Types of Shrinkage in the Metal Casting Process

1. Hōʻikeʻike

In modern manufacturing, dimensional pololei is non-negotiable.

Industries such as aerospace, aitompetitive, and energy demand precision-cast components with tight tolerances and defect-free microstructures.

One of the most persistent challenges in achieving these goals is metal shrinkage—the volumetric contraction of metals as they transition from a molten to a solid state and subsequently cool to room temperature.

Metal shrinkage occurs in multiple stages and is influenced by factors ranging from alloy chemistry to mold design.

Its effects differ significantly between ferrous and non-ferrous alloys, and its complexity increases with non-uniform or intricate geometries.

Addressing shrinkage is essential to avoid dimensional deviations, Potiwale, and mechanical failures.

2. Fundamental Mechanisms

Metal shrinkage arises primarily from thermal contraction a phase transformation effects. As metals cool, atoms move closer together, resulting in linear and volumetric contraction.

ʻo kahi laʻana, the linear shrinkage rate of aluminum alloys can range from 5.5% i 6.5%, while steels typically shrink around 2%.

Eia hou, shrinkage intensifies during Kūpuia, particularly in the mushy zone—a semi-solid state where feeding becomes difficult.

'Ōlelo interaction between cooling rate, alloy chemistry, and microstructure evolution determines whether feeding compensates for this contraction or defects like porosity develop.

3. Classification of Shrinkage in Metal Casting

Shrinkage in metal casting can be categorized based on the phase of the solidification process during which it occurs, the physical characteristics of the defects it produces, and its root causes.

Understanding these classifications enables foundry engineers to implement targeted design and process controls to mitigate casting defects.

Liquid Shrinkage

Liquid shrinkage refers to the volumetric reduction that occurs as molten metal cools within the liquid phase before the onset of solidification.

This type of shrinkage typically requires continuous feeding from risers to compensate for volume loss and avoid air aspiration or incomplete fills.

• Typical Magnitudes: Aneane 1% i 2% of volume loss in the liquid phase, varying by alloy.
• Nā hopena: Inadequate riser design or low metallostatic pressure may lead to poino, nā'ōpū anuanu, Oole surface shrinkage defects.

Solidification (Mushy-Zone) Shrinkage

During the transition from liquid to solid, metal passes through a “mushy” phase characterized by the coexistence of dendritic solids and interdendritic liquid.

Volume reduction during this phase is the most challenging to address due to decreasing permeability and feeding capability.

• Defect Types: Internal cavities and macro-shrinkage typically form in the last areas to solidify, particularly at thermal centers or poorly fed sections.
• Sensitive Alloys: Alloys with a wide freezing range (E.g., some copper and aluminum alloys) are particularly vulnerable.

Patternmaker’s (Solid) Shrinkage

After complete solidification, the casting continues to contract as it cools to ambient temperature.

This contraction, known as patternmaker’s shrinkage, is a linear dimensional reduction and is typically accounted for in the design of patterns and molds.

• Shrinkage Rates:

• • Gray Iron: ~1%
  • ʻAihue kīwī: ~2%
  • Apana Apana Aluminum: 4–6.5%

• Engineering Response: CAD models are scaled using empirical shrink factors to preempt dimensional deviation.

Macro-Shrinkage vs. Micro-Shrinkage

• Macro-Shrinkage: These are large, visible shrinkage cavities, often localized near risers, thermal centers, or in thick sections.
  They significantly weaken the structural integrity and are typically rejected in critical applications.
• Micro-Shrinkage: These are dispersed porosities on a microscopic level, often resulting from insufficient inter-dendritic feeding or localized thermal gradients.
  While they may not be visible externally, they degrade fatigue resistance, pressure containment, a me nā meaʻike.

Piping and Open Shrinkage

Piping refers to the characteristic funnel-shaped shrinkage cavity that forms at the top of a casting or riser due to progressive solidification from the periphery inward.
Open shrinkage is a related surface-connected cavity that indicates feeding failure.

• Industries Affected: Piping is common in steel castings for structural and pressure components where feeding requirements are high.
• Control Measures: Proper riser design, including use of insulating sleeves and exothermic materials, can significantly reduce or eliminate these defects.

4. Metallurgical Perspective

Solidification behavior is alloy-dependent and influences shrinkage characteristics:

Eutectic Solidification

Alloys like gray iron and Al-Si exhibit narrow freezing ranges. Solidification occurs almost simultaneously throughout the casting, reducing feeding needs but increasing the risk of gas porosity.

Directional Solidification

Preferred for structural castings (E.g., in steels or Ni-based superalloys), this allows predictable feeding paths.

