High Pressure Die Casting
High pressure die casting is a high-volume manufacturing method that can produce precision parts at great speed, efficiency, and exceptional tolerances. The process for high pressure die casting is an injection die casting method. What differentiates it from the other forms of injection die casting is the speed and pressure at which it injects molten metal into the die cavity. Unlike other injection methods, parts produced using high pressure die casting have excellent dimensional accuracy and smooth surface finishes.
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Types of High Pressure Die Casting
The basic principle of all high pressure die casting is to force molten metal into a tightly closed steel mold at high pressure and high speed. The cavity of the carefully formed and crafted die or mold has the detailed shape and design of the component to be produced. After the molten metal is injected into the mold, it is held under high pressure until it solidifies. The pressure, injection process, and mold design are common to all forms of high pressure die casting.
The two basic forms of high pressure die casting are hot chamber high pressure die casting and cold chamber high pressure die casting. The distinction between the two methods is how the molten metal is heated, placed in the casting machine mechanism, and injected into the die cavity.
Hot Chamber High Pressure Die Casting
All of the components for hot chamber high pressure die casting are included in one single casting machine. The piston, melting pot, heating mechanism, delivery method, and molten metal are connected together. A tube, referred to as the nozzle, is linked to the mold cavity. Pressure, provided by the piston, forces molten metal through the nozzle to the die. The bent curved shape of the nozzle has earned it the name of “gooseneck”.
The hot chamber high pressure die casting process is completed within the confines of a single machine. Parts and components are rapidly produced, ejected, and ready for shipping. The process begins with the loading of metal alloy ingots into the melting pot.
Once the melting pot is ready, the burner, at the bottom of the machine, heats the combustion chamber. As the ingots are melting, the piston or plunger is located at the opening between the plunger cylinder and the melting pot. Its position prohibits the heated metal from entering the cylinder, a design that prevents leakage and casting errors.
When the molten metal has reached the proper temperature, the plunger rises and draws under pressure a sufficient amount of molten metal from the melting pot into the cylinder. Once the cylinder is properly fed, the hydraulic or pneumatic powered plunger moves downward, cutting off the entrance to the melting pot and forcing the molten metal up the gooseneck, through the nozzle into the die cavity.
As the molten metal enters the die cavity, it crashes against the die wall and spreads, under pressure, evenly over the walls of the die. The die has a fixed half and movable half. In the middle of the fixed half is the opening to the nozzle through which the molten metal enters the die. The die is separated from the rest of the hot chamber high pressure die mechanism by a reinforced cover platen, which protects the die from any damage.
When the molten metal touches the steel walls of the die cavity, it begins to solidify, which starts with the metal closest to the surface of the die and moves to the center of the casting. The pressure is maintained until the full solidification of the casting, which varies in accordance with the type of alloy being processed.
After the metal is solidified, the plunger is retracted releasing the pressure on the casting. The halves of the die separate, and the ejection pins push the completed component out of the die. The complete process happens rapidly making it possible to produce large quantities of products in a short period of time.
The rapid solidification for hot chamber high pressure die casting makes it ideal for producing high volumes of parts in a short period of time. Fast cycle times and quick processing enables high levels of production for low melting point alloys.
Cold Chamber High Pressure Die Casting
The term cold used, in this reference, is a little misleading since it gives the impression that cold chamber high pressure die casting is a cold process, which is not the case. The difference between cold and hot chamber die casting refers to the methods used to deliver the molten metal to the die cavity. In the case of hot chamber high pressure die casting, a caldron of molten metal is included in the structure of the machine, which is the reason for the shorter cycle times.
With cold chamber high pressure die casting, the cylinder or chamber where the molten metal is delivered to the die cavity is empty at the beginning of the process. The melting pot and combustion chamber are not present in cold chamber high pressure die casting. The melted molten metal is heated and melted in a furnace not connected to the mechanism. The cylinder from hot chamber high pressure die casting is referred to as a shot chamber. The portioned molten metal is referred to as a shot.
The process for cold chamber high pressure die casting begins with the removal of a measured portion of molten metal from the melting furnace. The type of furnace varies in accordance with the manufacturer. Electric and gas furnaces are most common. The molten metal is collected using a ladle. At the top of the shot chamber is a hole where the ladle pours the molten metal into the shot chamber.
The shot chamber is like the cylinder of hot chamber high pressure die casting. The plunger is replaced by a ram that is similar to the rams used for the extrusion process. As with the plunger, the purpose of the ram is to create pressure. Once the ladle has delivered the molten metal, the ram moves along the shot chamber pushing the molten metal toward the die cavity.
The structure of the die for cold chamber high pressure die casting is the same as that used for hot chamber high pressure die casting with the two halves and ejection pins. As with hot chamber high pressure die casting, the cooling process begins as soon as the molten metal touches the surface of the die cavity. The ram remains in place until the casting solidifies, which may be slower for cold chamber high pressure die casting due to the size, intricacy and complexity of the parts being produced.
The reason metals for cold chamber high pressure die casting are heated in a furnace is because they have a higher melting point than the metals used for hot chamber high pressure die casting. Since the metals have a higher melting point, they are stronger and capable of producing stronger and more durable castings. The hardness and strength of the metals makes their castings ideal for demanding applications.
Conclusion
Hot and cold chamber high pressure die casting are necessary production methods for high quality parts. Cold chamber high pressure die casting can produce parts of any size and is constantly being perfected and improved to increase its use. Hot chamber high pressure die casting is ideal for producing high volumes of small parts quickly and efficiently. When determining which casting method to use for the production of a part or component, high pressure die casting manufacturers work closely with their customer to assist in choosing the right production method.
