Bulk bags with a liner

Four ways a liner is fitted

The film is the easy part. How it is held inside the bag decides how it behaves in filling and emptying.

• Loose: a tube of film dropped inside and gathered at the necks; lowest cost, but can shift or twist
• Tabbed or glued: fixed to the bag at points, by tabs sewn into the seams or by adhesive spots, so it stays in place when the bag hangs
• Form-fit: cut and welded to match the bag's shape, spouts included, so it fills square with no folds
• Baffled: a form-fit liner with its own corner baffles, made to go inside a baffle bag

Gauge, film and what the product needs

Liner gauge is the film's thickness, a spec field in its own right. Heavier film handles sharper granules and rough filling; lighter film is easier to fold and seat. Polyethylene covers most work, in low-density or linear low-density grades that stay supple in a cold warehouse. Polypropylene film and foil or metallised laminates are used where the product needs a stronger barrier against moisture or air. Tell us the product and how it is filled and we will ask the maker for the right film.

A liner is different from coated fabric. Coating closes the weave and stops sifting, but it is not a sealed bag, and vapour still migrates slowly across it. If the problem is moisture or contact with the fabric, a liner is the fix. If it is only fines leaking at the seams, coating may be enough.

Static type and the liner have to match

Film is an insulator. Drop an ordinary PE liner into a Type C or Type D bag and the protection the fabric gives can be cut off from the product. The bag and liner have to be chosen together, as a pair the maker has tested or approved for that static type. IEC 61340-4-4 deals with this; check it and the maker's documents, and follow your site's hazardous-area assessment.

For Type A and Type B bags the question is simpler, but still state the type in the request so the quote covers the right combination.

Film makers do produce dissipative and conductive liner grades for these cases, and some form-fit liners are built as a matched set with a particular static-protective fabric. Never swap in a cheaper generic liner on a reorder without asking; the pairing is the specification, not just the outer bag.

Filling and emptying bulk bags with a liner

Hang the bag and give the liner a gentle puff of compressed or blower air so it unfolds into the corners before product lands; wrinkles left behind become pockets that trap powder later. Run the liner neck up through the top and seal it to the fill head or tie it. After filling, close the liner first and the fabric top second.

At discharge, pull the liner's bottom spout through the fabric spout so product never touches cloth, then release the liner tie. A form-fit liner is less likely to be sucked down into the outlet and choke the flow, which is the main reason people pay for one.

Problems a liner solves, and ones it causes

Fertilizer blenders fight caking when prills pull humidity from the air. Pigment and carbon-black handlers fight staining dust that escapes through the weave. Salt and de-icer packers fight brine weeping out of the bag in a damp shed. Cement, lime and gypsum handlers want protection against early hydration, the slow lumping that ruins a binder. Resin and chemical shippers want a second skin around the product in case the outer fabric is scuffed. In each of those, a sealed film earns its cost.

Liners also bring their own snags. A loose one can creep, bunch or tear on a sharp edge of the fill head. Trapped air balloons the liner and slows filling unless the fill head vents it. A liner left untied at the bottom lets powder flood between film and fabric. Train operators on the sequence, and match the liner necks to the real spouts on the bag.

How lined bags ship

Lined bags fold into bulkier bales than unlined ones, so the same order can need more pallets and more space on the truck. Liners that come pre-inserted save labour at the fill line; liners packed separately keep bales slimmer but must be fitted on site. Either way we plan the load from the packed size and decide between LTL and a full trailer.

Think ahead to disposal as well. Recyclers generally want the polyethylene liner pulled out and kept apart from the polypropylene outer, because the two plastics are reprocessed separately. Emptying crews that strip liners at the unloader, and stack them in their own gaylord or cage, make the empties far easier to send onward.