Mylar is a brand name, in the same way that people say thermos or velcro. The material underneath it is BoPET, biaxially oriented polyethylene terephthalate, a polyester film that is stretched in two directions to make it strong, dimensionally stable and clear. Calling a bag mylar is common shorthand, but if you are writing a specification, ask for the actual film structure, because that is what determines whether it does the job.
What a mylar bag is usually made of
Very few barrier bags are a single layer of polyester. Most are a laminate of three or more layers, each with a purpose. The outer layer takes the print and the abuse. The middle layer is the barrier, either aluminium foil or a metallised film, and it is what blocks oxygen, moisture and light. The inner layer is a sealant, usually a polyethylene, which melts against itself under a heat seal bar to close the bag.
Strip the foil out and you have a clear polyester bag with modest barrier properties. Swap the foil for a metallised film and you get most of the barrier at lower cost, with a slightly less absolute block. Which structure you need depends on the product and the shelf life you are claiming.
Thickness, and the unit confusion
| Unit | Equals | Where you see it |
|---|---|---|
| 1 mil | 0.001 in | North American film quoting |
| 1 mil | About 25.4 microns | The conversion between the two systems |
| 1 micron | 0.001 mm | Metric film quoting, common on imported material |
Two quotes that look identical can be describing very different films if one is in mils and the other in microns. Confirm the unit and the total structure thickness, not just one layer.
What it does and what it does not
A good barrier laminate slows oxygen ingress, keeps moisture out or in, and blocks light, which together slow the reactions that spoil flavour, colour and potency. That is a real and useful thing. What it does not do is make a product shelf stable by itself, replace a proper fill and seal process, or fix a product that was already degrading. It also does nothing if the seal is bad, and seal quality is the most common failure in the category: contaminated seal areas, wrong dwell time and wrinkles at the edge all let air in through a bag that looks perfect.
Features worth deciding early
Zippers and slider closures make a bag resealable but they are not a barrier, so once the bag is opened the barrier argument is over. A tear notch makes opening predictable and protects the zipper below it. A degassing valve matters for coffee and other products that release gas after packing. A stand up gusset changes the shelf presence and the fill behaviour. Windows reduce barrier locally, so if a window matters more than absolute barrier, say so.
Printing on film
Film prints differently from board. Colours sit on a substrate that may be metallic or clear, so a white underprint is often needed to make the artwork read as intended. Plan for the seal areas, which must stay free of certain inks and coatings, and for the fact that a flexible pack distorts, so fine registration effects are harder than on a carton.
Minimum, price and lead time
Minimum order is 100 units, from about $0.48 per unit at bulk volumes, confirmed on your written quote. Plates, design support, US shipping and a digital proof before anything prints are included. We quote three lanes: Rush (expedited production plus air freight), Standard (air) and Cost saver (sea), from about 7 business days, plus 2 to 5 for delivery, and specialty imported items can run up to about 12 weeks. Rush carries about 7 to 25%, depending on the job.
Tell us the product, the fill weight, the shelf life you need and whether the bag stands or lies flat, and we will propose a structure at /quote/. To order directly, see mylar bags, and mylar bag dimensions covers sizing a gusseted pack.
