Metal casting is the process of pouring molten metal into a shaped space so that it will cool and harden in that form. For many items, this process can be less expensive than machining the part out of a piece of solid metal. Even though the idea behind it seems singular and simple, there are many ways foundries cast objects. How they make a metal casting mold will depend on the metals used, the size of the run, and the shape, symmetry, and complexity of the casting.
Before starting a production run, it is helpful to know some of the terms and methods from the foundry floor.
A glossary of metal casting terms
A mold is a cavity in a material that receives liquid metal and produces a cooled object in the shape of that cavity. Molds can be simple, such as the forms used to create ingots of metal, like a loaf pan forms bread, but most molds are for more complex shapes. These molds will be made in two parts. A pattern, or what the object is supposed to look like, is imprinted into the mold material. Half of the pattern is imprinted on one side of the mold and half on the other, and the halves are clamped together before the mold is filled. By making the mold in two parts, the pattern can be withdrawn before filling.
In horizontal molding, the top half of the mold is called the cope, and the bottom half is called the drag. In vertical molding, the leading half of the mold is called the swing, and the back half is called the ram.
If a mold is supposed to have internal spaces or holes, a core is often made. These cores are shaped like the internal space. The cores are usually held in place by extending past the casting and being held in place through core prints, which suspends the core like a bridge between two banks. The empty spaces around the core will fill with metal, and the core will be removed from the final casting, leaving a hole where it once was. If the core is very long, it might be supported by chaplets to prop it up. These are usually made of the same metal as the final casting as they sit in the space that will flood with material and become part of the final casting.
One of the important factors in choosing a casting method is dimensional tolerance. Dimensional tolerance is the variation acceptable in the size of the final product. Metal shrinks when cooling, and the type of casting influences by how much. If a product needs to be precise, a client may want a casting method that produces near net casting. This means that the product is very close to being the right size when it is shaken out of the mold.
Another consideration is surface finishing. How granular, bumpy, or rough can the surface of the casting be? What is acceptable for a cast iron pan is not acceptable for a wedding ring. Very smooth metal surfaces are usually created with machining, which is an extra cost: if shiny and smooth is a desired outcome, choosing a casting method with a finer finish may reduce machining costs.
Reusable and expendable metal casting molds
Reusable or permanent molds mean a single mold can create many items, whereas expendable molds are destroyed by the casting process. Although it may seem from a layman’s perspective that reusable molds must be more cost effective for a large production run, this is not always the case. Most iron and steel objects are made through expendable casting processes.
Using molds to reproduce objects is a very common process in manufacturing, and low temperature molding substances (resins, chocolate, wax, etc.) almost always use reusable molds. What makes metallurgy different is the high temperatures involved. These put a lot of strain on the mold. It is therefore not a surprise that alloys with a lower melting point like zinc, aluminum, magnesium, tin, or copper are more often those that succeed in reusable molding processes.
However, in some circumstances, even ferrous metals are poured into reusable molds. The complexity of design, choice of metal, and requirements for dimensional tolerance and surface finishing all influence whether reusable molds are appropriate.
Reusable molds: die casting, centrifugal casting, and more
Permanent molds are usually made of metal—one that has a higher melting point than the metal they are used with. Fluid metal is poured into these molds without any type of external pressure. Permanent cores have to be simple so they can be withdrawn for reuse from the finished casting.
These molds are sometimes used in iron casting, as well as with lower-temperature alloys. Often a series of metal-mold units are mounted on a turntable. The individual operations, such as coating the mold, placing the cores, closing the mold, pouring, opening the mold, and ejection of the casting, are performed as each mold passes certain stations. The molds are preheated before the first casting is poured so that it does not crack due to the difference in temperature.
The castings that come from this method cannot have walls as thin as those in other reusable methods, such as die casting. However, the castings are produced with “close tolerance,” which means that the size of the final casting can be more precisely predicted. Castings made this way are dense and fine grained. They have a smoother surface finish and avoid several types of defects.
This form of molding is durable enough to be used with iron, but it is not a preferred style for yellow brasses. Yellow brasses are high in zinc which will foul the mold or die.
