Die casting is a metal casting process that pours molten metal into a permanent metal mold cavity under artificial pressure that is applied through a machine. Because metal is forced into the die cavity under pressure, so it is also called “pressure die casting”. The pressure in practice ranges from 0.7-700MPA normally.
To learn more about the casting process related to expendable mold casting, please refer the article "Sand casting: the oldest craftsmanship shaped the world“ as a reference.
Material and Application
There exist die cavity cracking problems from repeating heating and cooling of die, the die wear increase with the temperature of molten metal. Because of the issue, normally low molten point nonferrous metals, such as aluminum, brass, magnesium, and zinc, are used in the casting process. The below table shows these material properties and their applications.
Die Structure and Process Operation
Die consists of two parts. One called the stationary die or cover die that is fixed on the casting machine. The second part called the ejector to die is moved out for the extraction of the casting. The casting cycle starts when the two parts of the die are apart. The lubricant is sprayed on the die cavity manually or by the auto lubrication system. The two die halves are closed and clamped. The required amount of molten metal is injected into the die. After the casting is solidified under pressure, the die is open and the casting is ejected out.
The cover dies consists of sprue also called biscuit, runner, and gates, and is also in contact with the nozzle of the gooseneck in the case of the hot chamber and with the shot chamber in case of cold chamber process.
The ejector pins move through the moving die to free the casting from the ejector die. Ejector pins must be sufficient enough so as to remove the hot casting without distortion, placement of ejector pin positions should be so that the pin mark left on casting is not objectionable.
The cores used are all metallic and are of two types. The fixed cores are the ones that are fixed to the die halves and parallel to the die movement. The others called slider cores that are not parallel with the die movement and hence are to be removed before casting is ejected out from the die.
Die may be a single cavity, multiple cavities (with several identical cavities), a combination cavity (with several different cavities) and unit die (simple, small dies that can be combined in two or more units in one master holding die).
Process Capability and Machine Selection
Die casting produce part at very short cycle, the tooling is a permanent metal tool, the cost is expensive, so it is only suitable for making volume part that has intricate structure and accurate dimension requirements, the part requires little or no subsequent machining or finishing operation. Because of the high pressure involved in the process, the thin-wall part may be produced, much thinner than those obtained by other casting processes.
The casting process may be classified as two types: hot chamber and cold chamber(Left figure showed).
The main difference between them is that in hot chamber, the holding furnace for the liquid metal is integral with the die casting machine as up figure indicated, the gooseneck is immerged in molten metal all the time, where in the cold chamber machine, the metal is melted in a separate furnace and then poured into the die casting machine with a ladle for each casting cycle.
Hot chamber process is used for most of the low melting temperature alloys, such as zinc, lead or tin. For materials, such as aluminum and brass, their high melting temperatures will attack gooseneck, and make it difficult to cast them by hot chamber process.
Design Consideration and Die Tooling
It is the same as other casting processes that the design of die casting shall place considerations on part geometric features and tolerance, and die features that are needed to produce the desired casting.
For example, sharp corners shall be avoided as much as possible to eliminate stress or cracking and tearing of metal as well as of the die; wall thickness is kept as even as possible to avoid shrinkage void or porosity.
Hot-working tool steels are normally used for the preparation of the dies, die insert, and cores. For zinc alloys, the normal die material is AISI P20 for low volume, and H13 for high volume, whereas for
aluminum and magnesium, H13, or H11 are used. For copper alloys, H20, H21, or H22 are used for die material.
Overflows are provided in the parting plane for the first metal which is normally cold, entering the die cavity, to solidify. The overflows are primarily provided for small components to provide enough heat input to the die so that no cold shuts occur. Also, overflows can be utilized for positioning the ejector pins so that no objectionable pin marks on the casting.
Process Advantage and Limitation
The advantage of the die casting process is:
- Economical for large volume production
- It is possible to make fairly intricate castings because of the use of a movable slider core
- Very thinner wall castings may be produced because of the high pressure applied
- Good surface quality and tight dimension tolerance may be achieved because permanent metal dies used.
- Better mechanical properties compared to sand casting because of fine-grained skin formed during solidification
- Very high production rate resulted from complete automation
- The size of casting is limited because of machine capacity
- Not suitable for all materials because of die material limitations.
- The air in the die cavity trapped in casting because of the high pressure applied in the casting process. So it is not suitable for castings that need heat treatment to strengthen its mechanical property
- Die and machine is very expensive and compensated through large volume production
In conclusion, die casting is a manufacturing process that has the most production efficiency in making nonferrous metal parts. Even tooling cost is relatively high, but it is compensable for the investment to make high volume production and its high production rate.