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Choose CNC machining for prototypes, low quantities, changing designs, or the tightest tolerances; choose metal injection molding (MIM) for small, intricate parts made repeatedly in the thousands or more; choose die casting for high-volume aluminum, zinc, or magnesium parts that can use draft and a parting line. The cheapest process is determined by geometry, alloy, tolerance, annual demand, tooling, and secondary operations—not by a universal break-even quantity. A near-net MIM or die-cast blank followed by CNC on critical surfaces is often the lowest-total-cost route.
What each process does
Metal injection molding (MIM)
MIM compounds fine metal powder with a polymer binder to create feedstock that can be injection-molded. The molded “green” part is then debound to remove most of the binder and sintered in a controlled furnace. During sintering, the powder particles densify and the part shrinks to its final dimensions. The method produces a near-net shape, so it is valuable when a small component has numerous features that would require many CNC setups or extensive material removal.
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Typical named MIM materials include stainless 316L, 17-4 PH, titanium, and cobalt-chromium. The process can therefore cover alloys that are outside die casting’s usual low-melting nonferrous range. That flexibility comes with obligations: the mold must be designed for predictable shrinkage, wall thickness should be reasonably uniform, and critical dimensions need a tolerance stack-up that includes molding, debinding, and sintering variation.
CNC machining
CNC machining is subtractive. A programmed cutter removes material from bar, plate, or billet until the remaining stock matches the CAD model and drawing. There is no dedicated production mold, so a supplier can begin after programming, workholding, and any soft tooling are prepared. Engineers can revise a feature without redesigning a hardened mold, which is why machining is usually the first choice for prototypes, pilot runs, and products whose design is still moving.
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Machining accepts a broad range of metals and plastics. It can also produce the tightest dimensions among the three routes when the machine, tooling, fixturing, material condition, and inspection method are appropriate. Its financial penalty is time: every pocket, hole, contour, and finish pass consumes machine minutes, and the removed material becomes chips or scrap.
Die casting
Die casting forces molten alloy into a hardened steel die at high pressure. The U.S. Government Publishing Office describes the primary distinction from MIM and permanent-mold casting as the speed with which metal enters the mold. Rapid filling supports short cycles and high output, but the die and casting machine represent a substantial initial investment.
Commercial die casting is concentrated in aluminum, zinc, and magnesium alloys, with lead and tin families also used in appropriate applications. The process favors repeatable, nonferrous components whose geometry can include draft, a planned parting line, vents, and provisions for flash removal. Gas porosity, shrinkage porosity, and incomplete filling must be managed through gating, venting, process controls, and—where necessary—machining or other finishing operations.
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| Axis | MIM | CNC machining | Die casting |
|---|---|---|---|
| How it works | Powder and binder feedstock is molded, debound, and sintered. | Cutting tools remove material from bar, plate, or billet. | Molten alloy is injected under pressure into a hardened die. |
| Typical economic fit | High-volume, small, complex parts. | Prototype, low-volume, and medium-volume work with flexible revisions. | High-volume, repeatable nonferrous parts. |
| Materials named in the reviewed comparisons | 316L stainless steel, 17-4 PH stainless steel, titanium, cobalt-chromium. | Broad range of machinable metals and plastics. | Mainly aluminum, zinc, and magnesium; lead and tin families are also used. |
| Indicative tolerance signal | About ±0.3% as-sintered in one 2026 guide; geometry and shrinkage control govern the result. | About 0.01 mm in one 2026 comparison table; supplier and feature dependent. | About 0.05 mm in one 2026 comparison table before finish machining; feature dependent. |
| Tooling | Dedicated injection mold plus debinding and sintering process control. | No dedicated production mold; fixtures and cutting tools are still required. | Hardened-steel die and casting equipment. |
| Main cost behavior | Tooling and qualification are amortized over produced volume. | Machine time, programming, stock, and scrap dominate. | High die investment followed by low cycle cost at volume. |
| Design watch-outs | Sintering shrinkage, wall uniformity, and tolerance stack-up. | Tool access, fixturing, workholding, and material waste. | Draft, parting line, vents, flash, porosity, and finish machining. |
The tolerance figures are indicative, not guarantees. The reviewed sources disagree on exact ranges because alloy, feature size, geometry, tool condition, and inspection method change capability. Put a datum scheme and inspection method on the drawing and have the supplier confirm critical dimensions before ordering production quantities.
How volume changes the financial decision
Low quantity or an unsettled design
CNC normally minimizes financial exposure when only a few parts are needed or revisions are likely. You pay for programming and machining rather than committing to a hardened production mold or die. A change to a hole, wall, or external contour is generally a CAM and fixture adjustment instead of a tooling rework project. The trade-off is a higher recurring price when each part requires long cutting cycles and consumes a large billet.
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Repeated production in the thousands
MIM becomes attractive when the same small, complicated shape is ordered repeatedly. The mold, debinding schedule, and sintering recipe require engineering and qualification, but those costs are spread across many pieces. A part that would need multiple orientations, tiny cutters, and extensive chip removal can emerge close to final shape in one molding operation, with CNC reserved for selected features.
