Metal injection molding parts are a breakthrough manufacturing method that produces incredibly strong, complex metal components for medium to very high annual volumes. The process works with minimum wall thicknesses of 0.012″ (0.3mm) and handles maximum thicknesses up to 0.30″ (8mm).
The MIM process needs several vital steps. Manufacturers mix fine metal powders with binders to create feedstock. They inject this mixture into precisely designed molds. Next comes binder removal, followed by sintering the components at high temperatures to achieve full density. The process demands strict adherence to injection mold design guidelines, especially when you have MIM parts that shrink by approximately 15-20% during sintering. MIM gives designers more freedom than traditional metal manufacturing methods. They can break free from conventional limits while working with stainless steel, nickel iron, copper, titanium, and other metal injection molding materials.
Manufacturers and designers need to master wall thickness management, ejection techniques, feature integration strategies, and gating control to optimize their MIM molding processes. This piece explores everything in steel injection molding to help create high-quality, economical metal components.
Wall Thickness and Coring Strategies in MIM Molds
Wall thickness design is the lifeblood of successful metal injection molding parts production. The right attention to this aspect will give a dimensionally stable manufacturing process that ended up delivering high-quality finished components.
Uniform Wall Thickness for Dimensional Stability
Uniform wall thickness throughout MIM components prevents many defects. Parts with uneven walls can suffer from distortion, internal stresses, voids, cracking, and sink marks. On top of that, it becomes nowhere near possible to control dimensions when wall thickness varies because it leads to non-uniform shrinkage during sintering.
The best results come from wall thickness between 1-6mm, which depends on the overall part size. Yes, it is possible to achieve thicknesses as low as 0.025mm (0.001in) or as high as 15mm (0.6in), but these extremes need special attention. Designers should avoid sections thicker than 12.5mm (0.5in) as they cause non-uniform shrinkage and take longer to sinter.
Coring Techniques to Reduce Mass and Cost
Coring serves as an affordable strategy in injection mold design guidelines to create uniform wall thickness throughout complex parts. This method strategically removes material, which then reduces weight, material costs, and processing times.
Through holes work better than blind holes when the length-to-diameter ratio goes beyond 4:1 because the core pin gets support at both ends. Blind holes formed by pins with support at just one end might end up off-center. This happens due to deflection from the feedstock flow into the cavity. That’s why blind hole depth usually stays limited to twice the core pin’s diameter.
The best direction for cored holes runs parallel to the mold opening and perpendicular to the parting plane. Designers can improve tooling function by changing one hole to a ‘D’ shape when holes sit perpendicular to each other. This change increases strength and reduces flashing.
Transition Ratios for Varying Wall Sections
Steel injection molding works best with consistent wall thickness, but design requirements sometimes need variations. These situations call for gradual transitions between different thicknesses.
Smooth transitions help reduce stress concentrations and prevent poor surface appearance from flow lines. Adjacent walls should stay within 40-60% thickness of each other when the thickness varies. The mold needs gating at the heavier section to pack the feedstock properly throughout the cavity.
Designing for Ejection: Drafts, Parting Lines, and Ejector Pins
Metal injection molding tooling design relies heavily on well-designed ejection mechanisms. MIM brings its own advantages and design requirements to get parts out smoothly, making it different from regular plastic molding.
Draft Angles in MIM: The Right Time to Use Them
MIM parts don’t need much draft angle compared to plastic injection molded parts. Paraffin wax in MIM feedstock works as a natural release agent, which means you can skip major draft angles. Many MIM parts work fine with straight walls since they barely shrink during molding.
All the same, you might just need draft angles between 0.5° and 2° in these cases:
- Features with high aspect ratios (thin wall sections or long core pins)
- External undercuts that must release from the tool
- Longer component sections where parts resist ejection
Your draft angle choice should fit within the tolerance of the affected dimension. External dimensions work with minimal or no draft because MIM material naturally pulls away from the cavity wall as it cools.
Smart Parting Line Placement
Parting lines show up where mold halves meet, leaving visible marks on finished parts. The best parting line management focuses on:
- Keeping parting lines away from working surfaces to avoid disrupting part function
- Using clever design tricks to hide parting lines along feature edges
- Putting parting lines on sharp edges to make mismatches less obvious
Single-plane parting lines make tools simpler and cheaper. Sometimes complex parting lines cost more but unlock special features that make them worth the extra expense.
