Metal Injection Molding Design Guidelines: Expert Solutions for Complex Parts

Metal injection molding design guidelines show why this manufacturing process stands out. MIM parts achieve impressive density levels of 94-97% and prove stronger than traditional pressed metal components. Engineers can create complex parts with undercuts, threads, and recessed features that would be tough to make through standard metalworking.

MIM brings together the best of plastic injection molding and powdered metallurgy. This combination makes it perfect for use in various industries like automotive, aerospace, medical, and consumer goods. The wall thickness can range from 0.012 inches (0.3 mm) to 0.30 inches (8 mm), while holding tight tolerances of ±0.3% of nominal dimensions. On top of that, it lets manufacturers add knurling, lettering, and logos right into the part without extra processing. The process also cuts assembly costs by combining multiple components into one solid piece that’s even stronger than before.

Designing for Complex Metal Injection Molding Parts

MIM process requires careful planning of geometry, wall thickness, and structural features to design complex parts. These injection molding design guidelines will help you tap into the full potential of this advanced manufacturing method.

Part Consolidation: Reducing Assembly Steps

MIM metal injection molding lets you combine multiple components into a single part. This approach:

• Cuts down assembly operations and related costs
• Makes supply chain simpler
• Creates more consistent and stronger parts

Real-world applications show remarkable results. An automotive manufacturer reshaped the scene by turning a 20-component assembly into a single MIM part. This led to 50% cost savings with better precision. The combined parts turned out stronger and matched the original design better. Another example showed how turning a four-component assembly into one MIM part cut out three assembly steps and their costs.

Mass Reduction with Coring and Ribs

Metal injection molding design guides focus on cutting mass through smart coring and ribbing. Coring holes work in several ways:

1. They line up cross-sections to keep uniform wall thickness
2. They use less material
3. They need less or no secondary machining

Best results come when coring holes run parallel to the mold opening direction and perpendicular to the parting plane. Through holes work better when the length-to-diameter ratio goes beyond 4:1. This happens because the core pin gets support at both ends instead of just one side.

Ribs and webs offer another quick way to strengthen metal injection molding parts. These features make thin walls stronger without creating thick sections that cause problems. They also help material flow better during molding, reduce distortion during sintering, and make the final part stronger.

Balancing Strength and Material Usage

MIM design works best with uniform wall thickness throughout the component. This prevents many possible defects:

• Parts getting twisted and stressed inside
• Holes and cracks forming
• Sink marks and uneven shrinkage

Wall thickness should stay between 1.3-6.3 mm (0.05-0.25 inches), though you can go outside this range sometimes. Areas needing extra strength should use multiple thin ribs rather than one thick rib. Successful injection molding design finds the sweet spot between using material efficiently and making parts strong enough.

Sometimes you can’t avoid varying wall thickness. In these cases, make smooth transitions between sections. This cuts down stress points and lets material flow properly during molding. Following these guidelines creates metal injection molded parts that are both strong and material-efficient.

Injection Molding Guidelines for Functional Features

MIM components need strict dimensional parameters to work properly. These specs ensure the material flows well, maintains structural integrity, and can be manufactured easily.

Wall Thickness Limits: 0.3mm to 8mm

Metal injection molding parts work best with wall thickness between 0.3mm (0.012″) and 8mm (0.30″) maximum. You must keep the wall thickness uniform throughout the component to avoid defects. Walls thinner than 0.3mm are hard to fill and pack completely because high shear forces limit flow or trap air. You should avoid sections over 12.5mm (0.5″) as they cause uneven shrinkage and take longer to sinter. The sweet spot for wall thickness lies between 1mm and 6mm, depending on the part size.

Minimum Feature Size: 0.3mm

MIM technology can create features as tiny as 0.3mm (0.012″) reliably. This lets you add intricate details that would be tough to make using regular manufacturing methods. But features smaller than this might warp during sintering. Designing with this minimum size helps ensure your part can be manufactured while working as intended.

