Abstract: Through analysis of engine cylinder products and die-casting process parameters, based on CAE simulation, mold pouring system, cooling and heating system, vacuum system, etc. are designed to achieve high-quality engine cylinder die-casting production, with a product qualification rate of more than 98% and a production cycle of less than 90s. This article can also provide effective ideas for other product development and mold design, promote systematization and standardization of product development and mold design.
Engine cylinder used in fuel vehicles and hybrid cars has a complex die-casting shape and a casting weight of more than 30.2Kg. There are water channels and oil channels inside cylinder, and air tightness requirements are different. Higher design requirements are put forward for cooling water control, temperature field control, and vacuum system of mold, which have always been focus and difficulty of cylinder die casting. In order to solve these problems, this article explains product review and mold development of engine cylinder blocks from following aspects.

Figure 1 is an engine cylinder block used in fuel vehicles and hybrid cars. Its basic information is as follows:

 

Dimensions: 381mm long * 376mm wide * 281mm high, wall thickness as shown in Figure 2.
Wall thickness: Main wall thickness of product is 4.5mm, the thickest wall thickness is 33.9mm, and average wall thickness is 6.73mm, which is suitable for die-casting production.
Net weight of finished product: measured from 3D drawing, net weight of finished product is 17.4Kg
Material: AlSi9Cu3(Fe)
Projected area of finished product: 1300cm²
Projected area of engine cylinder block casting can be calculated from product dimensions, and size of die-casting machine clamping force can be preliminarily analyzed. Main purpose of analyzing wall thickness is to determine filling time and solidification time of product, determine filling speed range, and lay foundation for selection of other process parameters later. Analysis of material can determine its mechanical properties and flow properties, and provide a preliminary basis for temperature of pouring aluminum liquid. By analyzing basic information of engine cylinder casting, product characteristics requirements can be obtained, laying foundation for subsequent mold development.

Cylinder products mainly have following key points.

Yellow part shown in Figure 2 shows oil channel, which has high sealing requirements. Main oil channel is directly die-casted and drilled in the middle. This has high requirements on the surface quality of casting hole, and head eccentricity (caused by aluminum liquid flushing) needs to be strictly controlled. To ensure position of main oil channel, a special inspection fixture is usually used, as shown in Figure 3.

 

 

 

Engine crankshaft surface position, as shown in yellow part in Figure 4, has high requirements for pores and hardness. Attention should be paid to cooling and machining allowance of this surface during die casting. Cylinder casting requires limited porosity, and control of cooling water should be considered in mold design. In the area where porosity is required to be limited, cooling water flow rate should be increased and mold temperature should be controlled at a low level. At the same time, it is possible to consider adding a mesh design to change fluidity and ensure quality requirements of product.

 

As shown in Figure 5, wall thickness at cross oil channel of engine cylinder is thick, which is easy to produce shrinkage holes and lead to leakage. Thickening treatment is performed here to isolate thickened area and do a good job of cooling. Mold design adds forced cooling here to control mold temperature. Wall thickness should be reduced in terms of product improvement to ensure quality of cylinder product.

 

Bottom of water pump joint surface shown in Figure 6 has a maximum wall thickness of 33mm, which is very easy to produce shrinkage holes, which penetrate adjacent oil channels and cause leakage. When designing gate, it is necessary to focus on shrinkage compensation effect at the bottom of this water pump joint surface, and consider forced cooling at the same time. A mesh design can be added here to change flow characteristics of aluminum water.

 

Red part shown in Figure 7 is the area around oil channel hole. Wall thickness is thick and it is easy to have shrinkage holes, which leads to leakage. It is necessary to focus on pressure-boosting shrinkage effect. At the same time, product wall thickness can be changed without affecting oil channel oil pump connection characteristics, which is conducive to improving product quality.

 

As shown in Figure 8, screw hole in red part is very close to oil channel hole, and weight reduction groove is too small, which is easy to stick to mold, damage surface layer, and cause leakage. When designing mold, it is necessary to focus on cooling here, increase cooling water channel, and improve cooling effect.
Above analysis provides a basic design basis for mold parting surface design, pouring system design, cooling system design, vacuum system design, etc.

As shown in Figure 9, use of 4 slider structures is conducive to mold opening and casting ejection. As shown in Figure 10, pouring system of mold adopts a single-side feeding method to reduce mold manufacturing cost. At the same time, according to filling mold flow analysis, this structure can ensure reliability of cylinder filling and is conducive to exhaust.

 

 

Parting surface design of mold is shown in Figure 11. Such a parting surface design is conducive to layout of pouring system and overflow system. Key dimensions of casting are left inside fixed core of fixed mold and moving mold cavity, and not in slider part of mold, to ensure dimensional accuracy of die-casting. At the same time, avoid gate from being eroded and product deformation affecting subsequent machining.

 

As shown in Figures 12 and 13, cylinder adopts a single-sided pouring system scheme and a double vacuum method. According to previous analysis of engine cylinder casting product, use of such a pouring system design is reliable, and vacuum design can effectively reduce gas content of each part of mold cavity.

