With rapid development of domestic industrial production, China's mold industry has also developed rapidly. Currently, China's total die-casting mold output is second only to United States, making it a veritable die-casting powerhouse. Die-cast parts are widely used in automobiles, motorcycles, machinery, and electronic products. Larger parts such as automobile engine blocks, cylinder head covers, chain covers, gearbox housings, and clutch housings are all made using aluminum alloy high-pressure casting. To meet requirements of mass production, higher demands are placed on quality, lifespan, and reliability of die-casting molds. This study takes a manual transmission car clutch housing as an example, analyzes its structural characteristics and parting surface selection, optimizes ejection mechanism of clutch housing die-casting mold based on product defects and maintenance issues encountered during production. This provides a reference for development of similar die-cast parts.
Automotive clutch is an assembly installed between engine and transmission, transmitting power between engine and transmission system. Clutch housing connects, supports, and protects internal components of clutch, improving stability of clutch operation. Figure 1 shows a manual transmission car clutch housing with outer dimensions of 477 mm * 358 mm * 130 mm and an average wall thickness of 3.5 mm. From casting structure, housing casting consists of two cavities on opposite sides. One side, connecting to engine flywheel assembly, has a cavity with a diameter of ϕ276.5 mm * 122 mm; the other side, connecting to transmission housing, has a stepped cavity with a maximum diameter of ϕ189 mm. There are multiple thick-walled areas formed by bolt mounting bosses on both sides and laterally. Casting material is AlSi9Cu3(Fe), and its weight is 6.1 kg. After machining, casting requires a sealing test; under 0.1 MPa pressure, leakage is less than 20 mL.
Die-casting molds mainly consist of two parts: a moving mold and a fixed mold. Contact surface between moving and fixed molds is parting surface. Parting surface is generally chosen at section with the largest outer contour dimension of casting to ensure smooth separation of casting from mold cavity after mold opening. Simultaneously, it must ensure that die-casting remains on moving mold side after mold opening, allowing ejection mechanism of die-casting machine to smoothly and steadily eject casting, thus simplifying mold structure. Considering shape characteristics of clutch housing, parting surface is chosen at the largest end face of both cavities, transitioning with a lateral slope to form a zigzag parting surface. Cavity surfaces (or core surfaces) with consistent draft angles are formed in one cavity. To ensure die-casting remains on moving mold side after mold opening, side with greater clamping force must be set as moving mold. Magnitude of clamping force is mainly related to factors such as surface area, wall thickness, and draft angle of formed part enclosed by casting. Therefore, larger assembly cavity on the side where clutch housing connects to engine is located in moving mold, thus determining main parting surface of casting. Based on changes in local shape of casting, main parting surface was refined to determine parting surface of clutch housing. Figure 2 shows parting surface of clutch housing.

 

Figure 1: 3D morphology of clutch housing

 

Figure 2: Parting surface of the clutch housing
1. Moving mold 2. Moving mold insert 3. Casting
During mass production, clutch housing exhibits deformation and micro-crack defects, as shown in Figure 3. Deformation of casting blank accounts for more than 60% of scrapped parts. Deformation occurs at mating surface between clutch housing and transmission housing, with lower part warping up by 1-2 mm. During mass production, defects caused by deformation are compensated for by increasing machining allowance of bottom surface and side holes of blank, resulting in significant waste. Micro-cracks occur at lower bolt bosses, without obvious traces, with a low defect rate, and occur in the middle to late stages of production. Analysis shows that both types of defects in casting are caused by varying positions and directions of clamping forces due to structure of casting. Figure 4 shows stress state of mold at the moment of mold opening. Clamping forces of moving mold and fixed mold are distributed vertically and in opposite directions. At the moment of mold opening, clamping force of moving mold forcibly separates housing from fixed mold. Although there is an ejecting force from punch below fixed mold, due to low strength at ingate, punch and ejecting force at the moment of mold opening separate sprue from casting at ingate, failing to assist fixed mold in ejection. In the early stages of mold use, casting deforms due to local clamping force of fixed mold. In the middle and late stages of mold life, as cracks appear on mold cavity surface and mold surface becomes rough, local clamping force of fixed mold increases, causing micro-cracks to appear in some castings.

 

Figure 3: Location of defects in clutch housing

 

Figure 4: Stress diagram of clutch housing during mold opening
Fixed mold ejection mechanism of die casting molds commonly uses disc spring ejection and hydraulic ejection. Hydraulic cylinder ejection is commonly used in applications with small ejection areas and concentrated ejection forces. However, in clutch housing, ejector rods for fixed mold ejection are distributed in lower half of housing cavity, with a total of 6 ejector rods located at bolt bosses around cavity circumference. Given large ejection range, disc spring ejection is employed. A 3D schematic diagram of fixed mold ejection mechanism is shown in Figure 5, and its structural composition is shown in Figure 6. Working process is as follows: When mold closes, moving mold insert plane contacts reset rod of this mechanism. Under action of mold closing force, disc spring is compressed. Reset rod drives ejector rod fixing plate and ejector rod to reset. After mold closing, reset rod moves to be flush with parting surface. At this time, limit post mounted on ejector plate is flush with die-casting machine's fixed mold plate. When mold opens, compressed disc spring is released. Under thrust of disc spring, ejector plate and ejector rod fixing plate move, pushing ejector rod and reset rod higher than parting surface, thus achieving simultaneous mold opening and ejection of casting from fixed mold. Fixed mold ejection distance is designed to be 3-5 mm. Due to small ejection distance, a separate ejection guide mechanism is not required. Reset rods are designed at four corners of ejector plate, simultaneously serving as ejection guides.

