1 Preface

Wire drawing die is an important tool for wire production in wire and cable industry. It is the key to realize normal continuous drawing and ensure the quality of drawn products. In order to achieve the best service life of the drawing die and obtain high-quality drawing products, it depends not only on the material of the drawing die itself, but also on the pass design of the die and other matching conditions in use.

At present, with the wide application of high-speed wire drawing machine, the problem of the service life of drawing die is increasingly prominent. Based on the production practice of copper wire manufacturers and some new development of wire drawing dies at home and abroad, this paper briefly analyzes the factors affecting the service life of wire drawing dies.

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

The quality of drawing die itself is an important factor affecting its service life. The quality of stay wire die is related to die core material, pass design and processing technology. Improving die core material, designing reasonable pass structure and improving processing technology are conducive to improving the service life of die and wire quality.

2.1 core material

At present, the core materials of wire drawing die used in domestic production are mainly cemented carbide, natural diamond and artificial polycrystalline diamond.

Cemented carbide is a powder sinter of tungsten carbide and cobalt with high hardness. It has high hardness, good wear resistance, strong impact resistance and low price. It is an excellent material for drawing die, widely used in drawing thick and medium wire. At present, hot isostatic pressing (hip) treatment and ultrafine grain process are used abroad to reduce porosity and improve the hardness of the alloy, and rare metals are added to develop the surface coating process to improve the surface strength of the alloy. The results show that by improving the composition and structure of cemented carbide, controlling the fluctuation value of carbon content and refining the carbide particles, the properties and service life of cemented carbide can be improved.

Natural diamond has the characteristics of high hardness and good wear resistance, and the surface finish of drawn wire rod is very high. Due to the anisotropic structure of natural diamond, its hardness is also anisotropic, which makes the wear of die hole uneven and the products not round. In addition, the price is expensive and rare, and it is generally used as fine wire drawing die or finished drawing die with high surface quality requirements.

Synthetic Polycrystalline Diamond is a non directional polycrystalline. It has the advantages of high hardness, good wear resistance and strong impact resistance. There is no anisotropy in hardness, wear is even, die life is long, suitable for high-speed drawing. Because of the quality problems such as coarse grains and poor polishing performance in domestic polycrystalline mould, polycrystalline mould is often used as transition mould instead of finished mould by domestic manufacturers. However, with the improvement of the internal quality and processing level of polycrystalline mold, there is a trend to replace the expensive natural diamond as the finished mold.

2.2 pass design of stay wire die

Under the same material condition, using different pass design, the service life of the mold is very different. Therefore, improving the pass design is an important way to improve the service life of the die. Drawing die passes are generally divided into curve type (R type series) and straight type (cone type series).

It seems that the curve type is better than the straight type from the point of view of the uniform deformation of wire rod in the drawing die. Therefore, in the past, the R-type series used by the Soviet Union in the 1950s was widely used in China to formulate the drawing die manufacturing specifications. This kind of pass was designed under the guidance of the theory of "smooth transition" at that time. Its pass structure can be divided into five parts according to the working properties: "entrance area, lubrication area, working area, sizing area and exit area". The junction of each part requires "chamfering" and smooth transition, grinding the whole pass into a large arc with different curvature. Under the condition of drawing speed at that time, this kind of pass mold can still be applied. From the end of 1970s to the beginning of 1980s, with the increase of wire speed, the service life of wire die has become a prominent problem. In order to meet the requirements of high-speed stay wire, t. maxwall and e. g. kennth of the United States put forward the theory of "linear type". The theory emphasizes the lubrication and wear factors in the drawing process, and points out that the improved pass pattern of the linear drawing die should have the following characteristics:

(1) The longitudinal section line of each part of the pass must be straight. The drawing force of straight working cone is the least.

(2) The joint part of each part of the die must be obvious, so that each part can give full play to its own role, avoiding the reduction of the transition angle to the actual length of the sizing area.