Very high-volume nonferrous production
Die casting can deliver rapid cycles once the die and machine are commissioned. For aluminum, zinc, or magnesium components whose geometry accepts draft and a parting line, the low cycle time can outweigh the initial die investment. Budget separately for die maintenance, trim or flash removal, porosity controls, and any machining of bores, sealing faces, or datums.
Do not use a single unit-count rule to declare a winner. Actual crossover depends on geometry, alloy, number of cavities, tolerances, labor, energy, scrap, machine utilization, and the supplier’s existing capacity. Request volume-specific quotations and compare the complete five-year cost, not only the quoted piece price.
When CNC machining is the better first choice
- Prototype or pilot quantities: no hardened production mold or die is needed.
- Design changes: revisions can be made in the CAD/CAM and workholding plan without restarting a molding qualification.
- Tight tolerances: machining is generally the strongest starting point for close dimensions, provided the supplier confirms the feature-level capability.
- Unusual alloys: CNC can work with many machinable metals that are not practical for MIM feedstock or die casting.
- Large or simple parts: the tooling and furnace steps of MIM, or the die investment of casting, may not earn back their cost.
Check the design for cutter access, adequate tool clearance, fixturing surfaces, thin-wall deflection, and the amount of stock that must be removed. A low quoted hourly rate can be overwhelmed by many setups or a poor material-removal ratio.
When MIM is the better first choice
- Small, intricate geometry: ribs, multiple bosses, fine external features, and complex three-dimensional shapes can be molded together.
- Large repeat orders: tooling and process qualification have enough pieces over which to amortize.
- High-performance alloys: 316L, 17-4 PH, titanium, and cobalt-chromium are named examples in the reviewed material guidance.
- Near-net economics: less stock removal can reduce cutting time, material waste, and the number of setups.
Design for a consistent feedstock flow and predictable densification. Sudden wall changes, isolated thick sections, and unsupported features can produce distortion or uneven shrinkage. Define which dimensions are critical after sintering and which can be corrected by a light secondary operation.
MIM production sequence
- Feedstock and alloy selection: select powder, binder system, and target density for the required mechanical and corrosion properties.
- Mold design: scale the cavity for expected sintering shrinkage, establish gates and ejectors, and check wall transitions and parting surfaces.
- Injection molding: mold the feedstock into a green part while controlling fill, temperature, pressure, and cycle consistency.
- Debinding: remove binder in a controlled schedule that preserves shape and leaves a stable brown part.
- Sintering: heat the brown parts through the debinding and densification profile; record furnace conditions and lot traceability.
- Inspection and finishing: measure critical datums, assess density and defects, and CNC-machine threads, bores, bearing seats, or sealing faces when the drawing requires it.
One 2026 comparison guide reports over 97% density and roughly ±0.3% as-sintered tolerance for MIM. Those figures are guide-level signals, not a blanket specification for every alloy, size, or feature.
When die casting is the better first choice
- High annual demand: the die cost can be spread over a large run.
- Aluminum, zinc, or magnesium: these are the principal material families identified for the process.
- Repeatable housings and structural shapes: ribs, bosses, and broad surfaces can be produced quickly when draft and parting are designed in.
- Low per-cycle cost is important: once running, the process can fill and eject parts rapidly.
Die-casting design and quality checks
- Add draft so the casting releases from the die without damaging walls or sticking.
- Choose the parting line deliberately and keep critical appearance or sealing features away from unavoidable flash where possible.
- Provide vents and overflow strategy for air evacuation and stable filling.
- Use ribs and uniform wall transitions instead of isolated heavy masses that encourage porosity or shrinkage.
- Plan trim, flash removal, shot-blast or other finishing, and CNC machining for precision bores and datums.
- Define acceptable porosity, leak-rate criteria, density checks, and non-destructive inspection in the purchase specification.
A die-cast part may be inexpensive per cycle yet expensive in total if the alloy, pressure, porosity controls, or required machining are mismatched to the design. Validate those items before committing to die steel.
Materials and performance trade-offs
| Requirement | Most natural starting point | Reason | Qualification to carry into the RFQ |
|---|---|---|---|
| Broad alloy choice or a material outside common casting families | CNC or MIM | CNC covers many machinable materials; MIM examples include stainless, titanium, and cobalt-chromium. | Confirm machinability, powder availability, heat treatment, and final properties. |
| Aluminum, zinc, or magnesium housing at high volume | Die casting | Fast filling and repeatable cycles suit these nonferrous alloys. | Specify die life, porosity limits, draft, flash, and secondary machining. |
| Small stainless component with complex geometry | MIM | Near-net molding can replace many cutting operations. | Model shrinkage and verify density, distortion, and critical dimensions. |
| Prototype in an evolving alloy or geometry | CNC | No dedicated production mold or die is required. | Expect higher piece cost if cutting time and material removal are high. |
Material suitability is not interchangeable between MIM and die casting. MIM begins with powder and binder and can process several high-performance steels and alloys; die casting injects molten metal and is materially concentrated toward lower-melting nonferrous alloys.
