Introduction: Microwave ovens are mainly composed of magnetrons, oven cavity, oven door, waveguide, microwave stirrer, timer, and power regulator. Among them, oven door is entrance and exit of food, and its requirements are very high: heating status of food inside oven can be observed from outside of door, while preventing microwave leakage. Microwave oven door mainly consists of door panel, inner cover, glass plate, handle, and metal mesh. This article introduces structure and design experience of a microwave oven inner cover injection mold.

Microwave oven inner cover plastic part (Figure 1) is made of modified polypropylene (PP) with 30% talc filler (PP+30TD), and has a shrinkage rate of 0.8%. Plastic part is frame-shaped with 13 undercuts inside, and lifter mechanism is quite complex.

 

Figure 1 Microwave Oven Inner Cover Plastic Part

Annual demand for microwave oven inner covers exceeds 10 million, which is huge. Therefore, mold adopts a 1*4 layout to meet production capacity. In addition, it is an appearance part and is assembled with sheet metal parts, so appearance quality and dimensional requirements are high. To ensure quality of plastic part, shorten molding cycle, and reduce amount of gate material, mold adopts a hot runner system, and introduces melt of each hot runner nozzle into two product mold cavities on two parting surfaces through cold runner. Compared with traditional stacked molds, it eliminates 6 hot runner nozzles. Since melt can enter product cavities on two parting surfaces through cold runner, there is no need to set up a transfer hot runner system in the middle of two products, thereby greatly reducing distance between two parting surfaces and thus reducing mold thickness. Mold gating system consists of hot runner nozzles, hot runner manifolds, other positioning parts, and cold runners, as shown in Figure 2.

 

Figure 2 Product layout and gating system

Microwave oven inner cover plastic part has 13 undercuts A1~A13, as shown in Figure 3(a). This mold needs to design 13*4 lifters. Conventional lifter structures, including lifters, guide slides, and guide blocks, will lead to an increase in mold thickness. How to shorten length of lifters and reduce mold thickness to be suitable for small-tonnage injection molding machines is a technical challenge of lifter structures.

 

Figure 3 Product undercuts
As shown in Figure 3(b), undercuts A1~A13 are all located inside product, and due to obstruction of inner ring side wall of product, a slider structure cannot be used. To shorten height of lifter and thus reduce mold thickness, this mold design uses a "push block + small lifter" structure, as shown in Figure 4(d). This achieves function of a conventional lifter while reducing mold thickness. This structure mainly consists of a small lifter 5, a push block 4, a push plate 3, and a push plate 11. Push block 4 is connected to push plates 3 and 11 by screws. When mold is closed, small lifter 5 is fixed in inclined grooves of mold core 6 and 15, as shown in Figure 4(a). When mold is opened, mechanical hook structure 21 (see Figure 5) controls parting surface I and parting surface II, ensuring that their opening distances are same, as shown in Figure 4(b). As mold continues to open, push plates 3 and 11 separate from mold plates 7 and 17, respectively. Push plates then drive push block 4 to apply an ejection force to small lifter 5, thereby causing small lifter to move within inclined grooves of mold core 6 and 15, thus completing unhooking process, as shown in Figure 4(c).

 

1. First parting surface 2. Second parting surface 3. Ejector plate 4. Push block 5. Small angled ejector 6. Mold core 7. Template plate 8. Product 10. Molding template 11. Ejector plate 15. Mold core 17. Template plate
Figure 4. Exploded view of lifter structure and its operation.

 

1 - First parting surface; 2 - Second parting surface; 3 - Ejector plate; 4 - Push block; 5 - Small angled ejector; 6 - Mold core; 7 - Template plate; 8 - Product; 9 - Rear mold base plate; 10 - Molding template; 11 - Ejector plate; 12 - Front mold base plate; 13 - Locating ring; 15 - Mold core; 16 - Hot runner plate; 17 - Template plate; S1 - Grade 1 hot nozzle; S2 - Manifold plate; S3 - Grade 2 hot nozzle
Figure 5. Structure diagram of new stacked mold for microwave oven inner cover.
Undercut amount of plastic parts of microwave oven inner cover, A1~A13, is 1.5mm. Including safety distance, unhooking stroke distance is set to 4.3mm. Ejection distance of push plate 3 and push plate 10 is 35mm. Ejection angle of small lifter 5 is α=arc tg4.3/35=7°. Compared with conventional lifter structure, this "push block + small lifter" structure significantly shortens lifter structure, eliminates conventional lifter guide plate and ejection plate, resulting in a substantial reduction in mold thickness and cost. Due to application of "push block + small lifter" structure, product layout, and gating method, number of hot runner nozzles and mold thickness are greatly reduced. Mold cost is reduced by 40% compared to traditional stacked molds, and tonnage of injection molding machine is reduced by 36% (from 600t to 380t).

