Optimisation of die-casting moulds for automotive transmission cases (2023-03-10)1 、Preface
Automobile rear drive transmission box body is the main part of the automobile rear drive transmission box body, is also the core of the automobile drive system key parts, transmission box internal assembly transmission gears and change gears, so the transmission box body size requirements of high precision; Due to the transmission box body has a large volume, uneven wall thickness, inside and outside of the cavity structure is complex and so on, the design of the blank die-casting mould puts forward a very high demand; Due to the casting of its own product characteristics are Cavity is large, wall thickness is not uniform, poor structural rigidity, easy to casting in the larger deformation, thus resulting in the position tolerance super poor, local machining allowance is not stable and other defects, highlighting in the subsequent machining of the position of the degree of super poor.

The material of this transmission case is aluminium alloy (grade: AlSi9Cu3 DIN 1706); the maximum outer contour size of the casting is 270mm×210mm×220mm, the maximum wall thickness is 16.5mm, the minimum wall thickness is 3.5mm, and the average wall thickness is 8.5mm; the quality of the casting is 3,680 grams; the appearance of the contour is required to be clear and intact, and the mechanical strength meets the requirements of the use of the casting, and the internal casting is required to be dense. The internal organization of the casting is required to be dense, internal defects (porosity, shrinkage) porosity with reference to VW-50097 porosity standard, to meet the requirements of D5-1. 
This casting machining parts basically all in the product inside, castings need to machine parts see Figure 1 red surface, according to the product size and product structure, the casting machining allowance is temporarily set to 0.7mm, to be blank after trial processing and then make appropriate adjustments.

2、Die-casting mould scheme design 
3、2.1 Die-casting machine and die-casting technique selection
4、Because the shape of the transmission box is more complex, after completing the three-dimensional design of the casting, we choose the mould parting direction as shown in figure 2, the whole mould projected area of 60,000mm2; Considering the complexity of the structure of the mould, combined with the customer's actual configuration of the die-casting machine, we chose the DM1000 die-casting machine for the design. 
5、According to the structural characteristics of the casting, we set the die-casting process as shown in table 1.

2.2 Selection of parting surface
For this kind of complex structure of the transmission box casting, the selection of parting surface will have a certain impact on the complexity of the mould structure, the setup of the mould insert (slider) and the use of the press; for the parting of this transmission box, we focus on the consideration of the casting's shaping performance, and adopt the three-slider extraction design shown in Fig. 2, with the position of the closed box cover as the lower slider, the mounting foot as the upper slider, and the movement of the transmission input port as the left slider. 

The spreader is set under the lower slider, and the inlet gate is mainly arranged on the end face of the hinged box cover to feed the material. Five inner gates are arranged around the lower core and two auxiliary gates are used near the upper slider to improve the mould filling performance of the castings at the far end. This design scheme, the mould structure is compact, the appearance of the moving and fixed mould two parts to molding, The machining benchmark of castings is reliable, which is beneficial for subsequent blank machining.2.3 Mould flow analysis 
2.3.1 Mould flow analysis filling temperature
According to the setting of die-casting process parameters, aluminium pouring temperature is 650 ° C, die flow analysis figure 3 shows that the filling front temperature is greater than the recommended temperature of 578 ° C, the appearance of the product is not easy to appear cold segregation, pattern and filling insufficient phenomenon, therefore, it can be considered that this runner arrangement basically conforms to the design expectations.

2.3.2 Mould flow analysis filling speed
Determine the internal gate speed of 20-60m/s based on relevant information; based on the product wall thickness and the complexity of the product, we chose the pressure injection speed of 3.5m/s, at this speed the inner gate speed is 50m/s; from the mould flow analysis Figure 4, the inner gate filling speed is suitable.

2.3.3 Mould flow analysis material tracking
From the material distribution results Figure 5, each runner is clearly distributed and the runner design is reasonable.

