The 3 Real Boundaries of Metrological Assurance for Prepackaged Goods—Net Content, Negative Deviation, and Absolute Value: Where Printers Most Often Go Wrong on “500g × 24 Bags” Orders

Last winter, a nut-processing client approached us to print composite flexible bags for 500g×24 units. The specifications they provided were beautifully drafted: net content 500g, allowable deviation ±3%, aluminum foil composite. None of the earlier terms required any discussion; the only issue was the small print on the final line—“Must comply with the requirements of JJF 1070 for the measurement assurance capability of net content in quantitative packaged products.”

The customer's intention is to place the product on supermarket shelves. The supermarket's inspector will randomly select 3 bags and weigh them on a net-content scale. If the weight is less than 485g, that bag will be deemed nonconforming and the entire batch will be returned.

The reality on the packaging side is that the filling line for composite bags operates in the customer’s factory, not ours. There is only one thing we can control: how much the bag can hold. In other words, the bag’s net weight capacity.

The First Boundary: Does the Printing Factory Manage the Net Content Itself?

Most procurement specialists believe that “net content is a filling matter and has nothing to do with the printing plant.” This is only partly correct. The net content value is set on the filling line, but whether the bag can accommodate this value must be determined during the printing and plate-making stages.

We took on an interesting project: the customer switched from the original 60g bag to a 75g bag because “the unit price of nuts has gone up, so filling the bag a little more makes it look more economical.” The net content on the specification sheet was changed from 60g to 75g, but the bag format was not changed. He assumed that the bag would “stretch anyway.”

During trial production, the width of the filling opening did not match the bag’s heat-seal edge width. The volume of the 75g nuts was approximately 30% larger than that of the 60g nuts, causing the filling opening to spread open and leaving only 6mm of the heat-seal area, resulting in weak sealing. The customer filled 200 bags for testing, and 11 bags burst.

Our conclusion to the customer is straightforward: the bag cannot accommodate 75g. Either the bag format must be changed (with the tooling costs recalculated), or the bag material must be increased from 60 microns to 75 microns (adding 0.12 yuan to the cost per thousand bags). The customer chose to change the bag format.

The boundary is this: the brand is responsible for the net content value, but the printer must raise any written objections to whether the bag can accommodate that value during the plate-making stage. If no objection is raised, the printer will be the primary party responsible when problems occur, because the printer made the packaging and had the ability to identify such issues during the proofing stage.

The second boundary: negative deviation and the average are two different things

This is the concept most likely to cause confusion among laypeople, and the one we explain to customers most often.

Suppose the customer requires 500g of nuts. Negative deviation means that the weight of an individual bag must not be less than 485g (for example, a tolerance of -3%). Average weight means the total weight of all bags in the batch divided by the number of bags; the average must not be less than 500g.

The logic of JJF 1070 is that the average value must be sufficient. A certain negative deviation is permitted for individual bags, but it must never exceed the limit. Both requirements must be met simultaneously.

Once, a customer told us: “Our filling line is set to 490g, and the spot checks have always passed.” We asked how they conducted the spot checks. They said: “Our own quality control takes 3 bags, weighs them, and checks whether they are all above 485, then releases them.”

This is a typical case of failure. The fact that all 3 bags are qualified does not mean the average value is qualified. Random sampling by supermarkets or third-party inspection agencies is conducted using the GB/T random number table, with 8 bags sampled to calculate the average. If the customer's production line is running 10g overweight overall, it is useless for the bags produced by the printing plant to have even more net capacity—the filling line is off, so the bags will retain excess capacity.

Here is how we remind our customers: “Your specifications state a negative tolerance, but we make the bags according to the average capacity. Please provide us with the average capacity value, as this is more important than the negative tolerance value.”

The third boundary: absolute content error versus relative content error

This procedure is unique to products sold by net weight and is completely different from that for products sold by the piece.

Piece-count products (such as “10-piece packs”) have discrete tolerances: 9 pieces fails, with no intermediate state. Net-weight products are continuous: 499g and 500g differ by only 0.2%, but under strict quantitative packaging requirements, this difference can be measured.

Therefore, JJF 1070 imposes an absolute-value constraint on average net content. For example, for a nominal content of 500g, the average must not be lower than 500g minus a certain absolute amount, nor may it exceed a certain absolute amount, preventing "overfilling" to deceive consumers. This is a hard constraint on the filling line.

The practical problem we encounter when making bags is that the bag format specified by the customer is designed for "maximum capacity," such as being able to hold 520g. This prevents bag breakage when the filling line overfills. However, this design has two consequences—

First, the bottom and sides of the bag are left blank, giving customers the impression that it “does not look solid”; second, the visible printing area on the front of the bag is reduced, compressing the high-resolution graphic area requested by the customer.

We usually offer customers two options: a bag designed at 2% above the nominal capacity and a bag designed at 8% above the nominal capacity. This allows customers to choose between cost and tolerance for deviations themselves, rather than having us make the decision for them.

An Account We Got Wrong

Last year, a customer developed a laundry detergent pouch labeled 2kg. During the plate-making stage, we designed it for 2.05kg on the grounds of “allowing a 2.5% filling tolerance.” The customer raised no objection and proceeded directly to production.

By the 3rd batch, the weighing sensor on the customer's filling line had drifted, resulting in an actual fill weight of 1.94kg/bag. By the time the issue was detected through spot checks, 8000 bags had already been produced.

When the customer held us accountable, they said: “The bag could not hold enough, so we did not fill it properly.” We pulled out the plate-making confirmation sheet from that time—it listed the net capacity of 2.05kg, confirmed and signed by the customer. The fault lay with the filling line, not the bag. Nevertheless, the dispute continued for two weeks.

After that, we changed our approach: on each plate-making confirmation sheet, the three fields related to net content (nominal value, average-capacity design value, and negative-deviation limit) must be listed on a separate line, with the customer signature field immediately adjacent. Otherwise, the customer is signing off on “what the bag looks like,” not “how much the bag can hold.”

Three Things the Printing Plant Can Actually Do

First, include the bag format’s net capacity specifications in the quotation at the quotation stage. Do not put them on the design confirmation sheet, as customers generally do not review that document carefully.

Second, when the customer changes the declared net weight, proactively conduct a version-capacity review and document the review findings in writing. This step takes less than ten minutes but can prevent most subsequent disputes.

Third, require the customer to provide a physically filled sample during the prototyping stage, rather than merely examining empty bags. Empty bags do not reveal changes in shape once filled, nor do they reveal problems caused by the heat-sealing area being stretched.

None of these three services is provided for an additional charge, but each will significantly alter the printer's position in the chain of responsibility.

Further Reading

This article addresses the intersection between metrology and packaging standards. If you are dealing with other compliance-related issues, you may want to continue reading these articles: