A liner bag is an inner bag, usually polyethylene, fitted inside a flexible intermediate bulk container (FIBC) or polypropylene sack to protect the contents from moisture, contamination and sifting. It is needed whenever the product is hygroscopic, fine enough to leak through the woven fabric, sensitive to contamination, or subject to hygiene rules that the outer bag alone cannot meet. Many bulk products such as coarse minerals or resin pellets need no liner, but food ingredients, chemicals, fine powders and anything shipped through a humid port usually do.
Woven PP fabric is strong but not airtight or watertight, even when coated. Coating closes most of the weave, but pinholes, stitch holes and seams still let fine particles and moisture through.
Types of liner
- Loose tubular polyethylene liners are the simplest and cheapest. They are inserted into the bag at the mill or at your filling plant and tied or heat-sealed after filling. They can crease and trap product in the corners.
- Form-fit liners are shaped to match the bag's baffles, spout and dimensions and are usually tabbed or glued into place so they do not shift during filling. They give a cleaner discharge and are the standard for food and pharmaceutical applications.
- Aluminium barrier liners add a foil layer for oxygen and moisture protection where the product is sensitive, such as some chemicals, flavours and pharmaceutical intermediates. They are heavier and more expensive.
- Conductive or static-dissipative liners are required when the outer bag is Type C or Type D, because a standard polyethylene liner can undo the static protection of the outer bag.
- Sack liners for PP woven sacks are usually a loose polyethylene tube or a laminated inner film, used for fertiliser, seed, chemicals and milled food products.
When a liner is needed
Three checks settle the question. The first is whether the product absorbs moisture, cakes or loses quality if it gets damp. The second is whether it is fine enough to sift through a coated fabric, which usually means a particle size below a few hundred microns. The third is whether the customer or regulator requires a contact surface that meets food, pharmaceutical or chemical rules.
If any of these applies, a liner is probably needed. The liner material, thickness, attachment and the sealing method at your plant should be specified together, because a well-made liner is useless if it tears when inserted or cannot be sealed with the equipment you have.
Specifying it properly
State the liner type, the film thickness, the material grade and whether it must meet FDA or EU food-contact requirements. Say whether the liner is to be fitted at the mill or shipped separately. Ask for the liner to be made in the same clean room as a food-grade bag if that is the application. Confirm the discharge method, because a liner that works well with a spout discharge may be awkward with a bottom-opening bag.
How Creston & Co helps
We check liner fitting and material at the mill, send sealed samples to an NABL-accredited lab where food-contact or barrier claims need confirmation, and carry out pre-shipment inspection and loading supervision. Weekly updates keep you informed, we keep one approved supplier plus one backup, and the starting point is a free 20-minute sourcing review.