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Abstract
It is well known that an extrudate leaving a circular die swells to a larger diameter. The ratio of the extrudate diameter to the die diameter is called extrudate swell B. A plot of B vs. the length over diameter ratio of the die, L/D, indicates that two mechanisms seem to give rise to extrudate swell.
The one is independent of the L/D ratio and is even present for infinitely long dies. This contribution arises from the orientation created while the polymer flows through the die and is related to the first normal stress difference. This contribution is named B$\sb\infty$. A theory developed by Tanner to predict this type of swell was modified and extended and an equation developed which gives B$\sb\infty$ as a function of the power law index only.
The second contribution depends on the L/D ratio of the die and is the larger the shorter the die. An equation to predict this type of swell was developed. It is based upon the assumption that the energy associated with the entrance pressure drop is consumed to stretch the melt at the die entrance. While inside the die, the stretched polymer is kept in the same shape, i.e. stress relaxation can take place during its residence time inside the die. That stretching ratio which corresponds to the stress still present when the extrudate leaves the die gives rise to a contraction and thus a corresponding increase in diameter. Methods to measure the stress-relaxation behavior of the polymer and the stretching ratio at the die entrance were developed. These methods utilize the same capillary rheometer used to determine extrudate swell.
The theory was compared to experimental measurements obtained on a variety of polymers. Excellent agreement between experiment and theory was obtained.
