Unpreheated metal molds cannot be poured. This is because the metal mold has good thermal conductivity, the liquid metal cools quickly, and the fluidity is drastically reduced, which easily causes 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 150°C.

Metal type preheating methods include:

(1) Preheating with a blowtorch or gas flame; (2) Using a resistance heater; (3) Using an oven for heating, its advantages The temperature is uniform, but it is only suitable for small pieces of metal molds; (4) First bake the metal mold on the furnace, 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. The pouring temperature of metal molds 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

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Zinc alloy 450～480 Tin bronze 1100～1150

Aluminum alloy 680～740 Aluminum bronze 1150～1300

Magnesium alloy 715～740 Cast iron 1300～1370

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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. The longer the metal core stays in the casting, the greater the force that holds the core due to the shrinkage of the casting, and therefore the greater the core pulling force required. The most suitable residence time of the metal core in the mirror is when the 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. To ensure stable quality and normal production of metal mold castings, 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 mold 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 the casting mold requirements are strong. Cool parts. 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. During the metal mold 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 different alloys, coatings may have a variety of formulas. Coatings are basically composed of three types of substances: 1. Powdered refractory materials (such as zinc oxide, talc powder, zirconium 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; it should have high refractoriness; does not produce a large amount of gas at high temperatures; does not react chemically with the alloy (except for those with special requirements), etc. 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 painted metal molds because the cooling rate of castings is still too high and white spots are prone to appear in castings. 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 that of 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. 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 mold 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.