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Closing, sealing, and reinforcing are primary considerations in preparing the outside of a parcel for mailing. The main materials for closing, sealing, and reinforcing mailing containers are adhesives, bandings (strappings), staples and steel stitching, and tapes (gummed and pressure-sensitive). Friction closures, screw caps, and locking devices are used to close and seal cans and similar containers.
Adhesives include glue, paste, cement, mucilage, cold emulsion, and thermal plastic. An adhesive used to close a container is adequate if, when the container is opened, at least a 50-percent fiber tear occurs on the surface to which the adhesive is applied. An adhesive used on tapes or box flaps must remain serviceable in temperatures from -20° to 160° Fahrenheit.
For box flaps, the adhesive should cover at least 50 percent of the box flaps and be applied no more than 1/4 inch from the box flap ends.
Alternatively, four strips of hot-melt adhesive may be used on each part of the box flap where the outer flap lies over the inner flap, but only if the strips have the following characteristics:
• Are 3/16 inch wide each (after compression).
• Are no more than 1-1/2 inches apart (with the first strip no more than 1/2 inch from the center seam).
• Extend to the full width of the inner flap (unless hot-melt adhesive is applied to 25 percent of the area where the outer flap lies over the inner flap).
Bandings (strappings) include both metallic and nonmetallic banding and pressure-sensitive filament tape. Flat steel banding should have smooth or plastic-coated edges, and banding ends should be covered or protected. Twine or cord should not be used to close and reinforce a container; if used, it must be of at least 20-pound tensile strength and secured at an intersection at least once on each side of the container.
When a mailing container is closed and reinforced with banding (strapping), at least one band must encircle the length and a second band must encircle the girth. Loose strapping, especially metal, is not acceptable because it does not reinforce the container and because it constitutes a danger to Postal Service employees and mail processing equipment. The bands should be tight enough to depress the corners of the box.
Staples and steel stitching are acceptable for closing a mailing container if they are spaced for the load type as follows:
• For an easy load or an average load, not more than 5 inches apart.
• For a difficult load, not more than 2-1/2 inches apart.
Staples must not be more than 1-1/4 inches from the ends of the box or
mailing container. If a box's staple spacing does not meet these
requirements, the box should be banded to compensate for the gap in the
staple closure, or a strip of 3-inch-wide reinforced tape should be applied to
the box between the staples (as shown in the illustration).
Envelopes, flat-size mailing containers, or cards that have partially opened staples are not acceptable for mailing.
Tapes are used to close, seal, and reinforce mailing containers. Commonly available tapes have pressure-sensitive adhesive or gummed adhesive, with either reinforced (filament) or nonreinforced backings. However, certain types of tapes (such as cellophane and masking tapes) are not strong enough for reinforcing.
The illustration shows how sealing tape can be combined with banding or reinforcement tape to produce an effective closure for boxes with flaps and for telescoping boxes.
Pressure-sensitive tape is acceptable for closing and reinforcing mailing containers. This type of tape is available with various cloth, paper, or plastic backings, both plain and reinforced. Except for pressure-sensitive filament tape, tape used to close and reinforce mailing containers may not be less than 2 inches wide. Nonreinforced plastic tapes must be at least as strong in the cross direction as in the machine (long) direction.
Pressure-sensitive tape works best when applied to clean surfaces at temperatures above freezing. In below-freezing temperatures, the tape should be rubbed down after it is applied.
Cellophane and masking tapes may not be used to close or reinforce containers. These tapes may be used, however, on envelopes to augment adhesive closures or on bags to cover staples.
Gummed tape is acceptable for closing and reinforcing mailing containers if the adhesive is adequately activated, usually with water. If the adhesive is improperly activated or the fibrous material of the container absorbs the water, the gummed tape cannot stick to the container.
Gummed tape is applied correctly if it remains attached to the container during handling and transportation and its removal causes delamination or at least a 50-percent fiber tear on the surface to which the tape is applied.
The tape should be kept away from freezing temperatures for at least 1 hour before its application. Even properly applied gummed tape tends to crack in extremely cold temperatures.
Gummed paper tape is available in two types:
• Reinforced Paper (Kraft). Reinforced paper (kraft) tape is acceptable for closing and reinforcing irregular-shaped or soft-wrapped packages. A mailing container closed with reinforced paper tape is more durable than one closed with nonreinforced paper tape.
• Nonreinforced Paper (Kraft). Nonreinforced paper (kraft) tape is acceptable for closing mailing containers if the tape is of at least 60-pound basis weight. This tape is inadequate for reinforcement. When applied, the tape should extend at least 3 inches over the adjoining side of the container.