Blow up ratio

In the tubular process a thin tube is extruded (usually in a vertically upward direction) and by blowing air through the die head the tube is inflated into a thin bubble. This is cooled, flattened out and wound up. The ratio of bubble diameter to die diameter is known as the blow-up ratio, the ratio of the haul-off rate to the natural extrusion rate is referred to as the draw-down ratio and the distance between the die and the frost line (when the extrudate becomes solidified and which can often be seen by the appearance of haziness), the freeze-line distance. 

Example 4.3 A plastic shrink wrapping with a thickness of 0.05 mm is to be produced using an annular die with a die gap of 0.8 mm. Assuming that the inflation of the bubble dominates the orientation in the film, determine the blow-up ratio required to give uniform biaxial orientation. 

A plastic film, 0.1 mm thick, is required to have its orientation in the transverse direction twice that in the machine direction. If the film blowing die has an outer diameter of 100 mm and an inner diameter of 98 mm estimate the blow-up ratio which will be required and the lay flat film width. Neglect extrusion induced effects and assume there is no draw-down. 

Design a die which will produce plastic film 0.52 mm thick at a linear velocity of 20 mm/s. The lay-flat widfli of the film is to be 450 mm and it is known that a blow-up ratio of 1.91 will give the necessary orientation in the film. Assume that there is no draw-down. 

Now it makes the solution simpler to assume that the blow up ratio is given by Dft/Di (ie rather than Db/D ). Also this seems practical because the change from D to D is caused solely by inflation whereas the change from D to Db includes die swell effects. 

Table 2 Properties of the PE Resins and Their Blow Film Samples (Blow up Ratio-2) [56]...

Figure 11 S11F/S33F ratio vs. blow up ratio for different...

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. 

Extrusion conditions = 1.25 mil film blown at 2 and 1 blow up ratio using 63.5 mm, 20D smooth bore extruder, die gap 80 mil, melt temperature 193°C. 

Most LLDPE produced worldwide is made into thin film, either melt blown or cast from the melt. Blown film is produced by extrusion of LLDPE melt through a circular die with a large diameter, up to 100-120 cm. and a narrow gap, usually less than 1 mm. The tube of molten polymer expands from internal air pressure and forms a bubble of a larger diameter, Typically, the blow-up ratio, the ratio of the final film tube diameter to the circular-die diameter, is between 1.5 1 and 4 1. LLDPE cast film is manufactured by depositing potymer melt on a rotating heated... 

Since we know the thickness reduction and the draw ratio of the film, we can compute the blow-up ratio,... 

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). 

The blown products, such as upward blown film, are basically natural for providing orientation (Figure 5.19). The blow-up ratio determines the degree of circumferential orientation, and the pull rate of the bubble determines the longitudinal orientation. 

Biaxial orientation leads to isotropic properties in blown film, that is, properties that are eqnal in the two primary directions the film was stretched (i.e., parallel to the flat bit ). Orientation in the machine direction of the film is controlled primarily by take-up ratio, defined above. To control orientation in the transverse direction, we measure something called the blow-up ratio, while the forming ratio provides an indication of the degree of isotropy. 

The film thickness and the degree of uniaxial and biaxial orientation of the material are controlled by the blow-up ratio and the haul-off rate. The haul-off rate controls the film orientation in the machine direction, while the blow-up ratio controls the orientation in the transverse direction. Common values for the blow-up ratio are in the range of 1.5-4, depending on the material and desired film thickness. Take-off speeds are usually around 10 to 50 m/min. 

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