How PP Woven Fabric Is Manufactured

The six stages from polypropylene granule to finished sack, what is measured at each one, and which defect in a delivered bag traces back to which stage.

A woven sack looks simple, and most of what makes it good or bad is decided long before it is sewn. The strength of a bag is set in the few seconds a strip of plastic spends being stretched on the tape line. Its weight is set at the loom. Its moisture barrier is set at the coater. By the time a bag reaches the stitching table, the only things left to get wrong are the cut and the seam. Knowing which stage controls which property tells a buyer what to ask a factory, and tells a factory where to look when a complaint arrives.

The process at a glance

1. Resin and masterbatch

The starting material is polypropylene homopolymer in a grade made for tape, usually called raffia grade. Small amounts of masterbatch are metered in with it: pigment if the fabric is coloured, a UV stabilizer if it will sit outdoors, as in our UV stabilized PP woven fabric, and calcium carbonate filler, which makes the tape easier to split and less glossy and also lowers cost. Filler is the ingredient to ask about. A little helps processing. A lot replaces polymer with chalk, and the fabric keeps its weight while losing strength. For food-contact sacks the resin and every additive need to be food-contact grades, which is settled here and cannot be corrected later.

2. Tape extrusion: where strength is made

The blend is melted and pushed through a flat die as a thin film, which is chilled at once in water or on a cold roll. Blades slit the film into strips. The strips then pass through a hot oven between two sets of rollers, the second running several times faster than the first, so each strip is drawn out to several times its length. Stretching lines up the polymer molecules along the tape and turns a weak film into a strong tape. Too little draw gives a tape that stretches and stays stretched; too much gives one that is strong on paper and splits along its length on the loom. The tapes are then relaxed on warm rollers so they do not shrink later, and wound onto bobbins. Denier, the weight of 9,000 metres of tape, is checked here because every later weight depends on it.

3. From bobbin to creel

Hundreds of bobbins are loaded onto a creel, the rack that feeds warp tapes to a loom. It is unglamorous work and it shows in the fabric. A bobbin wound soft or uneven feeds at a different tension from its neighbours, and the fabric comes off with a slack or tight stripe along its length. A tape that breaks and is not rejoined leaves a missing end: a thin line running down the roll where sifting and tearing start.

4. Circular weaving

A circular loom weaves a tube. The warp tapes are arranged in a ring, and shuttles carrying the weft travel round and round inside it, laying one continuous spiral of weft through the warp. The tube is flattened and wound as a roll; its lay-flat width becomes the width of the bag, with no side seam. Mesh, the number of tapes per inch each way, and tape denier together give the fabric weight in GSM, a relationship our guide to GSM, denier and mesh works through with examples. Our standard PP woven fabric roll comes off the looms as a plain weave at 60-120 GSM, 20-90 inches (50-230 cm) wide, in roll lengths from 500 to 5,000 metres on 3 or 6 inch cores.

5. Coating and printing

Woven fabric has gaps at every crossing, so it breathes and it leaks fine powder. Where the product needs a barrier, the fabric goes through an extrusion coater: a curtain of molten polypropylene falls onto the moving fabric and is pressed against it by a chilled roll, leaving a continuous film bonded to the weave. Coating weight is quoted in GSM. Our PP laminated woven fabric roll carries 15-30 GSM of coating on a total of 80-130 GSM. Printing follows on a flexographic press, on the coated face where there is one, because a smooth film takes ink more cleanly than bare tapes. Coated and printed fabric is also sold before conversion, as the printed PP woven fabric roll. Our custom printed PP woven bag runs six colours with register held to plus or minus 0.5 mm.

6. Cutting, sewing and baling

The roll is cut to bag length. A heated blade melts the tape ends together as it cuts, so the mouth does not fray; a cold cut is used where the mouth will be hemmed. The bottom is folded once or twice and stitched, or a valve is formed for bags that fill on a spout packer, as on the valve version of our PP woven cement bag. Liners are inserted and tacked if the order calls for them. Finished bags are counted into bundles, pressed and strapped into bales, and labelled with the lot. Fabric sold as rolls skips this stage and is wrapped and labelled at the winder.

Tracing a defect back to its stage

The same map is useful when choosing a supplier. A factory that runs its own tape lines controls stages one and two, where strength and honest weight are decided. One that buys fabric and only converts it controls stage six. Neither is wrong, but they can promise different things. Our own sequence, from tape extrusion through circular looms to coating and conversion, is shown on the infrastructure page, and the checks made at each point are listed under quality standards.