This paper introduces structural characteristics and technical requirements of an automobile engine crankcase. Based on its structural characteristics and quality requirements, when designing die-casting mold structure, an inlay-type structural design is adopted for special-shaped parts with deep mold cavities and complex structures in local areas. At the same time, failure mold analysis of special-shaped core inlay structure was carried out, design calculation and verification were carried out on local cooling capacity of mold, push rod size and instability, bolt strength, etc. to ensure reliability of mold inlay structure design.
Foreword: Die-casting mold is an important process equipment for die-casting production. Its quality directly affects shape, size, and accuracy of die-casting parts, especially structural design of mold. It can also affect mold cost and production efficiency.
Design of die-casting mold forming parts is core part of die-casting mold design. Its structure is mainly determined based on shape, structural characteristics and processing technology of casting. For simple-shaped castings, a monolithic structure cavity can be used. Mold structure is simple, strength and rigidity are high, surface of die casting is smooth and there are no traces of inlays. However, die-casting parts are currently developing in the direction of multi-function, integration, and large size. Therefore, die-casting mold cavity often adopts a mosaic structure according to casting structure. When designing mosaic structure, in addition to meeting quality requirements of casting, it is also necessary to ensure that mosaic structure mold has good strength, rigidity, toughness and surface quality. This article takes automobile engine crankcase as an example to introduce design method of inlay structure of special-shaped cores. Through reasonable design of inlay structure, quality of castings can be ensured and life of mold can be extended.

Figure 1 shows crankcase body of a certain automobile engine. Material is ADC12. Outline dimensions of casting are 442 mm * 358 mm * 173 mm, and weight mass is about 4.5 kg. This product has a complex structure. There are lateral depressions and side holes in three directions on the outside of box. There are multiple bolt mounting bosses on flange surface. Inside of box has a multi-layered shape with different depths, and there is a hollow part in the middle. Especially in area A shown in the figure, there are two deep triangular cavities. These two deep cavities are detrimental to manufacturing and maintenance of molds, forming of castings. Castings are prone to quality problems such as sticking and scratches inside triangle, which can seriously lead to leakage and mold damage.

 

Figure 1 3D view of crankcase
Sealing test of crankcase products requires that cavity pressure is 100 kPa and leakage is less than 20 m³/min, which requires casting cavity to have good appearance quality and internal quality.

Based on above structural analysis and technical requirements of crankcase, when designing die-casting mold insert structure, focus was placed on structural design of two special-shaped deep cavities in area A shown in figure. In view of characteristics of crankcase body where cavity is deep, structure is complex, heat is concentrated during die-casting, and it is easy to be damaged, an inlay-type structural design is adopted. Structural form is shown in Figure 2.

 

(a) Assembly drawing of moveable mold cavity insert (b) 3D drawing of special-shaped insert
Part 1: Moving mold insert; Part 2: Special-shaped core; Part 3: Push rod
Figure 2 Inlay structure of special-shaped core
Forming point of special-shaped core starts from main parting surface of mold, with a maximum outer contour of approximately 45 mm * 45 mm, a height of 154 mm, and a demoulding angle of 2.5°. Since outer contour is irregular in shape, there are two push rod holes on the side that need to be designed. Push rod holes occupy a small semicircle on the edge of core. In order to ensure assembly accuracy and accuracy of push rod holes, special-shaped core of this inlay needs to be processed by wire cutting, and connection method is fixed by bolts. Since core is wrapped by surrounding metal during die-casting and generates a lot of heat, a cooling water channel needs to be designed in core to cool core. Structural diagram of inlaid special-shaped core is shown in Figure 3.

 

Figure 3 Special-shaped core
Advantages of using an inlaid structure core for crankcase are as follows.
(1) Good manufacturing process. Due to special-shaped deep cavity, tool cannot enter, so it can only be processed by EDM, and polishing is difficult. Inlaid structure can solve processing technology problems, facilitate processing, and improve surface quality of mold.
(2) It is conducive to design of independent cooling water. For locations with high local temperatures in mold, single-point independent cooling water with a nozzle structure is used to independently control flow and pressure of cooling water to meet cooling requirements.
(3) Conducive to maintenance, repair and replacement of wearing parts.
(4) Use of inlay gaps is beneficial to casting exhaust.

