Why Precision Casting May Not be as Good as You Think: Limitations and Disadvantages

Precision casting, often referred to as ‘lost wax casting’ or ‘investment casting’, is a metal casting process that is used to create intricate and detailed metal casting components. As the name ‘precision’ suggests, it includes not only lost wax casting but other casting methods that are used to create castings with tight dimensional tolerances, such as pressure die casting or lost foam casting, in most cases, we often consider precision casting as investment casting.

Through this unique manufacturing technique, we can easily create parts and components with very complex shapes and exceptional accuracy, such as medical devices like surgical instruments or car parts like shifting forks. However, as promising as it seems, is this really the incredible method you need to meet all your casting requirements? Today, we will take a deep dive into this specific metal casting method and find out whether or not you can benefit from it.

What Makes Precision Casting So Precise?

Technically speaking, any casting method that can create metal components with complex details and tight tolerances, typically ranging from CT5 to CT7 (often used for commercial castings), or in some cases CT4 to CT6, can be considered ‘precision casting.’ However, people most commonly associate precision casting with investment casting or lost wax casting. This is because, out of all casting processes, investment casting can truly achieve remarkable precision in castings in comparison to other methods, regardless of the size and weight of the castings, which typically range from a few grams up to 50 kg.

Before I explain the limitations of the precision casting process, I’d like to first discuss why this method can create parts that other methods cannot. So, what’s making such a huge difference in the precision of the final cast?

Wax Pattern Accuracy

First and perhaps the most important reason why precision casting can produce parts with tight tolerances is the wax pattern. The use of wax patterns in investment casting is central to its high accuracy. Wax can be injected and formed with very fine details and excellent surface finish. Unlike sand or ceramic molds that may have some inherent textural roughness, wax patterns are smooth and can replicate intricate designs precisely.

Wax patterns allow for predictable and uniform shrinkage, which can be precisely accounted for in the design stages. And since we can predict shrinkage in early stages of the process, we can then control the required final dimensions and therefore achieve very tight tolerance of the final castings.

Shell Mold Precision

Another important reason why precision casting can achieve high accuracy compared to casting methods such as sand casting or shell molding is due to its ceramic shell precision. After the wax removal, the ceramic shell that forms the mold for the molten metal is capable of capturing very fine details, much finer than those typically achievable with sand casting. The ceramic material can hold tighter tolerances and finer details because of its strength and stability at high temperatures.

The surface quality of the ceramic molds directly impacts the surface finish of the final metal part. Since these surfaces are created from the high-resolution wax models, they are inherently smoother than those created in sand casting or shell molding, which can be grainier.

The Pouring Process

Finally, when we compare metal casting methods like sand casting and shell molding to precision casting, one distinct difference is the controlled pouring and cooling. Investment casting processes involve controlled pouring of the metal into the mold and equally controlled cooling, which helps in reducing warpage, distortion, and internal stresses that might affect the dimensional accuracy. The fluidity of the molten metal in investment casting is generally higher compared to thicker, more viscous metals in other casting processes. This allows the metal to flow into all parts of the mold, replicating the detailed features accurately.

The Process of Precision Casting

To better understand precision casting, we need to first look at the whole process. I will explain this process with comparison to other casting techniques like sand casting and shell mold casting.

Pattern Creation

In precision casting, the process begins with the creation of a detailed wax pattern, typically through an injection molding process where wax is injected into a metal die. This wax pattern is a precise replica of the final part and can capture very fine details.

Unlike sand casting, where the mold is created from compacted sand that can leave a rough surface texture, the wax pattern used in precision casting allows for much finer detail and a smoother surface finish. Shell mold casting uses a sand-resin mixture to create a harder mold than traditional sand casting, providing better surface finish and detail than sand casting but still not as refined as precision casting.

Assembly

Multiple wax patterns are assembled onto a wax tree using a sprue, allowing for multiple parts to be cast simultaneously. This assembly is then dipped into a ceramic slurry, differentiating it significantly from other methods.

