
Torn off / ripped label

A label tear is often visible from the top (protruding piece); a tear (or crack) is more likely to be seen from the sides. Treatment is facilitated by the presence of an area of the can not covered by the label (i.e. strong contrast at this point).
Faulty crimping

There are different types of crimps, the finest being a few hundredths of a mm. The size of the smallest seam visible to cameras depends on camera resolution and the area to be observed (e.g. to have a minimum of 2 pixels representing the smallest defect, on a 100 mm diameter cylindrical box and 5MP cameras, a seam defect can be seen down to 0.1 mm).
Denting

A dent is represented by a break in contrast, easy to see from the front, but more complex from the sides (particularly in the case of highlighted, unlabeled cans). If the dents can be fine, a total of 6 cameras (one every 60°) may be needed.
Folded label / air bubble under label

A fold is finer to see than a tear, especially if it can be mistaken for a box decoration. You'll need to present different manufacturing orders to the system to help it adapt and differentiate between folds/air bubbles and decorations resembling them.
Optical features
How to ensure good visibility of defects
The can has complex optical properties that must be taken into account in any defect detection system.
Brilliance: this product reflects light strongly, requiring the use of polarizers (both on the lens and on the lighting).
Sharpness: cans are often used in their cylindrical form. In front view, the depth of field (sharpness range) must be maximized to see clearly both the closest point (in the center) and the furthest points (towards the edges). To maximize depth of field, we need to minimize aperture, but we also want to minimize exposure time (otherwise, motion blur). Strobe lighting is therefore the best choice.
Optical integration
360° can control
Cylindrical cans can be rotated on themselves.
This lends itself particularly well to low production volumes, as each product has to pass through a rotation station.
If this isn’t possible, or for cube-shaped cans, you’ll need a control station large enough for 360°: 4 cameras that scan 90° each, with a minimum spacing between two cans equivalent to one can diameter.
For 100mm diameter cylindrical boxes, allow 800mm for the control station footprint.
Suppliers
Recommended suppliers for this application
2D color matrix cameras: Baumer
Short focal length lenses: VS Technology
White LED bars: Effilux
TIPS
What about multi-format management?
An additional complexity may be the passage of several production orders on the line (different volumes or types of label), particularly with regard to depth of field.
The areas to be observed and the brightness settings (exposure time and gain) can be set automatically on the software side at the start of production.
If size differences between products are too large, liquid lenses should be used to adjust focus to OF changes (and avoid manual adjustments for each new production run).