Abstract: This paper introduces structure of small brackets inside automobile engines and main difficulties in die-casting production. Small bracket die casting generally uses a multi-cavity mold. ProCAST was used to numerically simulate flow filling of a multi-cavity small bracket mold. Based on simulation results, gating system design was optimized, achieving flow balance across multiple cavities. Results show that numerical simulation analysis software can play a significant role in design of multi-cavity molds, shortening mold development cycle.

Automotive engines contain various small brackets that support various moving functional parts (camshafts, drive shafts, etc.). They are generally divided into an upper cover and a lower body, each with a semi-circular arc. They are assembled together, machined, then fitted with bearings. Figure 1 shows common shape of a small bracket. With trend towards lightweight automotive product design, these small brackets are becoming increasingly lightweight, often using die-cast aluminum alloy instead of original steel material, weighing only 20-50 g for ease of assembly.

 

Figure 1 Outline of small bracket
Top cover of small bracket part has one mating surface and two locating pin holes that need to be machined. The lower body connects top cover to engine block, so it generally has two mating surfaces and four locating pin holes that need to be machined. Semi-circular holes need to be machined after complete assembly at engine factory.

Mold structure for small brackets is simple, generally using a split mold without sliders. Uneven casting wall thickness easily leads to shrinkage cavities and porosity in thicker areas (see Figure 2). They support rotating shafts within engine and withstand certain motion loads and vibrations; therefore, high internal quality requirements are placed on die-cast bracket parts. Internal quality requirements are implemented according to standard #2 in Figure 3.

 

Figure 2 Shrinkage Cavities and Porosity Inside a Small Bracket

 

Figure 3 Internal Quality Control Standards for a Small Bracket
If excessive pores exist inside casting, it is easy for it to break during use, causing damage to the entire engine. Figure 4 shows a small bracket that broke during an engine durability test due to large internal shrinkage cavities.

 

Figure 4 Small Bracket Breaks During Durability Test Due to Internal Shrinkage Cavities

Small brackets are produced in a horizontal cold chamber die-casting machine using a multi-cavity mold. Generally, gating system balance of a multi-cavity mold is achieved through symmetrical design or by establishing identical runners from gate to each cavity. During trial production, a short-shot method is used to compare quality of undercast parts, allowing for fine-tuning of gate cross-sectional dimensions. This significantly increases trial production cycle of die-casting mold and may even shorten its service life. With advent of numerical simulation software, die-casting filling and solidification process can be simulated more accurately, guiding gating system balance design of multi-cavity molds, improving design accuracy, and shortening development cycle.
1. Feeding Methods
Small bracket die casting often employs two different feeding methods. See Figure 5. Feeding method 1 has advantage of simple mold parting, with gate overlapping mating surface, allowing for gate removal through subsequent machining. Its disadvantages include inconsistent temperatures on both sides of semi-circular arc, resulting in differences in internal quality, and poor gate compensation for thicker wall areas. Feeding method 2 feeds from semi-circular arc parting line. Its advantages include direct compensation for thicker wall areas, consistent temperature on both sides of semi-circular arc, and easier assurance of internal quality. Its disadvantages include stepped mold parting, increased production difficulty, gate marks remaining at parting line between moving and stationary molds on semi-circular arc. If semi-circular arc end face is not machined, workload for gate cleaning will significantly increase, leading to higher production costs.
Both feeding methods are used in actual production. When semi-circular arc end face is machined, method 2 is often preferred; if semi-circular arc end face is not machined, method 1 is often used.

 

Figure 5 Feeding method for small brackets
This project will use ProCAST software, taking 2JA-2 cam bracket as an example, adopt feeding method 2 to simulate die casting filling and solidification process. Based on simulation results, distribution of castings and runners, as well as shape and size of runners, will be repeatedly improved to complete balanced design of gating system of a multi-cavity mold.
2. Balanced design of cavity feeding
2JA-2 cam bracket is made of ADC12 die-cast aluminum alloy, with a volume of approximately 9.5 cm³ and a mass of approximately 25 g. It is produced using a Yizumi 1800 kN horizontal cold chamber die-casting machine, with 6 pieces produced per mold.
1) Preliminary scheme
Balanced design of feeding for different cavities in a multi-cavity mold should consider following factors:
(1) Consistency of casting quality in each cavity Ensure that castings in each cavity obtain same or similar forming conditions such as filling pressure, filling time, and filling temperature, so as to facilitate adjustment of process parameters. Generally, ensuring that length and cross-sectional dimensions of runners in each cavity are consistent allows for simultaneous filling of all cavities.
(2) Casting Production Efficiency: During mass production, while ensuring forming quality, runner length should be shortened as much as possible to reduce cross-sectional area, thereby shortening filling and cooling time and forming cycle.
(3) Casting Yield (Ratio of Casting Net Weight to Mold Weight): Mold weight of casting can be understood as material scooping weight. The larger ratio of casting net weight to mold weight, the less material loss in die casting production.
(4) Mold Dimensions: Size of die casting machine template should be considered to ensure symmetrical distribution and prevent uneven stress.
Considering point 1, a 6-cavity mold is best designed with each of 6 cavities having its own branch runner, length and cross-sectional area of each branch runner should be completely consistent. Considering above factors, gating system design for 2JA-2 cam support adopts scheme shown in Figure 6.