After it is determined that inlay structure is used locally, it is necessary to conduct a failure mode analysis on inlay structure based on structural characteristics of special-shaped core and combined with production experience, so that measures can be taken in mold design to eliminate hidden dangers. Crankcase body adopts a mosaic structure core. Due to small size and complex structure of special-shaped core, a cooling water channel must be set up to meet quality requirements of casting. In order to realize function of mold, bolt connection and push rod push-out are designed, so main failure modes that may exist are: ① Due to limitation of design space and insufficient capacity of cooling water channel, core temperature is too high, castings produce surface quality such as mold sticking and scratches; ② Local holding force is too large, castings stick to mold and push rod breaks; ③ Fatigue failure and fracture of connecting bolts, etc.
Based on above analysis of mold failure, it is necessary to perform relevant calculations and checks on cooling capacity of core, push rod size, and bolt strength to avoid various failures during use. At the same time, problems existing in trial production are analyzed and design is optimized to provide a process reference for mass production of casting.

In die-casting molds, design of cooling system is conducive to controlling temperature of mold so that internal heat reaches a state of dynamic balance, thereby ensuring product quality. During die-casting production process, core is wrapped in high-temperature molten metal for a long time, and temperature rises. Due to affinity of aluminum and iron at high temperatures, a pair of diffusion couples are formed, which easily causes mold sticking, thus affecting quality of casting and life of mold. Therefore, cooling water design of core is one of key elements of core structural design. According to structural shape of special-shaped core, cooling water channel is designed at the center of special-shaped core to form a step hole, with a front end diameter of 7 mm and a minimum distance of 8 mm from forming surface to ensure rapid cooling of forming part. Diameter at rear end is 10 mm to increase cooling capacity.

Heat transferred into mold during die casting process can be estimated using formula (1):
Q in =mqn/3600 (1)
In formula: Qin is heat transferred into mold core (kW), m is weight of die-cast metal wrapping special-shaped core (kg), q is heat emitted by die-cast metal from pouring to pushing out (kJ/kg), see Table 1, n is number of die-casting pieces per hour. For local area of crankcase special-shaped core, m is 0.16kg, q is 888 kJ/kg, n is 30, and substituting into equation (1), we get Q=1.18kW.

Table 1 Heat dissipated from die-casting alloy from pouring to pushing out

Q out =Q′L (2)
In formula: Q is heat taken out by cooling water channel (kW); Q′ is heat taken away from mold by cooling water channel per unit length (kW/cm), see Table 2; L is length of cooling water channel (cm), as shown in figure 3 shown. Substituting numerical values into equation (2), we get Q=0.052×8+0, 041×9=1.31 (kW).

Table 2 Heat absorbed by cooling water channel per unit length from mold
After calculation, heat Q out brought out by cooling water channel in local area of special-shaped core is greater than heat Q in transferred into mold core, and cooling capacity meets requirements. During trial production process, pure water with a pressure of 800 kPa was used to cool core. Cooling water flow was controlled by adjusting valve opening of flow valve in core cooling water return pipeline. It was finally determined that water flow rate of special-shaped core was between 0.8 and 0.9. L/min, temperature of core is controlled at 160~220℃ when mold is opened to meet process requirements.

When designing a die-casting mold, ejection force of the entire casting is usually estimated to determine diameter and number of push rods to be used. For large die-casting molds, in order to reduce failure of wearing parts during use of mold and improve the overall efficiency of equipment, when mold space allows, try to use a large-diameter push rod. This product has a special structure. There is a large local tightening force at special-shaped core and design space of push rod is limited. Therefore, it is necessary to calculate local push-out force of special-shaped core to determine whether diameter and number of push rods around special-shaped core meet requirements.

Stress situation when die casting is pushed out is shown in Figure 4. Calculation formula for push out force is:
Ft =F resistance cosα-Fbsinα=AP (μcosα-sinα) (3)

 

Figure 4: Force diagram of die-casting part when it is pushed out
In formula: Ft is pushing force required when die casting is demoulded (N); Fb is tightness of die casting to mold parts (N); K is safety factor, generally 1.2; P is extrusion stress, perpendicular to core surface. For aluminum alloys, generally P=10~12 MPa. A is lateral area of tight core of die-casting part. Lateral area of special-shaped core of crankcase body is 18788 m㎡; μ is friction coefficient of die-casting alloy against mold cavity, which is 0.2~0.25; α is draft angle, and draft angle of special-shaped inserts is 2.5°. Substituting above data into equation (3), local push-out force of special-shaped core Ft=3878 N.