 

Figure 5: 3D Schematic Diagram of Fixed Mold Ejection Mechanism
1. Ejector Rod 2. Reset Rod 3. Disc Spring 4. Disc Spring Support Column 5. Limiting Column 6. Ejector Plate 7. Ejector Rod Fixing Plate

 

Figure 6: Structural Composition Diagram of Fixed Mold Ejection Mechanism
1. Moving Mold Plate 2. Reset Rod 3. Fixed Mold Insert 4. Fixed Mold Ejector Rod 5. Ejector Rod Fixing Plate 6. Ejector Plate 7. Disc Spring 8. Disc Spring Support Column 9. Limiting Column
As shown in Figure 6b, fixed mold insert has a circular stepped transition shape in central forming area, mainly composed of six bosses. Ejection force of each part is calculated, and calculation results are shown in Table 1. The total ejection force of fixed mold core ∑F_eject = 42,393 N.
Disc springs are characterized by their small size, high load capacity, and convenient combination. They can be used to achieve different loads and strokes by single-piece pairing or multi-piece stacking, depending on requirements of space, load, and stroke. Using disc springs in fixed mold ejection mechanism of this mold effectively reduces installation space and ensures strength of fixed mold plate while maintaining sufficient ejection force.
Based on fixed mold ejection structure of this mold, two locations for installing disc springs are designed, symmetrically positioned longitudinally, horizontally, and laterally (see Figure 5). Based on required ejection force, two disc springs with an outer diameter of ϕ50 mm and a thickness of 3 mm are selected and used in parallel. To meet ejection distance requirements, six sets are used in series. Parameters of selected disc springs are shown in Table 2. A schematic diagram of disc spring structure is shown in Figure 7.

Table 1. Ejection Force of Fixed Mold Core

Table 2. Disc Spring Parameter Table

 

Figure 7. Schematic Diagram of Disc Spring Structure
Moving mold ejection mechanism mainly consists of ejection elements, reset elements, guide elements, limiting elements, and structural elements. Typical structure of moving mold ejection mechanism is shown in Figure 8. In Figure 8a, moving mold base plate of ejection mechanism is a flat plate structure, with limiting pins fixed to it. In Figure 8b, moving mold base plate of ejection mechanism is stepped, integrated with support plate, and internal steps serve as limiting mechanism for ejection. Both structures share a common problem: when replacing easily worn parts such as ejector pins or moving mold cooling water, moving mold base plate must be disassembled. Especially for large molds with large volume and weight, and large connecting bolts, disassembling moving mold base plate is time-consuming and labor-intensive, severely impacting production efficiency. Therefore, moving mold ejection mechanism needs to be optimized when replicating molds.
Optimized structural scheme of clutch housing moving mold ejection mechanism is shown in Figure 9. Moving mold base plate adopts an integrated structure. During assembly, push plate guide pillars are first installed on inner stepped surface of moving mold base plate. Four push plate guide pillars are distributed at four corners of inner cavity of moving mold sleeve plate. Positioning pins are installed on mating surface between moving mold base plate and moving mold sleeve plate. Then, moving mold base plate with push plate guide pillars and positioning pins is installed onto moving mold sleeve plate and fixed with bolts. Next, push rod, reset rod, push plate guide sleeve are installed on push plate in sequence and then fixed to push plate with bolts to form ejection assembly. Ejection assembly is then installed into moving mold base plate. Finally, four pressure-cap type limit blocks are fixed. During maintenance and replacement of vulnerable parts, only four small limit blocks need to be removed to take out ejection assembly with push rod, which is convenient, safe, and reliable.

 

Figure 8. Structural Composition of the Moving Mold Ejection Mechanism
1. Moving Mold Base Plate 2. Push Plate Guide Sleeve 3. Push Plate Guide Pillar 4. Push Plate 5. Push Rod Fixing Plate 6. Limit Pin 7. Push Rod 8. Reset Rod 9. Support Plate

 

Figure 9. Structural Composition of the Clutch Housing Ejection Mechanism
1. Moving Mold Base Plate 2. Limit Block 3. Push Plate 4. Push Rod Fixing Plate 5. Push Plate Guide Sleeve 6. Push Plate Guide Pillar 7. Positioning Pin 8. Cooling Spray Pipe 9. Push Rod 10. Reset Rod 11. Moving Mold Insert 12. Moving Mold Sleeve Plate
Clutch housing is produced on a 16,000 kN die-casting machine with an annual output of over 300,000 pieces. Optimized ejection mechanism is reflected in replication mold. After production verification with more than ten molds, quality of casting products has been significantly improved, eliminating deformation and cracks caused by excessive local clamping force in static mold. At the same time, mold maintenance time has been shortened, and production efficiency has been improved. Mold uses a disc spring ejection mechanism in fixed mold. Due to alternating stress and mold temperature during use, disc springs undergo plastic deformation over time, resulting in reduced free height and weakened elasticity. To avoid risk of disc spring failure, all disc springs are replaced during stress-relief maintenance disassembly of mold cavity after 20,000 mold cycles. Integrated moving mold base plate has a service life of over 500,000 cycles. When replacing mold, it can be reused after maintenance and inspection, significantly reducing mold manufacturing costs.
Conclusions
(1) Disc spring ejection mechanism in fixed mold overcomes clamping force of fixed mold to achieve casting ejection. Disc spring combination used in this design consists of 2 discs in parallel and 6 sets in series, with an ejection force twice that of a single disc and an ejection distance six times that of a single disc. This structure is simple, reliable, and improves product quality.
(2) For large die-casting molds, moving mold ejection mechanism uses an integrated moving mold base plate and a pressure-cap type limiting block structure, simplifying mold assembly and disassembly, reducing mold maintenance time, and improving production efficiency.