Stretching dies account for a very large proportion of the entire stamping die industry. Our common cups, the casing on the motor, and almost most of the products have more or less products that need to be stretched. For stretching The design of the mold does not mean that it can be calculated according to the conventional algorithm. There are too many processes full of variables, especially the stretching of some non-rotating bodies, which is prohibitive.
Because there are too many factors to consider in the design of the drawing die, such as the drawing coefficient, whether it has reached the limit of the material, the determination of the spring force, the direction of stretching, whether it is stretched upwards or downwards, often cannot One-time molding requires several trials to achieve the desired result, and sometimes the mold may be scrapped. Therefore, accumulating experience in practice is of great help to the design of the drawing die.
In addition, the size of the cutting material also plays a role that cannot be ignored in the production trial of the entire mold. So most of the time, when we design some irregular deep drawing parts, we often reserve a blank step in the mold design stage.
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1. Stretch material
When the customer's requirements for the material are not very strict and repeated mold trials fail to meet the requirements, you can try another material with better tensile properties. A good material is half the battle. Stretching must not be ignored. Cold-rolled thin steel sheets for stretching mainly include No. 08Al, 08, 08F, 10, 15, and 20 steels. Among them, No. 08 steels are used in the largest amount, which are divided into boiling steels and killed steels. Boiling steels are low in price and good in surface quality, but The segregation is serious, and there is a tendency of "strain aging". It is not suitable for parts that require high stamping performance and strict appearance requirements. Killed steel is better, with uniform performance but higher price. The representative grade is aluminum killed steel 08Al. Foreign steel has used Japanese SPCC-SD deep drawing steel, and its tensile properties are better than 08Al.
2. The surface finish of the mold
When deep drawing is performed, the two sides of the die and blank holder are not sufficiently ground, especially when drawing stainless steel and aluminum plates, deep drawing scars are more likely to occur, and in severe cases, tensile fractures may occur.
3. Determination of blank size
More wrinkles, less cracks is our principle. The positioning design of the blank must be correct. The diameter of the blank of the drawing part of the rotating body with a simple shape will not be thinned. Although the thickness of the material changes, it is basically the same as the original thickness. Close, it can be calculated according to the principle that the area of the blank is equal to the area of the stretched part (if there is trimming, the trimming allowance must be added). However, the shape and process of the stretched parts are often more complicated, and sometimes they have to be thinned and stretched. Although there are many 3D software that can calculate the expanded material, their accuracy cannot meet the requirements of 100%.
The solution: sample.
A product has to go through multiple processes, and the first process is generally a blanking process. First of all, it is necessary to carry out the calculation of the expanded material, and have a general understanding of the shape and size of the blank in order to determine the overall size of the blanking die. Do not process the punch and die dimensions of the blanking die after the mold design is completed. The blank is first processed by wire cutting (when the blank 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 blank is finally determined, and then the convex and concave molds of the blanking die are processed.
experience 1
Inverting the process, try the drawing die first, and then process the blanking edge size of the blank, which will get twice the result with half the effort.
4. Stretch coefficient m
Stretching coefficient is one of the main process parameters in stretching process calculation, and it is usually used to determine the sequence and times of stretching.
There are many factors that affect the stretching coefficient m, including material properties, relative thickness of materials, stretching method (with or without blank holder), stretching times, stretching speed, radius of punch and die fillet, lubrication, etc.
The calculation and selection principles of the tensile coefficient m are the key points introduced in various stamping manuals. There are many methods such as calculation, table lookup, and calculation. .
experience 2
The relative thickness of the material, the stretching method (referring to whether there is a blank holder), and the number of stretches are not easy to adjust when repairing the mold, so be careful. It is best to find a colleague to check it when choosing the stretch coefficient m.
Five, the choice of processing oil
The choice of processing oil is very important. The way to distinguish whether the lubricating oil is suitable is that when the product is taken out of the mold, if the temperature of the product is too high to be touched by hands, the choice of lubricating oil and the method of lubrication must be reconsidered, and lubricating oil is applied to the die , or cover the film bag on the sheet.
experience 3
In the event of stretching and cracking, apply lubricating oil on the die (not on the punch), and cover the workpiece with a plastic film of 0.013–0.018mm on the side of the die.
6. Workpiece heat treatment
Although it is not recommended, it is still necessary to say that during the stretching process, the workpiece is hardened due to cold plastic deformation, which reduces its plasticity and increases its deformation resistance and hardness. In addition, the mold design is unreasonable, so it is necessary to carry out Intermediate annealing to soften the metal and restore plasticity.
Note: Intermediate annealing is not necessary in the general process. After all, it will increase the cost. It is necessary to choose between increasing the process and increasing the annealing. Use with caution!
Annealing generally adopts low temperature annealing, that is, recrystallization annealing. There are two things to pay attention to when annealing: decarburization and oxidation. Here we mainly talk about oxidation. After the workpiece is oxidized, there are scales, which have two harmful effects: the effective thickness of the workpiece is thinned, and the wear of the mold is increased.
When the company's conditions are not available, ordinary annealing is generally used. In order to reduce the generation of scale, the furnace should be filled as much as possible during annealing. I also used the earth method:
1. When there are few workpieces, it can be mixed with other workpieces (premise: the annealing process parameters should be basically the same).
2. Put the workpiece in the iron box and weld it before loading it into the furnace. In order to eliminate scale, pickling should be carried out according to the situation after annealing.
When the company's conditions are available, nitrogen furnace annealing, that is, bright annealing, can be used. If you don't look closely, it is almost the same color as before annealing.
experience 4
When dealing with metals that are hardened by cold work or when there is no other way to crack in the test mold, the intermediate annealing process should be added.
Seven, add a few points
1. The size on the product drawing should be marked on one side as much as possible to make it clear whether to ensure the external size or the internal cavity size, and the internal and external dimensions cannot be marked at the same time. If there are such problems in the drawings provided by others, you should communicate with them. If they can be unified, they should be unified. If they cannot be unified, you must know the assembly relationship between the workpiece and other parts.
2. For the last process, the size of the workpiece is outside, mainly the die, and the gap is obtained by reducing the size of the punch; the size of the workpiece is inside, mainly the punch, and the gap is obtained by increasing the size of the die;
3. The fillet radius of the convex and concave dies should be designed with a small allowable value as much as possible to bring convenience to subsequent mold repairs.
4. When judging the reason for the cracking of the workpiece, you can refer to: the cracks caused by poor material quality are mostly jagged or irregular, and the cracks caused by the process and mold are generally relatively neat.
5. "More will wrinkle, less will crack". According to this principle, adjust the flow of the material. The methods include adjusting the pressure of the blank holder, increasing the drawing bead, trimming the radius of the punch and die fillet, and cutting the workpiece on the workpiece.
6. In order to ensure wear resistance and prevent tensile scratches, the punch and die and blank holder must be quenched, hard chrome plating can also be used, and the surface can also be treated with TD. When necessary, tungsten steel can be used as the punch and die.