Sand casting and shell mold casting do not use a wax pattern or ceramic slurry, leading to differences in mold stability and surface finish. The ceramic shell in precision casting is much stronger and capable of withstanding higher temperatures, providing finer details.

Shell Building

After the wax patterns are assembled, they are repeatedly dipped in a ceramic slurry and sprinkled with fine strewing materials until a thick ceramic shell forms. Each layer must dry thoroughly before the next is applied, creating a strong mold capable of handling the molten metal.

Shell mold casting also involves heating a sand-resin mixture to create a shell, but the repeated layering of ceramic slurry in precision casting results in a much stronger mold. Sand casting, on the other hand, uses a simpler method where sand is packed around a pattern and then the pattern is removed, which does not allow for the same level of detail or strength.

Wax Removal

The wax in the assembled tree is then melted and removed, leaving a hollow ceramic mold. This step is critical as it ensures that the mold is now ready for the metal pouring.

This “lost wax” process is unique to precision casting. Sand casting and shell mold casting involve removing a solid pattern from a sand or resin-sand mold, which can disturb the mold’s integrity and detail.

Metal Pouring

Molten metal is poured into the hollow ceramic mold. The high heat tolerance of the ceramic allows for the use of metals with high melting temperatures, enabling the creation of high-strength components.

Both sand and shell mold castings pour molten metal into sand-based molds, but these molds do not offer the same heat tolerance or stability, impacting the types of materials that can be used and the precision of the final product.

Shell Removal

Once the metal has cooled and solidified, the ceramic shell is removed to reveal the finished casting. This part of the process allows each casting to have a high-quality surface finish.

In sand casting, the sand mold is also broken away, but it generally results in a rougher surface that often requires more finishing work. Shell mold casting provides a better surface than sand casting but still requires more post-casting treatment than precision casting.

As you can see, the precision casting or lost wax casting method includes more steps in the casting process than sand casting or shell molding. As a result, metal castings created by this method often showcase tighter tolerances and a smoother surface finish than those produced by sand casting and shell mold casting. In the next section, I will discuss the major advantages of precision casting.

Advantages of Precision Casting

At this point, you’re probably wondering, isn’t this article all about the limitations and disadvantages of precision casting? How come you’ve spent so much effort talking about its precision and how much better this method is compared to other casting processes? Don’t worry, we will get to that, but for now, we need to understand why it is so effective before we discuss its shortcomings.

High Dimensional Accuracy and Precision

As mentioned earlier, precision casting method is mainly used to create castings with complex shapes and intricate details. It is known for its ability to produce components with exceptional dimensional accuracy and tight tolerances. The process can achieve as tight as ± 0.005 inches per inch, which is much tighter than those typically achievable with other casting methods like sand casting or shell molding.

Excellent Surface Finish

Parts produced through precision casting have a very smooth surface finish, typically ranging from 125 to 150 microinches. Depending on the types of metals used, some parts can be even smoother. You can also add extra surface treatments, such as mirror polishing, to make the parts shine like a mirror. Typically, the ability to produce parts with smooth surfaces minimizes the need for additional surface finishing processes, saving time and reducing manufacturing costs. However, this will still depend on your specific needs and the functionality you aim to achieve with them.

Ability to Cast Complex Shapes

The flexibility of the wax models used in investment casting allows for the creation of parts with complex geometries that would be extremely difficult or impossible to achieve with other casting techniques. This includes undercuts, hollow sections, and shapes with high levels of detail.

Integrity and Reliability

The precision casting process allows for the production of parts with superior mechanical properties. The ceramic molds used can withstand high pouring temperatures, ensuring that the castings have minimal defects and excellent mechanical properties.

Disadvantages and Limitations of Precision Casting

Now that you should have a solid understanding of what precision casting can achieve by following me to this part of the article, we’ve talked extensively about how remarkably effective precision casting appears to be—and indeed, it is. However, nothing is perfect; everything has its limits, including precision casting. I will now explain some of its major drawbacks that may change your mind.

