Machine Design #95: Plastic Ribs and Bosses — Add Stiffness Without Creating Sink Marks
Plastic ribs and bosses must preserve function, manufacturability, inspectability, cost, and serviceability when the boundary changes.
Function before a number
Write the input, expected result, acceptance limit, failure symptom, and measurement method before choosing a thickness, tolerance, datum, or hole pattern. Assign an owner to every value and change.
Core checks
- Functional stiffness and load path: record value, source, method, owner, and pass/fail evidence.
- Rib or boss thickness relative to wall: record value, source, method, owner, and pass/fail evidence.
- Draft, radius, and mold release: record value, source, method, owner, and pass/fail evidence.
- Sink, warp, cooling, and cosmetic face: record value, source, method, owner, and pass/fail evidence.
- Screw, insert, or fastening load: record value, source, method, owner, and pass/fail evidence.
- Mold trial, measurement, and acceptance: record value, source, method, owner, and pass/fail evidence.
| Failure mode | Symptom | Verification |
|---|
| Function not defined | Over-tight or weak design | Rebuild the functional stack |
| Datum not real | Inspection and assembly disagree | Try the real fixture |
| Documents out of sync | Correct number, wrong revision | Baseline every reference |
Ribs and bosses: it is all about proportion to the wall
Ribs and bosses are both places where the plastic thickens locally, which means both are places at risk of sink marks and voids. Both remain usable, but they have to be proportioned to the wall they attach to.
| Feature | Relation to the wall | Why |
|---|
| Rib root thickness | Clearly thinner than the wall | A rib root as thick as the wall guarantees a sink mark on the opposite face |
| Rib height | Limited by how well the mould fills | Too tall is hard to fill and hard to release |
| Rib root fillet | Filleted, never a square corner | A square corner is a stress raiser and obstructs the flow |
| Draft on the rib | Mandatory, on both side faces | The rib is the deepest feature along the opening direction |
| Spacing between two ribs | Wide enough for a mould core between them | Ribs too close leave a thin steel blade that breaks easily |
A paradox worth remembering: a thicker rib is barely stiffer but sinks noticeably more. To gain stiffness, increase the rib height or add more ribs, do not thicken them.
The root fillet: a two-way trade-off
A rib root fillet does two things — it reduces the stress concentration and it helps the plastic flow into the rib. But too large a fillet adds material exactly at the junction, creating the very thick region we are trying to avoid.
So the root radius is a trade-off: large enough not to crack and to fill, small enough not to create a thick region. This is a point to ask the mould maker about rather than choose by feel.
Bosses: do not leave them standing alone
A boss is a plastic column for a screw or a locating pin — also a thick region. Three principles:
- Make it hollow, not solid. A solid boss is the classic thick region. The boss wall follows the same
proportion rule as a rib.
- Do not let a boss stand free in the middle of a flat area. A lone boss is both weak and prone to sinking
on the opposite face. Tie it to the nearest wall with support ribs, or place it against the wall.
- Fillet the boss root and give it draft, exactly as for a rib.
For a screw boss, the wall thickness also has to withstand the tightening force and the hoop stress as the screw goes in — so decide it together with the screw type and the intended torque, not separately.
A quick check before freezing the geometry
- Look at the face opposite every rib and every boss: is it a visible surface? If so, the sink mark risk is a
real problem, not a theoretical one.
- Count the locally thickened regions. Many scattered thick spots usually means the structure should be
rearranged, not each spot adjusted.
- Check whether any two ribs sit too close — think from the mould side: between them is a thin steel blade
that has to withstand the injection pressure.
The concrete numbers for ribs and bosses
The earlier pass said ribs and bosses must be judged in proportion to the wall. This section gives the numbers to put on the drawing.
| Dimension | Common mark | Why |
|---|
| Rib thickness at the base | 0.5–0.6 × wall thickness | Thicker, and the rib base sinks on the opposite face |
| Rib height | ≤ 3 × wall thickness | Taller ribs are hard to fill and warp easily |
| Spacing between ribs | ≥ 2 × wall thickness | Ribs too close overheat locally and are hard to cool |
| Rib draft | ≥ 0.5° per face | So the rib pulls out of the mould without dragging |
| Rib root radius | ~0.25–0.5 × wall thickness | Lowers stress, but too large a root becomes a thick mass |
Bosses: tie into the wall and fight the sink mark
A boss (a post for a screw or a pin) is a local thick mass, so left solid it almost always sinks on the opposite face. Three ways to handle it:
- Tie the boss to the wall with gussets rather than leaving it standing alone — it is both stiffer
and gives the melt a path in.
