Because there are too many factors to consider in the design of a stretching die, such as the drawing coefficient, whether the material limit is reached, the determination of spring force, the direction of stretching, whether it is upward or downward, often cannot be formed in one go. It requires multiple trials to achieve the desired results, and sometimes there is a possibility of the die being scrapped. Therefore, in practice, we constantly accumulate experience, It is very helpful for the design of the drawing die.
In addition, the size of the cutting size also plays a crucial role in the production trial of the entire mold. So most of the time, when we design some irregular deep drawn parts, we often leave a blank step in the mold design stage.
1、 Stretch material
When the customer's requirements for materials are not very strict and repeated mold trials cannot meet the requirements, they can try again with a material with good tensile performance. A good material is half the success, and for stretching, it cannot be ignored. The cold-rolled thin steel plates used for stretching mainly include 08Al, 08, 08F, 10, 15, and 20 steel, with the largest amount being 08 steel, which is divided into boiling steel and killed steel. Boiling steel has low prices, good surface quality, but severe segregation, and a tendency towards "strain aging". It is not suitable for parts with high requirements for stamping performance and appearance. Killed steel has good performance, uniform performance, but high price, and is represented by aluminum killed steel 08Al. Foreign steel has used Japanese SPCC-SD deep stamping steel, and its tensile properties are better than 08Al.
2、 Mold surface finish
When conducting deep drawing, the grinding of both sides of the concave mold and the edge holder is insufficient, especially when drawing stainless steel and aluminum plates, which is more prone to deep drawing scars and can lead to tensile fracture in severe cases.
3、 Determination of blank size
More wrinkles and less cracks are our principles. The positioning design of the blank should be correct. The diameter of the blank of a simple shaped rotating body stretching part is constant in thin stretching. Although the material thickness varies, it is basically close to the original thickness. It can be calculated based on the principle that the blank area is equal to the area of the stretching part (if there is trimming, the trimming allowance must be added). However, the shape and process of stretching parts are often complex, and sometimes they need to be thinned for stretching. Although there are many 3D software available for unfolding material calculation, its accuracy cannot meet the requirements 100%.
Solution: Test portion.
A product needs to go through multiple processes, and the first process is usually the material cutting process. Firstly, it is necessary to carry out material unfolding calculation to have a rough understanding of the shape and size of the blank, in order to determine the overall size of the blanking die. After the mold design is completed, do not process the dimensions of the convex and concave molds of the blanking die. 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 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
The reverse arrangement process involves testing the drawing die first, and then processing the cutting edge size of the blank, achieving twice the result with half the effort.
4、 Tensile coefficient m
The stretching coefficient is one of the main process parameters in the calculation of stretching process, which is usually used to determine the order and frequency of stretching.
There are many factors that affect the stretching coefficient m, including material properties, relative thickness of the material, stretching method (referring to the presence or absence of edge holders), stretching times, stretching speed, radius of convex and concave die fillets, 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 deduction, table lookup, and calculation, which are both auspicious and auspicious. I also chose according to the book, and there is no fresh information. Please read the book.
Experience 2
The relative thickness of the material, stretching method (referring to the presence or absence of edge pressing rings), and stretching frequency are difficult to adjust during mold repair, and caution must be exercised. It is best to check with colleagues when selecting the stretching coefficient m.
5、 Selection of processing oil
The selection of processing oil is very important. The method of distinguishing whether lubricating oil is suitable is to reconsider the selection and lubrication method of lubricating oil when taking the product out of the mold. If the temperature of the product is too high to be touched by hand, it is necessary to apply lubricating oil on the concave mold or cover the thin film bag on the sheet.
Experience 3
When encountering tensile cracks, apply lubricating oil on the concave mold (not on the convex mold), and cover the surface of the workpiece against the concave mold with a plastic film of 0.013-0.018mm.
6、 Workpiece heat treatment
Although it is not advocated, it should still be noted that during the stretching process, the workpiece undergoes cold work hardening due to cold plastic deformation, resulting in a decrease in plasticity, an increase in deformation resistance and hardness. In addition, due to unreasonable mold design, intermediate annealing is required to soften the metal and restore plasticity.
