This article is part 2 of 2. Part 1 covers the basic design strategies for water-tight enclosure design.
Find Part 1 here: Designing IP-rated enclosures Part-1
This article explores the common components used for sealing, plus how and where to use them.
O-rings
O-rings are the answer to most sealing questions. Ubiquitous and available in a huge range of sizes and materials, the main challenge with using O-rings is how to pick the right one.
O-rings are frequently used on shafts (piston seal), on the inside wall of pockets (rod seal), or on the face of a mating part, like a gasket (axial seal). They can be used on slow-moving parts or fixed parts. The groove that an O-ring sits inside is called a gland. The design of the gland is critical to the sealing performance and the longevity of the seal.
When designing an O-ring gland, online calculators will get you most of the way. Apple Rubber and Eriks calculators have proven to be easy to use and are my go-to calculators: applerubber.com
O-rings are available in different cross sections. Round is the most common and works for most applications, but they are also available in square, cross, and more.
O-rings as irregular face seals
O-rings are supplied as circular rings, but that's not the only way to use them. For an axial seal arrangement where two faces are mated together, they are often routed into a rectangular shape or an irregular shape matching the shape of the part. When used this way, they are similar to a gasket, but cheaper and can be sourced off-the-shelf, making them much easier to maintain through the product's life.
Use generous corner radii for glands like this; the O-ring will be difficult to install with tight corner radii. When designing an irregular gland like this, the online calculator tools are still useful but have to be used carefully. The compression and free space values still hold, meaning the depth and width measurements found by the calculator can be transferred to the gland design. The inner and outer radius of the gland (face seal) and inner and outer diameter of the O-ring need to be converted. Use the measurement tools in CAD to measure the outer perimeter (or inner for vacuum seals), and with appropriate conversion, compare that to the calculator's gland outer diameter. Be careful using pre-stretch for this type of seal; it does not help the sealing or assembly, so it's usually best to have no stretch in the design for this type of seal.
Avoiding face seals
Face seals require a lot of fasteners to apply even pressure to the O-ring. Consider that most small O-rings have significantly less than 1mm compression; just a small amount of flex will compromise the seal. The sealing components need to be rigid and have very low compression set.
Wherever possible, using an annular O-ring arrangement (piston seal) is preferable. It requires fewer fasteners and guarantees a reliable seal. A small vertical wall with a gland machined in the side is often the easiest solution. This approach can be used for round parts or irregular shapes (convex corners only). Where space is limited, it is still often preferable to use a smaller O-ring in an annular arrangement than a thicker O-ring as a face seal.
Lubricating O-rings
Lubrication does two things. It lets the ring slide into the gland without twisting, rolling, or picking up a nick during assembly - pair it with a lead-in chamfer on any edge the ring passes over. And it fills the micro-roughness in the sealing surfaces, which improves sealing at the low pressures. It's even more important on moving seals as it greatly reduces friction.
Silicone grease is the go-to for most O-rings (NBR, EPDM), but never on silicone O-rings, where the oil swells the rubber. Food-safe silicone grease is available for anything sensitive.
A correctly designed O-ring gland should not require grease to make it seal, but experience has shown that using grease on an O-ring does make a significant improvement to sealing performance.
Shaft Seals
Anything that rotates or slides through the enclosure wall is the hardest sealing problem in the product. Every dynamic seal leaks eventually; you are designing for a service life, not a permanent solution.
Shaft seals are specially designed for rotating and sliding shafts. Use these when the amount of movement is substantial, is very frequent, or fast-moving. These seals do not require a gland; they usually press into a pocket in the enclosure and have a lip that squeezes onto the surface of the shaft.
They come in various types for different applications, pressure ranges, temperature ranges, and material compatibility.
Shafts need to be machined with a very high surface finish for a reliable and long-lasting seal. The surface of the shaft also must be relatively hard to avoid the seal wearing the surface over time. Always design a chamfer lead-in on the shaft to avoid damaging the seal and apply lubricant before assembly. If a lead-in is not practical, an assembly tool can be designed to guide the seal onto the shaft safely and then removed.
