It is a linear polymer, having short, branched structure ( Figure 1 ) formed by the co-polymerization of ethylene molecules with olefins by Zeigler-Natta catalysis reaction [5] . It has shorter chain branching and less shear sensitivity, therefore when it is subjected to film processing during extrusion operation, it easily allows for a faster relaxation of polymer chains and does not get stiff or harden during elongation. This characteristic is important for film formation as in case of blow film extrusion moulding and LLDPE molecules maintain high strength and toughness simultaneously without undergoing degradation [6] . This is the reason that it finds extensive application for packaging and wraps polybag production. In the blown film extrusion process, it becomes extremely viable to get a monolayer polyform of uniform thickness. As this film bubble rises up, it is stretched further and so for such purposes, the elasticity and melt flow index properties play an important role [7] . The shorter chains with a little branching ensure the proper flow of heated resin, avoiding any breakage of bubbles under high pressure. Therefore, LLDPE structure is excellent when it comes to overall maintenance of structure and adequate blow-up ratio, which will be discussed in the further sections.

The rheological property of raw material (LLDPE) is important as the melt flow index is defined as the weight of polymer flowing through a die section per 10 minutes. Higher the melt flow index, lower the viscosity [8] . It is needed that in case of film blow extrusion process, the melt flow remains between the value of 10 to 5 gm/10min. This is necessary for the film tube to rise up the blower and then get relaxed and rolled between sheets during cool down. The linear low-density polyethylene comes in the form of pellets that are melted in the extruder machine. It is very essential to ensure that there is proper, homogenous melting of pellets in the extruder lest there will be chances of collapse of film bubble from the die itself. The quality is checked so that there is no defect in the pellets of these raw materials. The pellets serve as the starting point of which inspection and quality check become crucial to ensure the product quality at the end stream. The pellet size, shape and texture influence the uniform mixing in the extruder section in which the shearing takes place between particles [9] . The LLDPE melting point is sharply at 122˚C [10] , however for preventing the degradation of resins at high temperature as the operation is performed in a continuous manner, it is necessary to set the temperature, an hour earlier at 100˚C of internal heaters inside the extruder and finally, at a uniform temperature of 20˚C - 25˚C when the operation is successfully occurring as now, the cumulative temperature reaches enough to allow the softened heating of resin pellets without degradation or improper mixing.

The blown film extrusion line consists of certain parameters like blow-up ratio, lay-flat width, draw-down ratio and frost line height. These elements along with die size, die gap, film thickness, and speed of nip rollers control the dimensions and sizing of a blown film tube, which must undergo pressing and winding in further stages.

The BUR indicates the maximum amount of stretching for a particular material in the crosswise or perpendicular direction to the length [14] .

Mathematically, BUR = D_b/D (1)

where D_b is the diameter of the bubble and D is the diameter of the die. For a normal reference, BUR = 2:1. A blow-up ratio greater than 1 means that the bubble has been blown to a diameter greater than the diameter of the die orifice. It also shows that the poly wrap film has been thinned and it acquires the orientation towards the transverse direction.

It is the measure of the extent by which a tube can be stretched or drawn without getting ruptured [15] . It is also represented as the ratio of the die hole gap to the film thickness at blow-up ratio BUR = 1.

Draw-Down Ratio = Die Gap/Film Thickness × Blow-up ratio (2)

If a draw-down ratio is greater than 1, it shows that the melt has been pulled away from the die orifice at a much faster rate than it is being issued from the die orifice. In that case, the film has been thinned and acquires orientation towards the machine direction.

It is the width of the film flattened up between the nip rolls [16] .

Mathematically, lay-flat width = (π D/2) × BUR (3)

It is the height from the face of the die at which the bubble tube melt freezes. The freeze line height influences the optical property of the film due to reorientation and relaxation of molecules at the point of freeze line [17] . If the freeze line height is higher, poorer optical properties will be there and brittleness increases as freeze line height shifts down. Also, the below-given formulas become essential in quality analysis of blown film produced by varying the parameters.

Film lay-flat width = 1.57 × D_b × BUR (4)

BUR = 0.637 × (film width/D_b) (5)

One must note that the die diameter (D_b) used is of the standard value of 50 mm and the die gap is between 0.5 to 3 mm.

The sizing is decisive in maintaining the overall product quality owing to the requirements of the customers. The speed of the nip rollers and idlers determine the size of the flat film in terms of its thickness and width. The units to measure the length and width of a plastic film employed in the polybag industry are generally in inches and the thickness is measured in gauge. One gauge is equal to 10⁻⁵ inches and that is equal to 0.254 microns. The plastic film is always measured in the gauge system. Thus, with the help of a micrometre, we can easily measure the thickness of plastic film in microns and convert them into gauge. There are different range of gauge of plastic films that are used of packaging and wrapping purposes. The desired value of gauge is either probed by a micrometre from one plastic bag as in the extruder process it is automatically set according to the pre-defined system values. The recycle size of plastic also is in the range of 2 × 70 inches (width by length) and the gunny bags that are used are in the size range of 20 × 30 inches or 22 × 24 inches. The 3 × 7 inches is the common dimensional size for a small packaging polybag to be supplied in the consumers across the market.

It is the heart of the polybag manufacturing plant as it controls the temperature, AC current, DC motor, speed of nip rollers, idlers as well as connected to the air blower and compressor for ON/OFF and emergency shut off. It has six temperature knobs each of four regulates the temperature of the extruder section where pellets are melted and two of which control the temperature of the die set. It also has a speed control unit which regulates the speed of nip rollers to adjust the thickness of the plastic film and its width.

