What is metal mold casting

Overview Metal mold casting, also known as die casting, is a casting method that pours liquid metal into a metal mold to obtain castings. Casts are made of metal and can be reused many times (hundreds to thousands of times).
Metal to Casting vs. Sand Casting: There are many technical and economical advantages. ​

(1) Castings produced from metal molds have higher mechanical properties than sand mold castings. For the same alloy, its tensile strength can be increased by about 25% on average, its yield strength can be increased by about 20% on average, and its corrosion resistance and hardness are also significantly improved;

(2) The precision and surface finish of castings are higher than those of sand castings, and the quality and size are stable;

(3) The process yield of castings is high and liquid metal consumption is reduced, generally saving 15 to 30%;

(4) No sand or less sand can generally save 80 to 100% of modeling materials; In addition, metal mold casting has high production efficiency; it reduces the causes of defects in castings; the process is simple and easy to implement mechanization and automation. Although metal mold casting has many advantages, it also has shortcomings. like:

(1) Metal type manufacturing costs are high;

(2) The metal mold is airtight and has no yield, which can easily cause defects such as insufficient washing of castings, cracking, or castings exposed to sunlight;

(3) When casting metal molds, the working temperature of the mold, the pouring temperature and speed of the alloy, the time the casting stays in the mold, and the coating used are very sensitive to the quality of the casting and need to be strictly controlled. . The castings that metal mold casting can currently produce have certain restrictions in terms of weight and shape. For example, ferrous metals can only be castings with simple shapes; the weight of the castings cannot be too large; the wall thickness is also limited, and smaller castings have smaller walls. Too thick to be cast. Therefore, when deciding to use metal mold casting, the following factors must be taken into consideration: the shape and weight of the casting must be appropriate; there must be sufficient batch size; and the time limit for completing the production task must be allowed. Characteristics of the formation process of metal mold castings: There are significant differences in performance between metal molds and sand molds. For example, sand molds have air permeability, but metal molds do not; sand molds have poor thermal conductivity, metal molds have good thermal conductivity, and sand molds have yielding properties. , while the metal type does not wait. These characteristics of the metal mold determine that it has its own rules in the casting forming process. The impact of changes in gas state in the cavity on casting molding: When filling metal, the gas in the cavity must be discharged quickly, but metal has no air permeability. As long as the process is slightly neglected, it will have a negative impact on the quality of the casting. . Characteristics of heat exchange during the solidification process of castings: Once the molten metal enters the mold cavity, it transfers heat to the metal mold wall. The liquid metal loses heat through the mold wall, solidifies and shrinks, while the mold wall gains heat, increases the temperature and expands at the same time, resulting in a “gap” between the casting and the mold wall. Before the “casting-gap-metal mold” system reaches the same temperature, the casting can be regarded as cooling in the “gap”, while the metal mold wall is heated through the “gap”. The impact of metal molds that hinder shrinkage on castings: Metal molds or metal cores have no yield during the solidification of castings and hinder the shrinkage of castings. This is another feature of it. Metal mold casting process 1. Preheating of metal. Metal molds that are not preheated cannot be cast. This is because the metal mold has good thermal conductivity/the fluidity of the liquid metal is drastically reduced upon cooling, which can easily lead to defects such as cold shut, insufficient pouring, inclusions, and pores in the casting. When a metal mold that has not been preheated is poured, the mold will be subject to strong thermal shock, doubling the stress, making it extremely easy to damage. Therefore, the metal mold should be preheated before starting work. The appropriate preheating temperature (ie, working temperature) depends on the type of alloy, casting structure and size, and is generally determined through experiments. Generally, the preheating temperature of metal type is not lower than 1500C. ​

The preheating methods for metal type are:

(1) Preheat with a blowtorch or gas flame;

(2) Use resistance heater;

(3) Heating in an oven has the advantage of uniform temperature, but it is only suitable for small metal parts;

(4) First bake the metal mold on the stove, and then pour liquid metal to heat the metal mold. This method is only suitable for small molds because it wastes some molten metal and reduces the life of the mold. 2. Pouring of metal mold The pouring temperature of metal mold is generally higher than that of sand casting. It can be determined experimentally based on the alloy type, such as chemical composition, casting size and wall thickness. The data in the table below are for reference. Pouring temperature of various alloys Alloy type Pouring temperature ℃ Alloy type Pouring temperature ℃ Aluminum-tin alloy 350～450 Brass 900～950 Zinc alloy 450～480 Tin bronze 1100～1150 Aluminum alloy 680～740 Aluminum bronze 1150～1300 Magnesium alloy 715 ~740 cast iron 1300~1370 Due to the chilling and airtightness of the metal mold, the pouring speed should be slow at first, then fast, and then slow. During the pouring process, the liquid flow should be kept as smooth as possible. 3. Casting ejection and core-pulling time If the metal core stays in the casting longer, the force that tightens the core due to the shrinkage of the casting will be greater, so the greater the core-pulling force required. The most suitable residence time of the metal core in the mirror is when die casting is cooled to the plastic deformation temperature range and has sufficient strength. This is the best time to pull the core. If the casting stays in the metal mold for too long, the temperature of the mold wall will increase, requiring more cooling time, which will also reduce the productivity of the metal mold. The most suitable core pulling and casting release time is generally determined by experimental methods. 4. Adjustment of metal mold working temperature To ensure the stable quality of metal mold castings and normal production, we must first make the temperature change of the metal mold constant during the production process. Therefore, every time you pour it, you need to open the metal mold, leave it for a period of time, and then pour it again when it cools to the specified temperature. If natural cooling is relied on, it will take a long time and reduce productivity, so forced cooling is often used. Cooling methods generally include the following:

