4.2 Article

Effect of extrudate swell on extrusion foam of polyethylene terephthalate

Journal

JOURNAL OF CELLULAR PLASTICS
Volume 57, Issue 6, Pages 911-925

Publisher

SAGE PUBLICATIONS LTD
DOI: 10.1177/0021955X20973599

Keywords

Die design; foaming; foams; extrudate swell; extrusion; polyethylene terephthalate; PET; cyclopentane; density; cell size

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The study investigates the impact of extrudate swell on the expansion ratio of foam in extrusion, revealing a correlation between extrudate swell and foaming, and how it affects the final shape of the product. Additionally, a simple heuristic model is developed to predict extrudate swell based on geometrical parameters and rheological characterization of the fluid, showing that the foaming mechanism of polyethylene terephthalate changes with extrudate swell, with lower density foam achieved using dies with higher extrudate swell.
The effect of extrudate swell on extrusion foam of thermoplastic polymers is presented using an experimental approach supported by a modelling of the phenomenon. Its understanding is fundamental in designing the geometry of a die for extrusion foam and to predict foaming. The extrudate swell is the swelling of a viscoelastic material due to a fast elastic recovery after being subjected to stresses. We show that there exists a link between the extrudate swell and foaming, performing experiments with simple and complex extrusion dies to measure the expansion ratio. It was found that the expansion ratio is anisotropic and the anisotropy in the expansion of the foam is due to the extrudate swell that affects strongly the final shape of the product and it cannot be neglected in standard application for extrusion foam. A simple heuristic model was developed to predict the extrudate swell from geometrical parameters and rheological characterization of the fluid. It was found that the foaming mechanism of polyethylene terephthalate, blown with cyclopentane(,) changes as function of extrudate swell and the lowest density foam is achieved using the die that has the bigger extrudate swell.

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