Definition
A film liner (PE insert) is a single-use insert of polyethylene film — most often LLDPE — that lines the inside of a container, octabin or other means of transport and forms a clean, sealed barrier between the bulk material and the walls of the hold. Thanks to the liner, an ordinary 20’ or 40’ sea container turns into a vessel for loose granulate: the material pours directly into the film, and after delivery it is poured out in its entirety, without touching the dirty container walls.
In the practice of the terminal in Chorula, the liner is one of three basic “bulk packagings”, alongside the big-bag (FIBC) and the octabin. Each of them solves the same problem — how to carry tens of tonnes of granulate cleanly and without losses — only at a different cost and with different discharge logistics.
Why a liner is used: the barrier function
Technical and food-grade polymer granulate is a clean product, and that state of cleanliness must be maintained all the way. A sea container is a universal means of transport — everything is carried in it, from machines to chemicals — so its floor and walls are never “food clean”. Pouring granulate straight into such a container would mean contact with dust, rust, residues of previous loads and moisture condensing on the steel walls during temperature changes at sea.
The liner cuts the product off from all of this. It serves four functions at once:
- cleanliness barrier — the granulate does not touch the container walls, there is no contamination by foreign material or dust;
- moisture barrier — a sealed PE film protects hygroscopic materials (polyamide, some PE, additives) from moisture from the air and condensate;
- easier discharge — the material pours out of the bag in its entirety, without residue pressed into the corners of the container;
- limiting electrostatics — dissipative versions reduce the accumulation of charge on the film during fast transfer.
This is why the transoceanic import of granulate from Asia and Korea travels almost entirely in liners, rather than loose or in big-bags — a liner combines maximum use of the container’s volume with cleanliness at the level of unit packaging.
Types of liner
Liners differ above all in what they line and how they are filled.
Container liner — a large bag lining a whole 20’ or 40’ container. This is the most common form for granulates: it is fastened to the container walls and doors with straps and battens, and filled through spouts in the front wall. After delivery the container goes to a tilting ramp and the material pours out by gravity.
Form-fit liner — a liner shaped to fit the interior, with profiled corners and walls. It adheres better to the walls, creases less during filling and gives a more even distribution of the load, which matters with heavy mineral materials.
Silo trailer / tank liner — a thin insert placed into the tank of a silo trailer or a tank container, used where the same vehicle carries one material at one time and another at the next and cross-contamination must be avoided without costly cleaning. In typical single-granulate loops at the terminal, a liner in a silo trailer is not needed, because the silo always carries the same type of plastic.
Big-bag and octabin liner — a small tubular insert placed into a single bag or carton. This is an entirely different scale from a container liner, but the same principle: the film separates the product from the fabric or cardboard of the packaging.
In terms of circulation they are also divided into single-trip (single-use — the standard for granulates) and reusable (rare, only in closed loops of a single material, where there is no risk of contamination).
A separate group are liners with additional equipment: with a moisture-absorbing pouch for hygroscopic materials, with ventilation for products requiring air removal, or with an additional partition stabilising the load on longer routes. In terminal practice we mainly encounter standard single-layer and co-extruded liners for polymer granulates — exotic versions are the domain of the chemical and food industries with specific requirements.
Film materials and parameters
The basic raw material of liners is polyethylene, most often in the LLDPE (linear low-density) variety, valued for its tear resistance and flexibility, and HDPE where greater stiffness and a barrier are needed. Co-extruded (co-ex) films combine several PE layers of different properties: a load-bearing layer, a barrier layer and a contact layer.
| Parameter | Typical range | Note |
|---|---|---|
| Material | LLDPE, LDPE, HDPE, co-ex | LLDPE dominates with granulates |
| Film thickness | approx. 80-200 µm | thicker for mineral and heavy materials |
| Capacity of a 20’ container liner | approx. 18-24 t / tens of m³ | depending on bulk density |
| Food-grade version | yes / no | no regranulate, raw material approved for food |
| Antistatic / dissipative version | yes / no | for flammable materials and fast transfer |
Liner manufacturers work under quality and environmental management systems — ISO 9001, ISO 14001, and for food films additionally ISO 22000 and food-safety certification. Food-grade film is a clean raw material without recyclate and without additives that could migrate into the product. Antistatic and dissipative versions matter where fast transfer of pellets causes electrostatic charging of the granulate and a risk of spark discharges arises.
Filling and discharge
A container liner is filled through spouts sewn into the front wall. The material is fed by gravity from a hopper, by conveyor or — with long 40’ containers — by a chute through a telescopic trough reaching deep into the bag. The film fills from the back to the doors, evenly displacing the air. The filling rate must be matched to the material: too fast a stream generates dust and electrostatic charge, too slow prolongs the operation and blocks the station. For light PE/PP granulates a moderate flow is maintained, for heavy mineral materials what counts is mainly the even distribution of mass in the container.
