# Thin walls

Evaluating wall thickness with unsupported span, material, manufacturing process and final functional requirements.

Reviewed: 2026-11-01

Tags: parts-and-design, features, manufacturability

Review thickness with unsupported span, height, material and support. A wall held flat during cutting can move after release.

For the small-deflection cantilever example, doubling thickness from 1 to 2 mm increases bending stiffness **eightfold** and reduces predicted end-load deflection to one eighth. Doubling unsupported length instead increases deflection eightfold. This assumes unchanged width, material, load and support. It does not model buckling or residual stress.

Start by recording the wall's minimum thickness, unsupported height and length, neighboring floors/ribs, openings, material condition and loading. A short web enclosed on several sides behaves differently from a tall free-standing fin of the same thickness. Specify whether dimensions apply before finishing, after coating, in a fixture or in the free state.

## Separate the constraints

| Constraint | Question to resolve |
| --- | --- |
| Feature production | Can the selected process consistently create the wall? |
| Process survival | Does it survive cutting, ejection, cleaning and finishing? |
| Dimensional acceptance | Is its thickness and free-state shape within requirements? |
| Service function | Does it resist the actual load, temperature and environment? |
| Handling/assembly | Can it be gripped, joined and transported without damage? |

A successful prototype addresses only the conditions it actually experienced. Production orientation, stock, resin grade, support layout or finishing can change the result even if nominal thickness stays unchanged.

## Machined walls: the workpiece can deflect too

Protolabs Network recommends minimum wall thicknesses of **0.8 mm for metals** and **1.5 mm for plastics** in its general CNC design guidance. It labels 0.5 mm and 1.0 mm respectively as feasible values requiring case-by-case examination. Those are supplier design guidelines, not guaranteed minima for every height, material or tolerance. Its explanation links reduced thickness with lower stiffness, vibration and reduced achievable accuracy; plastics add warping and heat-softening concerns. [Protolabs Network, How to design parts for CNC machining](https://www.hubs.com/knowledge-base/how-design-parts-cnc-machining/)

Thin-wall machining requires a plan for both sides of the wall and for the material that supports it during intermediate operations. Tool reach and the remaining floor can be as important as final thickness. Review clamping, cutting-force direction, residual-stock removal and what happens when the part is released. A wall held flat in the fixture may not remain flat after unclamping.

Sandvik's shoulder-milling guidance explicitly varies the strategy with wall height/thickness ratio and considers both cutter and wall stability. For moderate ratios it describes alternating sides using waterline or step-support approaches. For very large ratios above 30:1, it describes approaching final thickness in stages so thicker material below supports the section being cut. The page also discusses retaining a support leg while machining thin bases. These are process-planning examples, not declarations that every 30:1 wall is producible. [Sandvik Coromant, Shoulder milling—thin deflecting walls](https://www.sandvik.coromant.com/en-us/knowledge/milling/shoulder-milling)

Design changes worth evaluating include shortening the unsupported height, adding a return or rib, increasing the root radius, or permitting temporary support material. Check tool access and final removal access before adding a support. Avoid prescribing a generic speed/feed reduction as the solution: the stable process depends on the particular tool, wall, engagement and fixture.

## Molded walls: fill and cooling change the problem

Uniform wall thickness is a central injection-molding objective. Protolabs explains that thickness variation can contribute to sink, warp and inaccurate parts, and recommends evaluating thickness against the selected material's range. Its guidance also discusses ribs and gussets as alternatives to simply making a wall thicker. A thick rib junction can create a local mass concentration even when the main wall is uniform. [Protolabs, Injection Molding Wall Thickness Guidelines](https://www.protolabs.com/resources/design-tips/improving-part-design-with-uniform-wall-thickness/)

As a bounded example, Protolabs's ABS injection-molding page gives a recommended wall range of **0.045–0.140 inch**, equivalent to **1.143–3.556 mm**. This is guidance for that molding service/material family, not acceptance evidence for an arbitrary ABS grade, long flow path or thin enclosure. Part geometry, gate location and mold construction still require review. [Protolabs, ABS Injection Molding](https://www.protolabs.com/services/injection-molding/abs/)

Evaluate the whole flow path rather than an isolated thickness measurement. A thin region remote from a gate can be harder to fill than a short thin region nearby. Draft, ejection, weld-line location and the intended cosmetic surface also belong in the assessment. A machined or printed prototype demonstrates geometry and selected functions but does not demonstrate mold filling or ejection of the production design.

## Printed walls: support and post-processing matter

Formlabs's **Form 2** design-specification page bounds its recommendations to **Clear Resin at 100 µm layers on Form 2**. It distinguishes walls connected to other walls on two or more sides from walls supported on fewer sides. It also warns that thin features can absorb washing solvent such as IPA and deform with prolonged immersion. Preserve these printer/material/process qualifications; do not substitute an old Form 2 guideline for the settings of a different printer or resin. [Formlabs, Design specifications for 3D models—Form 2](https://formlabs.com/support/Design-Specs/)

HP's Multi Jet Fusion design guidance discusses thin, long parts becoming distorted through non-uniform cooling and shrinkage. Thus an additive process without conventional machining forces still has geometry-dependent distortion risks. Assess build orientation, material, cooling, cleaning access and required post-processing as part of the same manufacturing proposal. [HP, How to design a 3D print model](https://www.hp.com/us-en/printers/3d-printers/learning-center/how-to-design-3d-print-model.html)

Do not equate layer thickness or image resolution with reliable wall thickness. Determine whether the feature is present in the generated build data and whether it survives removal, washing, support removal and final conditioning. Specify the required finished wall, not just a nominal mesh surface separation.

## Original stiffness comparison

For a simplified rectangular cantilever strip under an end load, take width `b`, thickness `t` in the bending direction and length `L`. In the small-deflection elastic beam model, `I = bt³/12` and tip deflection is `FL³/(3EI)`. Assume the same material modulus, width, length, load and boundary condition for both designs.

For `b = 20 mm`, increasing thickness from **1 to 2 mm** changes the second moment of area from **1.667 to 13.333 mm⁴**. That is an eightfold increase in bending stiffness in this model and reduces the predicted deflection to one eighth. Conversely, doubling unsupported length at unchanged thickness increases predicted tip deflection eightfold under the same end load.

These original calculations illustrate geometric sensitivity, not the capacity of a specific part. They omit plate action, local buckling, shear deflection, residual stress, anisotropy and large deformation. A closed box, curved shell or rib-supported panel needs an appropriate structural model; simply applying the strip formula can misrepresent its behavior.

## Drawing and acceptance

Specify local minimum thickness where it matters, especially at pockets, fillets, countersinks, intersections and tapered surfaces. A nominal wall with a generous bilateral tolerance can leave a substantially thinner local section. Keep coating buildup distinct from load-bearing substrate thickness.

Agree the inspection state and method. Probe force or clamping can move a compliant wall; a thickness reading alone cannot establish flatness or profile. Include handling and packaging where damage could occur after acceptance. For sealing or pressure service, validate the relevant function and defects rather than treating manufacturability guidance as a pressure rating.

The release should identify the process, material/condition, minimum wall and support geometry, critical finished tolerances and the service function being demonstrated. When changing process, repeat the relevant assessment rather than carrying over a single minimum-thickness number.

## Source scope

The cited technical web pages do not display a technical revision edition. Their recommendations are used as retrieved process-specific guidance. Formlabs explicitly identifies its Form 2, Clear Resin and layer-height test context; none of the numerical recommendations above is represented as a general standard or guaranteed design allowable.

## Related reference

[Part features](index.md)