Figure 1 Workpiece structure

Shell of rice cooker is connecting piece between middle plate and base of rice cooker. There is a handle to avoid empty space in the middle, and there is a support installation position for fixing  cover of rice cooker on rear side. Figure 1 shows structure of rice cooker shell, material is 430 stainless steel, thickness of material is 0.35mm, and thickness of film is 0.04mm (to prevent scratches on parts during transportation and manufacturing, film is coated on the surface of material). During forming, upper end is turned inwards with a full circle and has a slope of 2 °, lower end is rolled inwards. Manufactured parts are exterior parts, and there must be no defects such as scratches, paint drop, pad printing, deformation, uneven unevenness.

Upper and lower structures of part are both flanged inward and rounded. When forming, inner and outer surfaces of part must be clamped to roll smoothly, rounded punch needs to be able to shrink before part can be taken out. In order to ensure dimensional accuracy of part, round convex punch must be uniformly expanded and contracted each time, otherwise it will cause pad printing and deformation. Difficulty of forming cooker shell is pre-rolling, rolling and flanging. Process arrangement of parts is shown in Figure 2.

 

Figure 2 Process arrangement

Process description: ①Upper part of workpiece has a slope of 2 °. In order to ensure height of flanging, it is necessary to cut off a part of upper sheet, circumference accuracy of sheet needs to be controlled within ± 0.1mm, and both ends of sheet must be cut. In order to ensure positioning, a positioning gap should also be made at lower end of sheet; ② In order to connect sheet to a cylindrical shape, it is necessary to flang both ends of sheet; ③ Set a joint on the sheet and press it tightly into a cylindrical shape; ④Shape of molded parts is oblong, and shape of sheet is round. In order to avoid manually pressing sheet from a circle into a circle and then putting it into mold, which reduces production efficiency and prevent sheet from scratching surface of molded parts, an automatic tooling pre-forming process is added to shape sheet from a circle to an oval; ⑤Sheet must be pre-flanged before rolling, an upper part of parts is a step, so it needs to be folded into a bevel first; ⑥Sheet metal is rolled and flanged into a certain shape and size; ⑦Punch forming of support hole and grounding wire hole.

Combined with above description of each forming process, difficulty of forming rice cooker shell is compound forming of round and flanging of process ⑥. Mold structure is shown in Figure 3, and design content of mold is now introduced.

 

Figure 3 mold structure

1. Cylinder 2. Lower die base 3. Screw 4. Support plate 5. Spring 6. Mold core mounting plate 7. Rolled parts 8. Screw 9. Fixing plate 10. Guide sleeve 11. Spring 12. Side plate 13. Guide Column 14. Outer cover 15. Outer inclined wedge 16. Screw 17. Upper die base 18. Flanged parts 19. Fixing seat 20. Connecting rod 21. Inner inclined wedge 22. Small slider 23. Manufacturing part 24. Large slider 25 . Positioning block 26. Reset spring 27. Reset spring 28. Guide post 29. Linear bearing 30. Linear guide 31. Pressure plate 32. Reset spring 33. Reset pin

Cylinder 1 drives internal wedge 21 downward to push small slider 22 outwards. Due to guiding effect of fixed seat 19, small slider 22 pushes large slider 24 to move around under guidance of guide post 28 and pressure plate 31. Finally, cylinder 1 reaches bottom dead center, large and small sliders also reach prescribed position, outer sides of large and small sliders form complete curve of inner side of workpiece. After workpiece is formed, cylinder 1 rises and drives inner wedge 21 upward, return spring 27 pushes small slider 22 to reset. After small slider 22 is reset, reset spring 32 pushes large slider 24 to reset, leaving room for parts to take parts .
Design of outer cover mechanism: Outer cover of rice cooker is an appearance part, and no surface quality defects are allowed, gap between outer cover and mold core must be stable and consistent, otherwise it will cause deformation of workpiece or formation of pad marks. Upper and lower positions of outer casing mechanism are ensured by guide post 13 and guide sleeve 10, linear bearing 29 and linear guide 30, left and right positions are ensured by outer inclined wedge 15.

