Designing a stretching die involves numerous factors, such as the drawing ratio, whether the material's limits are reached, spring force determination, and the stretching direction (upward or downward). Often, it requires multiple trials to achieve the desired result, and there's even a possibility of die failure. Therefore, accumulating practical experience significantly aids in stretching die design.

Additionally, the size of the blank significantly impacts the entire die production and trial process. Therefore, when designing irregular drawn parts, a margin is often reserved during the die design phase.

I. Stretching Material

If customer material requirements are not stringent and repeated die trials fail to meet requirements, consider using a material with better stretching properties. Good material is half the battle; this is crucial for stretching. Cold-rolled thin steel sheets used for stretching mainly include 08Al, 08, 08F, 10, 15, and 20 steel, with 08 steel being the most widely used. It is divided into rimmed steel and killed steel. Rimmed steel is inexpensive and has good surface quality, but it has severe segregation and a tendency for 'strain aging,' making it unsuitable for parts with high stamping performance and strict appearance requirements. Killed steel is better, with uniform properties but a higher price; a representative grade is aluminum-killed steel 08Al. Overseas, Japanese SPCC-SD deep-drawing steel has been used, exhibiting superior stretching properties to 08Al.

II. Die Surface Finish

During deep drawing, insufficient polishing of the die cavity and blank holder surfaces, especially when drawing stainless steel and aluminum sheets, easily leads to drawing marks, and in severe cases, stretching cracks.

III. Determination of Blank Size

Our principle is 'too much wrinkles, too little cracks'. Blank positioning design must be correct. For simple-shaped rotational stretching parts, in constant-thickness stretching, although the material thickness changes, it remains very close to the original thickness. The blank diameter can be calculated based on the principle that the blank area is equal to the stretched part area (plus trimming allowance if trimming is required). However, the shape and process of the stretched part are often complex, sometimes requiring thinning stretching. Although many 3D software programs can perform blank development calculations, their accuracy cannot reach 100%.

Solution: Trial material.

A product undergoes multiple processes, with blanking usually being the first. First, blank development calculations are performed to obtain a general understanding of the blank's shape and size, facilitating the determination of the overall dimensions of the blanking die. After die design, do not process the convex and concave die dimensions of the blanking die. First, use wire cutting to process the blank (for larger blanks, milling can be used followed by filing), and after repeated experiments in subsequent stretching processes, the blank size is finally determined, and then the convex and concave dies of the blanking die are processed.

Experience 1

Reverse process order: trial stretching die first, then process the blanking die edge size; this improves efficiency.

IV. Stretching Ratio m

The stretching ratio is one of the main process parameters in stretching process calculations, usually used to determine the stretching sequence and number of times.

Many factors affect the stretching ratio m, including material properties, relative material thickness, stretching method (with or without a blank holder), number of stretching times, stretching speed, convex and concave die corner radius, and lubrication.

The calculation and selection principles of the stretching ratio m are key points introduced in various stamping manuals, with many methods such as estimation, table lookup, and calculation. I followed the book's selection and have nothing new to add; please refer to the book.

Experience 2

Relative material thickness, stretching method (with or without a blank holder), and the number of stretching times are difficult to adjust during die modification and must be carefully considered. It is best to have a colleague check the selected stretching ratio m.

V. Selection of Processing Oil

The selection of processing oil is crucial. To determine whether the lubricating oil is suitable, if the product temperature is too high to touch after removal from the die, the choice of lubricating oil and lubrication method must be reconsidered. Apply lubricating oil to the die cavity or use a thin film bag on the sheet.

Experience 3

When encountering stretching cracks, apply lubricating oil to the die cavity (not the punch), and cover the workpiece side against the die cavity with a 0.013–0.018 mm plastic film.

VI. Workpiece Heat Treatment

Although not recommended, it's still worth mentioning that during stretching, the workpiece undergoes cold plastic deformation, resulting in work hardening, reducing its plasticity, and increasing its deformation resistance and hardness. Coupled with unreasonable die design, intermediate annealing may be required to soften the metal and restore its plasticity.

