Reference guide
Closure and Self-Seal Reference
The closure is the only join on a mailer made outside a factory, which makes it the part most likely to fail and the part most affected by how your team works.
This is the technical reference: how the bond actually forms, the four things that stop it forming, how storage affects it, and how to choose between the constructions available.
How the bond forms
Pressure, not folding.
A self-seal strip carries a pressure-sensitive adhesive under a release liner. The adhesive is already active; the liner is simply keeping it from touching anything.
The bond forms when the adhesive is pressed into contact with the film, and it forms in proportion to the pressure applied. Folding the flap places the adhesive near the film. Pressing it is what makes it stick.
That distinction accounts for most seal failures we are asked about. A packer who folds and moves on has created a bond that holds on the bench, survives the first handling, and fails somewhere in a sorting hub.
The four things that stop a seal forming
Work through these before concluding the adhesive was faulty. In our experience it usually was not.
Contents reaching the strip
Part of the adhesive bonds to a garment instead of to the film. That section never holds, and it is where the seal opens.
Partial pressure
Pressing the middle and not the ends. Seals open from the corners, so the ends are the part that most needs it.
Liner fragments
A liner torn off in stages leaves paper stuck to the adhesive. Those patches are dead.
Contamination
Dust, moisture or hand cream on the adhesive. Bench cleanliness affects seal performance more than people expect.
Choosing a construction
Four options, each solving a different operational problem.
| Construction | Solves | Cost position |
|---|---|---|
| Single adhesive strip | Sealing quickly, once | Baseline |
| Double adhesive strip | Returns without a second bag | Small premium, often self-funding |
| Peel strip with tear tab | Opening without scissors | Small premium |
| Printed tear line | Opening in the right place, preserving a returns strip | Print cost only |
| Tamper-evident | Proving whether a parcel was opened | Higher |
Storage and ageing
Polyethylene film is stable for a long time. The adhesive is not, and it is the component that decides how long a carton of mailers stays usable.
Heat accelerates ageing, humidity affects tack, and prolonged light exposure does both. A carton stored in a cool dark room performs consistently; the same carton in a garage through a summer will seal noticeably worse.
The practical response is to order to a rate you will use, keep cartons sealed until needed, store them flat and off the floor, and rotate so the oldest goes first. If you are using stock that has been sitting for a long time, test a bag before relying on it — peel, seal, and pull firmly.
Closures and returns
In categories where a meaningful share of orders come back, the closure stops being a packaging detail and becomes part of the returns process. A double strip with a printed tear line lets a customer open, repack and reseal the original bag with no extra packaging and no instructions from you. Whether that is worth the premium depends on your return rate, and the arithmetic is set out on the self-seal mailer page.
Questions about the closure guide
Why do poly mailer seals come open in transit?
Almost always because the adhesive did not meet clean film across its full length, or because the strip was folded rather than pressed. Contents reaching the strip line and partial pressing account for the large majority of failures.
How long do adhesive strips last in storage?
It depends entirely on conditions. Stored cool, dry and dark, they stay reliable for a long time; stored warm or in daylight they degrade noticeably. Rotate stock and test a sample from any carton that has been sitting.
Can a sealed poly mailer be opened and resealed?
Only if it was specified with a second adhesive strip. A single-strip mailer seals once — reopening it destroys the bond and it cannot be restored with the original adhesive.