Preform in a bevel gear MIM process is shown in Figure 1. Diameter of bevel gear of preform is φ21.78 mm, which is an important dimensional parameter. It affects bearing capacity and transmission efficiency of gear; thickness of gear is an important factor in determining its strength and durability, and thickness is 14.3 mm. Number of teeth determines transmission ratio of gear. More teeth also determine higher transmission accuracy and lower noise. Number of teeth is 22 teeth, module is 1 mm, and molding size accuracy is MT4. Preform shown in Figure 1 is an intermediate process in MIM process. Its molding material is POM+stainless steel 304 powder composite material. After preform is injection molded, it is degreased and sintered to obtain a metal part with a similar shape to preform.

 

Figure 1 Bevel gear preform
Preform injection bearing substrate uses impact-resistant POM, which has characteristics of softness, ultra-high elongation, and medium viscosity. POM can be divided into homopolymer and copolymer according to different chemical structures in its molecular chain. Difference between the two is that homopolymer has high density, crystallinity and melting point, but poor thermal stability, narrow processing temperature range (about 10 ℃), slightly low acid and alkali stability; copolymer has low density, crystallinity, melting point and strength, but good thermal stability, not easy to decompose, wide processing temperature range (50 ℃), good acid and alkali stability. Copolymer POM is now selected. When injection molding, a medium or higher injection speed is used. If injection speed is too slow, mold or melt temperature is too low, pores will appear on the surface of molded preform. Stainless steel 304 powder is now used as metal raw material, and fine powder particles are ground to 1 500 mesh gauze. Its fine particles help improve molding quality and reduce subsequent processing. After mixing with POM, granulation is performed to provide raw materials for injection molding of preforms.

Mold design uses CAD/CAM technology to ensure accuracy and manufacturability of design. Mold is mainly composed of injection mold cavity body (cavity + core + local insert), gate, runner, cooling system, etc.

Design of cavity needs to accurately match shape of bevel gear to ensure accuracy of gear after molding and avoid deformation during molding process. Parting surface PL is set using maximum outer edge contour line of preform as parting line, as shown in Figure 2, to obtain cavity plate insert 1 and moving plate 2. In order to achieve deformation-free ejection of molded preform, push tube 4 is used to push molded preform, and push tube 4 is sleeved on central cylindrical core 3. High-speed tool steel is used for mold parts to ensure high wear resistance and high temperature resistance. Cavity plate insert 1 and moving plate 2 are made of tool steel T8, central cylindrical core 3 and push tube 4 are made of T8A.

 

Figure 2 Parting design
1. Cavity plate insert 2. Moving plate 3. Central cylindrical core 4. Push tube
In order to ensure that molten plastic/metal mixture "feeding" can be smoothly and evenly injected into cavity to avoid cold shut and deformation, cavity is filled with a point gate, initial gate size is φ0.8 mm, and final size needs to be determined based on injection effect after trial mold.

According to cavity parting design shown in Figure 2, mold adopts a 2-cavity layout, mold structure is shown in Figure 3. A false three-plate mold structure is adopted, and mold is opened in 2 times. When PL1 surface is opened, pull rod 14 pulls runner condensate out of gate sleeve 21 and leaves it on ejector block 22, then ejector block 22 is ejected by spring 16. Spring 16 is in a compressed state when mold is closed. When PL1 surface is opened, ejector block 22 is automatically ejected by spring 16, and no additional control mechanism is required. PL1 surface opens before PL2 surface and is controlled by spring 25. Runner condensate is separated from the molded preform. After the PL2 surface is opened, the molded preform is ejected from the cavity plate insert 1 and remains on movable mold plate 2. Push tube 4 is used for final ejection and demolding of molded preform. PL1 surface is opened by spring 25 and is limited by pull rod 26. Upper pressure plate 17 is fixed to fixed mold plate 15, and push plate 10 is fixed to push rod fixing plate 11. Push plate guide column 13 is used to guide movement of push plate 10, and guide column 23 is used to guide movement of fixed mold plate 15. Pad 7 fixes central cylindrical core 3 on movable mold base plate 8, and gate sleeve 21 is fixedly installed on fixed mold base plate 18 by positioning ring 20.

