In actual die casting production process of rear cover housing of auxiliary box of JSD180 heavy truck transmission, production is often affected by difficulty of starting die. Various factors affecting difficulty of starting die casting are analyzed, and an improvement plan is formulated. By setting up undercuts on movable mold and disc spring ejection mechanism on fixed mold, problem of difficulty in starting die casting and distortion at the moment of mold opening is completely solved.
Aluminum alloy die casting belongs to metal mold casting. Aluminum metal liquid is quickly filled and cooled under pressure, so that casting has advantages of high dimensional accuracy, good surface quality, and dense organization. In addition, aluminum alloy die casting process also has characteristics of high efficiency and environmental protection. Therefore, it has been rapidly promoted in modern machinery manufacturing and other industries, has broad development prospects.

JSD180 die-cast aluminum alloy rear cover (Figure 1) produced by our company is one of important components of double intermediate shaft gearbox of heavy-duty vehicles. It is made of ADC12 aluminum alloy, weighs 7.6kg, has an overall size of 520 mmx345 mmx83 mm, and an average wall thickness of 6 mm. It is formed by die-casting.

 

Figure 1 JSD180 die-cast aluminum alloy rear cover casting

After machining, JSD180 rear cover shell casting has black skin in some areas of large mouth surface, as shown in Figure 2. In severe cases, production process adheres to fixed mold and it is difficult to remove mold core, as shown in Figure 3

 

Figure 2 Casting machining black skin

 

Figure 3 Casting sticking to the fixed mold

Difficulty in removing die casting is related to mold removal slope, Fe content, mold spray release agent quality, mold retention time, mold cavity surface roughness and mold structure. Difficult demolding can easily cause deformation of castings and black skin on the surface after machining. Although increasing machining allowance appropriately can reduce black skin phenomenon, it will affect surface quality, machining efficiency and subsequent assembly of finished parts.

Design value of machining allowance for large mouth of JSD180 rear cover shell is 1.5 mm. Five groups of castings were randomly selected for dimensional measurement. Measurement results are shown in Table 1. Actual machining allowance value is sufficient and far exceeds industry standard (1.0mm). It can be seen that main reason for black skin of rear cover is not small machining allowance.

Table 1 Machining allowance mm

Too small demolding angle will make it difficult for casting to escape from mold cavity. Demolding angle of mold core was actually measured, and none of them was less than 1.5°, which meets design specifications of die-casting mold industry.

Under normal circumstances, Fe is harmful to aluminum alloys, but for ADC12 die-cast aluminum alloys, Fe must be adjusted to a certain content. When it is lower than lower limit, there is a tendency to stick to mold; when it is higher than upper limit of 0.9%, fluidity of alloy decreases, tendency to hot cracking increases, and mechanical properties decrease. Therefore, Fe content is generally controlled between 0.7% and 0.9%. For this reason, chemical composition of problem castings was tracked, and test results within one month were analyzed by SPC. As shown in Figure 4, Fe content did not exceed standard.

 

Figure 4 SPC analysis of Fe content in ADC12 aluminum alloy

Release agent used on the surface of spray mold cavity has strong adhesion and good demolding effect. If release agent is not sprayed evenly and there is a shadow area, friction resistance of casting demolding will increase, resulting in aluminum sticking to mold surface or difficulty in demolding casting. Figure 5 shows situation after mold release agent is sprayed on the surface of fixed mold core during normal die casting production. It can be seen from photo that mold release agent is sprayed evenly, and no shadow area is found on the surface of mold cavity where spraying is not in place.

 

Figure 5 Effect of spraying mold release agent on fixed mold cavity

The longer mold retention time of die casting, the greater shrinkage of casting as temperature decreases, and tightening force of casting on mold core increases, making mold removal resistance greater. When mold retention time is reduced from initial 14 s to 13s, it is found that frequency of mold sticking of casting is reduced. The more appropriate mold retention cooling time is 12s, but casting still has problems of black skin processing and difficulty in removing fixed mold, which shows that mold retention time is a secondary factor.

Rough surface of mold cavity can also easily cause difficulty of casting demolding. When mold is taken off machine for regular maintenance, surface of mold core cavity is polished to ensure that roughness Ra value of forming surface is not greater than 0.4 μm. After measures are implemented, situation of casting sticking to fixed mold in production process has not been significantly improved. Deformation of casting demolding caused by roughness of mold cavity surface is also a secondary factor.

In order to ensure sufficient strength, external reinforcement ribs of die casting are arranged more and densely, and relative height is relatively high. In order to ensure appearance requirements of casting, external forming surface of casting is set on fixed mold side, and internal forming surface of casting is set on movable mold side. Problem caused by this is that effective wrapping area of casting formed by aluminum liquid after filling mold is greater than effective wrapping area of movable mold core, so resistance of casting when demolding fixed mold is greater than that of movable mold. After casting solidifies, it is more difficult to demold fixed mold than movable mold. Above analysis shows that imbalance of demolding positive force between movable and fixed molds is main reason for difficulty of casting demolding. Photos of fixed and movable mold cores are shown in Figure 6.

Basic principle of die casting is high-speed filling and high-pressure solidification of liquid metal. In die casting process, filling speed can be as high as tens of meters or even hundreds of meters per second. Gas involved in cavity is difficult to be completely discharged, and it is difficult to avoid pores in die casting. Therefore, processing allowance and draft angle are limited. However, there is little room for improvement in chemical composition of casting material, roughness of mold surface, spraying and mold retention time. In this way, we can only think of a solution from structure of mold.
Combined with above analysis, we made a disc spring ejection mechanism on fixed mold. Principle of this mechanism is: when movable and fixed molds are closed, ejector pin set on fixed mold core is pressed back by reset rod on parting surface; when mold is opened, casting is ejected from fixed mold core by elastic force of compression spring behind ejector pin; in addition, some undercuts (pits) are set on movable mold core to make casting have greater ejection resistance to movable mold, so that casting is easier to get out of fixed mold and attach to movable mold. After movable mold is in place, ejector pin on movable mold ejects casting. Figure 7a shows undercut (pit) set on movable mold, Figure 7b shows trace left after undercut residue is cut off after casting is ejected from movable mold, and Figure 8 shows layout of ejector pin of disc spring ejector mechanism on fixed mold core.

 

Figure 6 Mold core

 

Figure 7 Undercut

 

Figure 8 Layout of ejector pin of fixed mold
A total of 8641 pieces of production statistics before and after mold improvement were compared and analyzed. Proportion of black castings after machining decreased from original 17.02% to 0.30%. At the same time, problem of difficulty in removing casting from fixed mold was effectively improved.

(1) Main reason for difficulty in removing casting from fixed mold is that ejection resistance of JSD180 rear cover shell die casting is greater than demolding resistance of movable mold.
(2) Setting a disc spring ejection mechanism on fixed mold and making an undercut on movable mold can effectively solve problem of difficulty in removing fixed mold from rear cover shell.