 

Line diagram shown in Figure 14 is called p-Q2 diagram, which is used to calculate or calculate die-casting process parameters, matching status of die-casting machine and mold. P-Q2 graph uses injection pressure as ordinate and molten metal flow rate as abscissa. Blue curve is called pressure-flow characteristic curve of die-casting machine, and green curve is called mold pressure-flow characteristic curve. Red box is called die-casting process window, which is determined by maximum filling speed and minimum filling speed, maximum filling time and minimum filling time. Setting requirement of die-casting process parameters is that intersection of pressure-flow characteristic curve of die-casting machine and mold pressure-flow characteristic curve is within process window. The closer intersection is to middle position, the better. Intersection in the middle will increase process parameter margin. Following are specific data of cylinder and process parameters calculated by p-Q2. Clamping force of die-casting machine is 3000 tons:
Casting weight: 17.8Kg
Gate weight: 5.8Kg
Casting weight: 27.34Kg
Process yield: 59.25%
Projection area: 2450cm2
Applicable die-casting machine: 3000T
Punch size: φ170mm
Internal gate cross-sectional area: 16.17cm2
Punch cross-sectional area: 226.8cm2
Pressure chamber filling degree: 44.6% (empty stroke 1000mm)
Casting temperature: 660℃
High-speed stroke: 446mm
Filling time: 125ms
First fast speed: 0.3m/s
Second fast speed: 3.5m/s
Second fast switching: 540~560mm

Using process parameters determined in previous section, filling flow and temperature changes were simulated and verified.

 

Observing from right side of gate, molten aluminum enters mold cavity and fills smoothly, indicating that single-side feeding is beneficial for filling of engine cylinder mold, as shown in Figures 15 to 18. Through filling simulation analysis, it is confirmed that filling of molten aluminum in cavity is reliable.

 

Figures 19 to 22 show that the overall filling of casting is relatively smooth, and there is turbulence in crankshaft hole, which has risk of pores. Machining allowance is first reduced to a small value, and cooling is arranged at the same time. During filling process, entrainment of thick area is more obvious, such as crankshaft hole surface and outer cavity of cylinder liner. There is a possibility of pores in yellow dots in Figure 22. A vacuum design should be done here to eliminate or reduce pores and improve quality of casting.

 

Through analysis of solidification simulation process, last solidification position in product is red dot shown in Figures 23 to 25. During molding process, these parts are prone to shrinkage holes and other defects. In process of mold design, attention should be paid to these parts, point cooling and forced cooling should be added to achieve mold thermal balance.

 

Figures 26 and 27 are temperature field distribution observed from different directions. Control of mold temperature field is to ensure thermal balance of mold and gap between inserts under production state, and to control casting defects, dimensional accuracy and deformation of casting during solidification process. Cooling pipelines are added to necessary parts of mold to achieve simultaneous solidification of product.

Arrange cooling water channels according to solidification state in simulation. Focus on cooling in the areas with last solidification and high probability of shrinkage holes. Function of cooling water is to take away heat released by solidification of molten metal. Arrangement of cooling water channels needs to be calculated based on released heat.

 

Water cooling is the most commonly used cooling method for die-casting molds. Figures 28 and 29 are cooling water circuit layout diagrams of mold. Quality of water circuit layout directly affects molding quality and production cycle of product, and affects production efficiency of cylinder body.

 

Point cooling method is used to cool independent deep cavity part and slender core, which can realize delay control and flow control. As shown in Figures 30 and 31, high-pressure point cooling is used in crankshaft and water pump parts to achieve good temperature field control, which can greatly improve product quality and reduce defects. Diverter cone, gate sleeve and punch can be cooled by water supply from workshop cooling system. Dedicated mold temperature control system includes a pure water machine, a water temperature control machine, a mold temperature machine, etc.

 

As shown in Figure 32, there are 4 cylinder liner inserts installed in engine cylinder body, and temperature control requirements for mold are extremely high. Temperature change of die-casting mold will cause size change. Size of cylinder liner insert is fixed. Insert should be installed on mold, and it should be ensured that temperature change of these parts is not large.
Production of engine cylinder blocks requires design of cooling water circuits and control of cooling water flow, and use of a separate mold temperature control system. Mold core is arranged for point cooling, and cooling water is controlled by intermittent cooling.

 

As shown in Figure 33, engine cylinder block mold adopts a four-slide structure, and all four slides are opened after mold is opened. After mold is opened, product has a small clamping force on the mold, and it is reasonable to arrange ejection at cylinder sleeve.

 

As shown in Figure 34, cylinder block mold adopts a double vacuuming valve design, and overflow channel can use a double washboard type. Vacuuming area and vacuuming volume of vacuuming valve should be considered mainly, and volume of vacuum tank, volume of pipeline from vacuum tank to mold, and capacity of vacuum pump should also be considered. On basis of simulation, a vacuum channel is opened in part that is easy to roll up, and slag bag is connected.

According to characteristics of cylinder block product and technical characteristics of die-casting process, the whole process of engine cylinder block mold development is studied, and necessary conditions for each part of mold development process are given, which can provide a development process for cylinder block mold design. First, influence of cylinder structure on die-casting process parameters should be studied, then computational simulation should be used to provide favorable guarantees for the later pouring system design, cooling system design, vacuum system design, etc. According to given development process, reliability of mold design can be improved, quality of products can be greatly improved, and production efficiency can be improved.