In extrusion production, the profiles and billets may exhibit a defect commonly referred to as 'tail shrinkage' in the semi-finished parts after the head and tail are cut and inspected using alkali etching. The mechanical properties of profiles and billets containing this structure do not meet the required standards, posing safety risks. Additionally, when the produced profiles and billets undergo surface treatment or machining, the presence of this defect disrupts the internal continuity of the material, affecting subsequent surface finishing and precision processing. In severe cases, it can lead to hidden pattern scrapping or damage to cutting tools, which is a common issue in production.
Classification of Tail Collapse: "Tail collapse" is divided into two types: hollow tail collapse and annular tail collapse. 1) Hollow tail collapse: a hollow forms in the center of the end of extruded bars, with the cross-section showing a hole with rough edges or a hole filled with other impurities, and the longitudinal section resembling a funnel (conical shape), with the tip of the funnel pointing in the direction of metal flow. It mainly occurs in single-hole flat die extrusion, especially when the extrusion ratio is low, the product diameter is large, the walls are thick, or an oily extrusion spacer is used; it is more pronounced at the ends of the profiles. 2) Annular tail collapse: at both ends of products extruded from manifold dies (especially the head), discontinuous ring-shaped or arc-shaped collapses appear. On either side of weld lines, it is most noticeable as a crescent shape. For multi-hole products, the annular tail collapse is symmetric. Formation of Tail Collapse
Causes 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 on the side surface of the extrusion cylinder increases during extrusion, generating a force dN_cylinder. This force, together with the friction force dT_cylinder, when the balance condition of forces is broken (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. Excess material left for extrusion is too short 2. The extrusion pad has oil or is unclean 3. The surface of the ingot or billet is not clean 4. The cut length of the product tail does not meet the specifications 5. The liner of the extrusion cylinder is out of tolerance 6. The extrusion speed suddenly increases at the end of 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 applying oil to the extrusion gasket, lower the temperature of the aluminum rod before extrusion, use special convex gaskets, and adopt a reasonable length for the leftover 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 method of extruding with increased leftover material can be used.
Residual compressive stress should comply with the following regulations:
| Extruder (T) | Residual thickness (mm) |
| ＜800T | ≥15㎜ |
| 800-1000T | ≥18㎜ |
| 1200T | ≥20㎜ |
| 1600T | ≥25㎜ |
