functional surfaces (color, haptics, UV-

protection, etc.) Grain / structured surfaces Easier forming is possible (folding, corrugated panels, thermoforming...)

Materials And Applications Polystyrene is the most general polymer used in sheet extrusion. It is major plastic

material for the thermoformed packaging, and it gives competition to ABS & PP in the technical markets.

The end use applications of the sheet / film extrusion include margarine, tubs and pots for yogurt and desserts manufacturing of luggage, refrigerator liners, and shower units.

In the liquid packaging industry (wine boxes, juice cartons, etc.), the base if the process of plastic extrusion onto paper. In addition, there is also an aluminum layer present in this process.

In the food packaging, the plastic film is occasionally metalized. In the automotive industry, sheet is currently used to produce interior trim, panels, and

dashboards. Foamed polyolefin sheet, both cross-linked and non-cross-linked, is also used in automotive applications.

One of the main uses of extruded PS sheet is for thermal insulation materials for walls, roofs, and under floors.

Profile Extrusion

For manufacturing plastic pipes, there is a use of a process called profile extrusion. This process is used to generally produce plastic   products  with continuous cross-sections.

Typical examples include decorative molding, drinking straws, plastic evestroughing, window trimming and numerous variety of other products polymers that are melt into the hollow mold   cavity  under application of high pressure.A typical profile extrusion process works like this. The plastic is first fed in a pellet form into the Extruder. The material gets conveyed forward continuously by a rotating screw inside a heated barrel. Here it is being softened by heat and friction. The softened plastic is subsequently forced out via a die, directly into the cool water. The product gets solidified here. Afterwards it is conveyed onwards into the take-off rollers. It is actually the take-of-rollers which does the pulling act of the softened plastic from the die.

The die is basically a metal plate that is placed at one end of the extruder. It has a section cut out of its interior. This cutout, along with the speed of the take-off rollers, plays a determining factor in the cross-section of the product being manufactured Many grades of thermoplastic can be extruded based on the application. The two primary categories are rigid and flexible extrusions. Flexible materials are needed for profiles that are required to serve as seals (for example say refrigerator gaskets) while rigid thermoplastics are needed if structural integrity is required. Typical examples include rigid polyvinyl chloride, guttering and siding.

Features of Profile Extrusion Low Part Cost Low Tool Cost High Production Capability Shorter lead times in production Uniform cross section of parts Multiple materials/durometers are possible in the same

partMaterials for Profile Extrusion 

Generally two classes of materials are manufactured into profiles. This of course depends upon the application and needs of the final product. These two classes, rigid and flexible profiles, are differentiated based on the starting material and final properties of the profile.

Rigid profiles are typically hard and relatively inflexible, often found in building applications. They can be manufactured using different resins. Flexible profiles on the other hand are flexible. The parts get bent easily, and are often soft (Typical durometer hardness   range  for them is from 50–95 ascertained by Shore A test). They are also easily compressible and generally used as internal components of items. The following table summarizes a variety of resins that are used in the profile extrusion.

Typical Raw Materials For Plastic Profiles HDPE (High Density Polyethylene) LDPE (Low Density Polyethylene) LLDPE (Linear Low Density Polyethylene) PETG Flexible PVC Butyrate Polypropylene Polystyrene ABS

Typical examples of Profile Extrusion The following examples illustrate clearly illustrate the broad possibilities of Profile extrusion.

Window profiles Sealing sections Modular drawer profiles Decorative trims

Advantages And Disadvantages of Profile Extrusion The only main disadvantage of the profile extrusion is the limitations in the design possibilities, which is due to the linear nature of the process. It has various advantages as well that are as follows: 

Equipment widely available in all geographical areas Relatively low tooling costs Inexpensive process Product combinations possible Design freedom

Typical Applications & Design Possibilities There are number of applications where the process of profile extrusion is extremely useful. Also, there are many design possibilities with the use of this process. Typical applications and design possibilities of the profile extrusion process are as follows:

Windows profile Sealing sections Modular drawer profiles Decorative trim

Co-Extrusion Process

Coextrusion:

