Measures to improve the service life of stamping dies

       For deep drawing molds, adhesive wear is an important cause of mold failure. Generally, adhesion is prone to occur between materials with similar properties, so the corresponding mold material should be selected according to the different materials being drawn. If the drawn material is non-ferrous metal, the mold material can be cast iron, steel, and hard alloy; If the drawn material is black metal, then the mold material should be selected from non-ferrous metals, hard alloys, and steel materials with low affinity for them. For cold extrusion molds, if the unit extrusion force borne by the mold is large, high hardenability materials such as base steel and high-speed steel should be used, otherwise the un hardened material core will cause plastic deformation of the mold. If the convex mold is subjected to a large eccentric force, materials with high strength and toughness should be selected. The shape of the extruded workpiece is complex, the production batch is large, or the strength of the extruded billet is high. Choosing hard alloy or steel bonded hard alloy can improve the service life.
When selecting materials for cold heading molds, 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 small amounts of shrinkage cavities. Cold heading dies working under high load conditions require steel with high purity and strict control of sulfur and phosphorus content. Generally, steel with a carbon content of 0.8-0.9% has good toughness, with a carbon content of 0.95-1.05% indicating hardness and toughness, and a carbon content of 1.05-1.15% indicating hardness. The carbon content for large molds is set at the lower limit, while for small molds it is set at the upper limit.
(1) Reasonably design molds
On the premise of ensuring the quality of the punched workpiece, the punching die should choose a larger punching gap as much as possible to reduce the punching force and minimize the wear of the die. In order to improve the stiffness of the punch and enhance its ability to resist eccentric loads, in order to prevent the punch from bending, deforming or breaking during operation. Generally, the cross-sectional areas of the head and tail handle of the convex mold are approximately twice and four times the area of the working end face, respectively. If necessary, guide and protect the convex mold. Elastic unloading plates can be used to apply a certain amount of edge pressure to the sheet metal, in order to reduce the force on the convex mold caused by sheet metal slip or warping. To ensure uniform punching clearance during the stamping process and avoid uneven wear of the cutting edge, precise mold guide devices can be selected, such as using ball guide column guide sleeves.
The design of the clearance between the convex and concave dies of the deep drawing die should be reasonable: if the clearance is too small, the increase in frictional resistance will exacerbate the wear of the die; If the gap is too large, it will cause wrinkling of the parts and increase the wear of the mold; Uneven gaps can generate uneven internal stresses during mold operation, leading to a decrease in the lifespan of the mold. The working surface hardness of the mold should be high to reduce wear and tear. The surface roughness value of the mold should be low, and the surface roughness value of the drawn sheet metal should also be low to reduce the frictional resistance during deep drawing, which is beneficial for the plastic forming of the drawn parts and improves the life of the mold.
The structure of cold extrusion molds must have sufficient strength, stiffness, reliability, and good directionality. Adopting the optimal convex mold shape and, if conditions permit, using a process axis to transform single pure extrusion or reverse extrusion into composite extrusion, in order to reduce the unit extrusion force. The extrusion punch is not easily too long to prevent longitudinal bending. The transition part of the mold working component should be designed with a sufficiently large fillet radius to avoid stress concentration caused by sharp corner transitions. The parts of the concave mold that are prone to transverse cracking should adopt a segmented structure to eliminate stress concentration. Adopting a prestressed composite concave mold structure to prevent longitudinal cracking of the inner concave mold. The use of a stepped combination die has a greater ability to withstand radial internal pressure than a flat combination die of the same size.
At the entrance of the cold heading die's concave die, try to set a sufficiently large gradient fillet to avoid stress concentration, and make a draft angle in the direction of the die exit. To facilitate the flow of the billet in the mold cavity and reduce the load on the mold. Hard alloy or steel bonded hard alloy cold heading molds have high hardness, good wear resistance, and produce products with high precision. A combination structure of hard alloy or steel bonded hard alloy inserts can be used, and pre stress can be applied by adding sleeves to reduce or offset the cold heading force on the mold, in order to improve the service life of the mold. However, hard alloys have high brittleness, and when the mold shape is complex and works under high impact loads, hard alloys should not be used.

We should pay attention to the forging process of mold steel billets, eliminate the distribution of banded and networked carbides, and make the streamline and impact force direction perpendicular. In order to fully break down the carbides in the billet and make them evenly distributed in a dispersed state during forging, the method of high forging ratio and directional upsetting should be adopted.
