Is a deeper case gasket groove safer?

Answer: No. Every extra 0.5 mm of depth costs 4–5 points of compression, so 3.2 mm deepened to 3.7 mm takes a 29 % joint down to 18 %.

Can a single-lip case be converted to a double lip?

Answer: Not by swapping parts. The double channel is 6.5 mm against 5.5 mm and lid and base must be replaced as a pair.

What hardness should the cord be?

Answer: 40–50 Shore A for EPDM sponge. Below 40 gives under 0.2 MPa of contact pressure; above 55 adds 30 % to closing force.

Does the debris pocket need drains?

Answer: Yes. Drill a 1.5 mm drain every 150 mm, otherwise trapped water freezes at -20 °C and splits the outer lip.

Case Gasket Groove: Single vs Double Lip

Quick answer: A case gasket groove is set by three numbers, groove width, groove depth and lip radius, and it seals only between 20 % and 30 % compression. A single lip gives one sealing line and a light latch; a double lip adds a second line and a debris pocket for 25 %–35 % more closing force.

What the three groove dimensions fix
How a single lip holds the gasket
What a second lip buys you
Hitting 20 % to 30 % compression
Closing force and draft angle
FAQ

1. What the three groove dimensions fix

The channel around a case rim is not a trench cut by eye. Width, depth and lip radius pull against each other, and getting one wrong leaves the gasket loose in the channel or crushed flat.

Take an EPDM sponge cord 5.5 mm wide and 4.5 mm tall. A case gasket groove 6.0 mm wide and 3.2 mm deep compresses it to 29 %, inside the working band. Deepen it to 4.0 mm and compression falls to 11 %, enough for a 30 minute shower to reach the rim.

Width needs its own allowance. Sponge cord swells 3 %–5 % in heat or damp, and 0.2 mm of side clearance makes it bulge upward instead of outward, sending load into the groove wall rather than the lid rib; 0.25 mm per side, a 5.5 mm channel for a 5.0 mm cord, leaves room to recover.

Depth is the dimension that actually seals. Past 35 % compression the cell walls collapse, rebound drops below 70 %, and after 200 closures the cord stays set, which is why the drawing order runs backwards from habit.

2. How a single lip holds the gasket

A single-lip case gasket groove looks like a one-sided retaining wall. One wall rises 1.5–2.0 mm into a lip, the other stays open, and the gasket seats against that lip while the lid rib flattens it.

Case gasket groove around the closed rim: black case seen head-on, arched moulded handle, two orange-red latches at the top

Simplicity is the argument. No slide is needed, groove and lip mould in one shot, and 1° of draft releases the wall cleanly, so a station closes a case in under 20 seconds.

The weakness is equally plain: one sealing line around the whole rim. Trap a 0.3 mm grain of sand and that spot is the only failure path, and grit lodges at the lip root where only an angled air gun reaches it.

Lip radius belongs between R0.5 and R0.8. Tighter and stress piles at the root, cracking after roughly 5000 closures; looser and the effective lip height shrinks, so the cord slides sideways while compression still reads correct on paper.

Floor flatness gets missed. A moulded rim on a 520 mm case warps 0.5 mm, leaving the middle of the cord 0.3 mm above the rib; 60 °C mould temperature brings that under 0.2 mm.

3. What a second lip buys you

A double-lip case gasket groove raises a wall on both sides and traps the cord between them. The lips need not match: 1.8 mm inside and 1.2 mm outside is common, the taller one clamping, the shorter one shedding water.

That second lip pays in three ways. Sealing lines go from one to two with a 0.4 mm pocket between them, so when the inner line lifts over a particle the outer still holds. Debris sits in the pocket instead of on the sealing face, and the cord cannot shear sideways, loosening assembly tolerance from ±0.3 mm to ±0.5 mm.

The bill arrives as closing force, 25 %–35 % higher: where the single lip holds 0.35 MPa with two latches, the double lip needs three or four, halving spacing from 260 mm to 130 mm.

Tooling carries the rest: a slide or core pull adds 1200–2200 USD to the mould and 8 %–12 % to piece price, never amortised below 3000 units a year.

Item	Single lip	Double lip
Sealing lines	1	2
Groove width / depth	5.5 mm / 3.2 mm	6.5 mm / 3.4 mm
Lip height	1.8 mm	1.8 mm inner, 1.2 mm outer
Debris pocket	None	0.4 mm channel
Closing force	Baseline 100 %	125 %–135 %
Latches required	2	3–4

4. Hitting 20 % to 30 % compression

Compression is one line of arithmetic: free height minus groove depth, divided by free height. A 4.5 mm cord in a 3.2 mm case gasket groove sits at 29 %; in a 3.6 mm groove it sits at 20 %, the floor.

Below 20 % the seal is nominal. Sponge carries 0.1–0.2 mm of surface roughness, and without enough squeeze those peaks rest on the lid rib, cutting contact area to about 40 %.

Case gasket groove at the lid joint: black case turned slightly, double rim pressed shut, orange-red latches and moulded feet

Above 30 % the failure is slower but worse: cells collapse, rebound measures 65 % after 24 hours, and the second closing leaves a wider rim gap.

Measure with a projector or a calliper resolving 0.01 mm, three depth readings and five cord readings per section; a spread beyond 0.15 mm points at the tool or the cord. Call 25 % with a minus tolerance rather than a loose band.

Compression	Contact width	Rebound after 24 h	Verdict
15 %	0.9 mm	88 %	Too light, weeps
20 %	1.4 mm	85 %	Floor, usable
25 %	1.9 mm	82 %	Recommended
30 %	2.3 mm	76 %	Ceiling, watch force
35 %	2.6 mm	63 %	Crushed, set

5. Closing force and draft angle

More closing force is not more sealing. Past 120 N per latch an operator closing four tires, cycle time stretches from 25 seconds to 40, and half-latched closures climb from 2 % to 9 %.

Draft follows depth. At 3.2 mm, 1.0°–1.5° per side releases without scuffing. Push to 2° and the mouth opens 0.1 mm, costing 3 points of compression, so the 25 % on the drawing arrives as 22 %.

Weld lines must stay out of the groove floor, where a 0.05 mm mark passes water under peak load; move the gate to a corner radius and the weld lands in a lightly loaded bend.

PP shrinks 1.5 %–2.0 %, about 0.4 mm per side on a 520 mm rim, so cut the cavity 1.8 % oversize and the case gasket groove lands on nominal.

A silicone cast settles the first article: pour, demould after 30 minutes, slice and view at 20× to read lip height, depth and radius together.

Conclusion

A case gasket groove is decided on the drawing, not on the line. Width takes a 0.25 mm allowance per side, depth follows a 25 % compression target, and a lip radius of R0.5–R0.8 moves stress off the root. Single lip wins on simplicity; double lip trades 25 %–35 % more force for a second sealing line. Measure the cord before choosing the lip count.

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.