 The process of extruding two or more materials through a single die with two or more orifices arranged so that the extrudates merge and weld together into a laminar structure before chilling. Each material is fed to the die from a separate extruder, but the orifices may be arranged so that each extruder supplies two or more plies of the same material. Coextrusion can be employed in film blowing, free film extrusion, and extrusion coating processes. The advantage of coextrusion is that each ply of the laminate imparts a desired characteristic property, such as stiffness, heat-sealability, impermeability or resistance to some environment, all of which properties would be impossible to attain with any single material. This process that allows the combination of different materials and colors in a single sheet. This is done to achieve special properties which are specific to a certain polymer, or for aesthetic effects with color, or for economic reasons where an inexpensive material "sub-strata" is combined with a more expensive material "cap".

Co-extrusion may be employed in the processes of Film Blowing, Extrusion Coating, and Free Film Extrusion. The general benefit of the co-extrusion process is that every laminate ply imparts a required characteristic property like heat-sealability, stiffness, & impermeability, all of which are impossible to attain by using any single material. 

It is evident that co-extrusion is a better process than a single layer extrusion. For instance, in the vinyl fencing industry, co-extrusion process is used for tailoring the layers on the basis of whether these are exposed to weather or not. Generally, compound's thin layer is extruded that contains high-priced weather resistant additives. This extrusion is done on the outside, whereas inside there is an additive package which is more suitable for the structural performance and impact resistance. 

Tooling

Tooling is the assembly of precision machined components attached to discharge end of the extruder(s) that combines the multiple materials (in the case of coextrusion) and forms the final product shape. Coextrusion of film is accomplished by one of two distinctly different types of tooling design; one is Feedblock Coextrusion and the other is Multi-Manifold Coextrusion.

The designs are very different, the operating parameters are very different, but, the end result will be the same if all the defined rules for each are followed. One allows for broader material selection; the other allows for much more flexibility in the number and arrangement of layers.

Flat Die SystemIt can be said that the die system is the heart of any coextrusion line.  The die system is formed by the coextrusion feedblock, the flat die and the melt transfer adapters that transport the different molten polymers from the extruders to the feedblock inlet ports.   The quality of the coextruded film and the productivity of the process are greatly dependent on the design and performance qualities of the die system.The primary function of the die system is to form a multi-layered film that is uniformly distributed across the width of the die with thickness variations on the film and thickness variations on each individual layer within industry accepted tolerances (not to exceed ±2.5% for the total thickness and within ±15 to ±20% for each layer).

This die is the simplest shape that can be produced. Its circular cross-section will produce circular lines, like weed trimmer line or fishing line.

A tubing die is a little bit more complicated. It has a donut-shaped cross-section to make pipe and tubing.

This die is called a clothes hanger die, and will produce sheet or thin film. The cross section shows how the molten plastic from the extruder is spread out to produce a wide, thin sheet. In general, a product is considered sheet if it is thicker than 0.1mm (0.004"), and film, if it is thinner than 0.1mm (0.004"). However, these terms are sometimes used interchangeablyUpstream from the feedblock are the melt transfer adapters.  The design criteria of this capillary system must consider parameters such as material residence time, pressure drop and temperature control.  For instance, an excessive pressure drop could be addressed by increasing the pipe

diameter; however, this in turn would increase the residence time of the material and increase the possibility of material degradation.  Also, accurate wall thickness sizing and proper heater specifications are necessary to prevent the pipes from heating or cooling the melts that they transport.  It is the task of the designer to find the proper balance between all these variables.The coextrusion feedblock arranges the different melt streams in a predetermined layer sequence and generates as many melt streams as layers are to be in the final coextrudate. Once this is done, each stream adopts a planar geometry, meets its neighbouring layers and the final planar coextrudate is formed.

Feedblock film coextrusion tooling consists of a feedblock and a standard flat film die. The feedblock accepts the inputs of the various extruders (one for each material) and arranges them in a predefined, segregated, fluid sequence and discharges them in a square or rectangular shape for entry into the film die.

