Smooth production of a die-casting mold is a key link in successful development of a die-casting part, and good design of runner system is prerequisite for ensuring normal production of die-casting mold. Especially for products with special requirements (such as air tightness, surface roughness, etc.), during mass production, these special requirements often become key indicators for assessing success of a die-casting mold. We found in actual production that although there are many factors that affect quality of die-casting parts with special requirements, setting of gate position is often a link that cannot be ignored. Improper gate position setting will cause the entire mold to be scrapped, or mold life cycle will be greatly reduced. This article briefly explains impact of setting of gate position of die-casting mold on quality of die-casting parts in production practice.
Design of die casting runner system needs to be carried out after structural analysis of casting and determination of various requirements. General process for designing a gating system is:
(1) Select location of gate.
(2) Consider guiding flow direction of metal flow.
(3) Divide number of gate strands.
(4) Set shape and size of sprue.
(5) Determine cross-sectional area of gate.
In actual design, except that selection of gate location is the first step to be considered, above sequence is only a rough consideration, and order is not very strict. In fact, above aspects influence and restrict each other. When considering the latter step, it is likely that changes and adjustments to design made in previous step will be made. Therefore, it is necessary to comprehensively consider specific situation to design a pouring system that meets requirements. Since design of runner system has many factors that affect quality of die-casting parts, this article only discusses impact of selection of gate location on quality of die-casting parts.

During mold design, choice of gate location is often restricted by alloy type, casting structure and shape, wall thickness changes, shrinkage deformation, machine type (horizontal, vertical), and usage requirements of castings. Therefore, there are very few ideal gate locations for die castings. Among these factors that need to be considered, gate location can only be determined to meet the most important needs, especially some special needs. Gate position is first limited by shape of die casting, and other factors must also be considered. Generally speaking, you need to pay attention to following aspects.
(1) Gate position should be chosen where molten metal filling process is the shortest and distance from flow to various parts of cavity is as close as possible to reduce tortuous filling path and avoid excessive detours. It is recommended to use center gate as much as possible.
(2) Placing gate in the thickest part of die casting is beneficial to transmission of final pressure. At the same time, gate is opened in a thick-walled part, leaving room for increasing thickness of ingate.
(3) Gate position should make distribution of cavity temperature field meet process requirements and try to meet filling conditions for metal liquid to flow to the farthest end.
(4) Gate is located at a location where metal flow enters cavity without vortex and exhaust is smooth, which is conducive to elimination of gas in cavity. In production practice, it is very difficult to eliminate all gases, but trying to eliminate as much gas as possible according to shape of casting is an issue that should be considered during design. Exhaust problem should be paid special attention to castings with air tightness requirements.
(5) For frame-shaped castings, gate position can be placed within projection range of casting (see Figure 1). If a single gate is well filled, there is no need to use multiple gates.

 

(6) Gate should be positioned as far as possible where metal flow does not directly impact core, and metal flow should be prevented from hitting core (or wall). Because after impacting core, kinetic energy of metal liquid is dissipated violently, and it is also easy to form dispersed droplets that mix with air, which increases casting defects. After core is eroded, mold sticking will occur. In severe cases, eroded parts will form depressions, which will affect demoulding of casting.
(7) Gate position should be set at a location where it is easy to remove or punch gate after casting is formed.
(8) For die castings that require air tightness or do not allow presence of pores, ingate should be set in a location where molten metal can ultimately maintain pressure.

