A lid roughly 600 x 250 mm (24 x 10 in) with 3 mm (0.12 in) walls is a clamping problem before it is a gasket problem. Thin walls bow between screws, and IP68 means the seal must hold under continuous immersion, not just a splash. The concept that works is a flat gasket face with locating features, solid elastomer, close screw spacing, and molded compression stops so the lid bottoms out before it bends. Everything below assumes ratings set by the manufacturer of the finished assembly; IP68 is a rating claimed after testing, never a property a gasket alone gives you. For what the code actually tests, read what an IP68 rating means for an electronics enclosure.
The short answer
Choose one concept and commit to it:
- Seal: molded solid silicone or EPDM gasket, 50 to 60 Shore A, 2 to 3 mm (0.08 to 0.12 in) thick, compressed 20 to 30 percent at the stop.
- Geometry: flat face inside the screw line, with a shallow raised lip or tongue-and-groove ring that locates the gasket and traps it against pressure.
- Clamping: screws on 3 to 4 in (75 to 100 mm) centers, flanged heads or washers, bosses on both halves so the screws pull flange against flange instead of squeezing the thin wall.
- Stop: the molded lip bottoms out at target compression, so an assembler cannot over- or under-tighten the seal.
That combination holds immersion pressure on a thin lid and reduces assembly to placing one gasket, dropping the lid, and running the screws to the stop.
Pick the gasket concept before the material
On a thin lid, geometry does more work than elastomer choice. The two concepts worth building:
Shallow tongue and groove. A tongue on the lid sits in a groove in the body, with the gasket filling the groove. The walls of the groove stop the gasket from extruding outward under pressure and stop the lid from sliding during assembly. Keep the groove depth modest, around half the gasket thickness, so the 3 mm wall keeps its strength. The details are in tongue and groove gasket design for a sealed enclosure.
Raised lip with flat gasket. A continuous lip stands 1 to 2 mm (0.04 to 0.08 in) proud of the body flange, inside the screw line. The lid presses the gasket against this lip; the lip is the compression stop. This is easier to mold than a groove and self-locating. See raised lip or flat face gasket in a sealed enclosure.
A plain flat face with no locating feature is the third option, and the weakest here. It works on stiff flanges with dense screw patterns, which a thin 600 mm lid is not.
Choose the elastomer
Solid beats foam for immersion:
| Property | Solid silicone / EPDM, 50-60 Shore A | Closed-cell foam |
|---|---|---|
| Compression set | Low; recovers for years | Higher; takes a permanent set |
| Compression control | Needs a stop; force is higher | Forgiving; wide range |
| Pressure holding | Good with trapped geometry | Limited; cells can link up |
| Assembly simplicity | High: drop-in molded gasket | High: peel-and-stick |
| Best for | IP67 / IP68, repeated opening | IP65 and below, thick stiff flanges |
Foam is not wrong as a material; it is wrong for this job. The trade-offs are laid out in foam or rubber gasket for an enclosure door seal. If you use solid elastomer, mold the gasket to shape rather than cutting a flat sheet, so the corners are molded and the cross-section is uniform. A glued cord joint in a corner is a classic leak. Silicone handles wide temperature swings; EPDM resists water, ozone and many chemicals but not oils. Pick per the environment the box lives in.
How to design the seal step by step
- Define the gasket footprint. Run a continuous closed loop inside the screw line, with rounded corners of at least 3 mm (0.12 in) radius. Molded corners seal; cut corners do not.
- Set the gasket section. Start at 2 to 3 mm (0.08 to 0.12 in) thick solid elastomer, 50 to 60 Shore A. Target 20 to 30 percent compression at closure.
- Add the locating feature. Mold a shallow tongue-and-groove or a raised lip on the body flange. The lip height equals your target compressed thickness, so the lid seats on the lip at full compression. That lip is your stop and your gauge.
- Space the screws. Keep 3 to 4 in (75 to 100 mm) between screws along the loop. Where the long span of the lid crosses the middle, hold to the low end. Bosses on the lid and body, aligned to each other, keep the clamp load in the flanges instead of pulling the 3 mm walls.
- Spread the clamp. Use flanged screw heads or flat washers sized so the bearing area under each screw is at least four times the screw's core area. A screw head pulling straight into a thin wall dents it and unloads the gasket either side.
- Control torque, not feel. Specify a torque that reaches the stop without crushing past it. With a positive stop the torque window is wide, which is exactly what you want for simple assembly.
- Give the gasket a home. A shallow recess or the groove itself retains the gasket so the lid can be assembled with the box in any orientation and no adhesive. Retention beats stick-on for repeatability.
