Sand mold low-pressure casting process, casting of thin-walled parts

Posted on : On May 15th,2024 By  GREFEE

Thin walled castings process has been always a difficulty in industry, especially when low-pressure casting process is applied to the production of complex thin-walled aluminum alloy castings, the complexity of the sand casting system makes it difficult to ensure the quality of the castings.

Thus, to ensure the quality of castings, the system optimizing of sand mold gating system is essential. On the premise of meeting the casting quality needs, the number of the gating system should be reduced as much as possible to reduce the cost. 

In low pressure casting, as the small size of casting and thin wall, the gravity and the melt etal liquid impact to the cavity wall during the casting, the cavity shells often appear shrinkage, porosity and sand holes, etc. In the mean time, as aluminum alloy parts have strong contractility, the castings often come with defects.

Therefore, gating system designing, including type, location, and shape should be considered during low pressure die casting processing.

Principle of sand mold low-pressure casting process:

During the processing of sand mold low -pressure casting, due to the metal liquid, the gas pressure inside the cavity will push out the gas inside the shell so that a perfect sand mold can be obtained without pores, porosity and sand holes.

During the sand mold die casting, as metal liquid flowing fast in the cavity, so a vacuum pressure generated in the cavity, and there is a pressure gap between metal liquid and sand shell.

Since negative pressure is generated during the filling process, a certain pressure difference exits as well between metal liquid and shells. When the pressure reaches a certain rate, there will be a space in pressure gap, which is the so-called low-pressure area.

After the space appears, the inside of shell will turn to negative pressure. After low pressure area appears, the metal liquid will be impacting, shearing and rub the shell constantly. When the metal liquid flows to a certain height inside the mold cavity, due to the low pressure at that height, the pressure at that height can meet the filling requirements of the metal liquid.

After the metal liquid reaches the height, due to the influence of gravity and flow rates, the metal liquid will gradually flowing down.

When the pressure at this height is lower than the scourcing force of metal liquid to the shell, a decrease in pressure will be generated. When pressure decreasing to a certain value, the metal liquid will wash the shell off the casting with the function of gravity and flow rate.

Since the pressure decreasing and scouring force is changing constantly with the difference of pressure decreasing and flow rate, the shell will be washed out from casting. Thus, the higher the pressure decreasing or scouring force, the server the defects.

In sand mold die casting, the speed is fast when the cavity is filling out with the metal liquid. In the meantime, with the influence of gravity and flow rate, when the metal liquid reaches a certain height inside the cavity, the speed will gradually decrease. However, the pressure at this height will not change. When the pressure decreases to a certain value, the low pressure area will be generated.

When the metal liquid reaches a certain height, since the pressure at this height is low, so there is no negative pressure, then no scouring or shear action to shell during the flow process, and thus a vacuum negative pressure area will be exit at this height.

When th e metal liquid flow to the vacuum negative pressure zone, as the pressure is very low at this height, there will be no negative pressure inside the shell when the metal liquid getting up to this height, then the scouring will be and causing damage to the shell. While, as pressure is low at this height, so when metal liquid reaches to this height, the shell will be broken。

According to the above principles, in sand mold die casting process, there are pressure difference and vacuum negative pressure zone between metal liquid and shell. The pressure at this height is low and there will be a low pressure zone.

Upsides and downsides of sand mold die casting

Compared to traditional sand mold die casting, the low pressure die casting have below upsides:

Low pressure die casting does not have the additive equipemnts, like casting head and shell, so it is much cheaper than the traditional sand mold die casting. As the requirements for structure and size for casting is lower in sand mold die casting, as well as simplify the mold design, the cost is largely reduced.

Low pressure die casting does not have additional equipment, like casting head and shell, so it does not have metal liquid processing, which improves the surface quality.

Good surface quality. Low pressure die casting does not have additional equipment, like casting head and shell, so it does not have metal liquid processing, which improves the surface quality.

In the gating processing of low pressure die casting, there is only processes of metal liquid pouring and continuous solidification of the cavity wall of die-casting molds, so its production period is quicker.

As the filling speed of low pressure die casting process is higher than that of traditional sand mold die casting, the production rate is much better, too.

With the automated production, low pressure die casting does not have additional equipment, like casting head and shell, so it is easily to achieve automated production.

The low pressure die casting process requires constant heating, gas releasing and forming, so it largely reduces the environmental pollution.

As the filling speed of low pressure die casting is much higher than that of traditional sand mold die casting, the dimensional accuracy and surface quality are much higher, too. Meanwhile, as the good filling speed and flowing rate of meta liquid, castings with high precision production is more easy.

As the low pressure die casting does not require shell, so it makes the mechanical and automated production available, especially there is a lot flexibility and freedom in the gating system design. Meanwhile, without the heating and cooling of metal liquid, the surface quality of casting improves significantly as well.

No shell is being used during the low pressure die casting process, the defects are much less, especially during the production of thin wall parts.

Due to the use of vacuum casting and lost foam forming in low-pressure casting, the surface and internal quality of castings can be improved, and the internal quality and mechanical properties of castings can be enhanced. At the same time, as it does not require complex shell processing, it can shorten the casting production cycle and improve production efficiency.

The Application Prospects and Development Trends of Sand Mold Low Pressure Casting

With the development of low-pressure casting, the sand mold low-pressure casting process is constantly improving and gradually updating, especially the design, which has produced different types of castings that meet the needs of different customers. The application field of has also expanded from the initial casting of cast iron to low-pressure sand casting of aluminum alloys, magnesium alloys, and some refractory metals

Due to the rapid development of sand low-pressure casting, it has been widely used in aerospace, automotive, electronics and other fields. The products has been used in automotive engines, engine control systems, instrument control systems, and some are even used in the production of aerospace rockets, satellites, and missiles.

For example: the cover part of car body is made with low pressure die casting with different types, such as the engine hood, windshield, and wheel cover of a sedan.

Especially in recent years, with the continuous growth of domestic sedan production, the demand for sedan castings has also been increasing. Currently, the low-pressure casting of sedan panels has formed a certain scale and production technology.

With the continuous development of sand mold low-pressure casting, its application in the production of thin-walled metal parts is becoming increasingly widespread.Firstly, the gating system should be designed according to the quality requirements of the castings. Secondly, it is necessary to design the mold reasonably and use computer technology for simulation analysis. At the same time, it is necessary to make a reasonable selection of molding sand and shape the casting mold reasonably.

Through the above measures, the quality of castings can be effectively improved. Although the sand mold low-pressure casting process development has a long way to go, further improvement and refinement of the process are still needed.

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