What is Investment Casting?
Investment casting is a foundry (metal casting) process that uses a consumable pattern to produce a metal reproduction of the pattern. Typically, patterns are made of wax so the process is also known as the ‘lost wax’ process. The wax patterns are injection molded in aluminum dies. This injection molding step gives investment casting the ability to make very complex shapes. Following injection molding, clusters of the patterns are coated in multiple layers of ceramic to create a mold that exactly duplicates the outside geometry of the patterns. The wax is then melted out and the mold is fired to 1800F (typically) to increase its strength. While the mold is still hot molten metal is poured in. After the metal solidifies the ceramic shell is broken away, the parts are cut from their sprue and runner system, blast cleaned and then subjected to secondary processes such as heat treatment and machining.
Investment casting is the oldest of all the casting processes and traces its origins to ancient Egypt and China where naturally occurring waxes were sculpted and then packed in clay to create a mold. After melting the wax out of the clay, precious metals or bronze was then cast to create the finished piece of art or jewelry. Today the modern versions of investment castings use high-tech pattern waxes and ceramics to create molds capable of withstanding temperatures and pressures involved in casting the equally complex metal alloys used in industry, medical, defense and aerospace applications.
Why is it referred to as Investment Casting?
The term investment casting refers to the mold making process in which a pattern (typically wax) is surrounded, or ‘invested’, in ceramic. The earliest forms of investment casting used clay or plaster poured into a vessel that contained the wax pattern or a cluster of patterns creating what is called a ‘solid mold’. Today commercial investment casters use complex ceramic formulations that are applied in layers consisting of a wet slurry dip followed by a dry stucco application. Multiple layers of ceramic are used to create a ‘shell mold’ strong enough to withstand the pressures experienced when molten metal is poured into the mold.
What kinds of parts are suited for Investment Casting?
The beauty of the investment casting process is the design freedom the process affords engineers and designers. Complex shapes, tight dimensional tolerances, excellent repeatability from part to part, excellent surface finish and a wide variety of alloy choices make investment casting a very cost effective solution to complex manufacturing challenges.
Common applications for investment castings that are familiar to most include class rings and jewelry, knee, hip and other bio-implants, metal golf club heads and putters, rocker arms in internal combustion engines and the turbine blades in jet engines. Acra Cast specializes in commercial & industrial casting applications as well as aerospace and land defense castings
Speaking of defense, the investment casting industry is vital to our national security and is listed as a ‘Defense Critical Industry’ by the federal government.
What tolerances can Acra Casts Investment Castings hold?
Typically, a linear tolerance of ± .005 in. per inch is standard for investment casting. This varies depending on the size and complexity of the part. Subsequent straightening or coining procedures often enable even tighter tolerances to be held on specific dimensions. Parts that need to be held to a tighter tolerance will typically require post-casting machining.
It is important to note that although we can generally expect the final casting dimension to be within a total tolerance band of ± .005″ per inch, it is expected that the variation from casting to casting is small. For example, a casting dimension of 10 inches would be expected to fall within a tolerance band of ± .050″ (.005 x 10). That equates to a total tolerance window of 0.100″ (.050 x 2). However, the variation between castings would probably not exceed .010″. In other words, the predicted tolerance will fall within the advertised window of .005″/inch but only a small portion of that window will be used when we look at that dimension over a sample of castings produced.
Why would I use an Investment Casting?
Whether it involves aluminum, stainless steel or other alloys, our lost wax investment casting process can provide a cost effective and efficient solution in the following ways:
How does Investment Casting compare to other common manufacturing processes?
What can I expect for a lead time from Acra Cast?
Rapid Prototype: 1 – 2 weeks for casting, maybe longer depending on secondary operations.
Production: 8 – 10 weeks for first article samples. 6 – 8 weeks for production castings.
How can I be certain that investment casting is the correct process for my product?
Your first step is to call Acra Cast and discuss the application, design, material and quantities with our engineering personnel. We can make design and material recommendations to improve the castability of the part to improve its suitability for the IC process.
