AbstractUnvulcanized rubber compounds have been extruded from a constant rate extrusion rheometer having a low capillaryL/dratio. With a polychloroprene compound, the extrusion pressure decreased linearly as the piston height decreased and the results have been shown to be quantitatively in agreement with the suggestion that the reservoir and capillary behave as two extruders in series. With highly elastic rubber compounds, the extrusion pressure either increased continuously as the piston descended down the reservoir or increased to a maximum before falling to a final value. This suggests that as well as the reservoir height effect, there is an effect due to the viscoelasticity of the material. The height of the peak in the extrusion pressure versus piston height curve for natural rubber, was roughly proportional to the initial amount of charge in the reservoir and for both types of material, the final pressure was independent of the initial charge. These results would indicate that certain anomalous effects in extrusion rheometry are reflections of the behavior of the material in the reservoir as opposed to shear effects in the capillary. Even for rheometers with low capillaryL/dratios, consistent results can be obtained if the pressure drop across the capillary is calculated from observations taken when the reservoir is effectively empty.
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