Abstract: This paper calculates ejection force of core forming part of a cylinder front cover die-casting mold using formulas, analyzes limitations of ejection surface setting, adopts a special secondary demolding design to effectively solve cracking and deformation phenomena of casting during ejection. Design method and application process of a secondary demolding die-casting mold that facilitates casting ejection are detailed, and it is one of effective methods to improve casting quality.
Secondary demolding refers to two demolding processes during ejection of casting. The first demolding refers to partial core pulling in mold opening direction after die casting is completed. The second demolding refers to complete ejection of casting. A secondary demolding mold is also called a mold with pre-core pulling on mold opening side. In die casting production process, when local clamping force of die casting is too large, if casting is forcibly ejected, it is not accidental that casting will have quality problems. Therefore, it is necessary to achieve local early demolding before ejecting casting to reduce impact of such large clamping force on casting. Thus, a core pulling device is provided in mold opening direction. It is worth mentioning that secondary demolding device in this article can also be called a local early demolding device. Function of local early demolding is to remove large clamping force obstacle by core pulling before ejecting casting, so as to make die casting ejection process smooth.

1.1 Traditional ejection countermeasures
According to different ejection elements, previous casting ejection mechanism can be divided into push rod ejection mechanism, push tube ejection mechanism, push plate ejection mechanism, inclined slide ejection mechanism, gear transmission ejection mechanism and multi-element composite ejection mechanism. However, without exception, no demolding treatment is performed before ejection. As shown in Figure 1, moving mold view of cylinder front cover die-casting mold shows a row of four large cores in the middle of mold cavity. These cores will form four-hole joints in casting. Due to their large diameter, deep depth, small slope, and large local clamping force, four-hole joints are prone to cracking and deformation when mold is using a traditional ejection mechanism, resulting in a very low yield and high mass production cost. Analysis shows that among various traditional ejection methods, there are mechanisms that ensure relatively good quality, but none can solve fundamental problem of casting cracking and deformation during ejection.
Cylinder front cover is made of aluminum alloy. Diameter of die-casting mold core forming part is 3 cm, height is 6 cm, and draft angle is 1.5°.
Applying ejection force formula Ft = AP(μcosα-sinα), ejection force at four-hole joints of cylinder front cover die-casting mold shown in Figure 1 is approximately 6.8 t. Local clamping force is too large, so it is necessary to perform local demolding and unloading before ejecting casting.

 

Fig.1 Moving die drawing of cylinder front cover
1.2 Secondary Demolding and Ejection Strategies
Secondary demolding is an improvement on traditional ejection mechanism, and its ejection effect has undergone a qualitative change. In traditional ejection mechanism, ejection area is limited to the area around core, and it is difficult to set push rod. Even if push tube is used for ejection, ejection area is seriously insufficient. Because ejection area needs to be greater than or equal to thrust divided by allowable compressive stress, according to formula At ≥ Ft / [σ] = 6.8 / 0.5 = 13.6 cm2 (allowable compressive stress of aluminum alloy is 50 MPa), such a large ejection area requirement cannot be met by simply setting push tube on annular area around core to eject casting. If a secondary demolding and core-pulling mechanism is used, and core is pulled before casting is ejected, achieving partial early demolding, pushing surface will change from original local annular area to the entire projected area of casting, greatly increasing pushing area. As shown in Figure 2, secondary demolding and core-pulling mechanism is illustrated. Hydraulic cylinder installed above mold plate controls vertical slider 4 to move up and down via connecting rod 3. Vertical slider 4 slides up and down in guide rail 7 groove using its left-side guide step. Guide rail 7 is fixed to moving mold plate. Vertical slider 4 then uses its lower right reverse inclined step to cooperate with T-shaped inclined groove structure inside horizontal slider 5, controlling horizontal slider 5 to slide left and right. Horizontal slider 5 drives core 1 to perform a core-pulling motion, thus achieving partial early demolding of casting and significantly improving casting quality.

