Six Factors That Affect Gravity Die Casting Quality
Gravity die casting process design includes 6 factors listed as below. These factors are the key that determines gravity die casting quality.
1.Casting Pouring Position
Pouring position is directly related to the number of die cores and parting surfaces, the filling position of molten aluminum alloy, the smooth exhaust, and the complexity of die structure.
In short, the determination of pouring position aims to control the solidification of castings. Those castings realizing the sequential solidification can eliminate shrinkage porosity and shrinkage cavity, ensure the compactness of castings, improve the quality of castings, and reduce the rejection rate.
Therefore, the pouring positon should be the first concern when designing casting process.
2.Gating and Riser System
The casting quality is somehow determined by the gating and riser system, which should not only function as slag discharging, gas exhaust and feeding, but also ensure the reasonable solidification and cooling temperature field of castings.
The correct and reasonable gating and riser system should be based on the different structure of the castings, calculating the cross-sectional area ratio of the sprue, runner and ingate logically.
By computer simulation, the temperature field of the casting solidification process can be intuitively displayed, high-risk areas of castings where defects may occur can be shown as well, and then process design can be guided.
Therefore, the casting defects can be eliminated by adjusting the structure and size of the gating and riser system, the die structure, the cooling rate or the thickness of the coating layer.
Of course, in addition to considering the solidification temperature field when determining the position, the structure and the size of the gating and riser system, other factors as the structure of the castings, the smooth flowing of the molten aluminum alloy, the functions of slag discharging and gas exhausting should also be taken into account.
Design requirements of gating and riser system:
- The position and shape of the inner runner should conform to the solidification principle and feeding requirement.
- Smooth the flow of molten aluminum alloy to avoid severe turbulence. Prevent entrapment, absorption of gases and excessive oxidation of molten aluminum alloy.
- The linear velocity of molten aluminum alloy entering the die cavity should not be too high to avoid splashing and scouring the sand core.
- Make sure that the liquid level of aluminum alloy carries enough rising speed to avoid forming casting defects such as sand inclusion and cold shut.
3.The Positioning of Sand Core
The sand core positioning refers to the sand core head positioning in the die.
The core head is used to fix the sand core so that it has an accurate position in the die and can withstand the gravity of the entire sand core and the buoyancy of the aluminum liquid during pouring, preventing it from being damaged.
The core head should be able to exhaust the gas generated during pouring to the outside of the die in time. Meanwhile, for the convenience to locate sand core, the core head should be in clearance fit with the die, and designed with draft angle.
4.Working Temperature of Mold
The temperature difference of each part of the die plays a vital role in the cooling temperature field of castings.
For positions with thick walls on the casting, water cooling and air cooling systems are set at the corresponding positions of the die to ensure that these areas maintain normal working temperature, improve production efficiency, eliminate overheating and guarantee regular cooling temperature field.
In order to ensure a reasonable cooling temperature gradient of the castings and eliminate shrinkage porosity defect, embedding metal inserts with high thermal conductivity at the thick-wall areas of the die or adjusting the coating process of the mold release agent can be good options.
5.Die Structure Designing
- The limit between each part of the die should be reasonable.
- Sand cores should be positioned on the fixed modules of the die to ensure the accuracy of castings.
- Add gas extraction system at the proper position of the core head to make the gas produced by the sand core in the pouring process discharged well.
- Set up the cooling system at the proper position of the die to meet the solidification requirements of castings.
- As different casting alloys are with different shrinkage rate, take shrinkage into consideration when designing dies.
- The hydraulic cylinder for core pulling mechanism of the die should be equipped with cooling system to avoid damage caused by overheating of the cylinder during die preheating or pouring process.
6.Casting Temperature and Pouring Speed of Aluminum Alloy
Casting temperature has great influence on gravity die casting quality.
Lower pouring temperature means poorer fluidity of molten aluminum alloy, which is easy to cause poor filling and lead to casting defects such as cold shut, short filling, gas porosity and slag inclusion.
While, higher pouring temperature means higher shrinkage of the metal alloy and higher gas content, which will cause casting defects as shrinkage porosity, shrinkage cavity, coarse grain, blowhole, sand burn-on, crack and serious oxidation.
Therefore, to determine the proper pouring temperature, we should comprehensively considerate all the related factors as the alloy compositions, casting weight, casting wall thickness, casting structure, etc.
Pouring speed refers to the filling time of the molten aluminum alloy into the mold.
The higher pouring speed can make the aluminum liquid fills the die cavity quickly, reduces the oxidation, and balances the temperature of each part of the casting, which is favorable for simultaneous solidification.
However, too high pouring speed is easy to cause sand wash, gas in die cavity can’t be discharged in time and causes gas porosity.
The lower pouring speed increases the temperature difference of each part of the casting, which is beneficial to directional solidification, riser feeding and elimination of shrinkage porosity.
However, lower pouring speed means longer contact time of molten aluminum alloy with air, which will cause over oxidation and temperature reduction. Casting defects such as slag inclusion, sand burn-on, cold shut and short filling will occur eventually.
Therefore, in order to select the proper pouring speed, we should comprehensively considerate the factors as the casting structure and technical requirement. In addition, with the assistance of computer simulation technology, we can intuitively present the entire casting process, gather the correct data, which is able to guide the processing adjustment eventually.
The key points we should pay attention to when designing gravity casting process
Gravity die casting quality is determined by the design of casting process and we should pay attention to following points in the design:
- The design of pouring system should be as simple as possible, and the design of sprue, runner, etc. can eliminate turbulence. In order to prevent turbulence
- when pouring, tilting pouring can be applied in gravity die casting process.
- The uneven wall thicknesses tend to cause turbulence during pouring and hot spots when solidifying, which will lead to casting defects as gas porosity, shrinkage porosity, etc. So, the design of the casting should avoid uneven wall thickness and adopt smooth change between different walls.
- In order to avoid short filling, set up venting launders or venting plugs at the positions where the gas is easy to be blocked in the die cavity.
- To achieve sequential solidification, reasonably set risers for feeding at the proper positions.
- To ensure casting quality and improve production efficiency, add cooling system in the die.