How To Design The Die Casting Mold To Help Extend The Life Of The Mold

The correctness of the die casting mold design is directly related to the output and quality of the castings, and the process factors in manufacturing and production should also be taken into account. This article introduces in detail the issues that should be paid attention to in the whole process from die casting product design to die casting mold manufacturing.

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The importance of die casting mold design

The die casting mold is one of the three major elements of die casting production. The mold with correct and reasonable structure is a prerequisite for the smooth progress of die casting production, and plays an important role in ensuring the quality of castings (machine pass rate). Due to the characteristics of the die casting process, the correct selection of various process parameters is the determining factor for obtaining high-quality castings, and the mold is the premise for the correct selection and adjustment of various process parameters. The mold design is essentially a comprehensive reflection of the various factors that may occur in the die casting production. If the mold design is reasonable, there will be fewer problems encountered in actual production, and the casting machine pass rate is high.

On the contrary, the mold design is unreasonable. For example, when designing a casting, the wrapping force of the moving mold and the fixed mold is basically the same, but the pouring system is mostly in the fixed mold, and the casting is placed on the Guannan die-casting machine where the punch cannot feed the material after injection. The production cannot be normal, and the casting sticks to the fixed mold all the time. Although the surface finish of the fixed mold cavity is very clean, the phenomenon of sticking to the fixed mold still occurs due to the deep cavity. Therefore, when designing the mold, it is necessary to comprehensively analyze the structure of the casting, be familiar with the operation process of the die-casting machine, understand the possibility of adjusting the die-casting machine and process parameters, master the filling characteristics under different conditions, and consider the mold processing method, drilling and fixing form, so as to design a practical mold that meets production requirements.

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Issues to be noted in die-casting mold design

The design of the die-casting mold is mainly based on the shape of the die-casting. However, the mold design and size will affect the mold life.

(1) Cavity

High-strength steel is very sensitive to dead corners and gaps. Therefore, when designing the mold cavity, the changes in the wall thickness and ribs should be uniform and gentle, and a larger inner fillet radius should be used as much as possible. In order to reduce the possibility of metal erosion and thermal fatigue occurring near the gate, the cavity wall, core or insert should be as far away from the gate as possible.

(2) Cooling water channel

The cooling water channel should be located in a position that makes the temperature of the entire cavity surface as uniform as possible. From the perspective of cooling and mechanics, the pipe surface needs to be smooth.

(3) Runner, gate and overflow

To obtain the best die-casting effect, the cooling system must have a certain thermal balance with the "hot zone" (runner, gate, overflow and cavity). Therefore, the design of runner, gate and overflow is very important. In the parts of the cavity that are difficult to fill, overflow should be set to allow the die-casting metal to flow to these parts. In a single-mold multi-cavity mold with the same size, all runners must have the same runner length and cross-sectional area, and the gate and overflow must also be exactly the same. The location of the gate and the thickness and width of the runner are critical to the metal injection speed. The runner design should allow the metal to flow smoothly into each part of the cavity, rather than being injected in a jet-like manner. Too fast flow of the injected metal can cause mold erosion.
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Problems to be noted in the manufacture of die casting molds

(1) Machinability

The machinability of hot working tool steels of the martensitic system is mainly affected by non-metallic inclusions such as manganese sulfide and the hardness of the steel. Because the performance of the die casting mold can be improved by reducing the impurity content in the steel, such as sulfur and oxygen. The best structure for cutting is a spheroidized ferrite matrix with uniform distribution of good carbides in a spheroidized state, which gives the steel a lower hardness. Homogenization treatment gives the metal uniform machinability.

