For the deep drawing die, the adhesive wear is an important reason for the failure of the drawing die. Generally, the adhesion tends to occur between materials with similar properties, so the corresponding mold material should be selected according to the material to be drawn. If the material to be drawn is non-ferrous metal, the mold material may be cast iron, steel and hard alloy; if the material to be drawn is ferrous metal, the mold material is selected from non-ferrous metals, hard alloys and steel materials with low affinity. For cold extrusion die, if the unit is subjected to a large unit pressing force, a material with high hardenability such as base steel or high speed steel should be used, otherwise the core of the unhardened material will cause plastic deformation of the mold. If the punch is subjected to a large eccentric force, a material with high strength and toughness should be used. The shape of the extruded workpiece is complicated, the production batch is large, or the strength of the extruded billet is high. The choice of cemented carbide or steel-bonded cemented carbide can improve the life.

In the selection of cold dies, attention should be paid to the original structure and chemical composition of the steel. The steel should not have original structural defects such as segregation, inclusions and a small amount of shrinkage. For cold heading die working under high load conditions, the steel used for the mold should have higher purity, and the sulfur and phosphorus content should be strictly controlled. Generally, the carbon content of the steel is 0.8-0.9%, the toughness is good, the carbon content is 0.95-1.05%, the carbon content is 1.05-1.15%, the carbon content of the large mold is the lower limit, and the small mold takes the upper limit.

(1) Reasonable design of the mold Under the premise of ensuring the quality of the blanked part, the punching mold should use a larger punching gap as much as possible to reduce the punching force and reduce the wear of the mold. In order to improve the rigidity of the punch, the anti-offset capability is enhanced to prevent the punch from being bent or broken during operation. Generally, the cross-sectional area of ​​the head of the punch and the cross-sectional area of ​​the tail shank are respectively taken as 2 times and 4 times of the area of ​​the working end face, and the punch is guided and protected if necessary. An elastic discharge plate can be used to apply a certain blanking force to the sheet to reduce the force on the punch due to slippage or warpage of the sheet. In order to ensure uniform punching clearance during stamping and to avoid uneven wear of the cutting edge and cutting edge, precise mold guiding devices can be used, for example, using ball guide bushings.
The design of the convex and concave die gap of the drawing die should be reasonable: the gap is too small, the frictional resistance will increase the wear of the die; if the gap is too large, the workpiece will wrinkle and increase the wear of the die; the gap is uneven, and the die works. Uneven internal stress is generated, which reduces the service life of the mold. The working surface of the mold is tough to reduce wear. The surface roughness of the mold is low, and the surface roughness value of the drawn sheet is also lower to reduce the frictional resistance during drawing, which is beneficial to the plastic forming of the deep drawing and the life of the mold.
The structure of the cold extrusion die must have sufficient strength, stiffness, reliability and good guiding properties. The best punch shape is adopted, and the process shaft is used under the condition that the condition is permitted, and the simple positive or reverse extrusion is used as the composite extrusion to reduce the unit pressing force. The extrusion punch is not too long to prevent longitudinal bending. The transition part of the working part of the mold should be designed with a sufficiently large fillet radius to avoid stress concentration caused by the sharp corner transition. The concave mold is easy to be laterally cracked, and a split structure should be adopted to eliminate stress concentration. A pre-stressed combined die structure is employed to prevent longitudinal cracking of the inner die. The use of a stepped combination die has a greater ability to withstand radial internal pressure than a flat die die of the same size.
At the entrance of the die of the cold die, try to set a sufficiently large gradient fillet to avoid stress concentration and draft draft in the direction of the die. In order to facilitate the flow of the blank in the cavity and reduce the load on the mold. The hard alloy or steel-bonded hard alloy cold heading die has high hardness and good wear resistance, and the produced product has high precision. The combined structure of cemented carbide or steel-bonded carbide inserts can be used to apply pre-stressing by means of a sleeve to reduce or offset the cold heading force of the mold to improve the service life of the mold. However, cemented carbides are very brittle, and hard alloys should not be used when the mold is complex in shape and operates under high impact loads.

