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Rib Design for 3D Printing

A rib is a thin printed wall added to stiffen a larger flat panel without making the whole panel as thick as the rib needs to be.

What is the minimum rib thickness for FDM 3D printing?

Xometry states a safe minimum rib thickness to follow for FDM is 0.06 in, about 1.5 mm. Stratasys Direct adds that no thinning is needed for an FDM rib, and that ribs actually help avoid warping in thin-walled sections.

Do injection-molding rib rules apply to FDM, and do the bureaus even agree with each other?

Injection-molding rib rules do not apply to FDM, and the four bureaus that publish them do not agree with each other either. Their published ratios for rib thickness, height and spacing differ by up to roughly 2x from one bureau to the next.

Fictiv's article is titled for 3D printed parts, but every numeric rule in it below is labeled injection molding. Reading past the title and carrying its ratios into an FDM design is the trap worth naming.

Averaging the four bureaus into one house rib rule would invent a figure none of them actually published. The table below keeps each bureau's own number next to its own source instead.

RuleBureau figure (as published)Provenance
Rib thickness, percent of nominal wall0.5x nominalProtolabs Network, injection molding design guide
Rib thickness, percent of nominal wall60 percent or lessXometry, plastic ribs for injection molding
Rib thickness, percent of nominal wall60 percent or lessProtolabs, design stronger molded parts
Rib thickness, percent of nominal wall60 to 80 percentFictiv, ribs and gussets article
Maximum rib height3x rib thicknessXometry, plastic ribs for injection molding
Maximum rib height3x rib thicknessProtolabs Network, injection molding design guide
Maximum rib height3x wall thicknessFictiv, ribs and gussets article
Rib spacing2.5 to 3x wall thicknessXometry, plastic ribs for injection molding
Rib spacing4x rib thicknessProtolabs Network, injection molding design guide
Injection-molding rib rules by bureau, as each source states it, not reconciled

What this page does not tell you

  • This page shows injection-molding rib rules as a disagreement, not a resolved house rule. No FDM-specific figure exists for rib height or rib spacing, only for rib thickness.
  • The four bureaus below publish different ratios because each defines nominal wall, rib thickness and rib height on its own terms. None of that difference is explained on any of the four pages.
  • None of these figures were measured on the printers that will make your part.

When to print a test coupon instead

Print a small ribbed panel test coupon at the thickness and spacing you are considering before committing to a full run, especially when the rib has to stop warping or add stiffness without extra weight. Send the file through a review request and tell us which rib dimensions to check. A person reviews your request and follows up by email.

Sources

  • Mini-guide to FDM 3D printing, Xometry

    States a recommended wall thickness of 1.2 to 1.5 mm.

  • FDM Design Guidelines (2017 edition), Stratasys Direct

    States the recommended minimum wall thickness is generally 4x the layer or slice thickness, with a table mapping slice thickness 0.005 to 0.020 in to minimum wall 0.020 to 0.100 in, and a case-by-case absolute floor of 0.008 in. This 2017 edition also distinguishes a single-contour wall (2x slice thickness) from the general 4x recommendation.

  • How to Design Parts for Injection Molding, Protolabs Network

    States a molded rib thickness of 0.5x the nominal wall, a maximum rib height of 3x rib thickness, and a rib spacing of 4x rib thickness, for injection-molded parts.

  • Plastic Ribs for Injection Molding Design, Xometry

    States a molded rib thickness of 60 percent or less of the nominal wall, a maximum rib height of 3x rib thickness, and a rib spacing of 2.5 to 3x wall thickness, for injection-molded parts.

  • Design Tips for Stronger Molded Parts, Protolabs

    States a molded rib thickness of 60 percent or less of the nominal wall, for injection-molded parts.

  • Best Practices for Adding Ribs and Gussets to 3D Printed Parts for Structural Integrity, Fictiv

    Titled for 3D printed parts, but its numeric rules are labeled injection molding: rib thickness 60 to 80 percent of nominal wall, maximum rib height 3x wall thickness.

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Published 5 September 2026.