(2) Electrospark machining

The basic principle of electrospark machining is to discharge in a non-conductive medium between a graphite or copper electrode (anode) and a steel (cathode). The erosion of the mold is controlled by the discharge. During the operation, the negative electrode enters the steel to obtain the desired shape. The surface temperature of the steel in electrospark machining is very high, causing it to melt and evaporate. A brittle layer is produced on the surface that melts and then solidifies, followed by a re-hardened layer and a tempered layer. EDM has an adverse effect on the surface properties of the mold and destroys the processing properties of the steel. For this reason, as a preventive measure, EDM of steel after quenching and tempering and EDM of steel after annealing are used.

(3) Heat treatment

After machining, heat treatment is necessary to obtain the best high temperature yield strength, tempering resistance, toughness and ductility. The properties of steel are controlled by quenching temperature and time, cooling rate and tempering temperature. Too slow cooling rate during quenching can reduce the steel's destructive toughness. Fast cooling rate such as bath quenching can produce the best structure and thus obtain the highest mold life. In most cases, faster quenching cooling rate is adopted to give priority to the service life of the mold. Decarburization can cause early thermal fatigue. The mold should be cooled to 50℃-70℃ before tempering. To obtain a satisfactory structure, a second tempering is essential. The second tempering temperature should be determined according to the final use hardness required for the mold.

(4) Dimensional stability

During quenching and tempering, die casting molds usually deform or twist. The higher the temperature, the greater the deformation. Before quenching, a certain amount of processing is usually reserved so that the mold can be adjusted to the final required size through grinding and other processes after quenching and tempering. Machining stress, thermal stress, and structural deformation stress will affect dimensional stability. Therefore, during the die casting process, attention should be paid to the temperature and speed of heating and quenching so that the relative deformation range of the size can be controlled within the adjustable range.

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Die casting mold design and manufacturing help to extend the life of the mold

The life of the die casting mold will vary greatly with the design and size of the die casting mold, the type of die casting alloy, and the repair and maintenance of the mold. The life of the mold can be extended by proper treatment before and after die casting. There are several ways to extend the life of the mold:

(1) Proper preheating

The temperature difference between the mold surface and the molten metal should not be too large. For this reason, preheating is usually recommended. The preheating temperature depends on the type of die casting alloy and is usually between 150℃ and 350℃. The material preheating temperature should not be too high, otherwise the mold will be re-tempered during die casting due to the high mold temperature, especially the thinner ribs of the mold will heat up very quickly. It is important to preheat gradually and evenly. A constant temperature heating control system is best.

(2) Correct cooling

The mold temperature is controlled by the cooling water channel and the mold surface release agent. To reduce the risk of thermal fatigue, the cooling water can be preheated to about 50°C. A constant temperature controlled cooling system is also recommended, and cooling water below 20°C is not recommended. When the downtime exceeds a few minutes, the cold water flow should be adjusted so that the mold does not cool too quickly.

(3) Stress relief

During die casting, the mold surface produces thermal strain due to the temperature difference. This repeated strain will cause residual stress on the local surface of the mold. In most cases, this residual stress is tensile stress, which promotes thermal fatigue. Stress relief treatment will reduce the residual tensile stress of the mold, thereby increasing the life of the mold. Therefore, we recommend stress relief after a period of die trial, and then stress relief after 1000-2000 die castings and 5000-10000 die castings. This treatment can be repeated every 10000-20000 die castings until a small amount of thermal fatigue occurs in the die.

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Conclusion

To further improve the economic benefits of the die, heat treatment must be standardized. In addition to producing the best hardness and toughness match through heat treatment, excessive dimensional changes and deformation should be avoided as much as possible. The most critical factors in heat treatment are quenching temperature and cooling rate. Precautions such as correct preheating and appropriate stress relief will further increase the service life of the die. In each of these production steps, the quality has a big change. Only by pursuing the best quality in every production process can the best results be achieved.

References:

[1] Liu Wenchuan. Several issues in complex casting mold design [J]. Mold Industry, 2014, (02)

[2] Yu Bo, Xie Jianqing, Wang Ting. Development status and application of new hot working mold steel [J]. Heat Treatment Technology and Equipment, 2015, 29(6)