1, 5 and 1 yuan coins; Coins are made of aluminum-magnesium alloy, copper-zinc alloy and nickel-plated steel core. The whole coin manufacturing process can be summarized as three basic contents: material selection and cake treatment, design and molding, and stamping. Selection of materials and treatment of blank cakes (1) Choosing materials for making coins can use different metal materials. Common and relatively low-priced metals are used to cast low-denomination coins, while rare metals such as gold, silver and platinum are used to cast commemorative coins with investment and collection value. A good coin material needs to have certain physical characteristics, such as the metal texture should be soft, easy to process and shape, and at the same time have considerable hardness to withstand the wear and tear in the circulation process. Because there are few metals with the above characteristics, the material of coins is usually an alloy of two or more metals. In the United States and many other countries, commonly used metals are copper, zinc, nickel, iron, aluminum and so on. Among them, copper is an ideal material for coinage, both by itself and as an alloy. Gold and silver are usually alloyed with other metals to improve hardness, and copper is the first choice. The so-called "pure" ordinary gold and silver coins actually contain a small amount of other metal components. Of course, their value is mainly estimated by the content of precious metals in the coins. The specific process of making alloy is: (1) melting the selected metal into liquid alloy in a furnace, pouring it into an ingot (can), cooling it or pressing it into strips (thicker); (2) rolling the ingot or thick strip for many times in the hydraulic workshop to make it into a strip with a thickness meeting the requirements of the blank cake; (3) Stamping the strip into semi-finished blank cakes for further processing. (2) The blank cake processing coin is formed by stamping the blank cake. The quality of rough cake directly affects the quality of finished products, so the processing of rough cake is a very key link. The blank cake punched by alloy strip is rough, the surface is not smooth and there are burrs around it, which needs further finishing. The specific steps are as follows: (l) Put the biscuit into a columnar annealing furnace with a shape similar to that of a special mixer, and the annealing furnace rotates. Soften cookies at high temperature. (2) putting the annealed and softened biscuit into dilute acid or soap solution for cleaning. (3) using special mechanical equipment to trim and polish the blank cake. After the above treatment, the blank cake can be directly used to imprint coins. At the same time, in another mold workshop, the design and mold making work has begun. Design and Forming (I) Mould and its working principle It is hard for people who don't know much about metallurgy to imagine that hard metal can still flow around like liquid, but this is an objective fact, because solid metal can move and deform through its internal structure under pressure, just like turning steel plates into steel frames with various bending shapes in an automobile factory. Making a piece of metal into a patterned coin requires the help of a mold, and it is the front mold and the back mold, because the physical forces are mutual, that is, the forces are equal in magnitude and opposite in direction, and it is impossible for anyone to put a metal blank cake in the air to cast coins. The actual operation method is to fix one of the molds, put the blank cake on it, and punch it with another mold that can move up and down, so that the two sides of the blank cake can simultaneously imprint patterns under the interaction force. The mold is generally made of special steel, and the surface is engraved with patterns and hard, which can make the surface of coins appear mirror effect when casting. The original mold was made by hand, and the sculptor carved characters, numbers and various patterns on the surface of the mold with special tools. With the development of technology, engraving machine, electroplating, computer-aided design and other equipment and processes are widely used to replace manual work, and the efficiency and accuracy have been greatly improved. Because the coin surface is a relief mirror effect, every part of the surface pattern of the working die is concave, which is called "female die". For the sake of understanding, here is an example. Cover the coin with an aluminum foil, scrape it repeatedly with an eraser, and then remove the aluminum foil. You will find that there is a concave pattern on the side of aluminum foil that contacts the coin, because the pattern on the surface of the coin is convex. Similarly, if the pattern on the surface of the coin is concave, then the pattern on the surface of the working die is correspondingly convex. (2) The modeling process of pattern from plane to solid. Pattern design begins with the artist's plane drawing, and it takes a complicated process to turn the plane pattern into a beautiful and realistic three-dimensional relief on the coin: (1) The sculptor uses putty to express the plane pattern on the drawing in three dimensions and then turn it into a plaster mold or a resin mold; (2) Put the plaster mold or resin mold into the electrolyte, and make a copper mold blank with raised patterns by electroplating, which is also called copper mold or master mold; (3) The three-dimensional pattern on the master mold is indented and copied to another metal mold blank according to a given proportion through the synchronous stroke of the iron pen and the carving knife on the engraving machine, thus forming a master mold, also called a sub-mold, and the relief effect on its surface is completely consistent with the actual coin; (4) After the original die is quenched to increase the hardness, another die blank is repeatedly stamped on a large-tonnage hydraulic press to form a working die with concave patterns and mirror effect, and then the working die is quenched to increase the hardness for stamping. A female die can be used repeatedly and stamped into many working dies with the same patterns, which is why some circulating coins with the same patterns can be produced continuously all the year round. It should be said that the new molding technology is not only more standardized, but also the efficiency has been greatly improved, so that the original skilled sculptor needs to spend a whole day's work and finish it in a few hours. Before minting coins, the working die may only imprint hundreds of coins, but now the working die can imprint millions of coins. Modern coinage machine is a precise and efficient equipment system integrating stamping and automatic conveying. It continuously puts the blank cake in the stamping position at an extremely fast speed, and the stamping can be completed instantly, so as many as hundreds of coins pour out every minute. In the early days, when people cast coins with a hammer, they used the metal washer technology, which has been used until now, that is, cutting a round hole with the same diameter as the coin on the metal plate and putting the blank cake into the hole to prevent the blank cake from stretching and deforming due to pressure during stamping.