A variety of different designs for water-cooled permanent molds exist each with its own advantages and disadvantages.
Cooling Jacket Welded to Mold-In this design of water-cooled systems, the water jacket is welded directly to the back of the mold, Effective heat dissipation in this design is achieved by circulating water through the jacket. This method allows the back of the mold to be contoured (instead of remaining flat) in an effort to allow the cooling source (water) to reach throughout the mold and maintain uniform cooling.
This cooling system works well to reduce or eliminate hot and cold spots on the mold, drastically increasing mold life due to the elimination of thermal shock. The only limitation is that the permanent mold must be made of steel to allow the water jacket to be welded onto the back of mold. Low carbon steel water-cooled permanent molds with a wall thickness of 1-1.5-in, exhibit longer life because mold cracks and other deteriorated areas may be periodically repaired by welding.
Cooling Jacket Affixed to Casting Machine-Compared to the water jacket welded directly to the back of the mold, this type of cooling system, is more moderate because it provides indirect mold cooling. To ensure uniformity and the ability to use this cooling system on a variety of molds, both the face of the cooling system and the back of the permanent mold (where the cooling system interacts with the mold) must be flat. As a result, cooling is slower and less aggressive.
The water jacket can be made from a variety of materials, including steel and iron, and is suited for small and thin-wall casting jobs as well as operations with frequent mold changes. With small and thin-wall casting jobs, the less aggressive cooling approach works well. In addition, the requirement for flat faces on the back of the permanent mold and the face of the water jacket make mold changes easier.
The main drawback of this system is the air-gap that often occurs between the mold and cooling jacket. This gap is due to thermal-related stresses and distortion occurring in the mold during operation. As a result, some areas of the mold will not have contact with the water jacket and will not cool as effectively.
In permanent mold casting, the molten metal is poured into the steel die and flows only at the force of gravity. For the most part, permanent mold castings are produced by pouring the molten metal into the top of a die that has been made in the desired shape of the casting. There are many deviations from this simple approach in actual casting practice. Sometimes, the metal is poured into the mold at the top, but a runner is cut into the die that conveys the molten metal to the side or bottom of the casting cavity itself, so that the metal flow during filling is from the bottom or side of the mold. This is done to optimize casting conditions for the part being produced. Other times the metal may be poured into a basin beside the die and then the entire unit is tilted to achieve controlled filling of the mold. The flow of metal into the die cavity and the flow of heat from the metal to the die during solidification are two of the main criteria for successful casting production by the permanent mold process.
Dies for the permanent mold process are produced from hardened steel. The heat transfer rate is higher than from sand or ceramic molds used in other processes and this provides advantages in casting cycle time. Often, a ceramic coating is applied to the steel to protect it from the molten metal and to control the heat transfer rate from the molten metal to the die at only a slight increase in cycle time. The solidification of the molten metal must be controlled to prevent solidification shrinkage cavities in the final product. Also, the solidification rate is managed to optimize the microstructure of the resulting casting to achieve the design objectives. For high production castings, the dies are often water-cooled, further decreasing the cycle time and reducing the cost for each part.
Sometimes the casting design requires undercuts or hollow cavities in the casting that cannot be produced with conventional steel dies. In these cases, the die designer may choose a loose piece or an expendable core for the permanent mold process. Both of these options increase die and casting costs, but often the function of the product requires these features in the casting. When expendable cores are used, the process is called semipermanent mold casting. This subcategory of permanent mold casting refers to processes that utilize one or more expendable cores that are placed in the steel die before the production of each casting. The expendable core is then removed by vibration or heat treatment after the casting is removed from the die. Expendable cores for semipermanent mold casting are normally produced from sand with a binder to give them strength, similar to the cores utilized in sand casting processes.
The advantages of the permanent mold and semipermanent mold casting processes are: reasonable piece costs resulting from the high production rates achieved with metal molds (especially water-cooled molds) compared to sand and investment casting, and lower investment required for equipment when compared to low-pressure and high-pressure die casting. In addition, the use of expendable cores in semipermanent mold casting permits great design flexibility for castings.
One of the disadvantages of the permanent mold casting process is that metal dies are more expensive than patterns for sand casting or investment casting so the process is not economical for short runs. At low volume, it is difficult to overcome the high initial tooling cost and compete based on casting cost. In addition, since castings are filled with liquid metal under only the pressure of gravity, castings sections tend to be thicker in permanent mold casting than in the low-pressure and high-pressure die casting processes. Also, where material properties are critical, emerging concepts in high-pressure die casting, such as squeeze casting and semisolid metal processing, are creating new competition for permanent mold casting.
Typical parts produced in the permanent mold process include automotive parts such as aluminum pistons, steering knuckles, brackets, wheels, and pump impellers. Parts are also produced in zinc, brass, copper, lead, and even gray iron. Since the process has great design flexibility and is compatible with so many metals, the types of products that can be produced are almost unlimited.
Permanent mold casting can be used for high production rates and you can reuse the molds many times. Complex shapes can be formed with low labor costs using highly automated processes. The finished pieces have a good surface finish and good dimensional accuracy.
Permanent mold casting also produces little waste. Disadvantages include a higher cost to create the metal molds and only non-ferrous metals can be used due to their low melting points unless a specialized graphite mold is created. Molds also have a short shelf life due to erosion and thermal fatigue.
Applications Of Permanent Mold Casting
Permanent mold casting is used heavily in the automotive industry to create parts like gears, castings, suspensions, fuel injection housings, and engine pistons. Aircraft parts are also often made via permanent mold casting.
Cooling jackets control the direction of the flow of water for cooling. In most cases, the area must be flat. Jackets are suited for small and thin-wall castings and frequent mold changes. They aid in achieving a more uniform thermal profile, shorter process times, and repeatable ramping. Jackets also work to reduce hot and cold spots on the mold.
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