 

Figure 3 Mold structure
1. Cavity plate insert 2. Moving plate 3. Center cylindrical core 4. Push tube 5. Pad 6. Screw 7. Pad 8. Moving mold base plate 9. Limit block 10. Push plate 11. Push rod fixing plate 12. Guide sleeve 13. Push plate guide column 14. Pull rod 15. Fixed plate 16. Spring 17. Upper pressure plate 18. Fixed mold base plate 19. Screw 20. Positioning ring 21. Gate sleeve 22. Ejector block 23. Guide column 24. Guide sleeve 25. Spring 26. Pull rod

Cooling system can speed up cooling speed of mold and improve production efficiency. Cavity plate and mold plate are water-cooled. Cooling pipeline layout adopts two circular water channels W1 and W2 (see Figure 3). W1 is opened in fixed mold plate 15, and W2 is opened in fixed mold base plate 18. Pipeline diameter is φ10 mm to ensure uniform cooling of all parts of mold. Pipeline cooling effect is shown in Figure 4.

 

Figure 4 CAE analysis results

Moldflow2021 is used to perform CAE analysis on cooling effect of mold, and results are shown in Figure 4. As can be seen from Figure 4, cavity filling time is 2.034 s, filling of ends of each tooth is consistent and relatively balanced; filling pressure is about 3.7 MPa, and required pressure is relatively small. Main problem of molding is that weld line appears in the middle of tooth, and each tooth is accompanied by a pore. It is necessary to adjust assembly gap between cavity plate inserts or open an exhaust groove on parting surface to enhance exhaust in cavity and eliminate pores. Cooling pipeline is opened reasonably, temperature difference between inlet and outlet is less than 3 ℃, and cooling is effective.

Ejection mechanism is mainly composed of a push tube 4, a push plate 10, a push rod fixing plate 11, a guide sleeve 12, a push plate guide column 13, etc. Push plate 10 and push rod fixing plate 11 are fastened by screws to form a combined plate. Push plate guide column 13 is used for movement guide of combined plate. Limit block 9 is used to limit ejection height of combined plate. Ejection of preform is achieved by injection molding machine ejector driving push plate 10 to push push tube 4 placed thereon.

During processing of mold parts, due to high surface quality requirements of cavity plate insert 1, moving plate 2 and central cylindrical core 3 should facilitate demolding of molded preform to prevent its demolding deformation. High-precision CNC milling machines and EDM machines are required to process mold parts to ensure precision of each part. Surface roughness of cavity plate insert 1, moving plate 2, and central cylindrical core 3 is Ra0.8 μm. During processing, it is necessary to protect surface of mold parts to avoid scratches. Before mold is assembled, molding parts and guide parts must be cleaned and inspected to ensure that there are no impurities and defects. During assembly process, it is necessary to strictly follow design requirements to ensure that all parts fit tightly.
Mold trial and adjustment: During mold trial, filling condition, cooling effect and molding quality of cavity must be observed, and adjustments must be made according to actual situation. Adjusted mold must be tried again until requirements are met. After three mold trials, molded preform meets design requirements.

Mold structure is shown in Figure 3, and working principle is as follows.
(1) Mixing and granulation. POM particles and stainless steel 304 powder are evenly mixed and granulated.
(2) Injection. Mold is installed on injection molding machine. After mold is closed, heating is turned on. After POM+304 powder particles are melted, injection is completed. After injection is completed, wait for mold to be opened.
(3) PL1 surface is opened. Slider of injection molding machine drives parts below PL2 surface of mold to run. Due to action of spring 25, PL1 surface is opened first, completing separation of runner condensate, preform and removal of condensate. Opening distance of PL1 surface is 90 mm.
(4) PL2 surface opening. Movable mold part continues to move, mold opens at PL2 surface, molded preform is ejected from cavity plate insert 1, remains on central cylindrical core 3 and movable mold plate 2. Opening distance of PL2 surface is 105 mm.
(5) Push out. After PL2 surface is opened, injection molding machine ejector drives push plate 10 to push push tube 4 to push preform out from movable mold plate 2 and central cylindrical core 3, realizing complete demolding of preform. Pushing distance is 30 mm.
(6) Reset. Reset process is opposite to mold opening process. After reset is completed, mold starts next injection cycle.

Core process of stainless steel 304 powder metallurgy process includes raw material selection, powder preparation, mixing, preform molding, degreasing and sintering. Raw material must be metal powder with uniform particle size and stable chemical composition. Powder preparation is completed by mechanical alloying method or gas phase jet method. Proportion and time must be controlled during mixing to ensure uniformity. Preform molding stage uses a mold for injection molding. Preformed molded parts need to ensure uniformity of their density. Attention should be paid to problem that density in the middle area of preform may be low, and demoulding smoothness should be considered. Injection should be combined with product shape and injection pressure (usually not more than 120~150 kg/cm²).