Quality control is not a single inspection added at the end of production. At every step, from receiving the raw material to packing the bags, something is measured, compared and recorded. The final check is like a summary of this chain: does the bag match the jointly agreed dimensions and the expectations for its use?
For a business buying bags, understanding this step also shows what it needs to specify when placing an order. A check only means something if there is a measurable target.
- Width and length measurement The bag’s width, length, gusset and handle dimensions are compared with the specification.
- Visual check The film surface is inspected for gels, holes, streaks and print defects.
- Seal seam The continuity and strength of the seam are checked by hand and with test instruments.
- Thickness measurement The film is measured at several points to check thickness and its distribution.
- Record Results are entered on the checklist together with the batch information.
Checks throughout production
Checking starts with the raw material. The type and batch of the incoming granules are recorded, so that if a problem arises, it is possible to trace which raw material was used. During film extrusion, film width and thickness are monitored; on some lines these values are measured and recorded automatically throughout production.
Film rolls are also reviewed before moving to the next step: the evenness of the roll edges, winding hardness and obvious surface defects can be spotted at this point. Colour tone and registration are checked at regular intervals in printing, and bag length and seam appearance in cutting and sealing. These in-process checks stop defective product from building up; when a problem is caught early, losses stay limited.
Dimensions and thickness
On samples taken from finished bags, width, length, gusset depth and handle dimensions are checked with a ruler or template. How far dimensions may deviate from the target value should be set in advance; otherwise what is acceptable remains open to dispute.
Thickness is measured in microns and assessed not from a single point but from several readings taken at different places on the film. Distribution matters as much as average thickness: a film whose average is right but whose one edge is noticeably thin is more prone to tearing from that edge. Thickness tolerance is a value that should be stated in the order together with the thickness.
Bag weight is also a practical checking tool. Weighing a set number of bags quickly shows the overall deviation in thickness and dimensions. Especially for bags bought and sold by the kilogram, knowing the relationship between weight and count is important for both manufacturer and buyer: thicker film means fewer bags for the same weight.
For measurements to be reliable, measuring instruments need to be checked regularly and samples taken by the same method. Measurements taken from different places or by different methods can give different results for the same bag.
Strength: seal, tear and impact
The real test of a bag is use; strength checks try to simulate that use at the production site. Seal strength shows how well the seam holds when it is strained. In a simple hand check, the seam is pulled apart: with a good seal the seam does not separate; the film stretches or breaks, and the break usually occurs right at the edge of the seal line. If the seam separates, the layers have not fully fused.
Tear strength describes how easily a tear that has started will propagate, and is assessed separately for the machine direction and the transverse direction. Impact strength shows how well the film withstands a sudden load, for example the bag hitting the ground or a heavy product dropping into it. Puncture resistance is a separate property: it describes resistance to a sharp corner slowly forcing its way through the film. In laboratories, tensile strength, elongation and heat-seal properties are also measured with instruments.
Another method frequently used in practice is the load test: the bag is filled with a weight similar to its intended use and carried or left hanging for a set time. This test helps show how laboratory values translate into real use.
Print and appearance
- Registration: Whether the colours have shifted relative to one another and the position of the print on the bag are checked.
- Colour tone: The print is compared with the approved sample under the same light.
- Ink adhesion: Tape is stuck to the printed surface and pulled off to see whether the ink lifts.
- Surface defects: Holes, gels, streaks, wrinkles and contamination are looked for visually.
Deciding what to check
A good check starts with a good description. Having a written specification between the business buying the bags and the manufacturer reduces disputes both during production and at delivery. A specification covers the following headings:
- Dimensions: Width, length, gusset and handle dimensions, with acceptable deviations.
- Film: Raw material type or blend, colour and thickness, with thickness tolerance.
- Print: Approved design, colours and the position of the print on the bag.
- Use: The load the bag will carry and the conditions of use, and the strength checks to be carried out accordingly.
- Packing: Quantity per pack, label information and sample expectations.
When a nonconformity is found
When a deviation is found during checking, the first task is to understand how widespread the problem is. The affected rolls or packs are set aside, and additional samples are taken from the rest of the production. If the problem is limited to a single roll, that section can be separated and sent for recovery; if it is widespread, production is stopped and the settings are reviewed.
Finding the cause often means going back to earlier steps. An opening seam can stem from the seal setting or from film with uneven thickness. That is why keeping records at every step speeds up finding the root of the problem.
Samples, records and traceability
Samples set aside from each production batch are concrete evidence to refer to if a question arises later. Which roll a sample came from, on which date and with which raw material batch it was produced are recorded. These records make it easier to find the source of a problem and prevent the same defect from recurring.
Additional checks may be needed in areas such as food contact, medical use or special transport conditions; which checks are required is determined separately, based on the area of use and the rules in force.
Quality control is the last stop on a journey that began with the granule. But it is also the starting point for the next production run: lessons from the records feed back into raw material selection, line settings and sealing parameters. A bag that has passed this step is now ready for the product it will carry and for its user.