Ejector Pin Placement and Surface Effects
Ejector pins push out cooled parts and leave marks where they touch. Smart pin placement means:
- Putting pins where parts need the most force to eject (near bosses, cored holes, and ribs)
- Staying away from visible surfaces and critical functional areas
- Using bigger ejector pins spread evenly to share the load and prevent breaking
- Sleeve ejection becomes an option to minimize or remove ejector marks
Ejector pins work best by pushing rather than pulling parts during ejection. Flat surfaces make better contact points than curved areas, sharp angles, or steep slopes.
Feature Integration in Mold Design
Metal injection molding technology’s biggest advantage lies in feature integration. This allows manufacturers to create complex components that would be impossible or too expensive with regular manufacturing methods.
Molding Internal and External Threads
The MIM process can directly form both internal and external threads, which eliminates extra operations. Tapping internal threads works better and costs less than using unscrewing cores. External threads work best when placed along a parting line, so you don’t need to unscrew mold members. Thread tolerance stays accurate by using narrow flats—usually 0.005″—at the parting line. This creates a proper mold seal-off and reduces parting-line marks while minimizing flash issues in thread roots.
Undercuts and Collapsible Core Considerations
MIM handles undercut features well through collapsible cores or sliders that work for both external and internal undercuts. A split mold easily forms external undercuts on a parting line. Internal undercuts usually need slides or collapsible cores. The extra costs and possible flashing issues make these less attractive in MIM designs. While other metalworking technologies struggle with these features, metal injection molding makes them completely doable.
Ribs and Webs for Structural Reinforcement
Ribs and webs are the foundations of MIM designs. They reinforce thin walls and help avoid thick sections. These features boost strength and rigidity while improving material flow and reducing distortion. Rib thickness should stay below the adjoining wall thickness. When you need stronger supports, it’s better to use multiple ribs. This approach helps reduce mass while keeping the component strong.
Knurling, Logos, and Text Embossing in MIM
MIM parts can include decorative elements like knurls, logos, part numbers, and identification marks without extra costs. You can make these features raised or sub-surface, though raised (embossed) text looks better and reads easier. Sans-serif fonts like Century Gothic Bold give the best results. Serif fonts often have details that are too small to mill properly. A 0.2mm emboss depth works best as it shows up well without causing ejection problems.
Gating, Sink, and Flash Control in MIM Tooling
Quality gating systems and defect control techniques are the foundations of creating excellent metal injection molding parts. These elements affect part esthetics, dimensional accuracy, and production efficiency.
Edge Gate vs Submarine Gate Placement
The selection of gates requires balancing manufacturability, functionality, dimensional control, and aesthetics. Edge gates are most effective for low to medium volume applications when positioned at the parting line. You need to remove these gates manually, and they should be recessed to minimize vestige on functional surfaces. Submarine gates (or sub-gates) offer a different approach – they shear off automatically during part ejection, making them perfect for high-volume production. The best results come when gates connect to the thickest cross-section. This allows material to flow from thick to thin sections. Gating close to the center axis helps prevent sintering distortion in cylindrical components.
Sink Marks from Rib Intersections
Sink marks show up as shallow depressions on part surfaces, usually at rib intersections where thick sections form. These depressions occur due to uneven cooling. The outer surfaces solidify first, while thicker internal sections keep contracting. Rib thickness should stay between 40-60% of the adjoining wall thickness. A rib thickness of about 75% of the wall thickness will reduce sink potential effectively. This relationship minimizes mass at intersections and allows more uniform cooling rates across the part.
Flash Prevention Using Flat-on-Flat Seal-Offs
Flash creates a big problem in MIM because it forms metal burrs after sintering that are hard to remove. Several factors cause flash: parting line mismatches, worn tooling, improper clamping pressure, and material properties. Flat-on-flat seal-offs at mold interfaces will give a reliable way to prevent flash formation. This design creates precise mating surfaces that maintain tight tolerances even under high injection pressures. The seal-off areas should extend beyond potential flash zones with enough surface area to work properly.
