Thin-Walled Investment Casting and its Applications
Wall thickness refers to the distance between opposite sides or surfaces of a solid metal cast region. In a part like a cover the wall thickness is the distance between inner and outer surfaces.
Thin walls are difficult to cast, yet are often sought by part designers as a way of minimizing part weight. An advantage of investment casting over other casting processes is its superior thin wall capabilities. However, successful casting of parts with thin walls requires a high level of process expertise and close attention to detail. Here’s how we do it.
Reasons for Specifying Thin Walls
In investment casting a thin wall is generally considered to be a region of the part being cast less than 3/16” (4.8 mm) in thickness. Thin walls form part of the design of a part for several reasons:
- Minimize weight in non-load-bearing regions
- Where the function is to cover or block access to a piece of equipment
- To increase surface area (for heating or cooling)
- Reduce the overall size of an object or part
Producing Thin Walls in Investment Casting
In investment casting a mold is made by coating a wax pattern with a ceramic slurry. This slurry is then dried and the wax melted out. Metal is poured into the resulting cavity where it solidifies to form the part required.
Cavities in the cast part are created by placing cores in the mold cavity. In investment casting these cores are usually incorporated into the wax pattern, being placed in the pattern mold before it’s closed and wax is injected. In some situations, the design allows for cores to be added to the pattern before the ceramic slurry is applied.
Producing thin walls demands precision in core placement. Skilled investment casting specialists, (like those at Impro), can achieve walls as thin as 0.040” (1mm) although 0.080” (2mm) is a more practical target.
Challenges in Investment Casting Thin Walls
After core placement, the next challenge is getting metal to flow into the thin sections and cool without creating defects. The two problems here are:
- Castability/fluidity of the metal (related to viscosity and surface tension)
- Managing cooling rates
Foundry experts discuss metals in terms of their castability. Simply put, some metals flow better than others, and so will fill a thin section more readily. Factors involved in achieving a good fill are:
- Head (pressure) of metal pushing it into the section
- Speed of fill
- Venting – allowing air to escape in front of the advancing metal
- Rate of cooling
Cooling happens as heat is lost through the ceramic shell to the outside. Thin sections will cool faster than those that are thicker, resulting in metal solidifying in these regions first. This can lead to cracking and other defects.
Managing Cooling Rate
Cooling is controlled by adjusting the thickness of the ceramic shell, by preheating prior to pouring, and by raising the temperature of the metal. By carefully optimizing these parameters a skilled foundry specialist can ensure thin sections fill and cool consistently and without defects.
Applications for Thin-Walled Investment Casting
Thin walls are especially desired in applications where it’s important to minimize mass. Aerospace and automotive are therefore the primary sectors. Investment cast parts with thin walls include manifolds and housings, (like those for EGR components). Covers and other non-structural/non-load bearing parts are also candidates, along with parts needing thin ribs for heat transfer.
Partner with an Investment Casting Specialist
Thin walls are essential in some parts and desirable in others. Investment casting is capable of producing walls as thin as 0.040” but achieving consistent results requires careful optimization of all aspects of the process.
As a leader in investment casting, Impro has the resources and expertise necessary to produce high quality metal parts that meet and exceed customer expectations. If your part designs call for fine detail and thin walls, investment casting is probably the process you need. Contact us to discuss your application.