Based on undesirable defects that may occur in die casting, ideal filling effect that alloy liquid should achieve in die casting mold cavity is analyzed. Based on actual and theoretical discussions, eleven reasonable filling flow states of die casting alloy liquid in die casting mold cavity are proposed, and use of cases is demonstrated.
Defects such as pores, cold shuts and shrinkage holes in die castings are mainly related to filling and flow state of alloy liquid in cavity. Only with a reasonable filling flow state can ideal filling effect be achieved and qualified die castings can be produced. In die casting process, important thing is design of pouring system of die casting mold, which relies on pouring system to guide alloy liquid to flow and fill in mold cavity according to predetermined process. Here, from practical and theoretical analysis, reasonable filling flow state of die-casting alloy liquid in mold cavity and ideal effect to be achieved by flow filling.

Purpose of die-casting alloy liquid flow filling molding in mold cavity is to obtain qualified castings, but not all flow filling can obtain qualified castings. Due to variable structure and shape of casting, filling flow state of alloy liquid in various parts and stages of mold cavity is also complex and variable. Only a reasonable filling flow state can approach ideal filling effect. Die casting alloy liquid generally has following 8 ideal filling effects:
① Alloy liquid flows continuously and orderly, fills mold sequentially, and does not entrain air; ② Alloy liquid fills quickly and in a short time, casting shape is complete, and there is no cold shut; ③ Cavity is divided according to plan, alloy liquid starts at the same time or starts successively, fills evenly, ends at the same time, casting quality is uniform and guaranteed; ④ Alloy liquid flows steadily, each strand of alloy liquid docks with each other smoothly, does not collide with each other, and does not generate vortexes; ⑤ Alloy liquid is effectively drained and exhausted during filling process, casting is not loose and does not hold air; ⑥ Alloy liquid solidifies sequentially, effectively compensates for shrinkage, and has no shrinkage holes. ⑦ Alloy liquid is sprayed at ultra-high speed in inner gate, alloy liquid mist particles are accumulated and filled, alloy liquid is quickly cooled and solidified, without forming large pores and large shrinkage cavities. ⑧ Alloy liquid flows, fills at ultra-low speed in gate and cavity, and does not entrain gas.

Various filling and flow states of die-casting are related to comprehensive factors such as casting structure, pouring, overflow, exhaust system, mold temperature, mold structure, die-casting process, atmospheric environment, etc. They are extremely uncertain and variable, resulting in quality of die-casting parts often fluctuating and unstable. Therefore, all relevant factors need to be under control.
Here is a discussion and analysis from actual and theoretical aspects of die-casting. To achieve ideal filling effect, die-casting alloy liquid should have a reasonable filling and flow state in die-casting mold cavity.

Deep cavity parts, blind hole parts (see Figure 1), thin-walled parts, parts far away from gate, parts with many flow obstacles and large resistance that affect flow speed and flow rate, parts that require tortuous flow to reach, parts that require detour flow to reach, (filling along) ribs, heat sinks, threads, etc., are all difficult to fill parts (see Figure 2), and all require alloy liquid to be quickly directly or indirectly sprayed and filled. Multiple streams of alloy liquid can be used with one or more internal gates to fill different areas together or separately. This can achieve purpose of accurate area division, separate allocation of alloy liquid for centralized filling, and comprehensive solution.

 

Figure 1 Alloy liquid is concentratedly injected into deep cavity

 

Figure 2 Alloy liquid is injected into complex parts of casting

(1) Internal gate is set in deep cavity to allow alloy liquid to fill from deep cavity to parting surface, reducing flow, impact, twists and turns, and facilitating exhaust (see Figures 3 and 4).

 

Figure 3 Alloy liquid is filled from deep cavity to parting surface

 

Figure 4 Alloy liquid is filled from deep cavity to parting surface

(2). It is necessary to inject alloy liquid directly from one parting surface into deep cavity, then let alloy liquid flow from deep cavity to several other parting surfaces (see Figures 5 and 6).

 

Figure 5 Alloy liquid is directly injected into deep cavity and then flows from deep cavity to parting surface

 

Figure 6 Alloy liquid is directly injected into deep cavity and then flows from deep cavity to parting surface

(1) Inner gate is set in the middle part of cavity, allowing alloy liquid to fill from middle of cavity to surrounding parting surface, which is convenient for exhaust and reduces flow of alloy liquid (see Figure 7 to Figure 9).

 

Figure 7 Center gate fills from middle to periphery

 

Figure 8 Middle runner fills from middle to periphery

 

Figure 9 Middle injection chamber fills from middle to periphery

(3). It is necessary to inject alloy liquid of inner gate directly into middle of cavity, then let alloy liquid flow from middle of cavity to peripheral parting surface. This is conducive to filling complex part in the middle of casting first, then filling simple part on periphery (see Figure 10 to Figure 12).

 

Figure 10 Alloy liquid is directly injected into middle of cavity and then flows to periphery

 

Figure 11 Alloy liquid is directly injected into middle of cavity and then flows to periphery

 

Figure 12 Alloy liquid is directly injected into middle of cavity and then flows to periphery

(1) If alloy liquid first fills thick wall part and then fills thin wall part under pressure, alloy liquid is not easy to produce spray (see Figure 13), which is also conducive to sequential solidification, pressure increase and shrinkage compensation; but because speed and power of initial filling of thin wall are not large enough, it is only suitable for filling smaller and not very thin thin-walled castings; and because a large amount of alloy liquid stays in thick wall part to heat mold, shrinkage holes are easily formed in thick wall part.