The film die takes this combined stream of fluid materials and reforms it or distributes it into a flat thin film. The material sequence formed in the feedblock is exactly the same as the layer sequence of the final film structure

After the materials leave the feedblock and enter the die, they are no longer confined individually within steel channels. They are in intimate contact with each other and in the fluid state. They remain segregated because plastics, when melted, are part of a family of fluids characterized as being "non-Newtonian". Unlike Newtonian fluids, such as water or mineral oils which would readily co-mingle if discharged together, plastics will remain separate and distinct. The uniformity of the layers in the final film is a function of how carefully the primary rule of this type of coextrusion is followed. That rule is "adjoining materials MUST have a reasonable rheological match". Rheology is the flow performance of a given plastic material. It is graphically represented in the form of viscosity versus shear rate, which is the fingerprint of a specific material in terms of how it will flow when melted and pressurized. The rule simply states that the "fingerprint" of adjoining materials must be similar if layer uniformity of the final product is expected. The less the match, the less the uniformity that will result. When layer distortion occurs it happens in the die, not the feedblock, during the transition or distribution in shape from the square incoming stream to the wise thin film shape.

The flex lip adjusting mechanism is effective as a stand alone adjustment to correct flow imbalnace only if the due opening is less than about 0.040 inches. Larger openings require the addition of a restrictor bar (sometimes referred to as a choke bar) to balance flow.

The restrictor bar is a narrow bar about 1 inch wide that is positioned inside the die across the entire width of the die. It includes a series of adjustment screws that push and pull the bar for opening and closing the slot of material flow passing under it. This opening and closing of the flow passage across the die width conventionally used in conjunection with a flex lip as an additional and finer adjustment.

The flexible cantilevered lip, which defines the thickness of slot opening, is adjusted downward (reducing slot thickness) by means of a series of adjustable push screws. These screws are spaced approximately one inch apart across the entire width of the die. Thus, the die opening can be adjusted from a full or maximum opening to a lesser opening by adjusting the push screws.

It is important to note that, after adjusting the slit opening to produce a uniform thickness of film, the opening itself will not be dimensionally uniform across the width of the die. This variation in opening will compensate for any mismatch between the material rheology and the die's static internal distributor by acting as a variable flow valve.

Coextrusion feedblocks are grouped into two categories:  Fixed and variable geometry blocks.  In the upstream section of these blocks the so-called selector plug or selector spool is found.  This cylindrical shaped removable part is responsible for routing each melt stream into its final position in the coextrudate.  The plug, if required, also splits those streams with a material that

feeds more than one layer in the structure.  If a different layer sequence is required, it can be achieved by simply changing the plug.Fixed geometry blocks are most effective when the production line is devoted to only a few different products that are similar in their rheological behavior.  However, it is worth noting that these blocks have removable flow inserts that could be machined or replaced if required to process a wider spectrum of materials.    Variable geometry feedblocks are ideal for the coextrusion of high added value materials or when the scope of the production line is more diversified.  In general, these blocks feature movable internal components that can adjust the width distribution of an individual layer prior to meeting with neighboring layers and/or its velocity, which in turn affects its shear rate and viscosity.  Thus, problems inherent to coextrusion such as that of layer distortion and interfacial instability can be overcome with adjustments of the feedblock.  In spite of all the capabilities of coextrusion feedblock technology to address flow anomalies inherent to coextrusion flows, the production of an optimal coextrudate is only possible if the feedblock operates in conjunction with a die conceived and properly designed to process a coextrusion flow.  The perfect synergy between the die and the feedblock is what will guarantee a high quality product.A well designed die must guarantee that in the process of spreading the coextrudate coming from the feedblock the flatness of each individual layer is maintained within a tolerance of ±15 to ±20%.  It must also be designed so that the residence time is not excessive in order to prevent degradation problems or in some cases to prevent undesired heat transfer between layers.   The die must also be designed so that the pressure drop is kept at a level that is normal within the extrusion process.It is also critical that the die has the appropriate size, sufficient mass of steel and proper mechanical design to guarantee thermal stability and to minimize the so-called clam-shelling problem that manifests itself as an excessive deformation of the die lips when the die is subjected to the high pressures inherent to the extrusion of thin films. Recent advances in die technology have boosted the productivity of cast film production lines.  Special reference can be made to the so-called internal deckles.  Inserted on both ends of the die, the deckles allow changes to the film width and the consequent reduction of trim.  They can be fixed or adjustable and their length can exceed 20 inches.Edge encapsulation technology has been introduced in recent times to reduce the negative financial impact of material waste caused when the trim of the coextrudate is not recyclable.   The previous figure shows a band of a single material being coextruded side by side with a coextrudate.  The encapsulation material is of low cost, recyclable and has high mechanical properties. The encapsulation material mainly forms the trim, which allows for its reinsertion into the production process and great savings in material cost.  In addition, edge encapsulation technology is fully compatible with the internal deckle technology.