Mold adopts a hot runner + cold runner combined gating system with a total of 52 lifters. Mold dimensions are 920mm * 650mm * 585mm, and the total weight is 2770kg. Four sides of movable mold plate are equipped with 100mm wide square guide pillars 18 and bronze guide sleeves 19 at their center positions. Square guide pillars are made of 1.2510 steel, hardened, and have a hardness of HRC48-52. Because square guide pillars 18 and bronze guide sleeves 19 are located at the center of sides of ejector plate 3, ejector plate 11, and forming mold plate 10, they are less affected by mold's own thermal expansion. Compared with round guide pillars located at four corners of mold plate, they offer higher guiding and positioning accuracy and are less prone to seizing. Square guide pillars 18 and bronze guide sleeves 19 provide main guiding and positioning functions for mold. To support molding mold plate 10 after mold opening, four round guide pillars 20 are set at four corners of mold to provide auxiliary guidance. Guide pillars are made of SUJ2 material and have a diameter of 55mm. Mating position is equipped with an oil groove. Guide sleeve has an inner diameter of 55mm and is made of SUJ2 material. The overall assembly structure is shown in Figures 5-6.

 

3. Push plate; 7. Template; 9. Rear mold base plate; 10. Molding mold plate; 11. Push plate; 12. Front mold base plate; 16. Hot runner plate; 17. Template; 18. Square guide post; 19. Square guide sleeve; 20. Round guide post; 21. Mechanical hook structure; 22. Hook structure; 23. Resin spring.
Figure 6 Three-dimensional view of new stacked mold for microwave oven inner cover
Working process of mold is as follows: (1) Mold is closed and injection molding machine is high pressure locked. (2) Plastic melt enters mold cavity through hot runners S1, S2, and S3. (3) After entering cavity, melt enters product cavity of first parting surface through cold runner S4 of first parting surface; melt enters product cavity of second parting surface through cold runner S5 of second parting surface [Figure 2(c)]. Cold runners S4 and S5 are connected, and balance of injection of four products on two parting surfaces is ensured by adjusting size of connected position. (4) After filling, after holding pressure and cooling, injection molding machine rear seat plate moves away from injection molding machine barrel, thereby driving mold rear mold base plate 9. Mold first opens from first parting surface I between molding mold plate 10 and push plate 11. When injection molding machine pulls mold for 300mm, external hook structure 21 of mold reaches predetermined position, second parting surface II between molding mold plate 10 and push plate 3 begins to open. When injection molding machine pulls mold for 600mm, external hook structure 22 of mold reaches predetermined position. At this time, first parting surface I and second parting surface II each open by 300mm. (5) Injection molding machine continues to pull mold rear seat plate 9 for 70mm. At this time, hook mechanism between molding mold plate 10, push plate 11 and push plate 3 begins to pull push plate 3 and push plate 11. Among them, push plate 3 is fixed to mold core 6 and mold plate 7 (mold core 6 and mold plate 7 are fixed with screws and are a whole), and push block 4 is fixed to push plate 3 with screws. Then push plate 3 drives push block 4 to separate from mold plate 7, thereby driving small lifter 5 to move along inclined groove in mold core 6, realizing release of undercut and ejection of product. Similarly, push plate 11 is separated from mold core 15 and mold plate 17 (mold core 15 and mold plate 17 are fixed with screws and are a whole), and push block 4 is fixed to push plate 11 with screws. Then push plate 11 drives push block 4 to separate from mold plate 17, thereby driving small lifter 5 to move along inclined groove in mold core 15, realizing release of undercut and ejection of product. (6) Injection molding machine pulls mold back plate 9 to move 670mm, completing mold opening, release of undercut and ejection of product. Robot enters from first parting surface I and second parting surface II and takes out 4 products. (7) After molded plastic part is safely released from mold, injection molding machine pushes mold back plate 9 to close mold. Since resin springs 23 are installed on both first parting surface I and second parting surface II, and there is an 8mm preload, lifter will not damage molding surface during mold closing. Therefore, return sequence of first parting surface I, second parting surface II, ejector plate 3, and ejector plate 11 during mold closing is not important. After injection molding machine moves 670mm during mold closing, mold locks, and mold begins next injection molding.

(1) Application of new product layout of mold, hot runner and cold runner interactive gating method, and new lifter structure simplifies mold structure, reduces mold thickness, reduces cost by 40% compared with traditional stacked mold, reduces injection molding machine tonnage by 45%, and lowers production cost.
(2) Mold adopts a hook structure limit instead of conventional stacked mold gear and rack action control mechanism, reducing mold cost, mold opening and closing time, and shortening molding cycle by 10%.
(3) Mold guiding and positioning system adopts square guide pillars and bronze guide sleeves, which makes mold more accurate; mold structure is highly innovative, small lifter structure is safe and reliable, mold has been running well since mass production, and quality of plastic parts has met customer requirements.