Die-casting machine is basic equipment for pressure casting of non-ferrous metals and their alloys and has a complex structure. Mold clamping mechanism consists of a mold plate and a machine hinge, and is the key mechanism of die-casting machine. Each die-casting production cycle is accompanied by an opening and closing action of mold closing mechanism. Mold clamping and mold opening of mold closing mechanism mainly uses oil cylinder to push hinge system. Structure is shown in Figure 1. Hinge system rapidly expands thrust of oil cylinder to promote movement of mold plate. Machine hinge system is a typical multi-link mechanism. Unreasonable design of toggle rod (connecting rod) size will lead to insufficient expansion ratio of mold closing mechanism, large impact during mold closing process, low mold life, long mold opening and clamping time, and low die-casting efficiency. To achieve a large clamping force, it is necessary to increase cylinder thrust, which requires high energy consumption.
At present, design of die-casting machine hinges is mainly based on theoretical calculation methods. Since other complex structural parts in mold clamping mechanism and deformation effects of various parts of hinge system itself cannot be taken into account, calculation results have large errors, resulting in an unreasonable hinge structure design. Repeated design and manufacturing lead to long R&D cycles and high costs. Collaborative application of digital modeling, finite element method and kinematic simulation technology provides new solutions for design of die-casting machine hinges. Through numerical simulation technology, not only quantitative design of performance can be achieved, but also optimal design can be achieved, significantly shortening research and development cycle of hinge system and saving costs.
In response to actual problems reported by enterprise that large die-casting machine with a clamping force of 25,000kN has a large oil cylinder, high energy consumption, is accompanied by high abnormal noise during each mold opening and closing process, we carried out defect cause analysis and optimized design of hinge system based on numerical simulation methods    machine hinge analysis based on numerical simulation methods to increase stroke of movable mold base plate, while minimizing stroke of oil cylinder, achieve smooth mold plate movement without pauses and noise during mold opening and closing process.

Establish a 3D assembly model of die-casting machine's mold clamping mechanism with a clamping force of 25000kN. Materials of components are QT500 and No. 45 steel respectively. Physical property parameters of materials are shown in Table 1. Discretize components in assembly model respectively, define contact relationship and friction factor and other parameters between connecting parts, apply corresponding load of 25000kN, and constrain displacement freedom of fixed mold base plate. Established mold clamping mechanism model is shown in Figure 2.

 

Figure 1 Schematic diagram of machine hinge system

1. Mold clamping cylinder 2. Tail plate 3. Hook hinge 4. Cross head 5. Small hinge 6. Long hinge 7. Moving mold base plate

Table 1 Material physical performance parameters

 

Figure 2 Mold clamping mechanism model

 

Figure 3 Displacement cloud diagram of mold clamping mechanism
It can be seen that maximum displacement of mold clamping mechanism is 3.22mm, which is located at the end of Gorin column near tail plate. Ignoring local stress at unit contact boundary, maximum stress of mold clamping mechanism is 118MPa, located inside connection between hook hinge and tail plate, which is far less than yield strength of material (320MPa), so the overall structure of mold clamping mechanism can meet strength design requirements.

 

Figure 4 Stress cloud diagram of mold clamping mechanism

Table 2 Stiffness of mold clamping mechanism components and subsystems
Figure 5 shows hinge system of a 25000kN die-casting machine. It is a typical double-bent toggle structure with 5 hinges. Cross head, small hinge, hook hinge and long hinge are used as toggle levers respectively. Toggle levers are connected by rotating shafts. A, B, C, D and E are hinge points between hook hinge and tail plate, long hinge and hook hinge, movable mold base plate and hook hinge, hook hinge and small hinge, small hinge and cross head in upper half hinge. According to structure and working mechanism of hinge system, its motion geometric relationship is established, as shown in Figure 5.

 

Figure 5 Schematic diagram of motion relationship of machine hinge system

 

Figure 6 Mold clamping force calculation flow chart

 

Figure 7 Kinematics simulation model of machine hinge system

 

Figure 8 Kinematics simulation results of the machine hinge system
Expansion ratio, stroke ratio and speed ratio are all key parameters of hinge system. Goal of optimized design of machine hinge system is to have a large expansion ratio and stroke ratio in clamping state. During expansion of machine hinge system, speed in the middle of stroke is relatively high, speed in initial and final stages of stroke (close to mold clamping state) is relatively small. Machine hinge system is a typical multi-link mechanism. Without changing number of toggle levers, hinged positions of toggle levers and movable seat plate and tail plate, main influencing factors of system performance are length and stiffness of each toggle lever. Therefore, plane coordinate values of the three toggle lever hinge points in Figure 5, B, D, and E, are used as design variables, as shown in Table 3. Length of each toggle lever is controlled by design variables to optimize design of machine hinge system.

Table 3 Design variable table

Table 4 Sensitivity of design variables to design goals

Table 5 Coordinates of toggle joint hinge points

 

Figure 9 Comparison curve of performance indicators of machine hinge system before and after optimization

(1) Deformation of toggle bar has a great impact on performance of machine hinge system. Influence of toggle bar deformation should be considered in design process of machine hinge system.
(2) Calculate stiffness and strength of mold clamping mechanism through finite element method. When mold clamping force is 25000kN, local maximum stress of mold clamping mechanism is 118MPa, and the overall structure meets strength design requirements.
(3) Considering influence of toggle deformation, based on rigid-flexible multi-body dynamics modeling and kinematic simulation calculations, force expansion multiple of hinge system is 21.45, which is close to actual test result of 21.40, and numerical model is reliable.
(4) By optimizing spatial position of toggle joint hinge point of die-casting machine, hinge system can be optimized. After optimized design, force expansion factor of hinge system reaches 24.57, and stroke ratio is 1.08, which are 14.5% and 4.85% higher than before optimization respectively. During mold clamping process, impact on driving cylinder, hinge system, mold plate and mold is smaller.