What is Casting

Casting-metal thermal processing technology

Casting processing, Casting types,

Casting is a kind of metal thermal processing technology that mankind has mastered relatively early. And it has a history of about 6000 years. China has entered the heyday of bronze castings between 1700 and 1000 BC. The craftsmanship has reached a very high level.

Casting is a method of pouring liquid metal into a casting cavity suitable for the shape of the part, and after it has cooled and solidified, the part or blank is obtained. The cast material is mostly a metal that is originally solid but heated to a liquid state (for example: copper, iron, aluminum, tin, lead, etc.), and the material of the mold can be sand, metal or even ceramics. According to different requirements, the method used will be different.

Overview

Casting is a kind of metal thermal processing technology that mankind has mastered relatively early, and it has a history of about 6000 years. China has entered the heyday of bronze castings between 1700 and 1000 BC, and the craftsmanship has reached a very high level.

Casting refers to a processing method in which solid metal is melted into a liquid state and poured into a mold of a specific shape, and then solidified and formed. The cast metals include: copper, iron, aluminum, tin, lead, etc. The materials of ordinary castings are raw sand, clay, water glass, resin and other auxiliary materials.

Special casting molds include: investment casting, lost foam casting, metal mold casting, ceramic mold casting, etc. (Raw sand includes: quartz sand, magnesia sand, zircon sand, chromite sand, forsterite sand, cyanite sand, graphite sand, iron sand, etc.)

Early stage

The Simuwu Fangding Ding weighing 875 kg from the Shang Dynasty in China, the Yizun plate from the Warring States Period, and the translucent mirror from the Western Han Dynasty are all representative products of ancient casting.

Most of the early castings were tools or utensils used in agricultural production, religion, life, etc., with a strong artistic color. At that time, the casting process developed in parallel with the pottery process and was greatly influenced by the pottery.

Progression stage

In 513 BC, China cast the world’s first iron casting found in written records-the Jin Kingdom cast-shaped tripod, weighing about 270 kilograms.

Europe also began to produce iron castings around the eighth century. The emergence of iron castings has expanded the scope of application of castings. For example, in the 15th to 17th centuries, Germany, France and other countries successively laid many cast iron pipes to supply drinking water to residents.

After the Industrial Revolution in the 18th century, industries such as steam engines, textile machinery, and railways emerged, castings entered a new period of service to large industries, and casting technology began to develop greatly.

Modern times

Entering the 20th century, the rapid development of casting, one of the important factors is the advancement of product technology, which requires better mechanical and physical properties of castings, while still having good mechanical processing properties;

Another reason is that the development of the machinery industry itself and other industries such as chemicals and instrumentation have created favorable material conditions for the foundry industry. For example, the development of testing methods ensures the improvement and stability of casting quality and provides conditions for the development of casting theory;

The invention of the electron microscope has helped people to penetrate into the microscopic world of metals, explore the mysteries of metal crystallization, study the theory of metal solidification, and guide casting production.

Casting-smelt metal, manufacture molds, pour molten metal into the molds, and obtain a forming method of rough metal parts with a certain shape, size and performance after solidification
Casting is a process in which metal is smelted into a liquid that meets certain requirements and poured into a mold. After cooling, solidification, and cleaning, a casting with a predetermined shape, size and performance is obtained.

Because the casting blank is almost formed, it can achieve the purpose of avoiding mechanical processing or a small amount of processing, which reduces the cost and reduces the production time to a certain extent. Casting is one of the basic processes of modern device manufacturing industry.

Casting classification

There are two main categories: sand casting and special casting.

Ordinary sand casting

Using sand as the mold material, also known as sand casting, sand casting. Includes 3 types of green sand, dry sand and chemically hardened sand, but not all sand can be used for casting. The advantage is that the cost is lower, because the sand used in the mold can be reused. The disadvantage is that the production of the mold is time-consuming. The mold itself cannot be reused, and the finished product can only be obtained after being destroyed.

Sand mold (core) casting method. Green sand mold, resin self-hardening sand mold, sodium silicate sand mold, dry mold and surface dry mold, solid mold casting, negative pressure molding.
Sand core manufacturing method. It is selected according to the size, shape, production batch and specific production conditions of the sand core. In production, it can be divided into manual core making and machine core making as a whole.

Special casting

According to the modeling material. Can be divided into special casting with natural mineral sand and gravel as the main modeling material (such as investment casting, mud casting, shell casting, negative pressure casting, solid casting, ceramic casting, etc.) And special casting with metal as the main mold material. Such as metal mold casting, pressure casting, continuous casting, low pressure casting, centrifugal casting, etc.