Before filling, the liner is fastened to the container walls and doors with a system of battens, straps and loops. This is a key stage — a poorly fastened liner with a heavy material can slip, crease or tear on the edge of the door. That is why, when receiving a container, we always check the state of the fastening and the tightness of the film before proceeding to discharge.
Discharge is carried out by one of three methods:
- By gravity on a tipper — the container is tilted at an angle, the front wall of the liner is cut, and the material pours into a hopper or directly onto the vehicle. The fastest and most common method.
- By vacuum through spouts — the material is sucked out of the sealed liner; we describe a related technique in the context of pneumatic discharge of silo trailers. Used when there is no tilting ramp.
- Manually / mechanically with the bag — for smaller batches or difficult materials.
At the terminal in Chorula we most often empty the liner by gravity into a hopper, and from there transfer the material onto a silo trailer. We adhere here to the principle of transloading without pneumatics — the granulate descends by gravity, without being accelerated in an air stream, so that the pellets do not abrade against each other and do not form dust or “angel hair”. The load then travels by silo trailer ~60 m³ to the customer, ready for pneumatic discharge into their installation.
Liner versus big-bag and octabin — when to use which
The choice between a liner and bags is a logistical calculation. The liner wins when granulate travels far, by container, to a single customer taking the whole batch in bulk — then the maximum use of volume and the absence of fabric costs give the lowest cost per tonne. The big-bag wins when batches are small, go to different customers and must stand in the warehouse individually. The octabin is the golden mean: rigid, self-stacking, with a liner inside.
The difference is also visible at discharge. A big-bag is removed from the container one at a time by forklift or crane and transferred one by one — simple, but labour-intensive with large batches. A container liner gives bulk discharge: the whole container pours out in a few minutes on a tipper. For that speed, however, one pays with the need to have a tilting ramp and with the fact that the material must be received in its entirety — you cannot take “half a liner” and leave the rest for later, as you leave a few big-bags in the warehouse. That is why the liner is packaging for full, homogeneous batches, while bags are for fragmented turnover.
In environmental terms the liner too has its logic. One PE film for a whole container is less plastic per tonne of product than a dozen or so woven big-bags with liners, labels and straps. After emptying, what remains is a homogeneous stream of clean PE film, easy to recycle, whereas a used big-bag is a mixture of PP fabric, threads and a PE insert requiring separation. This is a real advantage when assessing the packaging footprint of an entire delivery.
At our terminal we handle all three paths. Material that arrived in a container liner from the port we discharge, transfer and send onward on silo trailers — or, if the customer needs it, we pack it back into big-bags or bags. A store of 2000 big-bags provides a buffer between the container delivery and the customer’s collection, and transloading of up to 200 tonnes per day allows whole container batches to be handled without a bottleneck. Importantly — we accept only materials that flow well and are not dangerous goods (non-ADR): PE, PP, ABS, PS, PA, PET and similar plastics.
The liner at the Chorula terminal
The liner ties two services together for us. On the inbound side — it is the form in which imported granulate most often arrives: we discharge a container with a liner, check the cleanliness and transfer it. On the outbound side — it is an alternative to the big-bag during transloading onto silo trailers, when the customer prefers to take the material in bulk by tanker. Between the two, the material waits in the warehouse, under controlled-moisture conditions, on a silo trailer ready for loading.
In terminal practice one more detail matters: after emptying, the liner is clean PE film, which we hand over for recycling as a homogeneous stream, without mixing it with big-bag fabric. For the plastics producers whose batches we handle, order in the packaging-waste stream and the declaration of material cleanliness are as important as the transloading itself. We describe the full logistics of carrying bulk materials by silo in the network hub at magnumchorula.pl/transport/.
Related topics
- Big-bag (FIBC) — a flexible container for granulate, types A-D and transloading.
- Octabins — a rigid cardboard container with a liner.
- Granulate import Asia - Europe — where container liners at the terminal come from.
- Transloading without pneumatics — why we transfer granulate by gravity.
Sources and author
This text was prepared by Aleksy Pasternak, a practitioner with over 30 years of experience in running a bulk-materials terminal in Chorula by the A4. More about the author: pasternak.me.
Substantive basis: the practice of discharging container liners and transloading granulate at the SMIALA terminal; Regulation (EC) No 1935/2004 and (EU) No 10/2011 (food contact); ISO 9001 / 14001 / 22000 management-system standards of film manufacturers; IEC 61340 (electrostatics). Operational values (200 t/day, a store of 2000 big-bags, a fleet of 26 DAF XF 480 Euro 6 and 31 silo trailers ~60 m³) — the terminal’s own data.
