Introduction to MIM Process
Metal Injection Molding (MIM) is a manufacturing process that combines powdered metallurgy with plastic injection molding. The process enables production of complex, high-precision metal parts in large volumes with excellent mechanical properties.
This guide walks through each step of the MIM process, from raw materials to finished parts.
Step 1: Feedstock Preparation
The first step in MIM is preparing the feedstock, which is a mixture of metal powder and polymer binder.
Metal Powder Selection
Powder Characteristics
Particle size: 5-20μm (fine powder for MIM)
Particle shape: spherical for optimal flow
Purity: high purity for consistent properties
Common Powders
316L stainless steel
17-4PH stainless steel
Ti-6Al-4V titanium
Fe-2Ni low alloy steel
Binder System
The binder system holds the powder particles together during molding and is removed during debinding.
Binder Components
Primary binder: polyethylene or polypropylene
Secondary binder: wax for debinding control
Additives: stearic acid for lubrication
Feedstock Composition
Metal powder: 55-65% by volume
Binder: 35-45% by volume
Mixed in twin-screw extruder at elevated temperature
Step 2: Injection Molding
The feedstock is injection molded into green parts using standard injection molding equipment.
Molding Parameters
Temperature
Barrel temperature: 120-180°C
Mold temperature: 40-80°C
Pressure
Injection pressure: 50-150 MPa
Holding pressure: 30-100 MPa
Cycle Time
Typical cycle: 30-120 seconds
Depends on part size and complexity
Green Part Characteristics
Green parts are the as-molded components before debinding and sintering.
Properties
Density: 55-65% of theoretical density
Strength: sufficient for handling
Dimensions: 1.15-1.20x final size (accounts for shrinkage)
Step 3: Debinding
Debinding removes the binder system from green parts, leaving a porous brown part.
Debinding Methods
Solvent Debinding
Solvent: heptane or other organic solvents
Time: 2-4 hours
Removes primary binder
Thermal Debinding
Temperature: 200-400°C
Time: 12-24 hours
Removes remaining binder
Catalytic Debinding
Catalyst: nitric acid vapor
Time: 4-8 hours
Fast debinding for specific binders
Brown Part Characteristics
Brown parts are the debound components before sintering.
Properties
Density: 50-60% of theoretical density
Porous structure
Fragile, requires careful handling
Step 4: Sintering
Sintering densifies the brown parts by heating them to high temperatures in controlled atmosphere.
Sintering Parameters
Temperature
Stainless steel: 1300-1400°C
Titanium: 1200-1300°C
Tool steel: 1100-1200°C
Atmosphere
Hydrogen: for stainless steel
Vacuum: for titanium and reactive metals
Nitrogen: for some alloy steels
Time
Soak time: 2-4 hours
Total cycle: 8-16 hours
Sintering Mechanisms
Densification
Particle bonding at elevated temperature
Pore elimination
Grain growth
Shrinkage
Uniform shrinkage: 15-20% linear
Isotropic shrinkage for consistent dimensions
Sintered Part Characteristics
Sintered parts are the final components with full density and properties.
Properties
Density: 95-99% of theoretical density
Mechanical properties: comparable to wrought material
Dimensions: within ±0.3% tolerance
Step 5: Secondary Operations
Secondary operations may be required for certain applications.
Common Operations
Heat treatment: for hardness/strength
Surface finishing: polishing, plating, coating
Machining: for tight tolerance features
Inspection: dimensional and material verification
Quality Control
Quality control is integrated throughout the MIM process.
Key Checks
Powder characterization
Feedstock viscosity
Green part dimensions
Brown part integrity
Sintered part properties
Final inspection
Conclusion
The MIM process enables production of complex, high-precision metal parts with excellent mechanical properties. Each step requires careful control to achieve consistent quality. Contact BRM engineering team for process optimization and technical support.