In the manufacturing process, it is necessary to strictly ensure the dimensional and shape accuracy of the mold to avoid leaving machining tool marks; The transition part should be smooth and free of minor defects to prevent stress concentration and cracking during use. After electrical machining and grinding, tempering should be carried out to eliminate machining stress.
The final polishing process of the deep drawing mold should be carried out in the same direction as the flow of the blank metal, and the concave mold cavity should be polished longitudinally instead of in a circular motion. Attention should be paid to cooling during polishing to prevent overheating and a decrease in mold hardness.
The shape of the cold extrusion punch after processing should be symmetrical, and the working part must be coaxial, otherwise the punch is prone to breakage under unilateral force. The lower the surface roughness value of the positive or negative extrusion concave die, the better. The method of grinding and then polishing can be used to reduce wear and improve the life of the mold.
The quenching hardness and hardening layer depth should be appropriately selected according to the working conditions and material properties of the cold heading die to prevent early failure. During heat treatment, attention should be paid to sufficient tempering. If the tempering time is insufficient and the stress is not completely eliminated, even if the hardness meets the requirements, the phenomenon of chipping will still occur. The tempering time is generally more than 1.5 hours.
(2) Correct selection of mold materials
When the production batch of punching dies is large, high-performance die materials with high strength, good toughness, and good wear resistance should be selected. Due to the poorer working conditions of convex molds compared to concave molds, the wear resistance of convex mold materials can be selected to be higher than that of concave mold materials.
(3) Adopting mold toughening treatment and surface strengthening treatment
The use of toughening and surface strengthening techniques to achieve excellent overall toughness, surface hardness, wear resistance, and adhesion resistance of molds is an effective way to improve the service life of various molds.
(4) Reasonable use and maintenance of molds
When operating the punching die, the depth of the convex die entering the concave die should be strictly controlled to avoid increased wear. After using the die for a period of time, the convex and concave die edges will inevitably show wear and wear grooves. At this time, repairing the mold in advance can reduce friction, prevent cracks caused by wear grooves, avoid additional bending moments caused by uneven clearance between convex and concave molds after wear, and improve the life of the mold. After the convex and concave molds are ground again, the cutting edge should be carefully ground and polished with a fine oilstone to remove grinding burrs and make the surface roughness Ra ≤ 0.10 μ m, eliminating potential damage hazards. When storing molds, a certain gap should be maintained between the upper and lower molds to protect the cutting edge.
Suitable lubricant must be applied between the deep drawing die and the drawn sheet metal to prevent direct contact between the die and the sheet metal and eliminate the conditions of adhesion and biting. During deep drawing, the relative motion between the mold and the sheet metal contact surface becomes the relative motion between lubricant molecules, which can greatly reduce friction and frictional heat, effectively reducing or preventing wear. The thickness, hardness, and microstructure of the drawn sheet metal must be uniform and consistent; The surface should be kept smooth and free of impurities, oxide scale, and rust to avoid premature wear of the mold due to uneven stress. If the surface roughness of the mold deteriorates after use, it should be promptly ground and polished.
A drawing machine with lower drawing speed should be selected to facilitate the flow of metal materials and reduce friction on the mold surface. The double action press has a slower deep drawing speed and a smoother and more uniform force than ordinary punching machines, which is beneficial for extending the life of the deep drawing die.
Cold extrusion molds should also be lubricated reasonably during operation, and phosphating treatment and lubrication should be used when extruding black metals. During the cold extrusion process, the mold temperature rises rapidly and should be cooled regularly. For heavy-duty molds, stress relief tempering treatment (160 ℃~180 ℃ insulation for two hours) should be carried out after thousands of squeezes, which can effectively improve the life of the mold. For the outer or middle prestressed rings that are repeatedly used, after multiple presses, they need to be insulated at 180 ℃ for two hours to relieve stress and undergo tempering treatment to prevent the outer ring from cracking. It is best to preheat the mold before use during low temperatures in winter to prevent the convex mold from breaking due to cold brittleness. It is necessary to establish a complete maintenance system and designate a dedicated person to adjust and repair the press and mold in a timely manner. During the storage and transportation of molds, rust prevention measures should be taken, and limited block protection should be provided between the upper and lower mold seats.
In order to reduce the friction coefficient during operation and prevent mold adhesion and biting, cold heading blanks should be treated with phosphating or copper plating. In most cases, the billet needs to be preheated before cold heading. Preheating can improve the processing performance of materials, reduce the possibility of cracking, and also increase the lifespan of molds. Lubrication should also be applied during cold heading, as good lubrication can reduce the surface roughness of the product and improve the lifespan of the mold. Lubrication is particularly important for cold heading of complex shaped workpieces.