Metal Shrinkage in Castings

Achieving tight dimensional tolerances remains a foremost concern in casting production.

As molten metal cools and solidifies, it inevitably contracts—sometimes predictably, other times unpredictably—depending on alloy chemistry, geometry, and process parameters.

Without proper control, shrinkage can introduce internal voids, distortions, and out-of-tolerance features that compromise both performance and cost.

In this comprehensive article, we examine the mechanics of metal shrinkage, its practical implications for ferrous and non-ferrous alloys, and the strategies foundries and designers employ to mitigate defects.

1. Introduction

Dimensional accuracy underpins the functionality of every cast component, from automotive engine blocks to precision aerospace housings.

Metal shrinkage refers to the reduction in volume and linear dimensions that occurs as an alloy transitions from liquid to ambient temperature.

Even a modest 2–3% linear contraction in steel or 5–8% in aluminum can lead to misfits, warping, or rejected parts if unaddressed.

By exploring shrinkage across simple versus complex geometries and contrasting ferrous and non-ferrous alloys, we lay the foundation for targeted design and process controls.

2. Types of Shrinkage

Understanding the distinct types of shrinkage that occur during the casting process is critical for achieving dimensional accuracy and structural integrity.

Shrinkage in metal castings typically progresses through three main stages—liquid shrinkage, solidification shrinkage, and solid (patternmaker’s) shrinkage—each with different implications for design, mold preparation, and defect control.

Additionally, shrinkage can be classified by its physical manifestation as macro-shrinkage, micro-shrinkage, or piping, depending on scale and location within the casting.

Liquid Shrinkage

Liquid shrinkage refers to the reduction in volume as molten metal cools from pouring temperature down to its solidification point, while remaining in a fully liquid state.

This shrinkage can range from 1% to 3% by volume, depending on alloy type.

Though generally not a concern for dimensional control, it is crucial to maintain open feeding paths from risers during this phase.

If the riser fails to supply enough molten metal, the casting may develop surface depressions or incomplete fill.

Example: Aluminum alloys may experience liquid shrinkage of 2.5%, necessitating careful riser design to maintain consistent mold fill during early cooling.

Solidification (Solid–Liquid) Shrinkage

This is the most critical form of shrinkage from a defect-prevention standpoint.

As the metal transitions from liquid to solid, it undergoes a significant volumetric contraction, typically 3% to 7%.

This shrinkage occurs within the so-called “mushy zone”, where both solid and liquid phases coexist.

If molten metal is not properly fed during this phase, macro-shrinkage defects such as voids, centerline porosity, or cavities can form.

Solidification shrinkage is highly sensitive to:

• Cooling rate and thermal gradients
• Solidification mode (eutectic, directional, or equiaxed)
• Alloy freezing range

Directional solidification, which promotes unidirectional heat flow toward the risers, is a widely adopted strategy to counteract these effects.

Solid (Patternmaker’s) Shrinkage

Once fully solidified, the casting continues to shrink as it cools to ambient temperature. This linear shrinkage typically ranges from 1% to 2.5%, depending on the alloy. For example:

• Carbon steel: ~2.0%
• Gray iron: ~1.0%
• Aluminum alloys: ~1.3% to 1.6%

Patternmakers accommodate this shrinkage by scaling the pattern dimensions using standardized shrinkage allowances.

This shrinkage is considered relatively predictable and uniform, though it may be non-uniform in castings with complex geometries or variable section thicknesses.

Micro-Shrinkage vs. Macro-Shrinkage vs. Piping

Type	Description	Typical Location	Causes
Micro-shrinkage	Fine, dispersed voids or porosity within the solid structure	Random or isolated regions	Dendritic solidification, poor feeding
Macro-shrinkage	Large, visible voids often found at the center or top of castings	Central or riser neck areas	Inadequate riser feed
Piping	Funnel-shaped cavity extending from the riser into the casting	Near riser–casting junction	Insufficient riser volume or delay in feeding

3. Solidification Modes and Their Effects

How a metal solidifies—its solidification mode—has a profound effect on shrinkage behavior, feeding requirements, and final casting quality.

