# Investment casting

Selecting and qualifying expendable-pattern investment castings, including shell and core design, shrinkage, melt atmosphere, machining allowances and acceptance.

Reviewed: 2026-11-01

Tags: processes, shaping, casting

For an invented **10.00 mm finished thickness** with **0.50 mm machining stock per face**, the casting must be **11.00 mm** before machining. If it contracts **2.0% relative to cavity size**, the model gives an **11.22449 mm cavity** (11.00 ÷ 0.98). Confirm the foundry's allowance convention and local cleanup stock at the machining datums.

Conventional lost-wax casting uses a reusable wax-pattern die but consumes the wax pattern and ceramic shell. Agree the shell, gating and cleanup route with the foundry. [Vulcan Engineering, process description, undated](https://www.vulcangroup.com/processes/investment-casting-process/)

## Understand the sequence before defining the part

| Stage | What it establishes | Handoff issue |
| --- | --- | --- |
| Pattern and assembly | A disposable representation of the component connected to the feed system. | Part geometry, process allowances and gate attachment locations. |
| Shell construction and pattern removal | The refractory mold and its internal cavity. | Alloy-compatible shell, drying and removal process. |
| Pouring and solidification | The metal component plus feed-system metal. | Filling, feeding, atmosphere and solidification control. |
| Knockout and finishing | Removal of shell and feed attachments, followed by ordered operations. | Accessible cleaning, gate witness limits and the delivered condition. |

This sequence follows Vulcan's shell-process description. It is not a universal temperature, layer-count or drying-time recipe. [Vulcan Engineering](https://www.vulcangroup.com/processes/investment-casting-process/)

## Cores and cavities need a removal route

Complex internal geometry is not automatically a machinable hole or a cavity that an unsupported shell can form. CERadvance supplies injection-molded investment-casting cores, including zircon-based and mullite-reinforced materials. Its LINE-OX description distinguishes mechanical removal from chemical leaching and relates the removal route to component complexity. This named example demonstrates why core material and removal access belong in the foundry design, rather than being left until a finished casting contains trapped refractory. [CERadvance, investment-casting cores, undated](https://ceradvance.co.za/product/investment-casting-cores/)

For a proposed passage, ask which surfaces are defined by a core, how the core is located, how its removal is achieved and how residual material is detected. Specify functional passage size and cleanliness separately from the external envelope. These are qualification questions, not permission to apply a generic leaching chemistry to every alloy or core. A CAD cavity alone does not prove a feasible manufacture-and-cleaning route.

## Shrinkage compensation is not feeding

SFSA distinguishes liquid shrinkage, liquid-to-solid shrinkage and contraction of the solid casting during cooling. Solidification shrinkage can produce voids; casting geometry and the foundry's feeding/thermal design must address it. Patternmaker's shrink is a dimensional compensation for the final solid dimensions. Increasing the pattern scale does not, by itself, establish a sound feed path to an isolated section. [SFSA, alloy characteristics and casting design, undated](https://www.sfsa.org/subject-areas/education/information-for-casting-designers/alloy-characteristics-affect-casting-design/)

The same SFSA guidance warns that predicted shrink allowances may require adjustment after sample castings. Agree the finished geometry and allow the foundry to establish the process tooling dimensions. For changes in alloy, local section or geometry, reconfirm the relevant compensation rather than reusing a single percentage as a universal material constant. Review heavy junctions and abrupt changes with the foundry while the design can still change. [SFSA, patternmaker's-shrink discussion](https://www.sfsa.org/subject-areas/education/information-for-casting-designers/alloy-characteristics-affect-casting-design/)

## Alloy and melt atmosphere are separate selections

MetalTek describes both air-melt and vacuum investment casting at Wisconsin Investcast. Its vacuum route uses a controlled environment for identified demanding applications and high-performance alloys. That is an available production route, not an inherent property of all investment castings or a guarantee of zero porosity. Specify the cast alloy and condition, required atmosphere and applicable acceptance criteria with the foundry. [MetalTek, investment-casting capabilities, undated](https://www.metaltek.com/metal-casting/investment-casting/)

The same producer distinguishes **pour weight** from **ship weight** in its published capacities. These quantities are not interchangeable: feed-system metal and material removed before shipment affect what the customer receives. Likewise, a foundry's maximum envelope does not demonstrate that any thin section, core arrangement or alloy can be combined at that envelope. Request confirmation for the actual part and route. [MetalTek, published capability limits](https://www.metaltek.com/metal-casting/investment-casting/)

## Tolerances, machining and delivery condition

Stainless Foundry identifies wax/plastic and die temperatures, injection pressure, shell composition, cooling, tree position and heat treatment as influences on investment-casting tolerances. Its page attributes the guidance to the **Investment Casting Institute Handbook, 1997 edition**. Use that as historical design guidance, not an automatically current contractual tolerance table. The producer explicitly states that needed tolerances vary with casting size, shape and foundry. [Stainless Foundry, tolerance discussion and source attribution](https://www.stainlessfoundry.com/investment-casting-foundry/investment-casting-foundry-tolerance/)

Specify which features are accepted as cast and which are machined. Define the locating datums, machining stock, permitted gate remnants and the stage at which dimensions are checked. A casting that contains enough nominal stock can still lack local cleanup after distortion or datum establishment. Do not replace a flatness requirement with a general linear tolerance or accept an unspecified straightening operation without agreeing the delivered condition.

## Original allowance calculation

Assume an invented flat blank must finish at **10.00 mm thickness** after removing **0.50 mm from each face**. Its required nominal as-cast thickness is therefore **11.00 mm**. Now assume, solely for this simplified model, uniform contraction of **2.0% from mold-cavity size to as-cast size**:

`cavity thickness = 11.00 / (1 − 0.020) = 11.22449 mm`

A rounded **11.2245 mm** cavity would give **11.0000 mm** under those assumptions. Simply adding 2% to 11.00 gives **11.2200 mm**, which contracts to **10.9956 mm**. The difference is **0.0044 mm**.

Use foundry trials and the agreed allowance convention for actual tooling. This model excludes wax, shell, alloy, local restraint, flatness and machining-datum effects.

## Material and inspection requirements

**ASTM A957/A957M-26**, shown active and updated **October 29, 2026** on the publisher's page, provides common investment-casting requirements for the listed steel and alloy product specifications. Its scope states that the individual product specification prevails in a conflict, and supplementary requirements apply when individually specified in the order. It is therefore not a standalone universal alloy designation or an instruction to require every possible examination. Identify the product specification, grade, condition, applicable edition and ordered supplements. [ASTM A957/A957M-26, public scope](https://store.astm.org/a0957_a0957m-26.html)

MetalTek's **October 12, 2022** inspection overview separates chemistry, finish, dimensions, mechanical properties and casting soundness, and discusses combining examinations for the customer's requirements. Retain those distinctions when defining acceptance: exterior appearance alone cannot establish internal soundness, and a dimensional report does not establish material properties. Specify examination coverage, stage and acceptance levels rather than requesting “NDT” without a method or criterion. [MetalTek, inspection methods and limitations](https://www.metaltek.com/blog/metal-casting-inspection-methods-what-they-tell-and-what-the-limitations-are/)

Provide the foundry with the complete drawing/model revision, cast material and delivered condition, internal-passage requirements, machined features and inspection requirements. Request a proposed rough-casting envelope, gate locations, core/removal plan and sample-part dimensional evidence before treating the design as production-qualified.