Investment casting is a metalworking manufacturing process with long history that traces back to thousands of years ago. In investment casting, molten liquid metal is poured into an ceramic shell mold. The mold is formed by using a wax pattern – a replica of the desired part. The pattern is dipped into ceramic slurry to form a layer coating. Investment casting is also called lost wax casting because after wax pattern is formed, it will be heated in the oven so that the wax pattern will be melted under high temperature. The lot production of investment casting is a repetitive process which increases production time and costs relative to other casting processes. However, as it is a pouring process different from forging, even if the part is complex, it can be also created by investment casting technique.
Investment casting can make use of most metal alloys, such as carbon steel, alloy steel, stainless steel, aluminum, bronze alloys, grey iron, ductile iron, ect. The benefit of investment casting process is it can cast metals with high melting temperatures that can not be molded in plaster or metal. Parts that are typically made by investment casting include those with complex geometry. High temperature applications are also common, which includes parts for the automotive, aircraft, and military industries.
During investment casting process, besides some main equipments, we still need to utilize a metal die, wax, ceramic slurry, furnace, molten metal. Below are the main steps of investment casting:
1. Pattern creation – Wax is injected into metal tooling to form wax pattern. One wax pattern will creat one piece investment cast part. So the quantity of wax patterns will be the same as the products. To creat internal structures, we may use cores on the pattern. After that, wax patterns are attached to gating system, which forms an assembly looks like tree. The moten metal will flow to the mold cavity by the gating system.
2. Mold creation – This tree assembly is dipped into ceramic slurry and form a coating layer outside, and then dried automatically to form a ceramic shell. This process is repeated until the shell is thick enough to withstand the molten metal it will encounter. The shell is then placed into an oven and the wax is melted out leaving a hollow ceramic shell that acts as a one-piece mold, that is lost wax casting is named.
3. Pouring – Melting metal ingots into liquid status, then preheating the ceramic shell mold in heating furnace under high temperature, and finally pouring the molten metal to full the mold cavity through sprue gate. Pouring is normally manual behavior under the force of gravity. Then cooling under room temperature for solidification of final shape. Thick parts will require more cooling time.
4. Casting removal – Break the parts from gating system, grinding to remove excess metal and shot blasting for smooth surface. Heat treatment is also sometimes used to improve the mechanical properties of products.
Why Investment Casting?
- Near net shape – for parts which normally require a lot of welding or fabrication, this process saves a great deal of labor and material.
- Complex shapes – very little need for machining.
- Hollow passages and thin walls can be easily achieved.
- Flexible size range – up to 78 inches (200cm)
- Flexible alloy selection – Steel, Nickel-Based Superalloy, Titanium, and Aluminum
Considerations When Using Investment Casting
Tooling cost: Expected usage rates are a crucial part of the calculation when selecting the right tooling option for investment casting. For low quantity requirements, investment castings may be more expensive than parts that are fabricated or cast using other methods if permanent tooling is pursued. For those applications, SLA or printed patterns may be a very cost effective alternative–even for a quantity of one.
Tooling amortization is a key factor to consider when determining whether investment casting brings the greatest value. The investment cast tool, a precision machined and assembled aluminum injection die, usually consists of multiple parts fitted together to produce the complex geometry of most investment cast components. This “front end” cost is not insignificant, but is often easily offset in a total cost analysis where the cost of unneeded subsequent machining and/or fabrication is excluded.As the only tool typically needed is for an injected wax part, tool wear is virtually non-existent.
Size Limitations: While it is possible to create investment castings in a range of sizes, and that capability continues to expand through the industry, there is an upper limit on that range that is less than other shaped technologies like sand casting.
Very small structures: Investment casting is an excellent choice for thin-walled applications, but very small internal shapes that use cores can present processing challenges. Holes typically cannot be smaller than 1/16” (1.6mm) or deeper than 1.5x diameter.
Timing: The multi-step investment casting process itself is more time consuming that other castings processes. But the entire process time to create a finished can be shorter than alternatives because of the reduced need for additional machining.
What may Affect the Dimensional Accuracy of Investment Casting Products?
In general, dimensional accuracy of investment casting products is influenced by structure of the part, casting material, moulding, shell making, roasting, pouring as well as other factors. Any unreasonable operation can change the shrinkage rate of investment castings, which may lead to the deviation of dimension accuracy of precision investment castings. Below are the main factors that affect dimensional accuracy of investment casting products.
1) Casting structure: a. If the wall thickness of an investment castings part is large, then it will cause large shrinkage rate. If the wall thickness is small, the result will be opposite. B. If the free systolic rate is large, then the block shrinkage rate will be small.
2) Casting material: a. The lower carbon content in investment casting material, the smaller the shrinkage rate will be. b. Casting shrinkage rate of common casting material: Casting shrinkage rate K=(LM-LJ)/LJ×100%, LM means the cavity dimensions, LJ refers to casting dimensions. K is affected by below factors: wax model K1, casting structure K2, cast alloy type K3, pouring temperature K4.
