Processes involving melt inflation

In a number of processes, air pressure is applied to a bubble of polymer melt to change its shape. We will examine one continuous process—blown film production—and two cyclic processes—blow moulding and thermoforming. All of them involve some melt extensional flow with a resultant thinning of the bubble, and at least one side of the polymer solidifies without the constraint of contact with a mould. 

The melt bubble is stretched vertically and circumferentially by a factor of 2 or more, so that an initial melt thickness of about 1 mm is reduced to between 250 and 100 pm. In the biaxial tensile flow, the melt stress in the hoop (H) direction can be calculated from the pressure p inside the bubble, the current bubble radius r and thickness t, using Eq. (C.22) of Section C.3. 

The longitudinal stress is also affected by tensile wind-up force T, therefore, Eq. (C.21) is modified to 

As there is no external control of the bubble shape, it is possible for shape instabilities to occur if excessive internal pressures are used, or if the polymer melt has an unsuitable extensional viscosity response. 

Cooling is provided by an annular air jet which blows upwards on the outside of the bubble with an initial velocity of about lms . The heat transfer coefficient becomes smaller as the air velocity falls but 

---