Solidification is not a uniform process; it varies significantly with alloy composition, cooling rates, and mold design.

Understanding the three principal solidification modes—eutectic, directional, and equiaxed—is essential for controlling shrinkage and minimizing internal defects such as porosity and voids.

Eutectic Solidification

Eutectic solidification occurs when a metal or alloy transitions from liquid to solid at a fixed temperature, forming two or more solid phases simultaneously in a very fine mixture.

This transformation happens rapidly, often across the entire casting cross-section at once, leaving minimal opportunity for shrinkage feeding.

• Common alloys: Gray iron, aluminum-silicon alloys (e.g., A356), and some bronzes
• Shrinkage characteristics: Low macro-shrinkage, but prone to micro-porosity if not properly controlled
• Feeding behavior: Requires minimal riser volume, but precise thermal management is essential

Example: Gray iron castings solidify through a eutectic reaction that produces graphite flakes.

The volumetric expansion caused by graphite precipitation can sometimes offset shrinkage, making gray iron relatively forgiving in terms of feeding.

Directional Solidification

In directional solidification, metal solidifies progressively from one end of the casting (typically the mold walls) toward a designated heat reservoir or riser.

This controlled thermal gradient allows molten metal to feed solidifying regions effectively, reducing shrinkage defects.

• Common alloys: Carbon steels, low-alloy steels, nickel-based superalloys
• Shrinkage characteristics: Predictable macro-shrinkage paths that can be managed with well-placed risers
• Feeding behavior: Excellent, if thermal gradients are maintained and hot spots are avoided

Example: In steel castings, directional solidification is deliberately engineered through the use of chills (which accelerate solidification) and insulated risers (which delay it).

This guides the solidification front from thinner sections to thicker, aiding in defect-free casting.

Equiaxed Solidification

Equiaxed solidification involves the simultaneous nucleation of grains throughout the liquid metal.

Solidification occurs randomly rather than following a predictable thermal gradient. This makes feeding and shrinkage control far more challenging.

• Common alloys: Aluminum 356 (in some casting methods), aluminum bronzes
• Shrinkage characteristics: High risk of internal shrinkage and micro-porosity
• Feeding behavior: Difficult to manage; prone to premature blockage of feeding paths

Example: In equiaxed aluminum castings, grains may solidify unpredictably in isolated areas, creating internal voids if the metal feed is blocked by earlier solidification. Simulation software is often used to anticipate such risks and adjust the gating design accordingly.

Implications for Porosity and Feeding Design

Each solidification mode influences how porosity develops and how feeding systems must be designed:

Solidification Mode	Porosity Risk	Feeding Complexity	Riser Efficiency
Eutectic	Low macro, possible micro	Moderate	High
Directional	Low if managed well	Low to moderate	High
Equiaxed	High (micro and macro)	High	Low

4. Key Influencing Factors

Metal shrinkage in castings is not governed by a single variable but rather by a complex interplay of metallurgical, geometrical, and process-driven factors.

Understanding these factors allows foundry engineers to design castings and processes that mitigate shrinkage defects, enhance dimensional accuracy, and improve overall casting performance.

Below are the primary contributors that influence shrinkage behavior:

Alloy Type and Composition

The alloy system being cast plays a foundational role in determining shrinkage characteristics.

Different metals and their respective alloys shrink at varying rates due to differences in density change during solidification and thermal contraction coefficients.

• Steel alloys typically exhibit volumetric solidification shrinkage in the range of 3–4%.
• Aluminum alloys may shrink 6–7%, though additions like silicon (e.g., Al-Si alloys) reduce shrinkage by forming eutectic structures.
• Copper-based alloys can show even greater shrinkage (up to 8%), depending on the presence of tin, zinc, or aluminum.

The inclusion of alloying elements can also change the solidification path (eutectic vs. equiaxed), thereby altering the feeding behavior and porosity tendencies.

