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Principles and Control Technologies of MIM Molding Shrinkage

Views: 24 Author: Site Editor Publish Time: 2025-05-30 Origin: Site

I. Fundamental Principles of MIM Molding Shrinkage

The shrinkage process in Metal Injection Molding (MIM) follows a typical three-stage dynamic evolution:

Primary Shrinkage Stage:
During injection cooling, volume change occurs in the binder system due to differences in thermal expansion coefficients, with a shrinkage rate of 1%-3%. This stage is primarily influenced by the melt cooling rate and mold temperature field distribution.
Secondary Shrinkage Stage:
In the debinding process, gradient removal of the binder leads to reconstruction of the porous skeleton, with a shrinkage rate of 3%-5%. This is positively correlated with the temperature-rate curve of the debinding process.
Ultimate Shrinkage Stage:
During high-temperature sintering, metal particles achieve densification through grain boundary diffusion and plastic flow, with a shrinkage rate of 15%-20%. This stage is critical for determining final dimensional accuracy.

Engineering Data Validation:
The total shrinkage rate is 20%-25% of the raw feedstock volume. Through multi-physics field collaborative control, the anisotropic shrinkage rate difference can be controlled within ±0.3%, meeting the ISO 2768-mK class precision tolerance requirements.

II. Key Factors Influencing Shrinkage Rate

2.1 Material Parameter Control

Parameter Category	Technical Indicators	Mechanism Description
Metal Powder Properties	Particle size distribution D10-D90, sphericity >0.9	Fine powder (D50 <10μm) enhances sintering driving force; spherical powder (sphericity >0.9) improves shrinkage uniformity.
Feedstock Formula	Solid content 92-96vol%, binder system	For every 1vol% increase in solid content, the sintering shrinkage rate decreases by 0.8%.
Powder Loading	Volume ratio ≥90% of theoretical value	Critical loading must exceed the densification threshold to suppress void formation.

2.2 Process Parameter Matrix

Parameter Category	Typical Process Window	Shrinkage Influence Weight	Action Mechanism
Injection Temperature	160-200℃	25%	Regulates melt viscosity, affecting initial density uniformity.
Mold Temperature	40-80℃	15%	Controls cooling gradient to reduce internal stress concentration.
Holding Pressure	60-100MPa	20%	Dynamic feeding determines green compact density.
Sintering Temperature	1300-1500℃	40%	Activates atomic diffusion, dominating the densification process.

2.3 Product Structure Design Guidelines

Wall Thickness Uniformity: Recommended wall thickness 2-5mm; non-uniform wall thickness requires a 1:50 transition slope.
Feature Size Ratio: Length-diameter ratio L/T <20:1 to avoid anisotropic shrinkage induced by slender structures.
Fillet Design: Transition fillet R >0.3mm to eliminate local shrinkage anomalies caused by stress concentration.

III. Engineering Technology System for Shrinkage Control

3.1 Prediction Modeling Technology

Theoretical Models:

Anisotropic shrinkage prediction based on the modified Tandon-Weng model, with an error ≤±1.2%.
Sintering shrinkage kinetics model built using the Arrhenius equation, with a goodness of fit R² >0.95.

Numerical Simulation:

Multi-physics field coupling simulation using COMSOL Multiphysics to achieve visual prediction of shrinkage nephograms.

3.2 Dimensional Compensation Technology Solutions

Compensation Method	Application Scenario	Technical Parameters	Compensation Accuracy
Mold Enlargement Coefficient	Conventional symmetric parts	1.15-1.25 (based on material certification)	±0.5%
Non-uniform Compensation	Heteromorphic complex parts	Local enlargement coefficient difference ≤5%	±0.3%
Iterative Correction Method	High-precision parts	Trial mold-inspection-correction cycle ≥3 times	Final tolerance ≤±0.05mm

3.3 Process Monitoring System

On-line Detection: Laser scanning diameter gauge (10kHz sampling) for real-time tracking of shrinkage dynamic curves.
High-temperature Monitoring: Infrared thermal imaging and high-speed camera systems to capture transient sintering deformation processes.
Statistical Control: Process capability analysis based on SPC, requiring Cpk ≥1.33.

IV. Shrinkage Characteristics of Typical Material Systems

Material Category	Linear Shrinkage Rate (%)	Anisotropic Ratio	Shrinkage Stability
17-4PH Stainless Steel	20.5±0.3	1.02	±0.3% fluctuation
Ti-6Al-4V Titanium Alloy	22.8±0.5	1.05	±0.5% fluctuation
WC-Co Cemented Carbide	18.2±0.2	1.01	±0.2% fluctuation

Data Source: MIMO Metal Materials Laboratory (CNAS certified), tests conducted in accordance with ISO 2740 standards.

V. Industry Application Standards and Cutting-edge Technologies

5.1 International Standard Systems

MPIF Standard 35: Specifications for MIM Material Properties and Test Methods.
ISO 2740: Standard for Density and Shrinkage Measurement of Sintered Metal Materials.
ASTM F2885: Biocompatibility and Precision Specifications for Medical MIM Components.

5.2 Technological Innovation Advances

Nano-powder Application: 50-100nm ultra-fine powder systems reduce shrinkage by 15% and enhance mechanical properties by 20%.
Microwave-assisted Sintering: Selective heating technology shortens the sintering cycle by 30% and reduces thermal stress deformation by 40%.
AI Process Optimization: Machine learning-based parameter prediction models improve the yield of complex parts by 5%-8%.

Technical Statement: The data in this white paper is derived from the R&D center of MIMO Metal Technology (ISO 9001 certified laboratory). Practical applications require process verification based on specific working conditions.