When to choose MIM over CNC machining for small metal parts

I have spent enough time in quoting meetings to know that the question "should we MIM this or machine it?" almost never has a clean answer. The part drawing arrives, someone says "it is small, let us just CNC it," and three months later the buyer is asking why each part costs $4.80 when the target was $1.20. Metal injection molding and CNC machining both make small metal parts. They overlap in part size, they overlap in materials, and they overlap in industries served. But they do not overlap in how cost scales with volume, and that is where most decisions go wrong.

This article breaks down the real crossover points where metal injection molding starts to beat CNC machining on cost, the tolerance and material trade-offs you accept when you switch, and the part geometry signals that tell you MIM is worth the tooling investment. I will use actual numbers from quotes we have run at Yujiaxin Tech over the past several years, not theoretical ranges.

What MIM and CNC machining actually compete on

Metal injection molding produces small, complex parts by mixing fine metal powder with a binder, injecting it into a mold, then removing the binder and sintering the part to near-full density. CNC machining removes material from a solid block or bar stock using cutting tools. The two processes meet in the part size range of roughly 0.1 to 100 grams, where both can physically make the part. Below 0.1 grams, MIM almost always wins because setup time for CNC swallows any margin. Above 100 grams, MIM tooling gets expensive fast and CNC or investment casting tends to take over.

The decision is not about which process makes a better part. It is about where the cost curves cross. CNC has a relatively flat per-part cost that barely drops with volume because most of the cost is machine time and operator time. MIM has a high upfront tooling cost, typically $8,000 to $25,000 for a single-cavity mold, but the per-part production cost drops to $0.20 to $2.00 depending on material and complexity. The crossover point, where total MIM cost (tooling plus per-part) falls below total CNC cost, is what you need to find.

From our quoting data, here is where that crossover lands for common scenarios:

• Simple part, 316L stainless, 5 grams: crossover around 3,000 to 4,000 pieces
• Complex geometry with undercuts, 17-4PH, 8 grams: crossover around 1,500 to 2,500 pieces
• Tight tolerance features, +/-0.025mm on critical dimensions: MIM may never win without secondary machining
• High volume, 50,000+ pieces, simple geometry: MIM wins by a factor of 3 to 5 on per-part cost

Those numbers are real, not estimates from a textbook. I have seen buyers skip MIM at 2,000 pieces and pay $3.20 per part for CNC when MIM would have landed at $1.40 per part including tooling amortization. I have also seen buyers force MIM at 800 pieces and pay $9.50 per part because the tooling had no time to amortize. Volume is the first filter, but it is not the only one.

Part geometry signals that favor MIM

Volume tells you whether MIM can pay back its tooling cost. Geometry tells you whether MIM can actually make the part well. There are four geometry signals I look for when recommending MIM over CNC:

1. External features that would require multiple setups in CNC. If a part has features on three or four sides that each need a separate CNC fixture, the machine time stacks up fast. A latch bracket with holes on two perpendicular faces, a slot on a third face, and a thread on the fourth can take 8 to 12 minutes of CNC cycle time. The same part in MIM is molded in one shot in 20 seconds of cycle time. The cost difference at 10,000 pieces can be $2.50 per part versus $0.80 per part.

2. Undercuts and internal features. MIM can mold undercuts with side actions or collapsible cores. CNC has to reach them with tools, which means longer setups, special fixtures, and sometimes impossible access angles. If your part has an internal groove or a side hole that requires a custom CNC fixture, MIM is worth pricing.

3. Wall thickness variation that is moderate. MIM handles wall thickness from 0.4mm to about 6mm well. Below 0.4mm, the part may not fill. Above 6mm, sintering density drops and sink marks appear. If your part has a uniform 1mm wall with a 3mm boss, MIM handles it. If it has a 0.3mm fin attached to an 8mm block, neither process is ideal without a redesign.

4. Feature density per unit volume. Parts with many features packed into a small space, like a small housing with threads, bosses, holes, and snap features, are where MIM shines. Each feature in CNC adds cycle time. Each feature in MIM adds zero cycle time because the mold already has it. This is why MIM dominates in small stainless steel latches, hinge knuckles, and electronic connector bodies.

