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+ What is "High-Pressure Die-Casting"?

+ What affects casting quality?
+ What are die-casting dies?


The following is paraphrased from the introduction of "A Study of Heat and Fluid Flow Phenomena in the Zinc Hot Chamber Pressure Die Casting Process", a thesis by Dr. Ho Siauw (1978).

> What is "High-Pressure Die-Casting?
The pressure die casting process is a method of transforming metal ingots into their finished products with a minimum amount of secondary machining. This process requires the ingots to be melted and then injected under high pressure into the cavity formed when two die halves are locked together. this inection pressure is externally applied and is produced by the hydraulic system of the die casting machine, which is transferred to the molten metal through a piston and cylinder arrangement, called the shot piston and shot sleeve.

There are two main types of pressure die casting process, namely the hot chamber process and the cold chamber process. In the hot chamber process the shot sleeve is fully immersed in the molten metal furnace, and together they form an integral part of the die casting machine. In the cold chamber machine, the metal furnace is a seperate part and the molten metal has to be ladled into the shot sleeve. Hence, a high melting alloy can be cast by the cold chamber process, and only low melting ones, such as Zinc alloy 3, are cast by the hot chamber process. The reason for this is that iron, the material used in making the shot sleeve and piston, is subject to ready chemical attack by the casting alloys at the high temperature.

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> What affects casting quality?
A good quality casting means that it is free from casting defects such as porosity, cold shuts and surface blisters.

In the pressure die casting process, casting porosity is mainly caused by air being trapped in the flowing molten metal during the casting injection, and to a lesser extent is caused by the fact that the molten metal shrinks as it is solidifying.

Cold shuts are caused by prematurely solidifying metal streams entering the cavity and forming a discontinuity in the casting. Generally, the remedy is to increase the molten metal and the die temperature.

Surface blisters are caused when the air trapped in the casting, which has been compressed by the high pressure at the end of the casting injection, bursts out to form blisters on the casting surface as soon as the die is opened. Such an occurence is the result of a localised high temperature and therefore, the solidifeid skin of the casting is not strong enough to withstand the pressure of the trapped air.

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> What are die-casting dies?
In the metal casting process the die is the essential piece of equipment. It consists of two halves, the cover die half and the ejector die half. the cover die half is fitted to the machine end platen which is part of the machine frame. Therefore, it is also called the fixed die half. The ejector die half, also called the moving die half, is fitted to the moving platen fixed on to the toggle mechanism.

The important design features (of a typical die design) apart from the casting cavity, are the metal feed system, the overflow, the air vents and the cooling or heating channels.

The metal feed system in the die consists of the runners, the flow channel in the sprue, sometime called the sprue runner, and the gates. Their functions are to provide a flow channel for the molten metal to fill the cavity and to distribute the metal to different parts of the casting cavity.

The overflows have many functions, but the main one is to receive the metal which enters the cavity first, because the first part of molten metal entering the cavity may contain a mixture of washed die lubricants and solidified metal particles. This mixture can cause undesirable effects on the quality of the casting.

The function of the air vents is to allow the air in the cavity and runner system to escape. the cooling and heating channels allow control of the die temperature.

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Source:
SIAUW TH. "A Study of Heat and Fluid Flow Phenomena in the Zinc Hot Chamber Pressure Die Casting Process" 1978.


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