Investment casting is used for parts that need high accuracy, excellent surface finish and fine detail. Achieving durability and performance goals requires these parts to be free from surface and internal defects. This blog explains the parameters we focus on to achieve these goals. First though, an overview of the investment casting process.

An Introduction to Investment Casting

All casting processes entail pouring molten metal into a mold cavity the size and shape of the part being made. The difference between the processes is in how that cavity is formed.

Investment casting starts with a wax pattern replicating the final part. Additional wax pieces are attached to form the galleries and gates that control the flow of metal into the cavity. This assembly is then covered with a ceramic slurry that dries to form a hard shell, after which the wax is melted out.

Once the metal has solidified, the shell is broken apart and the metal feeder system cut away from the cast part. From here the part is ready for finishing and heat treatment.

Critical Casting Parameters

The two most important parameters, in terms of their effect on the quality of cast parts, are:

• Fill rate: The speed with which the mold cavity fills with metal
• Solidification rate. The speed at which the metal freezes

The challenge with these is that process economics demand rapid filling and solidification to maximize throughput. However, defect rates go up as process cycle time is reduced. This puts the foundry in the position of needing to optimize both cycle time and part quality.

Managing Fill Rate

During casting, metal is poured into a cup at the top of the ceramic shell. This is kept full with gates close to the cavity controlling the speed with which metal flows. Rapid filling creates turbulence in the metal, which causes porosity and oxides. There’s also a possibility of particles washing off the inner walls of the ceramic shell, which will result in inclusions.

However, fill too slowly, and the metal starts to solidify before the mold is full. This leads to non-fills, especially in thin sections, and shrinkage defects. Thus, the mold designers look for an optimal gate cross-section that ensures complete filling, but at the slowest speed possible.

Managing Solidification

Solidification is a function of the cooling rate, which refers to how quickly heat is lost through the ceramic shell. The speed this happens at determines the crystal structure and hence hardness and toughness of the cast part. If it happens too slowly, some alloys will desegregate (where the elements crystallize at different temperatures).

Three factors that determine cooling rate are:

• Ceramic shell thickness
• Surface area-to-volume ratio of the part being cast: Smaller parts, or regions with a high surface area-volume ratio, cool faster than regions of larger mass
• Pouring temperature and fill speed: Hotter metal remains liquid longer, while slower fill speeds allow more cooling before the cavity is full

Cooling rate can be adjusted locally by varying ceramic shell thickness. However, part geometry often limits what’s possible. Thus, achieving an optimal solidification rate means juggling fill speed, metal temperature, and shell thickness.

Other Important Parameters

In addition to freedom from internal and external defects, producing a high-quality part also means meeting surface finish and geometry accuracy goals.

These are set primarily by the quality of the wax pattern. Any errors or imperfections in this will be transferred to the cast metal part, so it’s vital to ensure the pattern is correct. Surface finish is also influenced by the size of particles in the first layer of the ceramic shell.

A Complex, Multi-Variable Problem

Maximizing part quality from investment casting involves optimizing the speed the mold cavity fills and the metal solidifies. Process economics pushes the foundry to do these quickly, but go too fast or too slow and more defects will occur. Optimizing the process is therefore a complex problem best tackled by a combination of experience and advanced modeling software.

As a leader in investment casting, Impro has the resources and expertise to undertake this kind of optimization. If you’re seeking a source for high quality parts made by the investment casting process, contact us.