Wet pet food — including soft pouches, sauce pouches, and small bags in place of pet canisters — has seen rapid growth in China in recent years. However, it is more demanding in terms of packaging than dry food. Most after-sales issues encountered by manufacturers ultimately relate to sealing: either consumers complain about leakage, or returned products from e-commerce show sealing areas with residue. InvestigationPet Wet Food Packaging BagWhen troubleshooting sealing issues, the biggest efficiency loss isn’t due to a single parameter being improperly adjusted, but rather due to a disordered troubleshooting sequence: changing the film today, adjusting the temperature tomorrow, and suspecting the equipment the day after, only to find the problem still unresolved after going through the motions. This article provides a troubleshooting order from the perspective of a factory engineer, categorized into four typical phenomena, for domestic brand owners and contract manufacturers to use during line debugging.
Why is the sealing of wet food small bags more demanding than that of dry food?
The sealing of small pouches for wet food faces three adverse conditions simultaneously. First, the product is a viscous liquid or paste, and slight inaccuracies in filling can result in the paste entering the sealing area. Second, after the product is fully filled, the sealing area will be subjected to squeezing by consumers, and any minor unmelted points will eventually be detected. Third, wet food is typically a high-protein, high-moisture system; once the seal leaks, the microbial risk is an order of magnitude higher than that of dry food, so the problem cannot rely on consumer tolerance to be resolved. Understanding these three points explains why troubleshooting should follow the approach of classifying issues and eliminating them layer by layer, rather than adjusting parameters based on intuition.
Phenomenon One: Blurred Surface and Material Inclusion – First check the filling process, then check the material.
Contamination of the product on the inner side of the seal is the top cause of leakage in wet food small bags. The recommended troubleshooting sequence is as follows:
- Check if the filling quantity is stable.Sample and weigh to observe the variation in the amount of product in each individual bag. If the variation is large, first adjust the filling system, do not rush to change the sealing parameters.
- Check the cut-off and tail drag.The presence of tailing or dripping after thepaste is cut directly affects the cleanliness of the bag mouth. The valve condition, cutting timing, and filling speed should be evaluated together.
- Check the edge sealing width and product liquid level.Is the safety margin between the liquid level and the sealing line sufficient, and has it shifted due to equipment vibration?
- Read the materials last.Different heat-seal layer formulations have varying tolerance levels for mild material trapping. However, if the filling end hasn’t been thoroughly checked, switching to a new film first is likely to transfer the problem to the new film.
Actions that can be taken on the material side include: confirming the grade of the heat-seal layer with the film manufacturer, verifying the heat-seal performance under material jam conditions, and evaluating the heat-seal layer direction that is more tolerant to contamination when necessary. However, the order must not be reversed—first the equipment, then the parameters, and finally the materials.
Phenomenon Two: False Sealing and Insufficient Strength – Checking the Three Elements of Heat Sealing
The surface appears sealed, but it peels off in one piece with a light tear, which is called a false seal. False seal issues are investigated around three key factors: heat sealing temperature, pressure, and time.
Has the temperature entered the window?
Each composite film has its own heat sealing temperature window, and the width of the window depends on the heat sealing layer material and the overall structure. In actual production, the displayed temperature does not equal the actual temperature of the blade. Long-term operation can cause temperature drift and heating element aging, resulting in the actual blade temperature being lower than the displayed value. It is recommended to regularly measure the blade temperature using temperature paper or a spot thermometer and compare it with the film’s window data. Differences in heat sealing characteristics among different material systems can be referenced from our previous “Heat Sealing Temperature Guide: Heat Sealing Parameters and Settings for Different Packaging Materials》。
Pressure and Parallelism
Uneven sealing pressure and poor blade parallelism can cause some areas on the same machine to seal well while others fail to seal properly. On multi-row small bag lines, this phenomenon often manifests as a high rate of missed packages in a specific row, which is an important clue for identifying mechanical issues.
Time and speed synchronization
After speed increase, dwell time is shortened, and temperature and pressure combinations that were previously sufficient may no longer be adequate. The sealing window should be reconfirmed after any speed change, rather than continuing to use old parameters.
Phenomenon Three: Leakage and Seepage – From Bag Making Quality to Transportation Process
The underfilled package received by consumers may not necessarily be a direct result of the sealing process. A complete investigation chain includes:
- Bagging process.Edge sealing has wrinkles, channels, and scratches. The multi-layer film at the wrinkled areas is not fully fused, which is a typical source of chronic leakage.
- Online Leak Detection.Confirm the method for leak detection, sampling frequency, and record-keeping procedures. For factories with the capability, it is recommended to conduct full-filled product testing for wet pet food rather than only testing empty bags.
- Transport Simulation.Damage to the sealed edges from stacking pressure, drop tests, and vibration is not covered by factory inspections. Full-load transportation simulation is the only method that accurately reflects the actual experience of consumers.
