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Plastic covers
Items mailed in plastic covers can be processed by machine if the covers have the right properties in terms of strength and sliding characteristics, if they are properly sealed and if they do not acquire a strong electrostatic charge.
Standard letters (up to B5) in plastic covers cannot be processed by machine and are therefore classed as special items.
The following recommendations apply to large letters up to B4 format in transparent plastic covers/films.
Suitable films
Suitable films are strong, elastic and easy to heat seal, e.g. the widely used films made of polyethylene (LDPE and HDPE), if they comply with the following specifications:
| PDPE: (low density polyethylene) | |
|---|---|
| Minimum thickness: | 25 um |
| Thickness tolerance | max. 10% |
| Tensile strength along length | min. 20 N/mm2 |
| Tensile strength across width | min. 15 N/mm2 |
| Stretching along length | min. 150% |
| Stretching across width | min. 300% |
| Static friction and sliding friction coefficients:pure film | 0.30 - 0.50 (according to DIN 53 375 A) |
| Static friction coefficient: sealed film | 0.40 - 0.65 (item on item) |
| Sliding friction coefficient: sealed film | 0.35 - 0.60 (item on item) |
| HDPE (high density polyethylene) | |
|---|---|
| Minimum thickness: | 15 um |
| Thickness tolerance | max. 10% |
| Tensile strength along length | min. 40 N/mm2 |
| Tensile strength across width | min. 30 N/mm2 |
| Stretching along length | min. 200% |
| Stretching across width | min. 400% |
| Static friction and sliding friction coefficients: pure film | 0.30 - 0.50 (according to DIN 53 375 A) |
| Static friction coefficient: sealed film | 0.40 - 0.65 (item on item) |
| Sliding friction coefficient: sealed film | 0.35 - 0.60 (item on item) |
For films to be suitable it must be possible to write/print on them effectively and permanently and they must only build up a weak electrostatic charge (max. surface conductivity approx. 10 power 13 Ohm).
Film seal
As a general rule, it must be possible to heat seal the film material easily and it must satisfy the following requirements:
- Breaking load for all heat seals, especially in areas where the film is coloured (address strip): at least 60% of the film breaking load.
- Minimum value for longitudinal seams: 4 N/15 mm
- Minimum value for transverse seams: 6N/15mm
- With longitudinal seams, the film must be sealed along 95% of the length of the packaging in such a way that the film around the seam cannot be opened without being damaged (fusion sealing).
- With transverse seams, the film must be sealed along 95% of the width of the packaging such that the film cannot be opened around the seam without being damaged and the corners must be sealed along a length of 50 mm without gaps.
Tip:
Always avoid any seal using solid materials such as staples, paper fasteners or the like.
The least problematic films are those which fit the contents snugly on all sides and do not extend further at any point (e.g. shrink-wrapped films). However, for technical reasons relating to the heat-sealing process, films tend to be longer than their contents.
Please note the following permissible values for the difference (overhang) between the length of the packaging and the length of the contents.
- Mail items 0 - 5 mm thick:
- Max. extra length in total 20 mm
- Max. extra width in total 2mm
- Mail items 5 - 10 mm thick:
- Max. extra length in total 25 mm
- Max. extra width in total 3 mm
- Mail items 10 - 20 mm thick:
- Max. extra length in total 30 mm
- Max. extra width in total 3 mm
Address labels
The material on which the address is printed should have an opacity of at least 85% (ISO standard 1831) As it is impossible to give the address zone of the film this degree of pigmentation, we recommend the following options to ensure that the requisite opacity is achieved:
- Leave mail items (magazines/periodicals) white in the address zone and print the address in the white area directly on the item or on the film.
- Print the address on a loose sheet inserted in between the item and the film, the side length of the sheet being at least 95% of the length of the item.
- Print the address on a label with very good adhesive properties and attach the label to the item or film. The label on the film significantly improves machine-reading of the address as interference from reflections and creasing of the film is reduced.