Lip seals need lubrication to survive. A dry-running lip overheats, hardens, and cracks. For slow, intermittent rotation, an O-ring in a radial gland can work with a suitable dynamic squeeze and generous grease, but it isn't a substitute at speed. Sliding shafts drag dust and dirt on every stroke. Add a wiper ahead of the main seal to protect it and extend its life.
Design the seal to be replaceable, because it will probably need replacing. Use standard metric (almost goes without saying!) sizes to ensure replacements will always be easy to find.
Gaskets
There is a wide range when it comes to gasket design. For complex and demanding applications, find a good gasket specialist and lean on their expertise. Most enclosures I have designed have been able to be sealed by O-rings or other means, and the ones that do require gaskets have been relatively simple gaskets.
A gasket can follow any shape, seal around multiple penetrations, and accommodate a flange that isn't a clean closed loop. The trade-off is tooling, lead time, and a part number you have to keep supplying for the life of the product.
For low-volume products, die-cut or laser-cut EVA foam (or another closed-cell foam) is a very easy solution. It is extremely forgiving and works well. Beware of compression set and high temperatures. For a flat gasket like this, design in retention such as a perimeter wall or locate the gasket inside a pocket/gland. Add sealing ribs/beads to the components to apply localised pressure to the gasket to maximise sealing performance for a given squeeze. This will make the joint less sensitive to uneven clamping pressure and part flex and require lower clamp pressure overall.
For high-volume products, a custom moulded or overmoulded elastomer seal can become a better option. This seal can be designed with exactly the right shape and material for the application. Sealing features can be added to the shape of the seal so that sealing ribs/beads are not needed on the part surfaces.
Add compression stops to the parts to protect the gasket. A soft gasket will compress until something stops it, and an over-compressed gasket takes a permanent set and stops recovering. Design solid bosses or lands that bottom out at your target compression.
Pressure-Equalisation Vents
Air inside a sealed enclosure expands and contracts as the product heats and cools - environmental effects or just the electronics warming up. That pressure differential pushes against the seals from the inside, then pulls against them as it cools, and any weak point will eventually draw water in.
The fix is a pressure equalisation vent. A small membrane, usually housed in a plastic shell, that lets air and vapour pass while blocking liquid water. Gore is a well-known brand, but there are plenty of equivalents. They are cheap, need no maintenance, and are available in threaded, snap-fit, and adhesive versions to suit almost any wall.
Mount the vent on the underside of a product where it will be sheltered and protected. One vent is usually enough for the entire product, assuming the sealed regions are all connected.
Potting
Why seal an enclosure when you can easily seal the sensitive electronics inside? Potting is the prcess of encapsulating electronics in an epoxy or polyurethane resin. It is commonly used in electronics/PCB assembly. It is used to protect electronics from impact, water, vibration, etc.
Potting is generally a one-way process. Removing the potting resin is nearly impossible, which makes repair and rework impossible. It's critical to complete all testing on the product before potting so the electronics can be reworked if necessary.
When designing for potting, you must consider that the resin has water-like viscosity when applied. The boundary of the potting is usually created by a pocket in the enclosure or a secondary part within the enclosure. A dam can also be created with foam or a machine-applied adhesive into which the potting is then applied.
Potting material is relatively expensive, so reduce the potting volume as much as possible to save cost. Potting is usually applied by a CNC applicator - multiple parts can be arranged on the bed, and an accurately metered amount can be applied to each part.
Final Thoughts
There's no universal sealing solution, but there is usually an obvious one. Match the component to your production volume and your service expectations, design the parts around the seal rather than the other way around, and prototype early - sealing is one of the few areas where a cheap 3D-printed test will tell you almost everything you need to know before you commit to tooling.
Whether you're refining a concept or scaling up for production, reach out if you'd like help you get there faster.