Cooling Unit  The cooling unit is comprised of a primary quenching roll, a secondary roll, a motorized roll positioning system for proper vertical and cross machine direction alignment of the rolls, and in many cases a vacuum box and/or air knife.The rolls are typically chrome plated to achieve a better surface finish and to enhance the heat transfer process during film cooling.  The cooling agent is commonly water that circulates inside the rolls.  The primary quenching roll cools one side of the film while the secondary roll cools the opposite side of the film.The die is positioned above the primary quenching roll at an angle that varies from 45° to 90°.   The distance between the die lips exit and the roll ranges from 0.8 to 2 inches.The cooling system allows the line to operate at high speeds.  As the line speed requirement increases, so do the diameters specified for the rolls.

The rolls must be perfectly aligned with the web to guarantee a uniform tension and to minimize thickness variations across the width of the film.  In addition, the angular velocity of the rolls must be well controlled to prevent film thickness fluctuations in the machine direction.The use of a vacuum box, connected to the die fixed body, is necessary in certain applications, like that of Cast PP, that require a more efficient cooling.  PP materials, if not cooled aggressively, tend to form crystals that ultimately give rise to hazy films.The vacuum box removes entrained air between the primary quenching roll surface and the film to minimize the air barrier between the hot web and the roll.  This air barrier, if not reduced, acts as a thermal insulation cushion that impedes the film cooling process. The box also reduces the amount of necking in the film and the air gap and allows higher line speeds to be utilized.The vacuum box can be combined with an air knife or an air chamber to further enhance the web cooling.Automatic Gauge Control SystemInline measuring and control of film thickness distribution across its width is the function of the gauge control system or APC (Automatic Profile Control).  When the flexible lip on the die is manually controlled and the production process is well tuned, film thickness variations will be in the range of ±3 to ±5%.  In automatic mode, it is possible to reduce these variations by half.   The figure below shows an automatic die with the automatic control module mounted on the flex body of the die.  The so-called thermal translators or thermal bolts form the module.  The distance between the bolts is typically 1.125 inches. The gauge control system includes a radiation emission unit and a control console.  The radiation unit travels in the machine cross direction, scanning the film in cycles (measured in minutes).   Commonly, the radiation originates from a beta ray source; although, x-ray and infrared sources can also be used.  In general terms, the film thickness is determined as a function of the film radiation rate of absorption.  Thus, variations on the absorption rate translate into film thickness variations.         The control console is the interface between the control system and the automatic die.   Each adjustment point or thermal translator on the die is spatially correlated with a position on the film.  This is called mapping. The control system applies power to the thermal translators, as required, and the lip gap is regulated via thermal expansion of the adjustment element.   An important variable associated with APC is the time constant.  It is defined as the time needed for an adjustment element to elongate 62.3% of its maximum elongation.  The shorter the time constant the more responsive the system is, translating to gains in productivity. 

Corona TreatmentIn order to facilitate the adherence of inks or coatings onto the film surface it is necessary to apply a surface treatment.  Corona treatment is the most commonly used of the existing methods.  Corona treatment increases the surface energy of the film and consequently its surface tension.  The system includes a power source and the treatment station.  The power source transform 50/60 Hz plant power into much higher frequency power in a range of 10 to 30 KHz.   This higher frequency energy is supplied to the treatment station and is applied to the film surface by means of two electrodes, one with high potential and the other with low potential, through an air gap that typically ranges from 0.5 inches to 1 inch.  The surface tension on the film surface is increased when the high potential difference that is generated ionizes the air.  Corona treatment can be done inline or as a separate downstream process once the film is produced.  If performed inline, special consideration must be given to the potential generation of toxic ozone.   In some cases, it is necessary to provide a ventilation system in the production area.

