1 Introduction

    Pulling wire mold is an important tool for wire and cable industry to produce wire. It is the key to achieve normal continuous stretching and ensure the quality of stretched products. In order to achieve the service life of the wire drawing mold, obtaining a high-quality stretched product depends not only on the material of the wire drawing die itself, but also on the hole design of the mold and other fitting conditions in use.

    At present, with the wide application of high-speed wire drawing machines, the problem of the service life of the wire drawing die has become increasingly prominent. This paper combines the actual production of copper wire manufacturers and some new developments of wire drawing molds at home and abroad to make a brief analysis of the factors affecting the service life of wire drawing die.

    2 The influence of the quality of the wire drawing die on its service life

    The quality of the pull mold itself is an important factor affecting its service life. The quality of the drawing die is related to the core material, the hole design and the processing technology. The improvement of the core material, the design of the hole structure and the improvement of the processing technology are all beneficial to improve the service life of the mold and the quality of the wire.

    2.1 core material

    At present, the core material of the wire drawing die used in domestic production of copper wire is mainly made of cemented carbide, natural diamond and artificial polycrystalline diamond.

    Cemented carbide is a powder sintered body of tungsten carbide and metallic cobalt having a high hardness. It has high hardness, good wear resistance and strong impact resistance, and is low in price. It is a kind of drawing mold making material widely used for drawing thick and medium wire. At present, foreign countries use hot isostatic pressing (HIP) treatment, ultra-fine crystal technology to reduce porosity, improve the hardness of the alloy, and add rare metals, develop surface coating process, and improve the surface strength of the alloy. Studies have shown that by improving the composition and structure of cemented carbides, controlling the fluctuations in carbon content and refining the particles of carbides, the properties of the materials can be improved and their service life can be extended.

    Natural diamond has the characteristics of high hardness and good wear resistance, and the drawn wire has a high surface finish. Since the natural diamond has an anisotropy in structure, the hardness thereof is also anisotropic, the wear of the die hole is uneven, and the product is not rounded. In addition, it is expensive and rare, and is generally used as a thin wire drawing die or a finished wire drawing die with high surface quality requirements.

    Synthetic polycrystalline diamond is an unoriented polycrystal. It has the advantages of high hardness, good wear resistance and strong impact resistance. There is no anisotropy in hardness, uniform wear and long service life of the mold, suitable for high-speed drawing. Due to the quality problems of coarse crystal grains and poor polishing performance of domestic polycrystalline blanks, domestic manufacturers often use polycrystalline molds as transitional molds instead of as finished molds. However, with the improvement of the intrinsic quality and processing level of the polycrystalline mold, there is a tendency to replace the expensive natural diamond as a finished mold.

    2.2 Pulling die design

    Different hole designs are used under the same material conditions, and the die life is quite different. Therefore, improving the hole design is an important way to improve the service life of the mold. The wire drawing die type is generally classified into a curved type (ie, an R type series) and a straight type (ie, a tapered type).

    From the angle analysis of the wire deformation in the wire drawing mode, it seems that the curve type is better than the straight line type. Therefore, in the past, the R-type series used by the former Soviet Union in the 1950s was generally used to formulate the specifications for wire drawing die. This type of hole was designed under the guidance of the "smooth transition" at that time. Its pore structure can be divided into five parts: "inlet area, lubrication area, working area, sizing area, and exit area" according to the nature of work. At the junction of the various sections, a “chamfering” is required, a smooth transition, and the entire hole pattern is ground into a large curved surface with different curvatures. This type of hole mold is still applicable at the time of drawing speed at that time. By the end of the 1970s to the early 1980s, as the speed of the cable was increased, the service life of the cable die became a prominent problem. In order to meet the requirements of high-speed wire drawing, T.Maxwall and E.G. Kennth of the United States proposed the "straight line" theory. The theory focuses on the lubrication and wear factors during the drawing process. It is pointed out that the improved linear drawing die hole type should have the following characteristics:

    (1) The longitudinal section lines of each part of the hole type must be straight. The straight working cone has the smallest pulling force.

    (2) The intersection of the parts of the mold must be obvious, so that each part can fully exert its respective functions, avoiding the reduction of the actual length of the sizing area by the transition angle.

