Die Casting Air Leakage Guide – 7 Ways To Improve Tightness Of Die Castings

This article mainly analyzes the defects of poor air tightness of die castings and finds out the possible causes of defects. Poor air tightness of castings means that when a certain pressure is applied to the inside of a die casting, leakage occurs from the inside or outside of the casting, causing a pressure drop. If such casting is put into use, it may show oil leakage, air leakage, water leakage, etc. Poor air tightness is one of the most difficult problems to solve in the defects of die castings. There are many causes. Theoretically, any defect of die castings may lead to air leakage of castings. In most cases, it is caused by the combination of several defects.

Cause Analysis of Air Leakage in Die Castings

1.Gas in raw materials

Our common die-casting material is aluminum alloy (this article only takes aluminum alloy as an example to explain). In the production of die castings, because the main component of the gas in the aluminum liquid is hydrogen, the hydrogen content in the aluminum alloy liquid has a direct relationship with the number of pores in the castings. The porosity in the casting not only reduces the mechanical properties and corrosion resistance of the alloy, but also reduces the air tightness. At present, the main method to remove the gas in the aluminum liquid is to inject inert gas such as nitrogen into the aluminum alloy or add solid degassing agent to make the hydrogen dissolved in the aluminum liquid diffuse into the bubble. When the bubble floats to the surface of the aluminum liquid, the bubble bursts and the hydrogen escapes into the atmosphere, so as to achieve the purpose of removing hydrogen.

2.Impact of mold casting system

The gating system determines the design quality of a pair of die casting dies, and is also the main factor determining the quality of medium pressure castings in later production. As a system, it is composed of many elements, and its purpose is to make the alloy liquid enter the mold cavity with proper flow state

3.Equipment performance

Air hole, shrinkage cavity and cold shut in die castings are also the main reasons for air leakage of die castings, and equipment performance plays a crucial role in casting production. For products with strict air tightness requirements, appropriate die-casting models must be selected. At present, three-stage injection is basically adopted in die casting production of die casting machine. During the first stage injection, the injection punch moves at a slower speed, which is conducive to extruding the gas in the pressure chamber; In the second stage of injection, the ingate speed is extremely fast, and the aluminum liquid basically fills the mold cavity. At the same time, if the position of the secondary injection rate comes too early, the casting is prone to defects such as air holes. If the starting position of the secondary injection rate is too late, the casting is prone to cold shut and other defects. Generally, it is better to choose the starting position of the second stage injection velocity at the point where the molten metal in the cup just reaches the entrance of the ingate. Therefore, this stage is the key to the generation of pores, so the higher the speed is, the easier it is to generate vortex and form pores.

4.Operation method

In the process of die casting, due to the high volatilization point and large gas emission of some coatings, it has a direct impact on the porosity of castings. The release agent is mainly sprayed manually, and the amount is mainly determined by experience. If the spraying amount is too much and the spraying time is too long, it is easy to cause large gas volatilization. In addition, the mold temperature is low, and the volatilization is not timely, and other factors lead to large porosity. Therefore, the paint with low volatile point and small amount of gas shall be selected in the production process. At the same time, the blowing time can be appropriately extended to ensure that the dynamic and fixed molds are dried. For the mold with complex shell structure, it is necessary to dry the excess demoulding water or oil on the mating surface of the injection chamber and the punch, the cavity surface of the mold, the core pulling position and the runner.

5.Control of machining allowance

During the forming process of die castings, the mold cavity is filled at the fastest speed, so that the aluminum liquid can rapidly solidify in the mold to form products. Therefore, there will inevitably be air holes in the casting due to aluminum liquid entrainment, or shrinkage due to different densities between the solid and liquid. However, the surface layer of castings will also form a dense layer of fine grains due to rapid solidification, and the mechanical properties of these fine grain layers are relatively high. With the change of production process, the thickness of the dense layer of the structure will be different for different castings. In order to ensure the air tightness of castings, small machining allowance shall be used as far as possible in the later machining process.

6.Reasonably add recycled materials

Pinhole size reflects the spatial distribution density of pinholes and is an important factor affecting the air tightness of castings. There are many pinholes in the castings due to the combined effects of the micro pores and oxidation inclusions in the recycled materials. The scrap rate of individual castings in production is relatively high. Due to energy saving and consumption reduction, the actual production of scrap parts for recycling, as well as a substantial increase in the recycling amount of runner, will also lead to poor airtightness of castings. Therefore, when producing castings with air tightness requirements, the classification, treatment and use of recycled materials shall be strictly controlled, so that the proportion of recycled materials and new materials can be used in strict proportion when meeting the quality requirements. Otherwise, excessive use of recycled materials will increase the degree of pinhole of castings in subsequent production, failing to meet the requirements of air tightness, which is not conducive to the assurance of casting quality.

7.Select reasonable pressure chamber fullness

When the diameter of the punch and the die casting machine are selected, the weight of the metal liquid contained in the pressure chamber is also a certain value, but the weight requirements of the metal liquid poured each time for different castings are different. If the volume of metal liquid poured into the pressure chamber is insufficient (i.e., the filling degree of the pressure chamber is low), the gas in the pressure chamber cannot be eliminated as soon as possible during injection. With the high-speed push of the injection piston, the disordered flow in turbulent state is formed, which is easy to get involved in the gas, leading to defects such as air holes and insufficient pouring. At the same time, due to the existence of too much oxide skin in the pressure chamber, it is easy to form a diaphragm inside the casting, which leads to the reduction of local strength of the casting, and leakage is very easy to occur under the action of large leakage test pressure. Therefore, the reasonable filling degree of the pressure chamber can effectively reduce the porosity defects in the castings, thereby reducing the air leakage rate of the castings.