Metal casting works by simply heating metal until it is in a liquid state—molten—and pouring it into a mold. It is then allowed to cool, the mold is opened, and the final cast item is removed. It is the heating and molding of the metal that differentiates casting metal from forging in which the metal is merely heated until it becomes ductile, and then hammered into shape. Metal casting is generally less laborious than forging, and allows for metal to be formed into more complex shapes.

While the basic principles of metal casting are relatively simple to understand there are several different types of metal casting. These different casting types use different casting media, different mold types, and even different vertical or horizontal orientations of the mold. Depending on what type of metal is being cast, and what alloy of that metal is being used some casting techniques will work better than others. Some other casting techniques may be entirely ruled out. These different aspects make things a bit more complicated than just pouring molten metals into a mold.

How Does Metal Casting Work? An Overview

Metal casting falls into two broad groups depending on whether or not the mold is reusable. The types of casting can be further divided by what if any type of additional pattern—effectively an insert—is used inside of the mold, whether or not that pattern can be used repeatedly or is consumed during the casting process. These rough categories are:

Expendable Mold Casting		Permanent Mold Casting
Expendable Pattern	Permanent Pattern	Die Casting Gravity Casting Centrifugal Casting
Lost foam Casting Investment Casting	Sand Casting Plaster Molding Shell Molding Ceramic Molding

When it comes to answering the question, how does metal casting work? The answer tends to be a long one because of the sheer variety of different metal casting types. It is actually pretty close to impossible to provide anything but the most general overview because each casting type differs substantially in method and quality of product produced with only slight difference. There are also a range of additional modifications that can be introduced to each one to achieve a desired effect. However,  even a basic understanding though can be valuable.

Permanent Mold Casting

Permanent mold casting is one of the oldest metal casting techniques there are. The technique dates back to the bronze age when clay and stone casts were used as molds. The issue with these molds is that the surfaces can be airtight, and block the escape of gases from the cooling metal leading to gas pockets in the final metal casting. Although this an old concern, it is still with us today with a great deal of effort placed on allowing gases to escape either through permeable mold materials, or special ventilation points built into the molds. Modern permanent molds tend to be made of sterner stuff like steel, and as a consequence lead time to machine those molds is lengthy.

Permanent Mold Castings
Metal Casting Technique	Advantages	Disadvantages
Die Casting Die casting typically uses a metal mold, the die, in which molten metal is injected at very high pressures. The high pressure helps to ensure the metal fills every part of the mold. These can be used for metals like steel, but the high heats involved makes this rare.	High production rate High quality intricate parts Excellent surface finish	High dies costs Long lead times May be limited to nonferrous metals
Gravity Casting Gravity casting is something of a default. The mold can be metal, or ceramic, or any number of other lasting materials. The key factors is that the mold is meant to last and gravity is the only force that encourages the metal to fill the mold	High production rate Good quality Minimal setup costs	Limited parts shape/not intricate DIfficult to manage thin walls or part sections
Centrifugal Casting Centrifugal casting can refer to any casting process where a mold is spun to create centripetal force that pushes molten metal to the outer extremes of a circular mold.	High production rate Quality tubular parts	Limited shapes producible Expensive equipment Movement during casting.

As the lead time for these casting types is so long these permanent castings are used for very high production runs. Gravity and centrifugal casting tend to have issues with the molten material filling every part of the mold if some parts are thin, or intricate. That means these processes tend to be used with simple shapes. Pipes or ingots for industrial use for example. Die casting allows for much more intricate shapes, but limits on how much the mold can be heated means that nonferrous low melting point metals are the common casting metal for die molds.

Expendable Mold Permanent Pattern Casting

Sand and plasters, somewhat unintuitively, is a better material for casting due to the porosity of the sand allowing for any gases released from the cooling metal to escape from the mold. This leads to a casting that is more consistent throughout helping to prevent the formation of voids or other weak points. However, this does mean that these molds are expendable with single use being the norm. Due to the permeability of these materials the finish of the cast pieces can be rough with hybrids like shell or ceramic mold castings aiming to provide surface finishes.

Permanent patterns are effectively inserts that are slotted into the mold to make a space. These can be meant to channel the molten metal around them and leave a permanent channel. To create a hole for a bolt or other fastener for example, or a place to attach a nonmetallic fixture to the casting during its final assembly later. Intuitively permanent patterns tend to be less critical to the quality of the final metal casting than permanent ones.

