# Strength and warping

How resin and finish affect strength, how narrow necks break, and how flat panels and long bars warp.

**Use this when:** a part carries weight, has a narrow neck, or has large flat panels or long straight bars.

**Don't use this for:** minimum wall or detail sizes. Read Walls and holes, or Detail and lettering.

## Resin

Matte and glossy are finishes, not strength grades (`MAT-01`). Standard resin is strong but brittle: thin features snap when bent, snagged or dropped (`MAT-02`). For the materials you can order, see [Materials and finishes](https://womp.com/docs/print/materials.md).

## Breakage at narrow necks

A feature can pass every thickness rule and still snap because of what it carries. For each narrow neck, the breakage index is `R_b = m · L / d³`: the mass beyond the neck in g, the distance from the neck to that mass's centre in mm, and the neck diameter in mm (`BRK-01`). Stress rises in step with mass and lever, and falls with the cube of the diameter: doubling the diameter cuts stress 8 times.

![Diagram: a block with a thin neck carrying a round mass. The neck diameter d and the distance L from the neck to the centre of the mass m are marked.](/docs/diagrams/neck.svg)

*The breakage index is `R_b = m · L / d³` (`BRK-01`). Halving the neck diameter makes the stress 8 times higher. Widen the neck and fillet its root (`PRO-06`).*

| Rule | Name | Limit (mm, per side for gaps) |
| --- | --- | --- |
| `BRK-01` | Breakage index | For each narrow neck: R_b = m · L / d³ (g, mm). m = mass beyond the neck; L = neck to that mass's centre; d = neck diameter. |
| `BRK-04` | Flat necks | For a rectangular neck of width b and thickness t bending about t: σ ∝ m·L/(b·t²). Use d_equiv = (b·t²)^(1/3) in R_b. |
| `BRK-05` | Deflection is different | Tip sag of a round cantilever scales with m·L³/d⁴; use for floppiness, not breakage. |

## Warping

Warping is distortion without any load, from uneven curing shrinkage. The warp index is `R_w = L² / t`: the longest unsupported flat span over the wall thickness (`WRP-01`). Doubling the span makes warp about 4 times worse; doubling thickness only halves it. Ribs and curves usually fix warping better than extra thickness.

![Diagram: on the left a flat panel with its full span L marked; on the right the same panel with two ribs under it, and L marked as the span between ribs.](/docs/diagrams/warp-ribs.svg)

*The warp index is `R_w = L² / t` (`WRP-01`). Ribs divide a panel, so L becomes the largest span between ribs (`WRP-03`).*

| Rule | Name | Limit (mm, per side for gaps) |
| --- | --- | --- |
| `WRP-01` | Warp index | R_w = L² / t (mm). L = longest unsupported flat span; t = wall thickness. Derived from δ ∝ Δε·L²/t. |
| `WRP-03` | Effective span with ribs | Ribs, bosses and folds divide a panel; use the largest sub-panel span between stiffeners. |
| `WRP-04` | Thicker is not always safer | Very thick or uneven sections can increase shrinkage gradients; combine with WAL-06. |
| `WRP-05` | Mitigations | Add ribs, curve the surface, add edge returns, even out thickness, hollow thick regions, flag orientation. |

## Use instead

- **Wrong move:** thickening a large flat panel to stop it warping. **Why:** warp grows with the square of the span, and very thick or uneven sections shrink unevenly (`WRP-04`). **Right move:** add ribs to shorten the free span, curve the surface, or add edge returns (`WRP-03`, `WRP-05`).
- **Wrong move:** a heavy head on a thin neck. **Why:** breakage stress falls with the cube of the neck diameter. **Right move:** widen the neck and fillet its root at least 0.5 mm (`PRO-06`).

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Last checked: 2026-11-09. Womp version: Womp web app, November 2026.