Witness Line Management in Functional Areas
Witness lines leave unavoidable marks where mold components meet, and they affect both esthetics and functionality. Moving potential witness lines away from critical functional areas helps manage their impact. Small flats along the parting line ensure witness marks occur below the functional diameter in cylindrical components with external undercuts. Secondary tumbling operations can minimize witness line visibility in non-critical areas. This approach helps maintain component integrity while meeting aesthetic requirements.
Conclusion
Metal injection molding revolutionizes manufacturing with its unique design principles. Manufacturers who become skilled at these guidelines get major advantages when they produce complex, high-precision metal components. The right wall thickness management stops distortion and gives dimensional stability throughout the sintering process. Draft angles play a vital role for features with high aspect ratios or external undercuts that need easy release, though they’re nowhere near as large as in plastic molding.
Smart placement of parting lines, ejector pins, and gates affects both looks and performance. MIM’s feature integration capabilities set it apart from regular manufacturing methods. You can form threads, undercuts, ribs, and decorative elements directly without extra operations. The right design techniques help control sink marks, flash, and witness lines to keep component integrity while meeting visual needs.
A successful MIM mold design balances opposing elements. You need uniform wall thickness with complex geometries, tight tolerances despite 15-20% shrinkage during sintering, and multiple features that don’t compromise structural integrity. Designers must think about the whole manufacturing process from the original feedstock preparation to final sintering.
Companies following these design guidelines can tap into MIM’s exceptional ability to create intricate metal components. These components have excellent mechanical properties at medium to high volumes. MIM changes what engineers can do with stainless steel, nickel-iron, copper, titanium, and other metals. Small, complex metal parts are in high demand across industries. Metal injection molding remains a budget-friendly production choice for manufacturers who want design freedom with efficient production.
Key Takeaways
Metal injection molding (MIM) offers unprecedented design freedom for complex metal components, but success depends on following specific mold design principles that account for the unique characteristics of this manufacturing process.
• Maintain uniform wall thickness (1-6mm) throughout MIM parts to prevent distortion, internal stresses, and dimensional control issues during the 15-20% sintering shrinkage • Strategic gate placement at thickest sections and flat-on-flat seal-offs prevent flash formation and sink marks while ensuring proper material flow • MIM enables direct molding of complex features like threads, undercuts, ribs, and decorative elements without costly secondary operations • Minimal draft angles (0.5-2°) are needed due to natural mold release properties, but careful ejector pin placement prevents witness marks on functional surfaces • Proper parting line management and transition ratios between wall sections ensure both esthetic appeal and structural integrity in finished components
When executed correctly, these mold design guidelines transform MIM from a simple manufacturing process into a powerful tool for creating high-precision, cost-effective metal components that would be impossible with traditional metalworking methods.
FAQs
Q1. What are the key considerations for metal injection molding (MIM) mold design? The main considerations for MIM mold design include maintaining uniform wall thickness, strategic gate placement, proper draft angle application, careful ejector pin positioning, and effective parting line management. These factors ensure dimensional stability, prevent defects, and optimize part quality.
Q2. How does wall thickness affect MIM part quality? Wall thickness in MIM parts typically ranges from 1-6mm. Maintaining uniform thickness is crucial to prevent distortion, internal stresses, and dimensional control issues during the 15-20% shrinkage that occurs during sintering. Variations in wall thickness can lead to non-uniform shrinkage and defects.
Q3. Can complex features be directly molded in MIM? Yes, MIM allows for direct molding of complex features such as threads, undercuts, ribs, and decorative elements without requiring costly secondary operations. This capability gives designers greater freedom compared to traditional metalworking methods.
Q4. How are draft angles used in MIM compared to plastic injection molding? MIM typically requires minimal draft angles (0.5-2°) compared to plastic injection molding due to the natural mold release properties of the feedstock. However, draft angles are still necessary for features with high aspect ratios or external undercuts to ensure easy part release.
Q5. What techniques are used to prevent flash in MIM parts? To prevent flash in MIM parts, designers implement flat-on-flat seal-offs at mold interfaces. This creates precise mating surfaces that maintain tight tolerances even under high injection pressures. Additionally, proper gate placement and clamping pressure control help minimize flash formation.