Film Stretching and Properties

FIGURE 9.19 Modulus of elasticity of stretched and unannealed PPS films as a function of testing angles, for various stretch ratios. (Reprinted by permission of the publisher from Maemura et al., 1989.) 

The two main parameters of this process are the blow ratio (or blowup ratio), Br (or BUR), and the machine-direction draw (or draw-down) ratio. Dr. The blow ratio is defined as the ratio of the final tube radius, R, to the initial tube outside radius just downstream of the annular die, Ro (see also Figs. 9.20 and 9.21, and Section 1.2)  

FIGURE 9.21 Moving Cartesian coordinate system and cylindrical polar coordinate system, for die film blowing process. 

As far as the mechanical properties are concerned, the tear (test name Elmendorf tear), impact (test name dart drop), and tensile strengths give an indication of the mechanical strength of the tubular film. Some of these properties and the effect the processing parameters have on them were discussed in Section 1.2. The amorphous as well as the crystalline orientation developments during the blowing process depend on 

Commercially, the film blowing process is extensively used for the production of polyolefin (LDPE, HOPE, and PP) wrapping film. Mechanical strength, optical clarity, which depends on the degree and type of crystallinity for crystalliz-able polymers, and the uniformity of thickness (variations of about 5% for films with a length scale of 10 mm to 10 m is acceptable) are the three most important and general properties of the film. 

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