How to specify an FIBC.
An FIBC is not an off-the-shelf item. Six decisions determine the construction, and they interact: the wrong bulk density gives the wrong volume, the wrong safety factor gives a bag that cannot be reused. This guide takes the decisions in the order they need to be made.
If you read one paragraph.
Start with the bulk density of your product, because it sets the volume and therefore the dimensions. Then choose the safety factor by how many times the bag will be used: 5:1 for single trip, 6:1 for multi-trip. Next decide whether the product or the surrounding atmosphere is flammable, which fixes the type as A, B, C or D. Everything else, meaning construction, filling, discharge, coating and liner, follows from how you fill, lift, store and empty the bag.
Six decisions, in this order.
Product and bulk density
Bulk density determines how large the bag has to be. The volume you need is the net weight divided by the bulk density. One tonne of cement at 1 400 kg/m³ needs 0.71 m³. One tonne of plastic granules at 550 kg/m³ needs 1.82 m³, which is more than twice the bag for the same weight.
Allow roughly 10 percent extra height so the bag can be closed and lifted without being taut. Measure your own product if you can: table values vary with moisture content and particle size.
Load and safety factor
SWL, the safe working load, is the net weight the bag is built for. The safety factor states how many times that load the bag is tested against. 5:1 is single trip, 6:1 is multi-trip. A 5:1 bag must not be refilled after emptying, however sound it looks.
FIBCs for products not classed as dangerous goods are tested to EN ISO 21898. Dangerous goods require a UN certified bag under ADR, RID or IMDG, which is a separate regime with its own marking.
Static electricity and flammability
This is the decision with safety consequences, and the only one where the wrong choice can ignite something. Polypropylene is an insulator, so filling and discharge build static charge. Which type you need comes down to two questions: is the product flammable, and can flammable gas or vapour be present around the bag.
If you are unsure, work from the product safety data sheet and from an area classification. Do not guess from how the product looks.
Construction
Construction determines how the bag holds its shape when filled, which in turn determines how many fit on a pallet or in a container. A circular bag bulges, a 4-panel bag stands square. The difference shows up most clearly when you calculate container fill.
Filling and discharge
The top is chosen by how you fill and how dusty the product is. The base is chosen by how you empty and how well the product flows. A common mistake is choosing a flat base for a product that needs to be metered out, which means the bag has to be cut open and cannot be reclosed.
Coating, liner and certification
Coating and liner both deal with keeping moisture out and fines in, but they solve different problems. A coating laminated onto the fabric stops moisture from outside and dust from inside. A liner gives a separate barrier and can be changed between loads.
If the bag will be stored outdoors the fabric needs to be UV stabilised, and that stabilisation is quoted in hours of sun exposure. Uncoated polypropylene degrades visibly after a season in direct sun.
Typical bulk densities.
Indicative figures for working out volume. Your own measured density governs over this table, because moisture content, particle size and packing move the values considerably.
| Material | Bulk density kg/m³ | Volume per tonne m³ |
|---|---|---|
| Cement | 1 400–1 500 | 0,67–0,71 |
| Sand, dry | 1 500–1 600 | 0,63–0,67 |
| Aggregates and gravel | 1 500–1 700 | 0,59–0,67 |
| Fertiliser, granular | 900–1 100 | 0,91–1,11 |
| Grain | 750–800 | 1,25–1,33 |
| Wood pellets | 650–700 | 1,43–1,54 |
| Plastic granules (PP, PE) | 550–600 | 1,67–1,82 |
| Flour | 500–600 | 1,67–2,00 |
Type A, B, C or D.
The four types differ in how they handle static charge. The choice is driven by the product and by the atmosphere around the bag, not by price.
| Type | Fabric | Grounding | Use for |
|---|---|---|---|
| A | Plain woven polypropylene, with no static protection. | None | Products that are not flammable, in areas with no flammable dust, gas or vapour. |
| B | Plain polypropylene with a low breakdown voltage, under 4 kV. | None | Dry flammable powders. The low breakdown voltage prevents propagating brush discharge. Must not be used where flammable gas or vapour can be present. |
| C | Conductive threads woven through the fabric, linked to a grounding point. | Required | Flammable powders, and use in areas with flammable gas or vapour. The bag must be grounded throughout filling and discharge. |
| D | Antistatic fabric that dissipates charge without conductive threads. | None | The same applications as Type C, but where reliable grounding cannot be guaranteed every time. |
A Type C bag that is not grounded is more dangerous than a Type A bag, because the conductive threads collect charge that then has nowhere to go. If grounding cannot be assured at every fill and discharge, Type D is the correct choice.
Four constructions.
Circular (tubular)
Woven as a tube, with no side seams. Cheapest to make and strong around the circumference, but bulges when filled and loses its square shape.
U-panel
Base and two sides cut from one piece, with two panels sewn in. Holds its shape better than circular and is the most common compromise.
4-panel
Four separate sides sewn to a base. Stands squarest, which gives the best stacking and the best use of a pallet and a container.
Baffle
Internal baffles at the corners hold the bag square under load. Holds the most per floor area, and costs the most per bag.
Top and base.
Filling
- Open top
- Fastest to fill. For products that do not dust and do not need covering from above.
- Duffle top
- A skirt over the opening that is tied off after filling. Keeps dirt and water out without restricting filling speed.
- Filling spout
- Closed filling onto a dust-tight connection. Standard for powders and for products that must not be contaminated.
Discharge
- Flat base
- Cheapest. The bag is cut open to empty and cannot be reclosed, so it suits single trip use and products emptied in one go.
- Discharge spout
- Controlled discharge that can be closed between draws. Necessary when the product is metered out in stages.
- Full-open base
- The whole base opens. For poor-flowing and sticky products that bridge in a spout.
What we need to know.
With these seven details we can specify the bag straight away, without coming back with questions.
- What the product is, and its bulk density if you have measured it.
- Net weight per bag.
- Single trip or multi-trip.
- Whether the product or the surrounding area is flammable.
- How the bag is filled and how it is emptied.
- Whether it is stored outdoors, and if so for how long.
- Any requirement for food contact, UN certification or printing.
What we supply.
[COMPLETE: which types (A, B, C, D) and constructions Fortapac actually supplies, which certifications are held (food contact, UN, ISO), normal lead time and minimum order quantity. Do not publish anything here that the factory has not confirmed.]
Tell us what you are packing.
Give us the product, weight, filling method and handling requirements, and we will specify the right FIBC.