Most people never think about how a can gets sealed. They open it, drink from it, and move on. But the sealing process is one of the most critical steps in packaging — and getting it wrong means spoiled product, customer complaints, and potentially a recall.
Here's how it works, without the engineering textbook language.
The Double Seam
PET cans with aluminum lids use what's called a double seam. It's not glue, it's not welding — it's a mechanical interlock. The lid and the can body are physically rolled together to form a five-layer metal structure that's airtight and leak-proof.
The process happens in two steps:
First Operation: Forming the Hooks
A roller presses the lid's edge underneath the can body's flange. This creates two "hooks" — one from the lid (the cover hook) and one from the can body (the body hook). At this stage, they're loosely interlocked. Not airtight yet, but the basic shape is formed.
Second Operation: Compressing the Seam
A second roller with a flatter profile comes in and compresses those hooks together. This is where the sealing compound — a gasket-like material pre-applied to the inside of the lid — gets squeezed into any microscopic gaps. The result is a hermetic seal that keeps oxygen, moisture, and contaminants out.
Why This Matters for Your Product
The quality of the double seam directly affects:
- Shelf life: A poor seal lets oxygen in, which oxidizes flavors, degrades nutrients, and promotes microbial growth. For a cold brew coffee, that means the product goes flat and sour weeks before the printed expiration date.
- Carbonation retention: For sparkling beverages, even a tiny seam defect means CO2 escapes. The consumer opens a can that's supposed to be fizzy and gets nothing.
- Leak prevention: Obvious, but worth stating. A bad seam leaks. Leaking product damages other items in transit, creates a mess on retail shelves, and generates customer complaints.
What Can Go Wrong
Seam defects usually come from three sources:
- Improper roller pressure: Too much pressure crushes the seam. Too little leaves gaps. Modern seaming machines use servo drives that control pressure within 0.2 PSI — but older or poorly maintained equipment can drift.
- Lid or can body defects: If the lid's sealing compound is uneven, or the can body flange is damaged, the seam won't form correctly no matter how good the machine is.
- Speed vs. quality tradeoff: High-speed lines (300+ cans per minute) are impressive, but they require more precise control. Cutting corners on inspection to maintain speed is a common cause of defects.
How to Check Seam Quality
If you're working with a can supplier, here's what you should ask for:
- Seam teardown reports: The supplier should regularly cut open sealed cans and measure seam thickness, seam height, and overlap under a microscope.
- Vision inspection systems: Modern lines use cameras that check every single can for lid position and seam defects in real time.
- Leak testing: Pressure decay or vacuum decay tests on sample cans from each production run.
At CHINPK, every batch of PET cans goes through seam quality checks before shipping. We use aluminum easy-open lids with pre-applied sealing compound, and our seaming process is calibrated for consistent double-seam formation. See our can and lid combinations, or ask about our quality control process.