Permanent Pattern Casting Techniques
Metal Casting Technique	Advantages	Disadvantages
Sand Casting Sand casting is the most common form of metal casting. The mold is an expendable packing of sand around a cavity. The cavity is typically formed by a reusable or expendable pattern inserted into the mold that the sand is then packed around the pattern.	Low cost Intricate parts Wide range of materials Casts ferrous and high heat metals	Coarse surface finish Post casting cleanup required Loose tolerances Extra finishing work
Plaster Mold Casting In plaster mold casting plaster of paris is mixed and poured around an object. The object is removed, the mold is allowed to harden, and the resulting mold is sprayed with a specialized mold release.	Low porosity Fine finishes possible. Intricate parts	Confined to low heat alloys Limited size and volume Involved mold making process
Shell Mold Casting Shell mold casting uses a thin shell made of sand and resin to create a form with more detailed and complex shapes. These can be single piece or two piece molds.	Excellent finishes Good dimensional accuracy High production	Complicated mold making Expensive Limited part sizes
Ceramic Mold Casting Ceramic mold casting uses refractory ceramics that are capable of resisting high heat. This is used to cast iron, steels, and other alloys that only become molten under high heats.	Intricate part shapes High tolerances Fine surface finishes	Limited part sizes

Expendable mold permanent pattern casting is probably the most widespread type of metal casting. It is, however, not the metal casting technique that produces the most intricate and detailed parts. That is reserved for fully expendable casting techniques where the pattern is a soft material that can be carved into intricate shapes with fine details.

Expendable Mold and Expendable Pattern Casting

In expendable mold and expendable pattern casting the pattern is essentially the mold. It will be consumed or otherwise destroyed leaving behind a finished metal piece. This allows for desired pieces to be carefully carved into the much softer and easier to work with pattern material before casting begins. This allows for very intricate and detailed metal castings to be produced.

Expendable Pattern, Expendable Mold Casting
Metal Casting Technique	Advantages	Disadvantages
Lost Foam Casting In lost foam casting polystyrene foam is used to create a pattern of the part. This is then packed in sand. When molten metal is poured in the foam is vaporized leaving only the molten metal to fill the resulting cavity.	Intricate shapes Unlimited part sizes Wide range of metal for parts	Low strength pattern Multi-stage mold making Expensive
Investment (Lost Wax) Casting Investment casting uses a wax pattern of the part to be made. This wax pattern is coated in a refractory material, possibly a ceramic, and this is then packed into molding sand. A high heat molten alloy is poured in, the wax flows away, and a cast metal part is left in its place.	Intricate part shapes Excellent surfaces High accuracy and tolerances	Limited part sizes Expensive patterns Complex laborious mold making

Resins, waxes, and foams can be made in very intricate parts. When the liquid metal is poured into the mold these materials burn away leaving the molten metal cooling into the desired shape.This allows for the casting of some very complicated shapes.

The various different metal casting techniques listed above derive their differences from how gases are allowed to escape from the mold. Oftentimes the reason a mold is expendable is because of this in the case of sand casting. Other times the materials for the inserts provide the major difference in how metal casting works for each process. There are also a dizzying variety of ways these techniques can be modified. For example die casting can be used to cast the foam or wax for lost foam or investing casting speeding those processes up. Add in mechanical shakers to settle the sand around these patterns, and this allows for a rather laborious process to be used to produce highly detailed metal castings that comply with very high tolerance requirements quickly. While creating intricate metal parts is why most metal casting is done this isn’t the only reason. The process also offers some advantages in regards to internal stresses within the metal as well.

Why Cast Metal Plates?

The most common reason for metal casting work is to create complex shapes in metal quickly. It is much easier, and quicker to produce simple shapes like metal angles or c-channel by extruding semi-molten metal through a mold rather than fully encasing molten metals. Similarly, flat shapes like plates are more readily produced by squeezing metal that is heated up between two rollers than going to the effort to create a capacious flat mold that encloses more square footage than is really practical for a mold. Nevertheless, molded cast aluminum tooling plate in the form of Mic 6 and 5080 aluminum tooling plates are produced, and are a very sought after material.

This is because the different metal forming processes like casting vs forming distribute the atoms of material throughout the resulting metal in different ways. This affects the final properties of the resulting materials. For example cast aluminum plates can be much more dimensionally stable under changing thermal conditions specifically because of how the internal atomic structures line up as a result of being a cast product. These properties are very useful, and can be further enhanced by heat and other treatments post casting.

Indeed this is common with cast aluminum plates. Tooling plates are first cast to near net shape, stress relieved, and then machined to their final dimensions. The final result has substantially tighter tolerances than what is typical for an aluminum casting. Cast aluminum tooling plates have more in common with a precision part like a cast engine block, than the looser tolerances typical in a cast aluminum ingot or blank.

These additional properties are the reason that cast aluminum plates are so sought after. Not only are these plates dimensionally stable under thermal conditions, they are also stable under mechanical stresses. Their internal structures allow for them to be cut or milled away without the remnant metal becoming distorted due to their internal stresses. You can mill away one side without the other side pushing to fill the resulting void. This is the direct result of the casting process, combined with additional heat treatments. The only downside is that since these cast aluminums require so much effort to cast they can be hard to find, and expensive.