What Is Permanent Mould Casting?

The permanent mold casting refers to the casting process which use special metal mould (die) to receive the molten liquid cast metal. It is suitable to produce castings in large quantity. This cating proces is called metal die casting or gravity die casting, since the metal enters the mould under gravity.

Compared to the sand casting, shell mould casting or investment casting, in which a mould need to be prepared for each of the casting, the permanent mold casting could produce the castings with the same molding systems for each casting parts.

The mould material of permanent casting is decided by the consideration of the pouring temperature, size of the casting and frequency of the casting cycle. They determine the total heat to be borne by the die. Fine-grained grey cast iron is the most generally used die material. Alloy cast iron, carbon steel and alloy steels (H11 and H14) are also used for very large volumes and large parts. Graphite moulds may be used for small volume production from aluminium and magnesium. The die life is less for higher melting temperature alloys such as copper or grey cast iron.

For making any hollow portions, cores are also used in permanent mould casting. The cores can be made out of metal or sand. When sand cores are used, the process is called semi-permanent moulding. Also, the metallic core is to be withdrawn immediately after solidification; otherwise, its extraction becomes difficult because of shrinkage. For complicated shapes, collapsible metal cores (multiple piece cores) are sometimes used in permanent moulds. Their use is not extensive because of the fact that it is difficult to securely position the core as a single piece as also due to the dimensional variations that are likely to occur. Hence, with collapsible cores, the designer has to provide coarse tolerance on these dimensions.

Under the regular casting cycle, the temperature at which the mould is used depends on the pouring temperature, casting cycle frequency, casting weight, casting shape, casting wall thickness, wall thickness of the mould and the thickness of the mould coating. If the casting is done with the cold die, the first few castings are likely to have misruns till the die reaches its operating temperature. To avoid this, the mould should be pre-heated to its operating temperature, preferably in an oven.

The materials which are normally cast in permanent moulds are aluminium alloys, magnesium alloys, copper alloys, zinc alloys and grey cast iron. The unit casting weight ranges from several grams to 15 kg in most of the materials. But, in case of aluminium, large castings with a mass of up to 350 kg or more could be produced. Permanent mould casting is particularly suited to high volume production of small, simple castings with uniform wall thickness and no intricate structures.

Advantages of Permanent Mould Casting Process:
1. Because of the metallic moulds used, this proocess produces a fine-grained casting with superior mechanical properties
2. They produce very good surface finish of the order of 4 microns and better appearance
3. Tight dimensional tolerances can be obtained
4. It is economical for large scale production as the labour involved in the mould preparation is reduced
5. Small-cored holes may be produced compared to sand casting
6. Inserts can be readily cast in place

 

Comparison of Different Casting Processes
Items	Sand Casting	Permanent Mold Casting	Die Casting	Investment Casting	Chemically Bonded Shell Mold Casting
Typical dimensional tolerances, inches	± .010"	± .010"	± .001"	± .010"	± .005"
	± .030"	± .050"	± .015"	± .020"	± .015"
Relative cost in quantity	Low	Low	Lowest	Highest	Medium high
Relative cost for small number	Lowest	High	Highest	Medium	Medium High
Permissible weight of casting	Umlimited	100 lbs.	75 lbs.	Ounces to 100 lbs.	Shell ozs. To 250 lbs. no-bake 1/2 lb. - tons
Thinnest section castable, inches	1/10"	1/8"	1/32"	1/16"	1/10"
Relative surface finish	Fair to good	Good	Best	Very Good	Shell good
Relative ease of casting complex design	Fair to good	Fair	Good	Best	Good
Relative ease of changing design in production	Best	Poor	Poorest	Fair	Fair
Range of alloys tht can be cast	unlimited	Aluminium and copper base preferable	Aluminum base prepferable	Unlimted	Unlimited