Semi-enclosed shell stamping process and mold design

2021/08/20


Taking a semi-enclosed shell used in an amplifier as an example, we analyze its stamping process, design the corresponding die to ensure one-time forming for the interference problems and technical difficulties in the bending process, and improve the processing efficiency and accuracy.
The bending of closed parts usually requires more than two processes to complete, and the material is often unloaded manually or by cylinder in the stamping process. One-time forming of closed parts can reduce the number of processes and the number of dies, which is conducive to improving work efficiency and product accuracy and reducing production costs.

2 Semi-enclosed shell structure and process analysis

There are many semi-enclosed shells for instrument or amplifier assembly in aviation parts. The semi-enclosed housing part of an amplifier is shown in Figure 1, whose material is 3A21 rustproof aluminum plate, material thickness t = 2mm, and the main structure is symmetrical. From the relevant dimensional accuracy requirements of the drawing, it can be seen that the shape of the part, the inner cavity and the positioning dimensions of the hole are not required to be high in accuracy, reaching IT10 level, and the inner bending radius R of the part is 2mm, and the local thinning of the material is allowed to be 30%.


Figure 1 semi-closed shell parts

From the part structure, the shell is a five-sided fully enclosed, the sixth side is not open semi-enclosed structure, after the completion of the first bending, but also the sixth side of the structure to bend (see Figure 2). From the bending dimensions in Figure 1, the first bend is not very difficult to form, but the second bend flange part size is small and there is a problem that the mold can not be released after bending. In addition, the part requires a lot of hole processing after the bending is completed, so the size of the bending structure must be guaranteed. The second bend at the H / D ≈ 1.5 (where H is the bending height, about 6mm; D is the bending diameter, about 4mm), the flange bending is more difficult, especially the flange radius and flange height is similar, making the structure at the forming is easy to produce pulling crack, so the necessary process must be done. At the same time, because the upper part and the adjacent right angle bending forming part is shorter, and the other side is open, so its tangential force exerted on the excess triangle is not large, that is, the excess material in the rounded part is not exactly according to the pure radial flow in tension. On the contrary, because of the lack of material constraint in the tangential direction, the material flows mainly in the transverse direction, so it greatly improves the material deformation and enables it to be formed in one flap.


Fig. 2 Three-dimensional numerical model of the shell

3 Semi-closed shell stamping process analysis

(1) Process flow The main processing flow of the shell is shown in Table 1. 10 processes are involved in the shell processing, among which the main forming process is wire cutting and two bending processes. Since the part is a symmetrical part, the shape after unfolding is relatively uncomplicated, so the shape is not processed by drop die, but mainly by wire cutting and chemical etching, which can guarantee the processing accuracy and the strength of the material at the edge. After completing the unfolded shape, two bends are required, and there are three sides to be bent. The first bend completes the bending of the four long sides of the part, and the part is shown in Figure 3 after bending and forming; the second bending part is the semi-closed part at the top, and the part is shown in Figure 4 after bending is completed.

Table 1 The main processing process of the shell



Figure 3 First bending and forming


Figure 4 Second bending and forming

Shell in the completion of the first bending will lead to dimensional instability due to deformation, the second semi-closed part of the bending will therefore produce the problem of positioning deviation. Before and after the completion of bending, the shell will be processed with square holes and mounting holes, which need to be positioned by the shape. Therefore, after the first bending, weld forming and weld tumor removal, it is necessary to design a mold for shaping. A total of 3 sets of dies are required for shell processing, in addition to the first and second bending dies and the subsequent shaping dies, in order to achieve complete stamping and forming.

(2) Shell unfolding blank shape and size determination As the shell needs to be bent twice, as the primary guarantee of the bending shape size, the correct calculation of the unfolding size is very important, so the second bending notch part must be taken into account. After determining the unfolded blank shape, use the formula to calculate the shape size, if necessary, you can determine the final unfolded size by test bending method.

The final shape is reasonable, its bending forming part according to the calculation method of the conventional bending parts blank size can be calculated for this part bending angle are 90 °, the blank and unfold the length of.

Where, L is the total length of the blank (mm); l 1, l 2 for the straight edge length (mm); r for the bending radius (mm); x for the neutral layer displacement coefficient, according to Table 2; t for the thickness of the blank (mm).

Table 2 neutral layer displacement coefficient x value


The thickness of the shell part t = 2mm, bending radius r = 2mm, so the neutral layer displacement coefficient x = 0.32, according to the shell product drawing and unfolding diagram, the unfolding length of each section is calculated. In addition, according to the sheet metal bending requirements, the process hole 4× φ 1.5mm is designed in the part where 4 sides need to be bent, and the final unfolding diagram is shown in Figure 5.


Figure 5 Shell blank unfolding

4 Semi-enclosed shell stamping die design

Three sets of dies are required to complete the main process of semi-enclosed shell parts forming, including the bending die and shaping die for the main structure, and the bending die at the closure.

(1) Bending die design The bending of the whole structure of the shell is the first bending, which needs to be carried out with the help of bending dies. Figure 6 shows the bi-directional bending mold structure, need to be combined with special equipment for clamping, the main structure of the mold for bending type tire 10, its four faces for bending positioning surface, in the bending process, using the holes on the platen 2 for positioning, followed by bending of the four sides. Clamping block 9 is cylindrical, bending the part will be fixed in the vise, each stamping process can be completed one side of the bending, cylindrical structure can ensure that the four sides of the bending random rotation, and thus ensure a bending in place.