Attention: Intermediate annealing is not necessary in general processes, as it will increase costs. It is necessary to choose between adding processes and adding annealing, and use it with caution!
Annealing generally adopts low-temperature annealing, that is, recrystallization annealing. There are two things to pay attention to during annealing: decarbonization and oxidation. Here we mainly talk about oxidation. After oxidation of the workpiece, there is oxide skin, which has two harmful effects: thinning the effective thickness of the workpiece and increasing mold wear.
When the company's conditions are not met, ordinary annealing is generally used. To reduce the generation of oxide skin, the furnace should be filled as much as possible during annealing. I have also used the soil method:
1. When there are few workpieces, they can be mixed with other workpieces (premise: annealing process parameters should be basically consistent).
2. Place the workpiece in an iron box and weld it before loading it into the furnace. To eliminate the oxide scale, acid pickling treatment should be carried out according to the situation after annealing.
When the company has the necessary conditions, nitrogen furnace annealing, i.e. bright annealing, can be used. Without careful inspection, the color is almost the same as before annealing.
Experience 4
When dealing with metals with strong cold work hardening or when there is no other way to crack during mold testing, an 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 much as possible to clarify whether to ensure external or internal dimensions, and not both internal and external dimensions. If there are such issues with the drawings provided by others, they should be communicated 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 final process, the workpiece size is outside, mainly with a concave mold, and the gap is obtained by reducing the size of the convex mold; Within the workpiece size, the punch is the main component, and the gap is obtained by increasing the size of the concave mold;
3. When designing the fillet radius of the convex and concave molds, the allowable value should be as small as possible to facilitate subsequent mold repairs.
4. When determining the cause of workpiece tearing, reference can be made: the cracks caused by poor material quality are mostly serrated or irregularly shaped, and the cracks caused by process and mold are generally relatively neat.
5. More wrinkles, less cracks, "according to this principle, adjust the flow status of the material by adjusting the pressure of the blank holder, increasing the drawing rib, trimming the radius of the convex and concave die corners, and cutting the process port of the workpiece.
6. To ensure wear resistance and prevent stretching scratches, the convex and concave molds and edge holders must be quenched, hard chromium plated, or surface TD treated. If necessary, tungsten steel can be used as the convex and concave molds.
Design of the mold for the rear suspension spring support plate of automobiles. Quick inspection and mastery of the latest mold knowledge
1. Stamping process analysis
Nowadays, there are more and more friends studying mold design. Many people ask me if I have any information and what is better to read in the first book. Based on your needs, I have classified and managed some mold design materials. I hope you can have a promising future in the mold industry.
Figure 1 shows the rear suspension spring support reinforcement plate for automobiles, produced in bulk. The material is SAPH440, with a thickness of t=2mm. SAPH440 is an automotive structural steel with a carbon content of about 0.20%. The material has a yield strength of 305~395MPa, a tensile strength of 390~470MPa, and an elongation of ≥ 30%. It has good formability and is mainly used for structural components such as automotive frames and wheels that require high strength. The external dimensions of the parts shown in Figure 1 are 107.5mm × 149mm × 33mm, with high requirements for surface quality and accuracy, and complex shapes, ϕ The 7+0.1mm hole has assembly requirements and an accuracy level of IT9.
According to the external characteristics of the parts, the mold is designed as a structure of one mold and two parts (see Figure 2). The forming of the rear suspension spring support requires processes such as blanking, punching, forming, and separation. Due to the 2mm material thickness of the parts and the complex curved surface shape, it is advisable to use a combination of single process mold and composite mold for production.
2. Calculation of blank size
The commonly used blank calculation methods for sheet metal parts include experience method, neutral layer method, and assembly method. These methods are mostly applicable to specific shaped parts, but the surface shape of the rear suspension spring support part is complex, which is neither a standard bent part nor a standard drawn part. The deformation part includes bending, drawing, etc. It is difficult to calculate accurate blank dimensions using traditional blank calculation methods, The blank size was calculated using the finite element analysis method in UG software, as shown in Figure 3.