When push rod pushing mechanism pushes out, top surface of push rod bears pushing force. Formula for calculating cross-sectional area of push rod:
A≥KFt/（nб）　（4）
In formula: A is cross-sectional area of push rod (m㎡), K is safety factor, which is 1.25; Ft is pushing force (N), n is number of push rods, б is allowable stress of die-casting alloy (MPa). There are two push rod positions that can be set for special-shaped core of crankcase body according to its shape, and push rod hole needs to be machined with dynamic mold cavity, so n=2; die-casting alloy is aluminum-silicon alloy, and its allowable stress б= 50MPa. Substituting above values into equation (4), we get push rod diameter d = 7.86 mm. After rounding data, we design two push rods with a diameter of 8 mm.

For slender push rods, in order to ensure stability of push rod during operation, a single push rod needs to be checked for instability. If stability does not meet requirements, number of push rods needs to be increased or diameter of push rod needs to be increased. Stability check formula of push rod is as follows:

 

In formula: KW is stability safety factor, which is 1.5~3 for steel; eta is stability coefficient, eta=20.19; Ft is single push force (N); E is elastic modulus (N/c㎡), elastic modulus of steel E=2×107 N/c㎡; J is bending cross-sectional area of push rod (c㎡), and bending cross-sectional area of push rod of Φ8 mm is J=πd2 /64=0.049 c㎡; L is the total length of push rod (cm ), length of push rod at special-shaped core of crankcase body is L=52.8 cm. Substituting above values into equation (5), we get KW =3.66, which is greater than stability safety multiple of steel and meets stability requirements of push rod operation.
Mold was manufactured according to this design plan. During small batch trial production, push rods on both sides of special-shaped core were stuck and push out was blocked. After disassembly, push rods were found to be deformed. Further analysis shows that one side of special-shaped core is washed and impacted by inner runner, causing adverse effects on core and surrounding push rods. In addition to being affected by structure and shape of casting, push-out force is also affected by factors such as surface roughness and temperature of core, die-casting process parameters, and accuracy of push rod hole. Under influence of various adverse factors, safety factor needs to be increased. After unloading mold, disassemble mold, assemble special-shaped core and movable mold cavity, process push rod holes on both sides of special-shaped core, increasing diameter from Φ8 mm to Φ9 mm within maximum range allowed by part structure. Due to combined machining process, diameter of push rod is increased and accuracy of push rod hole is improved. Reproduction has verified that push rod operates smoothly and reliably.

Due to small size of special-shaped core, large local tightening force, and complex structure, special-shaped core is fixed by bolt connection. A maximum of three M8 bolts can be designed in three angular directions at the bottom of core to connect to movable mold plate. During pushing process of casting, bolt is affected by axial force, and its strength calibration formula is:

 

In the formula: б is actual stress borne by bolt (MPa), F is axial force borne by a single bolt (N), d is bolt diameter (mm), [б] allowable stress of bolt (MPa). Calculated based on uniform stress of three bolts, F=1.3Ft/3=1 680 N, and M8 bolt diameter Φ6.5 mm is substituted into equation (6) to obtain б=50.65 MPa. Mold uses high-strength bolts with an allowable stress greater than 80 MPa. Bolt strength check meets requirements, which can avoid casting deformation or mold failure caused by bolt failure during casting push-out process.

Crankcase body die-casting mold adopts a special-shaped core inlay structure in local deep cavity, which simplifies difficulty of mold manufacturing, simple process and convenient processing. Surface roughness after polishing of core is less than Ra = 0.4 μm, dimensional accuracy and appearance quality meet requirements. Mold has been verified in mass production, cooling effect of special-shaped core is good, and mold temperature is stable at process requirement of 160~220℃, as shown in Figure 5a. Casting has no appearance defects such as sticking and scratches caused by excessive mold temperature in local deep cavity, as shown in Figure 5b. Service life of special-shaped core reaches more than 80,000 mold times, which reduces difficulty of mold maintenance and manufacturing costs, ensures quality of castings. According to this mold structure, multiple sets of molds have been copied for mass production. Quality of castings is excellent, mold operation is stable and reliable.

 

(a) Mold temperature (b) Casting appearance
Figure 5 Mass production status

(1) For die castings with partially deep cavities and complex structures, mold design adopts a mosaic structure design, which can optimize processing technology and improve quality of mold.
(2) In order to ensure reliability of design, it is necessary to perform relevant reliability calculations on core of mosaic structure, such as local cooling capacity, push rod size and instability, bolt strength, etc.
(3) For locally special-shaped long cores, when calculating local push-out force, use conditions must be considered to appropriately increase safety factor, with safety factor ≥ 1.5.
(4) Special-shaped core of crankcase body adopts an inlaid structure. Through verification of mass production, product quality is stable and mold operation is reliable. Service life of special-shaped core reaches more than 80,000 molds, which reduces difficulty of mold maintenance, reduces manufacturing costs, and ensures quality of castings.