The only change in semi-permanent mold casting is that the cores used in the casting process might be expendable sand cores. More complex core shapes are possible with sand cores, because they do not need to be extracted intact from the final casting. As long as an opening in the casting is left to remove cores, they can be “shaken out” on a vibrating table, to drain like sand through an hourglass. The tolerance, density, and appearance advantages of permanent mold casting exist only in the section cast against the metal mold.
This colorfully-named casting style creates hollow castings without needing cores by creating a shell of material inside a mold. There are different ways to approach slush casting depending on how quickly the metal or other material sets: the caster can pour small amounts of the liquid into a mold and rotate to layer the inside with metal or can fill the mold completely and then pour excess material out after a specified cooling time. Zinc, aluminum, and pewter are metals that are commonly slush cast.
In true centrifugal casting, a water-cooled mold is rotated around its central axis at high speed while liquid metal is fed in. Centrifugal force pulls the liquid metal along the mold’s surface in an even layer. The casting and the mold must have even geometries around the axis of spin. This form of casting is therefore best for those molds that are roughly cylindrical or circular, like tubes or rings.
Objects cast in this method usually have a very low defect rate. Impurities end up close to the bore, or inner surface, of the casting, and can be machined away. Most pipes or fittings that will be used under pressure are cast centrifugally, because of the strength of their seamless structure.
Some small metal castings, like jewelry, are made using a centrifuge that swings an entire mold around a central point, pulling metal from a crucible as it whirls. These castings are not true centrifugal castings, but a form of pressure casting usually done with expendable molds.
Pressure casting methods use forces other than gravity to control the flow of metal into a permanent mold. Air or gas, vacuums, mechanical, or centrifugal forces are all used in pressure casting. These methods allow foundries to precisely control the rate at which a mold fills: gravity always works with the same force, but man-made forces can be varied.
Vacuum casting pulls metal into a mold when the mold is depressurized, and the vacuum created pulls liquid metal up from a reservoir below. The vacuum must stay on while the metal cools, and so this method is mostly used for thin walled castings. It provides excellent surface finish. Low pressure castings invert this process by pressurizing the furnace where the liquid metal sits, rather than creating a vacuum in the mold. The metal is pushed through risers into the mold cavity.
All die casting machines (below) also use some form of pressure to help create castings.
Die casting machines consist of a basin holding molten metal, a metallic mold or die on two plates, and an injection system that draws the material and forces it under pressure into the die.
The process for die casting starts with an open mold. Nozzles spray the mold with a lubricant to help prevent the part from sticking. The two halves of the mold are then closed, and the closed mold is injected. The new casting is given a moment to cool before the die opens. Ejector pins push the new casting from the die, and then the process starts again.
There are two forms of metal injection in die casting. Cold-chamber die casting works like a syringe: before each die is cast, an injection chamber must be filled with molten metal, and then a piston pushes the injector’s contents into the die. Hot-chamber or gooseneck die casting works by immersing the chamber of the injection system into the molten metal, where the shape of the system means the injector refills itself. Hot-chamber die casting pushes this material into the mold either with a piston or with air pressure.
Gooseneck systems are more prone to corrosion because they sit in a bath of melted metal. For this reason, they’re usually used with aluminum or aluminum-zinc alloys that have a lower melting point. The piston or cold-chamber injection die caster can be used for the higher temperatures needed to melt brass and bronze, because the injector is not continuously exposed to the heat.
Even metal parts we consider completely machined, rolled, or otherwise worked have often had their start on the foundry floor. Continuous casting creates blooms, billets, and slabs, which are different sizes of simple metal shapes, by extruding them through a permanent form. This casting process creates the raw material for worked steel.
The continuous casting process starts high above the factory floor. Molten metal is fed into a funnel that controls the rate of casting. The funnel fills a mold below it, which is a simple form, usually 20-80 inches long, and shaped on its width like a square, circle, or rectangle. The mold walls are cooled so that the exterior of the casting freezes as it passes through. As the metal leaves the form it is solidifying, but still pliable. This allows the continuous casting machine to bend it so that the finished produced comes out horizontally. A series of wheels guide the slab to a conveyer belt while cooling sprays solidify the surface. Gas jets on the horizontal surface cut the continuous metal piece into manageable lengths, so they can be lifted and stacked.
Expendable molds: investment casting, sand casting, and more
Expendable mold methods are the clear winners when it comes to casting ferrous metals. They are cost effective because they do not have to be sturdy for the high temperatures involved.
