What a liner actually does
Woven polypropylene breathes. Uncoated fabric lets humid air in, and even coated fabric leaks dust at the stitch holes. A film liner closes those gaps. Hygroscopic powders stay dry, fine pigments stay inside, and food ingredients sit against a clean film instead of the woven face.
Wikipedia's summary of FIBC construction notes that a PE or PP film liner or laminate is used for moisture or dust protection. That is the job in one line. Everything else is about matching the liner to the bag and to the machine at either end.
Liner styles
Four constructions cover nearly every request we see. They differ mainly in how the film is held in the outer bag, and that decides how it behaves when product rushes in or out.
| Style | How it sits and where it fits |
|---|---|
| Loose liner | A tube or bag of film placed inside, held only at the top and spout. Cheapest to make, fine for free-flowing granules, but it can twist or pull down during discharge. |
| Tabbed or glued | Film attached to the inner fabric by sewn tabs or adhesive spots, so it stays up while the bag empties. A common middle choice for powders. |
| Form-fit | Cut and welded to the bag's inner dimensions, with spouts that line up with the fabric spouts. Fills square, empties cleanly, and suits automated stations. |
| Baffled | A form-fit liner with internal baffle panels that match a baffle bag, keeping the filled shape square for tighter trailer and warehouse use. |
Film types, and gauge as a spec field
Low-density polyethylene (LDPE) is the usual starting point: soft, sealable, and a decent moisture barrier. Linear low-density polyethylene (LLDPE) is tougher against tearing and puncture, which helps with angular grains or sharp crystals. Blends of the two are common.
Where oxygen, light or aroma matter, makers laminate foil or other barrier layers into the film. Those cost more and handle differently, so ask how the maker wants them sealed and folded.
Static is the trap. A plain polyethylene liner can store charge on its own surface, even inside a Type C or Type D body. For those bags, ask for a conductive or dissipative liner that the maker has qualified with that exact outer bag. Follow your site's hazardous-area assessment and check the maker's documents and IEC 61340-4-4 for how the combination was tested.
Film thickness is stated in mils or microns, and it trades puncture resistance against flexibility and cost. There is no single right gauge for a given product. A heavy angular mineral and a light starch may want very different films.
So we treat gauge like SWL: you or the maker name it, it goes on the quote, and the tag or the liner's own marking confirms it on arrival. If you are reordering, copy the figure from your last delivery's paperwork rather than recalling it.
Filling with a liner in place
The liner's fill spout fits over the fill head first, then the outer bag's spout goes over that. Some lines inflate the liner with a short blast of air before product flows, which seats it against the fabric and stops folds trapping powder.
- Tie the liner spout separately from the fabric spout once full.
- Let entrained air settle out before closing, or the liner balloons on the trailer.
- Keep the liner top folded in, away from the loops and the hook.
- Avoid vacuum-sealing unless the maker has approved it for that design.
Discharging without dragging the film out
The common failure is the liner following product down the spout and choking the valve. Tabbed, glued and form-fit liners resist this because they are anchored. For a loose liner, open the fabric spout, then pull the film spout through and clamp it before untying it.
Discharge stations with a spout access chamber make this easier and keep dust contained. Massaging paddles help bridging powders let go of the film walls. Whatever the setup, nobody reaches under a suspended bag to free a snag.
Matching the liner to the product
Think about what goes wrong without one. Sugar, salt and many fertilizer blends draw water from humid air and cake into lumps that refuse to flow. Flour and starch sift through the weave and coat the trailer. Some ingredients pick up odours from a warehouse, and some oxidise or turn rancid when air reaches them. Each failure points to a different film.
Coarse resin pellets, by contrast, rarely need more than coated fabric, although regrind with fines may. Pigments and very fine minerals benefit from a well-anchored liner mainly to keep dust off people and equipment during emptying. If the only goal is moisture, a heat-sealed polyethylene liner is often enough; if shelf life is measured against oxygen, ask the maker about laminated barrier film.
What to put on the request
Tell us the outer bag spec, then the liner: style, film, gauge, whether the spouts are sealed or open, and whether the bag is Type C or D. Add any food-contact requirement. Lined bags for food or pharma are always new bags. Lined bags with a static rating need the liner and body matched by the same maker.
Used lined bags are rarely worth chasing. A film that held someone else's product cannot be cleaned to a known standard, so a lined job almost always starts from fresh stock.
Questions
Does a liner slow down filling?
A little, at first. Crews need a minute to dress two spouts onto the head. Inflating the film beforehand and choosing a form-fit design usually recovers that time through quicker, cleaner emptying.
Do I need a liner for sand or gravel?
Usually not. Coarse aggregate does not mind humidity and does not sift through the weave. Uncoated or coated fabric alone is normal.
Can I buy liners separately and put them in myself?
Some lines do, with loose liners. For tabbed, glued, form-fit or static-rated liners, buy them installed by the bag maker so the fit and qualification hold.
Will a liner change the SWL?
The SWL belongs to the woven outer bag and is printed on its tag. A liner does not raise it, and a liner that adds weight counts toward your fill total.
How is a lined bag recycled?
Recyclers generally want the film pulled out and kept apart from the woven fabric, since the two materials are processed differently. Empty, fold and bale them separately when booking a pickup.
Sources
Standards named here (ISO 21898, IEC 61340-4-4, NFPA 77) are summarised, not quoted: read the standard and the bag maker's documents before you rely on a rating.