Basics

What is die casting?

| Editor: Steffen Donath

Learn all about die casting, its advantages and disadvantages.
Learn all about die casting, its advantages and disadvantages. (Source: Mika Baumeister (Unsplash))

Die casting is the process where molten metal is forced into moulds under high pressure. Here is everything you need to know about it.

Unlike other casting processes, such as powder metallurgy, most metals used in the casting of dies are nonferrous. Examples include aluminum, magnesium, zinc, lead, and pewter. The process was invented just about 150 years ago so that printers could make pieces of movable type more easily and quickly than they could using older methods.

Types

Hot-chamber casting and cold-chamber casting are the two basic categories. There are also secondary types, such as vacuum, squeeze, and semi-solid. Hot-chamber is the most common because it combines the liquefying and injecting procedures in the same apparatus. It is also sometimes known as gooseneck casting because the chamber is fed through a goosenecked apparatus.

For a variety of reasons, the pieces created through hot-chamber casting are more susceptible to corrosion. This means that hot-chamber casting is best suited for metals with a low melting point, such as lead, zinc, and magnesium. Additionally, because of the relatively low temperature that is suited for hot-chamber casting, this method works best with single metals and not alloys.

Cold-chamber casting removes the combustion chamber from the injection apparatus and uses what is basically a straw to form the conduit for the molten metal. Its chief advantage is that it can heat various metals to their melting points, even those that are higher than that of others, because the melting chamber can be constructed of material with ultra-high melting points, such as tungsten. It is, therefore, perfect for making alloyed pieces.

The process's biggest drawback is the slower operational speed. Not only does it take extra time to pump the molten material, but because the material also must be heated in a separate location, it is also more expensive. For metals with higher melting points, such as copper, manganese, and molybdenum, it is the only method available if you want to avoid the corrosion problems later.

Low-pressure casting operates vertically. Instead of pressing the liquid metal into a mould with a powerful piston, the system uses a maximum of 1 bar to push the metal upwards into the mould. This enables the operator to add additional molten metal to the mould to correct for any imperfections that occur when the metal cools and shrinks. In this way, the operator can create complex geometric shapes with incredible accuracy. The chief metal used with this method is aluminum. In fact, this method is the most common form for an aluminium die cast. The only real drawback to this method is that it's slower even than cold-chamber die casting.

Vacuum casting reverses the position of the mould and the liquid reservoir from low-pressure casting. Also common for aluminium die casts, the vacuum procedure basically sucks the gas out of the chamber, and the metal flows into the mould because of the reduced pressure. It is, therefore, a secondary system that assists other casting processes. Because the vacuum eliminates air and other gases from the chamber, the chance of structure-threatening bubbles and excess porosity is reduced or eradicated. This procedure creates items that are particularly well-suited to heat treatments afterward.

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Squeeze casting is a hybrid procedure that borrows the best characteristics of other processes. The liquid metal is usually magnesium or aluminium and is is poured into the bottom half of a die. The machine then presses the top half of the die downward with immense pressure, which forces the metal into all portions of the metal die cast. Because the pressure is applied slowly, the quality of the produced pieces is exceptional. The method produces extremely dense pieces that are great for automotive parts or other parts that have high safety standards and low tolerance for imperfections.

Another method that produces ultra-dense products that are safe for repetitive-motion pieces is semi-solid die casting. In this method, which is also called Thixoforming, a machine cuts the material to be moulded into slugs. These are then heated to a point between full melting and staying solid. The ideal state is roughly 40 % solid. This is the method for creating the pieces that require the tightest tolerances and least porous finished products. As such, it is much more expensive than other methods. Additionally, the temperature band in which the casting occurs must be maintained to 10ths of a degree. This requires not only high-end equipment but also superbly trained operators with lots of special knowledge.

Metal die cast steps

No matter the primary or secondary casting method, there are five general steps germane to all processes:

1. Clamping — Each half of the mould must be held solidly in place. Before the fresh molten metal is injected, each half must also be thoroughly cleaned and lubricated. The clamping, cleaning, and lubrication time gets longer depending on part size and the number of cavities to be filled. The clamping must be complete so that there are no leaks when the molten metal is inserted, which would result in disaster.

2. Injection — Once the halves of the mould are secure, the liquid metal is transferred to a chamber. From there, the operator injects it into the mould. This method varies by apparatus and procedure. The pressure used can be immense, often as high as 20,000 psi, or gentle. The pressure holds the metal motionless inside the mould until it solidifies. Typically, the injection takes place in less than 0.1 seconds, which keeps the molten metal from hardening too soon. Different materials, however, require different injection times.

