There are many variables that affect the size and shape quality of rubber profile extrusions. First, the physical properties of the rubber compound vary according to the viscoelastic properties of the natural rubber, the carbon black particle size and structure, the uniformity of mix dispersion and the aggregate heat history of the compound before it reaches the extruder, among other factors. The consistency of the rubber flow through the die varies with the heat history applied by the extruder and the resulting viscoelastic properties of the compound under the pressure and temperature conditions at the die. These properties, combined with the flow-limiting geometry of the die, result in varying degrees of die swell once the rubber exits the die (ref. 1). As the extrusion proceeds through downstream operations, variations in heating and cooling affect the final geometry. Pullers, cutters, and windups further vary the line tension and cause distortion in the profile geometry. In addition, feed interruption during batch change causes variations in die pressure that affect the flow rate.
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