Blown films thickness ratio

In their study on LLDPE resins containing 1-butene, 1-hexane, and 1-octene comonomers, Kalyon and Moy [30] found a significant variation in their film thickness when measured around the circumference of tubular bubbles processed under identical conditions. The samples blown with a blow-up ratio of two, exhibited more significant variation in thickness than those prepared with a blow-up ratio of three. However, film processed at a higher blow-up ratio has been found to have less variation in thickness. 

The bubble diameter is normally always much greater than the die diameter. This bubble diameter divided by the die orifice diameter is called the blow-up ratio (BUR). The BUR is usually 1.5 to 4.0, depending on the plastic being processed and the thickness required. The bubble diameter must not be confused with the width of the flattened double layer of film between the nip rolls. The width of this double layer is 1.57 times the bubble diameter and is called the blown-film width (BFW). 

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. 

There are some characteristic parameters in the blown film process (see Fig. 24.1) the blow-up ratio (BUR), which is the ratio between the final radius (Of) and the radius at the die exit (Uq) the thickness ratio (TR) calculated as the ratio of thickness at the die exit (//q) and the final film thickness (//f) and the draw ratio (DR) defined as the ratio of take-up roller velocity (Vf) to the extrusion velocity (Vq). The stretching force (F ) is the force needed to take up the bubble by the roller system (Fig. 24.1). 

After extrusion, blown-film is often slit and wound up as flat film, which is often much wider than anything produced by slot-die extrusion. Thus, blown-films of diameters 7 ft. or more have been produced, giving flat film ofwidths up to 24 ft. One example is reported [16] of a 10-in. extruder with 5-ft diameter and a blowup ratio of 2.5, producing 1,100 Ib/h, or polyethylene film, which when collapsed and slit in 40 ft wide. Films in thicknesses of 0.004-0.008 in. are readily produced by the blown-film process. Polyethylene films of such large widths and small thicknesses find extensive uses in agriculture, horticulture, and building. 

See references (11,48,50, and 51) for the effect of blowing conditions on film properties of LLDPEs. The result in this table was obtained for the following blown film conditions blow-up ratio = 2.5 1 die gap = 2.54 mm output = 32 kgh film thickness — 25.4 mm die size = 102 mm frost line height = 230 mm melt temperature = 483... 

As noted earlier in sections 2 and 3, LCPs are different from typical random coil polymers, in that the orientation that is produced in the die does not relax quickly. For this reason, the transverse stretch from internal air pressure and the longitudinal stretch from the take-up speed enhance the orientation from the die, and do so within a relatively short distance from the die lips. Typical blown film conditions would be 2 to 1 blow-up ratio, with the bubble reaching full diameter within a height equal to one to two times the die diameter. Typical LCP film thickness is 12 to 75 micrometers (0.0005 to 0.003 in.). 

Tubular-Blown Film Process. This process is more flexible with regard to the permissible polymer viscosity mismatch, control of film orientation balance in the transverse and machine directions through blow-up ratio, and easy randomization of film-thickness variations. Production rates are limited by flow rates per circumferential length of die (pressure drop) and cooling rates (heat transfer). 

Tubular linear low-density polyethylene (LLDPE) film of thickness equal to 34.1 qm is produced (Kanai and White, 1984) by a blown film operation with draw ratio equal to 4. The annular die has an inner diameter 1.387 cm and outer diameter 1.496 cm. Calculate the pressure of the air to blow a bubble (i.e., the internal overpressure) of the given specifications and the axial tension to draw such a bubble. Consider that LLDPE is Newtonian with viscosity 720 Pa s, the process is isothermal at 180 °C, and LLDPE freezes at an axial distance of 15 cm. The mass fiow rate of the polymer is 0.21 g/s, and its density is 0.919 g/cm-. 

Consequently, Eigure 9.22 applies to this blown film case. The thickness ratio is easily calculated to be... 

Film blowing. A tubular 50 pm thick low density polyethylene film is blown with a draw ratio of 5 at a flow rate of 50 g/s. The annular die has a diameter of 15 mm and a die gap of 1 mm. Calculate the required pressure inside the bubble and draw force to pull the bubble. Assume a Newtonian viscosity of 800 Pa-s, a density of 920 kg/m3 and a freeze line at 300 mm. 

The concentration of a surface active impurity in an aqueous solution is 10 M the surface concentration at the liquid-air interface is 10 ° mol cm . To remove the impurity, air is blown into the solution, causing the formation of a foam that contains bubbles with volumes of about 0.1 cm and thickness of about 10 cm. (a) Estimate the amount of impurity contained in 1 cm of foam film and the approximate ratio of the amount adsorbed to that remaining in the bulk solution, (b) How many square centimeters (m ) of film should be produced and removed... 

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