Note: Intermediate annealing is not necessary in general processes, as it increases costs. A choice must be made between adding processes and adding annealing; use cautiously!

Annealing generally uses low-temperature annealing, i.e., recrystallization annealing. Two points to note during annealing are decarburization and oxidation. Here, we mainly discuss oxidation. Oxidized workpieces have oxide scales, which have two disadvantages: reducing the effective thickness of the workpiece and increasing die wear.

When company conditions are not suitable, general annealing is usually used. To reduce oxide scale formation, the furnace should be filled as much as possible during annealing. I have also used a makeshift method:

1. When there are few workpieces, they can be mixed with other workpieces (provided that the annealing process parameters are basically consistent).

2. Weld the workpieces in an iron box before placing them in the furnace. To remove oxide scales, acid washing should be performed after annealing as needed.

When company conditions allow, nitrogen furnace annealing, i.e., bright annealing, can be used. The color is almost the same as before annealing.

Experience 4

For metals with strong work hardening or when stretching cracks occur during die trials and there are no other solutions, add an intermediate annealing process.

VII. Additional Points

1. Dimensions on product drawings should be marked on one side as much as possible, clearly indicating whether external or internal dimensions are guaranteed; do not mark both internal and external dimensions simultaneously. If there are such problems in drawings provided by others, communicate with them; unify if possible, otherwise, understand the assembly relationship of the workpiece with other parts.

2. For the final process, if the workpiece dimensions are external, the concave die is the main die, and the gap is obtained by reducing the convex die size; if the workpiece dimensions are internal, the convex die is the main die, and the gap is obtained by increasing the concave die size;

3. When designing the convex and concave die fillet radius, use the smallest allowable value as much as possible to facilitate subsequent die modification.

4. When judging the cause of workpiece cracking, refer to: Cracks caused by poor material quality are mostly sawtooth or irregular in shape, while cracks caused by processes and dies are generally more regular.

5. "Too much causes wrinkles, too little causes cracks." Adjust the material flow according to this principle. Methods include adjusting the pressure of the blank holder, adding drawing beads, modifying the convex and concave die fillet radius, and cutting the workpiece.

6. To ensure wear resistance and prevent stretching scratches, the convex and concave dies and blank holder must be quenched, hard chrome plated, or surface TD treated. If necessary, tungsten carbide can be used for the convex and concave dies.

Rear Suspension Spring Seat Plate Design for Automobiles; A Must-Read for Die Makers; Master the Latest Die Knowledge

1. Stamping Process Analysis

More and more people are learning die design now. Many people ask me if there are any materials and what the first book to read is. Based on your needs, I have categorized some die design materials, hoping that you will have a bright future in the die industry.

Figure 1 shows a rear suspension spring seat reinforcement plate for automobiles, mass-produced. The material is SAPH440, with a thickness of t=2mm. SAPH440 is a structural steel for automobiles, with a carbon content of around 0.20%. This material has a yield strength of 305~395MPa, tensile strength of 390~470MPa, and elongation ≥30%, with good forming properties. It is mainly used for automotive frames, wheels, and other structural parts with high strength requirements. The outer dimensions of the part shown in Figure 1 are 107.5mm×149mm×33mm, with high surface quality and precision requirements. The shape is relatively complex, and the ϕ7+0.1mm hole has assembly requirements, with a precision grade of IT9.

According to the shape characteristics of the part, the die is designed as a 1-die-2-part structure (see Figure 2). The forming of the rear suspension spring seat requires blanking, punching, forming, and separation processes. Since the part thickness is 2mm and the surface shape is a complex curved surface, it is advisable to use a combination of single-process dies and compound dies for production.

2. Calculation of Blank Size

Common blank material calculation methods for sheet metal parts include empirical methods, neutral layer methods, and patchwork methods. These methods are mostly suitable for parts with specific shapes. However, the surface shape of the rear suspension spring seat part is complex, neither a standard bending part nor a standard deep drawing part. The deformation part includes bending and deep drawing, making it difficult to calculate the precise blank size using traditional blank material calculation methods. The finite element analysis method in UG software was used to calculate the blank size, as shown in Figure 3.

