A Guide to the Different Types of Metal Casting Processes

This article will be your friendly guide to the different types of casting processes. We will explore how each metal casting process works. I will help you understand the big ones, like sand casting and die casting. By the end, you will know how to choose the right type of casting process for your own project. This knowledge will help you save time and money.

What Exactly is the Metal Casting Process?

Let’s start with the basics. What is metal casting? Imagine you want to make a metal toy soldier. You can’t just carve it from a block of steel. That would be too hard and take too long. Instead, you can use casting. The metal casting process is simple in theory. First, you create a mold. A mold is a hollow container with the shape of the toy soldier inside. This hollow space is called the cavity. This mold is key to a good casting.

Next, you heat a metal, like tin or aluminum, until it becomes a liquid. This liquid is called molten metal. Then, the molten metal is poured into the mold. You let the metal cool down and become solid again. Once it has had time to solidify, you open the mold and take out your brand-new metal toy soldier. That’s casting in a nutshell! This basic casting process is the foundation for all the different types of metal casting we will talk about. Every type of casting follows this same idea of pouring molten metal into a mold.

This manufacturing process has been used for thousands of years to make tools, art, and weapons. Today, we use casting to manufacture everything from engine blocks to jewelry. The quality of the final casting depends a lot on the quality of the mold and how the molten metal is handled. A good casting is strong and has the exact shape you want. This process works with almost any metal or alloy.

Why Are There So Many Different Types of Casting Processes?

You might be asking, if the basic idea is so simple, why do we need so many types of casting processes? That’s a great question. In my experience, the answer comes down to four things: the type of metal, the shape of the part, how many parts you need, and your budget. Each metal casting process has its own advantages and disadvantages. There isn’t one perfect casting method for everything.

Some types of casting processes are great for making thousands of identical parts very quickly. Others are better for creating one or two very detailed and complex parts. For example, some casting methods use a mold that you can only use once. Other casting methods use a sturdy metal mold that is reusable thousands of time. The metal you choose also matters. Some metal alloys work better with certain types of casting.

The goal is to get the best possible final product for the lowest cost. That’s why engineers and designers must choose the right type of casting. A bad choice can lead to a weak part, a poor surface finish, or a high cost. Understanding the different types of metal casting processes helps you make the smart choice to manufacture your parts. This is why learning about each casting technique is so important.

Sand Casting: Is It the Oldest and Most Common Casting Method?

Yes, it is! Sand casting is one of the oldest and most popular casting methods. I’ve seen this process used for huge, heavy parts, like manhole covers and large pipes. The reason sand casting is so common is that it is very flexible and cheap. The sand casting process uses a mold made from, you guessed it, sand. It’s not just any sand, though. It’s a special kind of bonded sand, often called green sand, that can hold its shape.

To start the sand casting process, a pattern of the final part is pressed into the sand to create the mold cavity. The pattern is then removed, and the two halves of the sand mold are put together. After pouring the molten metal and letting it cool, the sand mold is simply broken away to get the finished casting. Since the mold is destroyed, you need to make a new sand mold for every single casting. This type of casting is great for parts with complex shapes and for casting metals with high melting points, like iron and steel.

The great thing about sand casting is the low cost of the sand used for the molds for casting. However, the surface finish of a sand casting isn’t very smooth. The parts from sand casting often need more work, like sanding or machine work, to make them look good. Also, the dimensional accuracy, or how close the part is to the perfect size, is not as good as with other casting methods. But for many casting uses, sand casting is the perfect choice. This casting process is a workhorse.

What is Permanent Mold Casting and When Should You Use It?

Now, let’s talk about a casting process that uses a mold you don’t have to break. Permanent mold casting uses reusable molds made from metal, usually iron or steel. These reusable molds can be used over and over again to manufacture thousands of parts. This makes the process faster than sand casting once the initial metal mold is made. The casting process is still simple. Molten metal is poured into the metal mold, often just using gravity.

This type of casting that uses gravity to fill the mold is called gravity die casting. The metal cools and solidifies inside the mold. Then the mold is opened, and the casting is taken out. Because the mold is made of metal, it cools the molten metal faster than a sand mold does. This can create a final casting with better mechanical properties, which means it’s stronger. The surface finish and dimensional accuracy are also much better than sand casting.

Permanent mold casting is great for metals like aluminum, magnesium, and copper alloys. It’s often used to manufacture automotive parts like pistons and gears. The main downside is the cost of the mold. A metal mold is expensive to make. So, this casting process is best when you need to manufacture a large number of parts. The cost of the mold is spread out over all the parts, making each casting cheaper in the long run.

Die Casting: How Does High Pressure Shape Your Metal Parts?

When you need to make a lot of metal parts very fast, die casting is one of the best choices. I’ve always been amazed at the speed of a die casting machine. The die casting process is similar to permanent mold casting because it uses a reusable metal mold, called a die. The big difference is how the molten metal gets into the mold. Instead of using gravity, die casting uses high pressure.

In high pressure die casting, molten metal is forced into the mold cavity under very high pressure. This pressure fills every tiny corner of the mold quickly. This allows you to create parts with very thin walls and complex shapes. The pressure is held until the metal solidifies. The result is a high-quality casting with an excellent surface finish and great dimensional accuracy. This is a very popular casting process. The casting isn’t going to need much finishing work.

Die casting is perfect for metals with lower melting points, like zinc, aluminum, and magnesium. You see die casting parts everywhere. Your car’s door handles, the body of your power drill, and even some computer parts are made with die casting. The die casting machine and the molds are very expensive, so this method is only used for high-volume production. When you need to manufacture tens of thousands of parts, die casting is a fantastic casting option. There is also low-pressure die casting and vacuum casting, which are types of die casting that use less pressure.