You can see real examples of these part types in our electronics industry work and automotive components, where feature-dense small parts are common.

Where CNC still beats MIM

I do not recommend MIM for everything. There are clear cases where CNC is the right call, and being honest about them saves everyone money and time.

Tolerance requirements below +/-0.05mm. MIM sintering shrinkage is typically 17 to 22 percent, and while the shrinkage is predictable and repeatable, holding tolerances tighter than +/-0.05mm on as-sintered parts is hard. For +/-0.025mm or tighter, you either need a secondary CNC sizing operation or you should just CNC the whole part. MIM sizing, where the part is pressed into a calibrated die after sintering, can hold +/-0.03mm on some dimensions, but it adds cost and limits which features can be sized.

Material requirements outside the MIM range. MIM works with a growing list of materials, including 316L, 17-4PH, 420, 440C, Fe-2Ni, and various tool steels. But if you need titanium Ti-6Al-4V, aluminum 6061, or copper alloys, MIM options are limited or not cost-effective. For titanium parts, CNC machining is still the default for most volumes under 50,000 pieces.

Prototyping and low volume. Below 1,000 pieces, MIM tooling cost is almost never justified unless you are already committed to high-volume production and just need prototypes for testing. For 100 to 500 parts, CNC machining or even investment casting for more complex geometries will be cheaper.

Parts that need frequent design changes. A CNC program change costs a few hours of programming. An MIM mold change can cost $500 to $5,000 depending on what needs to move. If your design is still iterating, stay in CNC until it is frozen.

Cost comparison with real numbers

Let me walk through a specific part to show how the math works. We recently quoted a 316L stainless steel latch body, 12 grams, with two threaded holes, a side slot, and a countersunk hole on the top face. Here is how the two processes compared:

CNC machining:

• Material: 316L bar stock, $0.85 per part raw material
• Machine time: 6.5 minutes on a 3-axis VMC, two setups
• Machine rate: $45/hour (including operator and overhead)
• Per-part cost: $4.88 for machining plus $0.85 material = $5.73 per part
• Total at 5,000 pieces: $28,650

MIM:

• Tooling: $14,000 for a 2-cavity mold
• Material: 316L MIM feedstock, $0.15 per part
• Injection and sintering: $0.65 per part
• Secondary tapping (threads need a light secondary tap): $0.20 per part
• Per-part cost: $1.00 production plus $0.15 material = $1.15 per part
• Tooling amortization at 5,000 pieces: $2.80 per part
• Total at 5,000 pieces: $19,750

At 5,000 pieces, MIM saves $8,900, or about 31 percent. At 10,000 pieces, the gap widens because the $14,000 tooling amortizes further: MIM total is $25,500 versus CNC at $57,300. At 20,000 pieces, CNC costs $114,600 and MIM costs $37,000. You can see why the crossover point matters so much. Below 3,000 pieces on this part, CNC would have been cheaper.

One thing buyers often miss is that MIM per-part cost does not drop much with volume. The $1.15 per part is roughly the same at 5,000 and 50,000 pieces. What changes is the amortized tooling cost per part, which drops from $2.80 at 5,000 pieces to $0.28 at 50,000 pieces. CNC per-part cost can drop 10 to 15 percent with volume through batch efficiency, but the curve is shallow compared to MIM's tooling amortization curve.

Material and tolerance trade-offs

When you move a part from CNC to MIM, you trade design flexibility for cost. But you also trade some material and tolerance capability. Here is what changes:

Density: MIM parts typically reach 96 to 99 percent of theoretical density. CNC parts are 100 percent because they are cut from solid. For most applications, the density difference is invisible. But for parts under high pressure or requiring hermetic sealing, the remaining porosity in MIM can matter. You can specify a target density of 98 percent or above in your purchase spec, and a good MIM supplier can hit it consistently.

Mechanical properties: MIM 316L reaches tensile strength of about 520 MPa and elongation of 40 percent. Wrought 316L bar stock, which is what CNC parts are cut from, reaches about 600 MPa with 50 percent elongation. The difference is real but acceptable for most non-critical structural applications. For safety-critical parts, you need to specify MIM material properties in your spec and verify them with test bars from the same sintering lot.

