With rapid development of automobile and communication technology, especially under guidance of energy conservation, emission reduction and new energy technology, lightweight magnesium and aluminum alloy die castings account for an increasing share of the entire automobile. In recent years, output of large-scale die-casting parts in China has shown a sustained and rapid growth trend.
There are a large number of die-casting equipment manufacturing companies in our country, but company's new product development process is mainly based on empirical design and analogy design, and products are mainly die-casting machines with a clamping force less than 25,000kN. According to traditional design methods, the overall mass of ultra-large die-casting machine is greater than 300t, single mass of some key components will be greater than 60t, exceeding load-bearing capacity of heavy-duty gantry machining centers commonly used at home and abroad. As a result, ultra-large die-casting machines with a clamping force greater than 35,000kN required for die-casting of large structural parts such as automobile engine blocks, gearbox casings, new energy vehicle battery packs, and chassis are dependent on imports, severely restricting development of domestic die-casting equipment manufacturing industry. Therefore, there is an urgent need to study innovative design methods such as optimized design and lightweight design to achieve independent development of high-performance ultra-large die-casting machines.
Ultra-large die-casting machine consists of a mold clamping mechanism, an injection mechanism, a machine base and a control system. Function of machine base is to fix and support mold clamping mechanism and injection mechanism of die-casting machine to ensure efficient implementation of die-casting process. It is a key component of super-large die-casting machine. This project aims at practical problem that machine base of a super-large die-casting machine with a clamping force of 35000kN is large in mass and high in cost compared with similar foreign products, carries out structural optimization design of base, conducts application research on joint optimization design technology of base with variable density topology optimization algorithm and size optimization algorithm. Finite element method was used to calculate stiffness of machine base under four working conditions, then stiffness was used as a constraint to optimize machine base topology. Based on unit relative density cloud diagram in optimization results, 18 steel plates were reduced from initial design; on this basis, a size optimization model of machine base was established, based on unit thickness distribution cloud diagram in optimization results, final optimal design plan for machine base was determined. A comparative study of optimization plan and initial design under same working conditions is conducted to provide reference for development of die-casting machines.
Establish geometric model of machine base based on DM3500 die-casting machine design drawing, see Figure 1. Since machine base is welded by 206 steel plates, thickness of steel plates ranges from 4 to 100mm, and there are 12 types of steel plates with different thicknesses. According to thickness and function of steel plates, all steel plates are grouped and pre-processed. Base model is discretized using plate and shell elements, with number of units being 123,407. Grid model is shown in Figure 2. Base material is Q235 steel, and its physical performance parameters are shown in Table 1.

 

Figure 1 Geometric model of DM3500 die-casting machine base

 

Figure 2 Finite element mesh model of DM3500 die-casting machine base

Table 1 Physical property parameters of Q235 steel
Load of machine base mainly comes from die-casting machine peripheral auxiliary equipment such as clamping mechanism, injection mechanism, hydraulic oil, spray robot, and soup feeder. Calculate mass of all above components and systems. Mass is rounded upward in tons. Load borne by machine base is shown in Table 2. Due to different application requirements of die-casting machine users, positions of tail plate and movable seat plate on machine base will be adjusted according to size of mold. Therefore, during numerical simulation process, it is necessary to fully consider impact of loads on machine base at various extreme positions that may exist in machine base. Four load conditions were considered at the same time, namely: (1) ordinary mold clamping condition; (2) working condition of movable seat plate close to fixed seat plate; (3) working condition of movable seat plate close to tail plate; (4) ) working condition of tail plate and movable seat plate are far away from fixed seat plate. Above four working condition loads cover the entire guide rail of machine base, which can effectively avoid situation where stiffness of local load-bearing mold plate of machine base does not meet design requirements due to large deformation during installation or use.

Table 2 Machine base load table

 

(a) Displacement cloud diagram

 

(b) Stress cloud diagram
Figure 3 Numerical simulation results of machine base under normal working conditions

Table 3 Summary of numerical simulation results under different working conditions

 

Figure 4 Relative density cloud diagram of machine base topology optimization unit

 

Figure 5 Geometric model after topology optimization of machine base

 

(a) Displacement cloud diagram

 

(b) Stress cloud diagram
Figure 6 Numerical simulation results after machine base topology optimization

Table 4 Numerical simulation results under different working conditions after topology optimization
After topology optimization, structural layout of machine base has been determined, but stiffness still has room for further optimization. Using size optimization method, thickness of steel plates grouped by different functions and thicknesses is defined as design variable. Upper and lower limits of steel plate thickness change are 2 times and 1/2 of original thickness respectively. Constraint condition is still that maximum displacement of machine base guide rail under four working conditions is all ≤0.1mm, and optimization goal is to minimize the total mass of machine base. Size optimization design model was numerically simulated based on Optistruct solver. Thickness distribution cloud diagram of machine base after 12 iterative calculations and before optimization is shown in Figure 7.

 

(a) Unit thickness cloud chart before optimization

 

(b) Unit thickness cloud chart after optimization
Figure 7 Comparison of unit thickness clouds before and after frame size optimization

 

(a) Displacement cloud diagram

 

(b) Stress cloud diagram
Figure 8 Numerical simulation results after optimization of machine base size

Table 5 Summary of numerical simulation results under different working conditions after size optimization

(1) Variable density topology optimization method is suitable for conceptual design stage of ultra-large die-casting machine base to determine spatial structure layout of machine base. Number of steel plates in DM3500 die-casting machine base is reduced by 18 pieces, mass is reduced by 0.47t, and weight is reduced by 2.5%. .
(2) Size optimization method is suitable for detailed design stage of ultra-large die-casting machine frame. By changing thickness of steel plate while ensuring the overall rigidity and strength of machine base, mass of DM3500 die-casting machine base can be reduced by 5.98t, and weight can be reduced by 31.6%.
(3) Combined application of SIMP-based variable density topology optimization method and size optimization method is an effective method for lightweight design of ultra-large die-casting machine bases.