What Is Double Seam Inspection? A Guide to Can Seam Analysis
Double seam inspection checks the joint that closes a metal can: the seam in which the edge of the can end and the flange of the can body are rolled together. Once the can is closed, the double seam is the only joint that keeps the product sealed from the outside, so can makers and food and beverage canners measure it at regular intervals during production.
How is a double seam formed and inspected?
In the seamer, the end is placed on the filled can body and two rolling operations form the seam. The first operation roll curls the edge of the end around the flange of the body. The second operation roll presses the seam flat and tight. The result is five layers of interlocked metal, three from the end and two from the body, with a sealing compound in between.
Two methods are used to inspect the seam:
Teardown: the outside dimensions of the seam are measured with micrometers and gauges, then the end is stripped off so that the body hook and the end hook can be measured and examined.
Cross-section: the seam is cut through with a seam saw, and the section is magnified on a projector, where the hooks and their overlap are measured on the image.
Main applications
Beverage cans: checking the seams of 2-piece aluminum cans.
Food cans: checking the seams of 3-piece tinplate cans.
Aerosol cans: measuring the seams of aerosol cans.
Empty cans: checking the flange width and dome depth of cans before filling.
Seamer set-up: checking every seaming head after a roll change or a can size changeover.
Key concepts: the seam parameters
Seam thickness and seam length: the outside dimensions of the seam. Seam length is also called seam height.
Body hook and end hook: the lengths of the two hooks folded inside the seam.
Overlap: the length over which the two hooks overlap. The longer the overlap, the longer the path a leak would have to follow. It is also expressed as an overlap rate.
Tightness: how firmly the second operation roll has compressed the seam, judged from the wrinkles on the end hook after teardown.
Countersink: the depth from the top edge of the seam down to the end panel, measured just inside the seam, next to the chuck wall.
Accuracy: seam dimensions are checked to the hundredth of a millimeter. The AT2E STG-1 measures the seam thickness from 0 to 5 mm with an accuracy of ±0.01 mm, and the SeamCheck Plus projector has an accuracy of 0.005 mm (0.0002 in).
Why a cut seam needs a second measurement
Cutting through a seam is like slicing a sandwich: the cut shows every layer, but pressing the knife also squeezes it a little. After the seam saw, the seam can be slightly deformed, so the thickness measured on the projector image can differ from the real one. A seam thickness gauge measures the actual thickness on an uncut part of the seam.
This is why the two methods are combined. The projector gives the internal parameters that cannot be seen from outside, such as hooks and overlap, and the gauges give the actual outside dimensions. On the SeamCheck Plus, 9 parameters are measured on the image, and the actual seam thickness, countersink and tightness can be added from separate gauges, for 12 parameters in total. A clean cut also matters: a saw with a linear guide rail, such as the AT2E SS-1, gives a clearer section than a typical seam saw.
Calibration of seam measurement
Seam gauges and projectors are checked against references of known size:
Check the zero of each gauge on its zero base or calibration block before a series of measurements.
Check the projector with its calibration piece: the SeamCheck Plus is supplied with a 2 mm calibration piece and can be calibrated at any time.
Record the deviations, and adjust or recalibrate if they exceed the acceptable limit.
Connecting the gauges to the SeamCheck software or to an SPC system, through the DTB-1 data transporting box, also avoids manual transcription errors.
AT2E double seam equipment
AT2E offers equipment for each stage of seam inspection, from cutting to reporting:
SeamCheck Plus: automatic double seam projector with UHD camera, 60x magnification, automatic line positioning in 1 second and SeamCheck software.
ANSS-2: antinoise seaming cutting saw with double blade, for 2-piece and 3-piece cans up to 66.5 mm (2.6 in) in diameter.
SS-1 and SS-2: seaming cutting saw with linear guide rail for 2-piece, 3-piece and aerosol cans, 40 to 130 mm in diameter; the SS-2 version cuts vertically.
STR-1: seam stripper for aluminum and steel cans that strips the double seam in a single motion, without damaging or distorting the end hook, so that both hooks can be measured.
DSMM-1: double seam measuring minidesk for seam thickness, seam length, countersink and can height in one operation.
STG-1 and SLG-1: seam thickness and seam length gauges, accuracy ±0.01 mm.
CG-D: digital countersink gauge for cans and ends, 0 to 12 mm.
CSM: can seam micrometers for food, beverage and aerosol cans, accuracy ±3 µm.
FWG-1 and CDDG-1: flange width gauge for empty cans and dome depth gauge for beverage cans.
Conclusion
A double seam is only a few millimeters of folded metal, yet it carries the whole seal of the can. Measured regularly, with clean cuts, calibrated gauges and the same parameters on every seaming head, it helps reveal a drifting seamer before its seams fall out of specification.
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AT2E is an ISO 9001 certified company (AFNOR Certification).
Request a quote: info@at2e.com
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