A snap fit can locate a cover and hold it in place, but it cannot seal one. Formed metal features touch at points and lines, not along a continuous compressed surface, so water and dust have an open path the whole way around the joint. If the box needs a NEMA type or IP code, a snap-fit or slip-fit cover is not an option; you need a flat flange, a continuous gasket and fasteners, or a welded seal you never reopen.
This matters most with small deep-drawn steel or stainless boxes. They are a common choice for sensors, transmitters and potted electronics, and the natural instinct is to want a lid that pops on like a battery cover. The physics of a formed joint says otherwise.
What a snap fit actually does
A snap fit is a retention feature. A formed ridge on one part snaps into a groove or undercut on the other, and elastic deflection during assembly holds the two parts together. The same idea appears as a slip fit, where a shallow drawn cover simply slides over the open end of the can.
Both give you three things: location, so the cover sits square; retention, so vibration and handling do not shake it off; and repeatable assembly with no tools. What neither gives you is compression of a gasket around the perimeter. The contact between the parts is a set of discrete points and short lines. Between those points there is nothing but an air gap.
Ingression works on the weakest path. A rating test does not care that the box is mostly closed; it cares whether water or dust can follow the joint around. A point-contact joint fails that test every time. The NEMA enclosure types guide explains what each rating demands of the joint.
Why a formed joint cannot be a seal
A seal needs a soft, continuous material squeezed uniformly along the whole joint line. A rubber O-ring works because it fills the gap everywhere the groove goes. Formed sheet metal cannot do this:
- Draw tolerances on a deep-drawn part are loose compared with machined flanges. A small can may vary by several tenths of a millimetre over its perimeter, so a ridge meets its groove deeper in one corner than in another.
- Metal-on-metal contact seals nothing. Even a tight line contact leaves a capillary path that water jetting or condensation will find.
- Forming an undercut deep enough to grip also thins and work-hardens the flange, so the joint is not repeatable part to part.
- A gasket riding on an undulating drawn lip gets pinched at the high spots and unloaded at the low spots. Compression that varies that much is not a seal.
This is why you see snap lids on cookie tins and grease caps, where the formed lip and some grease handle splash, but never on a box carrying a rating plate.
What each closure method gets you
| Closure method | Retention | Dust tight (IP6x) | Water jet (IPx5/x6, NEMA 4) | Submersion | Reopenable |
|---|---|---|---|---|---|
| Slip-fit cover, no seal | Yes | No | No | No | Yes |
| Slip-fit cover with grease or O-ring in groove | Yes | Maybe, uncontrolled | No | No | Yes |
| Snap features on formed lip | Yes | No | No | No | Yes |
| Flat flange, continuous gasket, screws | Yes | Yes | Yes | Sometimes | Yes |
| Welded cover, hermetic feedthroughs | Yes | Yes | Yes | Yes | No |
The table assumes a drawn can of the kind used for sensors and electronics, roughly 2 in (50 mm) across. Ratings are manufacturer ratings for a specific design; nobody can grant them to a snap-fit joint after the fact. Where NEMA types and IP codes overlap, the NEMA 4 vs 4X vs IP66 guide covers the details.
The practical fixes for a small drawn box
Flanged cover with a gasket and screws. Have the open end of the can trimmed square and a flat flange formed or machined on the cover. A die-cut closed-cell gasket, 1/16 to 1/8 in (1.5 to 3 mm) thick, sits in a groove or against a land, and small screws every 2 to 4 in (50 to 100 mm) pull it down evenly. This is the standard route when you still want to open the box. Watch the screw pattern near corners: corners unload first, so put a screw close to each one.
Welded and potted. If the unit is truly non-serviceable, weld the cover and bring the wires out through glass-to-metal feedthroughs or potted gland fittings. That is a hermetic path and survives submersion. You give up serviceability entirely, so it only suits potted electronics.
