The top 25 most frequently asked questions about aluminum casting.
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The Basics
1. What’s a casting?
While the word casting (noun) can be used to describe the finished product, casting (verb) can also be used to the describe the manufacturing method.
Metal casting is the process of pouring molten metal into a custom mold and shaping the metal as it solidifies.
2. Are there different casting processes?
There are four common casting methods: sand casting, die casting, investment casting, and permanent mold casting.
3. What metals are used for casting?
Iron, aluminum, bronze, zinc, steel, and copper are common metals used in casting. At Batesville Products, we cast aluminum (A356) and zinc (ZA5 and ZA12).
4. What aluminum alloys can be cast?
There are a variety of aluminum casting alloys, but we prefer A356. This alloy is preferred because of its castability, strength, weight, and corrosion resistance. A356 is also medical-grade and food-safe.
Aluminum ingot. A356.
Properties of Aluminum
5. Is cast aluminum heavy?
Compared to other metals like zinc or iron, cast aluminum is very lightweight. The weight of the casting depends on the size, wall thickness, hollows, and density of the part.
For example, this large casting used on building corners is about twenty pounds.
6. Is cast aluminum strong?
Cast aluminum is very strong, known for its high strength-to-weight ratio. Since it can withstand weight and stress, it’s often used in industrial machinery.
To further increase strength and remove flexibility properties, aluminum castings can be heat treated. T5 and T6 are common heat treatments.
7. Where does aluminum come from?
At Batesville Products, we use primary aluminum. Rio Tinto, a North American based aluminum producer, uses an advanced hydropower damming system to produce aluminum. Did you know that 40% of the cost of making aluminum is electricity? This hydropower system gives a competitive edge in comparison to coal burning smelters.
However, lots of aluminum in use today is recycled, or secondary. Aluminum is fully recyclable and can be recycled indefinitely without losing quality. Did you know 75% of all aluminum ever produced is still in use today?
8. Can cast aluminum be painted?
There are special air-dry or bake-on primers and paints for aluminum castings.
9. Can cast aluminum be powder coated?
Powder coating is one of the most common cast aluminum finishes. Know that castings must be prepped, cleaned, and degassed (or heat treated) before powder coating for the best results.
10. Can cast aluminum be anodized?
Aluminum castings can be anodized or chromated to provide corrosion resistance, oxidation protection, wear resistance, strong insulation, heat transfer properties, and an aesthetic appearance.
11. What can you cast with aluminum?
You can cast almost anything with aluminum! From frying pans to dental chairs, fan blades to electrical housings. Here are some examples.
Where’s aluminum used? Aluminum is used to make products in hundreds of industries, including…
12. Will cast aluminum rust?
Unlike iron, aluminum will not rust. To protect against other forms of corrosion, aluminum can be anodized or coated.
13. Will cast aluminum bend?
Aluminum is known for its high ductility. Unlike other metals, aluminum will bend before it breaks.
14. Can aluminum be welded?
Cast aluminum can be welded. It’s possible to cast different aluminum parts and welded them together to assemble your final product.
However, casting your product into 1 piece rather than welding many pieces together saves a lot of time and money. We recommend working with one of our expert engineers to see how you can turn your project into 1 piece.
Custom permanent mold castings manufactured by Batesville Products
Permanent Mold Casting
15. What’s permanent mold casting?
Permanent mold casting is the gravity-fed process of pouring molten metal into a reusable steel mold.
16. How’s permanent mold casting different from die casting?
Both processes use a reusable steel mold to form metal castings.
The main difference between die casting and permanent mold casting is the way the metal is fed into the mold. Die castings are quickly pressure injected, while permanent mold castings are gravity-fed by a slow tilt. This slow tilt pour means less internal porosity that could become an issue during machining.
Die casting tooling is more complex, meaning it can be up to five times more expensive than permanent mold tooling. Therefore, die casting process is ideal for high volume production runs (example: 50,000 pieces per year). It’s commonly used in the automotive industry. Permanent mold is ideal for moderate volume production runs (example: 5,000 pieces per year).
17. Why permanent mold?
Permanent mold is known for high-quality surface finish, low-porosity, high precision, repeatability, and easy secondary machining.
Permanent mold castings are denser than die or sand castings. Permanent mold is ideal for moderate volume production (500-20,000 pieces per year).
18. How much does permanent mold cost?
9 factors impact the cost of a casting.
The primary cost driver is mold tooling. Permanent mold casting is less expensive than die casting, but more expensive than sand casting.
Sand molds are one-time use, while permanent molds are reusable steel tools. Sand casting’s low-cost tooling and high piece price makes it ideal for low-volume production. However, you will often see a return on investment on permanent mold tooling if you are producing over 500 pieces per year.
Die casting molds are complex steel molds that take longer to produce and can be up to five times more expensive than permanent mold. Therefore, you see best ROI on high volume production in die casting, and moderate volume production in permanent mold.
