MIM 穴・スロット・アンダーカット設計ガイド

Can MIM Produce Holes, Slots and Undercuts?

Yes, MIM can produce many holes, slots and undercuts directly in the molded green part. These features are one reason MIM is selected for small, complex precision metal parts. However, each feature must be judged by tooling motion, core pin stability, feedstock flow, ejection path, shrinkage behavior and inspection requirements.

MIMA design guidance explains that cored holes can reduce cross sections, support more uniform wall thickness and reduce or eliminate machining operations. It also notes that the preferred hole direction is parallel to mold opening and perpendicular to the parting plane, which is why feature orientation matters before tooling. See MIMA complex design guidance.

核心的な結論： Not every hole, slot or undercut has the same MIM risk level. Direction, depth, access, function and tolerance determine the review priority.

From a design review perspective, the safest features usually follow the mold opening direction, have enough surrounding material, avoid sharp transitions and do not create thin unsupported tool steel. The highest-risk features are deep small holes, long narrow slots and internal undercuts that block normal ejection.

How Hole Direction Affects MIM Tooling

Hole direction is one of the first checks in a MIM DFM review. A hole is not only a geometry feature; it usually implies a core pin, shutoff surface, slide or post-sintering machining decision.

Holes Aligned with the Mold Opening Direction

Holes aligned with the mold opening direction are usually easier to mold than side holes or angled holes. The core pin can be supported more directly, the mold structure is simpler and ejection is easier to manage. This does not mean every vertical hole is automatically low risk. A very small, very deep or tight-tolerance hole can still create core pin deflection, wear or dimensional variation.

In practice, a product engineer should mark which holes are functional and which are non-critical. A cosmetic vent hole, weight-reduction hole and precision assembly hole should not be treated the same way. Critical holes may require tighter inspection, secondary sizing or machining after sintering. For a deeper tolerance strategy, review MIM公差.

Side Holes and Cross Holes

Side holes and cross holes are possible in MIM, but they often require slides or side actions. This changes the mold from a simpler open-close tool into a more complex tool with moving elements. The additional shutoff surfaces can become flash-sensitive areas, especially when the hole is near a visible surface, sealing surface or assembly interface.

A common mistake is to treat side holes as “free complexity” because MIM can form complex geometry. In production, side holes should be reviewed for their actual function. If the hole only supports assembly clearance, it may be redesigned. If it is a critical alignment or fluid passage feature, the supplier may need to review whether direct molding, post-sintering drilling or another process sequence is more stable. Related tooling issues are covered more fully in MIM金型設計.

Deep or Small Holes

Deep or small holes create a different risk. The core pin may become long and slender, which can lead to pin deflection, wear, breakage or hole position variation. Blind holes are usually more sensitive than through holes because the core pin may be supported from only one side.

The practical limit depends on material, feedstock behavior, hole depth, hole diameter, mold layout and tolerance requirement. Instead of applying one fixed rule to all projects, the better approach is to identify the hole function and ask whether it must be molded directly. For tight-tolerance holes, post-sintering machining may be more reliable than forcing the hole to be fully net-shaped in the mold.

Slot Design Risks in MIM Parts

Slots can reduce weight, create clearance, support assembly functions and help balance section thickness. However, a slot in a MIM part also creates tool steel, filling, flash and shrinkage questions. CAD shape alone does not show whether the matching mold steel will be strong enough for repeat production.

Open Slots vs Closed Slots

Open slots are usually easier to mold and inspect than deep enclosed slots. Feedstock has a clearer flow path, the tool structure can be more robust and ejection is often less complicated. Closed, deep or narrow slots are more sensitive because they may require thin mold steel, create flow hesitation or increase flash risk at shutoff areas.

Slot ends should avoid sharp internal corners where possible. Rounded slot ends and smooth transitions help reduce local stress concentration, improve filling behavior and reduce the risk of cracking during green part handling, debinding or sintering. Slot design should be reviewed together with MIM肉厚設計.

