A packaging machine should be accurate enough to meet the legal quantity rules in your market while giving away as little product as possible. Accuracy depends on the filler type, the product and the pack size, so it must be stated for your product, not in general.
Most markets use an average quantity system. Under it, a more consistent machine lets you set the target fill closer to the declared weight without breaking the rules. That saves product on every pack.
How accuracy is expressed
Makers often quote accuracy as plus or minus a number of grams, or as a percentage of fill weight. The clearer measure is the standard deviation of fill weight across a run of consecutive packs at rated speed.
Ask for the figure on your product, at your pack size and speed. A plus-or-minus figure without a sample size, speed and product means little.
- Multihead weighers are usually the most accurate choice for granules and irregular pieces.
- Auger fillers are good for powders, but accuracy varies with powder density and flow.
- Volumetric cup fillers are simple and fast, but they measure volume, so fill weight drifts when bulk density changes.
- Piston and pump fillers are usually consistent for liquids at a steady temperature.
The legal rules, in general terms
In the EU, packs carrying the ℮ mark follow Council Directive 76/211/EEC. Its three rules are often summarised as follows. The average content must not be less than the nominal quantity. Only a small share of packs may fall short by more than the tolerable negative error. No pack may fall short by more than twice that error. Tolerable negative error values are set in tables by pack size.
The UK applies similar rules under its packaged goods regulations. In the US, officials check packages using the methods in NIST Handbook 133. India has its own legal metrology rules for packaged goods.
The packer or importer is legally responsible for pack quantities, not the machine maker. Confirm the rules for your market with your local weights and measures authority before you set your target.
Specifying and testing accuracy
- State the required standard deviation, or the plus-or-minus range at a stated confidence, for each product and pack size.
- State the speed at which that accuracy must be achieved.
- At the factory test, weigh at least 100 consecutive packs from a run with your product, as described in FAT and SAT.
- Consider a check-weigher at the end of the line to reject underweight packs and give you records.
- Repeat the weight check at the site acceptance test, because product density and humidity at your plant may differ from the factory trial.
Next step
Put the accuracy target in your packaging machine specification, so every maker quotes against it. We supervise weight trials at the factory and check the data before shipment. To discuss your line, book a free sourcing review.