FIBC Fabric GSM and Coatings Explained

GSM is the mass of the woven polypropylene fabric in grams per square metre, and it is the number that most closely tracks the strength of the bag body. Coating is a separate layer of polymer applied over that weave to close the gaps between tapes, which slows moisture vapour and stops fine powder from sifting through. An uncoated bag for a general mineral load typically sits at 140 to 170 gsm; a coated bag for a fine powder or a damp route typically sits at 160 to 200 gsm of fabric plus 20 to 35 gsm of coating. Reading the two figures as one number is the most common specification error we see in incoming enquiries.

What GSM does and does not tell you

GSM describes the mass of the weave, not the strength of the finished bag. Two fabrics at 160 gsm can differ in tensile and tear performance, because tape width, tape count per 10 cm, draw ratio and the resin used all change how the load is carried. GSM also says nothing about the seams, the loop attachment or the base, which is where bags usually fail. A specification worth comparing pairs the fabric figure with a tape count and a minimum tensile figure, so that a supplier cannot meet the GSM by weaving heavier, narrower tapes at a lower count.

What each coating actually does

• Extruded coating: molten polymer pressed onto one or both faces of the fabric. Common for fine powders and products that must not sift; adds weight and reduces breathability.
• Laminated coating: a pre-made film bonded to the fabric. Even coverage and a smoother surface, usually at a higher cost than extrusion.
• Micro-perforated coating: coated but vented, used where the product has to release air during a fast fill.
• Uncoated: the lowest cost and the most breathable option, and the correct one for coarse, damp-tolerant loads such as gravel, construction waste or firewood.

How coating changes filling and discharge

A coated bag holds air. Filling one with a fine powder at a high rate without venting pushes product back up the fill spout and can lift the bag off the pallet. Where the fill rate is high, the answer is normally a vented panel or a micro-perforated coating rather than more fabric. On discharge the picture reverses: a coated bag releases a fine powder more completely than an uncoated one, because the powder does not key into the weave. Buyers comparing quotes should therefore compare the fill rate and the product behaviour, not the price per bag alone.

QINFENG’s first-hand experience

Coating weight is where quotes diverge most. We have cut open bags from other sources at the same nominal fabric weight and found coating at the low end of its tolerance with thin coverage along the fold lines, which is exactly where sift first appears. On our own coated work the coating weight is recorded per batch, and sift is checked by filling a sample bag with the customer’s product where we hold a sample of it. For fine mineral powders we also inspect the filled bag after 24 hours of settling, because a bag that looks tight at the fill head can relax and show sift marks along the base fold the next day.

Where the numbers stop being the whole answer

GSM and coating weight describe the fabric as delivered, not the bag as used. UV exposure, damp storage, abrasion from the product itself and repeated handling all change performance over time, and none of them appear in the fabric figure. Where a customer specifies a minimum GSM we treat it as a floor rather than a target, and where a market sets its own construction requirement, that requirement has to come from your written specification. Third-party testing is arranged on request and quoted separately.