Dimensional Change Resulting From Swell And Drawdown Of Extruded Polymers

James F. Stevenson, Stevenson Polytech, LLC

Understanding and predicting how extruded profile dimensions change along an extrusion line contribute significantly to the design of shaping dies and the effectiveness of dimensional control. Upon exiting the extrusion die, the dimensions of nonsymmetric profiles change non-uniformly along the line due to swell and drawdown. This article reports on size and shape changes for drawn profiles of rubber and plastic flowing through rectangular dies.

In previous work on shape change during drawdown, Griffith and Tsai concluded that drawdown from the equilibrated (relaxed) swell state for rectangular and t-shaped profiles was generally symmetric. However, small systematic deviations from symmetric shape change in their data, e.g., correspond to the shape change during drawdown reported here. Earlier work by Stevenson proposed size and shape change analysis, which is further developed here and linked with predictive models fit to data.

A systematic way is proposed to analyze and model changes in individual extrudate dimensions in terms of size (symmetric) and shape (non-symmetric) changes along the extrusion line. Shape change data are fit to a two-constant power law model, and a three-constant model which accurately predicts the asymptotic dimensional change at large draw ratios. This analysis will be applied to a rubber compound and a rigid polyvinyl chloride (PVC) compound.

The utility of this understanding will be illustrated by modeling dimensional changes when the draw ratio is in error and when the compound swell properties deviate from specification (different from those for which the die was designed). The pattern of the percentage deviations of end-of-line (cold) dimensions from specification will be used to distinguish between drawdown and swell as specific sources of variation.

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