Get familiar with the die casting process along with the most common defects and fixes.
October 29, 2020
Die casting: Process, equipment and applications
In our previous blog, we learnt of the history of die casting – how this versatile system of mass-producing identical metal parts originated in the printing industry of the mid-1800s. In this piece, we explore the process of die casting and more. Read on to know about:
Steps in the die casting process
Equipment and material used
Common defects and their fixes
Applications of die casting
HOW DIE CASTING 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 die casting machine. 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:
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.
Injection: The die cavity is injected with molten metal. This is done by pouring the metal into a channel or chamber in the die casting machine that allows it to flow into the die.
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.
Ejection: After the metal has cooled, the die halves are opened and the casting released with the help of an ejection system.
Trimming: During cooling, the metal in the machine’s injection system gets attached to the casting. These are trimmed.
DIE CASTING TECHNIQUES
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 die casting and low pressure die casting.
High pressure die casting: 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 die casting 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 die casting: 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 die casting accounts for 20% of light metal casting.
Gravity die casting: 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.
Vacuum die casting: When a vacuum is introduced in pressure die casting, 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.
DIE CASTING EQUIPMENT
There are two types of machines in pressure die casting, 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.
The die is the custom tool in die casting. Its two halves are attached to the die casting machine. One is mounted on a fixed steel frame or platen and is called the cover die. The other is mounted on a moveable platen and is called the ejector die. When the two halves come together, they form an internal cavity into which the metal is injected. When the casting is ready, the clamping system holding the die halves together separates them by pushing the ejector platen forward, which in turn ejects the casting from the ejector die. There are four types of dies:
Single cavity die: It has one cavity and can only make one casting at a time. It is often used to make complex components.
Multiple cavity die: This die has multiple, identical cavities and is used for mass production.
Combination die: It has multiple cavities that are not identical. It produces more than one type of component, but these components usually go into assembling a single product. Hence, a combination die is also called a “family die”.
Unit die: This is a small die that is inserted in a larger die frame with other unit dies. While the die frame is a fixed component of the die casting machine, the unit dies can be replaced. Unit dies are a cost-effective way of producing small, simple parts.
The metals and alloys used in die casting have different characteristics and work in different ways. It’s important to know which metal is right for your needs.
Easiest to cast, high strength, high plasticity, long-lasting, wide range of shapes possible
Lighter flame jet, door handles, smart watch bezel, automobile parts such as seat belt latch
Lightweight, corrosion-resistant, excellent conductor of heat and electricity, high dimensional stability (stable to changes in temperature, humidity)
Camera parts, cellphone frame and front cover, medical equipment, automobile parts
Lightest metal used for casting, capable of thin walls, durable, high thermal and electrical conductivity, superior finish, fully recyclable
Medical/lab equipment, camera housing, tablet frame, video game cover
Hard, high corrosion resistance, good electrical and thermal conductivity, good mechanical properties, aesthetic look
Air conditioner tubes, heating, refrigeration and plumbing components
Corrosion-resistant, low melting point, strong, bright and attractive appearance, high recyclability
Water pipes, electrical socket terminals, lamps, furniture components
COMMON DIE CASTING DEFECTS AND FIXES
Cracks: Also called hot tears, cracks might appear in the casting due to metal shrinkage during solidification, damage to the die cavity, use of excessive force during ejection or lack of lubrication. Solutions: Lower melting temperature, maintain operating temperature of die during casting, adjust cooling by applying more die spray.
Porosity: Porosity is the presence of holes or air pockets in the metal. There are many causes for it – trapped air in the metal, metal impurity, die design flaws, metal shrinkage, too much lubrication in the die, and so on. Porosity can be of two kinds:
Gas porosity, or the formation of air bubbles inside the casting as it cools. Solution: Melt the metal in a vacuum or use more through degassing steps in the metal melt stage.
Shrinkage porosity, or the creation of cavities in the casting due to metal shrinkage. Solution: Maintain high pressure while injecting liquid metal into the die.
Flash: When molten metal seeps out of the die cavity and solidifies, it is called flash. Solution: Adjust clamp force to ensure die halves are tightly shut.
Fragmentation: When the die cracks due to injection force, it is called fragmentation. Solutions: Use a fracture-resistant metal with high strength and plasticity when tooling the die, polish the die surface.
Lamination: This is a surface defect where a layer of metal forms a separate surface on the casting. It is usually caused by the metal cooling before it mixes. Solutions: Increase metal and die temperatures, reduce fill time (the time taken to fill the die with liquid metal).
Ejector marks: These are small indentations on the casting formed during ejection. Solutions: Lower ejection speed, allow adequate cooling time.
Discolouration: A discolouration of the casting surface caused by one or more components in the release agent. Solution: Change the release agent.