abstract
Using formulas to calculate the pushing force on the core forming part of the cylinder front cover die-casting model, and analyzing the limitations of the pushing surface setting, a special secondary demolding design is adopted to effectively solve the cracking and deformation phenomena of the casting during pushing. This article provides a detailed introduction to the design method and application process of secondary demolding die casting molds for casting, which is one of the effective methods to improve the quality of castings.

Secondary demolding refers to two demolding processes during the casting process. The first demolding refers to the partial core pulling in the mold opening direction after die casting, while the second demolding refers to the complete ejection of the casting. The secondary demolding mold is also known as a mold with pre core pulling on the opening side. In the process of die-casting production, when the local clamping force of the die-casting part is too large, if the casting is forcibly pushed out, the quality problem of the casting is not accidental. Therefore, it is necessary to achieve local advance demolding before pushing out the casting to reduce the impact of this large clamping force on the casting. In this way, a core pulling device is equipped in the direction of mold opening. It is worth mentioning that the secondary demolding device in this article can also be referred to as a partial advance demolding device. The function of partial advance demolding is to remove larger clamping force obstacles through core pulling before pushing out the casting, so as to ensure a smooth process of pushing out the die casting.

one
Analysis of the Launch Process of Die Casting Production
1.1 Traditional Launch Strategies
According to different ejection components, the previous casting ejection mechanism forms can be divided into push rod ejection mechanism, push tube ejection mechanism, push piece plate ejection mechanism, inclined slider ejection mechanism, gear transmission ejection mechanism, and multi-element composite ejection mechanism, but without exception, they have not undergone demolding treatment before ejection. The dynamic mold view of the cylinder front cover die-casting mold shown in Figure 1 shows a row of four large cores in the middle of the mold cavity, which will form the four connected holes of the casting. Due to the large diameter, deep depth, small slope, and high local clamping force, the cracking and deformation phenomenon of the four connected holes is more severe when the mold is pushed out using traditional pushing mechanisms, resulting in extremely low yield and high production costs. Through analysis, it can be seen that among the various traditional casting methods mentioned above, there are mechanisms that ensure relatively good quality and effectiveness, but none of them can solve the fundamental problem of casting cracking and deformation during casting casting.
It is known that the material of the cylinder front cover is aluminum alloy, and the diameter of the die cast model core forming part is 3 cm, the height is 6 cm, and the mold inclination is 1.5 °.
Using the formula of ejection force F t=AP (μ cos α - sin α), calculate the ejection force at the four connected holes of the cylinder front cover die-casting mold shown in Figure 1 to be about 6.8 t. The local clamping force is too large, so it is necessary to perform local demolding and unloading before ejecting the casting.

1.2 Countermeasures for secondary demolding
Secondary demolding is an improvement on the traditional ejection mechanism, and its ejection effect has undergone a qualitative change. Traditional push out mechanisms only limit the pushing area around the core, making it difficult to set up push rods. Even when using push tubes to push out, the pushing area is severely insufficient. Because the pushing area needs to be greater than or equal to the pushing force divided by the allowable compressive stress, according to the formula A t ≥ F t/[σ]=6.8/0.5=13.6 c ㎡ (the allowable compressive stress of aluminum alloy is 50 MPa), such a large pushing area requirement cannot be met by simply setting a pushing tube on the annular area around the core to push out the casting. If a secondary demolding and core pulling mechanism is used, and the core is pulled out before the casting is pushed out, to achieve local early demolding, the pushing surface will change from the original local annular area to the entire projection area of the casting, and the pushing surface will be greatly improved. As shown in Figure 2, the secondary demolding and core pulling mechanism diagram, the hydraulic cylinder installed above the mold sleeve plate controls the vertical slider 4 to move up and down through the connecting rod 3. The vertical slider 4 uses its own left sliding step to slide up and down in the sliding groove of the guide bar 7, which is fixed on the moving mold sleeve plate. The vertical slider 4 then cooperates with the T-shaped inclined groove structure inside the horizontal slider 5 through its own reverse inclined step at the bottom right and controls the horizontal slider 5 to slide left and right. The horizontal sliding block 5 drives the core 1 to perform core pulling and inserting movements, thus achieving partial premature demolding of the casting and significantly improving the quality of the casting.

two
Design of secondary demolding core pulling mechanism
2.1 Working principle of secondary demolding core pulling mechanism
The appearance of the secondary demolding core pulling mechanism is shown in Figure 3.