3. Cooling — The metal obviously cools as it solidifies, and the operator cannot open the mould until the material inside it is fully solid. The length of time this requires is governed by certain properties of the metal, such as viscosity, surface tension, rate of solidification, the shape to be created, and the maximum diameter of the wall of the casting.

4. Ejection — Once the item to be cast solidifies, the halves open, and the ejection mechanism pushes out the finished product. Depending on the material's properties, this cycle time varies. The preprogrammed time must include the time it takes for the finished piece to be collected from the machine either by measuring how long it takes to fall into a collecting apparatus or how long a conveyor or other method takes to remove the piece from the chamber. Only after the piece is fully ejected can the mould halves be clamped together again for the next piece.

5. Trimming — No piece ever forms without flash or other detritus on its surface, and these must be removed before the piece is usable. This can be done with specially designed cutters or saws, or each piece can be trimmed in a particular press. The material removed in this way can easily be reused as part of the next batch of molten metal, although it might need reconditioning so that it matches the correct purity or alloy balance of the next required metal.

Casting defects

Much of casting involves the prevention and rectification of defects. Certain pieces, too, may have slight faults that don't matter for their intended use but that would usher in a catastrophe in another situation. Therefore, all operators should be cognizant of which defects disqualify which pieces from which uses.

Shrinkage defects occur during certain parts of casting. Usually, they result in rough or jagged edges. They can be cylindrical defects called “pipes” that burrow into the centre of the piece, concave indentations in the surface, or even hidden air pockets that occur because of uneven cooling. All of these will reduce the structural integrity of the piece.

When the air pockets multiply, it's called porosity. A single, tiny air pocket might not be enough to disqualify a part from use, but a part honeycombed with such pockets is worse than useless. Vacuum-assisted casting processes are designed to prevent this kind of defect.This extreme example shows that even within the core of the part, giant air bubbles, which are called blowholes, can ruin an otherwise sound part.

Sometimes, defects occur during the pouring or mixing processes. For example, if a mixture calls for 58 % aluminium and 42 % zinc, having 43.5 % zinc might cause what's known as a cold shut. This is where the metals don't mix properly at a certain spot inside the part. The fracture line along where the metals didn't mix correctly creates a weak spot that can fail at the most inopportune times.

A misrun is where the material doesn't fill the mould completely or properly. Usually, this is because the molten material is either at the wrong temperature for one of the components therein, or some other outside force has changed the properties of the chamber.

An inclusion occurs when something unwanted gets mixed in with the desired material. This could be a broken piece of mould insulation, excessive lubricant, or even part of a ladle that chips off and melts. These contaminants become “included” in the mixture and either spoil the balance or create other faults in the mixture.

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Items generally created through casting

As already mentioned, automotive parts form a large chunk of the products produced through casting. Complex, solid pieces like four-wheel drive transmission cases made from aluminum and the gears that go into those cases are prime examples.

For boats, there are products like outboard motor swivels and propellers. For aircraft, parts made from lightweight pure metals and alloys, particularly in seats, are essential in saving weight and mass to increase lift. Planes, for example, that weigh less don't need as much power to get them airborne, which results in lower fuel costs.

In everyday life, everything from camera housings to tablet frames to satellite TV housings are cast from a variety of materials using one or more casting processes.

Advantages and disadvantages

Dimensional accuracy is quite fine using these methods, and the smoothness of the facing is exquisite, often down to 40 microinches. The tolerances achieved are in the range of 2 1,000ths of an inch too.

Most processes also obviate the need for secondary machining operations because of their accuracy. The tensile strength of parts produced ranges as high as 60 ksi, which is twice the strength of the steel used to erect buildings.

Disadvantages include a high overall cost to maintain both peak performance and low defect rate. The problem of porous defects can never be completely eliminated, which adds to the overall cost. For many items, other processes are more cost-effective than casting.

Also, the size of the components created is limited to 24 inches in length or width and between one ounce and 20 pounds. Because of the high cost and limited markup opportunities, production runs must be quite large to make up the deficit.

It is also not possible to work with metals that aren't high-fluidity, obviously because the metals need to flow easily into empty places in the moulds. Therefore, the kinds of products made are limited to those made from a short list of metals and alloys. Items made through casting must also not need special hardening or excessive post-process heating, both of which could cause the expansion of air inside minuscule, porous spaces. Once that air expands, the part will crack and be useless. Production rate is also severely limited by the capacity of the casting machinery.

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