Corner Radii: Minimum 0.1mm Recommended

While you can technically achieve a 0.1mm (0.004″) corner radius, bigger radii make better quality parts and your mold lasts longer. Sharp corners inside often create empty spaces during injection. Sharp corners outside mean you need hard-to-make sharp internal corners in your tools. A practical rule is to keep edge radii above 0.127-0.152mm (0.005-0.006″). Your best bet is to use radii of 0.2mm or more to get better injection quality and avoid defects.

Draft Angles: 1° to 5° for Ejection Ease

Draft angles help parts come out of the mold smoothly. You should use between 1° and 5°, based on:

• Surface texture – Smooth surfaces just need 1-2°, light textures need 3°, and heavy textures need 5° or more
• Cavity depth – A good rule is to use 1° for every inch (25mm) of cavity depth
• Feature length – Long parts without enough draft might get stuck during ejection

Adding even tiny 0.5° draft angles works much better than none at all, especially when you want to keep good surface finish quality and make your tools last longer.

Tooling and Mold Design for MIM

The foundations of successful MIM components lie in proper tooling design. This approach deals with everything from material flow to part ejection. The quality of parts and manufacturing efficiency depend on gate selection, ejector placement, and actuation systems.

Gate Types: Edge, Submarine, and Removable Post

Gates let molten feedstock enter the mold cavity, and each type brings specific advantages. Edge gates are the most common design. They sit at the parting line with larger cross-sections that help material flow. Their simple design makes them perfect for flat parts with medium or thick sections. Submarine gates sit below the parting line. They allow automatic trimming during component ejection through a tapered channel. These gates work great for smaller components but might take longer with larger parts. Removable post gates work like direct sprue gates. They sit at the broadest cross-section of a part with a circular vestige. The filling should be as close to the center axis as possible for cylindrical parts to prevent sintering distortion.

Ejector Pin and Parting Line Placement

Ejector pins need careful placement to reduce their effect on part appearance and function. These cylindrical components push to remove finished parts from the mold core. A minimum distance of 3.5mm should exist between waterlines and ejector pins to avoid interference. The pins work best on flat surfaces. They should stay away from R angles, sharp angles, steep slopes, and areas near gate positions. Parting lines create witness marks where mold halves meet. Designers can hide these marks by stepping the parting line down feature edges or placing them on non-cosmetic surfaces.

Flash and Witness Line Management

Flash appears when plastic melt enters mold gaps and creates extra material along part edges. This issue cannot be eliminated. High-requirement products should not have flash over 0.05mm. Products with lower requirements can accept up to 0.1mm. Too much injection pressure, weak clamping force, and poor mold parallelism lead to flash. Better processing precision, thicker mold plates, and adjusted product placement in the mold can help manage this issue.

Use of Hydraulic and Electric Actuators

Modern MIM tooling now uses electric servo motors more than traditional hydraulic systems, especially in medical cleanroom applications. Servo motors keep things cleaner without leak risks. They use power only when needed, unlike always-running hydraulic systems, and work accurately within microns. Hydraulic systems still excel for slides, core pulls, and ejection systems where high force matters. Both options need reliable control systems, sensors, and locking mechanisms to stay still during high-pressure injection phases.

Post-Sintering Operations and Tolerances

MIM components’ final quality relies on post-sintering operations that meet exact specifications. Manufacturers use several techniques to boost performance characteristics and dimensional accuracy after the main processing.

As-Sintered Tolerances: ±0.3% of Nominal

MIM parts reach tolerances of ±0.3% to ±0.5% of nominal dimensions after sintering. Part geometry, material choice, and processing conditions affect this precision level. Complex parts with multiple dimensions have tolerance capabilities that depend on specific features and design configuration. Most components reach their final dimensions right after sintering without extra processing. Additional operations become essential when applications just need greater precision.

Heat Treatment for Strength and Hardness

Metal injection molded components exist in an annealed state after sintering. Heat treatment changes their microstructure to boost physical properties like shear strength and wear resistance. This vital process has four stages: sintering to bond and densify, annealing to relieve stress, hardening to improve wear resistance, and tempering to boost toughness. Heat treatment methods use specialized equipment such as sealed quench furnaces, ECM furnaces, vacuum heat treatment furnaces, and tempering furnaces. Steel grades like SS 17-4PH, M2, S7, 4605, SS-420, and HK 30 need heat treatment to achieve their best mechanical properties.

