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Twist-off caps are used to hermetically close glass containers used to store sterilised food. The caps feature a safety button (flip panel) with a snap action which produces a characteristic noise when the jar is opened. This sound is an audible signal to the consumer that the food is safe.
As twist-off caps are formed in process which does not involve trimming, earring after drawing must be very low. This is particularly important for small-diameter caps. The tinplate must have good flatness (for printing), tight tolerances on thickness and yield stress (to make formation of the flip panel consistent), and low anisotropy (to avoid earring during drawing).
The exact pressure resistance of a twist-off cap comes from a combination of the packaging steel’s thickness and hardness.
The forming process requires very high strength grades such as TH580 and TH620. Steel thickness is only linked to the diameter of the cap.
When the drawing ratio (Ø/height) is less than 3, controlled anisotropy is mandatory to ensure low earring.
The thickness and yield stress of the steel used to create the flip panel (safety button) must be consistent.
Metallic coating: Tin coating E2.0/2.0 or 2.8/2.8