The compressed air output from the air compressor and the storage tank enters the centre hole of the spiral core rod of the die head through the outlet valve of the tank. The gas enters the centre of the bubble tube through the centre hole of the mandrel and blows out the plastic to form the bubble-shaped tube that rises upwards and gets stabilized, elongated and cooled before collapsing and pressed through the nip rollers. The pressure is set between 0 - 140 psi.

The extruder is mainly composed of screw, barrel, hopper, reducer and drive motor; through the belt drive reducer, the reducer drives the screw to rotate in the barrel. The screw facilitates the uniform mixing of melted resin throughout the various sections of the extruder. When working, it depends on the surrounding plastic to support centring. The gear reducer motor drives the screws at a rpm of 30 - 70.

The bubble is slowly compressed from a round shape to a flat double layer, between the gap of the nip rolls, and further the static current is applied for smoothening the stretch film as it enters the traction idler system [18] . The collapsing frame guides the mono-layer bubble and folds it into double flat film between the nip roller whose speed of rotation determines its thickness and width.

The lay-flat film is further rolled upon idler rollers and the cuts are made according to the sizing requirements and it is passed through the slitting cage, and it is now rolled over the reels that are rotating in the bender portion. The reels are then sent into the decker machine for stitching and cutting purposes to prepare the final polybag product. The constant gap winding is employed for the rolling-over operation onto different sizes of cones. The cutting operation is performed in two segments. Initially, the sizing of product is determined and how many roles we require further for putting into the decker press for stitching and edge-cutting purposes. Maximum of four rolls is employed in the bender housing system so that it does not affect the productivity, and should not be time-consuming, as in the case of long plastic film. For instance, if there is a bubble air tube of 24 inches and after laying flat into double paper film its width becomes 12 inches. Now, if three cutters are used to slit the flat film, there will be four different flat films each of size 3 inches and the length is determined further in the decker press. This is how cutting helps in setting the required size standards. In the decker machine, the pulley belt system train is used to convey the four to five wraps of plastic film over the ledge of decker for size cutting and pressing and finally stitching. The cutting is done according to the size set in the decker machine and the speed is varied of the pulley system if the film starts to lose grip or fall of. Before sealing, it must be carefully noted the edge margin or else the seal might be made at wrong axis. The air is blown with mouth to open the film to take it off the belt and the static electric current is used for the polybag to press properly while stitching and sealing. Once all these factors are considered, the final polybag product is filled into storage bags where it is transported to printing and colouring.

The temperature of the extruder as well as the die set is controlled to ensure that the bubble does not collapse and there is uniform distribution of polymer content over the entire film after elongation.

The air pressure is important for deciding the width and thickness of bubble tube and the back pressure facilitates the emission of molten material of polymer from the die orifice.

The speed of rollers controlled by DC influences the thickness as well as the lay-flat film width [18] . Lower speed will give thick film, and higher speed will give thinner overall films.

The data given below is based on the results produced by the simulator software, blown film extrusion simulator developed by Dr. Kirk Cantor, Professor of Plastics and Polymer Engineering Technology at the Pennsylvania College of Technology. The co-relationship between the volume of air and nip roller speed with that of other four dependent variables such as film thickness, bubble diameter, lay-flat width and frost line height has been established through this simulation. The figures and plots help us to deduce the results to get a better insight into the actual blown film extrusion process.

It can be inferred from the above plots that the film thickness and bubble diameter are a function of both air flow volume and nip roller speed. As shown in Figure 3 , if air flow volume is increased, the film thickness decreases and the bubble diameter increases, which in turn increases the blow-up ratio.

In Figure 4 , it can be inferred that both the lay-flat width and the frost line height of the plastic film are dependent on-air flow volume. To manufacture a plastic film of lower thickness, we need to increase the nip roller speed at a constant air flow volume as observed in Figure 5 . Frost line height controls the optical properties of the film. Thus, increasing the air flow volume will decrease the frost line height and this will eventually increase the brittleness of the film. In Figure 6 , however the reverse trend is noticed as we can observe that increasing the nip roller speed increases both the lay-flat width and frost line height. It can be concluded that both nip roller speed and air flow volume are the major process control parameters to significantly affect the properties of a plastic film as found in the actual blown film process.

The variety of challenges emerge during the entire blown-film operation ranging from the correction of gauge and cutting dimensions [20] - [23] . The rheological properties of the polymer also influence the freeze line height [24] , and it must be carefully analysed. Sometimes, due to high cling, the stitching becomes difficult, and it results in wastage of material. The similarity in the processing conditions produce the same mechanical properties of the film [22] [25] . Generally, it becomes difficult in maintaining such conditions during scale-up. The mandrel die set may clog with incompletely melted pellets. The challenge in the working of the die is to maintain the uniform velocity distribution of polymer melt exiting through the die hole [23] [26] . The bubble may collapse or may not even rise due to inconsistent or poor flow due to leakage in the annular section of die [24] [27] - [30] . The pressure can sometimes cause problems and lead to premature rupture of bubble tube and the geometry of the collapsing frame also causes problems resulting in production of the wrinkled film [25] [31] - [35] . Therefore, this continuous process must be strictly monitored to ensure that the product of desired quality is being manufactured and cleared of all defects.