There are various small brackets in automobile engine, which support various moving functional parts (camshaft, transmission shaft, etc.). It is generally divided into an upper cover and a lower body. Each of upper cover and lower body has a semi-circular arc. They are assembled together, processed by a boring machine, and loaded with bearings. Figure 1 shows appearance of a common small bracket. As requirements for lightweight automotive product design increase, aluminum alloys are often used to replace original cast steel brackets. Mass of a single aluminum alloy bracket is 20 to 50g. This article introduces structure of small brackets in automobile engines and main difficulties in die-cast production. Small stent die-casting generally uses one mold with multiple cavities. ProCAST software was used to numerically simulate flow filling process of a small stent with multiple cavities in one mold. Designed pouring plan was optimized based on simulation results to achieve flow balance in multiple cavities.

Upper cover of small bracket parts has a joint surface and two locating pin holes that need to be processed, and lower body connects upper cover and engine block. Therefore, there are generally 2 joint surfaces and 4 locating pin holes that need to be processed, semi-circular arc holes need to be assembled and bored at engine factory. Small bracket molds have a simple structure and generally adopt form of split molds without sliders. However, wall thickness of casting is uneven, shrinkage cavities and shrinkage porosity are likely to occur in wall thickness area (see Figure 2). They support rotating shaft in engine, bear certain motion loads and vibrations. Therefore, they have high requirements for internal quality of bracket die-casting parts. Internal quality requirements are implemented according to No. 2 standard in Figure 3.

 

(a) Upper cover

 

(b) Lower body
Figure 1 Outline drawing of small bracket

 

Figure 2 Shrinkage holes and porosity inside small stent

 

Figure 3 Internal quality control standards for small stents

 

Figure 4 Small stent broke during durability test due to internal shrinkage cavities.
Small bracket die-casting often uses two feeding methods, see Figure 5. Advantage of feeding method I is that mold parting is simple, gate overlaps joint surface, and gate can be removed through subsequent processing. Disadvantage is that temperatures on both sides of semi-circular arc are inconsistent, internal quality is different, and gate has a poor feeding effect on wall thickness area. Feeding method II feeds from semi-circular arc parting line. Advantage is that wall thickness area is directly fed, and temperature of wall thickness areas on both sides of the semi-circular arc is consistent, so internal quality is easier to ensure. Disadvantage is that mold has stepped parting, which makes production difficult, castings will have gate traces left at parting line of semicircular arc dynamic and static molds. If semi-circular arc end face is not processed, workload of gate cleaning will be greatly increased.

 

Figure 5 Small bracket feeding method

 

Figure 6 Preliminary design plan of gating system

 

Figure 7 Numerical simulation results of preliminary design scheme
In order to make mold structure compact, initial flow channel adopts a distribution method in which 1 stream is divided into 3 streams, and 3 streams are further divided into 6 streams. When 1 strand is divided into 3 strands, process of middle runner is obviously shorter than that of runner on both sides, and it flows directly down from sprue. Flow channel resistance of middle runner is obviously smaller than that of the other two, resulting in unsatisfactory filling results. Therefore, it was improved by adjusting cross-sectional size of three-strand runner, increasing flow resistance of middle runner, and reducing flow resistance of runner on both sides, but effect was found to be average. Reason is that length of middle runner is much shorter than runner on both sides, and middle runner has one less flow channel turn, resulting in smaller flow resistance. In order to balance length and flow direction of lateral runner of each cavity, distribution of small bracket pouring system was improved, as shown in Figure 8. One main flow channel is divided into 4 branches, and the two middle lateral runner branches are further divided into two, forming 6 branch feeds. The two branch runner in the middle are poured into two cavities respectively, and branch runner on both sides are poured into one cavity each. By increasing depth of the two middle runner, cross-sectional area is increased to achieve purpose of controlling flow rate.

 

Figure 8 Improvement plan for small bracket pouring system

 

Figure 9 Numerical simulation results of improved plan

 

Figure 10 X-ray flaw detection results
After 6 months of quality tracking and X-ray detection (see Figure 10), internal quality of bracket can 100% meet customer requirements. According to No. 1 standard, pass rate is about 90%, and there is no obvious difference in quality of each of 6 cavities. In addition to design of mold pouring system, internal quality is also closely related to production management, especially control of mold temperature, material temperature and spray management. Simulate and optimize design of gating system of a multi-cavity mold to achieve a flow balance of alloy in multiple cavities, guide adjustment of die-casting process, achieve better die-casting quality, reduce mold trial time, reduce scrap rate, and shorten production cycle.