PP Woven Bag Quality Checklist
A receiving inspection for woven sacks: how many to sample, twelve checks with the tool each needs, and what to do with a lot that fails.
A factory test report says what the factory found on the day it tested. It does not say what is in the container at the buyer's door. Most problems with woven sacks, short weight, wrong size, weak seams, print that rubs off, can be caught at receiving in under an hour with a scale, a tape measure and a magnifier, before a single bag reaches the filling line. This checklist is written for that hour. It assumes no laboratory, and it marks the few checks that do need one.
Before the goods arrive
An inspection compares goods with something. Agree that something in writing when the order is placed: a specification sheet with a tolerance on every number, and a signed sample kept by both sides. Without a tolerance, a bag one centimetre short is an argument. With one, it is a pass or a fail. Keep the approved sample in a sealed bag away from light, because it is the reference for colour, feel and construction that no document captures.
How many bags to check
Checking every bag is impossible and checking one is meaningless. The usual method is a sampling plan under ISO 2859-1, which gives a sample size for the lot size and an acceptable quality limit for each class of defect. In practice, draw bags from several bales, from the top, middle and bottom of the load, not all from the bale nearest the door. Measure a small fixed number, such as ten, for weight and size, and look over a larger number for visible faults. Write down which bales the samples came from; if a problem appears, that record shows whether it runs through the lot or sits in a few bales.
The twelve checks
| Check | How | Tool |
|---|---|---|
| 1. Count and labels | Bales against packing list; bags per bale on two bales | None |
| 2. Width and length | Bag flat on a table, measured edge to edge | Steel rule |
| 3. Bag weight | Weigh ten bags, take the average | Scale reading to 1 g |
| 4. Fabric GSM | Weigh a 10 x 10 cm single-layer square, multiply by 100 | Scale reading to 0.01 g |
| 5. Mesh | Count tapes in one inch, both directions | Pick glass or magnifier |
| 6. Weave faults | Hold against light: missing tapes, holes, stripes | Light |
| 7. Bottom seam | Stitches per 10 cm, fold depth, thread ends secured | Rule |
| 8. Coating | Try to peel at a cut edge; pour water into a bag and watch | Knife, water |
| 9. Print | Against approved sample; press and pull adhesive tape | Sample, tape |
| 10. Liner, valve, handle | Present, right size, attached as specified | Rule |
| 11. Drop test | Fill to weight, close as in production, drop flat and on its end | Product, scale |
| 12. Smell and cleanliness | Open a bale; no oil, dust or odour, especially for food | None |
Every check needs a number to be compared with, and that number comes from the agreed specification. As reference points from our own range: the PP woven cement bag is 90-115 GSM in 45 x 75 cm and 50 x 80 cm; the PP laminated fertilizer bag is 80-100 GSM including a 15-25 GSM coating; the printed PP woven rice bag is 65-85 GSM; the PP laminated sugar bag is 70-90 GSM; the custom printed PP woven bag holds print register to plus or minus 0.5 mm; and the standard PP woven fabric roll runs 60-120 GSM. A supplier's published range tells you what is normal; the order states which point in that range you bought.
Weigh the bags first
If only one check is made, weigh the bags. Weight catches a lighter fabric, a shorter cut and a thinner coating at once, because all three remove material. Work out what a bag should weigh from the specification, twice the width times the cut length times the GSM, and compare the average of ten. Our guide to GSM, denier and mesh shows the calculation. A lot that averages on target but swings widely from bag to bag is also telling you something: the loom or the cutter was not under control.
The drop test
Every other check is a proxy for whether the bag survives being filled, closed and thrown. Fill sample bags with the real product to the real weight, close them the way the line will, and drop each one flat on both faces, on each edge and on its end from the height your specification states, onto a hard level floor. A bag passes if nothing ruptures and no product escapes. National sack standards define the height and number of drops for their own products; use those where they apply. The test costs a few bags and an hour, and it finds weak seams and under-strength fabric that no measurement with a ruler will.
What needs a laboratory
- Breaking strength
- A strip test on a tensile machine, in warp and weft. Compare with the specified figure; our standard fabric is 800-2,000 N per 5 cm and our cement bag exceeds 3,500 N in the warp.
- Seam strength
- A tensile test across the stitched bottom. The seam should fail later than the specification requires, not at the first stitch.
- UV resistance
- Retained strength after accelerated weathering. It takes days, so ask for the report with the shipment.
- Moisture barrier
- Water vapour transmission of coated fabric; our PP laminated fabric roll and fertilizer bag are specified below 5 g per square metre per day.
- Food-contact compliance
- Migration testing and a declaration for the material, issued for the resin and additives used.
For those five, the practical route is to ask for the supplier's report on the production lot and to send one retained sample to an independent laboratory from time to time. For sacks that touch food, read our guide to food-grade woven bag standards for which documents mean something and which do not.
Classing what you find
| Class | Meaning | Examples |
|---|---|---|
| Critical | Unsafe or unusable | Seam opens in the drop test; contamination in a food sack |
| Major | Likely to fail in use or be rejected by the customer | Under weight beyond tolerance; wrong size; print that lifts |
| Minor | Visible, does not affect use | Slight shade difference; a loose thread end |
When a lot fails
Stop, do not sort quietly and carry on. Set the lot aside, photograph the faults next to a ruler and the bale label, and keep the failed samples. Send the supplier the numbers: how many checked, how many failed, on which check, from which bales. A good supplier will want a joint re-inspection and can usually trace the cause from the bale and lot numbers. Using part of a failed lot before the claim is settled weakens it, because the evidence is then in a customer's warehouse.
Frequently asked questions
How many woven bags should I inspect from a shipment?
Use a sampling plan such as ISO 2859-1, which sets the sample size from the lot size. As a working minimum, measure ten bags drawn from different bales and look over many more for visible faults.
What is the most important test for a woven sack?
A drop test with the real product at the real fill weight. It checks fabric and seam together under the load the bag will see.
How do I check GSM at receiving?
Cut a 10 x 10 cm square from one layer, weigh it on a scale reading to 0.01 g and multiply by 100. Average several squares.
How can I test whether print will rub off?
Press adhesive tape firmly onto a printed area and pull it away sharply. Ink that comes off on the tape will come off in handling.
Can I check UV stabilization on arrival?
No. It cannot be seen or measured without weathering equipment. Rely on a test report for the lot and, for critical uses, an occasional independent test.