    (3) Extend the height of the entrance area and the working area so that the wire enters the middle section of the working hole of the die hole, and the wedge shape formed by the inlet taper angle and the upper half of the working cone angle is used to establish a "wedge effect", and the surface of the wire material is formed more densely. A strong lubricating film that reduces wear and is suitable for high speed drawing.

    (4) The sizing area must be straight and of reasonable length. If the sizing area is too long, the friction of the wire will increase, and the wire will be easily reduced or broken after being pulled out of the die hole; the sizing zone is too short, and it is difficult to obtain a wire with stable shape, accurate size and good surface quality, and the die hole is also Will wear out too badly.

    The wire drawing die designed by the linear theory has a service life of 3 to 5 times higher than that of the R type wire drawing die.

    2.3 mold processing level

    The influence of the level of mold processing on the quality of the mold is mainly reflected in two aspects: one is the hole size of the drawing die, and the other is the surface finish of the die hole.

    The grinding process of foreign wire drawing molds generally adopts high-speed mechanical grinding machine and metal grinding needle with diamond-plated surface. The equipment runs smoothly, the specifications and usage of grinding needles are standardized, and the product precision is high. The hole size of the mold is detected by a contour recorder and an aperture measuring instrument, and the surface finish is checked by a microscope dedicated to the inspection of the wire drawing die.

    Many domestic manufacturers still use backward equipment and use manual operation to grind the hole type. Therefore, the following problems exist: the hole type parameters fluctuate greatly, it is difficult to process a straight working cone; the sizing area and the work area intersection Easy to grind out the transition angle, so that the wire will produce secondary compression in the sizing area, increase the external friction, shorten the length of the sizing area, and shorten the service life of the mold; the frequency of wear of the worn grinding needle varies from person to person. Not standardized, resulting in poor consistency of the hole type. The detection method is also backward, and can only be detected by visual inspection or simple tools such as a magnifying glass and a microscope, and the focus is on the surface finish of the mold, and the size of the hole cannot be effectively detected, let alone control.

    3 The effect of stretching conditions on the service life of the mold

    During the stretching process of the wire, the process conditions affecting the service life of the die mainly include: the effect of the reverse tensile force P', the compression ratio of the pass, the lubricant, and the surface quality of the wire.

    3.1 Anti-pull force P`

    The force that the wire receives in the die hole during stretching is: the positive pressure N of the die wall, the frictional force T, the drawing force P, and the pulling force P'.

    According to the equilibrium condition of the force when pulling the wire and the yield criterion of the metal material, O. Hoffman, G. Sachs uses the micro-element analysis method to derive the axial tensile stress σPX when the cable is pulled.

    σPX=σS1+ 1- 2B+σλ 2B (1)

    B=μ/tgα

    The positive compressive stress σNX of the mold wall is

    σNX=σS-σPX (2)

    Where σs is the yield strength; Rx is the die hole radius from the wire inlet x; σλ is the tensile stress at the wire population; RO is the die hole radius at the wire entrance; Rf is the die hole radius at the wire exit; μ is the friction coefficient ; α is the working cone half angle.

    The compressive stress on the mold wall is gradually reduced from the entrance of the wire mold to the outlet, so that it is just when it is imported. This is the mechanical cause of the annular wear groove often occurring at the entrance of the wire mold.

    Due to the effect of the reverse tension, the compressive stress at the entrance of the wire mold can be significantly reduced, and the lubricant can enter the working area, reduce the friction between the wire and the mold wall, slow the ring wear and mold rupture, and prolong the service life of the wire mold. . However, the excessive back tension will increase the drawing stress when the wire is pulled, which may cause the wire to be reduced in diameter or broken.

    3.2 pass compression ratio

    When other stretching conditions are constant, the greater the compressive stress on the mold wall, the greater the frictional stress, and the more severe the mold wear. Studies have shown that the compressive stress σN on the mold wall can be expressed as

    σN=σb/1+(3)

    Σb=(σb1 十σb2)/2

    Q=F/sinα≈F/αα is very small)

    F=f1-f2,α=

    σN==(4)

    In the formula, σb1 and σb2 are the tensile strength of the wire before and after the stretching; f1 and f2 are the cross-sectional areas of the wire before and after the stretching; Q is the contact area between the wire and the working area of the mold; σb is the average of the wire of the pass. tensile strength.

