Die Casting – How to Manufacture Successfully in China

Die casting: Process, equipment and applications

Read on to know about:

• Steps in the process
• Different techniques
• Equipment and material used
• Common defects and their fixes
• Applications 

HOW IT WORKS

To recap, die casting is a way of manufacturing metal objects by filling a reusable mold – called a die – with molten metal and letting the metal cool to take the shape of the mold. The resultant object is called the casting. Apart from the metal and die, the process requires a furnace and a tool. Alloys of non-ferrous metals such as zinc, copper, aluminium, magnesium, lead, pewter and tin are used. A casting can be made within seconds and in just five steps:

1. Clamping: A die comprises two halves. The two halves are heated, sprayed with a chemical release agent – to aid in ejection of the casting – and clamped tightly shut.  
2. Injection: The die cavity is injected with molten metal. This is done by pouring the metal into a channel or chamber that allows it to flow into the die.
3. Cooling: When the die is filled, the metal is allowed to cool and solidify, taking the shape of the cavity. Cooling time depends on the metal used, the casting’s wall thickness, and how complex the die is.        
4. Ejection: After the metal has cooled, the die halves are opened and the casting released with the help of an ejection system.
5. Trimming: During cooling, the metal in the machine’s injection system gets attached to the casting. These are trimmed.  

TECHNIQUES

1. Pressure die casting: Here, pressure is used to inject molten metal into the die. Depending on the amount of pressure used, there is high pressure and  low pressure processes.

• High pressure: The use of high pressure and high speed during injection results in castings with thinner walls, high precision and a smoother finish. The casting cycle is fast. High pressure as a process accounts for 50% of light metal casting. On the flip side, it requires higher investment and operational costs, is limited to soft alloys such as zinc and aluminium, and produces castings with lower strength.
• Low pressure: Molten metal flows into the die at a lower pressure, creating geometrically complicated parts (such as car wheels) with high strength, dimensional accuracy and minimum material wastage. You can use this technique to create castings that weigh up to 150 kg. However, the casting cycle is longer and the process unsuitable for parts with thin walls (minimum wall thickness 3 mm). It is also limited to metals with low melting points. Low pressure production accounts for 20% of light metal casting.

1. Gravity die casting: Is defined by a) the use of gravity to press molten metal into the die, and b) the use of a permanent or semi-permanent mold. Liquid metal is poured into the mold from above, usually with a ladle. This technique produces heavier, denser, larger castings with high strength and surface finish. It is simpler than pressure die casting and costs less to operate. But it can only attain a minimum wall thickness of 3-5 mm and has a longer casting cycle, making it unsuitable for long production runs.
2. Vacuum die casting: When a vacuum is introduced in a pressure process, it is called vacuum-assisted die casting. The vacuum is drawn both in the die cavity and shot sleeve (a tube through which the plunger that injects the metal into the die moves). The vacuum gets rid of the air in the die and shot sleeve, reducing gas entrapment and significantly improving the density, strength and smoothness of the casting.         

EQUIPMENT

1. The machine

There are two types of machines in the pressure process, depending on the metal used:

• Hot chamber machine: It uses metals with low melting points (zinc, lead, magnesium, tin). A typical hot chamber machine has an integrated furnace with an open pot to hold the molten metal. The metal flows through an inlet into the hot chamber, from where it is forced into the die with a vertical plunger. The plunger seals the metal in until it solidifies. Since the hot chamber is always in contact with the molten alloy during the casting process, alloys with high melting points cannot be used as these would damage the injection system.        
• Cold chamber machine: Unlike a hot chamber machine, the furnace and holding pot in a cold chamber machine are located separately. Therefore, such a machine can use metals with high melting points (aluminium, magnesium, copper, brass). The molten metal is ladled into the cold chamber, from where it is injected into the die with a plunger at high pressure.

1. The die

Niclas Bengtson Member since May 20, 2021