Correctness of die casting mold design is directly related to output and quality of castings, 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.

 

Die casting mold is one of the three major elements of die casting production. Mold with correct and reasonable structure is a prerequisite for smooth progress of die casting production, and plays an important role in ensuring quality of castings (machine pass rate). Due to characteristics of die casting process, correct selection of various process parameters is determining factor for obtaining high-quality castings, mold is the premise for correct selection and adjustment of various process parameters. Mold design is essentially a comprehensive reflection of various factors that may appear in die casting production. If mold design is reasonable, there will be fewer problems encountered in actual production, and casting machine pass rate is high.
On the contrary, mold design is unreasonable. For example, when designing a casting, wrapping force of moving mold and fixed mold is basically same, but pouring system is mostly in fixed mold, and casting is placed on Guannan die-casting machine where punch cannot feed material after injection. Production cannot be normal, and casting sticks to fixed mold all the time. Although surface finish of fixed mold cavity is very clean, phenomenon of sticking to fixed mold still occurs due to deep cavity. Therefore, when designing mold, it is necessary to comprehensively analyze structure of casting, be familiar with operation process of die-casting machine, understand possibility of adjusting die-casting machine and process parameters, master filling characteristics under different conditions, and consider mold processing method, drilling and fixing form, so as to design a practical mold that meets production requirements.

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

High-strength steel is very sensitive to dead corners and gaps. Therefore, when designing mold cavity, changes in 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 possibility of metal erosion and thermal fatigue occurring near gate, cavity wall, core or insert should be as far away from gate as possible.

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

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

 

Machinability of hot working tool steels of martensitic system is mainly affected by non-metallic inclusions such as manganese sulfide and hardness of steel. Because performance of die casting mold can be improved by reducing impurity content in 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 steel a lower hardness. Homogenization treatment gives metal uniform machinability.

Basic principle of electrospark machining is to discharge in a non-conductive medium between a graphite or copper electrode (anode) and a steel (cathode). Erosion of mold is controlled by discharge. During operation, negative electrode enters steel to obtain desired shape. Surface temperature of 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 surface properties of mold and destroys processing properties of steel. For this reason, as a preventive measure, EDM of steel after quenching and tempering and EDM of steel after annealing are used.

After machining, heat treatment is necessary to obtain the best high temperature yield strength, tempering resistance, toughness and ductility. Properties of steel are controlled by quenching temperature and time, cooling rate and tempering temperature. Too slow cooling rate during quenching can reduce 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 service life of mold. Decarburization can cause early thermal fatigue. 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 final use hardness required for mold.

During quenching and tempering, die casting molds usually deform or twist. The higher temperature, the greater deformation. Before quenching, a certain amount of processing is usually reserved so that mold can be adjusted to 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 die casting process, attention should be paid to temperature and speed of heating and quenching so that relative deformation range of size can be controlled within adjustable range.

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

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

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

During die casting, mold surface produces thermal strain due to temperature difference. This repeated strain will cause residual stress on local surface of mold. In most cases, this residual stress is tensile stress, which promotes thermal fatigue. Stress relief treatment will reduce residual tensile stress of mold, thereby increasing life of mold. Therefore, we recommend stress relief after a period of die trial, 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 die.
 

To further improve economic benefits of 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 service life of die. In each of these production steps, quality has a big change. Only by pursuing the best quality in every production process can the best results be achieved.