Experimental and Theoretical Study of the Flow Characteristics of Rubber Compounds in an Extruder Screw

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Abstract
We describe an experimental and theoretical study of the mechanisms of flow of rubber compounds in a screw extruder. Two compounds, a typical synthetic rubber (butadiene-styrene copolymer-cis-1, 4 polybutadiene) based passenger tire tread (PTT) and a natural rubber based truck tire tread (TTT), were investigated in a 1½ inch NRM cold feed screw extruder using a screw with a constant channel depth and constant channel pitch along the whole screw length. The rubber was cured in place and the screw removed. The screw exhibited starvation behavior except near the die. The length of the rubber strip in the screw is proportional to the pressure developed at the die. Marker studies showed transverse circulating flows beginning in the region immediately downstream from the hopper. These observations indicate that cold feed extruders exhibit a metering region flow, albeit sometimes starved, from entry port to die. Under these conditions the extruder is independent of the pressure developed in the screw channel. A simple non-Newtonian isothermal mathematical model for the metering zone is described. The rubber compound is considered as a pseudoplastic material with a yield value. A comparison of theory and experiment shows that a one dimensional isothermal model quite accurately predicts the flow rate pressure development of the rubber compound in our screw (with a shallow prismatic channel) when starvation is considered. The yield value of the rubber compound influences screw characteristic curves only for very high values of the dimensionless pressure gradient.

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