Tungsten carbide drawing die occupies a very large proportion in the entire stamping die industry. Our common cups, motor housings and almost most products have more or less products that need to be stretched. The design of the drawing die does not mean that it can be calculated according to conventional algorithms. There are too many processes full of variables, especially the stretching of some non-rotating bodies, which is prohibited.
Because there are too many factors to consider when designing the drawing die, such as the drawing coefficient, whether it reaches the material limit, the decision of the spring force, the drawing direction, whether to pull up or down, often it cannot be formed at a time and requires multiple tests. In order to achieve the desired effect, sometimes the mold may be scrapped. Therefore, the continuous accumulation of experience in practice is of great help to the design of the drawing die.
In addition, the size of the cutting material also played an important role in the trial production of the entire tungsten carbide drawing die. Therefore, when designing some irregular deep drawing parts, a blank is often reserved in the mold design stage.
1. Stretching material
When the customer's requirements for the material are not very high, and the repeated mold trials do not meet the requirements, you can change to a material with good tensile properties and try again. Good materials are half the battle. For stretching exercises, this cannot be ignored. Cold-rolled thin carbide sheets for drawing mainly include 08Al, 08, 08F, 10, 15, and 20 carbides. The largest is 08 carbide, which is divided into boiling carbide and killed carbide. The rimmed carbide has low price and good surface quality. The segregation is more serious, and there is a trend of "restraint and aging". Not suitable for parts that require high stamping performance and appearance. Killed carbide is better and has uniform performance, but the price is higher. The representative brand is aluminum-killed carbide 08Al. The foreign carbide adopts Japanese SPCC-SD deep drawing carbide, which has better tensile properties than 08Al.
2. The surface finish of the mold
When deep drawing, the two sides of the die and the edge holder are not sufficiently ground, especially when drawing stainless carbide plates and aluminum plates, scars are more likely to occur, which may lead to tensile fracture in severe cases.
3. Determination of blank size
More wrinkles and less cracks is our principle. The blank positioning design must be correct. The diameter of the drawn part of the rotating body with simple shape is not reduced. Although the thickness of the material has changed, it is basically the same as the original thickness. The degree of closure can be calculated based on the principle that the area of the blank is equal to the area of the stretched part (if trimming must be added to the trimming allowance). However, the shape and process of stretched parts are often complicated, and sometimes thinning and stretching are required. Although there are many three-dimensional software that can calculate the unfolded material, its accuracy cannot reach the 100% requirement.
Solution: sample.
A product has to go through multiple processes, and the first process is generally a punching process. First of all, it is necessary to calculate the unfolded material and have a rough understanding of the shape and size of the blank to determine the overall size of the punching die. The die and blanking process of the mold is carried out after the mold design is completed. First use wire cutting to process the blank (when the blank is large, it can be milled by a milling machine and then clamped). After repeated tests in the subsequent stretching process, the size of the blank is finally determined, and then the punch and the die of the punching die are processed.
In the flip-up process, the drawing die is tested first, and then the blank size of the blanking edge is semi-finished.
4. Stretch coefficient m
Stretch coefficient is one of the main process parameters in the calculation of the stretching process, usually used to determine the sequence and number of stretching.
There are many factors that affect the stretch coefficient m, including material properties, relative thickness of the material, stretching method (referring to whether there is a blank holder), stretching times, stretching speed, punch radius, lubrication, etc.
In the process of repairing the mold, the relative thickness of the material, the stretching method (referring to whether there is a blank holder), and the number of stretching are not easy to adjust. You must be careful. When selecting the stretch coefficient m, it is best to check with a colleague.
5. Selection of processing oil for tungsten carbide deep drawing die
The choice of processing oil is very important. The way to judge whether the lubricant is appropriate is when the product is taken out of the mold. If the product temperature is too high to be touched by hand, you must reconsider the choice of lubricant and the lubrication method, and apply lubricant on the cavity or on the thin plate. Put a film bag on top.
If there are tensile cracks, apply lubricant on the concave mold (the convex mold is not coated with lubricant), and cover the workpiece with a 0.013-0.018mm plastic film on the side of the concave mold.
6. Heat-treated workpiece
Although it is not recommended, it still needs to be explained that during the stretching process, due to cold plastic deformation, the workpiece will undergo cold work hardening, which will reduce its plasticity and increase deformation resistance and hardness. In addition, the mold design is unreasonable. Intermediate annealing softens the metal and restores its plasticity.
Note: The general process does not require intermediate annealing. After all, to increase the cost, you must choose between increasing the process and increasing the annealing, use it 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 discuss oxidation. After the workpiece is oxidized, oxide scale will be produced. This has two disadvantages: the effective thickness of the workpiece becomes thinner and the wear of the mold is increased.
When the company's conditions are not met, ordinary annealing is generally used. In order to reduce the generation of oxide scale, the furnace should be filled as much as possible during the annealing process. I also used the soil method:
1. When there are fewer parts, it can be mixed with other parts (premise: annealing process parameters should be basically the same).
2. Put the workpiece in the iron box and weld it to the furnace. In order to eliminate the oxide scale, pickling treatment should be carried out according to the situation after annealing.
When the company's conditions are available, nitrogen furnace annealing can be used, that is, bright annealing. If you don't observe carefully, the color is almost the same as before annealing.
When there is no other method to deal with the metal with strong cold work hardening or tensile cracks in the test mold, an intermediate annealing process is added.
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