 

Figure 6 Preliminary Design Scheme of Gating System
2) Numerical Simulation
Filling of 2JA-2 cam support was numerically simulated using ProCAST2016 software. A complete casting in STP format with overflow grooves and runners was imported (see Figure 6). After meshing casting, a virtual mold was added in VISUAL-CAST module. Then, initial parameters were set: ① In volume manager, materials and pouring temperatures of casting and mold were set; ② Heat transfer coefficient (HTC) of casting and mold materials was set; ③ Gravity direction was selected; ④ Process conditions were set, mainly referring to cooling method and pouring speed; ⑤ High-pressure casting (HPDC) was selected, and corresponding simulation parameters were set.
After completing above settings, "Start Simulation" was clicked to perform simulation. Simulation results were viewed in VISUAL-VIEWER mode, as shown in Figure 7. According to mold flow analysis results, filling speed of middle feed channel was much faster than that of left and right channels, making it difficult to set fast-pressure transition point. Pressure transmission effect of each cavity was also different, which did not meet design requirements, and design scheme had to be changed.

 

Figure 7 Numerical simulation results of preliminary design scheme
3) Improved Scheme
To achieve a compact mold structure, initial scheme used a distribution of one stream split into three streams, then those three streams were further split into six streams. When one stream was split into three, middle stream was significantly shorter than two side streams, and flowing directly down from sprue, middle stream had significantly less flow resistance than the other two, resulting in unsatisfactory filling. Therefore, cross-sectional dimensions of three streams were adjusted to increase flow resistance of middle stream and decrease flow resistance of side streams. However, this was found to have little effect. This is because length of middle stream is much shorter than side streams, and middle stream has one less flow turn, resulting in lower flow resistance.
To balance stream length and flow direction of each cavity, distribution of small support gating system was improved, as shown in Figure 8. Main runner split into four streams, and two middle streams were further split into two, forming six branch feeds to feed six cavities. Two branch runners in the middle pour two cavities respectively, while branch runners on both sides pour one cavity each. Cross-sectional area is increased by increasing depth of two middle runners to control flow rate.

 

Figure 8 Improved Design Scheme for Small Support Gating System
4) Numerical Simulation of the Improved Scheme
Simulation results of improved scheme are shown in Figure 9. According to mold flow analysis results, filling speed of four branch runners is similar, filling state of each cavity is small, fast-press start point is easy to set, and pressure transmission effect is also improved, meeting design requirements.

 

Figure 9 Numerical Simulation Results of Improved Scheme

1. Die Casting Process Parameters
Production uses a DM180 horizontal cold chamber die casting machine with a φ50 mm punch. Slow injection uses 25 molds, fast injection uses 5 molds; aluminum alloy insulation temperature is (660±15)℃. Main die-casting process parameters are: slow injection speed (punch) 0.25 m/s, fast injection speed 3.5 m/s, high-speed stroke 50 mm, casting specific pressure approximately 80 MPa. X-ray inspection combined with wire cutting and visual inspection were used, as shown in Figure 10.

 

Figure 10 X-ray inspection results
2. Verification Results
Through 6 months of quality tracking, X-ray inspection showed that internal quality of small bracket met 100% of customer's required #2 standard. Based on #1 standard, compliance rate was approximately 90%, there was no significant difference in quality among six cavities of mold. Internal quality depends not only on mold gating system design but also on management of production conditions, especially control of mold temperature, material temperature, and coating management.

Numerical simulation can conveniently and quickly guide design of gating systems for multi-cavity molds, achieving flow balance of alloy in multiple cavities, guiding adjustments to die-casting process, achieving better die-casting quality, reducing trial molding time, lowering scrap rates, and shortening production cycles. Simultaneously, to achieve 100% compliance with internal quality standards in mass production, it is essential to minimize or control variations in die-casting production process.