Section Thickness and Thermal Gradients

Geometric features have a major effect on cooling rates and local shrinkage behavior. Thicker sections retain heat longer and solidify more slowly, while thinner sections cool rapidly.

This creates internal thermal gradients, which dictate how solidification progresses through the casting.

• Thick sections are prone to hot spots and internal shrinkage voids.
• Abrupt section changes (e.g., from thick to thin) create localized stress zones and may block feeding paths, leading to shrinkage porosity.

Design best practices encourage smooth transitions and uniform section thickness to manage heat dissipation evenly.

Mold Material and Rigidity

The physical characteristics of the mold—particularly its thermal conductivity and rigidity—influence how heat is extracted from the molten metal, affecting both the rate and direction of solidification.

• Green sand molds offer flexibility and can accommodate minor shrinkage but may introduce warping due to their lower strength.
• Air-set or chemically bonded sand molds provide greater dimensional control but are less forgiving to thermal contraction, increasing residual stress.
• Permanent molds (e.g., die casting) enforce strict cooling rates due to their high thermal conductivity but require more precise shrinkage allowances.

Additionally, mold coatings and chills can be applied to locally control solidification times and feeding effectiveness.

Pouring Temperature and Rate

The temperature at which metal is poured affects both fluidity and the size of the solidification window.

Higher superheats can delay nucleation and promote equiaxed solidification, which may increase micro-porosity.

• Overly high pouring temperatures can cause turbulent flow, gas entrapment, and shrinkage voids.
• Conversely, low pouring temperatures may result in premature solidification and cold shuts, blocking feeding paths before shrinkage compensation occurs.

The pouring rate must also be optimized to ensure that all parts of the mold are filled before solidification begins, while avoiding mold erosion or turbulence.

Riser Design and Gating System

Proper riser and gating design is one of the most direct ways to combat shrinkage. Risers serve as reservoirs of molten metal that feed the casting as it contracts during solidification.

Key design principles include:

• Riser volume must be sufficient to compensate for solidification shrinkage.
• Riser location should be near hot spots to ensure molten metal is available where needed.
• Directional solidification should be promoted through the placement and sizing of risers, gates, and chills.

Advanced gating designs (bottom gating, pressurized vs. non-pressurized systems) influence how metal fills the cavity and cools, directly impacting shrinkage formation.

5. Compensation Strategies for Metal Shrinkage in Castings

Effectively mitigating metal shrinkage in castings requires a combination of precise design, predictive modeling, and well-executed process controls.

As shrinkage is an unavoidable physical phenomenon associated with cooling and solidification, foundries focus on compensatory strategies to ensure dimensional accuracy and prevent internal defects such as voids and porosity.

This section outlines key engineering techniques and technological innovations used to manage shrinkage in both ferrous and non-ferrous casting processes.

Pattern Scaling Rules and CAD Shrink Factors

One of the most fundamental approaches to compensate for shrinkage is adjusting the size of the casting pattern.

Since all metals contract to varying degrees upon cooling, patternmakers apply shrinkage allowances based on the expected contraction rates of specific alloys.

• For instance, carbon steel patterns typically include a 2.0%–2.5% linear shrinkage allowance.
• Aluminum alloys, due to their higher shrinkage, often require 3.5%–4.0% allowances.
• These values are implemented using “shrink rules” in manual processes or scaling factors in CAD models during digital design.

However, shrinkage is not uniformly distributed—areas with complex geometry or uneven mass may require localized adjustment.

Modern CAD software allows for region-specific scaling, improving accuracy for complex castings.

Riser Placement and Hot-Spot Control

Risers serve as reservoirs of molten metal that feed the casting during solidification, compensating for volumetric shrinkage.

Effective riser design is essential to promote directional solidification, ensure full feeding of thick sections, and eliminate shrinkage cavities.

Key riser design considerations include:

• Size: The riser must retain heat longer than the casting to remain molten while the casting solidifies.
• Location: Risers should be placed above or adjacent to hot spots—areas that solidify last due to mass concentration.
• Shape: Cylindrical or conical risers provide good volume-to-surface area ratios, slowing heat loss.
• Riser insulation: Use of insulating sleeves or exothermic materials can extend riser cooling time, enhancing feeding effectiveness.

