Seven points for attention in drawing mould design
source：Hongmaoda Mould Co.,ltd Writer： Stamping mould maker Time：2021-02-25 20:39 Browse：
The proportion of drawing mould in the whole stamping mould industry is very large. Our common cups, the shell on the motor, and almost most of the products have more or less some products that need to be drawn. The design of drawing mould can not be calculated according to the conventional algorithm. There are too many processes full of variables, especially the drawing of some non rotating bodies It's daunting to stretch.
Because there are too many factors to be considered in the design of drawing die, such as the drawing coefficient, whether it reaches the limit of material, the determination of spring force, the direction of drawing, whether it is upward drawing or downward drawing, it often can't be formed at one time, and it has to be tried for many times to achieve the ideal result, and sometimes the mould may be scrapped, so it is accumulated in practice Accumulated experience is very helpful to the design of drawing die.
In addition, the size of the cutting material also plays an important role in the trial production of the whole die. So most of the time, when we design some irregular drawing parts, we often reserve an empty step in the mold design stage.
1、 Stretch material
When the customer's requirements for materials are not very strict and the repeated mould test fails to meet the requirements, you can try again with a material with good tensile properties. Good material is half of the success, and the drawing must not be ignored. Cold rolled sheet steel for drawing mainly includes 08Al, 08, 08F, 10, 15 and 20 steel, among which 08 steel is the most used, which is divided into rimmed steel and killed steel. Rimmed steel has low price, good surface quality, but serious segregation and "strain aging" tendency. It is not suitable for parts with high stamping performance and strict appearance requirements. Killed steel has better performance, uniform performance but high price, which is a substitute The grade of the table is aluminum killed steel 08Al. Japanese spcc-sd deep drawing steel has been used in foreign steel, and its tensile property is better than 08Al.
2、 Finish of mould surface
When deep drawing, the two sides of the mould and blank holder are not fully ground, especially when drawing stainless steel plate and aluminum plate, it is more likely to produce drawing scars, which will lead to tensile fracture in serious cases.
3、 Determination of blank size
It is our principle that more wrinkle and less crack. The blank pos
itioning design should be correct. The blank diameter of the drawing part with simple shape is not thin. Although the material thickness changes, it is basically close to the original thickness. It can be calculated according to the principle that the blank area is equal to the drawing part area (if there is trimming, trimming allowance must be added). However, the shape and process of drawing parts are often complex, and sometimes they need to be thinned and stretched. Although there are many 3D software available for unfolding calculation, its accuracy can not meet the requirements of 100%.
A product has to go through multiple processes, the first process is generally blanking process. First of all, it is necessary to carry out unfolding calculation to have a general understanding of the shape and size of the blank, so as to determine the overall size of the blanking die. Do not process the punch and mould size of blanking mould after mould design. Firstly, the blank is processed by WEDM (when the blank is large, it can be milled by milling machine and then repaired by clamp). After repeated experiments in the subsequent drawing process, the blank size is finally determined, and then the punch and mould of blanking mould are processed.
In the reverse process, the drawing mould is tested first, and then the blanking edge size of the blank is processed.
4、 Tensile coefficient m
Drawing coefficient is one of the main parameters in drawing process calculation, which is usually used to determine the drawing order and times.
There are many factors affecting the drawing coefficient m, including material properties, relative thickness of material, drawing mode (with or without blank holder), drawing times, drawing speed, punch and mould fillet radius, lubrication, etc.
The calculation and selection principle of tensile coefficient m are the key points introduced in various stamping manuals. There are many methods, such as calculation, table checking, calculation, etc., which are auspicious and auspicious. I also choose according to the book. There is nothing new. Please read the book.
The relative thickness of the material, the drawing mode (with or without blank holder) and the drawing times are not easy to adjust during mould repair, so we must be careful. It is better to check with colleagues when selecting the tensile coefficient M.
5、 Selection of processing oil
The choice of processing oil is very important. When the product is taken out of the mold, if the temperature of the product is too high to be touched by hand, the selection of lubricating oil and the method of lubrication must be reconsidered. Apply lubricating oil on the mould or put a film bag on the thin plate.
In case of tensile crack, apply lubricating oil on the mould (not on the punch), and cover the workpiece with 0.013 – 0.018mm plastic film on the mould side.
6、 Heat treatment of workpiece
Although it is not advocated, it is still necessary to say that in the drawing process, due to the cold plastic deformation of the workpiece, cold work hardening occurs, which reduces the plasticity, increases the deformation resistance and hardness, and the mold design is unreasonable, so intermediate annealing is needed to soften the metal and restore the plasticity.
Note: in the general process, intermediate annealing is not necessary, after all, to increase the cost, to increase the process and increase the annealing in the choice, careful use!
Low temperature annealing, namely recrystallization annealing, is generally used for annealing. Two points should be paid attention to during annealing: decarbonization and oxidation. Here we mainly talk about oxidation. After oxidation, the workpiece has oxide skin, which has two disadvantages: thinning the effective thickness of the workpiece and increasing the wear of the die.
When the company's conditions are not available, ordinary annealing is generally used. In order to reduce the oxide scale, the furnace should be filled as much as possible during annealing. I have also used the method of soil:
1. When there are few workpieces, they can be mixed with other workpieces (premise: annealing process parameters should be basically the same).
2. Put the workpiece in the iron box, weld and seal it, and then put it into the furnace. In order to eliminate oxide scale, pickling treatment should be carried out according to the situation after annealing.
When the company has the conditions, it can use nitrogen furnace annealing, namely bright annealing. If you don't look closely, it's almost the same color as before annealing.
When there is no other way to deal with the metal with strong cold work hardening or tensile crack in the trial die, the intermediate annealing process should be added.
7、 A few additional points
1. The dimensions on the product drawing should be marked on one side as far as possible, so that people can make it clear whether the external dimension or the internal dimension is guaranteed, and the internal and external dimensions cannot be marked at the same time. If the drawings provided by others have such problems, they should communicate with them. If they can be unified, they should be unified. If they cannot be unified, they should know the assembly relationship between the workpiece and other parts.
2. For the last process, the size of the work piece is outside, and the mould is the main part, and the gap is obtained by reducing the size of the punch; the size of the work piece is inside, and the punch is the main part, and the gap is obtained by increasing the size of the punch;
3. The fillet radius of punch and mould should be as small as possible in the design, which brings convenience for subsequent mould repair.
4. When judging the cause of the workpiece crack, we can refer to: the cracks caused by poor material quality are mostly serrated or irregular shape, and the cracks caused by process and mold are generally neat.
5. According to this principle, the flow condition of materials can be adjusted by adjusting the pressure of blank holder, increasing the drawbead, trimming the radius of convex mould and concave mould fillet, and cutting the process opening on the workpiece.
6. In order to ensure wear resistance and prevent drawing scratch, punch and blank holder must be quenched, hard chromium plating or TD surface treatment can be used. When necessary, tungsten steel can be used as punch and blank holder.