Quick answer: A packaging bag is not "one plastic" — it is a film structure. BOPP / CPP / PE / PET, aluminium foil (AL), metallised polyester (VMPET), nylon (PA) and the new mono-material PE or PP families each solve a different problem: stiffness, sealing, strength, barrier or recyclability. The buyer's job is to name, in order, what the product needs protection from — moisture, oxygen, light, grease, puncture, aroma loss — and then choose the shortest structure that delivers it. Longer structures cost more, seal in a narrower window and are harder to recycle.
What each layer in a film structure actually does
Every flexible packaging specification is a list of layers written from the outside in, for example PET 12 / VMPET 12 / PE 60. The numbers are micrometres (micron, µm). Reading the structure correctly is most of the technical work, because each position in the stack has a job:
- Outer layer — print and stiffness. Usually BOPP or PET. It carries the printing, resists scuffing, and gives the bag its "body" so it stands up on a shelf. A reverse-printed outer layer holds the ink inside the laminate, protected by the film itself, which is why reverse printing survives abrasion better than surface printing.
- Middle layer — barrier or strength. This is where the technical differentiation happens. Aluminium foil, metallised polyester, EVOH or nylon sit here. A three-layer structure usually exists because of one specific middle layer.
- Inner layer — sealing. Almost always a PE or CPP sealant film, because it melts at a lower temperature than the outer layers and forms the heat seal. The sealant layer determines whether the bag seals cleanly in the presence of contamination such as powder or oil.
- Adhesive / tie layers. Not visible in most specifications, but they decide whether the laminate delaminates after retorting or after a year in a warehouse. Naming the adhesive type (solvent-based, solvent-free, water-based) is a legitimate question to a supplier.
Film family comparison: BOPP, CPP, PE, PET, PA, AL, VMPET
| Film | Main job in the structure | Typical thickness | Strengths | Watch out for |
|---|---|---|---|---|
| BOPP (biaxially oriented polypropylene) | Outer print layer; also a single-layer bag on its own | 15–40 µm | Very clear, high stiffness, low cost, good print surface | Poor barrier on its own; not a sealing layer on its own |
| CPP (cast polypropylene) | Sealant layer; retort-capable structures | 20–60 µm | High heat resistance, good for retort and hot fill, good clarity | Lower puncture resistance than PE; stiffer feel |
| PE / LDPE / LLDPE | Sealant layer; mailers and flat bags | 25–150 µm | Excellent sealability, tough, flexible, low-temperature performance | Soft, low stiffness; poor oxygen barrier alone |
| HDPE | Stiff PE variants, T-shirt and handle bags | 20–60 µm | Higher stiffness and strength than LDPE, better moisture barrier | Poor clarity; harder to seal cleanly |
| PET (polyester) | Outer layer needing heat resistance or strength | 12–25 µm | High tensile strength, heat resistant, excellent print surface | Costs more than BOPP; no sealability |
| PA (nylon) | Toughness and puncture resistance layer | 15–25 µm | Best puncture and impact resistance; thermoformable for vacuum packs | Moisture sensitive; expensive; often over-specified |
| AL (aluminium foil) | Absolute barrier | 6–9 µm | Blocks light, oxygen, moisture and aroma almost completely | Opaque; flex-cracks if unsupported; cannot go in a metal detector |
| VMPET (metallised polyester) | Light and partial oxygen barrier at lower cost than foil | 12–25 µm | Metallic look, moderate barrier, microwave-safe, lower cost than AL | Barrier is not comparable to foil; vulnerable to scratching |
| EVOH | Oxygen barrier inside a PE structure | 10–30 µm | Excellent oxygen barrier while keeping the laminate transparent | Barrier collapses with moisture, so it must be buried between PE layers |
Two facts are worth stating plainly because they are frequently misunderstood. First, metallised film is not a substitute for aluminium foil; the barrier difference is one to two orders of magnitude in oxygen transmission. Second, polyester and polypropylene are films, not bags — a "PET bag" in a quotation almost always means a laminate whose outer layer is PET, not a single-layer PET bag.