3、Actual production feedback and mould optimization
The castings produced after the mould opening of this scheme are shown in Fig. 6. At present, there are two following  problems after the feedback of mass production:
1. The coaxiality of the upper and lower holes is out of tolerance
6、Castings from the upper and lower two slider moulding hole, there is a coaxiality problem; blank position degree did not ensure that the blank drawing requirements within 0.6mm; machine processing leads to the casting hole unilateral machining after the existence of black skin, scrap ratio of about 3%.Analyse the reasons:
(1) There is a sliding assembly clearance under the slide block on the mould;
(2） The right side of the slider has a sprue on the face of its scouring, type into the left side of the oblique factors;
(3) Upper wall thickness of the product is 7mm, and there are thermal balance factors after the product is produced and taken, which leads to the product deformation。
2. Mold pulling injury  The internal position of the inlet slider below the product appeared to be scouring and strained phenomenon, accounting for about 3%.
Reason for analysis: from the simulation analysis and process settings, the punch 3.5m/s, the inner gate speed of 50m/s; analysis that the possible reasons for the sticking of the mould is that the speed of the inner gate is too fast or the temperature of the mould at the gate is too high; there may be the actual fast injection speed exceeds the pre-set speed value or caused the sticking of moulds under the lower sliding block at the gate.

3.1 Mould scheme optimization
In response to the feedback problem, in order to solve the problem of coaxiality of the upper and lower holes, we will change the mould parting to the scheme shown in Fig. 7; the position of the hinged box cover of the transmission box is designed as a fixed mould, the mounting foot is designed as a moving mould, and the drive input port is a downward sliding block. The diverter cone is set in the lower sliding block, the gate is mainly arranged in the lower sliding block at the end of the input face of the feeder, and then in the hinged box cover face additional feeder, to confirm the quality of air holes in this face and the inner ring after machining. 3.1.1 Mould flow analysis filling temperature
According to the setting of die casting process parameters, aluminium pouring temperature of 650 ℃, simulation results Figure 8 shows that the filling front temperature is greater than 578 ℃ of the recommended temperature, is expected to be less likely to appear in the future appearance of the cold segregation, pattern and fill the phenomenon of insufficiency, so it can be assumed that the arrangement of this runner basically conforms to the design of the expected.

3.1.2 Mould flow analysis filling speed
According to the data provided by North American Die Casting Association's Gating Manual P170, the recommended filling speed of aluminium alloy is 25.4-40.6m/s, and the speed is too fast to affect the life of the mould, and it is easy to erode the mould and stick to the mould. According to the wall thickness of the transmission box body, we choose the inner gate speed 40m/s, set the speed of 3.5m/s; from the simulation results of Figure 9, the inner gate filling speed is appropriate, there is no risk of cold segregation that occurs at a slow speed.

3.1.3 Mould flow analysis material distribution
Figure 10 shows this colour due to the inevitable mixing of the aluminium liquid entering the middle sprue of the casting after passing through the input hole. However, all the aluminium liquid is filled forward, and the aluminium water rushes to the end with a slag pack and exhaust, so it is judged that the design of the sprue is reasonable.

4. Conclusion
Optimized program to solve the two problems of the original program, see Figure 11, the original holes formed by the slider changed into a moving mold and static die core to molding; moving, static die by the guide pillar and the core of the fine positioning to ensure that the positioning accuracy is far better than the positioning accuracy of the slider; the original program on the lower down the block molding hole in the location of the degree of the new program has been completely resolved. The problem of falling blocks due to the erosion and sticking of the lower slider on the mold was solved by changing the position of the main gate and moving it to the input power interface. In addition, the wall thickness in this area of the product also has reinforcing ribs. The die-casting process was readjusted to adjust the speed of the inner gate within the set process range, and the problem of material shortage caused by erosion and sticking of the mold was partially solved.

Author of this article: Luo Yong from Chongqing Dongke Mold Manufacturing Co., Ltd

Part of the information from the "China die casting" magazine die casting practice, if any infringement, please contact to delete.