 

Figure 13 Alloy liquid first fills thick wall part and then fills thin wall part

(2). If alloy liquid directly passes through thick wall part from inner gate and is directly sprayed into thin wall part, alloy liquid has a greater impact force, which is conducive to filling larger and thinner thin-walled castings, is also conducive to sequential solidification, pressure increase and shrinkage compensation (see Figure 14); but thick wall part of casting will be filled last, which is easy to entrain gas.

 

Figure 14 Alloy liquid passes through thick wall and is directly sprayed into thin wall part

Filling from thicker wall thickness to very thick wall thickness and hot spot is shown in Figure 15. Due to very thick wall thickness and hot spot, alloy liquid has more heat and is easy to form shrinkage holes. If inner gate is set in a very thick part, degree of hot spot is further aggravated, which will promote formation of larger shrinkage holes. Since inner gate is relatively thin, it will solidify earlier than thick wall, and even earlier than very thick wall, so it is impossible to thoroughly compensate for very thick and hot spots. If inner gate is set in thick wall near very thick wall and hot spot, pressurized alloy liquid can be effectively transferred to thick wall through thick wall for compensation.

 

Figure 15 Filling from thicker wall to very thick wall and hot spot

(1) Wall parts are generally filled by jetting to far end first, then quickly reflowing, then filling position near inner gate (see Figure 1, 6). Pay attention to increasing exhaust capacity near inner gate.

 

Figure 16 Thick-walled parts are first sprayed to far end for filling, and then reflowed for filling.

(3). If thick-walled parts are filled from near to far, fill slowly forward (see Figure 17). Cold shut, unclear edges and corners, flow marks and vortex air entrainment should be prevented. It is believed that current classification of die casting gate filling speed is ultra-low speed less than 1 m/s, low speed 1-6 m/s, medium speed 6-20 m/s, high speed 20-70 m/s, and ultra-high speed 70-120 m/s. For aluminum alloy die casting, although 30-40 m/s is not easy to cause mold sticking, it can die-cast gate safety speed of ordinary castings. However, using current good mold materials, mold coating treatment and mold temperature control, in the case of no gate impact and mold sticking, for castings with porosity requirements, many use a gate speed of 40-70 m/s.

 

Figure 17 Slow filling of thick-walled parts from near to far

For medium-walled castings, full wall thickness is generally pushed forward to fill, which is conducive to exhaust and prevent vortex entrainment (see Figure 18). It is necessary to pay attention to early convergence of alloy liquids at near end, drainage and exhaust at far end.

 

Figure 18 Full wall thickness filling of medium-thickness castings

Pay attention to order in which alloy liquid reaches far end to prevent vortex gas encapsulation, strengthen discharge of cold materials and exhaust. Product in Figure 19 is a thin-walled casting, which is fully filled with a wider inner gate. In order to exhaust, more overflow ports are opened around casting for exhaust. Note that this pouring system is not a tapered runner. Width of cross runner is basically same as width of product. Feed runners are connected together to form a very wide runner, which causes flow speed of alloy liquid in feed runner to drop seriously, affecting increase in initial filling speed of inner gate.

 

Figure 19 Thin-walled castings are filled by jetting directly to far end

Fill part that can be exhausted last, which is conducive to draining and exhausting. Alloy liquid should first enter blind area and pit area where air is easily trapped, then fill area where air can be exhausted, and finally fill in direction of parting surface where air can be exhausted. Area where alloy liquid is finally filled should be able to exhaust gas that is finally blocked in cavity.

 

Figure 20 Alloy liquid first fills parting surface, casting has pores and cold shuts

 

Figure 21 Alloy liquid first fills deep cavity, casting has no pores and cold shuts

(1) Alloy liquids of adjacent inner gates should be merged together in parallel and smoothly as soon as possible after entering cavity (see Figure 22). Merge and fill forward, keep pace with each other, avoid backflow and flow marks, and prevent gas entrapment. Try to avoid mutual impact when the metal flows converge, and try not to converge far away from inner gate.

 

Figure 22 Alloy liquids of adjacent ingates should be smoothly converged as soon as possible

(2). Smooth convergence in a directional and fixed position: If alloy liquids of multiple ingates converge in a predetermined direction, position or end in cavity, they should not only converge as much as possible, but also pay attention to flow direction, position and size of exhaust after convergence to eliminate vortex and cold shut defects. As shown in Figure 23, flow direction and convergence of each ingate are not concentrated on bridge in the middle of product; after bridge, they should converge between two bridges and an overflow groove should be set between two bridges.

 

Figure 23 Smooth convergence in a directional and fixed position
(3) Alloy liquids flowing in opposite directions should not meet and converge in thin-walled area to avoid cold shut. If they converge in thin-walled area, overflow groove and exhaust groove need to be enlarged.

Alloy liquid flows from areas with high pore requirements of casting to areas with low pore requirements. Let fresh and pure alloy liquid filled later fill parts with high requirements for pores. Let alloy liquid that is entrained with gas and polluted by oxidation fill other parts.
Inner gate is directly shot at the side of casting wall with pores and surface quality requirements, which can prevent side from having cold shuts and pores during finishing. See Figure 24. Alloy liquid flow direction adjusted by j, k, and e feed runners is better.

 

Figure 24 Inner gate is directly shot at the side of casting wall with pores and surface quality requirements

According to ideal filling effect to be achieved by alloy liquid filling, 11 relatively reasonable filling flow states are flexibly used to design pouring system of die-casting mold, and alloy liquid is guided in cavity according to planned route, sequence, flow direction and filling flow state to fill various parts of cavity, so that mold design can be successful and good castings can be die-cast.