Narrow Slots and Weak Tool Steel Conditions

A slot in the part usually means a raised steel feature in the mold. When the slot is very narrow and deep, the corresponding tool steel may be thin and vulnerable to wear, damage or deflection. This is one of the most overlooked issues in CAD-driven design.

The question is not only whether the slot can be molded once. The real question is whether the slot can be produced consistently over the required production volume without excessive tool maintenance or dimensional variation. If a slot is narrow, deep and located near a critical surface, it should be reviewed before tooling release.

Slots Near Thin Walls or Functional Surfaces

Slots near thin walls, sealing areas, cosmetic surfaces or precision assembly features need extra attention. A slot can locally interrupt wall thickness, change feedstock flow, create uneven shrinkage or make a thin section more likely to distort during sintering.

If the slot is functional, its width, depth, end radius and inspection method should be defined clearly. If the slot is only for weight reduction or appearance, it may be safer to adjust its geometry to protect mold strength and production consistency.

Undercuts in MIM Design: Useful but Not Always Simple

Undercuts are one of the features that make MIM attractive for complex metal parts. Locking details, snap features, relief grooves, side recesses and retaining forms may be molded directly when the tooling path allows it. But undercuts are not automatically low-cost features. They must be reviewed against ejection direction, slide movement, parting line location, flash risk and production stability.

PIM International describes MIM design freedom as including blind and through holes, angled holes, undercuts, grooves, slots, external or internal threads, knurled surfaces and molded identification features. This supports the use of MIM for complex local features, but final feasibility still depends on part geometry and tooling review. Read the PIM International overview.

When Undercuts Are Suitable for MIM

Undercuts are more suitable when they are accessible, shallow, located near a practical parting line and do not require multiple complex mold movements. External undercuts are usually easier to manage than hidden internal undercuts because they can often be formed with split mold surfaces or side actions.

MIM can be a strong option when the undercut replaces multiple machining steps or reduces assembly complexity. This is especially relevant for small precision parts where CNC access is limited and the feature repeats across production volume.

When Undercuts Increase Tooling Risk

Undercuts increase risk when they block normal ejection, require complex side action, create tight shutoff surfaces or sit on a critical functional surface. These features can increase tooling cost, reduce molding speed, add maintenance points and create flash-sensitive areas.

When to Redesign an Undercut

An undercut should be redesigned or reviewed in more detail when it prevents straight ejection, requires several mold movements, creates a hidden shutoff surface, is located on a sealing or precision assembly surface, demands tight tolerance without practical inspection access, creates a fragile tool condition or can be replaced by an open groove, split feature or post-sintering machining step.

The best redesign is not always to remove the undercut. Sometimes a small change in opening direction, radius, relief angle or feature position can reduce tooling risk while keeping the part function.

Core Pins, Slides and Flash Risk Around Molded Features

Holes, slots and undercuts become production risks when they create unstable tooling conditions. Core pins, slides, inserts and parting lines are useful tools, but every added tooling element creates possible variation. This section explains the feature-level risk only; complete mold architecture belongs in the MIM金型設計 レビュー。.

核心的な結論： The risk of a molded feature usually comes from the tooling action needed to create it, not from the feature name itself.

For a deeper quality discussion around mold-related risks, see 金型設計がMIM部品の品質に与える影響.

How Holes, Slots and Undercuts Affect Filling, Debinding and Sintering

Feature design affects more than the mold. In MIM, fine metal powder and binder are formed into feedstock, the feedstock must fill the cavity, the green part must survive handling, binder must be removed during debinding, and the part must shrink during sintering without unacceptable distortion. Local features can influence each stage.

Filling and Short Shot Risk

Narrow slots, thin sections, deep pockets and complex internal geometry can interrupt feedstock flow. A feature near the end of the flow path may increase short shot or knit line risk. This does not mean the feature is impossible, but it should be reviewed with gate location and filling balance. Related flow-path decisions belong in MIMゲート設計.

