Sealing: What "Airtight" Has to Mean Before It Is Tested
A seal is a groove, a squeeze and a material. Get the groove wrong and no O-ring saves it.
Three Ways to Seal a Plastic Joint
| Type | How it works | Best for |
|---|---|---|
| O-ring in a groove | Elastomer compressed 15–25 % between two machined faces | Reopened joints, multi-part packs |
| Flat gasket | A die-cut disc pressed on a land; needs flatness, not depth | Caps, jars, wide mouths |
| Cone or plug | Two tapers meet; sealing by interference | Droppers, small closures, one-time seals |
Most designs that "leak" use a flat gasket where an O-ring belongs: the land is too narrow, the plastic is not flat, and the seal only works on the sample that was hand-tightened.
The Groove Is the Design
For a 2 mm cord O-ring, the groove is typically 2.2–2.4 mm wide and 1.5–1.6 mm deep, giving 20–25 % squeeze. Too shallow and the ring is crushed and takes a set; too deep and it never touches the mating face. On a plastic part the groove also needs a radius at the edges, or the ring is cut on assembly.
Material Follows Chemistry
Silicone covers most emulsions and survives −40 to 200 °C. EPDM is better with solvents and worse with oils. Nitrile is the default in general industry and the wrong choice for a fragrance. We ask for the formula's solvent before we specify the ring, because changing it later means a new groove depth.
The Test, Written Down
"Airtight" is not a specification. The line we work to is a vacuum hold — for example −0.3 bar for 30 seconds, no bubble, no drop — plus a shake test with the real formula and a 24-hour inverted hold. If a customer has their own standard, we test to theirs.
A pack that passes a vacuum test and fails a shake test is usually a thread issue, not a seal issue: the joint opens a fraction under load and the ring loses squeeze.
Left-Hand Thread, and Why It Matters to a Seal
If both joints on a multi-part pack turn the same way, opening the cap can unscrew the body. One side goes left-hand, so the load tightens the other joint instead of loosening it. This is a design decision, not a manufacturing one, and it has to be made before the tool is cut.
Need a Joint Checked?
Send the section and the formula — we return the groove and the test plan.