WinderIn simple words winders are used to convert the extruded film into rolls of material.  The winding process has to be such that the film preserves its properties and dimensions when these rolls are unwound and converted in other downstream processes. There are three basic types of winders; surface winders, turret or center winders, and center/surface winders.  Surface winders wind film through the contact between a large diameter drum and a winding shaft that is pressed against the drum with variable pressure.   Turret winders or center winders are any style of winding machine that use a driven shaft running through the center of the building roll or on chucks supporting the core to drive the building roll.   Finally, in the combination approach of a center/surface winder (or gap winder) a small gap is maintained between the surface winding roll or lay on roll and the winding roll.  A center drive system drives the winding roll independently of the surface drum. Films can be tacky or have some degree of slip, have high or low elasticity, thin or thick, the required roll diameter can be large or small; rolls can be narrow or wide, soft or hard. Winder technology is complex and the proper type of winder used in a particular application depends on all of the above variables.   The use of turret or center winders is typical in cast film applications.  With this type of winder the web tension decreases as the roll diameter increases.  This is controlled by the rotational speed of the winding spindle.  A lay on roll prevents or allows the entrapment of small amounts of air between the layers.  The latter is recommended for winding films with high tack or for winding soft rolls. In order to evenly distribute defects on the extruded film (thickness variations) a randomizer is used.  The randomizer moves the film back and forth, as it is slit and wound.  An alternative approach is to move the slitter and winder back and forth relative to the film.

Computerized Supervisory and Control SystemThe main components of a cast extrusion line have been enumerated and described.  These components do not act on their own but are integrated and governed by a computerized supervisory and control system. The main computer is the brain that couples and drives the controls of all the line components in an orchestrated way. The main tasks of the computer are:

To control start-up, shutdown and speed of the line; To monitor the weight of material fed into the extruders and to control the speed of the

extruders in order to maintain a constant throughput; To control all temperature zones and the temperatures of all the materials; To coordinate the interaction between the gauge control system, the response of the

automatic die and the line speed; To control the web tension; and To store and handle all product recipes, store operational data and control the alarm

system.

A good control system must provide operators with an easy to operate graphical interface or monitor system.   

Advantages of Co extrusion Process Combining rigid and flexible material is able to exploit the different and individual strengths of each material in to a single product. Another advantage is that it permits multicoloured products. This can range from decorative finishes to even product identification. As discussed

High quality mono-layer extrusion coatings in larger varieties of line speeds and widths

Use of lower cost materials for filling purpose, assists in saving on the amount of qualitative resins

Capability of making multi-layer as well as multi-functional structures that too in a single pass

Reduction in the number of steps required in general extrusion process Provides targeted performance with the use of definite polymers in particular layers Reduction in setup and trim scrap Potential for use of a recycle layer t It is possible to impart a desired distinct property. This could be in terms of stiffness,

resistance to chemicals, impermeability, heat-sealability

Disadvantages Of Co-extrusion As per a number of globally reckoned companies, there are some disadvantages related with the process of co-extrusion. Some of these disadvantages are as follows: 

Minor differences in physical properties are responsible for making a combination desirable, but these differences are also responsible for making the combination incompatible

For this process, polymers must have similar melt viscosities to sustain a laminar flow. All the viscosity differences may be more or less tolerable, according to the material location inside the composite structure along with the layer's thinness

Requires more sophisticated extruder and its operator. This implies extra maintenance cost of the equipment.

Demands considerable planning as well as forethought in the system design

Applications of Co extrusion The Co extrusion process is a widely used technique these days, producing many products for critical industrial applications. To understand the importance of co extrusion let us consider a very simple example of a striped drinking straw. It is a perfect example of a coextruded tube. For example, a white straw gets extruded from polypropylene, a colored polypropylene material could be coextruded in a smaller area for creating a striped tube. A relatively new application is the Co-extruded plastic films. These are having a wide use in packaging industry. Another emerging sector for co extrusion is the industrial films and laminations. The medical industry is another area where co-extruded tubing products are used. Further irrigation devices as used in the agricultural industry has a great use for co extruded stripes.

