MIM Design Guide — Engineering Your Part for Metal Injection Molding
Quick Answer
Designing for metal injection molding (MIM) means accounting for 15–20% linear shrinkage during sintering, maintaining uniform wall thickness, adding 0.5°–2° draft, avoiding sharp internal corners, and locating the parting line on a flat plane. Parts weighing 0.1 g to 200 g with wall thickness between 0.3 mm and 5 mm are ideal. At Emitech, every design receives a free DFM review before tooling begins, helping engineers eliminate warpage, cracking, sink, and dimensional drift before metal is cut. Upload your CAD model for engineering feedback within 48 hours.
Metal injection molding bridges plastic injection molding and powder metallurgy. It offers the geometric freedom of molding with the mechanical properties of wrought metal, but only when the part is designed with the process in mind. This MIM design guide explains the rules our engineers apply to every project, from material and wall-thickness selection to shrinkage compensation and gate location.
MIM design must account for both the molded "green" state and the final sintered metal part.
Is MIM the Right Process for Your Component?
Before optimizing a design for MIM, confirm that the process matches the part geometry, annual volume, and performance requirements. MIM is not a universal replacement for machining or casting; it excels in a specific envelope.
Choose MIM When:
- Geometry is complex. The part requires 5 or more CNC setups, has internal threads, undercuts, thin walls, or fine surface detail.
- Annual volume is high. Production exceeds 10,000–20,000 pieces per year so tooling costs amortize across many units.
- Part size is small. Weight is between 0.1 g and 200 g, with ideal range of 0.5 g to 50 g.
- Material is MIM-compatible. Stainless steel, low-alloy steel, titanium, cobalt chrome, Kovar, and soft magnetic alloys are common choices.
- Surface finish and tolerances fit. As-sintered Ra 0.8–3.2 μm and ±0.3% to ±0.5% tolerance are acceptable, or tight features can be finished by CNC.
Choose CNC Machining Instead When:
- Volume is low. Prototype or low-volume runs where tooling cost cannot be justified.
- Tolerances are extremely tight. Features requiring ±0.01 mm or better without secondary operations.
- Part is large. Dimensions exceed approximately 150 mm in any direction.
Many projects use a hybrid approach: MIM for the net shape and CNC for critical finishing. This balances material efficiency with precision.
5 Critical MIM Design Parameters
The five parameters below account for the majority of MIM defects. Addressing them early reduces tooling iterations, shortens first-article lead time, and improves production yield.
1. Wall Thickness
Uniform wall thickness is the single most important rule in MIM design. Thick sections shrink more than thin sections during sintering, causing warpage, cracking, and dimensional variation. Walls should generally be between 0.3 mm and 5 mm, with gradual transitions between sections. For parts below this window — walls of 0.1–0.3 mm at millimeter scale — see our micro MIM capability page.
- Minimum wall: 0.3 mm for simple geometries, 0.5 mm recommended
- Maximum wall: 5 mm; thicker sections may require coring or redesign
- Transition ratio: Avoid wall-thickness changes greater than 2:1
- Use ribs and gussets for stiffness instead of solid thick sections
2. Radii and Fillets
Sharp internal corners concentrate stress and impede feedstock flow. Generous radii improve molding, reduce stress, and prevent cracks during debinding and sintering.
- Internal radius minimum: 0.2 mm, preferably 0.5 mm or larger
- External radius: Add where possible to aid ejection
- Root fillets on gears and threads: Increase bending fatigue life
3. Draft Angles
Draft helps the green part release from the mold without damage. Vertical walls should include 0.5° to 2° of draft. Zero-draft features are possible but increase tool complexity and cost.
- Standard draft: 1° per side on vertical walls
- Deep cores: 2° or more to prevent green-part cracking during ejection
- Cosmetic surfaces: Draft may be minimized with polished tooling
4. Parting Line Location
The parting line should be placed on a flat plane to simplify tooling and minimize flash. Complex parting lines increase mold cost and create flash that requires secondary cleaning.
- Best practice: Flat, straight parting line on the largest cross-section
- Avoid: Parting lines across cosmetic surfaces or critical sealing faces
- Consider: Secondary machining for sealing surfaces when necessary
5. Undercuts, Cores, and Threads
MIM can produce internal threads, cross-holes, and undercuts using side cores or collapsible cores. Each core action adds tooling cost and cycle time, so keep core geometry as simple as the function allows.
- Internal threads: MIM threads typically require post-machining for tight fits
- Cross-holes: Possible but may create weld lines; place away from high-stress zones
- Side cores: Use only when essential; minimum core diameter typically 1 mm
Shrinkage Compensation & Mold Oversize
During sintering, the brown part densifies and shrinks by approximately 15–20% linearly. The mold cavity is intentionally oversized by this amount so the final part meets the drawing dimensions. Shrinkage is not uniform in every direction; it depends on material, wall thickness, part geometry, and sintering profile.
| Material Family | Typical Linear Shrinkage | Mold Oversize Factor |
|---|---|---|
| 316L stainless steel | 15–17% | 1.18–1.20× |
| 17-4 PH stainless steel | 16–18% | 1.19–1.22× |
| Low-alloy steels (4605, 8620) | 17–19% | 1.20–1.23× |
| Titanium Ti-6Al-4V | 14–16% | 1.16–1.19× |
| Cobalt chrome F75 | 16–18% | 1.19–1.22× |
Emitech uses material-specific shrinkage factors calibrated from production data. Moldflow simulation and first-article sampling validate the shrinkage model before mass production begins.
