Answering the 25 most asked questions about aluminum casting

We answered the top 25 most frequently asked questions about the permanent mold aluminum casting process.

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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 three common casting methods: sand casting, die casting, and permanent mold casting. Other casting methods include lost foam casting and investment 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?

The aluminum 300 series is common in metal casting. A380 is common in die casting. A356 is common in permanent mold casting and sand casting. This alloy is preferred because of its castability, strength, weight, and corrosion resistance. A356 is also medical-grade and food-safe.

Properties of Aluminum

5. Is cast aluminum heavy?

Compared to other metals, cast aluminum is very lightweight. The weight of the casting depends on the size, 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. Since it can withstand weight and stress, it’s often used in construction and 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?

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 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 machining and 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.

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. And both processes have a great as-cast surface finishes.

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 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 1,000 parts 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. This is smoother than sand casting (typically 300 to 560 RMS), but rougher than die casting (typically 20 to 120 RMS).

Designing for Permanent Mold

20. What’s draft? Why do I need draft?

Draft is also known as taper. It 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, walls should not be thinner than 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?

Permanent mold casting his high-precision, holding tight as-cast tolerances (some twice as tight as sand casting!). For even tighter tolerances, consider adding a quick secondary machining operation.

Read our Casting Design Guide for all tolerances and design specs.

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.

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Contact us to discuss your requirements of Complete equipment for lost foam casting. Our experienced sales team can help you identify the options that best suit your needs.

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.

For permanent mold castings with dimensions 0 to 6 inches, add a machine stock allowance of 0.045 inches. For dimensions 6 to 12 inches, add 0.060 machine stock allowance. See the chart for more.

More Questions?

Not to be confused with investment casting, lost-foam casting is a molding process used to create objects or 'castings' from a mold. It involves the use of a polystyrene pellets, which are inserted into the mold cavity. Molten metal is then poured into the mold cavity, which causes the polystyrene pellets to expand. As the polystyrene pellets expand, they form a solid casting.

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Lost-Foam Casting Pros:

Lost-foam casting offers several benefits, one of which is ease of use. Most casting processes are complex and require over a half-dozen steps. Lost-foam casting, however, is relatively simple and straightforward: The polystyrene pellets are injected into the mold cavity, after which molten metal is added. The heat from the molten metal will cause the polystyrene pellets to vaporize, resulting in the creation of a casting that can be ejected form the mold cavity.

Another benefit of lost-foam casting is its ability to create castings with high dimensional accuracy. In certain applications, dimensional accuracy is paramount. Automotive engines, for instance, are often manufactured using lost-foam casting because of its exceptionally high dimensional accuracy. Automakers can use it to build engine blocks in a specific size that offers optimal performance for their vehicles.

There's also no flash with lost-foam casting. Flash, of course, consists of excess and unwanted material that accumulates on a casting. With that said, flash is usually caused by draft. Lost-foam casting doesn't require the use of a draft, however, so it's naturally protected against flash. This casting process offers clean castings with high dimensional accuracy.

Lost-Foam Casting Cons

On the other hand, the castings created by lost-foam casting aren't particularly strong. With their low strength, they are naturally susceptible to damage, such as fracturing and breaking, when stressed.

When compared to other casting processes, lost foam is typically more expensive. Specifically, the molds used in lost-foam casting cost more than those used in other casting processes, which may deter some manufacturing companies from using it.

In Conclusion

Lost-foam casting is a casting process in which polystyrene pellets and molten metal is injected into a mold cavity. The molten metal causes the pellets to evaporate. Once evaporated, the hardened pellets form a casting that can be ejected from the mold cavity. Lost-foam casting is relatively easy to perform, offers high dimensional accuracy and is protected against flash. However, the castings are usually weaker and cost more to produce than those of other processes.

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In this article, we will look at lost foam casting, one of the metal casting methods used in modern casting.

What is lost foam casting?

Lost foam casting is a casting method that uses expanded polystyrene (EPS) material, also known as styrofoam, as a patterned material and is planted in silica sand with the compaction of sand molds using a vibration system on the mold frame.

Evaporative casting is another name for lost foam casting. This type of casting is classified as casting without binders.

Do you understand why it's called lost foam? Yes, because when the molten metal is poured into the mold, the EPS melts and evaporates, allowing the metal liquid to fill the space.

This casting method requires a layer of refractory material on the surface of the pattern to support the weight of the sand before the molten metal freezes. Refractory plating must be resistant to high molten metal temperatures. The coating of expanded polystyrene patterns with refractory materials is an important factor in producing high-quality castings using the lost foam casting method.

Lost foam casting stages

Creating patterns with EPS materials

The pattern material can be formed into the desired shape of the object using blocks. Furthermore, the pattern material can be polystyrene foam granules molded in metal molds and heated with moisture, causing the grains to bind together and form a pattern.

Creating a gating system (casting channel)

The pouring channel is constructed from polystyrene foam material by the design, and the forming process is identical to the pattern-making process. The runner (flow channel), sprue (inlet), and pattern are all included in the gating system. After the gating system is created, the pattern and gating system is assembled to form a cluster (pattern tree).

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The process of pattern coating with ceramic material.

Ceramic powder or powder dissolved in colloidal silica or a similar material. The process of coating the pattern by dipping to achieve coating homogeneity on all parts. When pouring liquid into the mold and producing a smooth cast surface, coating serves to hold molded sand into cast objects. Depending on the need, the coating can be done once or multiple times depending on the need, a thick coating will prevent gases from escaping from the mold and may result in cast object defects.

Insert the coated pattern into the molding frame.

The pattern is stockpiled with sand without binders until it is flush with the pattern's top. A vibratory machine is used in the compaction of the mold using the vibration method.

Pouring Liquid into Molds

After all of the processes are completed, the liquid is poured into the mold. If the pattern does not completely burn, it will be converted into a gas or residue by the metal liquid.

Mold Disassembly

Turning over the printed frame begins the disassembly process. Because cast objects do not contain a binder, they will easily separate from the molded sand.

The Advantages and Disadvantages of Lost Foam Casting

Aluminum alloy castings are frequently made using lost foam casting. Lost foam casting is also one of the most cost-effective and environmentally friendly casting methods.

There are numerous advantages to lost foam casting, including:

• Flexibility in designing casting patterns.
• Can create complex objects and shapes.
• There is no mold partition.
• Not wearing the core.
• It is cutting labor in its foundry.
• Molds made from polystyrene foam patterns are simple to make and inexpensive.
• Environmental pollution caused by binder and sand removal materials can be reduced because binders and sand can be directly reused.
• The use of foam molds increases dimension accuracy and casting quality when compared to conventional molds.

This casting method has some disadvantages, including:

Because of the influence of burnt polystyrene foam, the degree of porosity is greater.

Conclusion

That concludes the discussion of Lost Foam Casting, beginning with the basics and progressing to the advantages and disadvantages. If you have any questions or need more information, please leave them in the comments section or contact our Team directly.

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