Lock Latch MIM vs Die Casting: Process and Cost Comparison
Manufacturing Process Selection for Steel Lock Latches
A security hardware company developing a new line of high-security deadbolt locks required 150,000 steel lock latches per year. The latch design featured a 15 mm wide bolt face with a hardened steel roller insert, an 8 mm diameter actuating post, and side grooves for a retaining clip. The material specification was 17-4PH stainless steel for corrosion resistance in both interior and exterior applications, with a minimum hardness of 35 HRC after heat treatment. The dimensional tolerance on the bolt face width was ±0.08 mm, and the surface finish requirement was Ra 1.6 μm.
The two candidate manufacturing processes were metal injection molding (MIM) and investment casting combined with post-casting machining. Die casting with zinc alloy was ruled out because the customer specifically required stainless steel. A third option, conventional CNC machining from bar stock, was evaluated for reference but was deemed too expensive at the target volume of 150,000 units. This case study presents a detailed technical and economic comparison of MIM versus investment casting for this steel lock latch application.
Process Capability Comparison
MIM was the preferred technical choice due to its ability to produce near-net-shape components with complex geometries. The lock latch design included an undercut groove that would be difficult to cast without a complex core, but MIM could produce the feature directly from the mold. Investment casting could also produce the latch, but would require a secondary EDM operation for the undercut groove, adding cost and cycle time.
| Parameter | MIM (17-4PH SS) | Investment Casting + Machining | CNC from Bar (Reference) |
|---|---|---|---|
| Density after sintering | 97.5-98.5% | 99.5-100% (fully dense) | 100% |
| Dimensional tolerance (typical) | ±0.3% of dimension (±0.045 mm) | ±0.5% (±0.075 mm) | ±0.01 mm |
| Surface finish as-produced | Ra 1.0-1.6 μm | Ra 3.2-6.3 μm | Ra 0.4-0.8 μm |
| Undercut groove | As-molded, no secondary | Requires EDM or broaching | Requires broaching |
| Hardness as-sintered/as-cast | 25-30 HRC (solution treated) | 15-20 HRC (as-cast) | NA (bar-stock condition) |
| Hardness after aging/H900 | 38-44 HRC | 38-44 HRC | 38-44 HRC |
| Secondary operations | Deburring + heat treat only | Cut gates, machine bore, EDM groove, heat treat | Full machining + heat treat |
| Lead time to first article | 14-16 weeks (mold + qualification) | 10-12 weeks (wax die + qualification) | 2-4 weeks (setup) |
The MIM process achieved the tight ±0.08 mm tolerance on the 15 mm bolt face width without any secondary machining, whereas the investment cast part required a face milling operation to bring the tolerance within specification. The surface finish of the MIM parts was also closer to the Ra 1.6 μm requirement, with the as-investment-cast surface roughness averaging Ra 4.8 μm, requiring a secondary light grinding or glass bead blasting operation.
Cost Analysis at 150,000 Units Per Year
The total cost per part was calculated including material, processing, secondary operations, inspection, and packaging. The investment required for each process was amortized over the annual volume.
| Cost Category | MIM ($/part) | Investment Casting ($/part) | CNC from Bar ($/part) |
|---|---|---|---|
| Material (17-4PH powder / bar) | $0.65 | $0.42 | $0.88 |
| Tooling amortization (3-year) | $0.18 | $0.09 | $0.02 |
| Primary processing | $0.40 | $0.32 | $1.85 |
| Secondary operations | $0.08 | $0.45 | $0.30 |
| Heat treatment (H900) | $0.15 | $0.15 | $0.15 |
| Quality inspection | $0.05 | $0.08 | $0.10 |
| Packaging & logistics | $0.04 | $0.04 | $0.04 |
| Total per part | $1.55 | $1.55 | $3.34 |
The MIM and investment casting processes arrived at identical per-part costs of $1.55, but the cost breakdown was very different. MIM had higher tooling amortization due to the more expensive injection mold ($78,000 versus $40,000 for wax dies), but significantly lower secondary operation costs. The investment casting approach required $0.45 per part for secondary operations including gate removal, face milling, undercut EDM, and visual surface finishing. MIM required only $0.08 for simple sprue removal and light tumbling.
Technical Considerations Beyond Cost
While the per-part cost was equal, several non-cost factors influenced the final decision. MIM produced more consistent part-to-part dimensional variation because the injection molding process is inherently more repeatable than the multi-step investment casting process with its wax pattern, shell building, de-wax, and casting steps. The Cpk for the critical bolt face dimension was 1.55 for MIM versus 1.12 for investment casting in initial qualification trials.
However, investment casting had an advantage in corrosion performance. The investment-cast parts were fully dense, while the MIM parts at 98% density had micro-porosity that could serve as corrosion initiation sites in aggressive environments. For the customer's intended indoor commercial application, the MIM density was adequate. But for exterior-grade or marine applications, investment casting or MIM with hot isostatic pressing (HIP) would be preferred.
The final recommendation was MIM for this specific lock latch application, based on the superior dimensional consistency and lower secondary operation cost. The 16-week tooling lead time was acceptable for the customer's product development timeline. For future marine-grade or exterior-rated products where full density corrosion resistance is critical, investment casting or MIM with HIP was recommended.
Conclusion
This comparative manufacturing case study demonstrates that for stainless steel lock latches at volumes of 150,000 units per year, MIM and investment casting are economically equivalent at $1.55 per part, but the decision must factor in dimensional consistency requirements, surface finish needs, corrosion environment, and lead time constraints. MIM offers superior dimensional repeatability and surface finish with fewer secondary operations, while investment casting provides full density and shorter tooling lead time. The optimal choice depends on the specific performance priorities of the lock application.