A threaded or clamped ring. A screw-on cap with a captive O-ring, or a clamp ring pressing the cover down, gives even compression without a row of screws on a small part. Tooling cost is higher, but it assembles fast in volume.
Move the electronics instead. Sometimes the cheapest answer is a small rated enclosure with a proper gasketed door, with the potted assembly mounted inside. If the drawn can is only a mounting shell, it does not need a rating of its own.
Where the real leak paths hide
Even a gasketed cover can fail the rating if the rest of the box is not sealed too:
- Lead-throughs. Every wire crossing the wall needs a proper gland or a potted feedthrough; a single bad gland undoes a good gasket. The IP rating chart guide shows what each code assumes about the whole enclosure, not just the door.
- Mounting holes. Screws that pierce the wall of an outdoor box need their own sealing strategy: outside the flange, a captive washer, or a rear mounting plate.
- The joint between the box and its mounting surface, especially outdoors, where capillary paths form along the fixing screws.
What to specify
When you write the drawing or the purchase spec, state the closure explicitly:
- Joint type: flanged cover with continuous gasket, not snap or slip fit, whenever a rating is claimed.
- Gasket: closed-cell silicone or EPDM, compressed 25 to 30 percent, in a groove or against a defined land width.
- Fasteners: size and spacing, with a screw near each corner, and captive so they stay with the cover.
- Rating the design is rated to, as a manufacturer rating; a NEMA 4X / IP66 rating means the manufacturer rates the specified design to that level, and the box is not UL listed.
- Hermetic units: welded cover plus feedthroughs, with the leak rate or test method stated if it matters.
If the box is one of a kind or a small batch, a custom enclosure built to your drawing is the straightforward path: you define material, size and every cutout, and the drawing you approve is what gets built, delivered in 10 business days. The enclosure builder lets you configure the joint, gasket and cutouts in 3D and request a quote. For bigger panels with hinged doors the same logic scales up; the gasket and fastener spacing rules do not change.
The underlying rule is worth repeating once more: retention and sealing are different functions. A formed joint buys you retention cheaply. A rating costs you a gasket, compression and fasteners, and no forming operation can substitute for them.
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 |
|---|---|---|---|
| 12 × 10 × 6 in | 304 stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 16 × 14 × 8 in | 304 stainless | 16 x 14 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 20 × 16 × 8 in | 304 stainless | 20 x 16 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Can a snap-fit cover hold a NEMA 4 or IP65 rating?
No. A snap or slip fit only locates and retains the cover; it compresses a gasket at a few formed contact points, if at all, so water and dust paths remain along the joint. Ratings such as NEMA 4 / IP66 require a continuous compressed gasket around the joint, which means fasteners or clamps, not snap features. Ratings are manufacturer ratings for a specific design; a snap-fit box is never one of them.
What is the difference between retention and sealing?
Retention keeps the cover from falling off under vibration, handling or impact. Sealing keeps water, dust and oil out of the interior. Retention needs contact at a few points; sealing needs uniform gasket compression around the whole perimeter. A design that solves the first almost never solves the second, which is why snap lids exist on battery remotes but not on rated enclosures.
How do you make a small metal enclosure watertight without soldering?
Use a cover with a machined or formed flat flange, a continuous gasket sized for it, and screws at a spacing that keeps gasket pressure even, typically 2 to 4 in (50 to 100 mm) apart near corners. For a true hermetic seal, weld the cover and seal the lead-throughs with glass-to-metal feedthroughs or potted glands. Choose by how often you need to open the box.
Does gasket material matter on a small enclosure?
Yes. Closed-cell silicone and EPDM tolerate compression sets and temperature swings better than open-cell foam, which wicks water. A die-cut closed-cell sponge gasket of 1/16 to 1/8 in (1.5 to 3 mm) thickness, compressed about 25 to 30 percent, is a common starting point for a small drawn box. Match the material to the chemicals the box will see.