To improve the quality of mold manufacturing and processing, it is necessary to pay attention to the forging process of the mold billet, eliminating the distribution of the strip and the net carbide, so that the flow line and the impact direction are perpendicular. In order to fully break up the carbides in the billet during forging, it is evenly distributed in a diffuse form, and a high forging ratio can be used for the direction of the drawing.
In the manufacturing process, the dimensional and shape accuracy of the mold must be strictly ensured to avoid leaving machining marks; the transition part should be smooth, and there should be no small defects to prevent stress concentration cracks during use. After electrical machining and grinding, tempering should be performed to eliminate machining stress.
The final polishing process of the drawing die should be in the same direction as the flow of the blank metal, and the concave cavity should be longitudinally reciprocated rather than circularly polished. Care should be taken during cooling to prevent overheating and to reduce the hardness of the mold.
After the cold extrusion punch is processed, the shape should be symmetrical, and the working part must be concentric, otherwise the punch can be easily broken by one side. The lower the surface roughness value of the positive extrusion or back extrusion die, the better, the grinding and polishing method can be used to reduce wear and improve the life of the mold.
The quenching hardness and the depth of the hardened layer should be appropriately selected according to the working conditions and material properties of the cold boring die to prevent early failure. In the heat treatment, attention should be paid to full tempering, the tempering time is insufficient, and the stress is not completely eliminated. Even if the hardness meets the requirements, the blockage phenomenon will still occur, and the tempering time is generally more than 1.5 hours.

(2) Correct selection of mold materials When the production batch size of the blanking die is large, high-performance mold materials with high strength, good toughness and good wear resistance should be selected. Since the working condition of the punch is worse than that of the die, the wear resistance of the punch material can be selected to be higher than that of the die material.

(3) Using the strengthening and toughening treatment of the mold and the surface strengthening treatment, the toughening treatment and surface strengthening treatment technology are used to obtain excellent overall toughness and excellent surface hardness, wear resistance and anti-adhesion performance. An effective way to extend the life of various types of molds.

(4) Proper use and maintenance of the die blanking die operation should strictly control the depth of the punch into the die to avoid wear and tear. After the die has been used for a period of time, the convex and concave die edges will inevitably wear and wear grooves. At this time, the mold is repaired in advance, which can reduce the friction and prevent the crack caused by the wear groove, and avoid the additional bending moment caused by the unevenness of the convex and concave gaps after the wear, thereby improving the life of the mold. After the convex and concave molds are ground again, the fine oil stone is used to carefully grind and polish the cutting edge to remove the grinding burrs, so that the surface roughness Ra≤0.10μm, eliminating the hidden danger of damage. When the mold is stored, the upper and lower molds should maintain a certain gap to protect the cutting edge.
A suitable lubricant must be applied between the drawing die and the drawn sheet so that the mold and the sheet are not in direct contact, and the conditions of adhesion and seizure are eliminated. The relative movement of the mold-sheet contact surface during drawing becomes a relative movement between the lubricant molecules, which greatly reduces friction and frictional heat, effectively reducing or preventing wear. The thickness, hardness and microstructure of the drawn sheet are uniform and uniform; the surface is smooth and free of impurities, oxide scale and rust, so as to avoid premature wear of the mold. If the surface roughness becomes worse after the mold is used, it should be polished and polished in time.
A drawing machine with a lower drawing speed should be used, which is easy to draw the metal material and reduce the friction on the mold surface. The double-action press has a slower drawing speed, and the force is smoother and more uniform than that of the ordinary punching machine, which is beneficial to prolonging the life of the drawing die.
Cold extrusion die should also be properly lubricated when working. Phosphating and lubrication should be used when squeezing black metal. During the cold extrusion process, the mold temperature rises quickly and should be cooled regularly. For heavy-duty molds, it should be subjected to stress-relieving and tempering after two thousand extrusions (160 ° C ~ 180 ° C for two hours), which can effectively improve the life of the mold. For repeated use of the outer or middle layer prestressing ring, after multiple extrusions, it is necessary to be tempered and tempered at 180 ° C for two hours to prevent the outer ring from cracking. When the temperature is low in winter, it is best to preheat the mold before use to prevent the punch from breaking. A complete maintenance system must be established, and designated personnel should adjust and repair the presses and molds in a timely manner. During the storage and transportation of the mold, anti-rust measures shall be taken, and the upper and lower mold bases shall be protected by a limited block.
In order to reduce the friction coefficient during work and prevent the mold from sticking to the joint, the cold heading material should be phosphatized or copper plated. In most cases, the billet is preheated before cold rolling. Preheating improves the processability of the material, reduces the likelihood of cracking, and increases mold life. Lubrication should also be carried out during cold heading. Good lubrication can reduce the surface roughness of the product and increase the life of the mold. Especially for cold-formed workpieces with complex shapes, lubrication is more important.

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