Die lines on extruded profiles are a common surface defect seen in many metal extrusion processes. They usually appear as fine lines, streaks, or bands on the surface of the profile and can affect both appearance and, in some cases, product quality. These marks are often caused by issues related to the extrusion die, material flow, lubrication, temperature, or wear during production.One major cause of die lines is uneven metal flow through the die. When the metal does not pass smoothly and uniformly through the die opening, slight variations in pressure and speed can leave visible marks on the profile surface. This is especially likely if the die design has sharp corners, long land lengths, or areas where the flow path is not balanced. In such cases, the metal may stick and slip repeatedly as it moves through the die, creating a line pattern on the finished product.Another important reason is die wear or damage. Over time, the surface of the die can become worn, scratched, or slightly deformed due to continuous contact with hot metal under high pressure. Even small imperfections on the die surface can transfer directly onto the extruded profile. If the die is not polished properly or if it has surface buildup from oxidation or residue, the defect may become more noticeable. Regular inspection and maintenance of the die are therefore very important.Temperature control also plays a key role. If the billet, die, or extrusion speed is not properly controlled, the metal may overheat or cool unevenly during the process. Excessive temperature can soften the material too much and increase sticking, while insufficient temperature can raise resistance and cause rough surface flow. Both situations can lead to die lines. Stable and appropriate process temperatures help reduce this risk.Lubrication and surface condition of the tooling are also factors. Poor lubrication increases friction between the metal and the die, which can cause dragging and visible lines. Similarly, contamination such as dirt, oxide particles, or leftover material on the die surface may be pressed into the profile during extrusion. Clean tooling and proper lubrication help the metal flow more evenly and reduce surface defects.Material quality can contribute as well. If the alloy contains impurities, segregation, or inconsistent composition, it may not flow uniformly through the die. This can create localized differences in deformation and leave marks on the surface. In addition, improper billet preparation, such as poor cutting, heating, or surface cleaning, can introduce defects before extrusion even begins.To reduce die lines, manufacturers often improve die design, maintain better temperature control, polish and clean the die regularly, and ensure consistent material preparation. In some cases, adjusting extrusion speed or ram pressure can also help. Although die lines may not always affect the structural performance of the profile, they are often considered unacceptable in products that require a smooth and attractive finish.