The proportion of metal stretching molds in the entire stamping mold industry is large, and there are too many factors to consider in the design of stretching molds, such as the drawing coefficient, whether the material has reached its limit, the determination of spring force, and the stretching direction. Whether it is upward or downward, it cannot be formed in one go and requires multiple tests to achieve the expected effect. When designing irregular deep drawn parts, we often leave a blank step in the mold design stage.

1、 Stretch material

When repeated mold testing fails to meet the requirements, materials with good tensile properties can be used to try again. Good materials are half the battle. The most commonly consumed type is 08 steel, which is divided into boiling steel and killed steel. It has a low price, good surface quality, but severe segregation and a tendency towards "strain aging". It is not suitable for parts with strict requirements for stamping performance and appearance. Sedative steel is better, with uniform performance, but at a higher price.

2、 Surface finish of mould

When stretching, insufficient polishing on both sides of the mold and edge pressing ring, especially when deep drawing stainless steel and aluminum plates, is more likely to cause deep drawing scars, which may lead to tensile fracture in severe cases.

3、 Determination of raw material size

Our principle is to wrinkle more and crack less. The positioning design of the blank must be correct, which can be calculated based on the blank area and the area of the stretched part (if there is trimming, trimming allowance must be added). But the shape and process of stretching parts are often complex, sometimes requiring thinning and stretching. Although there are many 3D software that can perform unfolding calculations, their accuracy cannot meet the requirement of 100%.

Solution: Sample.

In the process of reverse arrangement, first test the stretching die, and then process the size of the blanking edge of the blank, which is twice the effort and half the result. A product needs to go through multiple processes, the first of which is usually the cutting process. After the mold design is completed, do not process the dimensions of the convex and concave molds of the drop mold. Firstly, the billet is cut by wire cutting (when the billet is large, it can be milled by a milling machine and then clamped). After repeated experiments in the subsequent stretching process, the size of the billet is finally determined, and then the convex and concave dies of the feeding die are processed.

4、 Tensile coefficient m

The hardware stretching coefficient is one of the main process parameters in the stretching process calculation, usually used to determine the stretching sequence and number of times. The relative thickness of the material, the stretching method (with or without edge rings), and the number of stretching times during mold repair are not easy to adjust, so be careful. It is best to check with colleagues when choosing the stretching coefficient m. There are many factors that affect the stretching coefficient m, including material properties, relative thickness of the material, stretching method (referring to whether there is edge pressure), stretching times, stretching speed, convex concave die fillet radius, lubrication, etc.

5、 Workpiece heat treatment

During the stretching process of hardware, due to cold plastic deformation of the workpiece, cold work hardening occurs, resulting in a decrease in its plasticity, an increase in deformation resistance and hardness, and an unreasonable mold design requiring intermediate annealing to soften the metal and restore its plasticity. Annealing generally adopts low-temperature annealing, namely recrystallization annealing. Two points should be noted during annealing: decarburization and oxidation. Here we mainly talk about oxidation. After oxidation, the workpiece will produce oxide skin, which has two hazards: thinning the effective thickness of the workpiece and increasing mold wear. In general, there is no need for intermediate annealing in the process, as it increases costs. Therefore, a choice must be made between increasing the process and increasing annealing. Use with caution!