Fig. 6 Bending type of shell tire
1-Locating pin 2-Pressing plate 3-Hexagonal screw 4-Hexagonal nut 5-Upper clamp plate 6-Screw for T-slot 7-Lower clamp plate 8 -Hexagon socket head cap screw 9 - clamping block 10 - bending type tire 11 - cylindrical pin

The material of bending type tire 10 is CrWMn, heat treatment is quenched to 50~55HRC to ensure the hardness of the part and prevent the part from breaking; the surface treatment technology is Ct.O (surface blackening and oxidation treatment); the type tire is a monolithic structure, the roughness value of the outer surface is required to reach 0.8mm. Therefore, milling machine is adopted first, then wire-cut slow-walking processing, while the shape is processed around 15 The rebound angle can be trimmed by work grinding in the process of mold testing. Mold assembly to control the gap between the pressure plate 2 and the bending type tire 10, the gap value is generally 1.1t (t is the thickness of the shell value). The screw holes on the clamping plate to ensure the accuracy of the clamping, should be made with the pressure plate.

(2) shaping die design After the shell bending is completed, the size correction also needs to use the shaping die (see Figure 7), shaping in the 63t hydraulic press. The mold can ensure the consistency of the size of the inner cavity of the shell by expanding the shape during the stamping process. The die is divided into three main parts, the stripper plate 3, the positioning plate 6 and the legs 7 are installed in the lower part of the machine, and the shell is placed in the positioning plate. When shaping, the mold tire 2 is placed on the top of the shell, then the upper mold pushes the handle 13 and the pallet 12 downward under the action of the hydraulic machine slider, which makes the mold tire all enter the shell slowly under the action of external force, and the upper mold continues to move downward until it reaches the depth of the shell, then the part will be naturally released from the stripper plate under the action of the insert plate 1.


Figure 7 Shell shaping mold
1 - insertion plate 2 - shaped tire 3 - demoulding plate 4, 8 - cylindrical pin 5, 10 - hexagonal thread 6 - positioning plate 7 - legs 9 -Plate 11-Top plate 12-Pallets 13-Handle

In the design of the mold, it is required that the center of the inner cavity is overlapped after the assembly of the stripper plate 3 and the positioning plate 6, and the four bottom surfaces of the legs 7 must be flush after the assembly of the stripper plate 3 and the legs 7. The main part of the shaping mold is the type tire, the structure is monolithic, in order to increase the strength, the material is T7A carbon tool steel, heat treatment quenching to 50 ~ 55 HRC, surface treatment technology for Ct.O. In order to make the shaping parts smoothly out of the mold, the outer surface roughness value required to reach 0.8μm, so its shape size rough machining using the number of milling, heat treatment quenching to 50 ~ 55 HRC, the final Formed size by the grinding machine to ensure. The majority of the remaining plate parts use the material 45 steel, heat treatment requirements for 30 ~ 35HRC, surface treatment Ct.O, in order to ensure that the appearance of the plastic parts are not damaged, the pallet selection of sandwich wood board.

Positioning plate 6, stripper plate 3 on the pin hole and cavity processing to ensure consistent position, mold assembly using a combination of positioning pins and screws to ensure that the stripper plate 3 and positioning plate 6 combination of the center of the inner cavity overlap. The legs and handles are mainly machined by number turning, and the length size can be corrected after assembly.

(3) semi-closed at the second bending mold semi-closed parts of the shell bending processing difficulties, Figure 8 is the second bending type tire, mainly to complete the remaining closed part of the shell bending. In order to ensure that after the completion of the bending tire can come off from the semi-closed cavity, the design of the bending mold design into the Haff structure - the most central part of the fixed oblique wedge, the other parts of the tire with it, the design of M8 screw holes in each part of the tire, easy to remove. When bending, first put the cushion in the shell, then put the tire and wedge in turn, and then bend by the clampers; after the bending is completed, first remove the wedge part, leaving space for other parts of the tire, and then take out respectively.


Figure 8 semi-closed at the second bending mold
1 - pad 2 - tire A 3 - tire B 4 - tire C 5 - wedge 6 - tire D

The materials used for mold parts 2 to 6 in Figure 8 are T7A, with heat treatment requirements of 50 to 55 HRC to ensure the hardness of the part disassembly; the pad 1 uses ordinary 45 steel. When the mold is processed, the base plate is made when the type tire and slant wedge are rectified, and the screw hole of M8 is used to form the whole part, and the shape size of each type tire is trimmed to ensure that the size of the type tire is 198mm×95.7mm×106mm and meet its tolerance requirements. After finishing the trimming, each type tire is marked with A, B, C and D according to the location shown in the drawing, which is convenient for use.

5 Conclusion

The shell studied in this paper is in semi-closed form. Through reasonable process arrangement and design of corresponding moulds, the interference problem of bending is avoided and the processing of closed-shaped parts is completed, which provides reference for the production of similar structural sheet metal parts in the future.