There is more than one way to bake a cake. Ingredients, temperature, bake time, and how many you are serving dictate your approach. Making brownies for the third grade class is not the same as baking an elaborate wedding cake for 350 degrees. The same is true when it comes to selecting a right metal casting process to manufacture any component. Which process offers the best all-in cost, considering requirements for features, material, quality, and quantity?
The list of methods to produce metal parts is long, and includes machining/hogouts, forging, fabrication, and more. In this article, we will just discuss various casting processes you might select to make an important component.
Sand casting has been around for ages and is used to make about 2/3 of cast products. It is a versatile method to produce a wide range of cast geometries including those with internal structures and passageways – from less than a pound to huge parts weighing several tons.
The process is versatile and cost effective, even in low volume because tooling cost can be minimized, or even eliminated, through 3D sand printing or direct machining of the mold. It is best suited for near-net shaped designs where some subsequent machining is employed for areas with tight surface finish or dimensional tolerance requirements. It is viable for just about any alloy except reactive metals and those requiring vacuum pouring.
Investment casting produces a finished metal part by creating a ceramic shell around a detailed consumable pattern, usually of wax or acrylic, which is ultimately melted or burned away. This method is selected because it produces an excellent “as-cast” surface finish. It can achieve thin walls, tighter tolerance, and shaped or non-symmetrical parts with fine detail including internal structures. Good process management can maximize the number of parts on a “tree” to be cost effective.
There are less expensive solutions when tolerance demands are less strict – heavier sections or parts that will be machined anyway – and upper size limits are much lower for investment castings than, say, sand castings. Tooling costs have historically limited investment casting to higher volumes, but processes like SLA and 3D pattern printing have made it viable for prototype or even single piece runs.
Continuous casting is a process where molten metal is drawn through a forming die as it solidifies into a desired shape. A profile may be created in both the ID and the OD of the part to create either a solid or hollow. Internal shaping, however, is limited by material cast as the high melting point of iron (as opposed to copper based alloys) makes shaping difficult due to die life considerations.
A continuous cast bar would typically be cut to the desired length and machined further. Continuous casting generates metal with sound structure and can be cost effective when used to reduce expensive machining processes like milling, and the material may be acquired in small quantities. It is not a good fit for more complex components with irregular shapes or intricate internal passageways.
Centrifugal casting is the technology of choice for severe service applications in configurations that can be developed with an axis of rotation. Molten metal poured into a spinning die solidifies directionally, centrifuging low density impurities to the ID where they are subsequently machined away. The result is material cleanliness that is typically superior to other casting or forging processes. The die will rotate either vertically or horizontally depending on the configuration of the part, and added OD shaping reduces post-cast machining. Nearly any material can be centrifugally cast including exotic vacuum alloys. Components from a couple of inches in diameter to 15+ feet can be produced. Limitations include a minimum of internal shaping, or parts that lack symmetry or cannot be safely balanced in casting process equipment.
There are many other casting processes and it may not be obvious which metalworking method is the best for your particular application. In the same way, you would call your Mom for the best cake-making advice, call a casting expert for guidance on the best casting method to specify.
With so many casting processes, it can be hard to decide which one to use. Therefore, it is important to understand each option and how it relates to production costs, production volume, casting size, tolerance, surface finish, complexity and more.
Compare and contrast with us the 5 main casting processes (sand casting, permanent mold, die casting, investment casting, and lost foam casting), so you can make your decision based on the facts.
Sand casting is one of the most common casting processes. In this process, molten metal is poured into a mold made of compacted sand.
There are 4 specific types of sand-casting processes, each with their own advantages and disadvantages: green sand, nobake, coldbox, and shell.
Green sand is the most common sand-casting process. Its components include sand, clay, water, and other additives.
A benefit of green sand casting is that it can be used for high volume or short/prototype runs. It is cost effective for many metal applications.
However, green sand casting cannot hold as tight of dimensional tolerances and surface finishes as other processes. Therefore, it typically requires more machining to reach the final dimensions and surface finish than other processes.
This process involves a simple sand and resin mixture that cures at room temperature through a chemical reaction.
Benefits of the nobake process include good dimensional control, uniform strength, and size flexibility.
Unfortunately, the nobake sand mixture has a limited shelf life and you must be conscious of the sand temperature during the process. Overall, this process can be very costly.
This resin process cures at room temperature, using a gas or vapor catalyst.
Coldbox sand casting can achieve complex parts with low labor and energy costs.
However, this process is limited to small and medium sizes. The core box design is an important factor. It is also only cost efficient in high volumes.
This resin coated sand process requires heat to cure.
Benefits of shell sand casting include high dimensional tolerances and surface finish, opportunity for complexity, and a long shelf life.
Yet, shell sand casting is only available for small to medium sized parts. It requires metal tooling, making it only cost efficient in high volumes.
The permanent mold process is our specialty. It involves pouring nonferrous alloys into a preheated, reusable steel mold. The steel mold is prepared with a coating that prevents sticking and lengthens mold life.
Permanent mold foundries are typically cleaner than sand casting facilities because they do not involve loose sand.
Different pouring methods can be used in permanent mold that impact the success of your part: static standard/gravity pour, tilt pour, and reverse tilt pour. To learn more about permanent mold pouring methods, watch this short video:
Die casting is similar to permanent mold casting. Die casting also involves a steel mold but uses pressure to inject molten nonferrous metals.
If you’re annually running tens or hundreds of thousands of parts, like an automotive company, you’re going to want to go with die casting.
Investment casting uses a wax (or thermoplastic) pattern that is layered in slurry and stucco to form a shell. The wax is melted out of the shell, the now hallow shell is cured, and the metal is poured while the ceramic shell is still hot.
In the lost foam casting process, the part or clusters of parts are coated in a slurry and sand. They are vibrated to create a compact mold. When the molten metal is poured, the pattern evaporates, eliminating the need for cores.
Metal stamping is a common alternative to casting. It is best for simple parts, as it involves using high pressure to shape a sheet of metal into your desired part.
To kick start your next project, we have many allies in the casting business. We are happy to look at your next project and help decide what casting process is right for you.
Give us a call at (812) 537-2275 or you can email us at sales@batesvilleproducts.com
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