(1) Air cooling: that is, blowing air around the metal type for cooling to enhance convection heat dissipation. Although the air-cooled metal type has a simple structure, is easy to manufacture, and has low cost, the cooling effect is not very satisfactory. ​

(2) Indirect water cooling: A water jacket is cast on the back or a certain part of the metal mold. Its cooling effect is better than that of air cooling, and it is suitable for pouring copper parts or malleable iron parts. However, for cast thin-walled gray iron castings or ductile iron castings, intense cooling will increase the defects of the castings. ​

(3) Direct water cooling: A water jacket is directly made on the back or part of the metal mold, and water is passed through the water jacket for cooling. This is mainly used for pouring steel parts or other alloy castings, and parts of the casting mold that require strong cooling. Because of its high cost, it is only suitable for mass production. ​

If the thickness of the casting wall is very different, when using a metal mold to produce it, one part of the metal mold is often heated and the other part is cooled to adjust the temperature distribution of the mold wall. ​

5 Metal mold coating During the metal mold die casting process, it is often necessary to spray paint on the working surface of the metal mold. The functions of the coating are: to adjust the cooling rate of the casting; to protect the metal mold from erosion and thermal shock of the mold wall by high-temperature molten metal; and to use the coating layer to store gas and exhaust gas. ​ Depending on the alloy, the paint may have a variety of formulas. The paint is basically composed of three types of substances:

1. Powdered refractory materials (such as zinc oxide, talc powder, zircon sand powder, diatomite powder, etc.);

2. Binder (commonly used water glass, syrup or pulp waste liquid, etc.);

3. Solvent (water). Please refer to the relevant manual for specific recipes. The coating should meet the following technical requirements: it should have a certain viscosity, be easy to spray, and be able to form a uniform thin layer on the surface of the metal mold; the coating should not crack or fall off after drying, and be easy to remove; have high refractoriness; and will not work at high temperatures. Will produce a large amount of gas; will not react chemically with the alloy (except those with special requirements), etc. 6 Sand-coated metal mold (iron mold sand-coated) Although coating can reduce the cooling rate of castings in metal molds, it is still difficult to produce ductile iron castings (such as crankshafts) using metal molds painted with coating because of the cooling rate of castings. It is still too large, and the casting is prone to white spots. If a sand mold is used, although the cooling rate of the casting is low, shrinkage porosity or shrinkage cavities are likely to occur at the hot joints. If a 4-8mm sand layer is applied to the surface of the metal mold, a satisfactory ductile iron casting can be cast. The complex sand layer effectively adjusts the cooling rate of the casting. On the one hand, it prevents the cast iron body from whitening, and on the other hand, it makes the cooling rate faster than sand casting. The metal mold has no collapsibility, but the thin sand compound can appropriately reduce the shrinkage resistance of the casting. In addition, the metal mold has good rigidity, which effectively limits the graphitization expansion of ductile iron, enables riser-less casting, eliminates porosity, and improves the density of castings. If the sand layer of the metal mold is resin sand, sand shooting process can generally be used to cover the sand. The temperature requirement of the metal mold is between 180 and 200°C. Sand-coated metal molds can be used to produce ductile iron, gray iron or steel castings, and their technical effects are significant. ​

7 Life span of metal type

The ways to improve the life of metal molds are:

1. Use materials with large thermal conductivity, small thermal expansion coefficient and high strength to make metal molds;

2. Reasonable coating technology and strict compliance with process specifications;

3. The metal type has a reasonable structure, and care should be taken to eliminate residual stress during the manufacturing process;

4. The grains of metallic materials should be fine. Process design of metal mold die castings According to some characteristics of the metal mold casting process, in order to ensure the quality of the castings, simplify the metal mold structure, and give full play to its technical and economic benefits, the structure of the castings must first be analyzed and a reasonable casting process must be formulated. 1 Technological Analysis of Casting Structure The technological quality of metal mold casting structure is a prerequisite for ensuring the quality of castings and giving full play to the advantages of metal mold casting. A reasonable casting structure should follow the following principles:

1) The casting structure should not hinder molding or shrinkage;

2) The thickness difference should not be too large to avoid large temperature differences among various parts, which may cause shrinkage cracks and shrinkage porosity in the casting;

3) Limit the minimum wall thickness of metal mold castings. ​

In addition, the accuracy and smoothness of the non-machined surface of the casting should be appropriate.

2 The pouring position of the casting in the metal mold The pouring position of the casting is directly related to the number of cores and parting surfaces, the introduction position of liquid metal, the feeding effect of the riser, the smoothness of the exhaust and the complexity of the metal mold, etc. .