Metal mold casting method
Use a metal with a higher melting point than the raw material to make a mold. Among them, it is subdivided into gravity casting method, low pressure casting method and high pressure casting method.
Subject to the melting point of the mold, the metal that can be cast is also limited.

Dewaxing casting method
This method can be the outer film casting method and the solid casting method.
Copy the object to be cast with wax first, then immerse it in a pool containing ceramic (or silica sol) and wait to dry. So that the wax replica is covered with a ceramic outer film. And the steps are repeated until the outer film is sufficient to support the casting Process (about 1/4 inch to 1/8 inch). Then melt the wax in the mold and pull out of the mold. After that, the mold needs to be heated many times to increase the hardness before it can be used for casting.
This method has good accuracy and can be used for casting of high melting point metals (such as titanium). However, due to the high price of ceramics, and the need for multiple heating and complicated production, the cost is quite expensive.

Molding

Gravity casting: sand casting, hard mold casting. Rely on the gravity of the metal itself to pour molten metal into the cavity.
Pressure casting: low pressure casting, high pressure casting. Rely on the additional pressure to instantly press the molten metal into the casting cavity.
The casting process usually includes
①Preparation of casting molds (containers that make liquid metal into solid castings). Casting molds can be divided into sand, metal, ceramic, clay, graphite, etc. according to the materials used, and can be divided into disposable and semi-permanent types according to the number of uses. And permanent mold, the pros and cons of mold preparation are the main factors affecting the quality of castings;
②Melting and pouring of cast metal, cast metal (casting alloy) mainly includes various types of cast iron, cast steel and cast non-ferrous metals and alloys;
③ Casting processing and inspection. Casting processing includes removal of foreign bodies on the core and surface of the casting. Removal of pouring risers, shoveling of burrs and drape seams, as well as heat treatment, shaping, anti-rust treatment and rough machining.

The casting process can be divided into casting metal preparation, mold preparation and casting processing.

Casting metal refers to the metal material used for castings in the production of casting. It is an alloy composed of a metal element as the main component and added with other metals or non-metal elements. It is customarily called a casting alloy, mainly including cast iron, Cast steel and cast non-ferrous alloys.

Metal smelting is not only a simple melting, but also includes a smelting process, so that the temperature, chemical composition and purity of the metal poured into the mold meet the expected requirements.

For this reason, various inspections and tests for the purpose of quality control must be carried out during the smelting process. And the liquid metal can only be allowed to be poured after reaching the specified indicators. Sometimes, in order to meet higher requirements, the molten metal has to be processed outside the furnace after being discharged. Such as desulfurization, vacuum degassing, refining outside the furnace, inoculation or deterioration treatment, etc.

Commonly used equipment for smelting metals includes cupola, electric arc furnace, induction furnace, resistance furnace, reverberatory furnace, etc.

Advantages:

It can produce parts with complex shapes, especially blanks with complex cavities;

Wide adaptability, metal materials commonly used in industry can be cast, ranging from a few grams to hundreds of tons;
Wide sources of raw materials, low prices, such as scrap steel, scrap parts, chips, etc.;
The shape and size of the casting are very close to that of the parts, which reduces the amount of cutting and belongs to non-cutting processing;
It is widely used. 40% to 70% of agricultural machinery and 70% to 80% of the weight of machine tools are castings.
Disadvantages: 1. The mechanical properties are not as good as forgings, such as coarse structure and many defects;
In sand casting, single piece and small batch production, the labor intensity of workers is high;
The quality of castings is unstable, there are many processes, and the influencing factors are complicated, which is easy to produce many defects.
Casting defects have an important impact on the quality of castings. Therefore, to lay a good foundation for choosing casting alloys and casting methods. The quality of the castings should be started, combined with the formation and prevention of the main defects of the castings.

Process flow

With the advancement of science and technology and the vigorous development of the foundry industry. Different casting methods have different mold preparation contents. Taking the most widely used sand casting as an example. Casting preparation includes two major tasks: preparation of molding materials and molding and core making.

Various raw materials used for modeling and core making in sand casting. Such as casting raw sand, molding sand binder and other auxiliary materials. And the molding sand, core sand, paint, etc. prepared from them are collectively referred to as molding materials. The task of modeling material preparation is to select suitable raw sand, binder and auxiliary materials according to the requirements of the casting and the properties of the metal. Then according to a certain ratio, they are mixed into molding sand and core sand with certain properties.

Commonly used sand mixers include roller type sand mixers, countercurrent sand mixers and continuous sand mixers. The latter is specially designed for mixing chemical self-hardening sand, continuous mixing, and high sand mixing speed.

product category

Common cast metals

1.Gray cast iron
Nodular cast iron
Malleable cast iron
Cast steel

Commonly used cast non-ferrous metals

Brass
Tin bronze
Aluminum alloy
Magnesium alloy