How Can Stamping Die Produce Qualified Products?
Many people are intimidated by drawing dies. This is because not only do many factors need to be considered during design, but more importantly, they often fail to achieve the desired result during die trials, requiring multiple revisions. Therefore, accumulating experience through practice is highly beneficial for drawing die design.
The following six points are summarized by our engineers:
1. Materials
Good materials are half the battle. When it comes to drawing, the importance of materials cannot be underestimated.
The main cold-rolled thin steel sheets used for drawing include 08Al, 08, 08F, 10, 15, and 20 steels. 08 steel is the most widely used, and is divided into rimmed steel and killed steel. Rimmed steel offers a low price and good surface quality, but it also exhibits severe segregation and a tendency to strain aging, making it unsuitable for parts requiring high stamping performance and strict appearance requirements. Killed steel offers superior performance, with uniform properties, but is more expensive. A representative grade is aluminum-killed steel 08Al. Foreign steels have used SPCC-SD deep-drawing steel, which has better tensile properties than 08Al. 1. If the customer's material requirements are not very stringent and repeated mold trials fail to meet the requirements, try another material.
2. Determining Blank Dimensions
For simple rotating parts, the blank diameter changes during the non-thinning process, but the material thickness remains very close to the original thickness. This can be calculated based on the principle that the blank area is equal to the area of the drawn part (adding a trimming allowance if trimming is required). However, drawn parts often have complex shapes and processes, and sometimes require thinning. While many 3D software programs are available for unfolding calculations, their accuracy cannot always meet the required accuracy.
How to solve the problem of product failure? Solution: Test material.
A. A product undergoes multiple steps, the first of which is generally blanking. First, perform unfolding calculations to gain a rough understanding of the blank's shape and size, which will help determine the overall dimensions of the blanking die. Do not modify the male and female dies of the blanking die after the mold design is complete. First, use wire EDM to machine the blank (larger blanks can be milled and then trimmed with a clamp). After repeated experimentation with the subsequent drawing process, the blank dimensions are finally determined, and then the male and female dies for the blanking process are machined.
B. Inversely arrange the process: test the drawing die first, then machine the blank's blanking edge dimensions, achieving twice the result with half the effort.
3: Stretch Factor(m)
The stretch factor m is one of the key process parameters in drawing process calculations. It is typically used to determine the order and number of stretches. Many factors influence the stretch factor m, including material properties, relative material thickness, stretching method (i.e., the presence or absence of a blank holder), number of stretches, stretching speed, male and female die corner radius, and lubrication.
The calculation and selection principles for the stretch factor m are key topics covered in various stamping manuals, and various methods are available, including deduction, table lookup, and calculation.
A. The relative material thickness, stretching method (i.e., the presence or absence of a blank holder), and number of stretches are difficult to adjust during die repair; proceed with caution!
B. Apply lubricant to the die, or wrap a film bag around the sheet.
C. If tensile cracking occurs, apply lubricant to the die (not the punch). Cover the workpiece with a 0.013-0.018mm thick plastic film on the die side.
4. Workpiece Heat Treatment
During the stretching process, the workpiece undergoes cold plastic deformation, resulting in cold work hardening. This reduces its plasticity, increases its deformation resistance, and increases its hardness. Furthermore, if the mold design is inappropriate, intermediate annealing may be necessary to soften the metal and restore its plasticity.
Note: In general processes, intermediate annealing is not required, as it increases costs. Choose between adding additional steps and annealing. Use with caution!
Annealing is generally performed at a low temperature, also known as recrystallization annealing.
There are two key considerations during annealing: decarburization and oxidation. Here, we will focus on oxidation. Oxidation creates a scale on the workpiece, which has two harmful effects: it reduces the effective thickness of the workpiece and increases mold wear.
When the company's conditions are limited, conventional annealing is generally used. To reduce scale formation, the furnace should be filled as full as possible during annealing. I've also used a few unconventional methods:
A. When there are fewer workpieces, they can be mixed with other workpieces (assuming the annealing process parameters are essentially the same).
B. Place the workpieces in an iron box, seal them with welds, and then load them into the furnace. To remove scale, pickling should be performed after annealing, as appropriate.
When the company's conditions are sufficient, nitrogen furnace annealing, also known as bright annealing, can be used. Without careful observation, the color will be almost the same as before annealing.
5. For metals with strong cold work hardening or when cracking occurs during test molds and no other solution is available, an intermediate annealing step can be added.
6. Finally, a few additional points:
A. Dimensions on product drawings should be marked on one side whenever possible to clearly indicate whether they are referring to external or internal dimensions. Do not mark both internal and external dimensions. If there are any issues with drawings provided by others, communicate with them. If consistency is possible, then it should be achieved. If not, the assembly relationship of the workpiece with other components should be understood.
B. For the final process, if the workpiece dimensions are external, the die is the primary factor, and the clearance is achieved by reducing the punch dimensions. If the workpiece dimensions are internal, the die is the primary factor, and the clearance is achieved by increasing the die dimensions.
C. When designing the corner radii of the punch and die, use the smallest allowable values possible to facilitate subsequent die repairs.
D. When determining the cause of workpiece cracking, consider the following:
Cracks caused by poor material quality are often jagged or irregular in shape, while cracks caused by the process or mold are generally more uniform.
E. "More leads to wrinkles, fewer leads to cracks." According to this principle, material flow conditions can be adjusted by adjusting the pressure of the blank holder, increasing draw ribs, adjusting the corner radii of the punch and die, and cutting process notches on the workpiece.
F. To ensure wear resistance and prevent stretch marks, the punch and die, as well as the blank holder, must be quenched and, if necessary, hard chrome plated.
If you are interested in mold, please feel free to contact us!
Foshan Davinci Technology Co., Ltd
Website: www.davinci-machine.com
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