Overjacketing And Tubing Extrusion

Overjacketing and tubing extrusion are highly preferred processes for extruding a range of plastic materials. The overjacketing extrusion process is discussed here. In the process of wire coating, the bare wire or some bundles of jacketed wires, filaments, etc. are pulled through the die's center, which is similar to the tubing die. The different materials that are used here, depends on the application. Basically, an insulated wire is a tube (thin walled) that is formed around bare wire. 

For coating a wire, there are mainly two different kinds of extrusion tooling used. These are called "pressure" tooling or "jacketing" tooling. The selection of the type of tooling to be used depends on the fact that whether the specific application needs intimate contact or if it needs polymer adhesion to the wire or not. In case there is the requirement of an intimate contact or adhesion, pressure tooling is used or else, jacketing tooling is selected. 

Jacketing Tooling vs Pressure Tooling 

Jacketing tooling differs with pressure tooling on the parameter of pin's position with respect to the die. In case of jacketing tooling, the pin is extended all the way and flushed with die. Upon feeding the bare wire through the pin, it does not directly makes contact with molten polymer unless it leaves the die. In pressure tooling, the pin's end is retracted in the crosshead, where it contacts with polymer at a higher pressure. 

Tubing Extrusion The process of tubing extrusion or extruded tubing process is used to manufacture products like medical tubing and drinking straws. This process is exactly similar to the process of regular extrusion, until the stage of die. To extrude the hollow sections, a pin or mandrel is placed inside the die and in various cases, positive pressure is applied through the pin onto the internal cavities. For specialty applications, it is required to make tubings with a number of lumens or holes. For such applications, tooling is created with the placement of more than one pin in die's center for the production of required number of lumens. In large number of cases, the pins are available with air pressure from a range of sources. This way, it becomes easy to adjust individual lumen sizes with the adjustment of pressure to the individual pins.

Crosshead Extrusion Process (Wire & Cable Coating)Crosshead extrusion process is widely used to coat wires and cables with a polymer. The basic procedure includes pulling of the wire / cable to be coated at a uniform rate via a crosshead die, where it is covered with the molten plastic. This extrusion process for coating is used in most wires and cables that find usage in telecommunication and electrical applications along with electronic industry. For more coatings, two extruders can also be used in tandem. 

Development Of Standard Two-Wire-Plus-Ground Cable By Crosshead Extrusion Consider the example of the development of standard two-wire-plus-ground cable, which is commonly used in home wiring. In the beginning, white insulation is used to coat one copper wire and black insulation is used for the other. In a secondary process, following procedures are followed:

A paper-wrapped copper wire is combined with the black & white insulated wires. This third wire is used for the ground.

All the wires are fed through die In the die, final insulating jacket is applied for protecting all the wires together. The

insulating jacket also assists in holding the three wires in a common plastic sleeve, which is used home wiring

Another approach to produce this product, is via the continuous process of production. In this, two extruders are used to individually apply white as well as back coating on the two conductive wires. The successive processes are as follows:

These two products brought together by using a third copper ground wire The three wires are sent via a third crosshead die, where the addition of exterior jacket

takes place

As all the three extruders are running at similar speeds, the end product is developed with minimal handling. A negative point of this process is the amount of production loss occurred, in case any of the three extruders is not running efficiently or there is some problem with only one of the extruders. Wire coating is generally done by the use of single screw extruders, in which the crosshead extrusion process is carried out. The job of the extruder is to melt the resin & forward it to the die at an even & constant melt pressure and temperature. The crosshead extrusion process is carried out by using a general equipment in the line, which includes following basic pieces:

Unwind station or some other wire / cable source for feeding the line Pretensioning station for setting the tension throughout the process Preheat station for preparing the wire for coating Crosshead die Cooling trough for solidifying the polymeric coating Test stations for assuring that the wire is suitably coated Puller for providing constant tension through out the process Winder for collecting the product

Typical Polymers Used In Wire Coating Applications There are various polymers that are used in wire coating applications by the crosshead extrusion process. The characteristics of these polymers, which make them ideal for this purpose are their flexibility, electrical properties, ability to withstand abuse, and durability. Typical polymers are as follows: 

Polyethylene Polyvinylchloride Polyamide Polybutylene terephthalate Thermoplastic elastomers Ethylene propylene copolymers Polypropylene Fluoropolymers

In the wire coating operation, cross linked polyethylene is used. The wire is extrusion coated with this polymer in this process. At the end of coating operation, polyethylene is cross-linked on to the wire.

