12.2.3 Sealing Advice The objective is that the sealed side seams cannot easily be lifted. This is achieved if the glue is close enough to the edge. The envelope will have a fold and an opening flap. It is recommended that it is sealed continuously and securely on the remaining sides which are not the closing flap i.e. the glue used to seal the sides must be in the form of a continuous seal and placed in such a position that the two elements of unglued paper cannot be easily lifted, separated, folded or torn. 12.3 Inserts Mailing items must not have contents that burst out of and cause damage to the Mailing Item or that move to such a degree that the Mailing Item buckles as it passes through our machines. It is therefore recommended that the largest paper insert movement is restricted as follows: • for Letters up to 2mm thick, no more than 30mm within the Mailing Item; and, • for Letters over 2mm and up to 5mm thick, no more than 20mm within the Mailing Item. It is recommended that the paper used for the insert has opacity of at least 85% (BS ISO 2471) to prevent any character on the reverse side showing through affecting the read of the Delivery Address or Mailmark barcode.
It is a requirement that the Mailing Items do not include any metal items that are larger than a standard paperclip or staple as our machines will reject these Mailing Items automatically. 12.4 Material Material requirements • envelopes must be made out of paper only; • you cannot use polythene, plastic or transparent materials or envelopes with apertures; and • if you wish to apply a Mailmark barcode to Letter format Mailing Items that are polywrapped, you can do so provided that you present, declare and pay for the items as Large Letters. • Opacity – the paper used for the envelope and the paper which the address is printed on is more than 85% opaque to prevent any character on the reverse side showing through (BS ISO 2471 paper and board); • Porosity – the paper has porosity of <700 ml/ minute; • Absorbency – the paper has absorbency of 15- 35gsm of water in one minute; and • Density – the paper has a density no less than 70g per square metre (gsm). Postcards are recommended to be made of paper with a density no less than 200gsm and should be at least 0.25mm thick. Material recommendations It is recommended that:
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