Keep in mind, all parts are custom. For a better cost estimate, please request a quote.
19. What’s the permanent mold as-cast surface finish?
Permanent mold castings generally have a surface finish of RMS 200-420.
Aluminum casting for food and beverage industry.
Designing for Permanent Mold
20. What’s draft? Why do I need draft?
Draft, also known as taper, is a slight angle applied to walls, moving away from the parting line. Draft is necessary in casting because it helps parts easily eject from the mold.
21. What’s the minimum wall thickness for permanent mold casting?
For permanent mold casting, we recommend a minimum wall thickness of 0.180 inches.
Avoid isolated thin or thick sections. These can negatively impact flow and solidification, and cause shrinkage.
22. What are as-cast tolerances for permanent mold casting?
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Permanent mold holds tight as-cast tolerances, almost double that of sand casting! For even tighter tolerances, consider adding a quick secondary machining operation.
Linear tolerance for features on the same side of the mold are 0.015 for the first inch. Add 0.002 for each additional inch. For concentricity, flatness, and other tolerances, view this design guide.
Linear tolerances for permanent mold aluminum casting
Secondaries
23. Why CNC machine a casting?
Castings are near-net shape of the final product. Sometimes, a secondary CNC machining operation is required to achieve tighter tolerances than permanent mold is capable of.
CNC machining also completes drilling, tapping, threading, or milling.
Casting then secondary machining is a great way to create a complex part. By first casting your project, you have a great starting point. Machining a near-net shape instead of a block of billet speeds up the CNC machining process. You can produce a complex part more efficiently.
Not sure if your casting needs CNC machining? Take this quiz.
24. What types of finishing options are available?
Permanent mold has a great as-cast surface finish. Aluminum casting finishing options are:
Zinc castings are preferred for mirror buffing or plating.
Not sure which finish to use? Use this flowchart to find your ideal finish.
25. How much extra material should be added for machining?
If you do not include a machine stock allowance in your design, the final part may be out of spec. Account for extra material if CNC machining your casting.
Machine stock allowance for permanent mold aluminum casting
More Questions?
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If you’re in the process of designing an aluminum part or product by die casting, you may feel overwhelmed by the number of factors to take into account at this stage. After all, perfecting the design phase is one of the most crucial steps in successfully and efficiently manufacturing die-cast products.
Luckily, our experts have extensive knowledge of how to design for optimum aluminum die casting production, and this article will give you some insight into what they know. We will cover how to design a product for efficient and effective manufacturing, as well as some of the most common considerations you need to take into account when designing a product to be made through die casting.
Aluminum Casting Design Best Practices: Design for ManufacturingDesign for Manufacturing (DFM) is a term often used in engineering. It refers to the process of optimizing production to make it as simple and cost-effective as possible. DFM focuses heavily on the manufacturing methods and processes used.
One of the main advantages of DFM is that it allows problems with the production method to be detected and solved early on in the design phase. At this stage, issues are much less expensive to resolve than when they are discovered during or after the production run. Applying DFM techniques allows for a reduction in the costs of manufacturing while maintaining a good or better standard of quality.
In order to optimize the production process of aluminum die casts, the following objectives should be targeted:
The best way to meet these optimization objectives is to take into account the design considerations discussed in the following sections during the design phase.
9 Aluminum Die Casting Design Considerations to Keep in MindThis section gives you an introduction to some of the most common considerations when designing a product to be made using die casting. These tips align with DFM best practices and are recommended by the North American Die Casting Association.
1. Parting lineAs you already know from our previous article, the die casting process involves a die that is usually divided into two halves (although it can be divided into more), one of them mobile and the other static, that come together to form the mold where the molten metal will be injected. The parting line of a die-cast component is the interface where the two halves of the die come together.
During the design process, choosing where the parting line will be positioned is one of the first things you have to decide, as it has an effect on other design specifications.
An important aspect of the location of the parting line is that it will also be the place where a common die-casting defect, known as flash, will be located. The flash has to be removed by a secondary process after the cast has solidified, which is why it should be designed to be easily accessible by the trimming machinery.
2. ShrinkageShrinkage is a very common and unavoidable phenomenon that happens in most castings, and aluminum is no exception. As the molten metal starts to cool down from melting temperature to room temperature, the cast will shrink towards its center.
Castings commonly shrink between 0.4-0.6% of their volume, which can help the product to be ejected from the external walls of the die. Unfortunately, it can also constrict around any internal protrusions on the mold, making ejection difficult under those circumstances. In these situations, you can make use of a draft to reduce shrinkage and facilitate easier removal of the cast.
3. DraftIn the context of die casting, a draft is a small taper or inclination that can be seen on the sides of the mold that allow for easy ejection. You might have seen this taper in other kinds of molds, such as baking pans for muffins and bread, where the sides have a slight angle instead of being completely vertical.
