ENGLISH EDITION

Transitioning from CNC Machining to MIM: Volume, Tolerances and the Cost of Finished Parts

Replacing CNC machining with metal injection molding becomes an economic decision about tooling recovery and the work still required after sintering. The comparison must use the same assembly function, material requirements and finished-part acceptance criteria.

A low molding price does not necessarily mean a low delivered cost. Precision bores, heat treatment, cosmetic finishing and inspection may remain. The useful measure is the total cost of delivering an accepted part, over a realistic period of stable demand and design.

1. Align the quotation scope

CNC costs include stock, machine occupancy, cutters, setups, handling, deburring and inspection. Deep narrow pockets and features on multiple faces can make machining less efficient. Before comparing processes, review whether larger non-critical internal radii, fewer setups or multi-part fixtures could reduce the existing CNC cost.

The initial CNC investment can include process planning, programming, dedicated fixtures and first-article verification. MIM introduces mold development, molding and sintering trials, setters, secondary machining fixtures and approval work. Recurring MIM costs include feedstock, molding, debinding, sintering, handling and any required sizing or finishing.

Both quotations should cover equivalent heat treatment, final dimensions, surface finish, inspection reports, assembly and packaging. Include program or fixture changes for CNC, and mold modification, maintenance and requalification for MIM. Comparing an unfinished part with a ready-to-ship component obscures the actual process difference.

2. Check geometry and material suitability

MIM injects a mixture of metal powder and binder, removes the binder and sinters the powder into a denser metal component. It differs from injecting molten metal in die casting. Shrinkage during sintering must be accounted for in tooling and process development.

Small parts that combine bosses, ribs or grooves and repeat in stable production are candidates for review. However, a CNC design does not automatically become a moldable design. Abrupt thickness changes, concentrated mass and difficult undercuts may add tooling and process complexity.

Agree on mold opening direction and acceptable locations for gates, ejector marks and parting lines. Consider how the part rests during sintering. A geometry without a stable support face may require a dedicated setter and extra handling. If several machined pieces become one molded component, compare assembly-level costs, including fasteners, joining and inspection.

The same alloy designation does not establish identical performance. Wrought stock and a sintered material can differ in density, residual pores, microstructure and heat treatment. A 316L designation alone cannot transfer the fatigue or corrosion qualification of an existing component to its replacement.

ISO 22068 addresses chemical composition and mechanical and physical properties of sintered MIM materials. It provides a basis for material specification, not automatic qualification of a particular assembly or service life. Define the relevant material condition and verify the properties required by the application.

3. Separate near-net-shape features from precision interfaces

Classify features by function: clearance envelopes, fits and threads, datum or sealing surfaces, and visible faces. Establish which features can remain as-sintered and which require additional machining. Treat dimensions, geometric relationships and surface finish as separate requirements.

MIMA describes an average process tolerance of ±0.3%, with secondary operations available for tighter requirements. At 20 mm, 0.3% equals 0.06 mm. This is an indicative calculation, not a guaranteed capability for a specific part or a general rule for flatness and position.

When secondary machining is necessary, define stock allowance, locating datums, support under clamping load and tool access. Insufficient allowance may fail to clean up the sintered surface; excessive allowance adds cutting time. Research on sintered 316L MIM components shows that non-uniform green density can contribute to directional shrinkage and distortion. Uniform CAD scaling alone is therefore not a complete dimensional control strategy.

A practical third route is to form the complex envelope by MIM and finish only selected functional surfaces by CNC machining. Validate the combination through early samples, dimensional distributions, tooling compensation and suitable sintering support.

4. Use delivery batches and accepted volume

An annual demand of 10,000 parts does not define the manufacturing plan. One large run and frequent small releases may have different setup, furnace scheduling and finishing costs. Specify order quantity, delivery frequency and cumulative volume separately.

More mold cavities increase output per molding cycle, but total throughput still depends on debinding, sintering, sizing, machining and finishing capacity. Identify the limiting step before converting molding cycle time into a delivery commitment.

Amortize tooling over credible demand for the approved design. A redesign can reduce the quantity made with the original mold, even when total product demand remains high. Separate committed orders from forecasts and account for inventory exposure.

Use accepted delivered quantity, rather than molding starts, as the cost denominator. Track losses at sintering, machining and cosmetic inspection. A rejection late in the route also consumes the cost of earlier completed operations.

5. Specify the final surface condition

Internal functional components and visible metal housings need different finishing assumptions. A polished, directionally brushed or uniformly matte surface may require dedicated operations after sintering. Coating and plating introduce their own acceptance requirements.

Specify the visible area, permitted process marks, edge condition, grain direction and approved appearance samples. Roughness alone does not define visual gloss or directional appearance. Decide when dimensions must comply: material-removing finishing and thickness-adding coatings can affect final fits. Include masking, final inspection and protective packaging where required.

6. Calculate the crossover with complete recurring costs

A first comparison can use total cost = initial fixed cost + accepted quantity × recurring cost per accepted part. Recurring cost must include the required secondary operations, inspection and yield losses. If MIM has higher initial cost but lower recurring cost, the crossover quantity equals the initial cost difference divided by the recurring cost saving.

The following figures are hypothetical, not a GPN quotation or measured production costs. Assume CNC initial cost of KRW 2 million and recurring cost of KRW 18,000 per accepted part. Assume MIM initial cost of KRW 30 million and recurring cost of KRW 8,000, including secondary machining, finishing, inspection and yield losses.

• At 1,000 accepted parts: CNC costs KRW 20 million; MIM costs KRW 38 million.
• At 2,800 parts: both routes cost KRW 52.4 million.
• At 3,000 parts: CNC costs KRW 56 million; MIM costs KRW 54 million.
• At 10,000 parts: CNC costs KRW 182 million; MIM costs KRW 110 million.

The crossover is 2,800 parts under these assumptions. If additional operations raise the MIM recurring cost to KRW 10,000, it moves to 3,500 parts. At KRW 14,000, it moves to 7,000. If CNC improvements instead reduce its recurring cost to KRW 14,000 while MIM stays at KRW 8,000, the crossover becomes approximately 4,667 parts.

There is no universal conversion quantity. If MIM has no recurring saving, higher volume cannot recover its additional initial cost within this model. The example assumes constant costs and excludes VAT, financing and inventory carrying costs. Actual programs require quantity-tier pricing, delivery timing, tooling maintenance or replacement, and inventory terms.

7. Make the decision from the drawing and production plan

Frequent design changes and uncertain demand favor retaining manufacturing flexibility while validating the product. Stable repeat demand for small complex geometry supports a detailed MIM quotation. Complex envelopes with a few critical interfaces justify reviewing MIM plus CNC finishing. Simple parts with short cutting cycles may benefit more from machining improvements.

For a comparable quotation, provide five groups of information: material and heat treatment; critical dimensions and datums; appearance and finish; order quantities and delivery frequency; and expected changes and launch timing. State what the existing CNC price includes.

CNC prototypes can validate assembly geometry, but MIM qualification remains necessary because the material history and thermal processing change. Confirm dimensions, assembly, application performance and stage yields on MIM samples and pilot lots. Then replace the preliminary assumptions with an accepted-part cost based on the proposed production route.

For a manufacturing comparison, contact gpn@gpn.kr with the current drawing and production plan.