Types of Shrinkage in the Metal Casting Process

1. Увођење

У модерној производњи, Димензионална тачност is non-negotiable.

Industries such as aerospace, аутомотиве, and energy demand precision-cast components with tight толеранције 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, порозност, and mechanical failures.

2. Fundamental Mechanisms

Metal shrinkage arises primarily from thermal contraction и phase transformation effects. As metals cool, атоми се приближавају, што резултира linear and volumetric contraction.

На пример, the linear shrinkage rate of aluminum alloys can range from 5.5% до 6.5%, while steels typically shrink around 2%.

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

Тхе 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: Приближно 1% до 2% of volume loss in the liquid phase, varying by alloy.
• Импликације: Inadequate riser design or low metallostatic pressure may lead to мисунс, хладно затвара, или surface shrinkage defects.

Стврдњавање (Mushy-Zone) Скупљање

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 (Нпр., some copper and aluminum alloys) are particularly vulnerable.

Patternmaker’s (Чврсто) Скупљање

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:

• • Сиво гвожђе: ~1%
  • Карбонски челик: ~2%
  • Алуминијумске легуре: 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, и механичка својства.

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. Металуршка перспектива

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.

Усмерено учвршћивање

Preferred for structural castings (Нпр., 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.

Са металуршког становишта, учињење зрна, инокулација, и alloy design play critical roles in minimizing shrinkage by promoting uniform solidification and improving feedability.

5. Дизајн & 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.

На пример, а 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. Да ублажим ово:

• 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. Међутим, complex castings demand region-specific scaling:

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

Riser, Камен & 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.

Дизајн за производност

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 мм tolerance requirements

6. Симулација & 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 топле спотове и 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. Контрола квалитета & Инспекција

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

• Radiographic Inspection (Рендген): Detects internal shrinkage cavities and macro defects
• Ултразвучно тестирање (Ут): Ideal for detecting porosity and internal discontinuities in dense alloys
• Dimensional Analysis (Цмм, 3D laser scanning): Validates shrinkage allowances and conformity to specifications

Foundries also implement Контрола статистичког процеса (Спц) 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. Закључак

Understanding the various types of shrinkage in metal casting—liquid, очвршћавање, 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, дизајн, simulation, and quality control.

Foundries that embrace predictive modeling, контрола у реалном времену, и collaborative design processes are better equipped to reduce waste, optimize cost, and deliver components that meet the highest standards of performance and reliability.

У Ово, 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 салес@цанинг-цхина.цом.