Die casting production process, due to the limitations of the process itself, inevitably produce some size of the casting internal pores, shrinkage, and other defects; especially the die casting of the thick part of the internal shrinkage, shrinkage is difficult to avoid the generation of loose, this article through an example of the solution to the problem of internal defects in the thick part of the die casting provides a new way.
Casting status analysis
Aluminum die-casting lid castings, material requirements: AlSi12; size: 230X198X75, blank weight 1.2Kg; maximum wall thickness of 8mm, the minimum wall thickness of 1.9mm, average wall thickness: 3.5mm; the original casting blank diagram is shown in Figure 1.
The product function and ID design needs, the outer side is flat, the inner side is rough die-casting surface, after adding the die pulling slope, the product wall thickness is very large in many places 8mm, at the same time the product assembly space is reduced, there is a contradiction, the general solution is to remove the material by machine addition, but the processing to remove too much material, need to process 5mm, a lot of air holes are exposed, according to the similar product estimate, the air holes are difficult to If the pores are close to the outer wall, there is a risk of leakage, which is not easy to identify, and if we increase the test leak, we will invest more in the test leak tooling and test leak process.
According to the customer’s blank design, the product needs to be produced by the die-casting process according to the customer’s design from the inner cavity on 3 sides of the line to reduce the wall thickness of the large position, reduce the wall thickness of the blank diagram, as shown in Figure 2.
Characteristics of the die casting process
The so-called die-casting is in the die-casting machine metal type pressure casting short, casting, the liquid alloy at high speed, high pressure, filling the cavity, and in high-pressure forming, solidification to form castings, is the highest production efficiency of the casting process; the die-casting process of high than pressure filling, greatly improve the fluidity of the alloy, metal liquid crystallization solidification and is under pressure. Therefore, the main characteristics of die-casting are molding under high pressure.
(1) molding under high pressure, product denseness, high mechanical strength, and surface hardness, but the product elongation is low.
(2) Fast filling, short cooling time, high production efficiency, and low cost in mass production.
(3) The product surface is rough, even up to Ra1.6-6.3.
(4) It is possible to produce parts with thin wall thickness.
(5) Fast filling, more gas involved inside, more pores in the product.
(6) Not heat treatable. The internal gas will expand during heat treatment, resulting in defects such as bulging or cracking of the product.
(7) A dense layer of about 0.5-0.8mm thick is on the surface, so the machining allowance is small (generally at 0.5mm). Too much machining will destroy the dense layer on the surface, resulting in reduced strength of the product and easy to make the machined surface defects such as porosity and shrinkage.
The above characteristics of die casting can be seen in the die-casting parts up to 8mm thick, the inner cavity of the machine deeper than 5mm. The internal will appear serious shrinkage, shrinkage, porosity, and other defects.
Die casting internal shrinkage, shrinkage, and porosity phenomenon exist for the following reasons.
(1) The aluminum alloy used in die casting is not enough to remove slag and gas, and the aluminum liquid contains high gas content.
(2) The residual mold release agent on the surface of the mold when the product is die-cast is not completely evaporated. When it comes into contact with the aluminum liquid, gas is generated.
(3) The casting temperature of the alloy is too high, and the temperature gradient distribution of the mold is unreasonable.
(4) The inner gate design is too thin, the area is too small, solidifying too early, and is not conducive to pressure transfer and metal liquid replenishment shrinkage.
(5) The casting design is unreasonable, and there is a hot joint that is difficult to make up for the shrinkage.
(6) low-pressure injection ratio and low pressurization pressure, resulting in poor local hot joints for replenishment and shrinkage.
So, can we solve it from the process to make the casting quality meet customer requirements?
As we all know, the density of solid aluminum alloy is 2.7X103 kg/m3, while liquid aluminum alloy’s density is around 2.4 X 103kg/m3. After the metal melt fills the cavity, when it turns from liquid to solid, the phase change shrinkage is bound to occur due to the difference in density; the die casting is cooled and solidified from outside to inside. This characteristic determines that when the casting wall thickness is larger, the mold gate area is thinner and solidifies in advance. Solidification, thick parts of the region’s center due to the gate solidification, can not effectively fill the shrinkage. The internal is bound to produce shrinkage, shrinkage, and other problems; this is the root cause of die casting shrinkage loose.
Therefore, the problem of shrinkage and shrinkage inside the casting is inevitable for aluminum alloy die castings, especially for thick die castings. Theoretically, it is very difficult to avoid.
To solve the way of die casting shrinkage defects
From the causes of shrinkage, shrinkage can be seen, from the process principle, to solve the casting shrinkage defects, only following the guiding principle of shrinkage; for a long time, people have carried out a lot of experimental work, the transformation of the die-casting machine, the process of improvement. Although the moderate wall thickness of the casting can be from the process, equipment measures to improve, however, aluminum alloy liquid solidification process of phase change shrinkage is an inevitable physical phenomenon, can not go against this natural law; for the wall thickness of the die casting, can only follow its laws, take the appropriate way to solve the problem.
Therefore, theoretically speaking, can not complete solving die-casting shrinkage and shrinkage problems from the die-casting process itself. To completely solve the problem can only go beyond the process and seeking a solution.
The way to solve the embedded live block method
The die-casting process itself can not solve the problem. Can the mold structure solve it?
From die-casting often uses inserts to change the local use of performance to get inspired; we can fully embed a movable live block inside the casting, pre-embedding the live block in the mold and aluminum alloy surrounded by molding out, to be knocked out of the live block after the product die-casting molding, live block to do the next die spare, live block repeatedly use, that is, to reduce production costs, the product also reduces weight to 0.37Kg, according to this idea, we in die-casting mold According to this idea, we selected a position on the die-casting tool, added some positioning positions, specifically for the production of live block H3 material, after improvement, each mold production of a product with 3 pieces of the live block, the improved blank with live block diagram, as shown in Figure 3.
The live block in the installation process, such as the mold live block fixed
After the live block is embedded in the mold, in the subsequent die-casting process, the live block will be stuck in the die casting when it is displaced under the impact of high-pressure aluminum water, so it is necessary to set the fixed structure in the live block structure to prevent the movement of the live block; as shown in Figure 4, we set three dovetail structures on the positioning of the three live blocks to play a positioning role when they are put into the die casting mold, which is used to prevent the live block from moving in the die casting process. It is used to prevent the movement of the live block during the die-casting process.
After the parts do live block for processing, the product’s weight is reduced after the cancellation of the machining process, and the surface quality reaches the quality requirements of customers.
Applicable conditions for solving partial thickness of large wall thickness with live block
From the structure, many products have this large wall thickness structure, a die-casting process caused by the mold is not suitable for internal extraction slider, many times the customer can only in industrial design, product shape and cost to make a balance, live block technology to solve this contradiction provides a solution, in the use of live block process, need to solve some problems in the process, such as the impact of the die-casting beat, the product in the case of uncooled take out the live block, the product deformation caused by the live block stuck, etc. The product deformation causes the live block to be stuck, etc.
In some projects, the overall demand is not large. In the case of wall thickness being particularly large, the overall structure of the product is not the more applicable and particularly complex case. When the product structure is complex, the slider is used too much. It is easy to fail.
This paper shows that the improvement of the mold embedded live block, to solve the die casting local wall thickness at the surface quality, the use of embedded live block method is a good choice, the use of this method relatively reduced the casting machine margin, to retain the casting surface dense layer to meet the casting requirements.
For more information on ways to improve the quality of die castings, please get in touch with us at V1diecast. Give you professional suggestions.
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