- Plan the test. Manufacturer rating means the finished assembly is tested: pressure-change or water-bath leak test per the rating you claim. Build a test lid with a pressure port early, before the molds are cut.
Checks when the design is done
- Gasket compression at the stop is between 20 and 30 percent everywhere, including corners. Corners compress more than straights if the section is not molded for it.
- The lid, screwed to the stop, shows no visible bow between screws. If you can see a scallop, add screws or stiffen the flange.
- The lid assembles in one orientation only, or any orientation, by design, not by accident.
- A leak test on a prototype at the immersion depth and duration behind the IP68 claim passes, and passes again after ten open-close cycles.
- Screw bosses and the sealing loop clear your internal components and mounting features; check this on the layout before the mold, using the same discipline as a cutout drawing for a custom enclosure.
Mistakes that cost the seal
No compression stop. An assembler torques until the lid dishes; the gasket is over-compressed at the screws and unloaded between them. The stop fixes this permanently.
Screws outside the gasket loop. The gasket must be inside the screw line. A loop outside the screws leaves the gasket unsupported between fasteners, and on a thin lid that gap opens first.
Cut sheet gasket with butt joints. Every joint is a leak under immersion. Mold the loop.
Trusting adhesive as the seal. Bonded lids can be sealed assemblies, but any contamination on a molded surface gives a hidden leak path and you cannot inspect it. If the lid opens ever, gasket and screws are the maintainable answer; sealing penetrations in a submersible electronics enclosure covers the same logic for penetrations.
Ignoring the pressure question. IP68 is claimed at a stated depth and duration. Air trapped in a sealed box heats in the sun and pushes outward; cooling water outside pulls inward. A trapped gasket in a groove handles both directions; a flat unrestrained gasket may not. If the box cycles in temperature daily, plan for that, and read condensation in a sealed enclosure for what happens to the air you seal in.
When to hand the sealing to the enclosure maker
If the box does not have to be your molded part, a formed stainless body with a gasketed door solves the same problem with the seal already engineered. Our wall-mount enclosures are rated NEMA 4X / IP66 by the manufacturer, not UL listed, and we build custom enclosures in 304 and 316L to your size and cutouts, delivered in 10 business days from your approval of the drawing. That route trades your mold for a proven flange, gasket and hinge assembly, and it is worth a quote before you commit tooling to a thin-walled lid. IP68 specifically, with its stated depth and time, is a rating the enclosure maker claims by test, so ask for the test basis when the box will sit under water rather than in rain; our standard boxes target 4X / IP66, and prolonged submersion is a conversation about the design first.
Enclosures for this job
Catalog sizes that suit the job this article answers, picked from its subject; open one for its full specifications.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 60 × 36 × 16 in | 304 stainless | 60 x 36 x 16 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 48 × 36 × 16 in | 304 stainless | 48 x 36 x 16 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 60 × 36 × 12 in | 304 stainless | 60 x 36 x 12 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 12 × 10 × 6 in | 304 stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
How close should screws be on a large thin lid?
Plan on 3 to 4 in (75 to 100 mm) between screws along the seal path for a lid near 600 x 250 mm with walls around 3 mm. Thin sections bow between fasteners, and any gap over about 4 in lets the gasket relax enough to leak under immersion pressure. Add screws near the middle of the long sides, where bowing is worst, and use flanged heads or washers so the clamp load spreads into the wall.
Foam or solid elastomer for an IP68 lid seal?
Solid elastomer, such as molded silicone or EPDM around 50 to 60 Shore A, is the safer choice for continuous immersion. Foam compresses easily, which suits thick or stiff flanges with coarse screws, but it takes a permanent set over time and its compression range is hard to control on a thin lid. A solid cord or a molded flat gasket with a defined stop gives you a repeatable seal every assembly.
Do I need a groove, or is a flat face enough?
A flat face works when the flanges are stiff and the screw pattern is dense. On a thin-walled lid near 600 mm long, a shallow tongue and groove or a raised lip pays for itself: it locates the gasket during assembly, traps it against pressure, and keeps compression on the gasket footprint instead of bending the lid. Keep the groove shallow so the thin wall is not weakened.
Can I just glue the lid on instead of using a gasket?
You can, and bonded lids are used on sealed enclosures, but you lose serviceability and the seal depends entirely on surface preparation. Any oil, release agent, or finger print on a molded part gives you a leak path you cannot inspect. If the lid never needs to come off, a qualified structural sealant is an option; if it might, use a gasket and screws and save the adhesive for a secondary bead if you ever need one.
