A Brief Analysis of the Causes and Elimination Methods of Taper Defects in Aluminum Profiles and Rods

During extrusion production, a defect commonly referred to as 'tail shrinkage' may appear in the semi-finished parts of profiles and bars after cutting the head and tail and checking with alkaline etching. The mechanical properties of profiles and bars containing this structure do not meet the requirements, posing safety risks. At the same time, when the produced profiles and bars undergo surface treatment or machining, the presence of this defect disrupts the internal continuity of the material, affecting subsequent surface finishing and precision machining. In severe cases, it can lead to hidden defects being scrapped or damage to cutting tools. This is a common problem in production. Here, this article briefly analyzes the causes of tail shrinkage and methods for its elimination.

Types of End Defects: "End defects" are divided into hollow end defects and ring-shaped end defects. 1) Hollow end defects: These form a hollow in the center of the extruded or bar-shaped material’s tail, with the cross-section appearing as a hole with uneven edges or a hole filled with other impurities. Longitudinally, it takes a funnel shape (conical), with the tip of the funnel pointing in the direction of metal flow. They mainly occur in single-hole flat die extrusion, especially when the extrusion ratio is low, the product diameter is large, the walls are thick, or when extruding through a die pad with oil contamination. They are more pronounced at the tail of such profiles. 2) Ring-shaped end defects: These appear as discontinuous rings or arcs at both ends of products from a multi-channel die (especially at the head). The defect is most noticeable as a crescent shape on both sides of the weld line, and for multi-hole products, the ring-shaped end defect is symmetric. Formation of End Defects.

Cause of tail shrinkage: The mechanical condition for the formation of tail shrinkage is: when the extrusion stage ends and the extrusion pad gradually approaches the die, the pressure during extrusion increases, generating a pressure dN_cylinder on the side surface of the extrusion cylinder. This force, together with the friction dT_cylinder, when it breaks the force balance condition (dN_cylinder - dT_cylinder) ≥ dT_pad, causes the metal around the extrusion pad area to flow backward along the edge into the center of the billet, forming tail shrinkage. The extrusion conditions that lead to tail shrinkage are: 1. The leftover extrusion material is too short 2. The extrusion pad is oily or dirty 3. The surface of the ingot or billet is not clean 4. The product's tail cut length does not meet specifications 5. The lining inside the extrusion cylinder is out of tolerance 6. The extrusion speed suddenly increases at the end of the extrusion.

Methods to eliminate tail shrinkage: Measures to reduce and prevent the formation of tail shrinkage: 1. Cut the press surplus and saw off the head and tail strictly according to process specifications, maintain the integrity of the extrusion barrel lining, prohibit oiling of the extrusion gasket, lower the temperature of the aluminum rod before extrusion, use special convex gaskets, and adopt a reasonable length for the residual material. 2. The extrusion tools and the surface of the aluminum rod should be clean. 3. Regularly check the dimensions of the extrusion barrel and replace unqualified tools. 4. Extrude smoothly. In the later stage of extrusion, the speed should be slowed down, an appropriate surplus thickness should be left, or the residual material method can be used to extrude.

Residual compressive stress should comply with the following regulations: