When it comes to investment casting of metal parts manufacturing factors such as design requirements, cost, and feasibility to manufacture dictate which casting process is most suitable to manufacture a product. This article describing investment casting is intended to help you make an informed casting decision.
Investment casting produces precise components while minimizing material waste, energy, and subsequent machining. It can also ensure the production of very intricate parts. This makes the investment casting process quite useful to design engineers.
The goal is to understand what is meant by investment casting. So, What exactly is the investment in “investment” casting? The term “invested” historically carries the meaning of “clothed” or “surrounded.” Investment casting employs a shell made of ceramic, plaster, or plastic that is formed around a wax pattern. The wax pattern is melted and removed in a furnace and metal is poured into the shell to create the casting.
What is investment casting used for? Let’s break down the investment casting production process for a greater understanding:
Creating the Pattern
Mounting the Wax Patterns and Creating the Tree
Creating the Mold Shell
Size range:
Although most investment castings are small, the investment process can produce castings weighing more than 1,000 pounds. This capability is limited to a relatively small number of investment casters and requires special expertise in handling. Most cast parts fall in the ounces to a 20-pound range.
Versatile and intricate shapes:
Investment casting provides consistent and repetitive close tolerances along with intricate passages and contours. Many of these configurations are impossible to produce. For example, where machine tools cannot reach. Achieving net-shape or near-net-shape cast components can dramatically reduce post-cast processing costs.
Investment casting is a good alternative to weldments or fabricating. Many components can be combined into a single casting. The more that are combined, the better the manufacturing efficiency. Converting multi-piece components to a single investment casting typically delivers more dimensional accuracy and reduced part complexity.
Accurate and Smooth Surfaces:
The ceramic shell used is built around smooth patterns produced by injecting wax into a polished aluminum die. A 125 micro finish is standard, and even finer finishes are not uncommon.
Investment castings contain no parting line because only one mold is used rather than two half molds (such as in the case of sand casting). Standards for surface blemishes and cosmetics are discussed and agreed upon with the customer based on the function.
Below is a comparison of relative surface finishes that can be expected from various casting process:
Casting ProcessRMS RangeDie20 – 120Investment60 – 200Shell Mold120 – 300Centrifugal – Standard tooling400 – 500Centrifugal – Permanent Mold20 – 300Static – Permanent Mold200 – 420Normal Non-Ferrous Sand300 – 560Normal Ferrous Green Sand560 – 900Dimensional Accuracy:
Typically, “standard” investment tolerances are +/-0.010” for the first inch and +/- 0.004” for each succeeding inch.
The design phase can result in a drawing that reduces or even eliminates previous machining requirements to produce the same part.
The cost of any part increases in proportion to the preciseness of its dimensional requirements. Whether if it’s castings machined parts or fabrications. A close design review will permit modification to tolerances, undercuts, blind holes, etc. to allow higher production yields and lower piece costs. If closer than cast tolerances are necessary, the machining required for investment casting will be less than conventional castings or fabricated components.
Quality and Integrity:
Casting integrity is an important feature of the process. Investment casting has a long history of serving demanding sectors such as gas turbine engine, petroleum, chemical, defense, and medical.
Tooling Cost:
For low quantity requirements, it may be more expensive than other methods if permanent tooling is pursued. For those applications, SLA or printed patterns may be a cost-effective alternative (even for a quantity of one).
Initial costs are another key factor when determining whether investment casting brings the greatest value. The investment cast tool usually consists of multiple parts fitted together to produce the complex components. This “front end” cost is not insignificant but can be easily offset by the lack of subsequent machining and/or fabrication.
Size Limitations:
It’s possible to create investment castings in a range of sizes. There is an upper limit on that range, which 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 challenges. Holes typically cannot be smaller than 1/16” (1.6mm) or deeper than 1.5 times in diameter.
Timing:
The multi-step investment casting process is more time consuming than other processes. The processing time can be shorter than other alternatives.
As with most materials and design decisions, a discussion with a metals expert can help drive the best decision. To learn more about the metal investment casting process or to determine if it is a fit for your application, please contact us.