3. Die Structure Design

1. Storage block 2. Limit pin 3. Upper die set 4. Upper die plate 5. Limit screw 6. Die handle 7. Ejector (upper die pressure plate) 8. Ejector spring 9. Concave die insert 10. Guide bushing 11. Guide pin 12. Lower die set 13. Ejector plate 14. Lower die plate 15. Convex die insert 16. Limit screw 17. Ejector spring 18. Hanging rod

The blank blanking die uses a sliding guide post inverted type punching die, a rigid ejector plate, and a cast steel guide post die frame, as shown in Figure 4. Die working process: The blank is roughly positioned by the stop pin and precisely positioned by the locating pin to ensure feeding accuracy. The workpiece ejector is designed inside the concave die. After the upper and lower dies are closed and punched, the part is ejected by the ejector, and the punched waste material is ejected from the lower die by the ejector plate.

1. Storage block 2. Limit pin 3. Guide pin 4. Upper die set 5. Concave die insert 6. Die handle 7. Locating pin 8. Guide bushing 9. Lower die set 10. Hanging rod 11. Rectangular spring 12. Ejector plate 13. Lower die plate 14. Convex die insert 15. Locating pin 16. Limit screw 17. Locating pin

Figure 5 shows the structure of the support forming die. According to the forming characteristics of the part, in order to reduce the manufacturing cost, the convex and concave dies use an insert structure. The concave die is on the upper die, and the convex die and ejector are on the lower die. Before forming, the sheet metal is placed on the ejector. During operation, the concave die moves down, and the ejector presses the sheet metal under the action of the machine tool's top rod force. After forming, the ejector pushes the part up. The forming die is precisely positioned using a self-made locating pin. Three locating pins are installed on the ejector of the lower die set, and three locating pin process holes are machined on the concave die plate of the upper die. The die's ejector device uses spring ejection. Since the part will be wrapped around the convex die after forming, eight SWM40-100 rectangular springs act on the ejector plate to eject the part wrapped around the convex die insert. The bottom of the spring directly contacts the lower worktable with a spring cover plate.

1. Storage block 2. Guide pin 3. Upper die set 4. Ejector plate 5. Punching convex die 6. Die handle 7. Punching convex die 8. Shoulder convex die sleeve 9. Guide bushing 10. Lower die set 11. Hanging rod 12. Backrest plate 13. Punching concave die insert 14. Guide block 15. Convex and concave die 16. Screw 17. Limit screw 18. Rectangular spring 19. Guide pin

Since the part uses a 1-die-2-part die structure, it needs to be punched and separated after forming. Figure 6 shows the structure of the part punching and separating compound die. This die mainly consists of an upper die set, an ejector plate, a punching convex die, a punching convex die, a concave die, and a lower die set. Due to the high punching accuracy requirements, the punching convex die is installed on the shoulder convex die sleeve and connected and installed on the upper die set together with the shoulder convex die sleeve via screws.

To ensure punching accuracy, an insert is designed on the punching concave die. The concave die insert and the concave die are installed on the concave die with an H7/n6 transition fit, as shown in Figure 7. The separating concave die consists of two concave die inserts. To ensure positional accuracy during punching, a backrest plate is installed on the left and right sides of the lower die set.

Figure 8 shows the structure of the separating convex die and concave die insert. During punching, the upper die set drives the convex die and pressure plate to move down and press the workpiece. The upper die set and convex die continue to move down to punch and separate the workpiece. The waste material slides down directly from the pressure machine table, and the part is ejected by the ejector plate.

A stamping process analysis was conducted on the reinforcement plate for the rear suspension spring seat of a vehicle, and a reasonable process plan was developed, clarifying the work content of three processes. Based on traditional design experience and using the computer-aided design software UG, three sets of dies were designed for blanking, forming, and punching separation of the rear suspension spring seat reinforcement plate. After die debugging and mass production, it was proven that the die structure is reasonable, the operation is normal, the part quality is stable and reliable, and the requirements for part precision and mass production are met.