Use of Chills and Insulating Sleeves

Chills are materials with high thermal conductivity (often iron or copper) placed in the mold to accelerate solidification in targeted areas.

Their use helps control the direction and rate of solidification, effectively drawing solidification fronts away from risers to promote directional feeding.

• Internal chills can be embedded in mold cavities.
• External chills are placed outside the casting surface.
• Insulating sleeves are applied to risers or mold areas to delay solidification, aiding feeding in heavy sections.

This strategic thermal management helps reduce internal porosity and ensures consistent structural integrity.

Advanced Simulation and Predictive Software

Modern foundries rely heavily on casting simulation software to visualize and optimize shrinkage control before physical molds are produced.

Software such as MAGMASOFT, ProCAST, and SolidCast simulates fluid flow, heat transfer, and solidification behavior within the mold cavity.

Benefits include:

• Prediction of shrinkage porosity and hot spot locations
• Validation of riser and gating system design
• Optimization of chill placement and mold insulation
• Evaluation of alternative alloys or mold materials

For example, simulations can reveal that a large aluminum housing has a high-risk hot zone near a mounting flange.

Engineers can then add a local riser and chill to improve feeding and minimize distortion.

Foundry Process Control and Monitoring

Even with sound design and simulation, shrinkage defects can occur if process variables are not controlled consistently. Critical process controls include:

• Pouring temperature: Too high can increase turbulence and shrinkage porosity; too low may cause incomplete fill or cold shuts.
• Mold preheat and coating: Affects initial heat transfer and mold-metal interaction.
• Cooling rates: Can be influenced by mold material, ambient conditions, and placement of castings in the mold box.

Real-time data acquisition through thermocouples, pyrometry, and thermal imaging supports proactive monitoring and adjustments during the pour and cooling phases.

6. Alloy Shrinkage Rates (Approximate)

Here is a comprehensive list of approximate alloy shrinkage rates for commonly used casting alloys, covering both ferrous and non-ferrous metals.

These linear shrinkage values are typically expressed as percentages and are essential for pattern design, tooling compensation, and accurate dimensional control in foundry operations.

Ferrous Alloys

Alloy Type	Approx. Linear Shrinkage (%)	Notes
Gray Cast Iron	0.6 – 1.0%	Low shrinkage due to graphite expansion during solidification.
Ductile Iron (SG Iron)	1.0 – 1.5%	Moderate shrinkage; nodularity affects volume contraction.
White Cast Iron	2.0 – 2.5%	Higher shrinkage; no graphitic compensation.
Carbon Steel (Low & Medium)	2.0 – 2.6%	High shrinkage; requires careful risering and feeding.
Alloy Steel (e.g., 4140, 4340)	2.1 – 2.8%	Varies with alloying content and cooling rate.
Stainless Steel (304, 316)	2.0 – 2.5%	High shrinkage; prone to internal voids if not properly fed.
Tool Steel	1.8 – 2.4%	Sensitive to temperature gradients and mold design.
Malleable Iron	1.2 – 1.5%	Similar to ductile iron but with post-solidification annealing.

Non-Ferrous Alloys – Aluminum-Based

Alloy Type	Approx. Linear Shrinkage (%)	Notes
Aluminum 356 (Heat-Treatable)	1.3 – 1.6%	Moderate shrinkage; influenced by T6 heat treatment.
Aluminum 319 / A319 (High Si-Cu)	1.0 – 1.3%	Lower shrinkage; good casting characteristics.
Aluminum 535 (Mg-bearing)	1.5 – 1.8%	More prone to porosity; benefits from chills.
Aluminum 6061 (Wrought)	~1.6%	Used in casting when T6 properties are needed.
Aluminum Alloys (General)	1.0 – 1.8%	Varies by composition and cooling strategy.