(3) Extend the height of the entrance area and the working area, make the wire into the middle section of the working cone of the die hole, use the wedge area formed by the entrance cone angle and the upper half of the working cone angle, establish the "wedge effect", form a denser and firmer lubricating film on the surface of the wire, reduce the wear, and be suitable for high-speed drawing.

(4) The sizing area must be straight and reasonable in length. If the sizing area is too long, the pulling friction will increase, and the wire will be easily reduced or broken after pulling out the die hole; if the sizing area is too short, it is difficult to obtain the wire with stable shape, accurate size and good surface quality, and the die hole will wear out quickly.

Through practical application, the service life of the drawing die designed by the linear theory is 3-5 times longer than that of the R-type drawing die.

2.3 mold processing and manufacturing level

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

High speed mechanical grinder and diamond plated metal grinding needle are widely used in the grinding process of stay wire mould in foreign countries. The equipment runs stably, the specification and use of grinding needle are standardized, and the product precision is high. The pass size of the mould is detected by the contour recorder and the aperture meter, and the surface finish is checked by the special microscope for checking the stay wire mould.

However, many domestic manufacturers still use backward equipment and use manual operation to grind the pass. Therefore, there are the following problems: the pass parameters fluctuate greatly, and it is difficult to process a straight working cone; the transition angle is easily grinded at the junction of the sizing area and the working area, which makes the wire produce secondary compression in the sizing area, increases the external friction, shortens the length of the sizing area and shortens the die Service life; the repair frequency of worn grinding needle varies from person to person, and the use is not standardized, resulting in poor consistency of pass pattern. The inspection method is also backward. It can only be inspected by visual inspection or simple tools such as magnifying glass and microscope. What's more, it pays attention to the surface finish of the mold. It can't effectively detect the hole size, let alone control it.

The influence of drawing condition on the service life of die

During the drawing process of the wire, the main technological conditions affecting the service life of the die are: the function of the reverse tension p ', the pass compression rate δ, the lubricant and the surface quality of the wire rod.

3.1 reaction force P`

During drawing, the forces acting on the wire rod in the die hole are: the positive pressure n of the die wall, the friction T, the drawing force P and the reaction force P '.

According to the equilibrium condition of pulling force and the yield criterion of metal materials, O. Hoffman and g. Sachs deduced the axial tensile stress σ PX of pulling line by the method of micro element analysis

σPX=σS1+ 1- 2B+σλ 2B (1)


The positive pressure stress σ NX of die wall is

σNX=σS-σPX (2)

Where, σ s is the yield strength; Rx is the radius of die hole at x from the wire inlet; σ λ is the reverse tensile stress at the wire population; RO is the radius of die hole at the wire inlet; RF is the radius of die hole at the wire outlet; μ is the friction coefficient; α is the half angle of working cone.

The compressive stress of the die wall is gradually reduced from the entrance to the exit of the die, and the maximum stress is at the entrance. This is the mechanical reason why the annular wear groove often appears at the entrance of the wire mould.

Due to the effect of the reverse tension, it can obviously reduce the pressure stress at the entrance of the wire mould, and it is beneficial for the lubricant to enter the working area, reduce the friction between the wire rod and the mould wall, slow down the annular wear and mold fracture, and extend the service life of the wire mould. But too large reverse tension will increase the drawing stress when pulling wire, which is easy to cause wire shrinkage or wire breakage.

3.2 pass compression rate

When other tensile conditions are not changed, the greater the compressive stress on the die wall, the greater the friction stress, and the more serious the die wear. It is shown that the compressive stress σ n on the die wall can be expressed as


σ B = (σ B1 ten σ B2) / 2

Q = f / sin α ≈ f / α  very small)



Where, σ B1 and σ B2 are the tensile strength of the wire before and after this pass of drawing; F1 and F2 are the sectional area of the wire before and after this pass of drawing; q is the contact area between the wire and the mold working area; σ B is the average tensile strength of the wire.

Let a = = 1 + - 1 (5)

For the convenience of discussion, σ B = 1, μ = 0.1 for this pass of wire drawing is set. The ratio a of the compressive stress σ n on the die wall and the average tensile strength σ B of the wire rod in this pass, and the relation curve between the parameters δ and α can be drawn by formula (5).