    Let A==1+-1 (5)

    For the convenience of discussion, σb=1 and μ=0.1 when the wire is stretched. From equation (5), the relationship between the ratio A of the compressive stress σN on the mold wall shown in Fig. 4 and the average tensile strength σb of the present pass wire and the parameters δ and α can be plotted.

    Since σN is proportional to the value of A, it can be seen from the figure: (1) When α is constant, the value of A decreases with increasing δ, that is, σN decreases with increasing δ; (2) When δ is constant, A value It increases as α increases, that is, σN increases as α increases. Therefore, in the case where the stretching conditions permit, increasing the pass compression ratio and appropriately reducing the working cone angle can reduce the compressive stress on the mold wall, and is advantageous for increasing the service life of the pull mold.

    3.3 Lubricants

    During the stretching process, the quality of the lubricant and the availability of the lubricant affect the service life of the wire drawing die. Therefore, the lubricant oil base is required to be stable, has good emulsifying properties, has excellent lubricity, cooling and cleaning properties, is easy to filter and precipitate copper powder, and maintains the lubrication state of the **** throughout the production process, so as to form a The layer can withstand high pressure without damage to the film, reducing the friction of the work area and improving the life of the mold.

    The stability of the lubricant pH has a great influence on the lubrication effect. Because when the copper powder in the lubricating emulsion is precipitated, the amount of fat in the lubricant is lowered, and the free alkali content is increased, so that the lubricating component on the surface of the wire is easily washed away, and the lubricating property of the emulsion is strongly lowered. When the emulsion is unstable and the fat content is too high, the emulsion will be stratified, and the fat component with the fine copper powder floating on the emulsion will make the copper powder difficult to precipitate and filter, causing the pores of the mold to be clogged and the lubricating effect to be deteriorated.

    3.4 Surface quality of wire

    If there is adhesion of oxide layer, sand or other impurities on the surface of the wire, this will adversely affect the service life of the wire drawing die. Because when the wire passes through the die hole, the hard, brittle oxide layer will cause the wire die hole to wear quickly and scratch the wire surface like the abrasive. Therefore, the heavily oxidized wire needs to be pickled before stretching. When stacking billets, pay attention to the cleanliness of the stacking site to avoid contact with sand and other impurities.

    4 The effect of the method on the service life of the wire drawing die

    During the long-term use of the wire drawing die, the mold wall is strongly rubbed and washed by the metal wire, which causes wear phenomenon. The most common is the occurrence of annular grooves (dents) in the wire population of the working area. The appearance of the pull mold groove groove exacerbates the wear of the die hole. Because the small core material of the core material peeled off due to looseness on the ring groove is brought into the working area and the sizing area of the die hole by the metal wire, it plays the role of the abrasive, and the wire entering the die hole aggravates the wear of the die hole like the grinding pin. . If it is not replaced in time for repair, the ring groove will continue to accelerate and expand, making the repair more difficult, and it is even possible to crack at the deeper part of the annular groove, so that the mold is completely broken and scrapped.

    It is known from experience that it is very economical to develop a set of normative standards, strengthen daily maintenance, and often overhaul molds. Once the mold has experienced any slight wear and polishing in time, the time it takes to return the mold to its original polished state is short, and the die size of the mold does not change significantly.

    In the case where the stretching process is constant, the method of using the stretching apparatus also has a large influence on the life of the mold. The tensile axis of the wire is asymmetrical to the centerline of the die hole, which will cause uneven stress on the wire and the wire drawing die, and the impact of mechanical vibration will also cause high stress peaks on the wire and the wire drawing die, both of which will accelerate. The wear of the mold.

    Especially for molds that are manually inlaid, if there is a significant deviation in the center line of the mold, the enlargement of the die hole is aggravated, resulting in uneven wear and making the die hole into an elliptical shape. In addition, frequent parking during the pulling process will also increase the wear of the mold, because the friction caused by the tensile stress at the time of pulling is much larger than the friction during normal drawing.

    5 Conclusion

    In order to extend the service life of the wire drawing die, in addition to selecting the appropriate die material, designing a reasonable hole size, improving the manufacturing level of the wire drawing die, and achieving the surface finish of the die hole to meet the process requirements, it is also necessary to determine a reasonable pass compression ratio and improve The use conditions of the pull mold, pay attention to the daily maintenance of the mold during use, should be frequently changed, and ensure a good lubrication effect during the stretching process.