Inside a Mylar Bag: Film Structures, Barrier Layers and What "Smell-Proof" Really Means

Ask three suppliers what a mylar bag is made of and you will get three answers, all of them partly right. The word describes the shiny, opaque, metallised appearance that became standard in cannabis packaging — and behind that appearance sits a stack of layers, each doing a different job.

Here is the stack, what each layer is for, and how to specify it.

The typical structure, outside in

1. Outer layer — usually PET (polyester)

The surface you print on and handle. Polyester takes print well, resists scuffs and gives the pouch its body. Reverse-printed PET — printed on the inside of the outer layer and then laminated over — is how the print stays protected and how the metallic sheen comes through the artwork.

2. Barrier layer — metallised film (VMPET) or aluminium foil (AL)

This is the layer that does the work. Metallised film deposits a thin layer of aluminium onto PET or another substrate; foil is solid aluminium. In plain terms:

• Metallised (VMPET) — good light and moisture barrier, gives the mirror-like finish, lighter and less expensive.
• Foil (AL) — the strongest barrier available in a flexible pouch: light, oxygen and moisture. Heavier, more expensive, and it creases.
• Clear / no barrier layer — for dry products sold fast, or where a window matters more than shelf life.

The barrier layer is also what makes a pouch opaque, which is a compliance or branding requirement in many markets, not just a design choice.

3. Inner sealing layer — PE (polyethylene)

The layer that touches your product and does the sealing. It is food-contact grade and heat-sealable, so the pouch can run on a form-fill-seal line or be sealed by a hand sealer. Seal strength and seal window are set with the film structure, not the artwork.

What "smell-proof" can and cannot mean

Smell-proof is a marketing term, not a standard. What it really describes is a combination of three things:

1. Barrier — foil or metallised layers slow the migration of volatile compounds through the film.
2. Closure — a zipper that actually seals across its full width. Most "leaks" in a pouch come from the closure, not the film.
3. Seals — clean, complete heat seals at the sides and top. A single channel in a seal is a path out.

So the honest version of the claim is: a well-made foil pouch with a good zipper and clean seals retains aroma far better than a plain film pouch, and it is the right answer for most products. It is not a hermetic seal, and no flexible pouch should be sold as one.

A smell test you can run yourself

Fill three sample pouches with your product. Seal one by hand, one with your production sealer, and leave one with the zipper closed only. Put each in a clean container for 24 hours, then open the container and smell it — not the pouch. What you are testing is the whole pack: film, seal and closure together. Do this before the bulk run and you will know whether your process, not just your film, is up to the claim.

Choosing a structure for your product

Product	Practical structure	Why
Flower, pre-rolls	Metallised or foil, opaque, with zipper	Aroma retention, light block, puncture resistance from stems and cones
Gummies, soft chews	Metallised, opaque	Moisture barrier; stiffness keeps the pack standing on shelf
Powders and concentrates	Foil, high barrier	Oxygen and moisture are what degrade the product
Dry snacks and confectionery	Metallised with a window	Barrier where it matters, product visible where it sells
Supplements and capsules	Foil, opaque	Moisture control and light block over a long shelf life

If you tell us the product, the target shelf life and where it is stored, we will recommend the structure and the film thickness and put both in writing in the quote. What we will not do is quote a premium structure when your product does not need it — most brands move to heavier barrier than necessary, and pay per pouch for it.

Finishes are separate from performance

Matte, glossy, holographic and soft touch are surface finishes. They change how the pouch looks and feels, not what it keeps out — with one practical exception: soft touch and matte surfaces show handling marks more readily than gloss. Foil, rose-gold foil and spot UV are add-ons applied on top of the structure, and they are quoted separately because they change the print process.

For what each finish looks like in hand before you choose, order samples in the finishes you are considering. It is the fastest way to settle a decision that is otherwise made from a screen.

Food contact and documentation

Pouches intended for food, supplements or anything ingested need food-contact grade materials and the documentation to match. Tell us what you pack and where you sell, and we will confirm the film structure and set out which documents we can supply with the order. Labels, warnings and market-specific compliance stay with the brand — we print the artwork you supply, which is the division of responsibility most regulators expect.

For size and format questions that come up alongside material choice, the sizing guide covers fill weight, gussets and standard dimensions.

Sources and standards

Primary sources for the rules and test methods referred to in this guide:

• ASTM F1249 — Water vapour transmission rate of plastic film (ASTM)
• ASTM D3985 — Oxygen transmission rate of plastic film (ASTM)
• 21 CFR 177.1520 — Olefin polymers for food contact (US eCFR)