Metal casting has been one of the oldest and most reliable manufacturing methods, even today when advanced processes such as additive manufacturing and CNC machining dominate production. Casting remains indispensable because it can produce complex geometries, large volumes, and strong components at relatively low cost.
However, one of the most persistent challenges in casting is shrinkage. As molten metal cools and solidifies, it contracts, leading to dimensional errors, porosity, and surface defects. If not properly managed, shrinkage can compromise both the functionality and strength of cast parts.
This guide explores what shrinkage is, why it occurs, how it affects cast metals, and the methods engineers use to minimize it.
What Is Shrinkage?
Shrinkage refers to the reduction in volume that occurs when molten metal cools and transitions into a solid inside a mold. It is an inherent material property, not a manufacturing error, but must be carefully managed to avoid defects.
The three main types of shrinkage in casting are:
1. Liquid Shrinkage
Occurs when molten metal cools from the pouring temperature down to the solidification point. This stage mainly reduces volume due to the contraction of liquid metal.
2. Solidification Shrinkage
Takes place during the phase change from liquid to solid. Metals and alloys typically shrink 3–7% during this transition, though the exact percentage depends on the alloy composition.
Aluminum alloys: High shrinkage, requiring larger risers.
Copper alloys: Also prone to significant shrinkage.
Steel and nickel alloys: Typically lower shrinkage rates.
3. Solid Shrinkage (Patternmaker’s Shrinkage)
Even after solidification, cast metals continue to contract as they cool to room temperature. This stage is usually compensated for by applying a shrinkage allowance during mold or pattern design.
Causes of Shrinkage Defects
Shrinkage defects result from a combination of material properties, casting parameters, and mold design issues.
Feeding System
The feeding system (risers, gates, runners) supplies molten metal during solidification.
Poorly designed risers or gates can fail to deliver enough liquid metal, creating internal voids or shrinkage porosity.
In sand casting, insufficient riser design is a common cause of shrinkage.
Pouring Temperature
Too high: Causes excessive liquid shrinkage, turbulence, and non-uniform cooling.
Too low: Causes premature solidification, blocking flow into thin sections.
Skilled operators must strike a balance to ensure smooth flow and controlled solidification.
Cooling Rates
Uneven cooling is often caused by variations in wall thickness or complex part geometry.
Thin sections cool faster than thick ones, creating unpredictable shrinkage.
Controlled directional solidification ensures solidification progresses uniformly from one end of the casting to another.
Alloy Composition
Alloying elements change the solidification behavior.
For example:
Aluminum & copper alloys → high shrinkage, prone to porosity.
Steels & nickel alloys → lower shrinkage, easier to control.
Foundries must account for alloy-specific shrinkage when designing patterns and risers.
Types of Shrinkage Defects
Shrinkage defects can be broadly divided into open (surface-visible) and closed (internal) types.
Open Shrinkage Defects
These appear on the casting surface and are visible to the naked eye.
Pipes: Conical depressions at the top of the casting caused by insufficient feeding.
Caved Surfaces: Shallow depressions caused by contraction during cooling. While often aesthetic, they may still require rework.
Closed Shrinkage Defects
These occur inside the casting and are much harder to detect without inspection techniques such as X-ray radiography or ultrasonic testing.
Internal Shrinkage: Voids and cavities within the part.
Macro-Shrinkage: Large internal cavities, usually caused by uneven cooling and poor mold design.
Micro-Shrinkage: Tiny voids spread throughout the material, often found in complex or thin-walled castings.
What Shrinkage Does in Casting?
Shrinkage directly affects the mechanical performance, accuracy, and quality of cast parts.
Compromises Structural Integrity
Internal voids act as stress concentrators.
Porosity reduces load-bearing capacity, making the part more likely to fail under stress or fatigue.
Reduces Dimensional Accuracy
Shrinkage changes part dimensions during cooling.
Foundries apply shrinkage allowances in pattern design to ensure final dimensions are within tolerance.
Complex geometries often require CNC machining after casting to achieve precise tolerances.
Affects Surface Finish
Surface defects such as caved areas or rough textures may appear.
High-quality applications (aerospace, automotive) require secondary finishing operations like grinding, polishing, or coating.
How To Minimize Shrinkage in Casting
Shrinkage cannot be eliminated but can be controlled and compensated using engineering techniques.
Optimize Mold Design
Properly positioned risers and feeders supply molten metal during solidification.
Gate design ensures smooth metal flow and prevents turbulence.
Reservoirs in the mold help compensate for localized shrinkage.
Control Cooling Rates
Directional solidification ensures the casting cools in a predictable path.
Chills (metal inserts placed in molds) accelerate cooling in thick sections.
Balancing cooling rates prevents both open and closed shrinkage defects.
Use Insulators
Insulating materials slow cooling in specific areas, maintaining liquid metal supply.
Combined with chills, they allow precise thermal control across the casting.
Porosity vs. Shrinkage in Casting: What’s the Real Difference?
Porosity and shrinkage often get mixed up, but they’re not exactly the same thing. Shrinkage happens when the metal cools and contracts, leaving behind voids or cavities. Think of it like the metal “pulling away” as it solidifies.
Porosity, on the other hand, usually comes from trapped gases or tiny air bubbles inside the casting. Instead of the metal contracting, it’s more about air or gas not escaping during the process.
In simple terms:
Shrinkage = metal volume loss during cooling
Porosity = gas pockets trapped inside the metal
Both can weaken the part, but they come from different causes and need different solutions to fix.
Die Casting Services
At Tops Precision, we specialize in producing precision cast components in metals and plastics.
Wide range of alloy choices including aluminum, zinc, and magnesium.
High-pressure die casting for superior surface finish and dimensional accuracy.
Secondary machining services for tight-tolerance requirements.
Conclusion
Shrinkage is one of the most common and critical challenges in casting, but it can be effectively managed through:
Careful mold and feeding system design,
Control of pouring temperatures and cooling rates, and
Accounting for alloy-specific shrinkage allowances.
By understanding the causes of shrinkage and applying proven countermeasures, engineers can ensure that cast parts achieve both structural integrity and dimensional accuracy.
Ultimately, choosing an experienced foundry or casting service provider is essential for minimizing shrinkage defects and producing high-performance cast components.