The working principle of the secondary demolding core pulling mechanism is as shown in the appearance diagram of the secondary demolding core pulling mechanism in Figure 3. A hydraulic cylinder is installed on the upper part of the dynamic mold sleeve plate 6, and the piston rod of the hydraulic cylinder is connected to the connecting rod 3 through a coupling to form a hinge mechanism, achieving linkage (the hydraulic cylinder is installed directly above part 3, omitted in the figure); The end of connecting rod 3 is placed in the T-shaped groove above the vertical slider 4 (as shown in Figure 4) and connected to the vertical slider 4 to form a hinge mechanism. Due to the constraint of the guide bar 7, the vertical slider 4 can only move in the vertical direction. Therefore, the hydraulic cylinder piston rod is linked with the connecting rod 3 and the vertical slider 4 to slide up and down as a whole; In addition, as shown in Figure 2, the transverse slider 5, the core pressing plate 2, and the core 1 are fixed together by locking screws 8. Since the core 1 can only slide in the form of a guide column in the core hole, the transverse slider 5, the core pressing plate 2, and the core 1 can only slide horizontally as a whole; Due to the guiding steps on both sides of the vertical slider 4, the vertical slider 4 can drive the horizontal slider 5 to move, so that the core 1 can be pulled and inserted under the drive of the oil cylinder piston rod. When the oil cylinder piston rod extends, it drives the connecting rod 3 and the vertical slider 4 to move downward, and the horizontal slider 5, core pressing plate 2, and core 1 move to the right as a whole, achieving the core insertion action of the mold. When the piston rod of the oil cylinder moves backwards towards the oil cylinder, it drives the vertical slider 4 of the connecting rod 3 to move upwards, and the horizontal slider 5, the core pressing plate 2, and the core 1 move together to the left, achieving the core pulling action of the mold and releasing the core 1 in advance. Due to the fact that the mechanism is mainly composed of vertical sliders driving horizontal sliders, the motion is reliable and the structure is scientifically reasonable.

2.2 Dimension design of secondary demolding core pulling mechanism
The secondary demolding and core pulling mechanism is the final solution determined after multiple calculations and motion simulations. The three-dimensional solid structure of the main component slider in the mechanism is shown in Figure 4. The schematic diagram of the secondary demolding mechanism is shown in Figure 5.

Method for determining the size of the secondary demolding core pulling mechanism:
(1) Both ends of connecting rod 3 are designed with T-shaped head structures, making the installation of the mechanism convenient;
(2) By slotting on the dynamic mold sleeve plate 6, the installation of the connecting rod 3 is convenient and the mechanism is compact;
(3) When the mechanism performs core insertion reset, it relies on the core mounting plate 2 and the bottom limit of the moving mold to achieve precise reset;
(4) Core 1 is equipped with a point cooling device inside, and the bottom of the core is sealed with an O-ring and fixed with a core pressure plate 3 to ensure that the cooling pipe does not interfere or leak during the movement of the horizontal sliding block 5;
(5) A guide bar 7 is installed on the left side of the vertical slider 4, ensuring reliable mechanism movement and easy disassembly of the vertical slider 4;
(6) A tortuous oil groove is set on the vertical slider 4 to store lubricating grease, so that the vertical slider 4 can drive the horizontal slider 5 to move smoothly;
(7) All parts of the mechanism have undergone two-dimensional and three-dimensional data processing to ensure reliable connection at all locations.
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conclusion
Although the process of die-casting casting is a simple part taking process, if not handled properly, it will seriously affect the quality of the casting, and even have endless consequences when the impact is not easily discovered. Therefore, for different die-casting structures, it is necessary to anticipate in advance and carry out necessary design treatments, such as setting up a pre core pulling mechanism in the mold opening direction to ensure that local demolding of the casting is achieved before casting, and the quality of the die-casting may be better. The device described in this article is mainly composed of simple slider components. The secondary demolding mechanism shown in Figure 5 is very simple, and the mold cost will not increase significantly. However, the quality of the casting during the ejection process will be greatly improved, which can be used as a reference for peers.

author
Zhang Yuxi
Ningbo University of Finance and Economics
This article is from: Casting Magazine