In the process of aluminum extrusion, along the extrusion direction, the longitudinal continuous lines are produced on the surface of the extruded aluminum products, which are called die marks.
The causes of mold marks come from three aspects: extrusion die itself, extrusion process and material of aluminum profile.In terms of die, the finish of extrusion die working belt, heating process and the length of working belt will have an impact on die marks.Mold with unpolished working belt or poor polishing effect will produce mold marks on the surface of extruded products. After a long time of production, the mold will be damaged by repeated heating and polishing, and the matrix of the mold working belt will be damaged or the nitriding layer will peel off, which will lead to the generation of mold marks. It needs to be re-nitriding and polishing before it is put into use.Mold heating time is too long, the higher the temperature, will make the mold work belt oxidation or partial annealing to reduce the strength (commonly known as "red envelope"), will also lead to the generation of mold marks;In addition, the longer the working belt of the die, the longer the friction time in the extrusion process, which will make the working belt stick to aluminum, resulting in the surface of the extruded aluminum coarsening, and even produce mold marks.
In terms of extrusion process, the higher the extrusion temperature, the higher the viscosity of aluminum alloy will increase the probability of mold marks on the surface of the extrusion product. At the same time, too fast extrusion speed will also lead to mold marks.When extruding 5××× ×× alloy and 7××× × alloy pipe, due to the high alloy content of the material, a small amount of granular hard compound phase is easy to precipitate at the exit part of the working belt of the extrusion die, resulting in mold marks on the surface of the product. Such mold marks are irregular and discontinuous in most cases.
