Gravity casting works by allowing molten metal to flow gently into a permanent mold under the influence of gravity alone. Listed below, you’ll find a detailed breakdown of the steps involved in gravity die casting:
1. Preheat the Die and Coat the Mold Cavity
Before starting the gravity die casting process, the die (also known as the mold) is preheated to a certain temperature to suit the charge metal. This prevents cold spots and ensures uniform solidification of the fill. This step helps to reduce thermal shock, improving the longevity of the die. Additionally, a release agent or coating is commonly applied to the inner surfaces of the die cavity. This coating helps facilitate the removal of the solidified part once the casting process is complete, by reducing sticking. Dies erode due to molten metal contact, especially in turbulent flow areas, because some casting metals can be quite abrasive. Die coating reduces this erosion, maintaining surface integrity for more cycles, whereas otherwise, the roughness would increase casting stick and defects.
2. Pour Molten Metal Into the Cavity
Once the die is preheated and coated, it is securely clamped together. Molten metal, typically aluminum or other non-ferrous alloys, is then poured into a pouring basin or runner system that leads to the die cavity. The molten metal enters the mold cavity under the force of gravity alone, hence the name “gravity die casting.” The carefully controlled pouring process helps minimize turbulence and air entrapment, resulting in better-quality castings. Filling from below allows the smooth upward rise of the charge meniscus and assists in driving out air and maintaining low turbulence.
3. Allow the Metal To Solidify
As the molten metal is introduced into the mold cavity, it cools rapidly. Solidification starts first when the charge contacts the relatively cooler surfaces of the mold and progresses inward toward the center of the casting cross-section. The metal fill cools rapidly in the mold cavity, solidifying first at the cooler mold surfaces and thinner sections, progressing inward. Preheated molds ensure consistent solidification rates, preventing premature solidification in thin sections that could block metal flow. Solidification time varies with metal type and part design. Mold preheating benefits include avoiding thermal shock, maintaining molten metal temperature for proper flow, and reducing thermal stress in solidifying casting. The time required for solidification depends heavily on the type of metal being cast, the mass of the tool and charge, and the design of the part.
4. Open the Die and Remove the Part
After the charge has fully solidified within the die cavity, the clamps holding the die closed can be released. The opened die reveals the solidified casting that will precisely mirror the cavity it fills, with good dimensional accuracy and great repeatability between casts. The application of the release agent during the initial steps should allow the easy removal of the casting from the die.
5. Machine the Part if Needed
Once the casting is removed from the die, it may undergo further processing, such as machining, to achieve the desired final shape and surface finish, remove vent, fill and flash attachments, and improve local precision in areas of higher tolerance than the casting method can achieve. The decision to machine the part depends on its intended use and the required level of precision, as well as the quality of the die machining/fit.
The production speed of the gravity die casting process varies widely as a consequence of various factors, including the material being cast, the complexity of the part’s design, and the size of the casting. The entire gravity die casting process typically takes anywhere from a few minutes for simpler castings to several hours for larger or more intricate parts that have longer cooling times. Cycle times can be reduced somewhat, although not without the potential for quality risks. Careful design of the casting process parameters, die design, and material selection allow for the balance of production and casting quality.
