By analyzing structure and formability of shell parts, we focused on detailed analysis and research on main aspects of mold design; we analyzed and designed parting surface position of mold, structure of pouring and overflow system, and ejection mechanism, a structure of multi-point gates on thick wall on the side was adopted; a specific structural design was made for core and cavity of mold, using an ordinary two-plate structure with cavity in fixed mold and core in movable mold; moreover, core and cavity are made into movable inserts for easy repair and replacement. Mold structure is simple and practical, fully meeting design requirements of mold.
Aluminum alloy materials have lightweight characteristics. With continuous development of casting and forming process technology, die-casting process and mold design of aluminum alloy materials have developed rapidly. Design of die-casting molds is an important part of die-casting forming and has an important impact on cost, efficiency and accuracy of finished products of the entire processing process. Therefore, many scholars at home and abroad have conducted research and analysis on die-casting molds. Through experimental comparative analysis, Ma Dongwei found that main factors affecting size of aluminum alloy styles are residual stress and changes in solid phase crystallization; Shi Baoliang conducted relevant analysis on typical parts of structural parts used in automotive industry, focusing on performance characteristics of aluminum alloy castings under high pressure; Jin K C designed thin plate die-casting mold using two geometries, proposed a new overflow system based on numerical simulation, conducted an actual vacuum die-casting test of part of channel without backflow, and manufactured a high-quality sample using proposed optimized mold design; Péter Szalva compared high-cycle fatigue behavior of high-pressure die-casting and vacuum-assisted die-casting, described how casting defects affect fatigue failure, found that vacuum-assisted die casting significantly reduces pore size and volume, reduces occurrence of oxidized flakes, and thus increases number of failure cycles. Research of above scholars is all about microstructure of product parts after die-casting and structural performance analysis of aluminum alloy castings. Structural design and simplification of die-casting mold have not been mentioned yet. Therefore, direction of this research is to design an aluminum alloy shell die-casting mold. This design can effectively avoid defects such as cold insulation, slag inclusions, bubbles, looseness, and unformed heat sinks caused by die-casting process.

Figure 1 shows shell parts, which are made of aluminum alloy die-casting. Casting structure is relatively complex and wall thickness is different. Wall thickness of end face of shell is 5 mm, wall thickness of surrounding sides is 3 mm, and wall thickness of five bosses on one side is 15 mm. There are 25 heat sinks on the back, which are relatively dense. Width of narrow end is 1 mm, slope of one side is 1.5°, and depth is 9 mm. Wall thickness is 5 mm with four ribs and 1.5 mm with six ribs.

 

Figure 1 Shell parts diagram
After a comprehensive analysis of structural characteristics of casting, flow direction and characteristics of aluminum alloy liquid in mold should be considered when designing mold, material flow direction and relationship with direction of heat sink should be reasonably selected; due to uneven wall thickness of castings, casting defects such as slag inclusions and looseness are prone to occur during die casting. Therefore, location of inner gate should be selected reasonably so that casting can be fully formed during die casting. Considering structure of shell and actual production conditions, this die-casting mold design adopts a one-mold-one-cavity structure.

According to structural characteristics of casting and design requirements of parting surface, large end surface of shell is selected as parting surface of movable mold and static mold. In order to facilitate demoulding of casting, finished product should be left on the side of movable mold. Draft angle of inner surface of shell is 2.5° on one side and depth is 48 mm. Tightening force of forming part can keep shell on core side, so core is selected to be on movable mold and cavity is on fixed mold. Structural form is shown in Figure 2.

 

Figure 2 Parting surface settings

According to design principles of die-casting mold pouring system, metal flow direction should be parallel to direction of heat sink to avoid defects such as cold insulation, slag inclusions, bubbles, looseness, and unformed heat sinks. In addition, position of inner gate should be set at thick wall, so that molten metal can fill thick wall first to avoid casting defects such as slag inclusions and looseness in thick wall. Therefore, in order to quickly fill mold cavity with molten metal, six ingates are provided on one side of shell boss. Runner adopts a stepped arc transition design to ensure sufficient filling speed. Sprue is equipped with a diverter cone, and diverter cone is designed with an arc transition structure, which can speed up filling speed of molten metal during die casting. Overflow tank should be set at the end of material flow direction. As shown in Figure 3, 13 overflow grooves and exhaust channels are provided on three sides of casting forming cavity.