As σ n is proportional to a value, it can be seen from the figure that: (1) when α is fixed, a value decreases with the increase of δ, that is, σ n decreases with the increase of δ; (2) when δ is fixed, a value increases with the increase of α, that is, σ n increases with the increase of α. Therefore, if the drawing conditions allow, increasing the pass compression ratio and reducing the working cone angle properly can reduce the compressive stress on the die wall, which is conducive to improving the service life of the stay die.

3.3 lubricants

In the process of drawing, the quality of lubricant and the sufficiency of lubricant supply affect the service life of drawing die. Therefore, the lubricant is required to have stable oil base, good emulsification, excellent lubricity, cooling and cleaning, easy to filter and precipitate copper powder, and keep the best lubrication state throughout the production process, so as to form a thin film that can withstand high pressure without being damaged, reduce the friction in the working area, and improve the service life of the mold.

The stability of lubricant pH value has a great influence on the lubrication effect. Because when the copper powder in the lubricating emulsion precipitates, it will reduce the fat content in the lubricant, increase the free alkali content, make the lubricating components on the wire surface easy to be cleaned, and strongly reduce the lubrication performance of the emulsion. When the emulsion is unstable and the fat content is too high, the emulsion will be layered, and the fat components containing fine copper powder float on the emulsion so that the copper powder is not easy to precipitate and filter, resulting in the blockage of the mold hole and the deterioration of lubrication.

3.4 surface quality of wire rod

If there are oxide layer, sand or other impurities adhering to the wire surface, it will bring adverse effects on the service life of the stay die. Because when the wire passes through the die hole, the hard and brittle oxide layer will cause the die hole of wire drawing die to wear and scratch the wire surface as abrasive. Therefore, the seriously oxidized wires need to be pickled before drawing. When stacking the billets, pay attention to the cleanness of the stacking site to avoid contact with sand and other impurities.

4 Effect of using method on service life of stay wire die

In the long-term use of the stay wire die, the die wall is subject to the strong friction and erosion of the metal wire rod, which will produce wear phenomenon. The most common is the ring groove (dent) at the wire rod population in the working area. The appearance of the ring groove of the stay wire die aggravates the wear of the die hole. Because the small particles of core material peeled off due to Looseness on the ring groove are brought into the die hole working area and sizing area by the metal wire, which plays the role of abrasive, while the wire entering the die hole intensifies the wear of the die hole like a grinding needle. If it is not replaced in time for repair, the ring groove will continue to accelerate expansion, making repair more difficult, and even there may be cracks in the deeper part of the ring groove, making the mold completely broken and scrapped.

From experience, we know that it is very economical to work out a set of standard, strengthen daily maintenance, and often repair the mold. Once there is any slight wear on the mold, polishing in time will take a short time to restore the mold to the original polishing state, and the hole size of the mold has no obvious change.

In the case of a certain drawing process, the use of drawing equipment also has a greater impact on the life of the mold. The tension axis of the wire rod is not symmetrical with the center line of the die hole, which will produce uneven stress effect on the wire rod and the wire drawing die, and the impact caused by mechanical vibration will also cause high stress peak value on the wire rod and the wire drawing die, both of which will accelerate the wear of the die.

Especially for the manual insert mold, if there is a significant deviation in the mold center line, it will aggravate the expansion of the mold hole, lead to uneven wear, and make the mold hole oval. In addition, if the drawing process is stopped frequently, the wear of the die will also be increased. This is because the friction caused by the tensile stress at the beginning of the drawing is much greater than that caused by the normal drawing.

5 concluding remarks

In order to prolong the service life of the drawing die, in addition to selecting the proper die material, designing the reasonable hole size, improving the manufacturing level of the drawing die, and making the surface finish of the die hole meet the process requirements, it is also necessary to determine the reasonable pass compression ratio, improve the service conditions of the drawing die, pay attention to the daily maintenance of the die in the process of use, wash and change it frequently, and ensure the tools in the process of drawing Good lubrication effect.