The quality of the wire drawing die has an important impact on its service life, which is related to the core material, hole design and processing technology. Improving the core material, designing a reasonable hole structure and improving the processing technology all help to improve the service life of the drawing die and the quality of the wire.
1. At present, the wire drawing dies used in the domestic production of copper wire mainly use cemented carbide, natural diamond and artificial polycrystalline diamond as the core material.
Tungsten carbide is composed of a high-hardness powder sintered body of tungsten carbide and cobalt metal. It has high hardness, good wear resistance and strong impact resistance, low price, is a high-quality drawing die making material, widely used in the drawing of coarse and medium wires. At present, hot isostatic pressing (HIP) treatment and ultra-fine grain process are used abroad to reduce porosity and improve alloy hardness, and rare metals and surface coating processes are used to enhance the surface strength of alloys. Studies have shown that by improving the composition and microstructure of cemented carbide, controlling the fluctuation of carbon content, and refining carbide particles, the material properties can be improved and the service life can be extended.
Natural diamond has high hardness and good wear resistance, and the surface finish of the drawn wire is very high. However, due to the anisotropy in the structure of natural diamond, its hardness is also anisotropic, so that the iw drawing die is worn unevenly and the product is not rounded. In addition, natural diamond is expensive and scarce, and is often used for fine wire drawing dies or finished wire drawing dies that require high surface quality.
Polycrystalline diamond is an adirectional polycrystalline material. It has high hardness, good wear resistance and strong impact resistance. Due to its non-anisotropic hardness, uniform wear and long service life of the drawing die, it is suitable for high-speed drawing. However, the domestic production of polycrystalline drawing die blank has quality problems such as coarse grains and poor polishing performance, and domestic manufacturers mostly use it as a transition drawing die instead of a finished drawing die. However, with the improvement of the intrinsic quality and processing level of polycrystalline diamond wire drawing dies, it is expected to replace expensive natural diamond as the choice of finished drawing dies.
2. The hole design of the drawing die is an important way to improve the service life of the die. Under the same material conditions, the service life of wire drawing dies with different hole designs varies greatly. Generally, the wire drawing die hole type is divided into curved type (R-type series) and straight type (tapered series).From the point of view of the uniform deformation of the wire in the drawing die, the curved type seems to be better than the straight type. Therefore, in the past, China generally used the R-type series used in the 50s of the last century in the former Soviet Union to formulate the production specifications of the drawing die. The hole pattern was designed under the guidance of the "smooth transition" theory at the time. According to the nature of the work, the hole structure can be divided into five parts: "inlet area, lubrication area, working area, sizing area, and outlet area", and the junction of each part requires chamfering to achieve a smooth transition, and grinds the whole hole type into a large arc surface with different curvatures. At the drawing speed at the time, the drawing die with this hole type was still suitable. However, by the late 70s to early 80s of the last century, with the increase in the speed of wire drawing, the service life of wire drawing dies became a prominent issue. In order to meet the needs of high-speed wire drawing, T. Maxwall and E.G. Kennth of the United States put forward the "straight line" theory, which especially considers the lubrication effect and wear factors in the drawing process, and improves the design of the straight wire drawing die hole type.
According to the design theory of linear drawing die hole type, the hole type should have the following characteristics:
(1) The longitudinal cross-section line of each part of the hole type must be straight, because the straight working cone can reduce the pull-out force.
(2) The junction of each part of the drawing die must be obvious, so that each part can give full play to its respective roles, and avoid the reduction of the transition angle to the actual length of the sizing area.
(3) Extend the height of the inlet area and the working area, so that the wire rod enters the middle section of the die hole working cone, and use the wedge-shaped area formed by the inlet cone angle and the upper part of the working cone angle to establish the "wedge effect". This results in a denser and stronger lubricating film on the surface of the wire, which reduces wear and is suitable for high-speed drawing.
(4) The sizing area must be straight and reasonable in length. If the sizing area is too long, it will increase the friction of the wire, and it is easy to cause diameter reduction or wire breakage after pulling out the die hole; if the sizing area is too short, it will be difficult to obtain a wire with stable shape, accurate size and good surface quality, and the die hole will quickly wear out of tolerance.
By adopting the linear theoretical design of the drawing die, in practical application, its service life is more than 3~5 times longer than that of the R-type drawing die.
3. The influence of wire die processing and production level on drawing die quality is mainly reflected in two aspects. On the one hand, it is the hole size of the wire drawing die, and on the other hand, the finish of the inner surface of the die hole. Foreign countries generally use high-speed mechanical grinding machine and metal grinding needle plated with diamond for grinding process, the equipment runs smoothly, the specification and use of grinding needle standardization, so as to achieve high precision of products. They use a profile recorder and a bore size measuring instrument to check the hole size of the iw drawing die, and a special microscope is used to check the finish of the hole surface. However, in China, many manufacturers are still using backward equipment and grinding hole shapes through manual operation. In this way, there are the following problems: the hole type parameter fluctuates greatly, and it is difficult to realize the straightness of the working cone; the transition angle is easy to grind out at the junction of the sizing area and the working area, which causes the wire rod to produce secondary compression in the sizing area, increases the external friction, shortens the length of the sizing area, and then shortens the service life of the die. In addition, because the frequency of repair of worn grinding needles varies from person to person and the use method is not standardized, the consistency of the hole pattern is poor. In terms of detection, China is limited to visual inspection or the use of simple tools such as magnifying glasses and microscopes, focusing on the finish of the inner surface of the drawing die, and there is nothing that can be done for the effective detection and control of the hole size.
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