Because processing changes the material.
The finished bag has been exposed to extrusion temperatures, stretching, cooling, printing, sealing, cutting, and storage. Each step can alter performance.
A resin may have good laboratory properties, while the finished film has:
- uneven dispersion;
- weak seals;
- excessive orientation;
- thickness variation;
- poor print adhesion;
- localized defects.
The consumer uses the finished product, not the resin pellet.
Material data is the starting point. Film data shows processing performance. Finished-product testing shows whether the actual bag works.
All three levels are useful, but they answer different questions.