Monofilament Fiber Extrusion 

The process of Monofilament Fiber Extrusion is highly useful for the development of products that are used in almost all walks of life. In fact, monofilaments are an essential part of our everyday work, making our work simpler along with making our leisure more enjoyable. For instance, monofilament finds usage for fishing line and to make strings for squash, tennis, racketball rackets and badminton. It is also used for making synthetic monoyarn which is used in the weaving process as well as in synthetic rope which finds usage in fences, construction, greenhouses, orchards, fastening, and many others. Various products that are developed using Monofilament Fiber Extrusion process are as follows: 

Decorative tapes Ribbons Webbing in lawn chairs Bristles for toothbrushes Grass trimmer lines Tire reinforcements Sewing thread for industrial textiles, apparel, home furnishings and floor coverings

Common Polymers Used For Producing Monofilaments  Polyamide (nylon 6, nylon 6,6, and nylon 6,10) Polyester Copolyester

Polypropylene Polyethylene Polyphenylene sulfide Polyvinyilidene fluoride

Process Of Monofilament Fiber Extrusion In the monofilament extrusion process, smaller extruders and longer lines are used. The products developed from this process have a very small diameter and many number of ends. Smaller diameter allows very swift cooling that too in a small space. On the cooling of filaments, they are drawn and wound onto the packages. The drawing process is done using comparatively large ovens having godet rolls on its every side. These rolls are used for drawing the filaments to the accurate denier (grams/9000 meters). After this process, the product is wound onto bobbins, wherein these can be utilized directly or by combination for producing some other products like rope etc. Steps Followed In A Monofilament Line

In general, 2.5 - 3.5 inch single screw extruder is used in most monofilament processes. A gear pump placed between the die and extruder provides consistent polymer flow &

pressure. Filaments are extruded vertically by circular dies into the cooling bath. Here, the

filaments are separated so that every filament runs separately via the line, where their wrapping process takes place. The wrapping is done on a bobbin or spool.

After quitting the water bath, filaments are dried & further and passed via godet rolls, which are controlling the speed as well as draw from the die.

A large oven placed between two godet roll stands heat the filaments for facilitating the filament drawing. Second godet roll stand operates at a much higher speed than the first and is responsible for the determination of the draw inside the oven. The number of godet rolls and ovens completely depends on the sophistication of line and the draw required. After the filaments exist the last godet roll, they may be passed through a laser micrometer for monitoring individual filament diameters. Lastly, every monofilament is wrapped on separate package for being used in subsequent operations. 

The products created by the process of Monofilament Fiber Extrusion are tested on the parameters of water shrinkage, hot air shrinkage, diameter uniformity, denier, color and filament smoothness.

Extrusion coating

Extrusion coating is a process of coating thermoplastic material onto a substrate such as woven fabric, paper, paperboard aluminum foil, PET, BOPP film etc. The resins most commonly used are polyolefins such as polyethylene, ionomers, ethylene vinyl acetate copolymers and polypropylene.Advatanges of extrusion coating are:As a process:Double sided coating can be done to achieve desired properties.Solvent/adhesive freeThickness of the coating can be varied depending on the end useHigher line speedsAs an end product;Provides moisture barrier properties

Avoid direct contact of content to the substrateProvide heat sealable characteristicsReduces loss of contentProcess of extrusion coating:Extrusion coating is a process in which a substrate is coated with an accurately metered film of molten plastics. Extruded thin molten film is pulled down on a substrate and into the nip between the chill roll and pressure roll below the die (fig. 1) The pressure between these two rolls forces the plastic onto the surface moving at a speed faster than the extruded film and drawing the film to the required thickness.