 

1. Core 2. Core Plate 3. Connecting Rod 4. Vertical Slider 5. Horizontal Slider 7. Guide Bar 8. Screw
Fig. 2 3D model of core-pulling mechanism

2.1 Working Principle of Secondary Demolding and Core-Pulling Mechanism
Appearance of secondary demolding and core-pulling mechanism is shown in Figure 3.

 

Fig.3 Appearance of secondary stripping mechanism-core-pulling device
Working principle of secondary demolding and core-pulling mechanism is as follows: As shown in Figure 3, upper part of moving mold plate 6 is equipped with a hydraulic cylinder. Piston rod of hydraulic cylinder and connecting rod 3 are connected together by a coupling to form a hinge mechanism, realizing linkage (hydraulic cylinder is installed directly above part 3, omitted in figure); end of connecting rod 3 is placed in T-slot above vertical slider 4 (as shown in Figure 4) and connected to vertical slider 4 to form a hinge mechanism. Since vertical slider 4 is constrained by guide bar 7 and can only move vertically, hydraulic cylinder piston rod, connecting rod 3 and vertical slider 4 move together as a whole, sliding up and down. Additionally, as shown in Figure 2, horizontal slider 5, core plate 2, and core 1 are fixed together by locking screws 8. Since core 1 can only slide horizontally within core hole as a guide post, the entire assembly of horizontal slider 5, core plate 2, and core 1 can only slide horizontally. Furthermore, because vertical slider 4 has guide steps on both sides, it can drive horizontal slider 5. Thus, driven by hydraulic cylinder piston rod, core 1 can be inserted or removed. When hydraulic cylinder piston rod extends, it drives connecting rod 3 and vertical slider 4 downwards, causing horizontal slider 5, core plate 2, and core 1 to move to the right, achieving core insertion action of mold. When hydraulic cylinder piston rod retracts, it drives connecting rod 3 and vertical slider 4 upwards, causing horizontal slider 5, core plate 2, and core 1 to move to the left, achieving core removal action of mold, and core 1 is demolded prematurely. Because this mechanism mainly consists of a vertical slider driving a horizontal slider, movement is reliable and structure is scientifically sound.
2.2 Dimensional Design of Secondary Demolding and Core-Pulling Mechanism
This secondary demolding and core-pulling mechanism was finalized after multiple calculations and motion simulations. Three-dimensional solid structure of main component, slider, is shown in Figure 4. A schematic diagram of the secondary demolding mechanism is shown in Figure 5.

 

Fig.4 3D structure of slider parts
Method for determining dimensions of secondary demolding and core-pulling mechanism:
(1) Both ends of connecting rod 3 are designed with T-shaped heads, making mechanism easy to install;
(2) By slotting moving mold plate 6, connecting rod 3 is easy to install, and mechanism is compact;
(3) When mechanism performs core insertion and reset, it relies on core mounting plate 2 and bottom surface of moving mold for precise reset;
(4) Core 1 is equipped with a point cooling device, and bottom surface of core is sealed with an O-ring and fixed with a core pressure plate 3 to ensure that cooling pipe does not interfere or leak during movement of horizontal slider 5;
(5) A guide bar 7 is set on the left side of vertical slider 4, ensuring reliable movement of mechanism and easy disassembly and assembly of vertical slider 4;
(6) A tortuous oil groove is set on vertical slider 4 to store grease, so that vertical slider 4 drives horizontal slider 5 smoothly;
(7) All parts of mechanism have undergone two-dimensional and three-dimensional data processing to ensure reliable connection at all points.

 

Fig.5 Schematic diagram of secondary stripping mechanism

Although die-casting ejection process is a simple part removal process, improper handling can seriously affect quality of casting. Furthermore, when impact is not easily detected, it can have far-reaching consequences. Therefore, for different die-casting structures, it is necessary to anticipate and implement necessary design measures. For example, setting a pre-core-pulling mechanism in mold opening direction can ensure that casting is partially demolded before ejection, thus improving quality of die-casting. Device described in this article mainly consists of simple sliding block parts. Secondary demolding mechanism shown in Figure 5 is very simple, and mold cost will not increase significantly, but quality of casting during ejection will be greatly improved, which can serve as a reference for peers.