Does more case rib design mean a stiffer lid?

Answer: No. Three ribs to twelve takes sag from 3.2 mm to 4.1 mm and adds 4% weight. Stiffness comes from height and section, not count.

Why cap rib height at three times the wall?

Answer: Past that the root turns into a heavy thermal node, sink runs from 0.1 mm to 0.3 mm, and scuffing gets likelier. A 3 mm wall means 9 mm; stiffer wants a ring.

Why are rotomoulded ribs thicker than moulded ones?

Answer: Powder under-fills a narrow groove. Below 4 mm the crest drops under 2.2 mm and dishes after demoulding.

Where does the 1 t, 24 h duty come from?

Answer: Five stacked cases at 200 kg each, times 1.5 for safety, gives 1 t; 24 h covers one sea leg, 40 °C a container in summer.

Protective Case Rib Design: Why More Is Not Better

Quick answer: Case rib design lives on ratios, not counts: rib height under three times the main wall, thickness at 0.6–0.8 of it, spacing at least twice it. Under 1 t for 24 h, three 22 mm ribs sag 3.2 mm while twelve 8 mm ribs sag 4.1 mm and weigh 4% more.

The Three Ratios That Govern Case Rib Design
Past the Ratios: Sink, Weld Lines and Dead Weight
Three Ribs Against Twelve Under 1 t for 24 h
Ring, Grid and Corner Gusset
Injection and Rotomoulding: Two Reworks
FAQ

1. The Three Ratios That Govern Case Rib Design

Four numbers decide if case rib design works: rib height at or under 3× the main wall, rib thickness at 0.6–0.8×, spacing at least 2×, and 1°–2° of draft. Meet all four and a 3 mm wall carries 1 t for 24 h.

Ratios let the rib freeze before the visible face. A rib as thick as the wall holds twice the heat of the walls beside it, so the last patch to shrink pulls a hollow into the surface; at 0.6× the root gap stays inside 0.05 mm.

More ribs is not more stiffness.

Stiffness follows second moment of area, and that follows rib height cubed: twelve 8 mm ribs do not add up to three 22 mm ribs. Pile on count and you meet sink and weld lines before a stiffness limit.

Item	Recommended	Cost of exceeding
Rib height over wall	≤3	Root sink, 0.1–0.3 mm
Rib thickness over wall	0.6–0.8	Surface hollow, deeper weld lines
Spacing over wall	≥2	Uneven cooling, 0.4 mm warp
Draft	1°–2°	Scuffing, 30% more ejector force

2. Past the Ratios: Sink, Weld Lines and Dead Weight

Break the ratios and the cost shows. A rib at 1.0× wall leaves 0.1–0.3 mm of sink at the root; paint needs sanding, and sanding past 0.15 mm erases the grain, so rework runs 12%.

case rib design: off-white slim case closed and facing forward, two same-colour latches on the front beam, recessed grip, smooth outer wall

Weld lines are the second bill. Every extra rib joins two flow fronts, and that joint keeps 40%–60% of body strength; twelve fine ribs form a net that a 1.2 m drop cracks first.

Fill pressure climbs with count: on one 380 t press, three ribs fill at 78 MPa and twelve at 92 MPa, up 18%, so clamp force rises and vent depth is re-cut from 0.02 to 0.03 mm.

Cooling follows: each 1.8 mm rib acts as a fin, so three to twelve takes the lid from 26 s to 34 s, about 6% of output. Dense case rib design saves material and pays in flatness.

Weight against stiffness is where the arithmetic fails: twelve fine ribs add 8% weight for 3% stiffness, three main ribs add 4% for 21%. That gap separates ratio from count.

3. Three Ribs Against Twelve Under 1 t for 24 h

One duty for both: 60 L body, 3 mm wall, 1 t over the top face, 24 h, 40 °C. Three main ribs sag 3.2 mm, twelve fine ribs 4.1 mm. Case rib design draws stiffness from height and section, not count.

The reason is the boundary. The top is held on four sides, so sag follows bending moment; main ribs cut the 380 mm span into two 190 mm spans and drop the moment to a quarter. Fine ribs only dice the plate and add dead weight.

Recovery is the second set. A day after unloading, the main-rib lid springs back 0.6 mm and keeps 2.6 mm of set; the fine-rib lid springs 1.1 mm and keeps 3.0 mm. Larger spring-back eats 15% of gasket compression.

State the basis or the numbers cannot repeat: 4 mm tetrahedra, two refinement layers at the root, PP at 1.1 GPa, 0.009 MPa on the face. Change the boundary and both figures move.

Layout	Ribs	Height	Sag	Added weight	Set
Main ribs	3	22 mm	3.2 mm	+4%	2.6 mm
Medium ribs	6	16 mm	3.6 mm	+6%	2.8 mm
Fine ribs	12	8 mm	4.1 mm	+8%	3.0 mm

4. Ring, Grid and Corner Gusset

A ring rib closes around the rim and turns wall bending into hoop tension, suiting thin walls that spread at the mouth. A grid rib runs both ways and makes each crossing a stress raiser. A corner gusset takes load off the corner post.

case rib design: blue slim case facing forward, two slim knuckles raised at the back edge, front latches and finger recess forming an arc

Lid and base share no load path. The base stands on four corner posts at 6.8 MPa under 1 t, so a 2 mm floor is enough. The lid takes face pressure, so ribs must run the short way into the corners. Run them the wrong way and the span changes. Along the long side the span is 380 mm; across the short one 280 mm, 1.8 times the sag at equal rib height. A ring is not free: closing the rim moves impact to mid-wall. Case rib design with a ring adds one horizontal rib there.

5. Injection and Rotomoulding: Two Reworks

An injection moulding needs draft on every rib, 1°–2° as a floor, costing 0.4–0.8 mm a side at 22 mm depth. A rotomoulded rib needs none; the price is wall uniformity.

Powder fuses on the mould wall and does not always reach the bottom of a groove, so the crest often measures 2.2 mm against a 3 mm wall, 27% thin; an R3 fillet at the groove floor lets powder through.

Injection holds a 1.2 mm rib without touching cycle time, while rotomoulding struggles below 4 mm, so at equal stiffness a rotomoulded case rib design goes taller and fewer. Crests need a radius too: R1.5 avoids whitening, and under R1 mm the ejector load concentrates and a drop starts a crack.

Stack	Load on the top	Height	Ribs	Thickness
1–2 high	≤300 kg	12 mm	3	1.8 mm
3–4 high	300–800 kg	18 mm	3 plus 1 ring	2.2 mm
5 or more	≥1 t	22 mm	4 plus ring and gussets	2.4 mm

Conclusion

Case rib design settles on one line: ratios decide, and the count is a result. Three main ribs at 22 mm high hold sag to 3.2 mm under 1 t for 24 h; twelve ribs at 8 mm give 4.1 mm, 4% more weight, 3% more stiffness and a net of weld lines. Check height-to-wall, thickness-to-wall and rib direction; the count then falls out.

This article discusses only the container (case body) side of specifications and design intent, and does not cover any handling, loading or use of the contents.