Common multilayer structures and what they are for
| Structure | Typical use | Why this combination |
|---|---|---|
BOPP / CPP | Biscuits, bakery, stationery, textile goods | Cheapest clear structure with good stiffness and clean sealing |
PET / PE | General retail bags, hardware, dried goods | Heat-resistant printable outer layer plus a tough sealant |
PET / VMPET / PE | Coffee, snacks, tea, detergent | Light barrier and moderate oxygen barrier while keeping a metallic or opaque look |
PET / AL / PE | Long shelf-life food, powders, pharmaceutical sachets | Near-absolute barrier; the reference structure when moisture and oxygen both matter |
PET / PA / PE | Frozen goods, liquids, sharp-edged items | Nylon middle layer for puncture resistance; retort or frozen performance |
PET / AL / PA / PE | Retort pouches, ready meals, pet food | Four layers because the product is cooked, stored and handled aggressively |
Kraft / PE or paper / PE | Gift bags, bakery, takeaway | Paper face for presentation; the PE inner liner provides the seal and grease resistance |
Mono-material PE and PP: when recyclability changes the answer
Multilayer laminates are technically excellent and practically hard to recycle, because PE, PET and aluminium cannot be separated economically. Stores in a growing number of markets now price or restrict mixed-material flexible packaging. The industry's answer is the mono-material structure: build the whole bag from one polymer family so that it can enter the PE or PP recycling stream.
The trade-off is real and should be stated honestly to buyers rather than sold as a free upgrade:
- You gain a recyclable, single-polymer bag — typically
Mono PEorMono PP, printed and sealed without a dissimilar lamination. - You give up some stiffness (mailers become softer), some heat resistance, and some barrier unless the structure adds an EVOH or a coated layer — which in turn complicates the recycling claim.
- You may need to change the bag: a mono PE stand-up pouch often needs a thicker sealant and a different seal temperature than the PET / PE pouch it replaces.
Mono-material is therefore a product decision, not an upgrade checkbox. Decide it before artwork, because the film family changes the bag's stiffness, its ability to stand on a shelf, and the sealing window the factory must run.
A five-question framework for choosing a structure
- What is the product protecting against? Name the single most important threat. Moisture, oxygen, light, grease, puncture, aroma escape or simply dust. Different threats point to different middle layers and this one answer removes most of the candidate structures.
- How long must it last? Two weeks in a local store and eighteen months in a container are not the same specification. Shelf-life targets are what justify aluminium foil; without a stated shelf life, foil is usually over-specification.
- Is the bag filled by machine or by hand? Automatic form-fill-seal lines need a consistent film thickness, low coefficient of friction and a wide sealing window. A hand-filled bag in a boutique shop does not.
- Must it be transparent? Transparency and barrier conflict. Metallised film and aluminium foil block light and force opacity, so if the product must be seen, the barrier usually has to come from a clear material such as EVOH.
- Does the destination market price mixed materials? If yes, weigh mono-material early. If no, a conventional laminate will usually deliver better protection per dollar.
How to verify the film you were quoted
Film is invisible in a finished bag, so verification has to be deliberate. Five checks answer most of the risk:
- Ask for the structure in writing as a layer list with micron thicknesses, for example
PET 12 / VMPET 12 / LDPE 60. A supplier who cannot state the structure is not the one to buy a barrier product from. - Ask for the total thickness and confirm it by micrometer on the delivered bag. A 90 µm laminate that measures 78 µm is a different product.
- Check the seal on the actual product, not on an empty bag. Powder, oil and crumbs are what cause leaks in the field.
- Confirm the print process (reverse printing inside the laminate, or surface printing). It changes both durability and cost.
- Keep a signed sample. The film structure, thickness and print process should all be fixed on a physical sample before mass production, so no party has to argue from memory later.
Dongyang Jinhe Packaging has produced OPP bags, zipper bags, PE bags and food bags for 16 years from a 1,000-square-metre factory in Dongyang, Zhejiang, supplying buyers in Europe, America, the Middle East and South Asia. If you are choosing a film structure for a specific product, send us the product type, the shelf life you need and the packing method — our team will reply within 24 hours with a structure and a sample plan.