As shown in Figure 2, it is the lower shell of an automotive air conditioning compressor. This product is made of ADC12 alloy and is produced on a DCC400 die-casting machine. Punch diameter is 70mm, net weight of product is 1040 g, slag bag weight is 267 g, and mold temperature is 180~ 220℃, temperature of aluminum liquid is 650~680℃.
After several small batch productions, it was found that air leakage would occur in varying amounts in the three locations shown in Figure 2, with the overall air leakage ratio even as high as 20% to 30%. Although some leaking product parts can meet customer's inspection standards after subsequent impregnation treatment, impregnation cost is high each time, resulting in a significant increase in production costs and mold cannot be put into normal production.
In order to improve this situation, we carefully calculated and analyzed die-casting process of several trial productions. For this mold, original die-casting process parameters are basically adjusted to optimal state. Is this problem caused by mold pouring and drainage system? Immediately, we used currently popular die-casting simulation flow analysis software to conduct mold flow analysis on filling process of mold, as shown in Figure 3.
From mold flow filling diagram, we can see that since gate is opened on the end face of product part, when molten metal is filled, leakage parts B and C shown in Figure 2 happen to be the end of molten metal filling. Because bottom of this product is thicker and circumferential barrel wall is thinner, end filled with molten metal cannot be effectively fed during final pressurization stage, and there is loose tissue inside, which ultimately causes air leakage here in casting and unqualified air tightness.
From above analysis, it can be seen that gate position of mold was not set properly, which caused casting to fail air tightness test. Adjusting process parameters can only slightly improve proportion of air leakage, but cannot fundamentally solve problem of air leakage in casting. In order to completely solve air leakage problem of lower shell, we decided to change gate position of mold. Three-dimensional casting blank after changing gate position is shown in Figure 4.
At the same time, we performed a simulation filling analysis on new design plan, and results are shown in Figure 5.

 

From simulation analysis after changing gate position, it can be seen that in new filling scheme, molten metal is basically filled sequentially, and flow is smooth. At the same time, a larger slag collection bag and a certain area of exhaust groove are designed at the end of flow of molten metal; judging from simulation results, modified scheme is significantly better than original design scheme.
Subsequently, we changed gate position of mold based on new design plan. After a small batch of mold trials, comprehensive air leakage rate of product parts dropped to about 3.3%, achieving expected change effect.

Casting shown in Figure 6 is an oil pump casing used in automobiles. Area shown in picture has air tightness requirements. This product is a relatively mature product. It has been used in molds many times and has been used in previous molds. There is a problem of poor air tightness of product during process. In order to improve this problem of original mold, we decided to redesign sprue system of product by changing gate position of die-casting mold.

 

(1) According to selection principle of gate position, use of center gate can minimize flow of molten metal, at the same time it is conducive to discharge of gas in cavity and improvement of air tightness of casting. Since there is exactly a through hole with a diameter of about 40mm in the middle of product, mold can be opened using center feeding. For solution using center gate, we conducted a simulation filling analysis. Figure 7 shows simulation of oil pump housing with center feed filling.
It can be seen from Figure 7 that after using center gate, poor filling occurs in the area with air tightness requirements, and a large area of gas is wrapped in it. This solution does not greatly improve quality and air tightness of casting, and might also bring adverse consequences, so this plan was ultimately rejected. After everyone’s discussion and research, we re-formulated plan for second set of gate locations.

 

(2) According to use requirements of die-casting parts, use multi-branch branch runner and open two branches in areas with air tightness requirements to strengthen filling and feeding of this area. According to design plan, new runner simulation results are shown in Figure 8.

 

It can be seen from Figure 8 that after molten metal enters mold cavity, filling of area with air tightness requirements is smoothly promoted, without spraying, air entrainment, etc., and gate is opened in the area with air tightness requirements. After filling is completed, pressurizing and shrinking effect of this area can be completed well, which can greatly improve density of this area of casting, effectively reduce occurrence of pores and shrinkage cavities, which is very beneficial to improving air tightness of casting.
When opening a new mold, we referred to second set of design plans, re-established sprue position and inner sprue parameters. After mold testing and mass production, air tightness of product has been greatly improved, and air leakage rate has dropped to less than 5%.

Through above analysis, we can see that there are many different design solutions for design of runner position of a mold. How to select the best design solution from many design solutions is a process that a well-designed die-casting mold must go through. With development of modern software technology, relying on advanced die-casting simulation software can help designers make better choices about sprue location, thereby avoiding problems discovered after mold is used and embarrassment of changing mold. At the same time, it can also save considerable mold modification costs for enterprise.