A Glossary of Investment Casting Terms: From Sprue to Shell Building.

Knowing the language isn’t just about sounding smart—it’s about communicating effectively with your foundry, catching potential issues early, and understanding the detailed quotes and process sheets they send you.

Let’s build a practical, working glossary. I’ll focus on the terms you’ll actually hear on the shop floor and see in technical documents, skipping the overly academic stuff.

The Foundry Floor Lexicon: A Practitioner’s Guide

Assembly (or “Tree Assembly”)
The process of welding multiple wax patterns onto a central wax stalk (the sprue) to create a “tree” or “cluster.” This is how multiple parts are cast in a single furnace pour. In my experience, how a tree is assembled—the angles and spacing—directly impacts yield due to heat distribution during solidification.

Burn-Out
The high-temperature oven cycle where the invested wax tree is melted and vaporized out of the ceramic shell, leaving behind a hollow, precise cavity. It’s a critical step; too fast and the shell can crack from thermal shock.

Cope and Drag
Terms borrowed from sand casting, but used in investment casting to refer to the top half (cope) and bottom half (drag) of the two-piece mold used to produce the wax patterns themselves. Mismatch here causes parting line flash on your wax, which translates to flash on your metal part.

Core (Ceramic Core)
A pre-formed, sintered ceramic insert placed inside the wax injection die. It becomes encapsulated in wax and then in the shell, creating internal passages in the final casting (e.g., cooling channels in a turbine blade). These are engineering marvels in themselves and a major cost driver. Designing for easy core removal (via leaching) is a whole sub-specialty.

Dewax
The initial, lower-temperature step to remove the bulk of the wax from the shell, usually via steam autoclave. It’s followed by the high-temperature burn-out. Getting this wrong can lead to shell cracks or “warts” from wax expansion.

Gate
The controlled connection point where the wax pattern is attached to the sprue. It becomes the channel through which molten metal enters the part cavity. Gating design is more art than science—it controls fill velocity, directional solidification, and is the last place to solidify (feeding shrinkage). Where they’re cut off leaves a “gate scar” that often needs grinding.

Investment
The verb and the noun. It means the act of building up the ceramic shell around the wax assembly. The investment is also the name for the ceramic slurry material itself (a blend of refractories like silica, zircon, and a binder).

Pattern (Wax Pattern)
The replica of your final part, made from injection-molded wax or (less commonly) 3D-printed polymer. This is the heart of the process. Its dimensional accuracy and surface finish directly dictate the quality of the final casting. There are different waxes for different needs: filled wax for stability, low-ash wax for superalloys.

Pouring/Casting
The moment of truth. The fired shell (now a ceramic mold) is filled with molten metal. It can be done in air (for steels), under a vacuum, or in a controlled atmosphere (for reactive alloys like titanium). The pour temperature is critical—a few degrees can mean good fill or cold shuts.

Riser (or “Feeder”)
A reservoir of extra metal attached to the wax pattern via a gate. It’s not part of the final component. Its sole job is to feed liquid metal into the casting as it solidifies and shrinks, preventing internal porosity. A well-designed riser is sacrificial weight you pay for to ensure soundness. Its size and placement are non-negotiable.

Shell Building (or “Stuccoing”)
The multi-step, repetitive process of creating the ceramic mold. The wax tree is repeatedly:

1. Dipped in a fine slurry (the “prime coat”).
2. Rained with coarse, sand-like stucco.
3. Dried in a controlled environment.
   This builds up layers (typically 6-9) to form a strong, permeable shell. The first coat determines surface finish; the later layers provide strength.

Shrinkage (Patternmaker’s Allowance)
This is the critical, hidden dimension. Wax and metal shrink at different rates. The pattern die is intentionally made larger than the final part to compensate for the total contraction of wax and metal. The allowance varies by alloy (e.g., aluminum ~1.3%, steel ~2.1%, superalloys can be 2.5%). Get this wrong, and the part is scrapped for being out of tolerance.

Sprue (or “Runner Bar”)
The central wax “trunk” of the tree assembly. All parts (via their gates) and risers connect to it. It’s the main conduit for molten metal from the pouring cup to every part cavity. Its diameter is calculated to ensure adequate metal flow to feed the entire tree before it solidifies.

Stucco
The coarse, refractory granules (like silica, zircon, or alumina-silicate) applied to the wet slurry layers to build shell thickness, create drainage paths for gases/wax, and provide mechanical strength. Different grades are used for different layers.

Wax Injection
The process of forcing liquid wax under pressure into the aluminum or steel pattern die to create the wax replicas. Injection temperature, pressure, and cycle time are tightly controlled to prevent defects like flow lines or sinks in the wax, which become defects in the metal.

Three Terms You Must Understand for Quality Discussions:

1. Cold Shut: A visible line or seam on the casting where two streams of molten metal met but didn’t fuse completely. Caused by metal temperature being too low or the section being too thin. It’s a crack waiting to happen.
2. Inclusion (Ceramic Inclusion): A piece of the ceramic shell or core that breaks off and gets trapped in the metal. A fatal defect in high-integrity castings. Shell strength and handling are key to prevention.
3. Porosity: Tiny voids within the casting. Shrinkage porosity (irregular, often near gates/risers) is a feeding issue. Gas porosity (round, shiny bubbles) is from trapped air or gas from the shell/metal reaction. HIP (Hot Isostatic Pressing) can often close this, but it’s an added cost.

My Final Advice: When you get a quote or process sheet, don’t gloss over these terms. If it says “8-layer zircon shell,” you know they’re using a premium refractory for high-temperature alloy. If they note “separately cast test bars,” they’re ensuring proper mechanical properties. This language is the blueprint of your part’s journey from wax to metal. Knowing it makes you a better, more informed partner in the process.

Keep this as a cheat sheet. It’ll demystify 90% of the conversation with any foundry engineer.