3) Wax model: a. In terms of wax injection temperature, wax injection pressure and keep pressure time, wax injection temperature is the most obvious factor that will affect the dimensions of investment casting products. The second influence factor is wax injection pressure. Keep pressure time is the least factor that affect the dimensional accuracy. b. Line shrinkage rate of wax material is around 0.9-1.1%. c. When wax models are stored, they will be further shrinked, and the shrinkage value is about 10% of total shrinkage, but after 12 hours of storage, the dimensions will be basically stable. d. Radial shrinkage rate of wax models only takes up about 30-40% in the length direction shrinkage rate. The effect of shooting wax temperature on the free shrinkage rate is much greater than the influence on block shrinkage rate (optimum temperature is 57-59℃, the higher the temperature is, the greater the shrinkage will be).
4) Shell making material: Adopting zircon sand, zircon powder, shop on the sand, shop on powder, because of its small expansion coefficient, only 4.6 x 10-6 / ℃, thus can be neglected.
5) Shell roasting: As a result of small shell expansion coefficient, when the shell temperature ois1150 ℃, is only 0.053%, thus can be neglected also.
6) Pouring temperature: the higher the pouring temperature, the larger the shrinkage rate will be. If the casting temperature is low, then the shrinkage rate will be small, so the pouring temperature should be appropriate.
What are the Benefits of Investment Casting over Sand Casting?
All casting methods are different and have their own benefits. Each casting process is the best way for one special design. As one of the oldest manufacturing techniques, the investment casting process is often superior to other types of casting, particularly in regards to surface finishes. So what are the benefits of investment casting over sand casting?
As the name implies, sand casting uses a silica-based material, like sand, to create a molding surface. Sometimes manufacturers add clay to help the sand particles bond more closely. A mold made of sand is somewhat flexible, so there’s less cracking, tearing or other types of flaws as the liquefied material cools and hardens. However, sand casting is less accurate than investment casting, investment casting tend to have a smoother surface finish and less machining is required. With investment casting, we can avoid many of defects caused by sand casting. Of course, the production cost is different. Investment casting may cost more, but in the long term, this is a more cost effective casting method with better working performance.
Investment Casting Methods
Investment Casting is a general concept for casting. There are two types of investment casting: water glass investment casting and silica sol investment casting.
Water glass investment casting: Water glass investment casting was first deveoped in the former Soviet Union, then introduced in China. So water glass casting is the most common investment casting method in China, but rarely seen in other countries. With this casting method, the production cost is much lower than silica sol casting process. We can achieve surface finish Ra12.5 and size tolerance CT7-8. Water glass casting is mostly used for manufacturing steel castings.
Silica sol investment casting: Silica sol casting is used mainly for stainless steel castings. The surface finish will be much better than that of water glass casting, but cost is much higher. In China, only about 25% factories are silica sol casting foundries.
Investment Casting Applications
For more than 25 years, CFS foundry has been supplying high quality, high precision investment castings for customers in application from military and aerospace to medical and dental, and nearly every industry.
We use our decades of experience and our proven casting techniques to deliver net shape or near-net shape cast parts with tight tolerances that will meet or exceed any industry standards. We will deliver custom investment castings that match your design exactly, in the material you need and the quantities you require.
Below are the main applications of investment casting products:
- Agricultural, Industrial & other Heavy Equipment
- Mining Industry
- Military & Defense
- Medical Devices & Equipment
- Firearms Parts & Components
- Hand Tools & Power Equipment
- Automotive/Truck Parts
- Door & Lock Security Hardware
- and many other industries
Chinese Investment Casting Foundries
Most foundries that produce investment castings just devote themselves to investment casting business only, and even do not provide machining service. They prefer to focus on a single casting process for all industries. In Ningbo, there are also some investment casting foundries that only supply bucket teeth with different types. That is because the production methods are different, such as investment casting vs sand casting, investment casting vs die casting. From these, you may understand why most investment casting foundries do not involve in other casting methods.
It is really a fact that investment casting foundries may offer higher prices than other casting methods. But its advantages will make the relatively high cost worthy, as investment casting can offer super quality and reduce machining cost. In this way, it is cost-effective.
Recent years, Chinese investment casting foundries are becoming popular for its exporting business to overseas market. There are lots of benefits of making investment castings in China. Price is the main factor that attracts foreign compaines to purchase investment castings from China. Of course, quality is also vital for any project. It is important for investment casting foundries to truly understand the design parameters of the investment cast product from customers. Good communication between foundries and customers will make all clear and avoid any misunderstanding. If you are not familar with investment casting technique, refer to our another article when to use investment casting, then you can know if your product is the right part for investment casting.