Extrusion coating line consists of:ExtruderDieChrome plated chilled rollBack up rollWinders/UnwindersAn extruder converts solid pellets of thermoplastic resin into a uniform, homogeneous melt. Extruders with L/D ratio of 24:1 to 26:1 are normally used for LDPE whereas for LLDPE, L/D ratio of 26:1 or above is recommended for better results. Die commonly used are flat, coathanger type with adjustable die lip. Melt extruded through the die comes in the nip formed by chrome plated chilled roll and soft rubber coated back roll. A large diameter chilled roll provides enough contact time to cool the product sufficiently to allow the film to be stripped from the roll without damage. This cooling requirement is often the limiting factor in the running speed for a particular product. The surface of the chilled roll may be bright, dull, matte, embossed depending upon the desired surface characteristics.

Factors influencing coating performance:

1. Melt temp: Extrusion coating lines are normally operated at higher temperature. The resins for extrusion coating contains lower level of antioxidants. Higher processing temperatures oxidizes the polymer partially and promotes good bonding to the substrate. Further it lowers melt viscosity promoting uniform flow of melt through the die. Higher temperature (>300 deg.C) is necessary to achieve acceptable level of adhesion to the substrate at high line speeds.2. Die gap: The die gap is the critical control point on any extrusion coating line. The slit die with die opening of 0.5 mm to 0.8 mm is commonly used. Higher die gaps give higher flow variation.3. Air Gap: The gap between the die exit and substrate on chilled roll is called air gap which need to be optimized with respect to coating material. Some oxidation of the melt takes place after exit from the die, facilitating adhesion of polymer to the substrate. Higher gap leads to higher necking and cooling of the melt, leading to improper adhesion of polymer to the substrate. For LLDPE/LDPE (1) blend, the air gap is normally maintained between 30 - 40 mm.4. Chilled roll temperature : It has influence on coating adhesion and stiffness. Optimum chilled roll temperature for polyethylene is 30 deg. c.

Material Selection for Extrusion Coating:The resin most commonly used for extrusion coating are LDPE, LLDPE and their binary blends. Each polymer offers unique advantages over the other and their binary blend strikes the balance of properties. Due to presence of long chain branching and broader molecular weight distribution in LDPE, it shows a good processability, superior hot melt strength. Further, LDPE shows a phenomenon called 'strain hardening' by virtue of which elongation visciosity increases with the line speed. Hence draw down to thinner gauges are possible at higher line speed without appreciable neck-in.Extrusion coating grades with LLDPE have shown that neck-in is high and increases with line speed. The increase in neck-in is attributed to the drop in elongational viscosity with increase in line speed. This phenomenon is termed as 'tension thinning and is due to narrow molecular weight distribution and absence of long chain branching. However LLDPE held a new distinct advantages over LDPE extrusion coating grade. The main advantage of LLDPE over LDPE is high mechanical strength which allows downgauging. The relatively lower thermo-oxidative stability of LLDPE over LDPE leads former to oxidize faster resulting in better adhesion to non porous substrates like aluminium foil and tapes. A marked improvement in sealing properties is observed when LLDPE is blended with LDPE.The most practical method to utilize LLDPE in extrusion coating is by blending with highly branched LDPE> Blending of polyolefins generally has an additive effect. LLDPE rich blends offer much better mechanical properties and sealing properties as compared to those of LDPE. The processing characteristics of such blends can be enhanced further by using narrower die gap (0.4 mm - 0.6. mm)Resin Characteristics for Extrusion Coating:Higher melt index-required for better adhesionAdditive freeAppropriate amount of antioxidantsReduce neck-in propertiesGood adhesion to substrate at higher line speeds

Applications Of Extrusion Coating The extrusion coated products are wide used in the segment of synthetics woven sacks. These are also used for coating on the non-raffia substrates that primarily includes coating on Al-foil, Paper, PET & BOPP substrates. The major application areas of extrusion coating are as follows:

Woven sacks for fertilizer packaging Tarpaulins Coating on conventional jute bags

Soap wraps Pharmaceutical strip packaging Cable wraps Coating on paper used in packaging of salt and sugar (sachets)