Polycrystalline die has the advantages of good wear resistance, long service life, low price, etc., and its application in wire drawing production is constantly expanding. However, the main reason why most polycrystalline molds can not be finished molds is that the wire diameter of the drawn wire will appear large and small, and the wire diameter is difficult to control.
In the drawing process of the wire, the process conditions that affect the service life of the die are mainly: the action of the reverse tension P, the compression rate of the pass, the lubricant and the surface quality of the wire.
The factors affecting tensile stress
1. Reverse tension
The reverse tension will increase the pulling force, which is not conducive to the process of wire drawing. However, the reverse tension can obviously reduce the positive pressure stress on the die wall, which is conducive to the lubricant entering the deformation zone and can reduce the friction between the wire and the die wall to a certain extent, reducing the die hole wear, and prolong the service life of the die. Of course, too much drawing force will increase the tensile stress too much, resulting in different stretching phenomena such as wire thinning or wire breaking. Therefore, a reasonable extension coefficient should be used in the mold allocation, so that the wire can maintain a normal sliding rate on the drum and avoid the negative sliding phenomenon.
The tensile force of the wire in the die hole is: The positive pressure, friction force, drawing force and reverse tension of the die wall According to the balance condition of the force when drawing the wire and the yield criterion of the metal material, the distribution of the tensile stress of the wire and the compressive stress of the die wall can significantly reduce the compressive stress at the entrance of the wire die, and help the lubricant to enter the working area, reduce the friction between the wire and the die wall, and slow down the ring aging damage and the rupture of the die. However, too large a reverse tension will increase the drawing stress of the wire, which is easy to cause the wire to shrink or break. When other tensile conditions are unchanged, the greater the compressive stress on the mold wall, the greater the friction stress, and the more serious the die wear.
2. Elongation coefficient
When other wire drawing conditions remain unchanged, increasing the elongation coefficient will increase both the tensile stress and the die wall positive pressure. When the tensile stress approaches or reaches the yield limit of the wire at the outlet of the wire die, unstable stretching will occur, causing the wire to become thin or broken. Therefore, the elongation coefficient of each pass should be reasonably determined according to different situations. Generally speaking, in order to ensure the size and surface quality of the wire, the elongation coefficient of the last stretch should be lower than that of the previous passes.
3. Friction coefficient
This is the most important factor in wire drawing.At present, most polycrystalline die should only be used as a bridge die, mainly because the polishing performance of the polycrystalline die is not good, which has a large friction coefficient, and the friction coefficient of the die hole is a variable, which is related to the material, surface state, pass shape and its surface finish. It is also related to various factors such as the material of the wire, the surface state, the drawing temperature, and the quality of the speed lubricant due to the constraints of objective conditions, although some factors can not be changed, we can still reduce the friction coefficient by improving the drawing conditions. For example, in the production of copper wire, you can use good quality oxygen-free copper rod and lubricant, control the drawing speed, and drawing temperature, and adjust the drawing process.
In short, we should pay attention to the wearing parts of the drawing die during the use of the drawing machine and replace them in time.
