The Influence of Die Casting Mold Design on Mold Life

Die casting mold design is another important factor affecting the life of die casting molds, and the quality of mold design directly affects the life of die casting molds. Therefore, in the design of die-casting molds, it is necessary to fully consider the factors that affect the mold life based on the characteristics of the castings, design the mold structure reasonably, and provide timely improvement suggestions or take reasonable measures to solve the casting design that affects the mold life in the early stage.

1. Is the mold strength sufficient

The strength of the mold is an important factor affecting its lifespan. Currently, most Chinese enterprises, including die-casting companies, pay special attention to the mold price when purchasing molds, and even believe that as long as qualified products can be die cast, it is enough. However, the size and strength of the mold have not been given enough attention. At the same time, many die-casting mold manufacturing companies often lower the price of molds in order to obtain orders, and save as much as possible during mold production to reduce costs. If the strength of the mold is not sufficient, it not only affects the reliability of the mold, but also affects its service life. Actually, from the perspective of die-casting production, such molds are not economical. If the strength and rigidity of the die-casting mold are not sufficient, the mold will crack prematurely under continuous impact and compression. And due to too few aluminum sealing surfaces, it can cause aluminum to seep through the mold, and if not cleaned in a timely manner, the mold can be pressed and damaged. Therefore, in mold design, it is necessary to fully consider the strength of the mold to ensure its lifespan.

Hardness is the main technical indicator of mold steel. To maintain its shape and size under high stress, the mold must have sufficient hardness. Cold work mold steel generally maintains a hardness of around HRC60 at room temperature, while hot work mold steel is generally required to maintain a hardness in the range of HRC40-55 depending on its working conditions. For the same steel grade, within a certain range of hardness values, hardness is directly proportional to deformation resistance; However, there may be significant differences in plastic deformation resistance between steel grades with the same hardness value but different compositions and structures.

The hot work mold with red hardness that works at high temperatures requires the stability of its structure and properties to maintain a sufficiently high hardness, which is called red hardness. Carbon tool steel and low-alloy tool steel can usually maintain this performance in the temperature range of 180-250 ℃, while chromium molybdenum hot work die steel generally maintains this performance in the temperature range of 550-600 ℃. The red hardness of steel mainly depends on its chemical composition and heat treatment process.

The compressive yield strength and compressive bending strength of molds are often subjected to high compressive and bending forces during use, so it is required that the mold material should have certain compressive and bending strength. In many cases, the conditions for conducting compressive and bending tests are close to the actual working conditions of the mold (for example, the measured compressive yield strength of the mold steel is in good agreement with the deformation resistance exhibited by the punch during operation). Another advantage of the bending test is that the absolute value of the strain is large, which can sensitively reflect the differences in deformation resistance between different steel grades and under different heat treatments and microstructure states.

2. Is the gate speed in the mold appropriate? It should be kept as low as possible

The design of the sprue in die-casting molds is not only an important factor affecting the quality of castings, but also directly affects the service life of die-casting molds. Therefore, it must be highly valued in the design of die-casting molds. The faster the sprue speed in the die-casting mold, the greater the impact on the mold cavity, the greater the instantaneous heating of the mold cavity, and the more likely the mold is to have surface cracks or fissures. The gate inside the die-casting mold is generally recommended to be between 30-70 meters per second, and while ensuring product quality, it should be kept as low as possible to reduce the impact on the mold and extend its lifespan. Under certain injection conditions, when the sprue area is too large, the filling speed will be too low, the metal will solidify prematurely, and even lead to insufficient filling; However, a too small sprue area can intensify spraying, increase heat loss, generate vortices, and draw in too much gas, exacerbating mold erosion and leading to early mold scrapping.

a. The size of the cross-sectional area of the sprue is often determined based on experience during the design and drawing process. Simply calculating the cross-sectional area of the sprue based on empirical formulas severs the close relationship between the cross-sectional area of the sprue and the filling speed and time. There is a risk of ineffective matching between the cross-sectional area of the sprue and the filling speed and time. The designer is not clear about the extent to which the design results can be modified within a certain process range.

b. The cross-sectional area of the sprue varies greatly, and in production practice, it is not uncommon for the cross-sectional area of the sprue in the mold to not match the die-casting part. When this mismatch is not significant, its performance is not very obvious. In the use of the mold, operators often feel that the mold is not easy to use; When the gap is large, it will be clearly manifested that the die-casting parts cannot be formed, the scrap rate is high, and the quality is unstable.

c. During the production process, there will be high requirements for process parameters, and slight fluctuations in the process can cause various defects on the surface of the die-casting parts. If such molds are produced on a higher performance die-casting machine, it is possible to smoothly produce qualified die-casting parts, but it is difficult to produce them normally on existing die-casting machines.

3. Is the strength of the mold slider locking block sufficient

Due to the high speed and pressure of die-casting, the impact force on the slider of the die-casting mold is very large. The specific pressure of die-casting is generally selected between 400-900 kg/cm2. Taking the projection of the slider at 100 × 100mm as an example, the force can be as large as 40-90 tons. Therefore, many die-casting mold companies generally design molds to save materials. The mold design is too small, the locking block is not large enough, and the strength is not enough, resulting in the slider not being able to lock. Not only does it affect the quality of castings, but it can also easily cause aluminum channeling, resulting in the slider getting stuck, causing deformation or cracking of the mold locking part, thereby damaging the mold and affecting its lifespan. Therefore, when designing molds, it is necessary to ensure that the strength of the mold slider locking block is sufficient to ensure the reliability of the mold and extend its service life.