After part is placed into mold core, positioning block 25 determines  circumferential position. Mold core spreads out and clamps inner surface of part, upper mold goes down. Fixed seat 19 is inserted into guide mechanism at the center of upper mold to ensure clearance between the mold core and the flange part 18. Outer inclined wedge 15 fixed on upper die base 17 descends, pushes outer cover 14 toward center, and clamps outer surface of workpiece. After workpiece is clamped, flanging part 18 performs flanging. After flanging is completed, mold core and outer jacket 14 continue to descend, part enters round part 7 to be rounded. Mold core mounting plate 6 and cylinder 1 are linked up and down with mold core. After part is formed, upper mold rises, return spring 26 pushes outer cover 14 to move outward and reset, spring 5 moves upward, mold core contracts, cylinder piston is ejected, part is pushed out of rolled part, and the part is taken out.

① Strength of core support spring 5 must be sufficient to provide force required to support upturning, otherwise core will drop first, making size of upturning too high and size of lower curling circle not enough, as shown at K in Figure 3, 6 springs with an outer diameter of ϕ30mm are finally selected; ② Gap between outer cover and core is thickness of part plus thickness of film plus 1.5 film thickness, for example, thickness of part is 0.35mm, thickness of film is 0.04mm, it need to add 0.06mm. Final gap between outer cover and mold core is 0.45mm. When gap is less than 0.45mm, thin film on the part will cause pad printing. When gap is greater than 0.45mm, it will cause deformation of parts (convex or concave), affecting appearance quality of part; ③Processing accuracy of part is ± 0.02mm, size of part is large, material will be deformed during forming, so it must be roughed first, and then finished after stress is fully released; ④Roughness of mating surface and sliding surface is less than Ra0.8μm, roughness will increase resistance of mechanism, wear rate is accelerated, service life of mold is shortened, stability is poor. Curved surface of molded parts is trimmed with medium wire or slow wire, surface is ground with a precision grinding machine.

 

Figure 4 Relationship between size of flange on the part and flanged part

Length of upper flange of workpiece must exceed upper surface of flange part, as shown in Figure 4 (I in Figure 3), otherwise flange plate will not reach edge of mold part, causing size of flange to be too short and material move downward, workpiece deform. Corresponding relationship between length of upturned edge and length of upper pre-turned edge is shown in Table 1.

 

Figure 5 Relationship between flanging size and flanging angle & curling circle

Relationship between flanging size and flanging angle & curling circle is shown in Figure 5. Lower flanging size must be 2.2 ~ 2.7mm, and flanging angle is 45 ° ± 5 °. If size of flanging is too small, front part of parts cannot touch bottom of rounding groove, causing rounding failure, as shown in Figure 5 (b); size of flanging is too large to allow parts to enter rounding groove smoothly, as shown in Figure 5 (c). If flanging angle is not enough, it will cause part to not fully contact side wall of round groove, and round fails, as shown in Figure 5 (d).

 

A. Length is too short

 

(B) Length is too long

Figure 6 Relationship between length of workpiece and joint

When length of part is too short, it will come out at connection when connecting (see process ③ in Figure 2). When mold is closed, surface of mold part will be scratched. Length of part needs to be increased, as shown in Figure 6 (a); When length of part is too long, material has nowhere to go. After mold is closed, a protrusion will be formed at joint (see Figure 6 (b)), and length of parts needs to be shortened.
When size of parts is same and material is different, such as printed tinned iron plate and 430 stainless steel plate, hardness and strength of material are different. Length of tinned iron plate should be less than length of stainless steel plate about 0.5mm to hold mold core and fold out same height dimension, otherwise size of upturned edge of printed tin-plated iron plate parts will be too short.