4 Permanent Metal Mold Casting Process that You Should Know!

Permanent Metal Mold Casting Process

What is Metal Mold Casting process?
Metal Mold Casting process (also known as Permanent Mold Casting process) is a metal casting process which uses permanent molds (or reusable molds) that are usually made of metal.
In simple words, the mold in which the molten metal is to be poured is made up of metal (and not sand).

Permanent metal mold casting method is used for casting metals like aluminum, magnesium, and copper alloys.

Metal like zinc, tin, iron, steel and lead alloys are also casted in graphite molds.

Various parts like gears, engine pistons, pipe fittings, wheels, gear housing, etc can be manufactured using permanent mold casting.

4 Metal Mold Casting Process

There are 4 types of process by which metal molds (or permanent molds) can be prepared.

They are;

Let me tell you a few important things about each of these permanent mold casting process.

#1) Gravity casting process

  • In gravity casting process, the metal mold is preheated at the temperature range of 150 °C to 200 °C, so that the molten metal can flow easily when poured. By preheating the mold, the thermal damage to the castings can be reduced.
  • After this, the inner walls of the metal mold is coated with the refractory material, which avoids the molten metal to stick with the metal mold. This eventually increases the mold life.
  • The sand cores or metal core are also placed inside the metal mold (if required).
  • After this, the molten metal is poured into the mold cavity.
  • As the metal solidifies, it is removed early from the mold to reduce the chance of hot tears.
  • Now for the next casting to be prepared, the exact process is repeated, but the preheating is not required as the heat from the previous casting is present in the metal mold. Also the refractory material lasts for several castings to be manufactured.
  • The metal mold casting process includes, 
    • Coating of mold, 
    • Pouring of molten metal, and
    • Removal of the final casting.
  • These are the repeated processes for each and every castings to be manufactured. Hence there are some automated techniques used in the permanent mold casting process.
  • The molten metal is poured in the mold cavity at the lowest practical temperature to avoid cracks and porosity.
  • This pouring temperature depends on different types of metal. For example, zinc metal is generally poured at around 371 °C, while grey iron is poured at around 1370 °C.

9 things about gravity casting that you should know

  1. The molds used in gravity casting consist of two halves.
  2. The metallic mold used in gravity casting is made up of grey cast iron, because it has the best thermal fatigue resistance.
  3. Materials like steel, graphite and bronze are also used for making metal molds, as they have good resistance to erosion and thermal fatigue.
  4. The mold is opened as soon as the molten metal solidifies. This prevents the hot tears.
  5. For the very first casting process, the metal mold is required to be preheated so that the thermal damage can be prevented to the casting.
  6. In metal mold casting, the cores are either made up of sand or metal.
  7. In metal molds, the venting of hot gases takes place from the small gaps present between the two halves of molds. For larger castings, several other small holes are made in the mold. These holes are so small that hot gases can pass through them, but molten metal can’t.
  8. Ejector pins are used for lifting the castings when the coating is not enough to separate the casting from the mold.
  9. These ejector pins are placed around the mold which may leave a round impression on the casting. This is removed later by the machining process.

#2) Slush process

Slush casting process
  • Slush casting process is a type of permanent mold casting process in which hollow castings are manufactured.
  • In slush process, the molten metal is poured in the metal mold as shown in the above image.
  • This molten metal is allowed to cool for a very short time such that only outer surface metal gets solidified.
  • The excess molten metal is then poured out of the hollow shell. (See above image).
  • And finally, the hollow casting is obtained when the halves of mold are separated.
  • The wall thickness of the casting may vary because of unequal solidification.
  • Slush casting process is generally used for manufacturing ornamental products, lamp bases, hollow statues, etc.
  • In slush casting, the metals having lower melting point are prepared.
  • The main advantage of slush casting is that it uses less material as compared to that of solid casting. Thus the final casting is lighter and cheap.
  • Generally, most of the hollow castings have a small hole from where the excess molten metal is poured out.

#3) Low pressure casting

Low pressure casting
  • Low pressure permanent mold casting uses a low pressure gas (between 20 kPa to 100 kPa) to push the molten metal into the mold cavity.
  • As shown in the above image, the low pressure is applied to the top part of the molten metal, and due to this, the molten metal rises up in the refractory tube and finally reaches the mould cavity.
  • The refractory tube is kept deeper in the molten metal so that clean molten metal is pushed to the mould cavity.
  • There is no wastage of molten metal as there is no runner and riser used in this process. The excess molten metal falls back to the ladle which can be reused.
  • Low pressure permanent mold casting is generally used for aluminum castings, magnesium castings and for some copper alloys.
  • Because of the constant pressure during the filling of molten metal, there is a low turbulence and it minimizes gas porosity.
  • The mechanical properties of the castings obtained by this process are better than those obtained with the gravity casting process.

#4) Vacuum casting

Vacuum casting process
  • Vacuum permanent mold casting has all the benefits of low pressure permanent mold casting. And by using this process, the amount of dissolved gases can also be minimized in the casting.
  • Thin walled profiles can be made with good accuracy using vacuum process.
  • The mechanical properties of casting obtained by this process is 10 to 15% better than castings obtained by the gravity process.
  • Vacuum casting process is used for manufacturing the jobs with smaller weight (i.e from 0.2 kg to 5 kg).
  • The disadvantage of vacuum process is that the tooling cost is higher.
  • The mold used in the vacuum casting process has short life. If steel or iron is used in vacuum casting process, then the mold life becomes extremely small. This is because steel and iron have higher melting temperatures.

External links:
Permanent casting example: Image by Janke, CC BY-SA 2.5, via Wikimedia Commons
Slush process: Image source (Science direct)

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