FABRIC WEIGHTS & MEASUREMENTS:
GETTING WHAT YOU PAY FOR
Of the many elements that go into making BOPP/WPP bag packaging, fabric weight has the most impact on price. Typically, BOPP bag manufacturers measure fabric weight in grams per square meter (GSM). For a long time, 85 GSM was the norm for BOPP bags. That has trended downward to 75 GSM; some producers are running 65 GSM or even 55 GSM bags on their filling equipment. Obviously, the lower the weight, the less fabric, and therefore, the lower your per-bag cost.
The lowest weights might run okay on manual filling lines, but most U.S. producers use automated filling equipment, and the less stiff the bag, the more challenging is filling it on automated equipment. (Some merchants also demand thicker bags.) Older swing-arm style filling machinery is particularly challenged by lower weights; if a bag is too thin, it will “creep” while in the cassette and slide out of position causing “mis-hangs.” Lower weights are handled a bit better by hangers that pick up bags lying flat.
You might see fabric weights measured by “denier,” typically calculated as square inches per pound. The different measurements can be correlated; for example, 85 GSM converts to a 900 denier.
All of these measurements and subsequent conversions can make it difficult to determine if your bags are truly measuring up to your specifications, that is, if you’re getting all the weight you’re paying for.
THE FOLLOWING PROCESS AND FORMULA SHOULD BE HELPFUL:
- Cut a measured square (accurately) and weigh it on a gram scale. Then convert to grams/square meter to get the overall structure basis weight.
- Subtract the basis weight of extrudate, typically 18 GSM, which is the polypropylene used to adhere the printed film to the fabric.
- Subtract the weight of the printed BOPP film, typically 16.2 GSM (18 micron).
- The remaining weight is
- the fabric weight, which in this case should be 75 GSM.If you’re looking to determine weight by denier, take the actual GSM weight, divide it by 85 and multiple by 900. E.g., 75 GSM divided by 85 GSM = 88.235% X 900 denier = 794.115 denier.
Remember this is an estimate and there will be some variation – most variance will come from extrudate – but you should be close enough to determine if your bags meet your fabric weight specification.
Understanding the measurements how to convert one specification to another will help you run a more effective cost analysis, leverage your packaging spend and make a smart BOPP packaging buy.