3. Mold structure design
1. Storage block 2. Limit column 3. Upper mold seat 4. Upper mold base plate 5. Limit screw 6. Mold handle 7. Ejector (upper mold press plate) 8. Discharging spring 9. Female mold insert 10. Guide sleeve 11. Guide column 12. Lower mold seat 13. Discharging plate 14. Lower mold base plate 15. Male mold insert 16. Limit screw 17. Discharging spring 18. Hanging rod
The blank blanking die adopts a sliding guide column inverted punch die, a rigid discharge plate, and a cast steel guide column mold frame, and its structure is shown in Figure 4. Mold working process: The blank is roughly positioned by the retaining pin and accurately positioned by the positioning pin to ensure feeding accuracy. A workpiece ejector is designed inside the concave mold. After the upper and lower molds are closed and punched, the parts are ejected by the ejector, and the cutting waste is ejected from the lower mold by the discharge plate.
1. Storage block 2. Limit column 3. Guide column 4. Upper mold seat 5. Female mold insert 6. Mold handle 7. Positioning pin 8. Guide sleeve 9. Lower mold seat 10. Hanging rod 11. Rectangular spring 12. Discharging plate 13. Lower mold base plate 14. Male mold insert 15. Positioning pin 16. Limit screw 17. Positioning pin
Figure 5 shows the structure of the support forming die. Based on the forming characteristics of the parts, in order to reduce manufacturing costs, the convex and concave molds adopt an insert structure. The concave mold is on the upper mold, and the convex mold and ejector are on the lower mold. Before forming, the sheet metal is placed on the ejector. During operation, the concave mold descends and the ejector presses the sheet metal under the action of the machine's ejector force. After forming, the ejector pushes the part up. The positioning of the forming mold adopts self-made positioning pins for precise positioning. Three positioning pins are installed on the ejector of the lower mold base parts, and three positioning pin process holes are machined on the concave template of the upper mold. The unloading device of the mold adopts spring unloading. As the parts are formed and wrapped on the convex mold, 8 SWM40-100 rectangular springs are used to act on the unloading plate, pushing out the parts wrapped outside the convex mold insert. The bottom of the spring is directly in contact with the lower workbench through a spring cover plate.
1. Storage block 2. Guide pillar 3. Upper mold seat 4. Discharging plate 5. Punching punch 6. Mold handle 7. Punching punch 8. Shoulder shaped punch sleeve 9. Guide sleeve 10. Lower mold seat 11. Hanging rod 12. Back support plate 13. Punching concave mold insert 14. Guide block 15. Convex concave mold 16. Screw 17. Limit screw 18. Rectangular spring 19. Guide pin
Due to the use of a mold structure with one mold and two parts, the parts need to be punched and separated after forming. Figure 6 shows the structure of the component punching separation composite mold, which is mainly composed of an upper mold seat, a discharge plate, a punching punch, a punching punch, a concave mold, and a lower mold seat. Due to the high precision requirements for punching, the punching punch is installed on the shoulder shaped punch sleeve and connected with the shoulder shaped punch sleeve on the upper mold seat through screws.
In order to ensure punching accuracy, an insert is designed on the punching die. The die insert and die are installed on the die using H7/n6 transition fit, as shown in Figure 7. The separation die is composed of two die inserts. To ensure positional accuracy during punching, a back support plate is installed on the left and right sides of the lower die seat.
Figure 8 shows the structure of separating the convex mold and concave mold insert. During punching, the upper mold seat drives the convex mold and pressing plate downward to compress the workpiece. The upper mold seat and convex mold continue to downward to punch and separate the workpiece. The waste material directly slides off the press table, and the part is pushed out by the unloading plate.
A stamping process analysis was conducted on the reinforcing plate of the rear suspension spring support of the car, and a reasonable process plan was developed, clarifying the work content of three processes. Based on traditional design experience and combined with computer-aided design software UG, three sets of molds were designed for the blanking, forming, and punching separation of the reinforcing plate of the rear suspension spring support of automobiles. After mold debugging and mass production, it has been proven that the mold structure is reasonable, the operation is normal, and the quality of the parts is stable and reliable, meeting the requirements of part accuracy and mass production.