Take-up ratio

An important process variable in all types of extrusion is the take-up ratio, defined in the box at the top of the next column. The take-up ratio not only controls product geometry, but it also has a significant effect on molecular orientation in the final product. Whenever polymer melt is stretched, such as an extrudate under a high take-up ratio,... 

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. 

To properly describe and control the bubble-forming process, certain quantities have been developed to characterize the process conditions that influence bubble geometry. These quantities are the take-up ratio (TUR), the blow-up ratio (BUR), and the forming ratio (FR). 

Draw Resonance appears as a continuous variation in bubble diameter. As in other extrusion processes, such as profile, it occurs when the melt is stretched too quickly (i.e., a high take-up ratio). Solutions act to reduce the take-up ratio, for example increasing the melt (screw) speed. 

Bubble Tears take place at the die lips and result when the stretching rate on the film is excessively high. When the film is drawn too fast or cooled too quickly, tears may occur. Potential solutions include increasing the die lip temperature and reducing the take-up ratio. 

Using the conservation of mass at the die face, determine the melt velocity and the take-up ratio. 

This equation is used to calculate the ratio of film speed to melt speed (i.e., take-up ratio). It provides quantification of the degree of machine direction (MD) stretching imparted on the melt by the process conditions. MD stretching is related to MD molecular orientation. Melt speed is difficult to measure, but its value is not required in order to calculate take-up ratio as described in the next section (Conservation of Mass). 

This equation states that the mass flow rate at any point in the system, such as through the die exit, is equal to the mass flow rate at any other point in the system, such as in the bubble above the frost line. It is shown in the general form and also rearranged to solve for the take-up ratio. 

This equation is sometimes used instead of the take-up ratio to provide an indication of the machine direction stretching. It represents the degree of thickness reduction from the die gap to the final film, both easily obtainable values. Of course, thickness reduction during processing occurs in the transverse direction as well as the machine direction. 

Low-density polyethylene (LDPE, Dow 6401, 0.92 g/cc and 1 g/10 min MFI) was used in this study. The polymer was extruded using a lab-scale 19 mm, 24 1 L/D ratio extruder (Alex James Associates, Greenville, SC) equipped with a cross-head die (Wayne Machine Co., NJ) of 50 mm diameter, 0.635 mm gap and a single-lip air ring. A die temperature of 195 C and polymer mass flow rate of approximately 36 g/min was maintained throughout the experiment. The bubble was extended uniaxiallyat a take-up ratio (TUR) of 7 and a blow-up ratio (BUR) of 0.4, so that a thickness ratio of 0.4 was obtained. 

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