Perforations are classified by bursting strength and type.
The bursting strength is referred to as "ties per inch" or TPI. The
perforations can be parallel or perpendicular to the fold of the flap. Perforations
can be used anywhere on the envelope, but are generally located on the flap.
They are most often parallel to the fold and are used to allow a section of
an extended flap to be detached and used for some other purpose. See examples
"Ties Per Inch" or TPI:
Some of the TPI's available to use are 6,8, 10 and 12.
A common size used is 8 TPI. The function of the perforation
on the envelope will help determine the proper TPI to use.
Be sure to discuss any special requirements with your vendor.
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