Common MIM Defects & Prevention
Most MIM defects are traceable to design, tooling, or process parameters. The table below maps common problems to their root causes and design-side prevention strategies.
| Defect | Root Cause | Prevention |
|---|---|---|
| Warpage | Uneven wall thickness, differential shrinkage | Uniform walls, gradual transitions, balanced ribs |
| Cracking | Sharp corners, thick sections, rapid debinding | Generous radii, coring, controlled debinding profile |
| Sink marks | Thick sections cooling slower than thin walls | Core out thick areas, maintain wall uniformity |
| Porosity / blisters | Incomplete debinding, trapped binder | Adequate venting, gradual thermal debinding |
| Dimensional drift | Incorrect shrinkage factor, uneven heating | Calibrate shrinkage, control sintering atmosphere |
| Flash | High injection pressure, poor parting-line fit | Flat parting line, proper clamping, optimized pressure |
Material Selection Impact on Design
Material choice affects shrinkage, strength, corrosion resistance, magnetic properties, and biocompatibility. The design should be optimized for the selected feedstock, not the other way around.
- Stainless steels (316L, 304, 17-4PH): Good corrosion resistance and strength; 17-4PH can be precipitation hardened for higher strength.
- Low-alloy steels (4605, 8620, 4140): Heat-treatable for high wear resistance; ideal for gears and locking parts.
- Titanium Ti-6Al-4V: Excellent strength-to-weight and biocompatibility; requires controlled sintering atmosphere.
- Cobalt chrome F75: High wear resistance and biocompatibility; common for orthopedic implants.
- Soft magnetic alloys (Fe-2Ni, Fe-8Ni, Fe50Co): Controlled permeability and density for electromagnetic applications.
For a full material comparison, visit our MIM materials page.
Economic Break-Even: MIM vs CNC Machining
MIM requires upfront tooling investment, so it is not always the lowest-cost choice at low volumes. The break-even point depends on part complexity, material, and annual quantity.
| Factor | CNC Machining | MIM |
|---|---|---|
| Tooling cost | Low to none | $15,000–$100,000+ |
| Part cost at low volume | Lower | Higher due to tooling amortization |
| Part cost at high volume | Stable or higher | Significantly lower |
| Geometric complexity | Adds setup cost | Little impact after tooling |
| Material waste | High (subtractive) | Low (near-net-shape) |
| Typical break-even volume | 10,000–50,000 pieces per year for complex parts | |
For a detailed process comparison, see MIM vs CNC machining.
MIM Tolerances & Secondary Operations
As-sintered MIM typically achieves ±0.3% to ±0.5% of dimension. Features requiring tighter tolerances can be finished by CNC machining, grinding, coining, or honing after sintering.
- As-sintered: ±0.3%–±0.5%
- Post-machining: ±0.02 mm or tighter
- Surface finish as-sintered: Ra 0.8–3.2 μm
- Surface finish after polishing: Ra 0.2–0.4 μm
Specify which dimensions are critical and which can remain as-sintered. This helps Emitech target secondary operations only where necessary, reducing cost.
Frequently Asked Questions
Q: What is the ideal wall thickness for MIM parts?
The ideal wall thickness is 0.3 mm to 5 mm, with 0.5 mm to 3 mm being the most reliable range. Uniform walls prevent warpage and cracking during sintering — design rules in our wall thickness & coring guide.
Q: How much does MIM shrink during sintering?
Linear shrinkage is typically 15–20%, depending on material and geometry. The mold cavity is oversized by this factor to produce a final part that meets the drawing. Shrinkage control details: MIM shrinkage explained.
Q: Can MIM parts have threads?
Yes, but molded threads are usually coarse and may require post-machining for precision fits. Fine threads and tight tolerances are best achieved by CNC secondary operations.
Q: What draft angle is recommended for MIM?
Standard draft is 0.5° to 2° per side on vertical walls. Deep cores and long draws benefit from larger draft angles to prevent green-part damage.
Q: Can MIM produce undercuts?
Yes, using side cores or collapsible cores. However, each core action adds tooling cost and cycle time, so undercuts should be minimized. See our undercut design guide for solutions.
Q: What tolerances can MIM hold without machining?
As-sintered tolerances are typically ±0.3% to ±0.5% of dimension. Tighter tolerances require post-sintering CNC machining or grinding — full data on our MIM tolerance page.
Q: At what volume does MIM become cheaper than CNC?
For complex parts, the break-even point is usually between 10,000 and 50,000 pieces per year. Simple parts may favor machining at even higher volumes — cost breakdown in our MIM cost guide.
Q: What file format should I send for a DFM review?
STEP, IGES, SolidWorks, or PDF drawings are preferred. Include material, annual volume, critical tolerances, and surface-finish requirements for the most useful feedback.
Get a Free MIM DFM Review
Send your CAD model, drawing, or sample photo to Emitech's engineering team. We will review your design for wall thickness, shrinkage compensation, draft angles, radii, parting line, and undercuts, and return a DFM report with recommended changes within 48 hours.
Free MIM Design Review
Upload your drawing and receive DFM feedback, material recommendation, and quote within 48 hours.
Submit Your Design →Source Custom MIM Parts from Emitech
Nanjing Emitech (ISO 9001:2015) delivers MIM from tooling through sintering and finishing, plus precision CNC machining for prototypes and secondary operations. Custom MIM parts · CNC machining services · MIM services · Request a quote