Surface fracture in axisymmetric paste extrusion - An experimental study

In the extrusion of pastes, fractures can be found on the surface of product. Such fractures compromise strength and are often unacceptable aesthetically. Here, experiments show that increasing the length of the die has a beneficial effect on surface quality in all pastes examined. The materials were cu-alumina pastes allied to those used for making catalyst that could additionally contain Bentonite clay and carbohydrates. Defects can be completely eliminated for ct-alumina pastes with starch additives. Increasing extrusion velocity increases the depth of fracture in pastes with glucose additives. Reduced frequency and depth of fracturing is found when using tapered dies. All the materials show a decrease in the depth of fracture with increasing extrusion ratio, i.e. the ratio of barrel to die diameter. For a sample paste, the product of the frequency of fracture and diameter of the extrusion die yields a dimensionless quantity independent of extrusion velocity and extrusion ratio, having a value of two.