Size Limitations

While precision casting is excellent for small to medium-sized components, it becomes less practical and cost-effective for very large parts. The size of components that can be effectively produced is limited by the capacity to handle and fire the ceramic molds without compromising their integrity.

The ceramic shells used in precision casting are not inherently strong enough to handle the weight of molten metal required for very large castings. The risk of mold cracking or breaking under the weight of large volumes of metal is significantly higher.

Creating large wax patterns can be very challenging because the wax can deform under its own weight, especially during removal from the mold and handling during the assembly and shell building process. This can drastically affect the dimensional accuracy of the final product.

Managing the heat during the pouring and cooling phases becomes more complex with larger castings. Larger volumes of metal require careful control to avoid thermal stresses, distortion, and cracking during solidification. Ensuring even cooling can be challenging, which might necessitate specialized cooling techniques.

The larger the item, the more challenging it is to maintain uniform quality throughout the casting. Issues such as porosity, inclusions, and cold shuts are more difficult to control in larger castings. The integrity of the metal can be compromised if these issues are not adequately addressed.

Costs Disadvantages

It goes without saying that the bigger the item, the bigger the costs. The costs associated with precision casting rise substantially with the size of the cast part. This includes the costs of materials for both the wax patterns and the ceramic shell, as well as increased labor and energy for handling, shell building, and curing. The initial setup and tooling costs also become more substantial, making the process less economical for large items compared to other casting methods. In comparison with sand casting, casting large components with the investment casting process is less ideal.

Material Restrictions

Although precision casting is versatile in terms of the materials it can handle, the process might not be suitable for certain alloys that have very high melting temperatures or those that are highly reactive with ceramic. This can limit the range of applications where precision casting is feasible.

Metals like tungsten, molybdenum, and some high-strength superalloys have melting points that can exceed the thermal resistance of ceramic molds used in precision casting. The intense heat can degrade the ceramic material, affecting the mold’s integrity and the quality of the casting.

Materials such as titanium and certain aluminum alloys are highly reactive with oxygen when molten. In precision casting, managing this reactivity can be challenging because any interaction between the molten metal and the mold material or atmosphere can introduce defects such as inclusions or embrittlement.

The feasibility of using expensive or rare materials in precision casting is not just a technical challenge but also an economic one. Precision casting can be a cost-intensive process due to the detailed and laborious steps involved. When combined with high-cost materials, the overall expense can make the process economically unviable, especially if large quantities or large-sized castings are required.

Longer Production Time

The production time in precision casting, or investment casting, is inherently longer due to the detailed and multi-step nature of the process. This extended timeline can be a significant limitation, especially when compared to other casting methods that may offer quicker turnarounds. In contrast, sand casting molds can be prepared more quickly because they do not require the multiple, time-consuming steps of wax pattern creation and ceramic shell building as in precision casting.

Die casting allows for much faster production because the molten metal is injected under high pressure into metal molds, which cool and solidify the metal much more rapidly than the ceramic molds used in precision casting.

As for shell mold casting, it involves even fewer and quicker drying and curing steps compared to the ceramic shells in precision casting, resulting in shorter overall production times.

I have listed these common issues and limitations that precision casting might have, in comparison to other casting methods such as sand casting, die casting, and shell molding. When finding precision casting foundries or services, you need to take these factors into consideration before making a decisive choice. In the next section, I will help you decide whether or not you should use precision casting for your metal casting project.

Is Precision Casting the Right Process for You?

Although the precision casting technique has its unique advantages and a robust capability for producing various casting components across a wide range of industries, we must not overlook its limitations. In many cases, it may not suit your requirements when it comes to final decision-making. Before you order your next batch of castings, let’s be sensible to avoid wasting your precious time and money.

