| 摘要 |
A partly-overlapped portion of a sheath film is meltbonded by heat to provide a seal which a user can tear open with ease. A heat sealing area of the sheath film is halftone printed to reduce the bonding area, thereby decreasing the bonding strength. A tearing portion is provided along the heat seal which is printed so that it has a bonding strength which gradually increases along the tear part until it reaches the bonding strength of the remainder of the heat seal. A curl section having minimal bonding strength relative to the tearing portion of the heat seal is induced to curl up using heat treatment to allow the user to grasp the sheath film at the tear part.
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