Debinding and Section Change Risk

Holes and slots can help reduce thick sections and improve wall balance. This can be beneficial because thick and uneven sections can make debinding and sintering more difficult. However, a slot that creates abrupt thickness changes can also introduce local stress, cracking risk or shrinkage imbalance.

The design target is not simply “add holes to reduce mass.” The better target is controlled and gradual section balance that supports molding, green part handling, debinding and sintering stability.

Sintering Distortion Around Slots and Undercuts

Long slots, open frames, thin bridges and unsupported undercut areas may distort during sintering. The part shrinks significantly during sintering, and the geometry must be able to move predictably. Unsupported cantilever-like sections, long open slots and uneven wall sections may need MIM sintering support design レビュー。.

核心的な結論： A slot is not only a clearance feature; it changes wall balance, shrinkage behavior and support requirements during sintering.

For more process context, see how debinding and sintering affect MIM part quality.

Inspection Checks for Molded Holes, Slots and Undercuts

A feature should not be judged only by whether it appears in the sintered part. For production use, the drawing should define which holes, slots or undercuts are functional, which are cosmetic, which are clearance-only, and which require dimensional verification. The inspection plan may vary by part size, feature geometry, tolerance class and application risk.

DFM Review Matrix for Holes, Slots and Undercuts

Use this matrix as an early design screen. It does not replace project-specific DFM review, but it helps identify which features need attention before tooling.

Molded Feature vs Secondary Machining Decision Table

Many MIM projects combine molded geometry with selective secondary operations. The goal is not to machine every precision feature, but to decide which features are stable enough to mold directly and which features need machining, sizing, drilling, tapping or functional verification after sintering.

This decision table should be used together with the drawing tolerance plan. A feature that is acceptable as-molded in one product may require secondary machining in another product if it controls sealing, bearing, alignment, wear, torque transfer or safety-related assembly.

When Should a Hole, Slot or Undercut Be Redesigned?

A feature should be redesigned when the geometry creates more production risk than functional value. This is especially important before tooling, because CAD changes are less expensive than mold steel changes after trial.

• A deep small hole requires a fragile core pin.
• A blind hole is too deep for stable molding.
• A side hole creates a flash-sensitive slide shutoff.
• A long narrow slot creates weak mold steel.
• A slot interrupts thin wall balance.
• An undercut blocks normal ejection.
• An internal undercut requires complex collapsible tooling.
• A feature is located on a sealing, bearing, alignment or cosmetic surface.
• A tight-tolerance feature cannot be inspected reliably.
• A molded feature would require excessive post-process flash removal.

How These Features Affect MIM Tooling and Part Cost

Holes, slots and undercuts affect cost when they change the mold structure, production stability or secondary operation plan. The cost issue is not only the presence of the feature; it is the manufacturing method required to create it.

For sourcing users, this explains why two parts with similar size and material can have different tooling and part costs. A compact part with straight-through holes may be simpler than a similar part with several side holes, hidden undercuts and critical slot tolerances. Related cost factors are discussed in コストを考慮したMIM設計.

What to Provide for a Feature DFM Review

For holes, slots and undercuts, a useful DFM review requires more than a screenshot. The engineering team needs enough information to understand function, risk and production requirements.

• 2D drawing with critical dimensions marked
• 3D CADファイル
• Material requirement or target mechanical properties
• Hole diameter, depth and tolerance
• Slot width, depth, end radius and function
• Undercut function and mating part information
• Cosmetic surfaces and functional surfaces
• Sealing, sliding, bearing or alignment requirements
• 想定年間数量
• Surface finish or post-treatment requirement
• 二次加工代
• 検査要件
• Application environment and load condition

核心的な結論： The more clearly a buyer explains the function of holes, slots and undercuts, the more accurately the supplier can judge tooling, tolerance and production risk.

If you are still collecting design risks before RFQ, also review MIM設計におけるよくあるミス.