 

Figure 3 Layout of pouring system and overflow system

This mold uses a push rod to push out casting. In mold design, position selection of push rod is crucial. Generally speaking, push rod position should be set at position where casting has the greatest tightening force on core and at thick wall of casting to prevent casting from being damaged when pushed out. After comprehensive consideration, all push rods adopt Φ8 mm round push rods. There are 6 push rods on the top surface of inner surface of casting and 12 push rods on the end surface of casting. In addition, 9 push rods are installed at sprue and runner, 13 push rods are installed at all overflow troughs. This design can fully meet launch requirements.

Improving die-casting production efficiency, as well as quality and density of die-casting parts and reducing thermal stress, largely depend on adjustment of mold temperature. Considering that die casting is a thick-walled casting and is produced in small and medium batches, during continuous operation, in order to maintain high quality and high productivity of casting, a water cooling device needs to be installed in mold to allow heat to be quickly discharged with circulating flow of cooling water. Mold adopts a relatively simple cooling system, and cooling water channel is set in cavity with higher mold temperature (i.e., fixed mold insert). Six Φ10 mm cooling water channels are set up along length of cavity. There are six water nozzles on each side of fixed mold and fixed mold insert is threaded (sealed). Water inlet pipe and water outlet pipe are set on side opposite operator, water nozzles on both sides are connected with soft water hoses (tightened with a tightening reed) to form a complete water cooling circulation system, as shown in Figure 4.

 

1. Fixed mold plate 2. Fixed mold insert 3. Movable mold insert 1 4. Movable mold insert 2 5. Gate sleeve 6. Diverter cone 7. Movable mold plate 8, Push plate fixed plate 9. Push plate 10. Moving Mold base plate 11. Push rod 12. Reset rod 13. Guide sleeve 14. Guide column 15. Pad 16. Push plate guide 17. Push plate guide 18. Faucet 19. Limiting nail 20. Hexagon socket bolt
Figure 4 Mold assembly drawing

Figure 4 shows final assembly structure diagram of this mold. This mold adopts an ordinary two-plate structure. Considering complexity of casting structure and cost factors of mold production, cavity and core parts of mold adopt movable inserts, which are embedded in movable and fixed mold plates respectively. Moving and fixed mold inserts, moving and fixed mold plates adopt H7/K6 transition fit, are connected and fixed with bolts. This design facilitates processing of mold forming part, as well as repair, replacement and size adjustment of forming part. Mold closing of moving and fixed molds adopts combination of four guide pillars and guide bushes to ensure stable and accurate mold closing. In order to ensure that push rod can slide smoothly, push rod is fixed in push plate and push plate fixed plate. A structure of four push plate guide posts and guide bushes is used to support weight of push plate and push plate fixed plate to ensure that push rod operates smoothly and will not deform. Four Φ20 mm reset rods are installed in moving mold plate and fixed in push plate. After push-out action is completed, when mold is closed, reset rod in movable mold drives all push rods to complete reset.

A domestic research institute currently has a 300 t cold chamber die-casting machine. Mold used is mold designed and developed this time. It adopts an ordinary two-plate structure with cavity in fixed mold and core in movable mold. Trial production of this aluminum alloy shell was completed by preparing aluminum alloy liquid and optimizing relevant parameters of die-casting process. Product produced after removing slag bag and sawing off gate is shown in Figure 5. This trial production effectively avoided defects such as cold insulation, slag inclusions, bubbles, looseness, unformed heat sinks caused by die-casting process, and achieved expected results.

 

Figure 5 Trial product sample

(1) Designing main forming part structure as an insert and processing it separately can not only control dimensional accuracy of casting, but also enable rapid repair, replacement and adjustment of wearing parts.
(2) Through structural analysis of casting, we chose to set up multiple internal gates at thick wall on the side of casting, and designed lateral runner into a stepped arc transition form, which not only ensures complete formation of heat sink, but also satisfies rapid filling of casting. requirements.
(3) By analyzing casting forming process, choosing to set up multiple overflow grooves at the end of metal flow direction can avoid casting defects such as cold shut, slag inclusions, bubbles, looseness, and unformed heat sinks during die casting.
(4) Structure of this mold is that core is in movable mold and cavity is in fixed mold. Main forming part adopts an inlaid structure, and mold adopts an ordinary two-plate structure. After mold testing, it was verified that mold operates smoothly and reliably, appearance quality and dimensional accuracy of die-casting parts fully meet product drawing requirements without any casting defects.