Tolerances, surface finish, and secondary operations
Use the process capability as a starting envelope, then assign tolerances by feature. A supplier’s nominal capability does not mean every hole, wall, or datum will hold it simultaneously after shrinkage, distortion, fixturing, and inspection uncertainty.
- CNC: specify datums, tool-access assumptions, burr limits, edge breaks, and the inspection instrument for each critical dimension.
- MIM: identify dimensions controlled in the sintered state, account for shrinkage in the mold, and reserve machining for threads, precision bores, bearing seats, or sealing faces that cannot reliably be held as-sintered.
- Die casting: separate as-cast dimensions from machined dimensions, locate the parting line and flash allowance, and state porosity and surface requirements.
The reviewed 2026 comparison table lists about 0.01 mm for CNC and about 0.05 mm for die casting or injection molding before finish machining. These are comparison values, not guarantees; feature size, alloy, tool condition, and inspection method can move the result materially.
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Hybrid routes can lower total cost
A hybrid route uses MIM or die casting to create a near-net blank, then CNC-machines only the surfaces that carry the functional tolerance. Typical candidates are bearing seats, sealing faces, threads, precision bores, and defined locating datums. This approach avoids machining away most of a billet while avoiding the demand that every critical feature be held in the molding or casting operation.
Hybrid costing must include transfer handling, datum establishment, fixturing, cleaning, and inspection between processes. If the secondary operation touches many surfaces or requires several setups, the apparent near-net savings may disappear.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to compare supplier quotes
Send every supplier the same CAD model, drawing, revision, and demand forecast. Ask for separate line items rather than accepting a single blended unit price.
- State demand: give initial lot size, annual quantity, ramp schedule, and a five-year forecast.
- Define material: identify alloy, condition, heat treatment, corrosion or wear requirements, and approved equivalents.
- Mark critical geometry: show smallest wall and feature, datums, geometric tolerances, surface finish, threads, sealing faces, and inspection points.
- Describe quality targets: specify expected density or porosity, leak requirements, mechanical properties, cosmetic zones, and sampling or 100% inspection.
- Separate tooling: request mold or die price, expected life, maintenance assumptions, ownership, storage, and what happens if the tool is transferred.
- Separate recurring charges: obtain piece price at several volumes, cycle time, secondary machining, debinding or furnace charges, packaging, and freight assumptions.
- Expose launch costs: ask for first-article lead time, engineering, process qualification, capability studies, fixtures, gauges, and inspection programming.
- Document loss assumptions: require scrap, rework, yield, and replacement-part assumptions instead of allowing them to remain hidden in the unit price.
- Calculate total cost: compare tooling, qualification, recurring pieces, maintenance, inspection, inventory, and likely design changes over five years.
Have the supplier identify which dimensions are guaranteed as molded, sintered, cast, or machined. That single distinction prevents an apparently low quote from omitting the operation needed to make the drawing pass.
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MIM parts drift out of size
Likely causes include an inaccurate shrinkage factor, uneven wall thickness, unstable debinding, furnace variation, or an uncontrolled tolerance stack-up. Recheck cavity scaling against measured lots, map distortion by location, stabilize the thermal profile, and move only truly critical features to a controlled machining step.
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MIM parts crack or warp during debinding
Investigate binder-removal rate, green-part density variation, unsupported geometry, and thick-to-thin transitions. Slow or staged debinding, revised gating, and more uniform sections can improve stability, but any change requires a documented requalification.
Die-cast parts leak or show internal porosity
Review shot profile, venting, overflow design, melt cleanliness, die temperature, and local heavy sections. Define a test method and acceptance limit; visual inspection alone cannot establish internal soundness. Machine critical bores only after the porosity risk and remaining wall thickness are understood.
Die-cast dimensions or flash are inconsistent
Check die wear, clamping, alignment, parting surfaces, trim tooling, and thermal balance. A drawing should distinguish the as-cast envelope from the post-machining requirement so flash is not treated as an unexpected defect.
CNC cost exceeds the estimate
Compare quoted and actual setup count, tool changes, cycle time, workholding, inspection, and material utilization. Reorient the part to reduce setups, enlarge internal radii where function permits, standardize hole sizes, and consider a near-net MIM or cast blank if the same geometry will be produced repeatedly.
A practical decision sequence
- Freeze the requirement: list function, environment, alloy, critical dimensions, finish, annual quantity, and expected product life.
- Screen material compatibility: remove processes that cannot supply the required alloy or properties.
- Classify geometry: note size, wall uniformity, draft, parting-line tolerance, cutter access, and the number of precision features.
- Estimate the cost drivers: tooling and qualification for MIM or die casting; programming, setups, machine time, and stock for CNC.
- Choose a prototype path: use CNC when the design is not proven, even if production may later move to MIM or die casting.
- Request parallel quotes: give identical drawings and ask for several volume points plus separate tooling and engineering charges.
- Validate samples: inspect datums, critical dimensions, finish, density or porosity, mechanical properties, and actual cycle or machining time.
- Recalculate lifecycle cost: include scrap, maintenance, inventory, secondary operations, and the financial effect of design changes.
The decision is strongest when the process is selected for the part’s complete cost and risk profile rather than for a headline tolerance or unit price.
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