A tolerance written as ±0.5 mm looks precise until you ask which dimension, measured how, on which sample. Most arguments about machine accuracy come down to a tolerance that was never defined in measurable terms.
Which dimensions actually carry a tolerance
- Cut-off length — the repeat length of the bag or the sheet.
- Bag width and gusset depth — set by the forming section.
- Seal position and seal width — the dimension that decides whether the product holds.
- Print registration — position of print relative to the cut or the seal.
- Stack or reel quality — how evenly the output can be packed.
Each of these drifts for a different reason, so a single blanket tolerance across the machine is misleading.
Typical achievable values
The ranges below reflect normal production on well-maintained converting machines. They are starting points for a specification, not a guarantee:
| Dimension | Typical production tolerance | Main source of drift |
|---|---|---|
| Cut-off length | ± 1 mm | Encoder, servo tuning, web tension |
| Bag width | ± 1 mm | Forming plate wear, film or paper width |
| Seal width | ± 0.5 mm | Sealing bar temperature and pressure |
| Print registration | ± 0.5 mm | Web tension, print mark contrast |
| Gusset depth | ± 1 mm | Material stiffness, folding geometry |
Machines with servo-driven sealing and closed-loop tension control hold the tight end; mechanical lines with fixed cams sit at the wider end. Ask which axis is closed-loop before accepting a tight number.
How to measure so the number means something
- Sample size. At least 30 consecutive pieces, taken after the machine has run for 15 minutes.
- Conditioning. Paper and film change dimension with humidity; measure in the production area, not in an air-conditioned office.
- Tool. Calipers for width, a steel rule or a template for length, a magnifier for seal width.
- Reporting. Give the mean and the range, not a single measurement. A machine that averages the nominal length but swings ±2 mm fails a ±1 mm specification.
What makes the tolerance drift
- Blade and anvil wear on the cut-off unit.
- Tension variation between a full and a nearly empty unwind.
- Temperature drift in the sealing bar over an eight-hour shift.
- Material lot change — 5% GSM difference is enough to move the fold line.
Writing it into the purchase order
Specify each dimension separately, with the sample size, the measurement method and the material to be used for the test. Add the conditions: speed at which the tolerance applies, and whether it holds over an eight-hour run. A tolerance without a sample size and a method is not a specification, it is a wish.