This draft has to be included in all cast surfaces that are parallel to the movement of the die, as this will facilitate easier ejection. Failure to include it can cause the cast to be very difficult, if not impossible, to remove without damaging it.
Keep in mind that the amount of draft, expressed in either millimeters or degrees, will vary considerably throughout most designs. For example, outside walls require only a relatively small draft as the cast shrinks away from them, but inside walls and holes require a larger draft as the metal shrinks and grips around them.
4. Wall ThicknessIn order to ensure rapid production, reduce material waste, and create a successful product, wall thickness has to be carefully considered.
A wall design that is too thin could prevent an adequate flow of the molten metal, making the metal solidify before the mold is completely filled. Thin walls can also be susceptible to warping when post-mold machining forces are applied. Excessively thick walls, on the other hand, not only waste casting material, but they increase solidification times, potentially eliminating the fast production cycle advantage of high-pressure die casting.
Wall thickness should also be as uniform as possible. This allows the molten metal to flow easily, which is essential for having an effective solidification process that ensures optimal cast strength and reduces the possibility of cast defects. If variations in thickness are necessary, gradual transitions should be used instead of sudden changes.
5. Fillets and RadiiFillets and radii are curved junctures between two surfaces that would otherwise create a sharp edge. The following image shows an example of a part that incorporates each of these features. The key difference is that fillets exist on the internal edges of the part while Radii are found on the external edges.
These rounded edges are extremely important in die casting design, as having them allows the metal to flow easily when injected. Sharp corners in the die create turbulence in the metal flow, which can reduce the strength of the cast component. Radii can also eliminate the need for trimming sharp corners and edges through a secondary operation.
6. BossesBosses are protruding features of cast products that are normally used as standoffs or mounting points. This video displays a good visual for what a boss is by showing a boss being added to a doorknob. Including bosses in your design can prevent the need for time-consuming boring operations as a secondary process.
Since major changes in wall thickness can cause uneven shrinkage and sinking that negatively impact a part’s appearance and even its integrity, bosses must be designed to maintain a uniform wall thickness relative to the surrounding part. A common method for maintaining a uniform wall thickness is to add a hole to the center of the boss.
You should also consider adding ample fillets at the location where the boss connects to the rest of the cast, to allow for proper metal flow. A proper draft angle must also be considered for bosses, as well as ribs to increase its strength.
7. RibsRibs are small bridges of material that can be added between walls to increase their strength without adding too much metal. They also help molten metal to reach every part of the die by increasing the available flow pathways.
The image below shows a cast aluminum product that includes a boss which will be used for mounting, and three ribs connected to an outer wall, which increases the structural soundness of the boss.
In accordance with best practices, ribs are usually added to a design in odd numbers or are otherwise offset from one another in order to prevent stresses impacting ribs that are adjacent to each other.
The image above shows an example of a design with an even number of ribs, going against best practices.
8. UndercutsUndercuts are known in manufacturing as recessed surfaces that cannot be accessed with a straight tool. The nature of undercuts can prevent die separation and cast ejection after solidification as the die will be essentially “gripping” the cast product, which is why careful design is important.
The following image shows a component with and without an undercut. You can see that the figure on the right has a depressed surface where the small and big cylinders connect which would make it impossible to eject from the die.
If an undercut is essential to your design, you can design your parting line around the undercuts. Another method is to use dies that are made up of more parts than just the core and cavity, like in the following animation, or by using semi-permanent molds.
These methods, however, increase the cost and complexity of the die considerably. Additionally, semi-permanent milds, which include the use of sand cores, cannot be used in high-pressure die casting processes.
9. Holes and WindowsAccounting for the holes and windows needed in a final die cast part during the design phase can significantly reduce the amount of post-molding machining required. Eliminating or minimizing drilling, milling, and other machining activities wherever possible by thoughtful mold design can drastically improve manufacturing times.
Holes and windows have the added benefit that they generally do not introduce too many new challenges when incorporated into a design. However, you should still take into account that they will make the molten metal flow more complex, which could lead to turbulence and potential casting flaws associated with turbulence. Adding fillets and radii to any edges on holes and windows can help mitigate this issue.
It is also important to remember that holes and windows in the cast can grip the die during ejection, meaning you have to incorporate drafts around them during the design phase.
ConclusionAs you have now seen, designing for aluminum die casting presents many challenges. This article gives you an idea of some of the most important aspects that have to be considered during the design stage, though it’s possible that you may encounter even more obstacles impacting your specific project.
In this article, we’ve briefly touched on secondary, or post-molding, operations as an aspect of die casting design, particularly since minimizing them is a good way to optimize the manufacturing process. To get a better understanding of what they entail, what applications they are relevant for, and more view our article on secondary operations for die cast parts.
If you have any doubts or need assistance in setting up your aluminum die casting project, please contact us for more guidance. You can also learn more about Gabrian’s aluminum high-pressure die casting services.
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