Sand casting is the most common method used for metal casting. It is manufacturing process at least three thousand years old: the first evidence of clay casting comes out of China, during the Shang Dynasty (c. 1600 to 1046 BC).
It is no wonder this process is still so popular: sand is cheap, plentiful, pliable, and able to take the heat.
Cores created out of sand are easy to remove: they can be shaken out with a vibrating table. Runners and gates, used to direct the metal into the mold cavity, are either cut by hand by an experienced molder or are created as part of the pattern.
The surface finish on sand cast items is often rough, and the dimensional tolerance not precise, so sand casting is great for producing large, rugged pieces from decorative fences to cast iron pans to car engine parts.
Shell molding is a form of sand casting that provides closer dimensional tolerances. It’s very similar to sand molding, only the sand is mixed with a resin. A mixture of sand and resin are poured over each half of hot metal molding pattern. This mixture melts and cools into a shell. The “shells” of the mold are brought together, and usually supported by a flask full of sand. With the resin providing extra support to the interior surfaces, these shells form a very precise mold.
Often, shell molding is used to produce cores for traditional sand casting. The resin gives the sand cores strength to keep shape, even when positioned over the void that will become a casting. These shell cores are often hollow, created in a hot metal mold in a process similar to slush casting. The two halves of the core mold are clamped and heated, and then filled with resin coated sand. The mold bakes until the shell wall is thick enough to support the size of the core and then the excess, uncured resinous sand is poured back out. When the two halves of the mold are split they reveal the sturdy core, now ready to be placed in the sand-casting mold create space in the casting.
Investment casting (lost-wax casting)
Sand casting is by far the most used form of metal casting, and yet there is one aspect of sand casting that makes it inappropriate for some projects. Sand casting patterns need to be removed from the mold they create, which can mean intricate pattern construction. Draft requirements, parting line placements, gates, risers, and cores: all of these require a pattern maker to carefully consider the pattern’s needs at each stage in the casting process.
The lost-wax, investment, or precision-casting process is an alternative to sand casting that can work with most grades of metal, even high-melting point ferrous alloys, and yet avoids some of these challenges of patternmaking in sand casting.
A designer for an investment casting makes an accurate metal die into which the wax or plastic patterns are cast. These patterns are assembled on a sprue also made of this material: the foundry worker uses a torch to melt the sprue enough to attach each pattern to it.
This assembly is then used to create a shell that will be used as the mold. It is sprayed, brushed, or dipped in a slurry of a fine-grained, highly refractory aggregate, and a proprietary bonding agent composed chiefly of ethyl silicate. This mixture is then allowed to set. The pattern is coated repeatedly with coarser slurries until a shell of the aggregate is produced around the pattern. The molds stand until the coating has set, after which they are heated in an oven in an inverted position so that the wax will run out and be collected for reuse. After the wax is removed, the molds are baked in a preheated furnace. The molds may then be supported with loose sand and poured in any conventional manner.
- When the castings have cooled, the shell around the investment casting is broken and shaken off using a vibrating table.
Investment casting provides superior surface finish, and high dimensional accuracy. There are no parting lines like there are in sand casting.
Full mold or foam casting process
The full mold or foam casting process is a combination of sand and investment cast processes. A foamed polystyrene pattern is used. Indeed, the foamed pattern may be made complete with a gating and runner system, and it can incorporate the elimination of draft allowance. Sometimes the pattern is removed prior to filling, but with some foams the pattern can be left in place in the mold to instantly vaporize when hot metal is poured in.
This process is ideal for casting runs of one or a few pieces, but sometimes foundries mass-produce foam patterns to create production quantities. There is extra expense for the equipment to make the destructible foam patterns, but often the economics of the total casting process can be favorable if the pattern is very complex.
Comparing casting processes
Consultation with manufacturers is helpful to find the most cost-effective way to cast a project. In general, ferrous metals will be cast using expendable molds, whereas non-ferrous metals have a wider range of possibilities, but there are exceptions even to this simple rule.
Going into the process with a clear understanding of the project’s needs will help choose the best process for casting. Does the design need to be a precise size? How thin do the walls need to be? What size and weight will the casting be in the end? What about the surface finish? Knowing in advance the answers to all these questions will help a savvy designer understand and guide their product through the casting process, finding the best, and least expensive, process to do their job.