Plastic Part Structure Design: Ribs, Bosses & Wall Rules

Most plastic parts do not fail because of a bad material choice or a bad mold. They fail because the structure was drawn as if it were a machined metal part. Plastic behaves differently: it shrinks when it cools, it conducts heat slowly, and it is far weaker at a sharp corner than the stress analysis suggests.

Figure 1. Plastic Part Structure Design: Ribs, Bosses & Wall Rules.

These are the structural rules that make the difference. They are not negotiable preferences — they are consequences of how the material behaves.

1. Wall Thickness: Uniform and Thin

Rule: keep wall thickness as uniform as possible.

Thick sections cool more slowly than thin ones. The thick section shrinks later, pulling material from the thin section, and you get:

• Sink marks on the surface above the thick section
• Voids inside the thick section
• Warpage, because different regions shrink at different rates
• Longer cycle time, because the thick section sets the cooling time

Typical wall thickness by material:

Thickness ratios to hold:

• Thin-to-thick transitions should not exceed 1 : 1.5 over a short distance. Where a step is unavoidable, taper the transition over at least 3× the thickness difference.
• Thick sections should be cored out rather than left solid. A solid boss that needs to be 8 mm thick is a design failure; core it and add ribs.
• Minimum wall for a flowable part is usually 0.8–1.0 mm, but thin walls need higher injection pressure, higher melt temperature and better venting.

Wall thickness and cooling time: cooling time scales with the square of wall thickness. Going from 2 mm to 4 mm does not double the cooling time — it roughly quadruples it, in that region.

2. Ribs

Ribs add stiffness without the mass (or the sink) of a thick wall.

Why ribs must be thinner than the wall: if the rib base is as thick as the wall, the junction becomes a thick section and sinks. The 0.5–0.6 ratio is the classic result — thick enough to carry load, thin enough to cool with the wall.

Additional rib rules:

• Cross-ribs (a lattice) are much stiffer than parallel ribs, but the intersections are thick sections — keep the rib network consistent and avoid double-thick junctions.
• Very tall ribs need a taper, or a "stepped" rib, to avoid being a cooling problem and a filling problem.
• Ribs near a hinge, a living hinge, or a sealing surface should be avoided or moved — they change the local stiffness and the flow.
• Do not put a rib directly opposite a gate; the flow will race down the rib and leave the surrounding wall short.

3. Bosses

Bosses are the local features that accept screws, inserts or locating pins. They are the most common source of sink marks in molded parts.

Boss design notes:

• A boss standing alone on a wall is a thick section and a sink. Support it with ribs that blend into the wall.
• Keep the boss's base radius generous (0.25–0.5 × wall) — a sharp root is a stress raiser and a crack origin under screw load.
• For bosses near a corner or an edge, keep the boss outer wall at least one wall thickness away from the part edge to avoid short molding and thin steel in the tool.
• For metal inserts, size the boss around the insert's knurl or groove diameter, not its nominal diameter.
• Keep the number of bosses per screw joint as low as possible. Every extra boss is a sink risk and a tool feature.

4. Fillets and Radii

Rule: radius everything that is not a functional edge.

• Internal corner radius: minimum 0.5 × the wall thickness, ideally 1.0 ×. Sharp internal corners multiply stress — a stress concentration factor of 3 is normal at a sharp corner in a plastic part.
• External corner radius: minimum 0.5 × wall thickness as well; a sharp external corner is a weak point and a difficult tool feature.
• A generous internal radius also improves flow and reduces molded-in stress.
• Where a radius is functionally impossible (a sealing edge or a mating face), specify the radius on the non-functional side and keep the sharp edge to a minimum.

5. Draft Angles

Draft is what lets the part leave the mold. Without it, the part drags on the steel, scuffs, and holds the cycle up.

Rule of thumb: add 1° of draft for every 0.025 mm of texture depth, on top of the base 0.5°.

6. Corners, Edges and Local Structure

• Avoid sharp internal corners anywhere a load path passes. Radius them.
• Avoid thick-to-thin abrupt steps. Taper over at least 3× the thickness difference.
• Design the parting line deliberately. Choose where the tool splits based on appearance and function, then make sure the geometry follows it — a parting line that runs across a cosmetic face will be visible forever.
• Plan the gate location with the structure. A gate placed at a structural thick section will create a sink and a warp; a gate placed to flow through thin sections first will short-shot.
• Consider the assembly. Structure that exists only to hold a screw is more expensive than structure that snaps together. See our DFMA guide for part-count reduction.

Figure 2. Quick reference: the key checks and reference values from this guide.

7. Structure Design Checklist

• [ ] Wall thickness uniform within 1 : 1.5 locally, tapered where it changes
• [ ] No solid thick sections — all thick regions cored out
• [ ] Rib thickness 0.5–0.6 × wall, spacing ≥ 2 × wall, draft 0.5°+
• [ ] Boss wall 0.5–0.6 × wall, supported by 3–4 ribs, generous base radius
• [ ] All internal corners radiused ≥ 0.25 mm, ideally ≥ 0.5 × wall
• [ ] Draft specified for every vertical surface, including textured faces
• [ ] Parting line chosen before the geometry is finalized
• [ ] Gate location consistent with the structure and the flow path

Conclusion

Plastic part structure design comes down to three habits: keep thickness uniform, keep transitions gradual, and radius every corner. Everything else — rib ratios, boss proportions, draft values — follows from those three, plus the shrinkage behavior of the material you chose.

If your part is being designed for the first time in plastic, our DFM engineers will review the 3D geometry against these rules and mark up every feature that will cause a molding problem before the tool is cut.