Surface Finishing Options and Roughness Levels

MIM parts have a sintered surface finish with a mean roughness of 32 millionths of an inch (0.8128 micrometers). Secondary operations can improve this to 16 millionths of an inch (0.4064 micrometers). Surface finishing techniques include:

• Disk finishing, vibro finishing, and magnetic deburring to get uniform smoothness
• Chemical treatments, bead blasting, and polishing to enhance aesthetics
• Passivation and electroplating to get specific surface properties

These processes remove ejector pin marks, gate marks, and parting lines left after molding.

Machining and Tapping for Tight Tolerances

Some applications need secondary machining to achieve very tight tolerances, despite MIM’s impressive precision. Turning, milling, honing, lapping, and electrochemical machining create features with precision beyond standard MIM capabilities. Tapping creates internal threads with better tolerances than direct MIM processing. MIM materials work well with standard machining techniques, which means manufacturers can perform all regular operations just like wrought metals. Manufacturers can add special features like undercuts or grooves that would be hard to include in tooling.

Conclusion

Metal Injection Molding revolutionizes manufacturing capabilities for complex metal components in many industries. This piece explores complete design guidelines that help engineers realize the full potential of this sophisticated process. Without doubt, MIM technology brings remarkable advantages. Parts reach density levels of 94-97%, show superior strength, and can create intricate geometries that conventional metalworking methods can’t achieve.

Success in MIM design starts with basic principles. Engineers must maintain uniform wall thickness, consolidate parts strategically, and reduce mass through coring and ribbing. These approaches cut material use and eliminate assembly steps while boosting structural integrity. Wall thickness ranges from 0.3mm to 8mm, with specific minimum feature sizes and draft angles. These parameters ensure optimal material flow and ease of manufacturing.

On top of that, proper tooling design creates the foundation for quality MIM production. Gate selection, ejector pin placement, and parting line management affect part quality and streamline processes. Parts achieve tolerances of ±0.3% to ±0.5% of nominal dimensions after sintering. Secondary operations can boost precision for applications that need tighter specs.

MIM offers a versatile manufacturing solution. It combines plastic injection molding’s geometric freedom with traditional metal components’ superior performance. The process needs careful attention to design guidelines. Yet the parts deliver great value through fewer assembly steps, better functionality, and smart material use. Engineers who become skilled at these design principles gain a competitive edge. They can produce complex, high-performance metal components that serve demanding applications in automotive, aerospace, medical, and consumer goods industries.

FAQs

Q1. What are the key advantages of Metal Injection Molding (MIM)?

Metal Injection Molding offers high part density (94-97%), superior strength, and the ability to produce complex geometries with features like undercuts and threads. It combines the benefits of plastic injection molding and powdered metallurgy, making it ideal for high-performance applications across various industries.

Q2. What is the recommended wall thickness range for MIM parts?

The ideal wall thickness for Metal Injection Molding parts ranges from 0.3mm to 8mm. However, for optimal results, a thickness between 1mm and 6mm is preferred, depending on the overall part size.

Q3. How does part consolidation benefit MIM manufacturing?

Part consolidation in MIM allows multiple components to be combined into a single part. This approach eliminates assembly operations, reduces supply chain complexity, improves part consistency, and enhances overall strength while potentially reducing costs.

Q4. What post-sintering operations are commonly used in MIM?

Common post-sintering operations include heat treatment for improving strength and hardness, surface finishing techniques like disk finishing and polishing, and secondary machining for achieving tighter tolerances. These processes enhance the performance characteristics and dimensional accuracy of MIM parts.

Q5. What tolerances can be achieved with Metal Injection Molding?

Typically, MIM parts achieve tolerances of ±0.3% to ±0.5% of nominal dimensions after sintering. However, secondary operations can be employed when applications require greater precision to achieve tighter tolerances.