Investment Casting: The Key to Producing Complex Metal Parts?

What if you need to create a metal part with an incredibly complex shape? I’m talking about shapes that are almost impossible to make any other way. For that, you would turn to investment casting. This metal casting process is sometimes called “lost-wax casting” because it starts with a pattern made of wax. It is an amazing type of casting for producing complex geometries.

Here’s how the investment casting process works. First, an exact replica of the part is made from wax. This is the wax pattern. Several of these wax patterns are attached to a wax “tree.” This whole tree is then dipped into a liquid ceramic mixture many times. A hard ceramic shell forms around the wax. Once the ceramic is dry, it’s heated in an oven. The heat melts the wax, which flows out, leaving a hollow ceramic mold. This mold is an exact copy of the part’s shape. Pouring the molten metal happens next, into this new ceramic mold. After the metal cools, the ceramic shell is broken away to reveal the finished casting.

Investment casting can produce complex metal parts with an amazing surface finish and incredible detail. It’s used in the aerospace industry to make turbine blades and in the medical field for implants. The process is more expensive and slower than sand casting or die casting. But for producing complex parts where detail is key, investment casting is the best type of casting. The ability to produce such complex shapes makes this casting process invaluable.

What is Plaster Casting for a Smooth Surface Finish?

Sometimes, you need a very smooth surface on your casting, but you don’t need the strength that comes from a metal mold. In these cases, plaster casting is a great choice. As the name suggests, this process uses a mold made from plaster, similar to the kind used for art projects. The process is a lot like sand casting, but the plaster mold provides a much better surface finish.

The plaster mold casting process involves pouring a plaster mixture over a pattern. Once the plaster sets, the pattern is removed, and the mold is baked to remove any water. This leaves a dry, hard mold ready for the casting. The molten metal is then poured into the plaster mold. This type of casting works best with softer metals that have low melting points, like aluminum and zinc, because the plaster mold isn’t as strong as a sand or metal mold.

The main advantage of plaster casting is the excellent surface finish and high dimensional accuracy it can achieve. It’s a good middle-ground casting option between the roughness of sand casting and the high cost of investment casting. However, making the plaster mold can take a long time because it needs to be baked. The mold is also not reusable, just like with sand casting. It’s a specialized casting process for specific needs.

How Does Centrifugal Casting Work?

Imagine spinning a bucket of water around your head. The water stays in the bucket because of a force pushing it outwards. Centrifugal casting uses this same force to create parts. This casting method is perfect for making parts that are hollow and cylindrical, like pipes or rings. It is a very clever type of casting.

In this metal casting process, a permanent mold is spun at high speed. While the mold is spinning, molten metal is poured inside. The spinning force, called centrifugal force, pushes the liquid metal against the inside walls of the mold. The metal spreads out evenly, creating a uniform thickness. The process continues until the casting solidifies. The final casting is very dense and strong because the force squeezes out any impurities or air bubbles.

There are different kinds of centrifugal casting, including roto casting. This casting process is great for producing high-quality pipes, wheels, and bushings. The parts have good mechanical properties. The main limitation of this casting method is that it’s mostly used for symmetrical, round shapes. You can’t make just any shape with this casting technique.

What Are Some Other Specialized Types of Metal Casting?

We’ve covered the big types of metal casting processes, but there are a few other specialized ones I should mention. These are used for very specific jobs. Each casting process offers unique benefits.

• Lost-Foam Casting: This is similar to investment casting, but instead of a wax pattern, it uses a pattern made of foam. The foam pattern is placed in a sand mold. When the molten metal is poured in, the foam instantly vaporizes, and the metal fills the space. Lost-foam casting is great for complex shapes.
• Continuous Casting: This is a process used to manufacture huge amounts of metal stock, like bars and beams. In continuous casting, molten metal is poured into a mold that is open at the bottom. As the metal cools and solidifies, it is slowly pulled out, creating a continuous, long piece of metal.
• Squeeze Casting: This method, also known as squeezing die casting, combines casting and forging. Molten metal is poured into a die, and then high pressure is applied as the metal solidifies. This process creates a very dense, strong final casting with no air pockets.

Each of these different types of metal casting processes was developed to solve a specific manufacturing challenge. For example, vacuum casting is used to create prototypes with very high detail.

How Do You Choose the Right Type of Casting Process and Avoid Defects?

Choosing the right casting process is the most important step. In my work, I always start by asking a few key questions: What metal or alloy am I using? How complex is the part’s shape? How many parts do I need to manufacture? What is my budget? And what kind of surface finish and dimensional accuracy do I need? The answers will point you to the best casting method.

The table below gives you a quick comparison of the main casting techniques.

No matter which casting process you choose, you can run into problems. These problems are called casting defects. A defect could be a crack, a hole, or a part that is the wrong shape. These casting defects are often caused by problems with the mold, the temperature of the metal, or how the casting is cooled. A good manufacturer knows how to control the metal casting process to avoid these defects and produce a high-quality final product. Making a good casting is both a science and an art. The goal is always a perfect final casting.

Key Things to Remember

I know we’ve covered a lot of ground on the different types of casting. Here is a quick summary of the most important points to help you remember everything about casting.

• Casting is a process where molten metal is poured into a mold to create a part.
• There are many types of casting processes because each one is good for different metals, shapes, and production numbers.
• Sand casting is cheap and great for large parts but has a rough surface finish. The mold for this casting is not reusable.
• Permanent mold casting and die casting use reusable metal molds to make many parts quickly with a good finish. Die casting uses high pressure for fine details.
• Investment casting uses a “lost-wax” process to create very complex parts with excellent detail.
• Choosing the right casting process depends on your project’s needs, like cost, quantity, and the final casting quality you desire.
• Avoiding casting defects is key to making a strong and reliable final casting.