Surface finish: As-sintered MIM surface finish is typically Ra 0.8 to 1.6 micrometers. CNC machined surface can reach Ra 0.4 to 0.8 micrometers. If your part needs a polished or mirror finish, MIM parts can be polished but the starting surface is rougher, so polishing takes longer. For most functional parts, the MIM as-sintered finish is fine.

Tolerance: MIM as-sintered tolerances are typically +/-0.3 to 0.5 percent of the dimension. For a 10mm dimension, that is +/-0.03 to 0.05mm. CNC can hold +/-0.013mm or better. If you have two or three critical dimensions that need +/-0.025mm, consider a hybrid approach: mold the part in MIM for the complex external geometry, then do a secondary CNC sizing or machining pass on just those critical features. This hybrid is what we do for 316L stainless parts that need both volume efficiency and tight tolerance on a few bores or thread positions.

Making the decision: a practical checklist

When a new RFQ comes in for small metal parts, here is the decision flow I use:

1. Volume: Is annual demand above 3,000 to 5,000 pieces? If no, CNC is likely the better choice. If yes, continue.
2. Part weight: Is it between 0.1 and 100 grams? If yes, MIM is viable. If heavier, consider investment casting or powder metallurgy.
3. Geometry: Does the part have features on multiple faces, undercuts, or high feature density? These favor MIM.
4. Tolerance: Are there critical dimensions tighter than +/-0.05mm? If yes, can those features be sized or machined secondarily? If no, CNC the whole part.
5. Material: Is the required material available in MIM feedstock? Check 316L, 17-4PH, 420, 440C first. If titanium or aluminum, stay in CNC.
6. Design stability: Is the design frozen? If still iterating, stay in CNC until it is locked.
7. Lifecycle: Will the part be in production for 2 or more years? MIM tooling lasts 100,000 to 300,000 shots, so a long lifecycle helps justify the investment.

If five or more of these check in favor of MIM, it is worth getting a formal MIM quote alongside the CNC quote. The comparison should include total tooling cost, per-part cost at the expected volume, and a break-even analysis showing where MIM total cost crosses CNC total cost.

You can read more about how we handle this comparison process in our how we work page, or check common buyer questions on the FAQ page. If you have a part drawing and want a side-by-side MIM versus CNC quote, that is the kind of work we do every day.

Common mistakes buyers make

The most common mistake I see is deciding the process before looking at the volume curve. A buyer gets a CNC quote for $5.73 per part at 500 pieces and assumes the price will scale linearly to 20,000 pieces. It will not. CNC pricing drops maybe 10 to 20 percent at higher volume, not 80 percent. So the buyer commits to CNC for a 20,000-piece order and pays $91,680 when MIM would have cost $37,000.

The second mistake is assuming MIM cannot hit tight tolerances at all. It can, with the right secondary operations. Sizing, coining, and light CNC machining of critical features after sintering can bring MIM parts to +/-0.025mm on selected dimensions. The cost of those secondary steps is still far below full CNC machining at volume.

The third mistake is not specifying density and material properties in the purchase spec. If you just say "316L stainless, MIM," you get whatever the supplier's default process gives you. If you need 98 percent density minimum, 520 MPa tensile, and 40 percent elongation, write it in the spec. A supplier like us will meet it, but only if you ask.

The fourth mistake is underestimating tooling lead time. MIM tooling takes 4 to 8 weeks. If your project timeline does not account for that, you will end up running CNC for the first batch while the mold is being built, which is fine if you plan for it, but painful if you do not.

Final thoughts

The decision between MIM and CNC for small metal parts comes down to volume, geometry, tolerance, and material. There is no universal answer. But there is a reliable method: get both quotes, plot the total cost curves against your expected volume, and see where they cross. If you are making 10,000 small stainless steel parts per year with moderate complexity, MIM will almost certainly save you 40 to 60 percent. If you are making 500 titanium parts per year with tight tolerances, CNC is the right call.

The worst thing you can do is pick a process based on what you did last time or what a colleague recommended without running the numbers. Every part is different. Every volume is different. Run the comparison, look at the crossover point, and let the math decide.