Taking aluminum alloy clutch housing die casting as an example, this paper introduces its structural characteristics and problems in actual production. In view of problem of casting deformation and micro-cracks caused by excessive local clamping force of fixed mold, a disc spring ejection mechanism was added to fixed mold, and selection of disc spring was designed, calculated and verified; in view of problem of moving mold ejection mechanism affecting efficiency in mold maintenance, structure of moving mold ejection mechanism was optimized, an integrated moving mold base plate and a pressure cap type limit block structure were adopted to facilitate mold disassembly and replacement of vulnerable parts. Through optimization design of clutch housing die casting mold ejection mechanism, production verification of multiple sets of replica molds has greatly improved casting quality and production efficiency, while reducing mold manufacturing cost.
With rapid development of domestic industrial production, China’s mold industry has developed rapidly. At present, the total output of die casting molds is second only to United States, making it a veritable die casting powerhouse. Die castings are widely used in automobiles, motorcycles, mechanical equipment and electronic products. Larger parts such as automotive 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, analyzing its structural characteristics and parting surface selection. Considering product defects and maintenance issues encountered during production, ejection mechanism of clutch housing die-casting mold is structurally optimized, providing a reference for development of similar die-cast parts.

Automotive clutch is an assembly component 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 perspective of casting structure, housing casting consists of two cavities in different directions on both sides. One side has a cavity with a diameter of ϕ276.5 mm*122 mm connected to engine flywheel assembly, and the other side has a stepped cavity with a maximum diameter of ϕ189 mm connected to transmission housing. There are thick-walled areas formed by bolt mounting bosses on both sides of casting. Casting material is AlSi9Cu3(Fe), and weight is 6.1 kg. After casting is processed, a sealing test is required. Under a pressure of 0.1 MPa, leakage is less than 20 mL.

 

Fig. 1 3D morphologies of clutch housing

Die casting mold is mainly composed of two parts: moving mold and fixed mold. Contact surface between moving mold and fixed mold is parting surface. Parting surface is generally selected on section with the largest outer contour of casting to ensure that casting can be smoothly separated from mold cavity after mold is opened; at the same time, it is necessary to ensure that die casting remains on moving mold side after mold is opened, casting can be smoothly and stably pushed out by ejection mechanism of die casting machine, thereby simplifying mold structure. Based on shape characteristics of clutch housing, parting surface is selected at maximum end face of both cavities, transitioning with a lateral slope to form a broken parting surface. Cavity surfaces (or core surfaces) with consistent draft angles are formed in one cavity. To ensure die-cast part 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 engine mating side of clutch housing is set in moving mold, thus determining main parting surface of casting. Based on local shape changes of casting, main parting surface is refined locally to determine clutch housing parting surface. Figure 2 shows parting surface of clutch housing.

 

Fig.2 Parting surface of 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 blanks accounts for over 60% of scrapped parts. Deformation occurs at mating surface of clutch housing and transmission housing, with lower part warping up by 1-2 mm. In mass production, defects caused by deformation are compensated by increasing machining allowance of bottom surface and side holes of blank, resulting in significant waste. Microcracks occur at lower bolt bosses, without obvious traces, with a low defect rate, and occur in mid-to-late stages of production. Analysis shows that both types of casting defects are caused by different positions and directions of clamping forces due to structure of casting. Force situation of mold at the moment of mold opening is shown in Figure 4. 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 a punch pushing force below fixed mold, due to low strength at ingate, punch and pushing force at the moment of mold opening separate transverse runner from casting at ingate, and cannot assist fixed mold in pushing out. In the early stage 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 the surface of mold cavity and mold surface becomes rough, local clamping force of fixed mold increases, causing micro-cracks to appear in some castings.

 

Fig. 3 Defect location of clutch housing

 

Fig. 4 Force diagram of clutch housing during mold opening
For castings with special structures like clutch housings, even if casting will detach from fixed mold and attach to moving mold after mold opening, excessive local clamping force of fixed mold has an adverse effect on casting. Therefore, it is considered to add an ejection mechanism at fixed mold. Ejection force of fixed mold ejection mechanism can overcome clamping force of fixed mold at the same time as mold opening, forcibly separating casting from fixed mold, and avoiding deformation of casting due to uneven force.

Fixed mold ejection mechanism of die casting molds commonly uses disc spring ejection and hydraulic ejection. Hydraulic cylinder ejection is often used in situations where ejection area is small and ejection force is concentrated. However, ejector rods of fixed mold ejection of clutch housing are distributed in lower half of housing cavity. A total of 6 ejector rods are set at bolt boss on circumference of cavity, and ejection range is large. Therefore, disc spring ejection is used. Three-dimensional schematic diagram of fixed mold ejection mechanism is shown in Figure 5, and structural composition is shown in Figure 6. Working process is as follows: When mold closes, moving mold insert plane contacts reset rod of mechanism. Under action of mold closing force, disc spring is compressed. Reset rod drives push rod fixing plate and push rod to reset. After mold closes, reset rod moves to be flush with parting surface. At this time, limit post installed on push plate is flush with die-casting machine's fixed platen. When mold opens, disc spring, which is in a compressed state, is released. Under pushing force of disc spring, push plate and push rod fixing plate move, pushing push rod and reset rod out above parting surface, thus realizing ejection of casting from fixed mold at the same time as mold opens. 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 rod is designed at four corners of push plate, which also serves as an ejection guide.

