You are paying for barrier. Oxygen, moisture and light are what a mylar bag is for, and there are exactly two places that barrier can fail — a hole in the film, or a seal that did not fully bond. Both can be present on a pouch that looks perfect, and both are worth catching before your product is inside.
A brand switches to a heavier mylar pouch expecting a longer shelf life, and gets the same stale product six months later. The film is fine. The problem is somewhere else: a pinhole half a millimetre across, or a seal that looked closed and never fully fused across its width.
Neither shows up in a photograph, and neither shows up when you squeeze the pouch. They are findable, but only if you test for them deliberately. Here is what to do, in the order that catches the most problems for the least effort.
A pinhole does not look like a hole. On a laminated foil pouch it typically appears as a tiny bright speck, a micro-abrasion, or nothing visible at all until you put light behind the film. It gets there in one of three ways:
The consequence is disproportionate to the size. A single pinhole is a permanent path for oxygen and moisture, and it does not heal. On a product where shelf life is the whole point — coffee, dried food, anything oxygen-sensitive — one pinhole in a case is enough to generate a complaint that gets attributed to the pouch generally. And the barrier you are protecting is a property of the film stack rather than the gauge alone — how film structures and barrier layers work explains which ply is doing which job.
This is the simplest useful test and it needs no equipment beyond a bright light. It is a standard practice among buyers who have been burned before.
It is not a laboratory method. It will not catch a sub-micron defect. It will catch the visible pinholes and fold cracks that cause most field complaints, and it takes about a minute per pouch.
For a quantitative check, the standard approach for flexible packaging is bubble emission testing, and you can approximate it on a bench:
Bubbles that appear and stop are usually trapped air escaping from a fold. A continuous stream from one location is a real defect. If you are checking a filled pouch, the same test on a sealed sample from your own filling line is the most realistic measurement you can make, because it tests your seal rather than the supplier's.
The laboratory version of this is ASTM D3078, bubble emission testing of leaks in flexible packaging. If you are specifying a test to be run by the supplier, that is the method to name.
Seal failures are more common than pinholes, and the causes are process-based rather than material-based: seal bar temperature, pressure, and dwell time drifting outside the process window, or product contamination — powder, oil or moisture — trapped in the seal zone during filling.
Three checks, in increasing order of rigour:
| Check | Sample size | What it catches |
|---|---|---|
| Backlight inspection | 3+ pouches from across the batch | Pinholes, fold cracks |
| Seal width measurement | Averaged over several points | Seals squeezed thin by over-temperature |
| Hand peel across the seal | 3+ pouches | Unbonded sections, channel leaks |
| Water immersion with pressure | 1-2 pouches | Continuous leak paths |
| Fill and seal with your own product | First production run | Contamination in the seal zone, real-world seal strength |
| Zipper function after sealing | Several pouches | Zipper interfering with the top seal |
Keep a record of each batch. When a defect appears six months later, the batch number and the inspection result from that delivery are what tells you whether it was a one-off or a process that has been drifting.
None of this requires a lab. It requires deciding what "good" means in numbers, testing for it when the shipment arrives, and keeping the record. The alternative is finding out from a customer.
Primary references for the test methods and food-contact rules referred to in this guide:
The questions buyers ask most about this topic.
Hold a sample pouch against a strong light in a dark room and look for pinpricks, bright specks or a line of light along a fold. Pay attention to the bottom gusset creases and side folds, where the film bends hardest and pinholes concentrate. Inflating the pouch slightly with air separates the film plies and makes fold defects easier to see. Use pouches from different points in the batch, not several from the top of the carton.
Because thickness is not the only variable. A pinhole from film handling or a fold crack at the gusset creates a permanent path for oxygen and moisture regardless of film gauge. A seal that pinches thin at a corner or has a channel running across it does the same. Both can be present on a pouch that looks and feels intact.
Three levels. Visually, run a fingernail along the seal and look for channels, wrinkles or a seal width that narrows at the corners. By hand, cut a strip across the seal and peel it apart - you should feel consistent resistance across the whole width, and any section that gives way with no resistance never bonded. Properly, use ASTM F88 peel strength testing and agree the numbers with your supplier before the order rather than assessing by feel.
ASTM D3078, bubble emission testing. A sealed pouch is submerged and pressure is applied, then you watch for a continuous stream of bubbles from a single point. That is a leak path. Bubbles that appear once and stop are usually trapped air escaping a fold rather than a defect.
If you fill and seal on your own line, seal quality becomes your process - and so does most seal failure. The supplier's responsibility stops at the sealant layer. That is not a reason to avoid self-sealing; it is a reason to test a sealed sample from your own line, with your own product, because contamination in the seal zone is the most common cause of micro-leaks and only your filling setup can reproduce it.
We reply with pricing and a dieline, usually within 24 hours — free design service, samples approved before bulk production.
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