- Film Resistance.Products containing bone particles and meat chunks require higher puncture resistance for the bag, as mentioned in the guidelines.Pet Dental Chews and Meat Jerky Packaging: Why Puncture Resistance Can’t Be Judged Only by Bag ThicknessIt is discussed in detail in the following section.
Phenomenon Four: Swollen Bag — Identify the Cause First, Then Take Action
Swelling bags are the most serious type of complaint in wet food packaging. The principle for handling such issues is to first identify the cause before discussing solutions.
- Gas residue causing expansion.Air trapped during filling and fluctuations in the nitrogen charging process can cause the bag appearance to look full. This type of bloating can usually be traced back to a specific production batch, and the product inside remains in normal condition.
- Gas-induced swelling caused by microbial gas production.Contamination of contents and unusual odors are often related to seal leakage, incomplete sterilization, or raw material contamination. Such issues must be isolated by batch and subjected to microbial testing; they cannot be simply attributed to the packaging.
The packaging plant can support in providing seal integrity data, batch retention samples, and reconciliation records, helping to identify the issue as originating from the filling, sterilization, sealing, or storage and transportation process. For microbial safety of high moisture foods and label framework, refer to FEDIAF the relevant guideline documents; the food contact requirements for packaging materials can be compared with National Standards Full Text Public System Current regulations of the GB 4806 series.
Sample Confirmation Checklist: Preventing Issues Before Mass Production
Based on the above four phenomena, we recommend that domestic customers complete the following confirmations before the wet food small bag production line is officially launched.
- Conduct filling trials with real products, recording the accuracy of dosage, tailing, and sticking issues.
- Test the heat sealing window and record the operational ranges of temperature, pressure, and time in the technical specifications, rather than just specifying a target temperature.
- Multi-column equipment confirms the sealing quality column by column to exclude mechanical parallelism issues.
- Full-filled finished products undergo leakage testing and transport simulation, with data retained.
- Confirm with the film manufacturer the management method for consistency of the heat-seal layer grade, to prevent window drift after batch change.
The forming logic of the bag structure itself — how three-sided sealing, four-sided sealing, and eight-sided sealing are formed — is described in the following.Bag Making Process Explained: How to Form Three-Side Sealing, Four-Side Sealing, and Eight-Side Sealing BagsAn article with illustrations is available for reference, suitable for pre-consultation before contacting the factory. The layered structure of composite materials can be referenced in “How Many Layers Does a Composite Packaging Bag Have? What Is the Function of Each Layer? Factory Structure Diagram.” If your product line includes large-sized main feed bags, the selection criteria can be compared with “Pet Food Packaging Bag Selection: From Specifications, Stacking to Post-Opening Usage Experience.”
Common Questions
Why do only some columns show false sealing when using the same machine and the same batch of film?
Check the pressure parallelism and heating bar condition first, then suspect the film. Poor sealing is usually concentrated in the fixed column, indicating that the heating bar temperature in that column is too low, the pressure plate is uneven, or there is a shift during bag movement, causing the actual temperature and pressure applied in the sealing zone to be lower than the set values. After calibrating these points, if only a few columns still have issues, check the thermal sealing layer batch differences of the film. The basis for judgment is whether the leak points are fixed: if they are in a fixed position, it is likely a problem with the equipment; if they are randomly distributed, it is more likely a material issue. Refer to the method for confirming the thermal sealing temperature window.
The seal strength test has passed, but why does the finished product still leak?
Because strength only indicates whether the seam can be torn, it does not reveal whether there are micro channels inside the seam. Test samples are usually taken from the most regular part of the seam, while leakage points often occur in areas such as the sealing zone with material trapping, the end of the seam, or positions where the bag is misaligned. To locate the leakage, the leaking package must be cut open to examine the cross-section and record the distribution of the leakage points on the bag. If the seam cannot be peeled off or if fiber-like pulling occurs when tearing, it indicates that the actual sealing condition differs from the sample.
Is a swollen bag the same as a leak?
Differences in handling approaches are significant. Swelling can be caused by microbial gas production or by insufficient sealing or material barrier properties allowing external gases to enter. First, check the batch in which the swelling occurred and the storage and transportation temperature, then compare with retained samples from the same batch: if it’s due to gas production, similar manifestations are often observed in different packaging forms within the same batch; if it’s a barrier issue, it usually accompanies defects in the sealing area or the material itself. Before determining the cause, do not directly adjust sealing parameters, as this may mask the true root cause. The relationship between layer structure and barrier properties can be referenced in the Composite Film Layer Structure Description.
Which step is most likely to be overlooked during the sample stage?
The most overlooked step is confirming the sealing performance using real product contents at actual production line speeds. Many samples are run with empty bags or water, resulting in visually appealing seals. However, when switching to products like pastes, oils, or those with particles, the situation changes completely. During confirmation, you should at least check for any material trapping in the sealing area, ensure the sealing edge is even, and examine the cross-section after tearing. Also, record the parameters used at that time. Most issues in mass production of pet wet food packaging bags can be identified early through these steps. Keeping sample records in the workshop is far more useful.