 

Fig. 5 3D diagram of fixed mold ejection mechanism
1. Push rod 2. Reset rod 3. Disc spring 4. Disc spring support post 5. Limiting post 6. Push plate 7. Push rod fixing plate

 

Fig. 6 Structural 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

Ejection force is divided into initial ejection force and subsequent ejection force. Initial ejection force mainly overcomes clamping resistance of casting, while subsequent ejection force mainly overcomes motion resistance when casting continues to be ejected after leaving core. When designing mold ejection, only initial ejection force needs to be calculated. Formula for estimating ejection force is:

In formula, A is perimeter of core forming part enclosed by casting, mm; L is length of core enclosed by casting, mm; P is compressive stress (enclosure force per unit area), perpendicular to core surface, and P is generally taken as 10~12 MPa for aluminum alloys; μ is friction coefficient of die-casting alloy on core (generally taken as 0.20~0.25); α is draft angle.
As can be seen from Figure 6b, mold insert has a circular step transition shape in central forming area, mainly composed of 6 bosses. Ejection force of each part is calculated separately, and calculation results are shown in Table 1. The total ejection force of mold core ∑F_push = 42 393 N.
Tab.1 Ejection force of fixed mold core

 

(1) Selection of Disc Springs
Disc springs have characteristics of small size, large load, and convenient combination. Different loads and strokes can be obtained through single-piece pairing or multi-piece stacking combinations according to different requirements such as usage space, load, and stroke. Using disc springs in fixed mold ejection mechanism of this mold can effectively reduce installation space and ensure strength of fixed mold plate while ensuring sufficient ejection force.
Based on fixed mold ejection structure of this mold, two locations for installing disc springs are designed, located at longitudinal, horizontal, and transverse center-symmetrical positions of fixed mold ejection mechanism, as shown in Figure 5. According to 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, six sets of series combinations are selected. Parameters of selected disc springs are shown in Table 2. A schematic diagram of disc spring structure is shown in Figure 7.
Tab.2 Parameters for disc spring

 

Fig.7 Schematic diagram of disc spring
(2) Push-out force verification
A disc spring with a diameter of ϕ50 mm and a thickness of 3 mm is selected. When its compression is 0.75 h0, load is 11,976 N. Two disc springs are used in parallel. Mechanism is designed with two disc spring push-out points. The total disc spring push-out force F = 47,904 N, which is greater than push-out force ∑F_push required for fixed mold core.
(3) Push-out distance verification
Under required operating conditions, compression of a single disc spring is f = 0.83 mm. This design uses 2 disc springs in parallel and 6 sets in series, as shown in Figure 6c. Push-out force is twice that of a single spring, and push-out distance is six times that of a single spring. Push-out distance is 4.98 mm, which meets requirement of maximum push-out distance.

Moving mold push-out mechanism mainly consists of push-out elements, reset elements, guide elements, limit elements, and structural elements. Structural composition of moving mold push-out mechanism is usually 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 a limiting mechanism for ejection. Both structures share a common problem: when replacing easily worn parts such as push rods or moving mold cooling water, moving mold base plate must be disassembled. This is especially problematic for large molds with large volume and weight, and large connecting bolts, making disassembly of moving mold base plate time-consuming and labor-intensive, severely impacting production efficiency. Therefore, moving mold ejection mechanism is optimized during mold replication.

 

Fig. 8 Structure diagram of moving mold ejection mechanism
1. Moving mold base plate 2. Ejector plate guide sleeve 3. Ejector plate guide post 4. Ejector plate 5. Ejector rod fixing plate 6. Limit pin 7. Ejector rod 8. Reset rod 9. Support plate

Optimized structure 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 moving mold sleeve plate's inner cavity; locating 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 post and positioning pin 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. When repairing, maintaining, or replacing vulnerable parts, simply remove four small limit blocks to take out ejection assembly with push rod, which is convenient, safe, and reliable. One-piece structure of moving mold base plate is shown in Figure 9c, with dimensions of 1400 mm * 1300 mm * 270 mm, made of No. 50 steel, and a hardness (HB) of 260–300 after heat treatment.

 

Fig. 9 Structure diagram of clutch housing ejection mechanism
1. Moving mold base plate 2. Limiting block 3. Push plate 4. Push rod fixing plate 5. Push plate guide sleeve 6. Push plate guide post 7. Positioning pin 8. Cooling nozzle 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 mold replication. After production verification with more than ten molds, quality of castings has been significantly improved, eliminating deformation and cracks caused by excessive local clamping force in static mold. Simultaneously, mold maintenance time has been shortened, and production efficiency has been increased. Mold uses a disc spring ejection mechanism in fixed mold. Because disc springs are affected by alternating stress and mold temperature during use, they 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.