MIM Solution for Automotive Sensor Housings: From Prototype to 500K Units

Project Background

A leading automotive sensor manufacturer approached BRM with a challenge: their existing sensor housing was assembled from four separately machined components, resulting in high assembly costs, inconsistent sealing quality, and supply chain complexity. The target application was an oxygen sensor housing for next-generation exhaust systems, requiring production of 500,000 units annually.

Requirements Analysis

Technical Requirements

The sensor housing needed to meet demanding specifications:

Parameter	Requirement	Achieved
Material	316L Stainless Steel	316L MIM, ≥97% density
Outer Diameter	18.00 ± 0.05mm	18.00 ± 0.03mm
Inner Bore	8.00 ± 0.05mm	8.00 ± 0.03mm
Wall Thickness	1.5mm nominal	1.50 ± 0.05mm
Thread	M12 × 1.25 internal	Molded M12 × 1.25, 6H
Sealing Surface	Ra ≤ 0.8 μm	Ra 0.6 μm (as-sintered + vibratory)
Annual Volume	500,000 units	500,000+ units/year

Project Challenges

1. Multi-part assembly — four components required welding and brazing, introducing leak paths
2. Tight tolerance — sealing surface required Ra ≤ 0.8 μm on a complex geometry
3. Material certification — automotive grade required full material traceability per IATF 16949
4. Cost target — per-part cost needed to be under $2.50 at volume

Solution

Process Selection: MIM Over CNC

After evaluating CNC machining, precision casting, and MIM, BRM recommended the MIM process for several reasons:

• Geometry consolidation: Four separate parts consolidated into a single MIM component, eliminating three weld joints and associated leak paths
• Tolerance capability: MIM's ±0.3% tolerance met all critical dimensions without secondary machining
• Volume economics: At 500,000 units/year, MIM's per-part cost was 40% lower than CNC machining
• Material properties: Sintered 316L achieved equivalent corrosion resistance to wrought material

Key Technical Parameters

The MIM process for this project involved:

Process Step	Parameter	Value
Feedstock	316L powder + binder	63% powder loading by volume
Injection Pressure	Barrel temperature	150-180°C
Debinding	Catalytic + thermal	2-stage, 16-hour cycle
Sintering	Temperature / Atmosphere	1360°C, hydrogen atmosphere
Sintering Time	Hold time	2 hours at peak temperature
Final Density	Percentage of theoretical	97.5% (7.92 g/cm³)

Innovation: Molded-In Thread

Instead of tapping threads after sintering (which adds cost and tool wear), BRM designed a specialized mold insert that formed the M12 × 1.25 internal thread directly during injection molding. This eliminated a secondary operation and improved thread consistency across all 500,000 units.

Implementation Process

Phase 1: Design and Mold Development (Weeks 1-6)

BRM's engineering team performed a comprehensive DFM review, optimizing wall thickness transitions and adding draft angles for mold ejection. The mold was designed with a 4-cavity configuration to meet volume requirements.

Phase 2: Prototype and Validation (Weeks 7-10)

Four prototype runs were produced, with dimensional inspection on 50 parts per run. Key findings:

• Run 1: Slight warpage on sealing surface — mold temperature profile optimized
• Run 2: Thread fill incomplete — injection pressure increased by 15%
• Run 3: All dimensions within tolerance — surface finish verified
• Run 4: Full validation — 50 parts measured, Cpk > 1.67 on all critical dimensions

Phase 3: Production Ramp-Up (Weeks 11-16)

Production ramped from 10,000 to 500,000 units over 6 weeks. Quality control included:

• 100% visual inspection on first 10,000 units
• Statistical sampling (AQL 0.65) for ongoing production
• Monthly material testing for density, hardness, and corrosion resistance

Results

Quantitative Outcomes

Metric	Before (4-Part Assembly)	After (Single MIM Part)	Improvement
Parts per Assembly	4	1	75% reduction
Assembly Operations	3 welds + 1 braze	0	Eliminated
Per-Part Cost	$4.20	$2.15	49% reduction
Leak Rate	2.3%	0.05%	98% improvement
Cycle Time	12 minutes	45 seconds	94% reduction
Annual Cost	$2,100,000	$1,075,000	$1,025,000 savings

Customer Feedback

"BRM's MIM solution consolidated four parts into one, eliminated our leak issues, and cut our per-unit cost by nearly half. The quality consistency across 500,000 units has been exceptional." — VP of Manufacturing, automotive sensor manufacturer

FAQ

Q: What materials were considered for this application? A: 316L stainless steel was specified by the customer for corrosion resistance in exhaust environments. Alternative materials like 17-4 PH could be used if higher strength is needed. Q: What is the typical lead time for production? A: Mold development takes 6-8 weeks. Once validated, production lead time is 4-6 weeks for 500,000 units with a 4-cavity mold. Q: Can MIM achieve the required surface finish? A: Yes. As-sintered surface finish of Ra 1.0-2.0 μm can be improved to Ra 0.4-0.8 μm through vibratory finishing or tumbling, meeting the sealing surface requirement.

Lessons Learned

1. Early DFM involvement prevented costly mold modifications — the initial design had three areas of inconsistent wall thickness that were corrected before tooling
2. Molded-in threads saved $0.35 per part versus post-sinter tapping — a significant saving at 500,000 units
3. Multi-cavity mold justified the higher initial cost through production efficiency — the 4-cavity mold paid for itself within 100,000 units
4. Statistical process control ensured consistent quality — Cpk monitoring caught a gradual mold wear trend before it affected part quality

For similar multi-part consolidation projects, contact BRM's engineering team for a free DFM assessment.