By controlling the thermal gradient, solidification progresses from thinner to thicker sections.

Equiaxed Solidification

Common in bronzes and some Al alloys, this involves random nucleation of grains, which can disrupt feeding channels and increase porosity.

From a metallurgical standpoint, grain refinement, inoculation, a alloy design play critical roles in minimizing shrinkage by promoting uniform solidification and improving feedability.

5. Hoʻolālā & Engineering Perspective

From a design and engineering standpoint, controlling shrinkage begins with smart geometry and targeted feeding strategies.

Effective parts not only reflect metallurgical understanding but also embody best practices in sectioning, pattern scaling, and thermal management.

Section Thickness & Thermal Gradients

Thicker sections retain heat longer, creating “hot spots” that solidify last and draw molten metal away from thinner regions.

ʻo kahi laʻana, a 50 mm-thick steel wall may cool at 5 °C/min, whereas a 10 mm section cools at 20 °C/min under the same conditions. To mitigate this:

• Uniform wall thickness minimizes extreme gradients.
• Rounded transitions (minimum fillet radius = 0.5× wall thickness) prevent localized thermal stress.
• When thickness varies by more than 3:1, incorporate internal chills or localized risers.

Pattern Scaling & Regional Allowances

Global shrinkage allowances typically range from 2.4% for carbon steels to 6.0% for aluminum alloys. Akā naʻe,, complex castings demand region-specific scaling:

• Thin webs (≤ 5 mm): apply 0.8× global allowance (e.g. 1.9% for steel).
• Thick bosses (≥ 30 mm): increase by 1.2× (e.g. 2.9% for steel).
  Modern CAD tools support multi-factor scaling, allowing direct mapping of local allowances to pattern geometry.

Riser, Goting & Chill Strategies

Promoting directional solidification requires strategic placement of feeders and temperature controls:

• Riser volume should equal 30-40% of the mass of the zone it feeds.
• Position risers directly above thermal hot spots, identified via solidification simulation or thermal analysis.
• Insulating sleeves around risers slow their cooling by 15–20%, extending feeding time.
• Chills made of copper or iron accelerate local solidification, diverting the solidification front toward the riser.

Design for Manufacturability

Early collaboration between design and foundry teams reduces shrinkage risk.

By integrating DFM guidelines—such as uniform sectioning, adequate draft angles (> 2° for sand casting), and simplified cores—engineers can:

• Lower scrap rates by 20-30%
• Shorten lead times by avoiding multiple pattern iterations
• Ensure first-pass success in high-precision components, such as engine housings with ±0.2 mm tolerance requirements

6. Simulation & Predictive Modeling

Modern casting operations leverage CFD-based thermal and fluid simulations to preemptively identify shrinkage-prone areas.

Using tools like MAGMASOFT®, Flow-3D®, or ProCAST®, foundries can:

• Predict nā wahi wela a feed paths
• Evaluate the impact of alloy selection, mold design, and pouring parameters
• Simulate multiple casting scenarios before physical production

Integrating simulation with CAD/CAM systems enables more accurate tooling design, significantly reducing trial-and-error iterations, waste, and lead time.

7. Honua mālamalama & Nānā

Defect detection is crucial in verifying casting integrity. Commonly used Nondestructive Testing (Ndt) methods include:

• Radiographic Inspection (X-ray): Detects internal shrinkage cavities and macro defects
• Ultrasonic Testing (U): Ideal for detecting porosity and internal discontinuities in dense alloys
• Dimensional Analysis (Cmm, 3D laser scanning): Validates shrinkage allowances and conformity to specifications

Foundries also implement Statistical Process Control (SPC) to monitor shrinkage variations across batches and continuously improve process capability.

8. Approximate linear shrinkage allowances for common casting alloys.

Below is a consolidated table of approximate linear shrinkage allowances for a range of commonly cast alloys.

Use these as starting points in pattern or CAD scaling—then validate with simulation and prototype trials to dial in final dimensions.

9. Hopena

Understanding the various types of shrinkage in metal casting—liquid, Kūpuia, and solid-state—is essential for producing structurally sound and dimensionally accurate components.

As alloys and part geometries become more complex, so too must our strategies evolve.

Mitigating shrinkage requires a multi-disciplinary approach involving metallurgy, Hoʻolālā, simulation, and quality control.

Foundries that embrace predictive modeling, real-time control, a collaborative design processes are better equipped to reduce waste, optimize cost, and deliver components that meet the highest standards of performance and reliability.

At ʻO kēia, 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].