If desired, then next step could be to produce a prototype casting using additive manufacturing technology to create a consumable pattern in lieu of a wax pattern. Using Rapid Prototyping technology to create a sample part is a low risk opportunity to prove your design using investment casting. After testing and evaluation and perhaps even several generations of prototypes to perfect the design a production tool can be built to make the castings in larger quantities and at a lower piece price.
Aren’t investment castings expensive? If so, how can they save money?
A common misconception is that investment castings are expensive. However, it’s important to look at ‘total cost’ of a finished part. If an investment casting can reduce the expense and time of machining a part from solid bar stock, or reduce the setup times and material removal required on a sand casting, or extend product life by reducing the time between failures on a welded heat treat fixture or a powder metal part that disintegrates under harsh use, then investment castings are a cost effective alternative. We wouldn’t be in business if we weren’t solving problems and saving money for our customers.
I heard castings are prone to porosity, is this true?
Porosity is an often misused term and understood concept. There are multiple potential sources and causes of ‘porosity’ in any casting. Some porosity can be caused by gasses picked up from the atmosphere by the molten metal. This is controlled with metal quality, inert gas blanketing, vacuum melting and proper de-gassing procedures at the melt deck.
Another source of ‘porosity’ is due to turbulence and splashing when the metal is poured causing air in the mold to become entrapped. Proper gate, runner and sprue design, good pouring technique and filtering are all used to reduce turbulence and reduce the entrapment of air.
Finally, what some refer to as ‘porosity’ is actually shrink inside the casting due to the volumetric changes that occur in the metal as it transitions from liquid to solid. Shrink can be reduced or eliminated by, first and foremost, a good casting design. Next, gate sizes and locations on the castings are critical. Finally, good alloy selection and proper control of pouring temperatures and part cooling is fundamental to reducing shrink inside a casting.
If the parts are properly designed and gated, and the manufacturing processes are controlled, porosity and shrink issues can be eliminated.
What about the integrity of an investment casting? Will I have problems with porosity and shrinkage that is usually nonexistent in bar stock or forgings?
Investment castings are used for many critical applications that require the parts to be x-rayed and meet specific soundness criteria (per AMS-2175). The integrity of an investment casting can be far superior to parts produced by other methods. Ultimately, the integrity of any casting starts with a good design. Beyond design, using quality materials and good process controls in the various stages of manufacturing ensures a sound, porosity-free casting.
At Acra Cast we test the chemistry of all incoming metal used for casting using an Optical Emission Spectrometer. We use this chemistry to create a melt formula used at the melt deck which ensures we achieve the prescribed chemistry specification in the finished casting. Additionally we practice precise temperature control and careful degassing procedures to avoid internal porosity due to hydrogen gas in the molten metal.
Finally we direct pour our castings from the melt bath to avoid the gas, slag, and temperature problems associated with ladle pouring. We pour most of our metal through a filter to trap slag and avoid non-metallic inclusions. Consequently, we produce high quality castings capable of passing X-Ray inspection with mechanical properties appropriate to the material selected.
What is the typical surface finish on an investment casting?
Good surface finish starts with the injection die and the production of a wax pattern with an excellent surface. Next, the first layer of ceramic applied to the pattern is designed to replicate extreme detail and create an interior mold surface that is very smooth and non-reactive when contacted by molten metal. As a result, surface finishes of 60 – 100 RMS are typical with some alloys producing smoother surfaces than others.
What type of tooling is required?
Aluminum injection dies are used to produce the wax patterns. Some dies are manually operated and these are typical on either very large parts or very low volume jobs. Dies that clamp into the injection press and are equipped with an ejector system are used for parts of medium to large volumes and where automating the die is practical. The advantages to automated dies are faster production times (reduced cost), improved wax pattern quality and longer die life.
We build our dies to very exacting standards and a typical automatic injection die is capable of producing tens of thousands of parts with no or minimal repair and rework.