- Keep the wall around the boss in proportion: the boss wall should be about 0.6 × the main wall,
like a rib, so it is not a solid mass.
- Radius the hole and the boss base: a sharp base concentrates stress when the screw is tightened;
add a small radius at the base.
For a boss taking an insert or a self-tapping screw, the outside diameter and boss wall must also carry the tightening load — see the article on inserts in plastic parts.
MINATA release checklist
- [ ] Function, boundary, and failure symptom are written.
- [ ] Datums, ownership, process, and mistake-proofing are clear.
- [ ] Six topic checks have evidence and pass/fail limits.
- [ ] Manufacturing, assembly, inspection, and maintenance were tried.
- [ ] Revision, supplier, material, and configuration records agree.
Good engineering is an explicit chain that survives manufacture, operation, maintenance, and change. For plastic ribs and bosses, evidence that function and variation remain reliable is the MINATA standard.
Frequently asked questions
Should a rib be as thick as the wall?
No. The rib root has to be clearly thinner than the wall, otherwise the opposite face will almost certainly sink. A thicker rib is also barely stiffer — to gain stiffness, increase the height or add ribs.
Is a larger root fillet always better?
No. A fillet reduces the stress concentration and helps the plastic fill the rib, but too large a fillet adds material right at the junction — creating the very thick region being avoided. It is a trade-off, so ask the mould maker.
Should a screw boss be solid or hollow?
Hollow, with the wall proportioned as for a rib. A solid boss is the classic thick region, producing a sink mark on the opposite face and a void inside. The boss wall also has to withstand the screw tightening force, so choose it together with the screw type and the torque.
Is a boss standing free in a flat area a problem?
It is both weak and prone to sinking. Tie the boss to the nearest wall with support ribs, or rearrange so it sits against a wall. A lone boss carries tightening force and side load noticeably worse.
Can two ribs be placed close together?
Consider it from the mould side: between two ribs sits a thin steel blade carrying the injection pressure, and too close together it breaks easily and the mould is hard to make. Ask the mould maker for the minimum spacing based on the rib height and the mould material.
Frequently asked questions, continued
Can a rib be as thick as the wall?
Better not. A rib as thick as the wall forms a mass at the rib base, cools slowly, and leaves a sink mark on the opposite face. Keep the rib thickness at about 0.5–0.6 × wall thickness.
How tall can a rib be?
About 3 × wall thickness. Taller and it is hard to fill to the rib tip and warps easily; if you need more stiffness, add several short ribs rather than one tall rib.
Bosses tend to sink — how do I reduce it?
Keep the boss wall at about 0.6 × the main wall (core it out rather than leaving it solid), tie the boss to the wall with gussets, and radius the boss base. A solid boss standing alone is a classic sink source.
A quick table for the shop floor
Ribs and bosses hold up when the situation, the way they are made and the purpose are lined up:
| Situation | How to make or control it | Purpose |
|---|
| Ribs | Thinner than the main wall, filleted root | Add stiffness, reduce sink |
| Screw bosses | Cored and supported by ribs | Avoid a thick mass of plastic |
| Spacing | Do not crowd ribs at one point | Reduce uneven shrinkage |
A worked case
A boss standing on its own splits when tightened, while a boss that is too thick sinks the opposite face. Tying the boss to the wall with two or three thin ribs is usually more stable, but check the flow and the weld-line position.
Conclusion
Plastic Ribs and Bosses is not paperwork done to make a file look tidy. It is how intent becomes a result that can be manufactured, assembled and measured repeatedly. A good drawing does not need the designer standing beside it to explain it; the structure of the information has to do that work.
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