Let’s Look at the Materials You Use

Precision casting, or investment casting, is suitable for casting various types of metals, but some metals are not as economical to cast using this technique. For example, if your products are aluminum-based, then you are better off using die casting methods such as pressure die casting or gravity die casting instead, because you can achieve very tight tolerances with die casting as well. This is because the high pressure used in injecting molten aluminum into steel dies ensures that the metal reaches all parts of the mold, capturing intricate details and sharp corners that might be challenging to achieve with the slower gravity-fed process used in investment casting.

If this is for larger production runs, then the cost would still be cheaper than investment casting because of the nature of die casting. And the time it takes would be significantly less than using the precision casting method. Die casting is known for its rapid production capabilities, particularly suitable for high-volume manufacturing. Aluminum, with its relatively low melting point, quickly fills the die and solidifies faster, allowing for quicker cycle times compared to the more time-intensive investment casting process. This efficiency is crucial when producing large quantities of parts.

Even though die casting may not be as precise as investment casting in terms of dimensional accuracy, combining it with machining can also be beneficial, and the overall cost may still be lower, especially for large production runs, since the tooling costs are typically higher for investment casting.

Check the Size of Your Components

When making a decision on which technique to choose, you also need to consider the required size of the components. For components that are excessively large, it would be ideal to choose sand casting over precision casting. Not only is sand casting more economical than investment casting, but it can also enhance the integrity and quality of the casting. If the component is large and requires tight tolerances in specific areas, you can combine sand casting with precision machining to achieve the desired tolerances.

Check The Quantity of Your Order

While precision casting can handle both small and large quantities, it would still be wise to choose a method that is sensible in terms of costs. Here, you would still need to consider the materials. As mentioned, aluminum is typically cast using the die casting process. However, if the quantity is small and you require tighter tolerances, then investment casting may be more suitable as the tooling cost is significantly lower than that of die casting. Otherwise, for large quantities, aluminum castings are better cast with die casting, combined with precision machining when tighter tolerances are required.

You will also need to consider the speed at which you need your castings to be made. The processes of die casting, sand casting, and shell molding can be much quicker than investment casting, and cast parts require much less time to be made. However, this is only for your reference. Our rapid drying silica sol investment casting method can be employed much quicker than the traditional lost wax casting method. This is because we have dedicated rapid drying chambers with layers of drying racks combined with a fully automated process, making our efficiency almost unmatched in the industry.

Check Your Budget and Future Plans

Needless to say, all the information I provide represents typical cases. Often, we blur the lines between small and large quantities, and between medium and large components. Nothing is definitive.

For example, suppose you are a supplier of certain medium-sized components that can be cast with either investment casting or shell mold casting, with additional machining. However, you’re not sure about the future market demand, so you may not want to invest heavily in the initial order. Typically, the tooling cost for shell mold casting is much higher than for investment casting, but the batch order would be cheaper for your current quantity with investment casting.

Now you must decide: Should I stick to investment casting for this ‘testing quantity’ since the overall cost would be lower because of the lower tooling cost compared to shell mold casting? But you’re not sure how well the market will react to this; perhaps in the future, you might require a very large quantity or need to change your whole design. In this case, precision casting would be ideal. However, if you believe from the beginning that your product has a future, then sticking to shell mold casting could result in lower overall costs for years to come.

Conclusion

Generally speaking, placing an order for your metal casting project is not an easy task. There are so many factors that you’ll need to consider, and this is just to choose a suitable casting process, let alone finding the right manufacturer. Luckily, we are here to help. Taiyuan Simis Investment Casting Co., Ltd is a professional precision casting manufacturer with over 40 years of experience. Our foundry workshops can produce various types of castings using investment casting, sand casting, shell mold casting, and die casting, combined with CNC machining workshops.

Simply contact us for any uncertainties in situations like those I have mentioned. We have sufficient experience to help you choose the most appropriate production technique. There are no components that can’t be cast; it’s only a matter of production time, costs, and suitability. Let us be your trusted partner for your next metal casting project.