Permanent mold casting is a metal casting process that employs reusable “permanent” metal molds. It is often referred to asgravity die casting因为通常在重力诱导的压力下通常将金属倒入管芯中而不是使用强制注射。一些制造商也使用其他过程，例如在高气体压力或真空压力下浇注熔融金属。
Permanent mold casting uses a strong metal mold, typically one made from steel, cast iron or a dense cast iron trademarked as Meehanite. The cost of producing the mold can be significant, but since it is permanent, the mold can be reused thousands of times. This combination of long mold life and large product volumes that can be manufactured with the same mold offsets mold production costs and makes the process very economical.
The first step of permanent mold casting is creating a custom mold. Each component of the mold is carefully machined from individual metal blocks, making sure that they fit together precisely, and open and close easily and accurately. The mold must also include a gate area to ensure that molten materials can enter safely.
Typically, before casting, the mold is preheated to between 300 degrees Fahrenheit and 500 degrees Fahrenheit. Heated molds permit smoother metal flow and reduce defects. Preheating also reduces the possibility of damage to the mold itself from hot molten metals.
A thin protective layer of clay or sodium silicate may also be applied to interior mold cavities to function as a thermal gradient and lubricant for easier removal of the cast part, and to additionally protect the mold from damage.
Permanent casting molds typically consist of at least two parts, including the two mold sections and any cores used to form complex features. All the parts need to be closed and held together with force prior to the pouring of molten metal inside. This is normally done with the help of machinery.
Molten material enters the mold from the top at a slow rate through a gate area accessed by an opening called a sprue.
Molten metal is left to slowly solidify within the mold and cool to a desired temperature. Since many metals somewhat contract in size during the cooling process, solid metal casting needs to be removed from the mold before it cools completely to prevent the mold from cracking. This is done by means of ejector pins that are built into the mold.
Permanent mold casting produces reduced levels of trapped gas bubbles inside metal parts while the nature of the mold allows the casting to solidify quickly and with only minor shrinkage. As a result, cast products have smaller metal grain structure, greater uniformity in every part of the cast part, and superior mechanical properties.
Permanent mold castings are also characterized by a good surface finish. Defects such as traces of metal flow lines are only cosmetic in nature, rather than functional. They can be easily removed during the finishing phase of the casting process.
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