An undercut is a feature that cannot be produced or released using a simple straight-line manufacturing movement. In prototype manufacturing, undercuts are important because they can affect cutting-tool access, machining setups, mold opening direction, tooling mechanisms, inspection and overall manufacturing complexity.

The presence of an undercut does not automatically mean that a prototype needs to be redesigned. The correct manufacturing approach depends on the geometry, material, tolerance, quantity, functional requirement and intended production process.

What Is an Undercut?

An undercut is a portion of a component that is hidden from the primary direction of tool movement, mold opening or part removal.

Typical examples include side grooves, hooks, retaining lips, internal channels, snap features, cross holes and recessed surfaces behind a wall.

In CNC machining, the question is usually whether the cutting tool and holder can physically reach the feature. In injection molding, the question is whether the molded part can be removed from the mold without damaging the component.

Common Types of Prototype Undercuts

External Undercuts

External undercuts occur on the outside of a component. Hooks, retaining lips, side grooves and recessed surfaces are common examples.

Internal Undercuts

Internal undercuts are generally more difficult because surrounding material can restrict access to the feature. Internal grooves, retaining features and enclosed channels are common examples.

Side Features

Side holes, transverse slots and cross features can act as undercuts relative to the primary manufacturing direction.

Can CNC Machining Produce Prototype Undercuts?

Yes. Many prototype undercuts can be CNC machined when the geometry provides sufficient tool access.

Depending on the feature, a manufacturer may use a different part orientation, specialized cutter, additional setup, 4-axis machining or 5-axis machining.

The important consideration is not simply whether the machine has enough axes. Tool diameter, tool length, holder clearance, workholding and collision avoidance all affect manufacturability.

For complex components, review CNC part orientation together with CNC setup planning before finalizing the manufacturing approach.

Tool Access Is Often the Real Constraint

A CAD model may show an undercut clearly, but the cutting tool does not have the same freedom as the CAD model.

During DFM, the manufacturer should consider:

• Cutting tool diameter
• Tool length
• Tool holder diameter
• Approach angle
• Part orientation
• Workholding clearance
• Machine travel
• Collision risk
• Number of machining setups

When 4-Axis or 5-Axis Machining Helps

Additional machine axes can provide access to features that cannot be reached efficiently from a conventional 3-axis setup.

However, multi-axis machining should not be selected simply because the part contains an undercut. Sometimes changing the part orientation or using a secondary operation is simpler.

The decision should consider geometry, tolerance, quantity, setup time, programming requirements and inspection strategy.

For basic machining fundamentals, see the CNC machining process guide .

Prototype Undercuts in Injection Molding

Injection molding introduces a different type of undercut problem. The component must be released from the mold after injection.

A conventional straight-pull mold cannot release every geometry. Depending on the feature, the tooling may require:

• Side-action slides
• Lifters
• Removable inserts
• Collapsible cores
• Unscrewing cores
• Split cores

For a deeper explanation, see Collapsible Core Injection Molding and Unscrewing Core Injection Molding .

How Undercuts Affect Prototype Tooling

The effect of an undercut depends heavily on the expected prototype quantity and the purpose of the prototype.

A small number of parts may justify a removable insert or a relatively simple tooling approach. Higher quantities or production-intent validation may justify more sophisticated mechanisms.

This is why prototype mold design should be considered together with the intended production path.

Prototype Undercut Manufacturing Methods

DFM Considerations for Prototype Undercuts

Undercuts should be reviewed together with the complete part design. A feature that is manufacturable in isolation may become difficult when combined with thin walls, deep pockets, ribs or restricted access.

Draft, Wall Thickness and Undercuts

For molded prototypes, draft and wall thickness can strongly influence how an undercut behaves during ejection.

Insufficient draft can increase ejection forces and surface damage. Undercuts positioned near thin walls, ribs or bosses can also introduce additional molding concerns.

The undercut should therefore be reviewed as part of the surrounding geometry rather than as an isolated CAD feature.

How Undercuts Affect Prototype Cost

An undercut does not automatically make a prototype expensive. The cost impact depends on how the feature interacts with the selected manufacturing process.

• Material and part size
• Geometry complexity
• Number of undercuts
• Required tolerances
• Number of machining setups
• Specialized cutting tools
• Fixture requirements
• Tooling mechanisms
• Secondary operations
• Inspection requirements
• Prototype quantity

For CNC prototypes, additional setups and specialized tooling can influence machining time. For injection molded prototypes, slides, lifters and other mechanisms can influence tooling complexity.

See the CNC machining cost guide for a broader explanation of machining cost factors.

Inspection of Prototype Undercuts

Hidden or difficult-to-access features should be considered during inspection planning.

Depending on the geometry and tolerance, inspection may involve:

• Vernier calipers
• Micrometers
• Height gauges
• Gauges
• Optical measurement
• CMM inspection

The inspection method should be capable of verifying the dimensions that actually control the function of the undercut.

Materials and Prototype Undercuts

Material selection can change the practicality of a manufacturing approach.

Aluminum, stainless steel, engineering plastics and other materials can all be used for prototype components, but their machinability, rigidity, dimensional stability and finishing requirements differ.

For machining projects, review aluminum CNC machining where applicable.

From Prototype to Production

The manufacturing strategy for an undercut can change as the product moves through development.

An undercut may initially be CNC machined or additively manufactured for engineering validation. Later, the same geometry may be produced using injection molding with slides, lifters, inserts or other tooling mechanisms.

This is one reason DFM should consider the expected production process before a prototype strategy is finalized.

For the transition between development stages, see Bridge Manufacturing: From Prototype to Production .