Let’s talk about why people use polyester sleeves to protect their comics. They’re expensive, and people often think of comic supplies as disposable items. It’s only a bag (box)…but what you store your comic in, might be the main reason your investment pays off. “Mylar” is still a term used for Polyester material. “Mylar” is actually a trade name for a now-defunct product once made by Dupont. The industry still uses the term to refer to all types of Polyester film. However, The Library of Congress (the authority on archival storage) acknowledges the trade name Melinex (also produced by Dupont) or an equivalent substitute as the proper material for indefinite archival storage of paper products.
That said, besides putting your valuable comics in the best archival material, the thickness of your polyester sleeves should be carefully considered. Which thickness should you use? Commonly know as Mylites – 1 mil polyester sleeves do not give the corner and edge protection needed for best storage practices. A backing board is essential if using them. 2 mil polyester is a good choice if the comic is going into storage. If your comic is being handled frequently, 4 mil polyester sleeves should be used. 4 mil polyester is semi-rigid. There’s no chance of the material flopping or the corners being crushed and allowing the comic to be accidentally damaged.
We make our own 4 mil Polyester sleeves in-house at Bags Unlimited. We offer two different style flaps. One is the open flap and the other a closed flap. The open flap sleeves have a fold-over flap that needs to be pre-creased before taping closed. The closed or “beaded” flap is made by sealing a third piece of polyester on the open edge. This preformed flap not only looks good but saves you from having to tape the sleeve closed.