4. Is the aluminum sealing surface of the mold cavity sufficient

Due to the high temperature, high speed, and high pressure of die-casting, aluminum leakage in die-casting molds occurs from time to time. The reasons for this are not only improper selection of die-casting parameters, too fast injection speed, excessive pressure, and insufficient locking force, but also unreasonable mold design and insufficient aluminum sealing surface. If there is aluminum leakage in the mold, it will directly affect the internal quality of the casting. On the other hand, due to the lack of timely or difficult cleaning during the die-casting process, the mold will be crushed, causing the mold parting surface to collapse. More seriously, in molds with slider structures, if aluminum penetrates into the gap between the sliders, it can cause the slider or slider seat to be crushed or even crack the mold frame due to the strong locking force during mold closing. This phenomenon often occurs in die-casting enterprises. The main reason is that die-casting enterprises or mold manufacturing enterprises make the molds too small and the aluminum sealing surface is not large enough in order to save costs. There is a phenomenon worth reflecting on. Japanese people are very frugal, but the die-casting molds made by Japanese people are basically larger than those made by most Chinese manufacturers. Why? Because if the cost of die-casting molds is only considered and the most critical characteristics of the mold, namely reliability, yield, production efficiency, and lifespan, are ignored, it will not be worth the loss. This is also the gap between Chinese die-casting molds and molds from advanced foreign countries. This concept is not only something that needs to be changed by die-casting mold manufacturing enterprises, but also something that die-casting mold using enterprises should reverse. Therefore, when designing molds, high attention should be paid to the aluminum sealing surface, especially in the sliding block area, which must be sufficient to ensure that there is no aluminum leakage and thus extend the mold life.

5. Is the mold cooling (heating) design reasonable, and is the mold temperature field reasonable

The design of mold cooling water is a crucial link in the design of die-casting molds. In most die-casting mold manufacturers in China, the pouring system is often highly valued.

The temperature of the mold may be too high or even reach the bottom where it cannot be produced normally; The mold may also be scrapped due to excessive deformation caused by high temperature; It will significantly reduce the lifespan of the mold; Another issue is that it can cause high-temperature oxidation failure on the surface of the mold.

Many manufacturers often study the filling part extensively when using mold flow analysis software, but do not study the temperature field changes of the mold much. In fact, in die-casting production, mold cooling and temperature field changes are crucial for production efficiency, casting quality, and mold life. If conditions permit, a dedicated die-casting mold temperature machine can be used, which can help improve production efficiency, enhance casting quality, and also extend mold life. During the use of aluminum alloy die-casting molds, the liquid aluminum alloy instantly fills the mold cavity, causing the mold surface to withstand high temperatures and resulting in an increase in surface temperature. The cooling forming of die cast aluminum alloy products is achieved by using the mold material as a thermal conductor to transfer the heat emitted during the solidification process of the aluminum alloy to the inside of the mold. Due to the irregular shape of the product formed by die-casting, that is, the uneven thickness of the product, the heat emitted by the casting product during the solidification process is different. In the process of mold design, it is necessary to effectively design the internal cooling water channel distribution of the mold to minimize the temperature difference on the mold surface. Only in this way can the temperature difference on the surface of the mold be reduced, thereby controlling the deformation of the die-casting product, reducing the occurrence of shrinkage holes, and ensuring the production capacity and pace of the shift. The control of mold temperature field needs to be considered comprehensively in the early stage of mold design. That is to say, it is necessary to set the shift output and production pace (such as 60 products/hour) in order to understand the heat power emitted by the mold during the production process. After understanding the thermal power emitted by different thickness parts of the mold during production, calculate the diameter of the cooling water channel, the distance from the surface, and the flow rate of the cooling water to control the surface temperature of the mold at 240 degrees. The common problem at present is that the establishment of temperature field is not based on calculation, and the distance between the cooling water channel and the surface, as well as the aperture of the cooling water channel, can be roughly estimated based on experience. As a result, it is difficult to achieve uniform surface temperature of the mold. When there is local overheating in the mold, a large amount of spray release agent is used to cool the mold. The result is that after a short period of use, such as 3000 cycles, thermal fatigue cracks appear on the surface of the mold. In the subsequent production process, due to the continuous exposure of the mold to severe cold/hot cycles, the early failure of the mold did not meet the predetermined 80000 to 100000 mold life requirements. This greatly reduces the lifespan of the mold. Therefore, optimizing the design of mold cooling (heating) is the key to improving mold yield and production efficiency, and it is also a very important way to extend mold life.

6. Is the vulnerable area fitted together

In order to extend the lifespan of the mold, complex cavities can be processed in blocks during mold design to simplify the process. For parts that are prone to mold cracking or damage, they can be assembled to facilitate maintenance and replacement, reducing mold manufacturing costs; On the other hand, patchwork seams can block the propagation of cracks. It also avoids local cracking from spreading to other parts, thereby extending the lifespan of the mold.

7. Reduce sharp corners of stress concentration

Sharp corners are prone to stress concentration, therefore, for die-casting parts, the sharp corners at the corners have a significant impact on the casting process, casting quality, and die casting mold life. Therefore, when discussing issues with casting designers in the early stages, it is important to pay close attention to the corners of the castings and maximize them as much as possible.

8. Adopting advanced technology

In order to extend the service life of molds, many scientific and technological personnel, including those in China, are actively exploring new technologies such as surface nitriding technology, titanium plating, surface micro grid life extension technology, and so on. Therefore, as a manufacturing enterprise of die-casting molds and a designer of die-casting molds, actively exploring new technologies, timely tracking advanced technologies, and timely adopting new technologies to improve mold life are also effective ways to enhance the level of die-casting molds and improve the technical level of enterprises. Huiwang Company has explored this area and applied many new technologies, achieving good results.