Copper-Based

Alloy Type	Approx. Linear Shrinkage (%)	Notes
Yellow Brass (e.g., C85700)	1.5 – 2.0%	High shrinkage; requires strong feeding systems.
Red Brass (e.g., C83450)	1.3 – 1.7%	Good flow; moderate shrinkage.
Silicon Bronze (C87300, C87600)	1.3 – 1.6%	Widely used in art casting; moderate shrinkage.
Aluminum Bronze (C95400)	2.0 – 2.5%	High shrinkage; directional solidification essential.
Tin Bronze (C90300, C90500)	1.1 – 1.5%	Lower shrinkage due to tin content.

Non-Ferrous Alloys – Nickel-Based

Alloy Type	Approx. Linear Shrinkage (%)	Notes
Inconel 718	2.0 – 2.5%	High-temperature alloy; needs precision casting control.
Hastelloy (C series)	1.9 – 2.4%	Used in corrosion-resistant applications.
Monel (Nickel-Copper)	1.8 – 2.3%	Good ductility; high shrinkage.

Magnesium Alloys

Alloy Type	Approx. Linear Shrinkage (%)	Notes
AZ91D (Die Casting)	1.1 – 1.3%	Light weight; fast cooling aids dimensional control.
ZE41 / ZE43 (Sand Casting)	1.2 – 1.5%	Requires control of hydrogen porosity.

Titanium Alloys

Alloy Type	Approx. Linear Shrinkage (%)	Notes
Ti-6Al-4V	1.3 – 1.8%	High-performance alloy; investment casting required.

7. Dimensional Tolerances and Standards

International standards align design expectations with process capabilities:

• ISO 8062: Defines casting tolerance grades (CT5–CT15) that scale with nominal size.
• ASME & ASTM: Provide industry-specific shrink allowances (e.g., ASTM A802 for steel castings).
• Trade-Off: Tight tolerances increase tooling cost and lead time; designers balance affordability against required precision.

8. Conclusion

Metal shrinkage presents both predictable and complex challenges in casting.

By combining metallurgical understanding—thermal contraction, phase-change dynamics, and solidification modes—with robust design and simulation tools,

Engineers and foundries can mitigate shrinkage defects, optimize feeding strategies, and achieve the tight tolerances modern applications demand.

Ultimately, success hinges on early collaboration between design and production teams, leveraging both experience and technology to transform molten metal into precision components.

At LangHe, we are happy to discuss your project early in the design process to ensure that whatever alloy is selected or post-casting treatment applied, the result will meet your mechanical and performance specifications.

To discuss your requirements, email [email protected].

FAQs on Metal Shrinkage in Castings

What is metal shrinkage in castings?

Metal shrinkage refers to the reduction in volume and linear dimensions that occurs as molten metal cools from its pouring temperature down to ambient temperature.

Why does metal shrink during casting?

First, thermal contraction causes the liquid metal to contract as it cools toward its freezing point.

Second, solidification shrinkage occurs when the metal transitions from liquid to solid, leading to additional volumetric contraction.

Finally, solid-phase shrinkage continues as the fully solid metal cools to room temperature.

What is the patternmaker’s shrinkage?

Patternmaker’s shrinkage is the linear contraction (typically 1–2%) that occurs after the metal has fully solidified and cools to room temperature; foundries compensate for it by enlarging pattern dimensions.

What factors influence shrinkage magnitude and direction?

Key factors include alloy composition (e.g., silicon reduces shrinkage in aluminum), section thickness (thicker areas cool more slowly),

mold material and rigidity (sand vs. permanent molds), pouring temperature/rate, and the design of risers and gating systems.

What role do risers and chills play in shrinkage control?

Risers act as molten-metal reservoirs to feed the casting during solidification shrinkage,

while chills (high-conductivity inserts) accelerate cooling in targeted areas, promoting directional solidification and preventing internal voids.

How is the shrinkage allowance calculated for a pattern?

Shrinkage allowance (%) = (Pattern dimension – Casting dimension) / Casting dimension × 100%.

Foundries derive these allowances empirically for each alloy and process, then implement them as CAD scale factors or pattern expansions.