Causes and elimination methods of tail shrinkage of aluminum molds and bars are analyzed

In the extrusion production, there will be defects commonly known as "tail shrinkage" in the semi-finished parts of the cut head and tail after the head and tail are inspected, and the mechanical properties of the type bar containing the tissue cannot meet the requirements, and there are potential safety hazards. At the same time, when the production of the profile bar to be surface treated or turned, due to the existence of the defect destroys the internal continuity of the material, it will affect the subsequent surface and finishing, and seriously cause the dark grain scrap or damage to the turning tool, etc., which is a common problem in production.

Classification of tail shrinkage: "tail shrinkage" is divided into two types: hollow tail shrinkage and annular tail shrinkage, 1) hollow tail: form a hollow in the center of the extrusion type, the tail end of the bar, and the cross-section is presented as a hole with an unsmooth edge or a hole with other impurities at the edge, longitudinally a funnel-shaped (cone-shaped), and the tip of the funnel is facing the direction of metal outflow, mainly appearing in the single-hole plane die extrusion, especially the tail of the profile with small extrusion coefficient, large product diameter, thick wall or extrusion gasket with oil stain is more obvious. Two. Annular tail shrinkage: discontinuous annular or arc shape at both ends of the extruded shunt mold (especially the head), and the crescent shape is the most obvious on both sides of the weld line, and the annular tail shrinkage of each hole product is symmetrical. Formation of a shrunken tail　

Reasons for the formation of tail shrinkage: The mechanical conditions for the formation of tail shrinkage are; When the advection phase is over, the extrusion gasket gradually approaches the die, and the extrusion increases and produces a pressure on the surface of the extruded cylinder side dN cylinder. This force, together with the frictional dT cylinder, forms a tail shrinkage when the equilibrium condition of the force (dN cylinder dT cylinder) ≥ the dT pad is disrupted, and the metal located around the extruded pad area flows backward along the edge into the center of the blank. The extrusion conditions formed by tail shrinkage are: 1. The extrusion residue is too short, 2. The extrusion gasket is oily or not clean, 3. The surface of the ingot or wool is not clean, 4. The length of the tail cutting of the product is not in accordance with the regulations, 5. The lining of the extrusion cylinder is out of tolerance, 6. The extrusion speed is suddenly increased at the end of extrusion.

Elimination method of tail shrinkage: measures to reduce and prevent the formation of tail shrinkage: 1. Strictly according to the process regulations to shear the pressure and sawing the head and tail, keep the inner lining of the extrusion cylinder intact, prohibit the extrusion of the gasket to make oil, reduce the temperature of the aluminum rod before extrusion, use a special convex gasket, and adopt a reasonable length of residual material. 2. The surface of the extrusion tool and aluminum rod should be cleaned, 3. Check the size of the extrusion cylinder frequently and replace the unqualified tools, 4. Smooth extrusion, in the later stage of extrusion, the extrusion speed should be slowed down, and the thickness of the pressure residue should be appropriately left, or the residual material should be increased by extrusion.