Point-to-point variation is different than the area derived from a specific amount ( weight ) of PE film. The word for total amount derived is called yield.
The industry standard is loosely defined as plus / minus 10 %. Smaller custom orders should allow wider tolerances.
Here’s what happens in real life: The operator sets up the order. When the conditions of width, resin recipe and polyethylene film thickness ( a tad on the thick side ) are right, the first roll(s) of custom saleable film is started. The target weight of the first roll of layflat tubing or first two rolls in the case of single wound sheeting will be “heavy” or over the target weight. Adjustments to the process conditions are then made. By the third roll of layflat tubing or “set” of sheeting, the roll weights will be lighter than the target weight thereby ensuring good yield.
Over the course of the run, the customer will get more area than what is referred to as the ” theoretical” yield. Theoretical yield is calculated by dividing the yield, or weight per 1,000 lineal feet, into the total pounds shipped. Example: 35″ X 4 mil sheeting weighs 56 pounds per 1,000 lineal feet ( the “yield” = 56 pounds ). If 3,158 pounds are shipped, the theoretical yield, or number of feet the customer should have gotten out of 3,158 pounds is 56,392.
What also happens in real life: If someone on the line in a printing / converting shop runs excess scrap, the tendency is to blame the film. The excuse is the extruder shipped “heavy”, or film thicker than ordered. Carefully selected samples from the scrap bin are offered as evidence. Samples are of course from the extruder’s setup rolls. The invoice is paid short and the dispute argument begins.
Two other common scenarios are failures resulting from gauge chiseling by either or both distributors or bagmakers. For custom bags:
1. Customer orders bags or film of a specific thickness.
2. Distributor shops it with RFQ of 10% lighter than the gauge called out
3. bagmaker quotes shaving another 10% or more
4. when the film order goes to the extruder’s shop floor, the operator has + – 10% leeway so right there the film could be 30% light.
5. the bags fail
6. the acrimony begins
Distributors sell film by the roll with the ostensible gauge on the tags taking advantage of the information asymetry. Plant managers have enough to deal with. They trust their distributor and have no idea how to keep them honest.
Film is ordered light of course.
Because we do not engage in this common practice we do not do business with many distributors. Our orders show the same information to the customer and to the shop floor.